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ICSE Class 10 Science★ ICSE Class 10 Science (Biology) · ~40 marks · Board: ICSE

Biology — Cell Division, Genetics, Plant Physiology & Human Body Systems

ICSE Class 10 Biology: MITOSIS and MEIOSIS — how cells divide and life reproduces. GENETICS — Mendel's laws and how traits are INHERITED. Plant PHYSIOLOGY — absorption, transpiration, photosynthesis. And the HUMAN BODY — circulatory, excretory, nervous, endocrine, and reproductive systems. Plus POPULATION and POLLUTION. ICSE Class 10 Science.

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By the end of this chapter you'll be able to…

1Describe the stages of mitosis and meiosis with diagrams; compare the two processes and explain their significance
2Apply Mendel's laws using Punnett squares for monohybrid and dihybrid crosses; explain sex determination in humans
3Explain absorption, translocation, transpiration, and photosynthesis in plants with limiting factors
4Trace the pathway of blood through the double circulation; describe structure and function of the heart
5Explain the structure and function of the nephron; describe kidney dialysis as an artificial kidney
6Describe the structure of the human eye; explain accommodation and common defects with corrections
7Outline the structure of the brain and spinal cord; describe a reflex arc; distinguish voluntary and involuntary actions

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Why this chapter matters

Biology is the largest section of ICSE Science — the Biology paper (Science Paper 3) is 80 marks, and this file's topics cover most of it. Cell Division (mitosis/meiosis) is 8–10 marks and highly structured — students who learn the phases and diagrams score full marks. Genetics (Mendel's laws, Punnett squares) is 6–8 marks and requires logical thinking. Human body systems (circulatory, excretory, nervous, reproductive) account for another 20+ marks. The content is detailed but highly predictable — the same diagrams and processes are tested every year. Diagram labelling alone can score 10 marks.

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A 5-minute refresher here will save you 30 minutes of confusion below.

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Biology — Cell Biology, Genetics, Plants & Human Body

1. Cell Division

Feature	Mitosis	Meiosis
Where	Body (somatic) cells	Germ cells (gonads — ovaries/testes)
Daughter cells	2. DIPLOID (2n). Genetically IDENTICAL.	4. HAPLOID (n). Genetically DIFFERENT.
Purpose	GROWTH and REPAIR	Produce GAMETES (sperm/egg) for sexual reproduction

2. Genetics — Mendel's Laws

Key Terms

Gene: Unit of heredity on a chromosome.
Allele: Alternative form of a gene. Dominant (T — tall). Recessive (t — dwarf).
Homozygous: Both alleles SAME (TT, tt). Heterozygous: Alleles DIFFERENT (Tt).
Genotype: Genetic makeup. Phenotype: Physical appearance.

Mendel's Laws

Law of Dominance: In a heterozygote, the DOMINANT allele masks the recessive.
Law of Segregation: Alleles SEPARATE during gamete formation. Each gamete receives ONE allele.
Law of Independent Assortment: Genes for DIFFERENT traits assort INDEPENDENTLY.

Monohybrid Cross (Tt × Tt): Phenotype Ratio = 3:1. Genotype Ratio = 1:2:1.

Sex Determination

Females: XX. Males: XY. 'The FATHER determines the sex of the child — the sperm carries either X or Y.'

3. Plant Physiology

Absorption by Roots

Water and minerals absorbed by ROOT HAIRS by OSMOSIS and DIFFUSION.
Active Transport: Movement AGAINST concentration gradient. Requires ENERGY (ATP).

Transpiration — Loss of water as VAPOUR from leaves

Factors: Temperature (↑). Humidity (↓). Wind speed (↑). Light intensity (↑). Cohesion-Tension Theory: Water pulled up through xylem by transpiration 'suction.'

Photosynthesis

6CO₂ + 12H₂O →(sunlight, chlorophyll)→ C₆H₁₂O₆ + 6O₂ + 6H₂O Limiting Factors: CO₂ concentration. Light intensity. Temperature. 'At any given time, one factor is LIMITING the rate of photosynthesis.'

4. Circulatory System

Heart: Four chambers. Double Circulation.

Blood Vessels

Type	Structure	Function
Artery	Thick, elastic	Carry blood AWAY from heart (O₂ rich — except pulmonary artery)
Vein	Thin, VALVES	Carry blood BACK to heart (O₂ poor — except pulmonary vein)
Capillary	ONE cell thick	EXCHANGE of O₂, CO₂, nutrients, waste

Blood Components

RBCs (haemoglobin — O₂ transport). WBCs (immunity). Platelets (clotting). Plasma (liquid — transports nutrients, hormones, waste).

Blood Groups (ABO)

A, B, AB, O. Universal donor: O—. Universal recipient: AB+.

5. Excretory System

The Kidneys — FILTERS

Functional unit: NEPHRON. ~1 million per kidney.
Process: Glomerular FILTRATION. Selective REABSORPTION. Tubular SECRETION.
URINE: water + urea + salts. Stored in bladder. Expelled through urethra.

6. Nervous System and Sense Organs

The Neuron

Functional unit. Dendrites (receive impulses). Axon (transmits). SYNAPSE (gap between neurons).

Parts of the Brain

Cerebrum: Intelligence, memory, voluntary movement.
Cerebellum: Balance, coordination.
Medulla Oblongata: Involuntary actions (heartbeat, breathing).

Reflex Arc

Stimulus → Receptor → Sensory neuron → SPINAL CORD → Motor neuron → Effector (muscle/gland). 'Bypasses the brain for SPEED.'

Eye

Cornea (transparent front). Iris (coloured — controls light). Pupil (opening). Lens (focuses). Retina (image formed — rods and cones). Optic nerve (to brain).
Accommodation: CILIARY MUSCLES change lens shape to focus near/far.

Ear

Outer (pinna, ear canal) → Middle (eardrum, ossicles — hammer, anvil, stirrup) → Inner (cochlea — hearing. Semicircular canals — balance).

7. Endocrine System (Hormones)

Gland	Hormone	Function
Thyroid	Thyroxine	Metabolism. Needs IODINE.
Pancreas	Insulin	↓ blood sugar. Diabetes if deficient.
	Glucagon	↑ blood sugar.
Adrenal	Adrenaline	'Fight or flight.'
Pituitary	GH (Growth Hormone)	'Master gland.'

8. Reproductive System

Male: Testes (produce sperm + testosterone). Female: Ovaries (produce eggs + oestrogen/progesterone).

Fertilisation: Fusion of sperm and egg → ZYGOTE (in Fallopian tube).
IMPLANTATION in uterus → EMBRYO → FOETUS. Gestation: ~280 days (9 months).

9. Population

BIRTH RATE. DEATH RATE. GROWTH RATE. Demography = study of population. 'India: 2nd largest population.' Problems of overpopulation: pressure on resources, unemployment, environmental degradation.

10. Pollution

Type	Causes	Effects
Air	Vehicles, factories, burning fuel	Respiratory disease, global warming
Water	Sewage, industrial waste	Waterborne diseases. Eutrophication.
Soil	Fertilisers, pesticides, dumping	Loss of fertility

Global Warming = increase in Earth's average temperature due to GREENHOUSE GASES (CO₂, CH₄). OZONE DEPLETION: CFCs → destroy ozone layer → more UV reaches Earth.

Key formulas & results

Everything you need to memorise, in one card. Screenshot this for revision.

Cell Division — Mitosis vs Meiosis

MITOSIS: 2n → 2n. Two identical daughter cells. PHASES: Prophase (chromatin condenses → chromosomes visible), Metaphase (chromosomes align at metaphase plate/equator), Anaphase (chromatids pulled to opposite poles), Telophase (nuclear envelope reforms) + Cytokinesis (cell division). MEIOSIS: 2n → n. Four genetically different haploid cells. MEIOSIS I: Homologous pairs separate (reduction division). MEIOSIS II: Sister chromatids separate (like mitosis). SIGNIFICANCE: Mitosis = growth, repair, asexual reproduction. Meiosis = gamete production, genetic variation.

ICSE DIAGRAM: Draw the metaphase diagram (chromosomes on the equatorial plate) — this is the most-tested phase. For mitosis: show 2n chromosomes aligned at the plate with spindle fibres attached to centromeres. For meiosis I: show PAIRED homologous chromosomes (bivalents/tetrads) at the equatorial plate — this is the KEY DIFFERENCE. Crossing over (exchange of genetic material between homologous chromosomes) occurs in Prophase I of meiosis — this creates genetic variation.

Genetics — Mendel's Laws

MONOHYBRID CROSS (Tt × Tt): Gametes: T, t. Punnett square: TT, Tt, Tt, tt. Genotype ratio: 1 TT : 2 Tt : 1 tt = 1:2:1. Phenotype ratio: 3 tall : 1 dwarf = 3:1. DIHYBRID CROSS (TtRr × TtRr): Phenotype ratio = 9:3:3:1 (two dominant : first dom, second rec : first rec, second dom : both recessive). TEST CROSS: Unknown genotype × homozygous recessive. If offspring are all dominant phenotype → unknown is homozygous dominant. If 1:1 ratio → heterozygous. SEX DETERMINATION: Female XX. Male XY. Sperm: 50% X, 50% Y. Father determines sex.

PUNNETT SQUARE METHOD: (1) Write the gametes of each parent on the top and left. (2) Fill each box with one gamete from each parent. (3) Count phenotypes and genotypes. CODOMINANCE: Both alleles expressed (neither dominant). Example: Blood groups: IA + IB → blood group AB (both A and B antigens expressed). ABO blood groups: IA (antigen A, dominant), IB (antigen B, dominant), i (no antigen, recessive). Genotype IAi = blood group A (A dominant over i). ICSE tests ABO blood groups every few years.

Photosynthesis

OVERALL EQUATION: 6CO₂ + 6H₂O →(sunlight + chlorophyll)→ C₆H₁₂O₆ + 6O₂. TWO STAGES: LIGHT REACTIONS (in thylakoids): Water photolysis → O₂ released; ATP and NADPH produced. DARK REACTIONS / Calvin Cycle (in stroma): CO₂ fixed using ATP and NADPH → glucose. LIMITING FACTORS: The factor in shortest supply limits the rate. (1) Light intensity. (2) CO₂ concentration. (3) Temperature (affects enzymes). (4) Water availability. TRANSPIRATION PULL: Water evaporates from stomata → creates tension → pulls water up through xylem (cohesion-tension theory).

ICSE EXPERIMENT: 'Prove starch is produced in photosynthesis' — Iodine test on a destarched leaf that was partly covered (one part in light, one in dark). Only the LIGHT part turns blue-black (starch present). VARIEGATED LEAF EXPERIMENT: shows chlorophyll is necessary (green parts turn blue-black, white/non-green parts do not). LIMITING FACTOR GRAPH: Rate increases with CO₂/light until the limiting factor changes — then the graph plateaus. ICSE tests interpreting these graphs.

Human Body Systems

DOUBLE CIRCULATION: Right side of heart → Pulmonary circuit (lungs) → Oxygenated blood → Left side → Systemic circuit (body). CARDIAC CYCLE: Systole (contraction) → Diastole (relaxation). BLOOD PRESSURE: 120/80 mmHg (systolic/diastolic). KIDNEY/NEPHRON: Glomerular filtration (blood pressure forces small molecules out of capillaries into Bowman's capsule). Tubular reabsorption (glucose, water, amino acids reabsorbed). Secretion (urea, H⁺, drugs secreted). Urine: water + urea + salts. EYE: Cornea (refraction) → aqueous humour → iris/pupil → lens (accommodation) → vitreous humour → retina (rods: night vision, cones: colour). Image formed on retina is INVERTED and REAL. REFLEX ARC: Stimulus → receptor → sensory neuron → spinal cord (relay neuron) → motor neuron → effector → response.

MOST-TESTED DIAGRAMS: (1) Heart with chambers and valves labelled. (2) Nephron with filtration, reabsorption, secretion labelled. (3) Cross-section of eye with parts labelled. (4) Neuron with cell body, axon, dendrites, myelin sheath. ACCOMMODATION OF EYE: Near object → ciliary muscles CONTRACT → ligaments RELAX → lens becomes MORE CONVEX (higher power) → sharp image. Far object → ciliary muscles RELAX → ligaments TAUT → lens becomes FLATTER (lower power). Common defect: Myopia (near-sighted) → concave lens correction. Hypermetropia (far-sighted) → convex lens.

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Common mistakes & fixes

These are the exact errors that cost students marks in board exams. Read them once, save yourself the trouble.

WATCH OUT

✗ Confusing mitosis and meiosis outcomes and ploidy

✓ MITOSIS: Starts with 1 diploid (2n) cell, produces 2 DIPLOID (2n) identical daughter cells. Occurs in somatic (body) cells. MEIOSIS: Starts with 1 diploid (2n) cell, produces 4 HAPLOID (n) genetically varied cells. Occurs in gonads (testes/ovaries). Key distinction for ICSE: In Meiosis I, HOMOLOGOUS CHROMOSOMES separate (not sister chromatids). In Meiosis II (and in Mitosis), SISTER CHROMATIDS separate. Meiosis I is the 'reduction division' — it halves the chromosome number.

WATCH OUT

✗ Writing transpiration as the cause of water absorption by roots

✓ Transpiration from leaves CREATES A TENSION (suction force) that PULLS water up through the xylem — this is the COHESION-TENSION THEORY. This is different from ROOT PRESSURE (which pushes water up from below — active process requiring energy, accounts for guttation). ICSE asks both mechanisms: root pressure (for short plants, at night when transpiration is low) and cohesion-tension/transpiration pull (for tall trees, main mechanism). Water enters roots by OSMOSIS — soil water potential > cell sap potential → water enters by osmosis.

WATCH OUT

✗ Reversing the accommodation of the eye

✓ For NEAR objects: ciliary muscles CONTRACT → suspensory ligaments become SLACK/LOOSE → lens becomes MORE CONVEX (more curved, shorter focal length, higher power). 'Near = muscles work harder → lens fatter.' For FAR objects: ciliary muscles RELAX → ligaments become TAUT/TENSE → lens becomes FLATTER (less convex, longer focal length, lower power). 'Far = muscles relax → lens thinner.' Remember: it's LIGAMENTS that pull the lens flat (not the muscles). Muscles contract → ligaments loosen → lens rounds up.

Practice problems

Try each one yourself before tapping "Show solution". Active recall > rereading.

Q1EASY· genetics-punnett-square

A man with blood group A (genotype IAi) marries a woman with blood group B (genotype IBi). What are the possible blood groups of their children? Give the ratio.

▶ Show solution

Father gametes: IA and i. Mother gametes: IB and i. Punnett square: IAIB (blood group AB), IAi (blood group A), IBi (blood group B), ii (blood group O). Possible blood groups: AB : A : B : O in ratio 1:1:1:1. So all four blood groups are possible, each with 25% probability. This is a classic ICSE genetics question. Note: Blood group AB shows CODOMINANCE — both IA and IB are expressed simultaneously.

Q2MEDIUM· photosynthesis-limiting-factors

A plant is kept at 25°C with adequate water. When light intensity is increased from 0 to 10,000 lux, the photosynthesis rate increases. But beyond 10,000 lux, the rate stays constant at the same CO₂ concentration. Explain what is happening and what you would do to increase the rate further.

▶ Show solution

At low light intensity: LIGHT is the limiting factor — increasing light increases photosynthesis rate. At 10,000 lux: the rate plateaus because LIGHT is no longer limiting — now CO₂ CONCENTRATION or temperature has become the new limiting factor. To increase the rate further: INCREASE the CO₂ concentration (e.g., add CO₂ to the greenhouse atmosphere). This is why greenhouses pump in extra CO₂ to increase crop yield. Alternatively, slightly increasing temperature (up to the enzyme's optimum, around 30–35°C) could also increase the rate if CO₂ is increased simultaneously.

Q3HARD· nephron-kidney-function

Describe the three processes that occur in the nephron to produce urine. State ONE substance present in the glomerular filtrate that is NOT present in urine and explain why.

▶ Show solution

THREE PROCESSES in the nephron: (1) GLOMERULAR FILTRATION: Blood at high pressure in the glomerulus forces small molecules (water, glucose, urea, salts, amino acids) through the capillary walls into Bowman's capsule. Large molecules (proteins, blood cells) remain in blood. (2) TUBULAR REABSORPTION: As filtrate passes through PCT (proximal convoluted tubule), loop of Henle, and DCT: ALL glucose, amino acids, vitamins, and most water and salts are actively or passively reabsorbed back into the blood. (3) TUBULAR SECRETION: Additional waste products (creatinine, H⁺ ions, drugs) are actively secreted from blood into the tubular filtrate. SUBSTANCE present in filtrate but NOT in urine: GLUCOSE. Reason: All glucose that is filtered is completely reabsorbed by active transport in the proximal convoluted tubule. Exception: In diabetes mellitus, blood glucose is so high that the reabsorption capacity is exceeded → glucose appears in urine (glucosuria).

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