Cell grows; organelles increase; proteins and RNA synthesised; preparation for DNA replication
S (Synthesis)
DNA replication — DNA amount doubles; each chromosome duplicates into 2 sister chromatids; histone proteins synthesised
G₂ (Second Growth)
Further cell growth; spindle proteins (tubulin) synthesised; DNA checked for errors; cell prepares for division
M Phase
Mitosis + Cytokinesis
Nuclear division then cytoplasm division → two daughter cells
Stages of Mitosis
🧬 Mitosis — Equational Division
Mitosis: parent cell (2n) → two daughter cells (2n). Occurs in somatic cells for growth, repair, asexual reproduction. Chromosome number is conserved.
Stage
Key Events
Memory Tip
Prophase
Chromatin condenses → chromosomes visible. Nuclear envelope and nucleolus disappear. Spindle fibres form (centrioles move to poles in animal cells). Chromosomes = 2 chromatids joined at centromere.
Prophase = Prepare (chromosomes appear)
Metaphase
Chromosomes align at equatorial plate (middle). Spindle fibres attach at centromeres. Best stage to count chromosomes — maximally condensed and spread out.
Metaphase = Middle
Anaphase
Centromeres split. Sister chromatids pulled to opposite poles by spindle fibres. Each chromatid is now called a chromosome. Cell elongates.
Anaphase = Apart
Telophase
Chromosomes at poles. Nuclear envelope reforms. Chromosomes decondense. Nucleolus reappears. Two nuclei formed.
Telophase = Two nuclei
Cytokinesis
Animal cells: cleavage furrow (actin ring pinches inward). Plant cells: cell plate (vesicles fuse at midline → new cell wall). Result: 2 genetically identical daughter cells.
Cytoplasm divides
Best stage to study chromosomes: Metaphase — chromosomes are maximally condensed and spread on the equatorial plate. Best stage to count chromosome number.
Meiosis — Reduction Division
🧬 Meiosis Overview
Meiosis: one diploid cell (2n) → four haploid cells (n). Two divisions: Meiosis I (homologous chromosomes separate) and Meiosis II (chromatids separate). Occurs in gonads to produce gametes.
Crossing over: During Prophase I, homologous chromosomes pair (synapsis) and exchange segments at chiasmata → genetic recombination → variation in offspring.
Independent assortment: Homologous pairs orient randomly at Metaphase I → each combination of maternal/paternal chromosomes possible in gametes.
Homologous chromosomes: Pairs of chromosomes (one from each parent) of same size, shape and gene loci but possibly different alleles. Also called bivalents when paired in Prophase I.
Stage
Key Events
Prophase I (longest)
Homologous chromosomes pair (synapsis) → bivalents formed. Crossing over at chiasmata. Nuclear envelope breaks down. Spindle forms.
Metaphase I
Bivalents align at equatorial plate. Each pair orients randomly (independent assortment). Spindle fibres attach to centromeres.
Anaphase I
Homologous chromosomes (not chromatids) pulled to opposite poles. Chromosome number halved.
Telophase I
Two cells form — each haploid but chromosomes still double-stranded (2 chromatids each).
Meiosis II
Similar to mitosis. Chromatids separate. Four haploid cells result. Each genetically unique.
Mitosis vs Meiosis
Feature
Mitosis
Meiosis
Cells produced
2 daughter cells
4 daughter cells
Chromosome number
Same as parent (2n → 2n)
Half of parent (2n → n)
Genetic composition
Identical to parent
Different (variation via crossing over + independent assortment)
Crossing over
Does not occur
Occurs in Prophase I
Number of divisions
One
Two
DNA replication
Once, before division
Once (before Meiosis I only — NOT between I and II)
Maintains species chromosome number across generations; creates genetic variation
Synapsis
Absent
Occurs in Prophase I
Significance of Cell Division
📋 Why Cell Division Matters
Mitosis:
Growth of multicellular organisms from zygote
Repair of damaged/dead cells (wound healing)
Replacement of worn-out cells (RBCs, skin cells)
Asexual reproduction (budding in Hydra, regeneration in Planaria)
Maintains exact genetic copies in all somatic cells
Meiosis:
Produces gametes (sperm and eggs)
Reduces chromosome number from 2n to n
Fertilisation restores 2n → species chromosome number constant across generations
Creates genetic variation (crossing over + independent assortment) → raw material for evolution
Source of variation that drives natural selection
Exam Q&A
Q1. Why is meiosis called a reduction division?+
Meiosis is called a reduction division because the chromosome number is halved. A diploid parent cell (2n = 46 in humans) produces four haploid daughter cells (n = 23). When two gametes fuse at fertilisation, the diploid number (46) is restored. This reduction ensures the chromosome number stays constant across generations in sexually reproducing species. Without this halving, each generation would double the chromosome number.
Q2. State the significance of crossing over.+
Crossing over occurs during Prophase I when homologous chromosomes exchange segments at chiasmata. Significance: (1) Creates new gene combinations on chromosomes — neither purely from mother nor father. (2) Produces genetic variation in offspring. (3) Genetic variation is the raw material for natural selection and evolution. (4) Explains why siblings (except identical twins) are not genetically identical even though they inherit genes from the same two parents.
Q3. At which stage of mitosis would you observe: (a) chromosomes most clearly, (b) a cleavage furrow?+
(a) Metaphase — chromosomes are maximally condensed and spread out on the equatorial plate. Best stage to count chromosomes and study their shape.
(b) Cytokinesis (following Telophase) — in animal cells, the actin ring contracts to form a cleavage furrow that pinches the cell into two. In plant cells, a cell plate forms instead (not a cleavage furrow) because plant cells have a rigid cell wall.
Q4. Why does DNA replication NOT occur between Meiosis I and Meiosis II?+
DNA replication occurs only once (during S phase of Interphase, before Meiosis I begins). Between Meiosis I and Meiosis II there is a brief interphase-like period (interkinesis) but NO S phase and NO DNA replication. This is essential — if DNA replicated again between the two divisions, the chromosome number would not be halved correctly. Meiosis II simply separates the already-replicated chromatids, similar to mitosis. This ensures gametes have exactly n chromosomes.
02
Chapter Two
Structure of Chromosomes & DNA
Key Terms
ChromatinLoosely coiled DNA + histone protein material in the nucleus during interphase. Not visible as distinct chromosomes. Condenses into chromosomes at the start of cell division. Exists as euchromatin (actively transcribed, loosely packed) and heterochromatin (inactive, tightly packed).
ChromosomeCondensed chromatin visible during cell division. Made of DNA wound around histone proteins. Humans: 46 chromosomes (23 pairs) in somatic cells. Each consists of 2 sister chromatids joined at centromere (after replication).
ChromatidEach arm of a replicated chromosome. Two identical sister chromatids are joined at the centromere. After centromere splits in Anaphase, each chromatid becomes an independent chromosome in a daughter cell.
CentromereConstriction point joining the two sister chromatids. Site where spindle fibres attach (via kinetochore). Its position determines chromosome shape: Metacentric (centre), Sub-metacentric (slightly off), Acrocentric (near end), Telocentric (at end — terminal).
GeneA specific sequence of DNA nucleotides occupying a fixed position (locus) on a chromosome. Codes for a particular protein/polypeptide. The functional and hereditary unit of inheritance. Humans have ~20,000–25,000 genes.
GenomeThe complete set of genetic information (all DNA) in an organism. Human genome = ~3 billion base pairs. Entire genome is present in every somatic cell (though not all genes are active in every cell).
DNA NucleotideThe monomer unit of DNA. Consists of: (1) Deoxyribose sugar, (2) Phosphate group, (3) Nitrogenous base (A, T, G or C). Base pairing: A–T (2 H-bonds), G–C (3 H-bonds). Chargaff's rule: %A = %T; %G = %C in any DNA sample.
Feature
Chromatin
Chromosome
Appearance
Loosely coiled, thread-like
Condensed, rod-shaped, distinct
Visibility
Not visible under light microscope
Clearly visible during cell division
Stage present
Interphase (non-dividing)
During mitosis/meiosis
Composition
DNA + histone proteins
Same — just more tightly coiled
Human Chromosomes
Type
Number
Description
Autosomes
44 (22 pairs)
Non-sex chromosomes. Same in males and females. Control body characteristics. Numbered 1–22 by size.
Sex chromosomes (allosomes)
2 (1 pair)
Female: XX (homogametic). Male: XY (heterogametic). Y is smaller than X and carries fewer genes. Sex of offspring determined by which sperm fertilises the egg.
Types of Chromosomes by Centromere Position
Type
Centromere Position
Arm Ratio
Shape during Anaphase
Metacentric
Centre — equal arms
1:1
V-shaped
Sub-metacentric
Slightly off-centre
Unequal but not extreme
J-shaped
Acrocentric
Near one end — very short arm
Extremely unequal
Rod/J-shaped
Telocentric
At end — one arm only
One arm = 0
I-shaped (rod)
DNA Replication (Semiconservative)
🔬 DNA Replication
Occurs during S phase of Interphase. Each strand of the double helix serves as a template for a new complementary strand.
Semiconservative replication: Each new DNA molecule contains one old (parent) strand and one new (daughter) strand. Proved by Meselson and Stahl (1958) using N¹⁵/N¹⁴ experiment.
Key enzyme: DNA polymerase adds complementary nucleotides to the template strand (A pairs with T; G pairs with C).
Result: One DNA molecule → Two identical DNA molecules. Each daughter cell gets an exact copy of the genome.
Exam Q&A
Q1. What is the role of the centromere during cell division?+
(1) Holds the two sister chromatids together after DNA replication until they need to separate. (2) Site where spindle fibres attach (via the kinetochore protein complex). During Anaphase, spindle fibres pull on the centromere, dragging each chromatid to opposite poles. When the centromere splits, the chromatids separate and each becomes an independent chromosome in a daughter cell.
Q2. Distinguish between a gene and an allele.+
Gene: A specific sequence of DNA at a fixed position (locus) on a chromosome that codes for a particular trait or protein. E.g., the gene for flower colour in pea plants.
Allele: One of two or more alternative forms of the same gene at the same locus. E.g., the allele for purple flowers (P) and the allele for white flowers (p) are both alleles of the flower colour gene.
Key distinction: Every gene has a specific location; alleles are the different versions (variants) of that gene. A diploid organism has two alleles for each gene — one on each homologous chromosome.
Q3. If one strand of DNA has the sequence ATCGGT, what is the complementary strand?+
Using base pairing rules: A pairs with T; T pairs with A; C pairs with G; G pairs with C.
Template strand: A – T – C – G – G – T Complementary strand: T – A – G – C – C – A
The complementary strand also runs antiparallel (opposite direction) to the template strand.
03
Chapter Three
Genetics — Mendel's Laws & Inheritance
Key Terms
GeneUnit of heredity; specific DNA sequence on a chromosome that codes for a trait.
AlleleAlternative forms of the same gene at the same locus on homologous chromosomes. E.g., T (tall) and t (dwarf) are alleles of the height gene.
DominantAllele that expresses itself even when only one copy is present (heterozygous). Written as capital letter (T). Masks the recessive allele.
RecessiveAllele expressed only when two copies are present (homozygous recessive). Written as small letter (t). Masked by dominant allele.
HomozygousTwo identical alleles for a trait: TT (homozygous dominant) or tt (homozygous recessive). Also called pure-breeding.
HeterozygousTwo different alleles for a trait (Tt). Also called hybrid. Shows dominant phenotype but carries recessive allele.
GenotypeThe actual genetic constitution (alleles present). E.g., TT, Tt, tt. Cannot always be determined by observation alone.
PhenotypeThe observable physical characteristic produced by the genotype (interacting with environment). E.g., tall or dwarf.
MutationA sudden heritable change in DNA (gene or chromosome structure/number). Can be spontaneous or induced (by radiation, chemicals — mutagens). Source of new genetic variation. E.g., sickle cell anaemia from a point mutation (GAG → GTG in haemoglobin gene).
VariationDifferences among individuals of the same species. Genetic variation (heritable — from mutation, crossing over, independent assortment) or environmental variation (non-heritable — diet, climate, exercise).
Test CrossCross between an organism of unknown genotype (showing dominant phenotype) and a homozygous recessive individual. If ALL offspring show dominant trait → unknown was homozygous dominant (TT). If 50% show recessive → unknown was heterozygous (Tt).
Back CrossCross between F₁ offspring and either parent. If crossed with the homozygous dominant parent = back cross. If crossed with recessive parent = test cross. Used to determine genotype.
Mendel's Three Laws
📜 Law 1 — Law of Dominance
When two pure-breeding individuals contrasting in one trait are crossed, only the dominant trait appears in F₁. The recessive trait is present but hidden. Example: TT × tt → all Tt (all tall; T dominates t).
📜 Law 2 — Law of Segregation (Purity of Gametes)
The two alleles for a trait separate during gamete formation so each gamete receives only one allele. Alleles do not blend. In F₂: 3:1 phenotype ratio; 1:2:1 genotype ratio.
Basis: During Meiosis I, homologous chromosomes (carrying the two alleles) separate into different gametes — so each gamete gets only one allele.
📜 Law 3 — Law of Independent Assortment
Alleles for different traits on different chromosome pairs segregate independently of each other during gamete formation. Dihybrid F₂ phenotype ratio = 9:3:3:1.
Basis: Random orientation of different bivalents at Metaphase I. Applies only when genes are on different chromosomes (not linked).
Monohybrid Cross — Tall × Dwarf
P generationTT × tt
(tall) (dwarf)
F₁ generation — all Tt (all Tall; T is dominant)Tt × Tt (F₁ selfed)
F₂ Punnett Square T t
T [ TT ] [ Tt ]
t [ Tt ] [ tt ]
F₂ Genotype ratio: 1TT : 2Tt : 1ttF₂ Phenotype ratio: 3 Tall : 1 Dwarf
Phenotypic ratio explanationTT = tall, Tt = tall (T dominant), tt = dwarf
Dihybrid Cross — F₂ Phenotype
🌱 Dihybrid — Round Yellow × Wrinkled Green
P: RRYY (Round Yellow) × rryy (Wrinkled Green) F₁: All RrYy (Round Yellow — both R and Y are dominant)
F₂ phenotype ratio (9:3:3:1):
9 Round Yellow (R_Y_)
3 Round Green (R_yy)
3 Wrinkled Yellow (rrY_)
1 Wrinkled Green (rryy)
ICSE note: ICSE only requires the F₂ phenotype ratio — NOT the full 4×4 Punnett square for dihybrid cross.
Incomplete Dominance & Codominance
Feature
Incomplete Dominance
Codominance
Definition
Neither allele is fully dominant — F₁ shows intermediate phenotype
Both alleles are fully expressed simultaneously in F₁
ABO blood group: I^A I^B → both A and B antigens present (AB blood group)
Sex Determination in Humans
♂♀ Sex Determination
Mother (XX) × Father (XY)Eggs: all X Sperm: X or Y (50:50)
XX → Female XY → Male
Ratio: 1 Female : 1 Male (50% probability each)
Sex is determined by the FATHER — Y chromosome determines male sex.
Mother always contributes X. If X-bearing sperm fertilises → XX (female). If Y-bearing sperm fertilises → XY (male).
Y chromosome: Smaller than X; carries SRY gene (sex-determining region Y) which triggers testis development in the embryo.
Gene carried on X chromosome only. Males (XY) have one X — one defective allele = affected (hemizygous). Females (XX) need two defective alleles to be affected; one defective allele = carrier (normal phenotype).
Why males are more often affected: Males have only one X chromosome. If it carries the recessive allele, there is no second X to mask it. Females have two X chromosomes and need both to carry the allele to be affected.
Genotype
Sex
Phenotype
X^N X^N
Female
Normal
X^N X^c
Female
Carrier — normal appearance, carries recessive allele
X^c X^c
Female
Affected (colour blind/haemophilic)
X^N Y
Male
Normal
X^c Y
Male
Affected — only one X, recessive expresses
Carrier Mother × Normal FatherX^N X^c × X^N Y
Gametes: Mother → X^N or X^c | Father → X^N or Y
Offspring:
X^N X^N → Normal female (25%)
X^N X^c → Carrier female (25%)
X^N Y → Normal male (25%)
X^c Y → AFFECTED male — colour blind/haemophilic (25%)
50% of sons affected. 50% of daughters are carriers.
A daughter can only be affected if FATHER is also affected AND mother is carrier/affected.
Inheritance of Blood Groups (ABO)
🩸 ABO Blood Group Genetics
Controlled by a single gene with three alleles: I^A, I^B, and i. I^A and I^B are both dominant over i; I^A and I^B are codominant with each other.
Blood Group
Genotype(s)
Antigen
Antibody
A
I^A I^A or I^A i
A
Anti-B
B
I^B I^B or I^B i
B
Anti-A
AB
I^A I^B
A and B
None (universal recipient)
O
ii
None
Anti-A and Anti-B (universal donor)
Exam Q&A
Q1. A carrier woman marries a normal man. What proportion of sons will be colour blind?+
Mother (carrier): X^N X^c Father (normal): X^N Y Gametes — Mother: X^N, X^c Father: X^N, Y
Offspring: X^N X^N (normal female), X^N X^c (carrier female), X^N Y (normal male), X^c Y (colour blind male).
50% of sons will be colour blind. Overall 25% of all offspring will be colour blind males.
Q2. Differentiate between genotype and phenotype with examples.+
Genotype = actual alleles present. Example: TT, Tt, or tt. Not directly visible.
Phenotype = observable physical trait. Example: tall or dwarf plant.
Key: TT and Tt have different genotypes but the same phenotype (both tall). Cannot determine genotype just by looking at phenotype unless organism shows the recessive trait (then must be homozygous recessive tt).
Q3. What is a test cross? How is it used to determine genotype?+
A test cross is the cross of an organism showing the dominant phenotype (unknown genotype: TT or Tt?) with a homozygous recessive individual (tt).
If unknown is TT: TT × tt → all Tt (all tall). 100% dominant phenotype in offspring. If unknown is Tt: Tt × tt → 1 Tt (tall) : 1 tt (dwarf). 50% dominant, 50% recessive.
By observing the offspring ratio, the genotype of the unknown parent can be determined. A test cross always uses the homozygous recessive parent because its genotype (tt) is certain — it contributes only recessive (t) alleles.
Q4. Parents with blood groups A and B have a child with group O. How is this possible?+
Group O child (genotype: ii) means both parents must each have carried a recessive i allele.
Father (Group A): genotype must be I^A i (heterozygous A — not I^A I^A) Mother (Group B): genotype must be I^B i (heterozygous B — not I^B I^B)
Cross: I^A i × I^B i Offspring: I^A I^B (AB), I^A i (A), I^B i (B), ii (O) 25% of children will have blood group O. This is possible because both parents were heterozygous — each carrying a hidden recessive i allele.
04
Chapter Four
Absorption by Roots — Osmosis & Ascent of Sap
Key Definitions
DiffusionMovement of molecules from higher concentration to lower concentration (down concentration gradient) without energy. E.g., CO₂ and O₂ exchange in leaves; perfume spreading in a room.
OsmosisMovement of water molecules through a selectively permeable membrane from higher water potential (dilute solution) to lower water potential (concentrated solution). Passive process — no energy required.
Water Potential (Ψ)Tendency of water molecules to move from one region to another. Pure water has highest water potential (= 0). Adding solutes lowers water potential (makes it more negative). Water moves from high Ψ to low Ψ.
ImbibitionAbsorption of water by colloidal substances (e.g., cell wall cellulose, dry seeds) without osmosis. Can generate huge pressure — e.g., seeds cracking a rock. E.g., swelling of dry seeds soaked in water; swelling of wood when wet.
Osmotic PressurePressure that must be applied to a solution to prevent osmosis into it. Higher solute concentration → higher osmotic pressure → greater tendency to draw in water.
Turgor PressurePressure exerted by the cell contents (vacuole + cytoplasm) against the cell wall when the cell is turgid. Also called wall pressure when referring to the resistance of the cell wall. Maintains firmness of non-woody plants.
Root PressurePressure generated in root xylem by osmotic absorption of water. Active process (energy needed for mineral ion uptake → creates osmotic gradient → water follows). Causes guttation and bleeding. Minor force in ascent of sap.
TurgidityCondition of a plant cell fully inflated with water. Cell vacuole presses against cell wall; wall exerts equal and opposite turgor pressure. Keeps non-woody plants upright.
FlaccidityCondition of a plant cell that has lost water — soft and limp. Precedes plasmolysis. Causes wilting in plants. Cell wall and protoplasm lose contact with each other slightly.
PlasmolysisShrinkage of protoplasm (cytoplasm + membrane) away from the cell wall when a plant cell is placed in hypertonic solution. Water exits by osmosis → cell vacuole shrinks → membrane detaches from wall. The gap between cell wall and membrane fills with hypertonic solution.
DeplasmolysisReversal of plasmolysis — plasmolysed cell returned to hypotonic/pure water → water re-enters by osmosis → protoplasm swells back to fill cell wall. Possible if plasmolysis was not prolonged (cells still alive).
Osmosis Effects on Cells
Solution
Effect on Plant Cell
Effect on RBC
Hypotonic (dilute — less solute than cell)
Water enters → turgid cell; vacuole swells; firm plant
Water enters → RBC swells → bursts (haemolysis)
Isotonic (same solute as cell)
No net movement → normal turgidity
No change in size or shape
Hypertonic (concentrated — more solute than cell)
Water exits → flaccid → plasmolysis → wilting
Water exits → RBC shrinks and becomes spiky (crenation)
Plant cells vs animal cells: Plant cells have a rigid cell wall — they become turgid (not burst) in hypotonic solution because the wall prevents swelling beyond a point. Animal cells (no cell wall) burst (haemolysis) in hypotonic solution.
Root Hair — Adaptations for Absorption
🌱 Root Hair Cell for Water & Mineral Absorption
Long and thin — huge surface area for absorption (root hair zone = ~400 cm² per cm of root)
Thin cell wall — permeable; easy water entry by osmosis
Large vacuole with concentrated cell sap — low water potential; draws water in from soil by osmosis
Selectively permeable membrane — controls what enters the cell
Many mitochondria — energy for active uptake of mineral ions (K⁺, NO₃⁻, PO₄³⁻) against concentration gradient
Close contact with soil particles — maximises absorption surface
Pathway of Water from Soil to Xylem
Water pathway through root (three routes)1. APOPLAST route: Through cell walls only (no membranes crossed).
Blocked at Casparian strip in endodermis → forced to cross membrane.
2. SYMPLAST route: Through cytoplasm (plasmodesmata) from cell to cell.
3. VACUOLAR route: Through vacuoles of successive cells.
Sequence: Soil → Root hair → Cortex cells → Endodermis (Casparian strip) → Pericycle → Xylem vessels → Up stem
Ascent of Sap — Forces
Force
Description
Importance
Transpirational pull (Cohesion-Tension theory — Dixon & Joly)
Evaporation from leaves creates tension/suction in xylem; pulls water column up. Most important force for tall trees.
Major — primary force
Cohesion of water
H-bonds between water molecules keep the continuous water column intact and unbroken under tension
Essential — maintains column
Adhesion
Water sticks to hydrophilic xylem walls — prevents column pulling away from walls; counteracts gravity
Supporting
Root pressure
Osmotic pressure from roots pushes water up; sufficient only for short plants; absent in conifers during winter
Minor (~0.1–0.2 MPa)
Capillarity
Water rises in narrow xylem tubes by adhesion + surface tension forces
Minor — helps in narrow vessels
Active vs Passive Transport
Feature
Active Transport
Passive Transport
Energy (ATP)
Required
Not required
Direction
Against concentration gradient (low → high)
Along concentration gradient (high → low)
Carrier proteins
Required (carrier/pump proteins)
May use channel proteins (facilitated diffusion) or none
Examples
Mineral ion uptake (K⁺, NO₃⁻) by root hair; glucose absorption in gut
Osmosis, diffusion, facilitated diffusion
Exam Q&A
Q1. What happens to a plant cell placed in a hypertonic solution?+
In hypertonic solution (more concentrated than cell sap): Water moves OUT of cell by osmosis (from high water potential in cell → low water potential in solution). Cell vacuole shrinks. Protoplasm (cytoplasm + membrane) shrinks and pulls away from the cell wall → plasmolysis. Gap between cell wall and membrane fills with hypertonic solution. Cell becomes flaccid → plant wilts. If returned to water → deplasmolysis (reversal). If permanently in hypertonic solution → irreversible plasmolysis → cell death.
Q2. Explain the cohesion-tension theory of ascent of sap.+
The Cohesion-Tension theory (Dixon and Joly) explains ascent of sap as follows:
1. Transpiration from leaves evaporates water from mesophyll cells → water potential of mesophyll cells falls → they draw water from xylem vessels in the leaf veins.
2. This creates a tension (negative pressure/suction) in the xylem vessels of the leaf, which is transmitted down to the stem and root xylem.
3. Cohesion of water molecules (H-bonds) keeps the water column in xylem unbroken as it is pulled up — the column does not snap.
4. Adhesion of water molecules to xylem walls prevents the column from pulling away from the walls.
5. Water is pulled from root xylem, which draws water from the soil by osmosis through root hairs.
This creates a continuous stream of water from roots to leaves. This is the major mechanism for tall trees.
05
Chapter Five
Transpiration
Definition & Types
TranspirationLoss of water as vapour from the aerial parts (mainly leaves) of a plant through stomata. A necessary consequence of keeping stomata open for CO₂ uptake for photosynthesis.
Stomatal (~90%)Through open stomata. Most significant type. Controlled by guard cells opening/closing. Occurs mainly during day when stomata open for photosynthesis.
Cuticular (~5–8%)Through the waxy cuticle on the leaf surface. Cannot be controlled — occurs even when stomata are closed. Reduced by thicker cuticle in xerophytes.
Lenticular (~1–2%)Through lenticels (pores in bark of woody stems). Very minor. Cannot be controlled.
Stomatal Opening — Potassium Ion Theory
🌿 Potassium Ion Exchange Theory (K⁺ Pump Theory)
Opening (in light): Light activates K⁺ pump in guard cells → K⁺ actively pumped INTO guard cells → osmotic pressure of guard cells rises → water enters by osmosis → guard cells become turgid → thick inner walls (less elastic) curve outward → stoma opens.
Closing (in dark/drought): K⁺ pumped OUT of guard cells → osmotic pressure falls → water leaves by osmosis → guard cells become flaccid → stoma closes.
Why guard cells open when turgid: Guard cells have unequal wall thickness — inner wall (facing stoma) is thick and inelastic; outer wall is thin and elastic. When turgid, the outer wall bulges outward, pulling the inner wall with it and opening the pore.
ABA (Abscisic acid): Triggers stomatal closure during drought stress by causing K⁺ to leave guard cells.
Factors Affecting Transpiration Rate
Factor
Effect on Rate
Reason
↑ Light intensity
Increases
Stomata open (K⁺ pump activated); also increases temperature slightly
↑ Temperature
Increases
More kinetic energy → faster evaporation; increases water vapour capacity of air; stomata open wider
↑ Humidity
Decreases
Smaller diffusion gradient between leaf interior (high water vapour) and outside air (already humid)
↑ Wind speed
Increases
Removes humid air at leaf surface → steeper concentration gradient → faster diffusion of water vapour
Water availability (scarcity)
Decreases
Stomata close (ABA released) to prevent wilting; less water to evaporate
CO₂ concentration ↑
Decreases
High CO₂ → stomata close (stomata close when CO₂ not needed)
Ganong's Potometer & Experiments
🧪 Ganong's Potometer
Measures rate of water uptake (not direct transpiration) in a cut leafy shoot. Air bubble in capillary tube moves as plant absorbs water. Set up in water to avoid air bubbles entering cut stem.
Limitations: (1) Measures water uptake, not actual transpiration — some water used in photosynthesis. (2) Artificial conditions — stem is cut. (3) Affected by temperature/pressure changes. (4) Cannot be used in the field for intact plants.
How to compare transpiration rates in different conditions: Run same leaf shoot under different conditions (light/dark, fan/still air) and measure distance bubble moves per unit time.
Experiment
Method
Observation
Conclusion
Loss in weight
Potted plant weighed at intervals; Vaseline on soil to prevent soil evaporation
Loss in weight = water lost by transpiration
Confirms plants lose water by transpiration
Cobalt chloride paper test
Blue CoCl₂ paper pressed on upper and lower surfaces of dorsiventral leaf
Lower surface paper turns pink faster (more stomata on abaxial surface)
Stomata more numerous on lower (abaxial) surface → more transpiration there
Bell jar test
Leafy shoot under bell jar; CaCl₂ (desiccant) or blue CoCl₂ inside
Air bubbled through warm lime water via transpiring plant in sealed bell jar
Lime water stays clear (CO₂ not being added — only water vapour)
Confirms transpiration = water loss, not CO₂
Adaptations to Reduce Transpiration (Xerophytes)
Adaptation
Effect
Example
Thick waxy cuticle
Reduces cuticular transpiration
Cactus, Nerium, Agave
Sunken stomata (crypts)
Humid air trapped in pit → less diffusion gradient
Pine needles, Nerium leaves
Needle-like/reduced leaves
Less surface area = fewer stomata = less transpiration
Cactus (spines), Pine
Stomata only on lower surface
Away from direct sunlight → cooler → less evaporation
Most dicot leaves
Leaf rolling
Traps humid air around stomata; reduces gradient
Marram grass
Stomata open at night only (CAM plants)
Cooler temperatures at night → less water loss
Cactus, Agave, Pineapple
Deep root system
Reaches underground water sources
Acacia, Cactus
Succulent stems/leaves
Stores water internally
Aloe vera, cacti
Guttation & Bleeding
GuttationLoss of liquid water (not vapour) from hydathodes at leaf margins/tips when root pressure is high and stomata are closed (usually at night or early morning). E.g., grass tips, colocasia, nasturtium. NOT dew — dew is atmospheric condensation; guttation fluid comes from inside the plant and may contain dissolved salts.
BleedingExudation of sap from cut or injured plant parts due to high root pressure. E.g., sap oozing from freshly cut tree branch or vine. More pronounced in spring when root pressure is highest.
Significance of Transpiration
Transpiration is called a "necessary evil": It is unavoidable (open stomata for CO₂) but has both beneficial and harmful effects. The benefits generally outweigh the drawbacks for most plants in normal conditions.
Advantages ("Necessary")
Disadvantages ("Evil")
Creates transpirational pull — drives ascent of sap in tall trees
Excessive water loss → wilting → death if prolonged
Evaporative cooling — prevents leaf overheating on hot sunny days
Under drought conditions, a serious survival threat
Helps transport dissolved minerals from roots to all parts
Plant expends energy building complex adaptations to reduce it
Maintains stomata open for CO₂ entry (photosynthesis)
In hot dry conditions, plant must close stomata → limits photosynthesis
Removes excess water from waterlogged soils (minor)
Can cause nutrient imbalance if salts accumulate as water evaporates
Exam Q&A
Q1. Why is transpiration called a necessary evil?+
Necessary: (1) Stomata must be open for CO₂ uptake (photosynthesis) — water loss is unavoidable. (2) Transpirational pull drives ascent of sap — without it, tall trees cannot get water to leaves. (3) Evaporative cooling prevents leaf overheating. (4) Helps transport dissolved minerals upward.
Evil: (1) Excessive loss → wilting → death. (2) Under drought, a serious survival threat. (3) Plant must develop expensive adaptations (thick cuticle, sunken stomata) to limit it. (4) When stomata close to limit transpiration, CO₂ entry also stops → photosynthesis limited.
Since water loss cannot be separated from gas exchange, it is "necessary." Since it causes water deficit and wilting, it is "evil."
Q2. How does the cobalt chloride experiment demonstrate the distribution of stomata?+
Blue cobalt chloride (CoCl₂) paper is pressed firmly onto both the upper and lower surfaces of a leaf (e.g., privet or Ficus) and held with a glass strip/clip. The leaf is left for a few minutes.
Observation: The paper on the lower (abaxial) surface turns pink faster than that on the upper surface.
Conclusion: CoCl₂ paper turns pink (blue → pink) when it absorbs moisture. Faster colour change on lower surface = more water vapour escaping = more stomata on the lower surface. This demonstrates that stomata are more numerous on the lower surface of dorsiventral (bifacial) leaves — an adaptation to reduce transpiration (lower surface is away from direct sunlight, so cooler).
06
Chapter Six
Photosynthesis
Definition & Equation
☀️ Photosynthesis
Process by which green plants use light energy to synthesise glucose from CO₂ and water in the presence of chlorophyll, releasing oxygen as a by-product.
Activation of chlorophyll: Chlorophyll absorbs light → electrons become energised (excited to higher energy level)
Photolysis of water: 2H₂O → 4H⁺ + 4e⁻ + O₂↑ (released as by-product — source of ALL atmospheric O₂)
Photophosphorylation: ADP + Pi → ATP (light energy used to make ATP — energy currency for dark reactions)
NADPH formation: NADP⁺ + H⁺ + 2e⁻ → NADPH (reduced NADP; carries hydrogen atoms to dark reactions)
Dark Reactions (Biosynthetic Phase) — Stroma
🌑 Dark Reactions (Calvin Cycle)
Do NOT directly require light but need ATP and NADPH from light reactions. Occur in stroma. Called "dark" reactions because they can occur in the absence of light — not because they only occur in darkness.
Main event (simplified): CO₂ + H (from NADPH) + ATP → Glucose (C₆H₁₂O₆).
CO₂ fixation: CO₂ + 5-carbon RuBP (catalysed by enzyme RuBisCO) → 3-carbon PGA → reduced using NADPH and ATP → G3P → glucose (and RuBP regenerated).
Note: Detailed Calvin Cycle equations not required for ICSE — only the overall concept.
Light vs Dark Reactions
Feature
Light Reactions (Photochemical)
Dark Reactions (Biosynthetic)
Location
Thylakoid membranes (grana)
Stroma of chloroplast
Light needed directly
Yes — directly uses photons
No — uses products of light reactions
Inputs
H₂O, light, ADP, NADP⁺
CO₂, ATP, NADPH
Outputs
O₂, ATP, NADPH
Glucose (C₆H₁₂O₆)
Key processes
Photolysis of water; photophosphorylation; NADPH formation
CO₂ fixation; Calvin cycle; glucose synthesis
Depend on each other?
Yes — light reactions provide ATP+NADPH for dark reactions
Yes — dark reactions regenerate ADP+NADP⁺ for light reactions
Experiments on Photosynthesis
Necessity tested
Experiment
Result
Conclusion
Light
Cover part of leaf with black paper (leaf previously destarched); iodine test after 4–6 hrs in light
Covered area: yellow-brown (no starch). Exposed area: blue-black (starch).
Light is necessary for photosynthesis
CO₂
One plant with KOH in bell jar (absorbs CO₂); control with water; iodine test after destarching
Plant with KOH: no starch (no CO₂). Control: starch present.
CO₂ is necessary for photosynthesis
Chlorophyll
Variegated leaf (green + white/yellow areas); destarched; iodine test
Green area: starch (blue-black). White/yellow area (no chlorophyll): no starch (yellow-brown).
Chlorophyll is necessary for photosynthesis
O₂ production
Hydrilla/Elodea in water, light; collect gas bubbles in inverted test tube; glowing splint test
Glowing splint rekindled → confirms O₂ released
Photosynthesis produces oxygen
🧪 Starch Test Steps (MUST KNOW in order)
Destarch: Keep plant in dark 24–48 hrs — existing starch is used up (converted to sugar and transported)
Perform experiment (expose to light/CO₂/etc.)
Boil leaf in water for 1–2 min — kills cells, softens leaf, makes it permeable
Boil in alcohol (ethanol) in a water bath — removes chlorophyll (decolourises leaf). NEVER heat alcohol over direct flame — highly flammable
Rinse in warm water — removes alcohol, re-softens leaf
Red and blue light most effective; green light least absorbed
Yes — depends on light quality
Law of Limiting Factors (Blackman, 1905): The rate of photosynthesis is limited by the factor present in the least favourable amount. Even if all other factors are optimal, the one limiting factor controls the overall rate.
Carbon Cycle
♻️ Carbon Cycle
CO₂ removed from atmosphere: Photosynthesis (plants fix CO₂ into glucose); dissolution in oceans
CO₂ returned to atmosphere: Respiration (all organisms), combustion (fossil fuels, wood), decomposition (microbes break down dead matter), volcanic eruptions
Human impact: Burning fossil fuels + deforestation → ↑ atmospheric CO₂ → enhanced greenhouse effect → global warming
Exam Q&A
Q1. Why is the leaf boiled in alcohol during the starch test?+
To remove chlorophyll (decolourise the leaf). Chlorophyll is green and would mask the colour change of iodine. Without removing it, you cannot see whether the leaf turns blue-black (starch present) or yellow-brown (no starch). Alcohol is heated in a water bath (not over direct flame) because alcohol is highly flammable — direct heating would cause a fire.
Q2. What is photolysis of water and why is it important?+
Photolysis = splitting of water using light energy during the light reactions: 2H₂O → 4H⁺ + 4e⁻ + O₂
Importance: (1) O₂ released → source of all atmospheric oxygen; essential for aerobic life. (2) H⁺ ions combine with NADP⁺ → NADPH → carries hydrogen to dark reactions for glucose synthesis. (3) Electrons replace those lost by excited chlorophyll → light reactions continue uninterrupted.
Q3. Name the raw materials and products of photosynthesis. Where is each used/produced?+
Raw materials: • CO₂ — absorbed from air through stomata; used in dark reactions (Calvin cycle) for carbon fixation • H₂O — absorbed from soil by roots; used in light reactions (photolysis) → split to release O₂ and H⁺ • Light energy — absorbed by chlorophyll in thylakoid membranes
Products: • Glucose (C₆H₁₂O₆) — produced in stroma (dark reactions); used for energy (respiration), cell wall (cellulose), storage (starch), transport (sucrose) • Oxygen — produced in thylakoids (photolysis of water); released through stomata; used by all aerobic organisms for respiration
07
Chapter Seven
Chemical Coordination in Plants
Plant Growth Regulators
Hormone
Produced at
Main Effects
Applied Use
Auxins (IAA — Indole Acetic Acid)
Shoot tips, young leaves, embryo
Cell elongation (primary effect); apical dominance; promotes adventitious root formation; causes phototropic/geotropic bending; fruit development without fertilisation
Rooting powder for cuttings; weedkiller (2,4-D at high conc. kills dicots); seedless fruits (grapes, tomatoes)
Ripening bananas/tomatoes in storage rooms; stored fruits kept ethylene-free to slow ripening
Tropic Movements — Detailed
TropismDirectional growth movement of a plant organ in response to a directional stimulus. Toward the stimulus = positive tropism; away = negative tropism. Caused by unequal distribution of auxin.
PhototropismResponse to light. Shoots → positive (grow toward light). Roots → negative (grow away from light). Mechanism: Auxins migrate to shaded side of shoot → more cell elongation on shaded side → shoot bends toward light. Adaptive value: leaves positioned for maximum photosynthesis.
Geotropism (Gravitropism)Response to gravity. Roots → positive (grow downward — anchoring, water/mineral uptake). Shoots → negative (grow upward — toward light). Mechanism in stems: Auxins accumulate on lower side of horizontal stem → more elongation on lower side → stem curves upward (negative geotropism).
HydrotropismResponse to water/moisture gradient. Roots grow toward moisture (positive hydrotropism). Can override geotropism when water is scarce. Ensures roots reach water sources in dry soil.
ThigmotropismResponse to touch/contact. Tendrils of climbing plants coil around a support (positive thigmotropism). Causes: rapid cell elongation on the side away from contact. E.g., pea tendrils, grapevine, Passiflora tendrils.
ChemotropismResponse to chemical stimulus. E.g., pollen tube grows toward the ovule following chemical attractants (positive chemotropism — directed by sugars and Ca²⁺ gradient). Fungal hyphae grow toward food source.
Apical Dominance
🌿 Apical Dominance
The apical (terminal) bud suppresses the growth of lateral (axillary) buds below it. Caused by high auxin concentration produced at the shoot tip. Auxin moves downward, inhibiting lateral bud development.
Practical use: Pinching off the growing tip (removing the source of auxin) → lateral buds grow out → bushy, branched plant. Used in pruning, hedge shaping, producing bushy tea plants and cotton.
Cytokinins counteract apical dominance: High cytokinin:auxin ratio promotes lateral bud growth.
Nastic Movements (Non-directional — NOT Tropisms)
Type
Stimulus
Example
Mechanism
Photonasty
Light (non-directional)
Flowers opening in day (sunflower, tulip), closing at night
Differential growth of upper/lower petal surface
Thigmonasty
Touch/contact (non-directional)
Mimosa pudica (touch-me-not) leaflets fold and droop when touched
Rapid loss of turgor pressure in pulvinus cells (motor cells) — water moves out via osmosis
Thermonasty
Temperature change
Tulip/crocus opening in warmth
Differential growth rates at different temperatures
Nyctinasty
Day/night cycle
Legume leaves fold at night (sleep movements)
Pulvinus cells lose/gain turgor rhythmically
Exam Q&A
Q1. How do auxins cause a shoot to bend toward light?+
When a shoot receives unilateral (one-sided) light: (1) Auxins produced at the tip migrate to the shaded side (away from light). (2) Higher auxin concentration on shaded side → more cell elongation (auxin causes cell walls to loosen → cells absorb water → elongate). (3) Lit side: less auxin → less elongation. (4) Differential growth → shoot curves/bends toward the light (positive phototropism). This places leaves in optimal position for photosynthesis.
Q2. How does the movement of Mimosa pudica differ from a tropic movement?+
Mimosa pudica (thigmonasty): Rapid non-directional response to touch. Leaflets fold and petioles droop when any part of leaf is touched, regardless of direction of stimulus. Caused by sudden loss of turgor pressure in specialised pulvinus cells (water moves out of motor cells rapidly). The response is reversible (leaves re-open after a few minutes). NOT caused by growth — no permanent change.
Tropic movement: Directional, slower response caused by unequal growth (differential cell elongation due to unequal auxin distribution). Direction of response depends on direction of stimulus. Permanent (the growth change is irreversible).
Key difference: Nasties are non-directional, often rapid, turgor-based, reversible. Tropisms are directional, growth-based, slower, permanent.
No nucleus: More space for haemoglobin → more O₂ carried per cell. Biconcave shape (from lack of nucleus) = increased surface area:volume ratio for faster gas exchange. Also more flexible — can squeeze through narrow capillaries.
No mitochondria: RBCs use anaerobic respiration for their own energy needs → do NOT consume the O₂ they carry. All O₂ is delivered to tissues. If they had mitochondria, they would use up the O₂ they carry.
Blood Clotting (Coagulation)
Clotting cascade (simplified)Injury to blood vessel
↓
Platelets aggregate at wound → release Thromboplastin (clotting factor)
↓
Thromboplastin + Ca²⁺ + other clotting factors
↓
Prothrombin (inactive, in plasma) → Thrombin (active enzyme)
↓
Fibrinogen (soluble plasma protein) → Fibrin (insoluble threads)
↓
Fibrin mesh traps RBCs, WBCs, platelets → CLOT (scab) formed
↓
Prevents further blood loss and pathogen entry; allows wound healing
Haemophilia: Genetic disorder — absence of clotting factor VIII (haemophilia A) or IX (haemophilia B). Blood does not clot normally. X-linked recessive disorder — affects males mostly. Even minor injury can cause uncontrolled bleeding. Treated with factor replacement therapy.
ABO Blood Groups
Group
Antigen on RBC
Antibody in Plasma
Can Donate To
Can Receive From
A
A antigen
Anti-B
A, AB
A, O
B
B antigen
Anti-A
B, AB
B, O
AB (Universal Recipient)
A and B antigens
None
AB only
All groups (A, B, AB, O)
O (Universal Donor)
No antigens
Anti-A and Anti-B
All groups
O only
Why transfusion incompatibility is dangerous: If mismatched blood is given, recipient antibodies react with donor antigens → agglutination (clumping of RBCs) → clots block blood vessels → haemolysis → kidney failure → death. Always cross-match before transfusion.
Rh Factor: Rh+ = Rh antigen present on RBC. Rh− = absent. Rh− mother carrying Rh+ foetus → at first birth, some fetal Rh+ RBCs enter maternal blood → mother develops anti-Rh antibodies. In subsequent Rh+ pregnancies, maternal antibodies cross placenta → attack fetal RBCs → erythroblastosis foetalis (haemolytic disease of newborn). Prevented by giving Rh− mother anti-D immunoglobulin (RhoGAM) at delivery.
Heart Structure & Cardiac Cycle
❤️ Cardiac Cycle
Systole: Ventricles contract → blood pumped out. AV valves close (→ "lub" sound). Semilunar valves open.
Diastole: Ventricles relax → fill with blood from atria. Semilunar valves close (→ "dub" sound). AV valves open.
Heart rate: ~72 beats/min at rest. Each beat takes ~0.8 seconds. Cardiac output = Heart rate × Stroke volume = ~5 L/min at rest.
Pacemaker (SAN — Sinoatrial Node): Located in right atrium wall. Generates electrical impulse that spreads across both atria → AV node → Bundle of His → Purkinje fibres → both ventricles contract simultaneously. Controls heart rate automatically.
Feature
Details
4 Chambers
Right atrium (RA), Right ventricle (RV), Left atrium (LA), Left ventricle (LV)
4 Valves
Tricuspid (RA→RV, 3 cusps), Bicuspid/Mitral (LA→LV, 2 cusps), Pulmonary semilunar (at base of pulmonary artery), Aortic semilunar (at base of aorta)
Left ventricle wall
Thickest — pumps blood to entire body (systemic circuit) at high pressure (~120 mmHg)
Right ventricle wall
Thinner — pumps blood to lungs only (short distance, low resistance, ~25 mmHg)
Valve function
Prevent backflow of blood — ensure one-way flow through heart
Double Circulation
Double Circulation PathwayPULMONARY CIRCUIT (right side):
Body → Superior/Inferior Vena Cava → Right Atrium → Tricuspid valve → Right Ventricle → Pulmonary Artery → Lungs (oxygenation) → Pulmonary Veins → Left Atrium
SYSTEMIC CIRCUIT (left side):
Left Atrium → Bicuspid valve → Left Ventricle → Aorta → All body organs → Veins → Vena Cava → Right Atrium
Blood passes through heart TWICE per complete circuit → double circulation.
🔄 Advantages of Double Circulation
High pressure maintained throughout body tissues — efficient O₂ delivery
Oxygenated and deoxygenated blood kept completely separate — no mixing (unlike single circulation in fish)
More efficient than single circulation — can sustain higher metabolic rates (warm-blooded mammals and birds)
Allows different pressures for pulmonary (low) and systemic (high) circuits
Bilobed gland behind sternum; largest in childhood, shrinks after puberty
Maturation of T-lymphocytes (T-cells) — cell-mediated immunity
Tonsils
Lymphoid tissue at back of throat; 3 pairs (palatine, pharyngeal, lingual)
First line of defence against inhaled/ingested pathogens. Swollen tonsils = tonsillitis.
Exam Q&A
Q1. Why does the left ventricle have a thicker wall than the right?+
The left ventricle pumps oxygenated blood through the systemic circulation — to all body parts via the aorta. This requires much greater force to push blood over this long distance against peripheral resistance (pressure ~120 mmHg systolic).
The right ventricle only pumps to the lungs (short distance, low resistance, ~25 mmHg).
Greater workload → thicker muscular wall → greater force generation. The left ventricle wall is approximately 3× thicker than the right.
Q2. Explain why valves in veins are important but not in arteries.+
Veins: carry blood back to heart at low pressure. Blood must flow upward in limbs against gravity. Without valves, blood would pool in legs due to gravity. Valves open when blood moves toward heart → close to prevent backflow. Skeletal muscle contraction also squeezes veins, pushing blood toward heart (muscle pump).
Arteries: blood flows under high pressure from heart. Pressure itself keeps blood flowing in the correct direction — no backflow risk. Valves would also be damaged by the high pulsatile pressure in arteries. (Exception: semilunar valves are present at the base of aorta and pulmonary artery to prevent backflow into ventricles during diastole.)
Q3. What is the significance of the hepatic portal system?+
The hepatic portal system ensures that all blood from the digestive system passes through the liver BEFORE entering general circulation. This allows the liver to:
1. Regulate blood glucose — absorb excess glucose from post-meal blood; store as glycogen; release glucose between meals to maintain ~90 mg/100mL. 2. Detoxify — remove alcohol, drugs, bacterial toxins, and convert ammonia → urea before blood reaches the rest of the body. 3. Process nutrients — convert excess amino acids to urea (deamination); synthesise non-essential amino acids.
Without the portal system, toxic substances and excess nutrients absorbed from the gut would directly enter systemic circulation — potentially fatal.
Produces urea from deamination of amino acids; bile pigments (bilirubin from Hb breakdown) excreted in bile into gut
Deamination; haem catabolism
Nephron — Structure & Urine Formation
Part
Process
Details
① Glomerulus + Bowman's capsule (Malpighian capsule)
Ultrafiltration
Blood filtered under high pressure (afferent arteriole wider than efferent → high pressure). Small molecules (water, glucose, urea, salts, amino acids, uric acid) → filtrate. Large molecules (proteins, RBCs) remain in blood. Non-selective. ~180 L of filtrate formed per day.
② PCT (Proximal Convoluted Tubule)
Selective Reabsorption
All glucose, all amino acids, vitamins, ~75% water reabsorbed by active transport and osmosis. Na⁺ reabsorbed (active). H⁺ and creatinine secreted into tubule. Highly metabolically active — many mitochondria.
③ Loop of Henle
Concentration of urine
Creates osmotic gradient in medulla. Descending limb: permeable to water → water exits by osmosis into hypertonic medulla. Ascending limb: impermeable to water → Na⁺ and Cl⁻ pumped out actively → medullary interstitium becomes hyperosmotic → allows further water reabsorption in collecting duct.
④ DCT (Distal Convoluted Tubule)
Fine regulation of ion balance
Water reabsorption (controlled by ADH). Na⁺ reabsorption (controlled by aldosterone). Secretes H⁺ → regulates blood pH. Secretes K⁺ when blood K⁺ too high.
⑤ Collecting Duct
Final concentration of urine
ADH (vasopressin) → makes collecting duct permeable to water → more water reabsorbed into hypertonic medulla → concentrated urine. Delivers ~1.5 L/day urine to pelvis.
Hormonal Control — ADH & Aldosterone
Hormone
Source
Stimulus for Release
Action on Kidney
Effect
ADH (Anti-Diuretic Hormone / Vasopressin)
Posterior pituitary
Low blood water content (dehydration); detected by hypothalamus osmoreceptors
Makes collecting duct more permeable to water → more water reabsorbed
Less, more concentrated urine; blood water rises → negative feedback → ADH release stops
Aldosterone
Adrenal cortex
Low blood Na⁺ or low blood pressure (detected by renin-angiotensin system)
Increases Na⁺ (and water) reabsorption in DCT and collecting duct
Raises blood pressure and blood Na⁺; less urine
Plasma vs Filtrate vs Urine
Substance
Blood Plasma
Glomerular Filtrate
Urine
Proteins
Present (large molecules)
Absent (too large to filter)
Absent (proteinuria = kidney disease)
Glucose
Present (~90 mg/100 mL)
Present (same as plasma)
Absent (all reabsorbed in PCT); present in diabetes (glucosuria)
Urea
Low conc. (~30 mg/100 mL)
Same as plasma (freely filtered)
High conc. (~2000 mg/100 mL) — concentrated ~70×
RBCs
Present
Absent (too large)
Absent (haematuria = kidney damage)
Water
~90%
Present (high volume — 180 L/day)
~95% water, 1.5 L/day
Creatinine
Low conc.
Present
High conc. — all excreted (not reabsorbed)
Kidney Failure & Dialysis
🔬 Kidney Failure & Haemodialysis
In kidney failure, nephrons cannot filter blood properly → urea and toxins accumulate in blood (uraemia) → can be fatal.
Haemodialysis (kidney machine): Blood pumped from patient's artery → through machine → dialysis fluid separated from blood by selectively permeable membrane. Urea and excess salts diffuse out down concentration gradient (dialysis fluid has normal plasma concentrations of useful substances → they are not lost). Blood returned via vein. Patient requires dialysis ~3× per week, ~4 hrs per session.
Kidney transplant: Long-term solution. Donor kidney implanted; immunosuppressant drugs prevent rejection. Transplant preferred over dialysis for quality of life.
Exam Q&A
Q1. Why is glucose absent in urine despite being present in glomerular filtrate?+
Glucose filters through the glomerulus (small molecule). But in the PCT, glucose is completely reabsorbed by active transport (glucose carriers + Na⁺ co-transport) back into the blood capillaries. Under normal conditions, all filtered glucose is reclaimed → none in urine.
Exception: In diabetes mellitus, blood glucose exceeds the renal threshold (~180 mg/100 mL). PCT reabsorption is saturated → excess glucose remains in urine → glucosuria (a diagnostic sign of diabetes). Glucose in urine also draws water → polyuria (excessive urine) → dehydration.
Q2. Distinguish ultrafiltration from selective reabsorption.+
Ultrafiltration (in Malpighian capsule — Bowman's capsule + glomerulus): • Occurs under HIGH HYDROSTATIC PRESSURE • NON-SELECTIVE — all small molecules pass into Bowman's capsule • Large molecules (proteins, RBCs) retained in blood • Passive process — no energy directly required
Selective Reabsorption (mainly in PCT): • Only useful substances (glucose, amino acids, most water, some salts) are taken back into blood • SELECTIVE — waste products (urea, excess salts, H⁺) remain in tubule • Uses active transport (energy required) and osmosis • PCT cells have many mitochondria and microvilli (brush border) to increase surface area for maximum reabsorption
Q3. How does ADH regulate urine concentration?+
When blood water content is low (dehydration): 1. Osmoreceptors in hypothalamus detect rise in blood osmolarity 2. Hypothalamus signals posterior pituitary → releases more ADH 3. ADH increases permeability of collecting duct to water 4. More water reabsorbed from filtrate into blood → less, more concentrated urine 5. Blood water rises → osmoreceptors detect this → ADH release reduces (negative feedback)
When blood water is high (overhydration): Less ADH released → collecting duct less permeable → less water reabsorbed → more, dilute urine produced. This is a classic example of homeostasis by negative feedback.
10
Chapter Ten
Nervous System
Neuron Structure
⚡ Parts of a Neuron
Cell body (Cyton/Soma) — contains nucleus and organelles; metabolic centre; makes neurotransmitters
Dendrites — short branched processes; receive impulses; carry impulse TOWARD cell body; many per neuron → increase receptor surface area
Axon — single long process; carries impulse AWAY from cell body toward next neuron or effector
Myelin sheath — fatty insulation around axon (made by Schwann cells in PNS, oligodendrocytes in CNS); speeds transmission; Nodes of Ranvier = gaps in myelin → saltatory conduction (impulse jumps node to node → faster)
Thinking, reasoning, memory, intelligence, voluntary movement, speech (Broca's area), understanding language (Wernicke's area), sight, hearing, smell, personality. Left hemisphere = language, logic, analysis. Right hemisphere = creativity, spatial awareness. Outer = grey matter (cell bodies = cerebral cortex). Inner = white matter (myelinated axons). Two hemispheres connected by corpus callosum.
Cerebellum (little brain)
Coordinates muscular movements; maintains posture, balance and equilibrium; fine motor control; timing of movement. Damage → ataxia (loss of coordination, staggering gait).
Pavlov's dog salivating at bell (associated with food), riding a bicycle, typing without looking, braking a car
Eye — Structure & Vision Defects
Part
Function
Cornea
Transparent; refracts (bends) light entering eye — responsible for ~70% of total refraction; fixed power
Iris (coloured part)
Controls pupil size via circular muscles (constrict pupil in bright light) and radial muscles (dilate pupil in dim light); regulates light entering eye
Lens
Biconvex, transparent, flexible; fine focuses light on retina; shape changed by ciliary muscles (accommodation); ~30% of refraction
Collects and directs sound waves into ear canal; helps determine direction of sound
External auditory meatus
Conducts sound to eardrum; lined with hairs and ceruminous (wax) glands that trap dust and pathogens
Tympanic membrane (eardrum)
Thin membrane; vibrates in response to sound waves; converts sound energy to mechanical energy
Middle Ear (air-filled)
Ossicles: Malleus → Incus → Stapes
Three smallest bones in body; amplify and transmit vibrations from eardrum (~20×) to oval window. Malleus attached to eardrum; stapes to oval window.
Eustachian tube (auditory tube)
Connects middle ear to pharynx; equalises air pressure on both sides of eardrum; opens during swallowing/yawning
Oval window
Membrane between middle and inner ear; stapes vibrates against it → transmits vibrations to cochlear fluid
Inner Ear (fluid-filled)
Cochlea
Snail-shaped, fluid-filled. Organ of Corti (contains hair cells — mechanoreceptors) on basilar membrane; hair cells bend → generate nerve impulses → auditory nerve → auditory cortex of temporal lobe
Semicircular canals (3)
Three mutually perpendicular fluid-filled canals; detect rotational/angular movement (dynamic equilibrium); NOT involved in hearing
Utricle and Saccule
Detect linear acceleration and head position relative to gravity (static equilibrium/balance)
Exam Q&A
Q1. How does the eye accommodate for near and far objects?+
Accommodation = automatic adjustment of lens shape to focus objects at different distances.
Near objects: Ciliary muscles contract → suspensory ligaments slacken → lens becomes more convex (thicker) → greater refractive power → converges diverging rays from near object onto retina.
Distant objects: Ciliary muscles relax → suspensory ligaments tighten → lens becomes flatter (less convex, thinner) → less refractive power → focuses parallel rays from distant object onto retina.
In presbyopia (ageing), lens loses elasticity → cannot become sufficiently convex → near objects cannot be focused → convex/bifocal lens needed. Also ciliary muscles weaken with age.
Q2. Distinguish between the functions of rods and cones.+
Feature
Rods
Cones
Light sensitivity
Very sensitive — function in dim light
Less sensitive — require bright light
Colour vision
No — only black and white (grey scale)
Yes — 3 types (red, green, blue cone pigments)
Acuity (sharpness)
Low — many rods converge on one bipolar cell
High — especially in fovea (1 cone: 1 bipolar cell)
Distribution
~120 million; all over retina except fovea
~6 million; concentrated in fovea; absent in periphery
Pigment
Rhodopsin (visual purple) — bleached by bright light
Iodopsin (3 types, sensitive to R, G, B wavelengths)
Colour blindness = absence or dysfunction of one or more types of cones (X-linked recessive).
11
Chapter Eleven
Endocrine System
Endocrine vs Exocrine
Feature
Endocrine (Ductless)
Exocrine (Ducted)
Duct
None — secretion directly into blood
Have duct — secrete onto surface or body cavity
Product
Hormones
Enzymes, sweat, saliva, bile, milk, mucus
Action
On distant target organs via blood (systemic)
Local effect on nearby surface/organ
Speed of response
Slow (seconds to days)
Fast (immediate)
Duration of effect
Long-lasting
Short-lived
Examples
Thyroid, pituitary, adrenal, islets of Langerhans, gonads
Mixed glands: Some organs have both endocrine and exocrine functions. Pancreas: exocrine = acini cells secrete digestive enzymes into pancreatic duct. Endocrine = Islets of Langerhans secrete insulin/glucagon into blood. Gonads (testes/ovaries): exocrine = produce gametes; endocrine = produce sex hormones.
Q1. How do insulin and glucagon maintain blood glucose levels?+
After a meal (blood glucose rises): β cells → secrete insulin → cells absorb glucose → liver converts glucose to glycogen → blood glucose falls to normal.
Between meals (blood glucose falls): α cells → secrete glucagon → liver breaks down glycogen → glucose (glycogenolysis) → blood glucose rises to normal.
They act as physiological antagonists — negative feedback loop — maintain blood glucose at ~70–110 mg/100 mL (homeostasis). In Type 1 diabetes, β cells are destroyed → no insulin → hyperglycaemia. Treatment: insulin injections.
Q2. Why is the pituitary called the "master gland"?+
The pituitary is called the "master gland" because it secretes tropic hormones that regulate the activity of other endocrine glands:
• TSH → stimulates thyroid to produce thyroxine • ACTH → stimulates adrenal cortex to produce cortisol • FSH/LH → stimulates gonads (testes/ovaries) to produce sex hormones and gametes • Prolactin → stimulates mammary glands for milk production • GH → stimulates growth of bones and muscles
The pituitary itself is controlled by the hypothalamus (via releasing hormones), making the hypothalamus-pituitary axis the primary control centre of the entire endocrine system.
Q3. Distinguish between adrenaline and cortisol in stress response.+
Adrenaline (from adrenal medulla): Rapid, short-term response to sudden/acute stress (e.g., perceived danger). Within seconds: ↑ heart rate, ↑ BP, ↑ blood glucose (glycogenolysis), dilates pupils and airways, redirects blood from gut to muscles — "fight or flight." Effect lasts minutes.
Cortisol (from adrenal cortex): Slower, sustained response to prolonged stress. Takes hours: ↑ blood glucose (gluconeogenesis from proteins), suppresses immune/inflammatory response, increases protein catabolism, promotes fat mobilisation. Effect lasts hours to days. Chronic cortisol elevation → Cushing's syndrome.
12
Chapter Twelve
The Reproductive System
Male Reproductive Organs
Organ
Function
Testes (2)
Spermatogenesis (in seminiferous tubules via meiosis); produce testosterone (Leydig/interstitial cells). Located in scrotum (outside body cavity) — ~2°C cooler → needed for sperm production
Epididymis
Coiled tube on posterior surface of testes; storage and maturation of sperms (they gain motility here); ~20 days
Vas deferens
Muscular tube; carries sperms from epididymis through inguinal canal → urethra during ejaculation
Seminal vesicles (2)
Secrete fructose-rich, slightly alkaline fluid (~60% of semen volume); provides energy for sperm flagellar movement; prostaglandins stimulate female reproductive tract contractions
Secrete lubricating fluid before ejaculation; neutralises acidic residue in urethra; prevents sperm damage
Penis
Organ of copulation; transfers semen into female reproductive tract during ejaculation; also excretion of urine (urethra serves both)
Sperm Structure & Adaptations
🔬 Sperm — Structure & Adaptations
Head: Haploid nucleus (23 chromosomes). Acrosome cap contains hydrolytic enzymes (acrosin, hyaluronidase) to digest zona pellucida of egg for penetration. Flat/streamlined for reduced drag.
Midpiece: Contains many mitochondria packed around central axoneme → ATP energy for flagellum movement; ~50–70 mitochondria.
Tail (flagellum): Long (50 μm); whip-like movement; provides motility to swim toward egg; axoneme (9+2 microtubule arrangement) powered by ATP.
No endoplasmic reticulum or ribosomes: No protein synthesis → all energy for movement. Nucleus tightly packed → minimal size.
Female Reproductive Organs
Organ
Function
Ovaries (2)
Oogenesis (produce eggs/ova via meiosis — one egg per month from puberty to menopause); produce oestrogen (follicle) and progesterone (corpus luteum)
Fallopian tubes / Oviducts (2)
Site of fertilisation (upper third — ampulla); ciliated epithelium and peristalsis carry egg toward uterus; muscular; fimbriae sweep egg from ovary surface
Uterus (Womb)
Pear-shaped, muscular (myometrium); endometrium (inner lining) thickens for implantation → sheds during menstruation; site of fetal development; contracts during labour
Cervix
Narrows at base of uterus; produces mucus (watery at ovulation → sperm entry; thick at other times → barrier). Dilates up to 10 cm during childbirth.
Vagina
Receives penis during copulation; birth canal; pathway for menstrual flow; slightly acidic pH (~4) inhibits infection
Key Reproductive Events
FertilisationFusion of haploid sperm (n=23) and haploid egg (n=23) in the fallopian tube (ampullary region) → diploid zygote (2n=46). Many sperm needed but only ONE penetrates the egg. Acrosome enzymes digest zona pellucida. After one sperm enters, zona hardens (cortical reaction) → prevents polyspermy. Takes ~12–24 hrs after ovulation.
CleavageRapid mitotic divisions of zygote as it travels down fallopian tube (3–4 days). Cell number increases but overall size stays same: zygote → 2 cells (morula stage) → 8 cells → 16 cells → blastocyst.
ImplantationBlastocyst (~100 cells) embeds into endometrium of uterus ~6–7 days after fertilisation. Trophoblast cells (outer layer of blastocyst) invade endometrium. Triggers production of HCG → maintains corpus luteum → maintains progesterone → pregnancy continues. HCG detected in urine → basis of pregnancy test.
PlacentaDisc-shaped organ formed from embryonic trophoblast + maternal endometrium; fully formed by ~12 weeks. Contains chorionic villi (fetal capillaries) bathed in maternal blood spaces (intervillous space). Fetal and maternal blood do NOT mix — exchange across thin membrane of villi by diffusion and active transport.
Placenta Functions(1) Nutrition: glucose, amino acids, vitamins, minerals: mother→fetus by diffusion and active transport. (2) Respiration: O₂: mother→fetus; CO₂: fetus→mother (by diffusion). (3) Excretion: urea, CO₂: fetus→mother (excreted by maternal kidneys/lungs). (4) Endocrine: secretes HCG (maintains corpus luteum early in pregnancy), oestrogen and progesterone (maintains pregnancy after 12 weeks when corpus luteum degenerates). (5) Immunity: maternal antibodies (IgG) cross placenta → fetal passive immunity. (6) Barrier (limited): prevents some pathogens but NOT all (rubella virus, HIV, syphilis cross placenta).
GestationDuration of pregnancy from fertilisation to birth. Humans: ~38 weeks (266 days) from fertilisation; ~40 weeks (280 days) from last menstrual period (LMP). Divided into 3 trimesters of ~13 weeks each.
ParturitionProcess of childbirth. Triggered by oxytocin (pituitary) → uterine contractions (positive feedback — more oxytocin released as contractions strengthen). Three stages: (1) Labour — cervix dilates (0→10 cm), membranes rupture; (2) Delivery — fetus expelled; (3) Afterbirth — placenta expelled.
Amniotic Fluid & Foetal Membranes
🫧 Amnion & Amniotic Fluid
Amnion = inner membrane forming amniotic sac (fluid-filled). Chorion = outer membrane. Amniotic fluid volume: ~800 mL at term.
Functions of amniotic fluid:
Cushions fetus from mechanical shocks and physical injuries
Maintains constant temperature and chemical environment around fetus
Prevents amnion adhering to fetal surface
Allows free movement of fetus — essential for normal limb, lung and digestive tract development
Lubricates birth canal during delivery; helps dilate cervix
Fetus swallows and breathes amniotic fluid — practises swallowing and breathing reflexes
Menstrual Cycle (~28 days)
Phase 1: Menstruation (Days 1–5)Progesterone and oestrogen levels fall
→ Endometrium breaks down → sheds → bleeding (menstrual flow ~50 mL blood)
Phase 2: Follicular Phase (Days 1–13)FSH from anterior pituitary → stimulates Graafian follicle to develop in ovary
→ Follicle produces increasing amounts of oestrogen
→ Oestrogen → rebuilds/thickens endometrium
→ Oestrogen → (positive feedback at high level) → triggers LH surge
Phase 3: Ovulation (Day 14)LH surge → mature Graafian follicle ruptures → egg released into fallopian tube
→ Egg survives 12–24 hrs; most fertile day
Phase 4: Luteal Phase (Days 15–28)Ruptured follicle → Corpus luteum (yellow body) → secretes progesterone + oestrogen
→ Progesterone: maintains thick, vascular endometrium for implantation
→ Progesterone: inhibits FSH and LH (prevents new follicle developing and new ovulation)
If NO fertilisation:Corpus luteum degenerates (day ~24)
→ Progesterone and oestrogen fall sharply
→ Endometrium loses support → sheds → menstruation begins (Day 1 of new cycle)
If fertilisation OCCURS:Blastocyst implants → secretes HCG → maintains corpus luteum
→ Corpus luteum continues to secrete progesterone → no menstruation → pregnancy
Sex Hormones
Hormone
Source
Role
Testosterone
Testes (Leydig/interstitial cells); stimulated by LH
Male secondary sexual characteristics (puberty): voice breaking (deepening), facial and body hair, pubic/axillary hair, penis/testis growth, muscular development, sebaceous gland activity. Stimulates spermatogenesis. Anabolic effects.
Oestrogen
Ovarian Graafian follicle (stimulated by FSH); placenta in pregnancy
Female secondary sexual characteristics: breast development, pubic/axillary hair, widening of pelvis, menstrual cycle onset. Rebuilds endometrium post-menstruation. Triggers LH surge (ovulation) at high levels. Maintains female physiology.
Progesterone
Corpus luteum (stimulated by LH); placenta in pregnancy
Maintains endometrium during luteal phase and pregnancy. Prevents further ovulation in pregnancy (inhibits FSH/LH). Promotes breast development for lactation. Basis of hormonal contraceptive pills.
HCG (Human Chorionic Gonadotrophin)
Blastocyst/early placenta
Maintains corpus luteum (prevents its degeneration) → corpus luteum continues producing progesterone until placenta takes over at ~12 weeks. Detected in urine → basis of home pregnancy test.
Twins
Feature
Identical (Monozygotic, MZ)
Fraternal (Dizygotic, DZ)
Origin
One zygote splits into two embryos (inner cell mass divides at blastocyst stage)
Two separate eggs fertilised by two separate sperms at same time (two ovulations)
Prevents implantation; copper versions also toxic to sperm
~99%
Tubectomy
Surgical (permanent)
Fallopian tubes cut, tied and sealed (ligated) → egg cannot travel to uterus; sperm cannot reach egg
>99%
Vasectomy
Surgical (permanent)
Vas deferens cut and tied → sperms cannot exit testes into semen (man still ejaculates, but semen contains no sperm)
>99%
Natural methods (abstinence, rhythm method)
Behavioural
Abstaining from intercourse or avoiding fertile period (days 10–18)
Variable (~75–85%)
Exam Q&A
Q1. State the functions of the placenta.+
1. Nutrition: Glucose, amino acids, vitamins pass from mother's blood to fetal blood by diffusion and active transport. 2. Respiration: O₂ passes from mother to fetus; CO₂ passes from fetus to mother (by diffusion — O₂ gradient maintained because fetal Hb has higher O₂ affinity than maternal Hb). 3. Excretion: Urea and other metabolic wastes pass from fetus to mother (excreted by mother's kidneys and lungs). 4. Endocrine: Secretes HCG (maintains corpus luteum early pregnancy), then oestrogen and progesterone (maintains pregnancy and endometrium). 5. Immunity: Maternal antibodies (IgG) pass to fetus — provides passive immunity to newborn for several months.
Important: Fetal and maternal blood do NOT mix. Exchange occurs across thin placental membrane.
Q2. Describe the events of the menstrual cycle from Day 1 to Day 28.+
Days 1–5: Menstruation — endometrium sheds due to falling progesterone/oestrogen. Days 1–13 (Follicular phase): FSH from pituitary → Graafian follicle matures → secretes oestrogen → endometrium rebuilds and thickens. Day 14 (Ovulation): High oestrogen triggers LH surge → follicle ruptures → egg released. Most fertile time. Days 15–28 (Luteal phase): Corpus luteum forms from ruptured follicle → secretes progesterone → endometrium becomes thick and vascular (ready for implantation). Days 24–28: If no fertilisation → corpus luteum degenerates → progesterone falls → endometrium sheds → Day 1 begins again.
If fertilisation occurs: blastocyst implants → HCG secreted → corpus luteum maintained → no menstruation.
13
Chapter Thirteen
Population
Key Terms
DemographyScientific study of human population — its size, structure, distribution, and dynamics (births, deaths, migration, age composition).
Population DensityNumber of individuals per unit area (e.g., per km²). India (~500/km²) = one of the highest population densities in the world. Affects resource availability, pollution, infrastructure pressure.
Birth Rate (Natality)Number of live births per 1000 people per year. India's birth rate has been declining (from ~40 in 1950s to ~18 now) but remains higher than death rate → population still growing.
Death Rate (Mortality)Number of deaths per 1000 people per year. India's death rate has dropped sharply (~7 currently) due to improved healthcare, sanitation, vaccines and antibiotics.
Growth RateRate of increase in population = (Birth rate − Death rate) + Net migration. A positive rate = population growing. India's annual growth rate ~1% now (down from ~2.2% in 1970s) but absolute numbers still increase greatly because of large base population.
Infant Mortality RateNumber of deaths of infants (<1 year) per 1000 live births per year. India's IMR has fallen dramatically from ~145 (1960) to ~28 (present) — but still high compared to developed nations. Key indicator of healthcare quality.
Life ExpectancyAverage number of years a newborn is expected to live. India: ~70 years currently (was ~32 in 1947). Increased due to better healthcare, nutrition, sanitation, reduced infant mortality.
Population ExplosionSudden rapid increase in population size beyond the carrying capacity of resources. Occurred in developing countries including India in mid-20th century when death rates fell sharply but birth rates remained high — demographic transition lag.
More waste, vehicle emissions, sewage per unit area; loss of biodiversity
Unequal wealth distribution
Gap between rich and poor widens; inadequate housing (slums), healthcare, education
Infrastructure overload
Hospitals, schools, transport, water supply insufficient for growing population
Methods of Population Control
✂️ Surgical Contraception (Permanent)
Tubectomy / Tubal ligation (Female sterilisation): Fallopian tubes cut, tied (ligated) and sealed. Egg cannot travel to uterus; sperm cannot reach egg. Permanent, highly effective (>99%). Done laparoscopically under general anaesthesia. Menstrual cycle and hormone levels unaffected — only fertility is prevented.
Vasectomy (Male sterilisation): Vas deferens cut and tied. Sperms cannot exit testes into semen. Permanent, highly effective (>99%). Simpler, cheaper, quicker procedure than tubectomy — done under local anaesthesia. Semen volume, sexual function and hormone levels unaffected.
Approach
Measure
How It Helps
Medical/Contraceptive
Family planning services; free contraception; sterilisation
Reduces unintended pregnancies; spaces births
Education
Female education; school enrolment of girls
Higher education correlates strongly with lower birth rates; women have more economic independence
Age at marriage
Raising legal minimum marriage age (18 for women, 21 for men in India)
Shorter reproductive period; more women in workforce/education
Incentives
Financial incentives for smaller families; free maternal healthcare for 2 children
Promotes small family norm
Awareness campaigns
Mass media; community health workers (ASHAs); Hum Do Hamare Do campaign
Changes attitudes toward family size; promotes contraception
Exam Q&A
Q1. What is the demographic transition? How does it explain population explosion?+
The demographic transition is the process by which a country moves from high birth rate/high death rate to low birth rate/low death rate as it develops economically.
Stage 1 (Pre-industrial): Both birth and death rates high → population stable at low level. Stage 2 (Early industrial): Death rate falls rapidly (better medicine, sanitation, food) BUT birth rate stays high → large gap between birth and death rates → rapid population growth (explosion). Stage 3: Birth rate begins to fall as women educated, urbanised, contraception adopted → growth slows. Stage 4: Both rates low → population stable again at high level.
India is in Stage 2→3 transition — death rate has fallen sharply, birth rate falling slowly → still growing, but rate declining.
Q2. Compare tubectomy and vasectomy as methods of population control.+
Feature
Tubectomy
Vasectomy
Sex
Female
Male
Organ operated on
Fallopian tubes
Vas deferens
Procedure
Laparoscopic; general anaesthesia; more complex
Minor surgery; local anaesthesia; simpler
Effect
Egg cannot reach uterus; sperm cannot reach egg
Sperm cannot enter semen; semen is sperm-free
Hormones affected?
No — menstruation continues normally
No — ejaculation and sexual function normal
Permanence
Permanent (reversal possible but unreliable)
Permanent (reversal possible but unreliable)
Both are >99% effective and are considered the most reliable non-hormonal contraceptive methods.
14
Chapter Fourteen
Human Evolution
Evidence for Evolution
🔬 Evidence Supporting Evolution
Palaeontological (Fossil record):
Fossils show gradual changes over time — transitional forms link ancestor to descendant
E.g., Archaeopteryx — transitional between dinosaurs and birds
Older rocks contain simpler forms; more complex organisms appear in younger rocks
Anatomical:
Homologous organs — same basic structure but different function (e.g., human arm, bat wing, whale flipper, horse leg — all derived from same ancestral forelimb = common ancestry)
Analogous organs — different structure but same function (e.g., butterfly wing and bird wing — convergent evolution, NOT common ancestry)
Vestigial organs — reduced, non-functional remnants of organs that were functional in ancestors (e.g., human appendix, coccyx, wisdom teeth, body hair, pinnae muscles) — evidence that organisms have changed from their ancestors
Biochemical/Molecular:
DNA sequence comparison — more closely related species share more DNA (humans and chimps share ~98.5% DNA)
Same genetic code (triplet codons) in all organisms — evidence of common ancestry
Similar proteins (cytochrome c, haemoglobin) across species
Embryological:
Early embryos of vertebrates (fish, amphibian, reptile, bird, human) look very similar — pharyngeal arches, post-anal tail in human embryo
Ernst Haeckel: "Ontogeny recapitulates phylogeny" (now known to be oversimplified but embryonic similarities real)
Biogeography:
Similar environments → similar organisms (convergent evolution)
Darwin's Galapagos finches — 13 species descended from one ancestor, adapted to different food niches
Human Ancestors (Hominid Evolution)
Ancestor
Time period (MYA = million years ago)
Cranial capacity
Key characteristics
Australopithecus ("Southern ape")
~4–2 MYA; Africa
~400–500 cc (ape-like)
Bipedal (walked on 2 legs) — freed hands. Ape-like face; large canines; prominent brow ridges; no chin; much body hair; short stature (~1.2–1.5 m). May have used simple tools. Several species: A. afarensis ("Lucy"), A. africanus.
Homo habilis ("Handy man")
~2.4–1.4 MYA; East Africa
~600–700 cc
First Homo species. More upright posture. Used and made simple stone tools (Oldowan tools = first known toolmaker — hence "handy man"). Reduced canines vs Australopithecus. Less body hair. Slightly larger brain. Ate meat (scavenging).
Homo erectus ("Upright man")
~1.9 million–110,000 years ago; Africa, Asia, Europe
~900–1100 cc
Fully upright; used fire (first hominid to control fire). More sophisticated tools (Acheulean hand axes). Reduced brow ridges. Less body hair. Migrated out of Africa. "Peking Man" and "Java Man" are examples. Longer legs. Communicated (possibly primitive language).
Neanderthals (Homo neanderthalensis)
~400,000–40,000 years ago; Europe and western Asia
~1400–1500 cc (LARGER than modern humans)
Heavy, prominent brow ridges; receding forehead; no chin; massive nose (warms cold air); stocky, muscular build (cold climate adaptation). Made sophisticated tools (Mousterian culture). Buried their dead (ritual/culture). Cared for sick/injured. Coexisted and interbred with early modern humans (Homo sapiens). Modern non-African humans carry ~2% Neanderthal DNA.
Cro-Magnon (Early Homo sapiens)
~40,000–10,000 years ago; Europe
~1350–1400 cc
Anatomically modern. Tall stature. Prominent chin; high forehead; reduced/absent brow ridges. Created art (Lascaux cave paintings, 17,000 yrs old). Used advanced tools (Aurignacian culture — needles, carvings). Wore clothing. Language. Less body hair. Buried dead with grave goods.
Homo sapiens sapiens (Modern humans)
~300,000 years ago – present
~1350–1450 cc
High forehead; no brow ridges; prominent chin; fully upright; minimal body hair; complex language; abstract thinking; culture and technology; writing; art; agriculture. Out of Africa ~60,000–100,000 years ago. Only surviving hominid species.
Trends in Human Evolution
📈 Key Evolutionary Trends (from ancestor → modern human)
Bipedalism — walking on two legs (freed hands for tool use, carrying, manipulation)
Increasing cranial capacity — brain volume from ~400 cc to ~1400 cc; increasing intelligence and cultural complexity
Reduction and change of canine teeth — from large (ape-like, for fighting/tearing) to small; diet changed; tools replaced teeth
Development of chin — absent in early ancestors; prominent in modern Homo sapiens
Reduction of brow ridges — from heavy supraorbital torus to absent (modern human)
High, vertical forehead — associated with larger, more developed frontal lobe (planning, language)
Reduction in body hair — adaptation to open savannah; sweating for thermoregulation in hot climates
Increased height and upright posture — changes in pelvis, spine curvature, foot arch
Tool use and cultural evolution — increasingly sophisticated technology; accumulated knowledge
Theories of Evolution
🦒 Lamarck's Theory — Inheritance of Acquired Characteristics (1809)
Key ideas:
Use and disuse: Organs used frequently become stronger and better developed; organs not used weaken, shrink and may disappear over generations
Inheritance of acquired characters: Changes acquired during an organism's lifetime are passed on to offspring
Internal vital force: Organisms have an inherent drive toward greater complexity/perfection
Example (giraffe): Ancestral giraffes stretched their necks to reach leaves on tall trees. This stretched neck was used constantly and was inherited by offspring. Over many generations, necks became longer and longer.
Vestigial organs (Lamarck's explanation): Organs that fell into disuse (appendix, wisdom teeth, pinnae muscles, coccyx) became smaller over generations as they were not used.
Criticism/Disproof: August Weismann's experiment (1880s): cut off tails of mice for 22 generations → offspring still had normal tails. Acquired characteristics are NOT inherited. Only genetic changes (mutations) in germline cells are inherited, not changes to somatic (body) cells.
🦋 Darwin's Theory — Natural Selection (1859, "On the Origin of Species")
Five key points:
Organisms produce more offspring than the environment can support (overproduction/superfecundity)
Struggle for existence — competition for limited food, space, mates; predation; disease
Natural variation exists among individuals in a population (some faster, better camouflaged, more disease-resistant, etc.)
Survival of the fittest — individuals with favourable variations survive and reproduce more; those with unfavourable variations die or reproduce less (differential reproductive success)
Favourable variations are inherited by offspring → over many generations, frequency of beneficial traits increases → population changes → new species may arise
Example — Peppered moth (Biston betularia) — Industrial Melanism:
Before industrialisation (~1850): Light-coloured (peppered) moths camouflaged on pale lichen-covered bark of trees → survived → reproduced. Dark (melanic) moths visible to bird predators → eaten → rare.
After industrialisation: Air pollution killed lichens; soot blackened tree bark. Light moths now visible on dark bark → eaten. Dark moths camouflaged on dark bark → survived and reproduced. Dark moth frequency rose from <1% to >90% in polluted industrial areas.
After Clean Air Acts (1950s–60s): Lichens recovered, bark lightened → light moths increased again. Dark moths declined. This demonstrates reversible natural selection.
Lamarck vs Darwin
Feature
Lamarck (1809)
Darwin (1859)
Mechanism
Use and disuse; inheritance of acquired characteristics
Natural selection acting on existing heritable variation
What is inherited?
Changes acquired during lifetime (somatic changes)
Only pre-existing genetic variations (germline mutations)
Direction of change
Driven by need/desire (purposive; toward perfection)
Driven by environment (random, non-purposive; no direction)
Giraffe example
Necks stretched during lifetime → longer necks inherited
Giraffes with naturally longer necks ate more → survived better → reproduced more → long-neck gene frequency increased
Accepted today?
No — disproved by genetics and Weismann experiment
Yes — confirmed by genetics, molecular biology, population genetics (Neo-Darwinism / Modern Synthesis)
Exam Q&A
Q1. How does the peppered moth example demonstrate Darwin's theory?+
The peppered moth (Biston betularia) exists in two forms: light (peppered) and dark (melanic). Pre-industrialisation: lichen-covered pale bark → light moths camouflaged → survived + reproduced. Dark moths visible → eaten by birds → rare.
Post-industrialisation: sooty black bark → dark moths camouflaged → survived + reproduced. Light moths eaten → declined.
This demonstrates Darwin's key concepts: (1) Natural variation (light vs dark colour, genetically based). (2) Struggle for existence (predation by birds). (3) Survival of the fittest — better-adapted variant survives. (4) Inheritance — survivors pass on colour gene. (5) Population change over time — frequency of dark allele increased in polluted areas. Evolution occurs, not because moths "wanted" to change, but because environment selected the pre-existing favourable variant.
Q2. What are homologous organs? How are they evidence for evolution?+
Homologous organs are organs that have the same basic structure and embryological origin but are adapted to perform different functions in different species.
Example: The forelimb of vertebrates — human arm (grasping/manipulating), bat wing (flying), whale flipper (swimming), horse leg (running). All have the same bones: humerus, radius, ulna, carpals, metacarpals, phalanges — but modified in shape and proportion for different uses.
Evidence for evolution: The only way to explain why these organs have the same basic structure despite doing completely different jobs is that they all evolved from a common ancestral forelimb (common descent). The modifications represent divergent evolution — adaptations to different environments and functions. This is strong anatomical evidence that these organisms share a common ancestor.
Household detergents and sewage; industrial effluents (heavy metals, dyes, chemicals); oil spills from ships; agricultural runoff (fertilisers, pesticides); thermal pollution (hot water from power plants)
Spread of infectious diseases (HIV, Hepatitis B/C) if improperly disposed; environmental contamination; requires colour-coded segregation and special disposal (incineration, autoclaving)
Radiation Pollution
X-ray machines; nuclear power plants; radioactive fallout from nuclear testing/accidents (Chernobyl, Fukushima); radon gas (natural)
Cancer (leukaemia, thyroid cancer, bone cancer); genetic mutations; radiation sickness; long-term environmental contamination; teratogenic effects (embryonic malformations)
Noise Pollution
Motor vehicles; industrial machinery; construction sites; loudspeakers/festivals; aircraft; trains
Decibels >85 dB prolonged (safe limit: 85 dB occupational; 55 dB residential day)
Noise-induced hearing loss (temporary or permanent); stress hormones released (cortisol, adrenaline); hypertension; sleep disturbance; psychological effects (irritability, aggression); affects wildlife behaviour and communication
Biodegradable vs Non-biodegradable
Feature
Biodegradable Waste
Non-biodegradable Waste
Definition
Broken down by microorganisms (bacteria, fungi) into harmless simpler substances (CO₂, H₂O, minerals)
Cannot be broken down by natural processes; persists for very long periods
Recycling; incineration; landfill; special disposal for hazardous waste
Bioaccumulation & Biomagnification
☠️ Bioaccumulation & Biomagnification
Bioaccumulation: Buildup of a toxic, non-biodegradable substance within a single organism over its lifetime (e.g., DDT accumulating in fat tissue of fish).
Biomagnification (Biological Magnification): Progressive increase in concentration of a toxic substance at each successive trophic level of a food chain.
Example — DDT in aquatic food chain:
DDT concentration increases at each levelPlankton (0.000003 ppm) → Small fish (0.5 ppm) → Large fish (2 ppm) → Fish-eating birds (25 ppm) → Top predator (eagles, ospreys: 75+ ppm)
→ DDT causes eggshell thinning in birds of prey → eggs crack → population crashes
→ Concentration factor: ~10 million× between water and top predator
Why it happens: Each organism consumes many from the level below and stores (not excretes) fat-soluble toxins → concentration increases at each level. Top predators most affected.
Other examples: Mercury in fish (Minamata disease in Japan — mercury poisoning from factory effluent → fish → humans); lead in food chain.
Effects on Climate & Environment
Greenhouse Effect & Global WarmingGreenhouse gases (CO₂, CH₄, H₂O vapour, CFCs, N₂O) trap infrared radiation from Earth's surface → atmosphere warms. Natural greenhouse effect essential for life (keeps Earth ~15°C instead of −18°C). Enhanced greenhouse effect from burning fossil fuels + deforestation → excess CO₂ → more heat trapped → global warming → melting glaciers and ice caps → rising sea levels → flooding of coastal areas; extreme weather events; shifts in climate zones; habitat loss.
Acid RainBurning fossil fuels releases SO₂ and NO₂ → react with atmospheric water vapour → H₂SO₄ (sulphuric acid) and HNO₃ (nitric acid) → acid rain (pH below 5.6; normal rain pH ~5.6 due to dissolved CO₂). Effects: damages trees (leaches minerals from soil; damages leaf cuticle); acidifies lakes/rivers (kills fish, amphibians); corrodes buildings/monuments — especially limestone and marble (CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂); reduces soil fertility. Examples: Taj Mahal damaged by acid rain from Mathura refinery.
Ozone Layer DepletionOzone (O₃) in stratosphere (~20–30 km altitude) absorbs harmful UV-B and UV-C radiation. CFCs (from refrigerators, air conditioners, aerosol sprays) and halons release chlorine and bromine radicals in stratosphere → destroy ozone: Cl + O₃ → ClO + O₂. One Cl atom can destroy 100,000 ozone molecules. Ozone hole over Antarctica discovered 1985. Effects: increased UV-B reaching Earth → skin cancer (melanoma), cataracts, weakened immune systems; disrupts marine food chains (kills phytoplankton — base of ocean food chain). Solution: Montreal Protocol (1987) — global ban on CFCs.
EutrophicationExcessive nutrients (nitrates and phosphates from agricultural runoff and sewage) → massive algal growth (algal bloom) in water bodies. Algae cover surface → blocks sunlight to aquatic plants. When algae die → decomposed by bacteria → bacteria consume all dissolved O₂ → hypoxic/anoxic conditions → fish and aquatic life die. E.g., Dal Lake (Kashmir), many rivers in India. Prevents recreational use; kills biodiversity; produces toxins (some cyanobacteria are toxic).
Control Measures
Measure
How it helps
CNG (Compressed Natural Gas) and unleaded petrol in vehicles
Reduces vehicular CO, SO₂, NO₂, particulate matter and lead emissions; cleaner combustion
Catalytic converters in vehicles
Convert CO, hydrocarbons and NO₂ to CO₂, H₂O, N₂ — reduces harmful exhaust emissions by ~90%
Treat and purify sewage (physical, biological and chemical treatment) before releasing into water bodies
Ban on polythene bags and single-use plastics
Reduces soil and water pollution; prevents drainage blockage; reduces harm to animals
Organic farming
Avoids synthetic fertilisers and pesticides; reduces soil, water and air contamination; maintains soil microbiome
Bharat Stage (BS) / Euro vehicular emission standards
Sets legal limits on vehicle exhaust pollutants; BS-VI (equivalent to Euro-VI) introduced in India 2020
Renewable energy (solar, wind, hydro)
Replaces fossil fuels; reduces CO₂ and air pollutant emissions; reduces dependence on coal
Montreal Protocol (1987)
Global treaty to phase out CFC production and use → ozone layer recovery (ozone hole shrinking)
Swachh Bharat Abhiyan
National campaign for clean India — promotes sanitation, waste management, open-defecation-free villages; built 110+ million toilets
Noise barriers and quiet zones
Soundproof barriers near highways; designated quiet zones (hospitals, schools); limits on loudspeaker use
3 R's of Waste Management
♻️ Reduce — Reuse — Recycle
Reduce: Use fewer resources in the first place — buy less, use durable goods, avoid over-packaging, reduce food waste
Reuse: Use items multiple times before disposing — glass bottles, cloth bags, repurposing containers; extends product life
Recycle: Convert waste materials into new products — paper recycling saves trees; metal recycling saves energy (aluminium recycling uses 5% of energy needed for new production); composting organic waste → fertiliser
Q1. Explain how acid rain is formed and state its effects.+
Formation: Combustion of fossil fuels (coal, oil) in power stations and vehicles releases SO₂ and NO₂. These react with atmospheric water vapour and oxygen → sulphuric acid (H₂SO₄) and nitric acid (HNO₃). These acids dissolve in rainwater → acid rain (pH below 5.6).
Effects: 1. Damages forests and crops — acids leach essential minerals (Ca²⁺, Mg²⁺) from soil; damage leaf cuticle → trees weaken and die 2. Acidifies lakes and rivers → kills fish, amphibians, invertebrates; pH below 5 kills most aquatic life 3. Corrodes buildings and monuments — especially marble/limestone: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂ (marble turns powdery) 4. Reduces soil fertility — kills beneficial soil microorganisms; makes minerals unavailable to plants 5. Damages human health — acidic aerosols cause respiratory irritation
Q2. What is the greenhouse effect? How does it lead to global warming?+
Greenhouse effect: Solar radiation (short wavelength) passes through atmosphere and heats Earth's surface. Earth re-radiates heat as infrared radiation (long wavelength). Greenhouse gases (CO₂, CH₄, H₂O vapour, CFCs, N₂O) in atmosphere absorb and re-emit this infrared radiation downward — trapping heat near Earth's surface (like glass in a greenhouse).
Natural greenhouse effect keeps Earth at ~15°C average. Without it → Earth would be about −18°C → uninhabitable.
Global warming = enhanced greenhouse effect: Human activities → ↑ atmospheric CO₂ (from burning fossil fuels: 280 ppm pre-industrial → 420 ppm now) + deforestation reduces CO₂ absorption → more heat trapped → global temperatures rising (~1.1°C above pre-industrial levels).
Consequences: Melting polar ice caps and glaciers → rising sea levels (coastal flooding); extreme weather (droughts, floods, cyclones, heatwaves); habitat loss (coral bleaching, species extinction); disrupted agriculture; spread of tropical diseases.
Q3. What is biomagnification? Give one example.+
Biomagnification (biological magnification) is the progressive increase in concentration of a persistent, non-biodegradable toxic substance at each successive trophic level of a food chain.
It occurs because: (a) animals eat many organisms from the trophic level below; (b) toxins are fat-soluble and stored (not excreted) → accumulate in each organism; (c) each predator concentrates what all its prey had stored.
Example — DDT in an aquatic food chain: Water (0.000003 ppm) → Phytoplankton (0.04 ppm) → Small fish (0.5 ppm) → Large fish (2 ppm) → Osprey/eagle (25 ppm+)
DDT concentration magnifies ~10 million times from water to top predator. High DDT in birds of prey → eggshell thinning → eggs break → population crashes (bald eagle nearly went extinct in USA due to DDT). DDT was banned in most countries in the 1970s.