Board Previous Year Questions

About This Section

This is the 'real exam practice' section. The 15 questions below are drawn from actual CBSE Class 10 Science Board papers across recent years. They reflect the patterns, marks distribution, and depth of detail that Board examiners look for.

Distribution:

• 1-mark questions — quick definitions and identifications (4 questions).
• 2-mark questions — short explanations and simple comparisons (4 questions).
• 3-mark questions — detailed answers, tables, mechanism explanations (4 questions).
• 5-mark questions — labelled diagrams + complete explanations (3 questions).

Total marks practice = 40 marks — equivalent to the entire chapter weight in a Board exam.

How to use this section

1. Try first, answer second. Cover the model answer with your hand and write your own response in a notebook.
2. Time yourself. 1-mark = 1.5 min; 2-mark = 3 min; 3-mark = 5 min; 5-mark = 8 min.
3. Compare to model answer. Note where you missed marks — usually due to missed keywords or NCERT phrases.
4. Mark scheme awareness. CBSE awards marks for specific points; reproduce them exactly.

What CBSE examiners look for

CBSE marking schemes typically award marks for:

• Correct labelled diagrams (when asked).
• Use of NCERT-canonical terms (e.g., 'ultrafiltration', 'translocation', 'transpiration pull').
• Step-by-step reasoning with clear sequencing.
• Examples (especially in comparison questions).
• Concluding sentences that explicitly state the answer.

Now let's begin.

PYQ 1: Define the term 'excretion'. [1-mark — CBSE 2019]

Model Answer:

Excretion is the biological process by which an organism removes harmful metabolic wastes (such as urea, uric acid, and excess salts) from its body.

Mark scheme points:

• Term 'biological process' — 0.5 mark.
• Mention of 'metabolic wastes' or examples (urea/CO₂/etc.) — 0.5 mark.

Common mistakes:

• Saying 'removing food' (wrong — that's egestion).
• Saying 'removing CO₂ only' (incomplete).

[Board Important] Use the exact NCERT term 'metabolic wastes' for full marks.

PYQ 2: Name the largest gland in the human body and state any one of its functions. [1-mark — CBSE 2020]

Model Answer:

The liver is the largest gland in the human body.

Function (any ONE):

• Produces bile, which emulsifies fats in the small intestine.
• Detoxifies harmful substances (alcohol, drugs).
• Converts excess amino acids into urea (deamination).
• Stores glycogen and releases glucose when needed.
• Stores vitamins (A, D, B12) and minerals (iron).

Mark scheme:

• Name 'liver' — 0.5 mark.
• Any correct function — 0.5 mark.

Common mistakes:

• Saying 'pancreas' (it's the second-largest gland, not the largest).
• Forgetting to give a function.

[Board Important] Liver is the answer; pancreas is a common trap option.

PYQ 3: What is the role of bile in digestion? [1-mark — CBSE 2018]

Model Answer:

Bile emulsifies fats — it breaks large fat droplets into smaller ones, increasing the surface area for lipase enzymes to act on. Bile itself does not contain any digestive enzyme.

Mark scheme:

• Use of word 'emulsify' — full mark.
• OR: 'breaks fat into small droplets so lipase can act' — full mark.

Common mistake:

• Saying bile 'digests fats' (wrong — bile only emulsifies; lipase does the digesting).

[Board Important] Word 'emulsify' is a CBSE keyword.

PYQ 4: Name the structural and functional unit of the kidney. [1-mark — CBSE 2019]

Model Answer:

The nephron is the structural and functional unit of the kidney.

Mark scheme:

• Just the word 'nephron' — full mark.

Optional extra (not needed for 1 mark but good for context):

• Each kidney has about 1 million nephrons.
• A nephron consists of Bowman's capsule, glomerulus, PCT, Loop of Henle, DCT, and collecting duct.

Common mistakes:

• Saying 'glomerulus' (wrong — that's a part of the nephron, not the unit).
• Saying 'Bowman's capsule' (same — partial only).

[Board Important] Simple recall — must answer 'nephron'.

PYQ 5: Why is diffusion insufficient to meet the oxygen requirements of multicellular organisms like humans? [2-marks — CBSE 2021]

Model Answer:

Diffusion is insufficient for multicellular organisms because:

1. Distance — most cells are far from the body surface. The diffusion distance is too great for oxygen to reach interior cells fast enough.

2. Specialised requirements — different cells need different amounts of oxygen at different rates. A simple diffusion gradient cannot match cells to their specific O₂ needs.

Hence multicellular organisms have evolved specialised respiratory systems (lungs) and circulatory systems (blood with haemoglobin) for active, targeted O₂ delivery to every cell.

Mark scheme:

• Mention of distance / depth of cells — 1 mark.
• Mention of size + specialised needs — 1 mark.

Common mistakes:

• Saying just 'humans are big' (vague).
• Not mentioning that specialised systems evolved.

[Board Important] Always mention BOTH distance AND specialised needs.

PYQ 6: List any two differences between aerobic and anaerobic respiration. [2-marks — CBSE 2019]

Model Answer:

Feature	Aerobic Respiration	Anaerobic Respiration
Oxygen	REQUIRED	NOT REQUIRED
Site in cell	Mitochondria	Cytoplasm
End products	CO₂ + H₂O	Ethanol + CO₂ (yeast) OR Lactic acid (muscle)
ATP yield	~38 ATP per glucose	~2 ATP per glucose

Any TWO clear differences for full marks.

Mark scheme:

• 1 mark per correct difference (give 2 = 2 marks).

Common mistake:

• Just writing 'aerobic uses oxygen, anaerobic doesn't' twice (counts as ONE difference).

[Board Important] Always write at least 3 differences in a TABLE for safety.

PYQ 7: What is meant by translocation in plants? Why is it essential? [2-marks — CBSE 2020]

Model Answer:

Translocation is the transport of soluble products of photosynthesis (mainly sucrose) from sources (leaves, where food is produced) to sinks (roots, fruits, storage organs, growing buds — where food is needed or stored). It occurs in the phloem.

Why it is essential:

• Photosynthesis happens only in green parts (mainly leaves), but every cell in the plant — including non-green roots, growing tips, and storage organs — needs glucose for energy and growth.
• Without translocation, the rest of the plant would starve.

Mark scheme:

• Definition with mention of 'phloem' — 1 mark.
• Why essential (food distribution) — 1 mark.

Common mistake:

• Confusing translocation with transpiration (xylem water transport).
• Forgetting to mention 'phloem'.

[Board Important] Use the NCERT phrase: 'transport of soluble products of photosynthesis'.

PYQ 8: Why do herbivores like cows have a longer small intestine than carnivores like tigers? [2-marks — CBSE 2021]

Model Answer:

Herbivores (like cows) eat plant material, which is rich in cellulose — a tough complex carbohydrate that is difficult to digest. They need:

• More time to break it down.
• More surface area to absorb the slowly released nutrients.
• Bacterial partners (in their stomach/intestine) to ferment cellulose into simpler compounds.

Hence herbivores evolved a longer small intestine to allow extended digestion.

Carnivores (like tigers) eat meat, which is rich in proteins — easier to digest and absorb. They need less digestion time, hence a shorter small intestine.

Mark scheme:

• Mention 'cellulose is hard to digest' — 1 mark.
• Mention 'longer intestine = more time/surface area' — 1 mark.

Common mistake:

• Just saying 'cows eat more' (wrong — quantity isn't the reason).

[Board Important] Always link to type of food and time/surface needed for digestion.

PYQ 9: Explain the role of xylem and phloem in transportation in plants. [3-marks — CBSE 2019]

Model Answer:

Plants have two transport tissues, each with specific roles:

Xylem

• Conducts water and dissolved minerals from roots to all other parts of the plant (especially leaves).
• Direction: upward only (unidirectional).
• Mostly dead cells (tracheids, vessels) with lignified walls.
• Driving force: transpiration pull (suction generated by water loss from leaves) + root pressure.
• No ATP needed — passive process.

Phloem

• Conducts food (sucrose) from sources (leaves) to sinks (roots, fruits, growing buds, storage organs).
• Direction: bidirectional — source to sink.
• Mostly living cells (sieve tubes, companion cells).
• Driving force: pressure-flow (Münch's hypothesis) — active transport.
• ATP needed — active process.

Together

Xylem and phloem run side by side in vascular bundles throughout the plant — roots, stems, and leaves. Together they form the vascular system that transports everything the plant needs.

Mark scheme:

• Role of xylem (water + minerals, upward) — 1.5 marks.
• Role of phloem (food, source to sink) — 1.5 marks.

Common mistake:

• Confusing the directions.
• Missing 'dead' vs 'living' point.

[Board Important] Always tabulate or use clear headings.

PYQ 10: Describe the process of digestion in human stomach with respect to the role of HCl, pepsin, and mucus. [3-marks — CBSE 2020]

Model Answer:

In the human stomach, gastric juice is secreted from the glands in the stomach wall. It contains three key components, each with a specific role:

1. Hydrochloric acid (HCl)

• Creates a strongly acidic medium (pH ~1-2).
• Three roles:
• Kills bacteria and microbes that came in with food (sterilization).
• Activates the inactive pepsinogen into active pepsin.
• Provides optimum pH for pepsin to work (pepsin works best at pH ~2).

2. Pepsin (a protein-digesting enzyme)

• Secreted in inactive form as pepsinogen by chief cells.
• Activated by HCl.
• Active pepsin breaks down proteins into smaller peptides.
• This is the start of protein digestion in our body.

3. Mucus

• A thick, slimy fluid secreted by mucous cells lining the stomach.
• Forms a protective coating on the inner surface of the stomach.
• Why essential: HCl is so corrosive that it could digest the stomach wall (which is made of protein, after all). Mucus is the wall's shield.
• Without mucus, peptic ulcers form.

Result of stomach digestion

Food is partially digested (proteins → peptides) and converted into a semi-liquid acidic mass called chyme, which then passes into the small intestine for further digestion.

Mark scheme:

• HCl role — 1 mark.
• Pepsin role — 1 mark.
• Mucus role — 1 mark.

[Board Important] Always cover ALL three components and ALL three roles of HCl.

PYQ 11: Differentiate between arteries and veins. Why does the left ventricle have a thicker wall than the right ventricle? [3-marks — CBSE 2021]

Model Answer:

Differences between arteries and veins

Feature	Artery	Vein
Direction of blood	AWAY from heart	TOWARDS heart
Wall	Thick, muscular, elastic	Thinner, less muscular
Lumen	Narrow	Wider
Valves	NO	YES (prevent backflow)
Blood pressure	HIGH	LOW
Blood content (usually)	Oxygenated (red)	Deoxygenated (purple)
Location	Deep inside body	Closer to skin

Exception: Pulmonary artery carries DEOXYGENATED blood; pulmonary vein carries OXYGENATED blood.

Why left ventricle is thicker

The left ventricle has a thicker, more muscular wall than the right ventricle because:

• Left ventricle pumps oxygenated blood to the ENTIRE BODY (systemic circulation) — every cell, every limb, every organ.
• This requires very HIGH pressure (~120 mm Hg systolic) to push blood through kilometres of vessels against gravity.
• High pressure needs a thicker, stronger muscle.

In contrast, the right ventricle only pumps deoxygenated blood to the nearby lungs (pulmonary circulation) at much lower pressure (~25 mm Hg). It doesn't need as much muscle.

Hence left ventricle wall ≈ 3× thicker than right ventricle wall.

Mark scheme:

• Differences (at least 3) — 2 marks.
• Reason for thicker left ventricle — 1 mark.

[Board Important] Always link 'thicker' to 'pumping to entire body / high pressure'.

PYQ 12: Explain how plants get rid of their excretory products. [3-marks — CBSE 2022]

Model Answer:

Plants don't have specialised excretory organs like kidneys. Instead, they use five methods to get rid of their wastes:

1. Gaseous wastes through stomata and lenticels

• O₂ (from photosynthesis) → out through stomata in leaves.
• CO₂ (from respiration) → out through stomata + lenticels in stems.
• Water vapour → out through stomata (transpiration).

2. Excess water by transpiration

• Loss of water as vapour from stomata serves both water transport AND water excretion.

3. Storage in vacuoles

• Wastes are stored safely in large central vacuoles inside plant cells.
• Examples: calcium oxalate crystals (spinach, taro), tannins (oak bark), anthocyanins (red/purple pigments).

4. Shedding of old parts

• Old leaves, bark, flowers, and fruits are dropped along with their accumulated wastes.
• Example: deciduous trees shedding leaves in autumn.

5. Secretion of gum, resin, latex, alkaloids

• Some plants store wastes as special secretions in ducts and cavities.
• Examples: Latex from rubber tree; resin from pine; gum from acacia; alkaloids like caffeine, quinine, nicotine.

Why plants don't need a specialised excretory system

Plants have slow metabolism, large vacuoles for safe storage, and the ability to shed waste-laden parts — so they don't need active kidney-like systems.

Mark scheme:

• At least 4 of the 5 methods, with examples — 3 marks.

[Board Important] Always give 4-5 distinct methods with examples.

PYQ 13: Draw a well-labelled diagram of the human respiratory system and explain the mechanism of breathing. [5-marks — CBSE 2020]

Model Answer:

Labelled diagram (write description if cannot draw):

A well-labelled diagram should show:

• Nostrils / nasal cavity — air entry.
• Pharynx — throat region.
• Larynx — voice box.
• Trachea (windpipe) — held open by C-shaped cartilage rings.
• Bronchi (singular: bronchus) — two main branches from trachea into each lung.
• Bronchioles — finer branches inside lungs.
• Alveoli — tiny air sacs at the end of bronchioles (site of gas exchange).
• Diaphragm — dome-shaped muscle below lungs.
• Ribs with intercostal muscles.

See section 5 image for reference.

Mechanism of breathing — two phases

1. Inhalation (Inspiration — air in)

• Diaphragm CONTRACTS → flattens (moves down).
• External intercostal muscles CONTRACT → ribs move UP and OUT.
• Chest cavity volume INCREASES.
• Pressure inside DECREASES (Boyle's law).
• Atmospheric air now at higher pressure → air flows IN through nose → trachea → bronchi → bronchioles → alveoli.

2. Exhalation (Expiration — air out)

• Diaphragm RELAXES → returns to dome shape (moves up).
• External intercostal muscles RELAX → ribs move DOWN and IN.
• Chest cavity volume DECREASES.
• Pressure inside INCREASES.
• Air now at higher pressure inside → air flows OUT.

Summary table

Phase	Diaphragm	Ribs	Cavity volume	Cavity pressure	Air flow
Inhalation	Contracts (DOWN)	UP and OUT	INCREASES	DECREASES	IN
Exhalation	Relaxes (UP)	DOWN and IN	DECREASES	INCREASES	OUT

Where does gas exchange happen?

In the alveoli — they have:

• Thin walls (one cell thick).
• Rich blood capillary network.
• Huge surface area (~80 m² total in adult).

O₂ diffuses from alveoli into blood (binds to haemoglobin in RBCs). CO₂ diffuses from blood into alveoli (exhaled out).

Breathing rate

Normal: 12-18 breaths per minute at rest. Increases during exercise.

Mark scheme:

• Labelled diagram — 2 marks.
• Mechanism (inhalation + exhalation) — 2 marks.
• Role of diaphragm + intercostal muscles + pressure/volume changes — 1 mark.

[Board Important] 5-mark question — MUST include labelled diagram. Don't skip the diagram even if it's not perfect.

PYQ 14: Draw a labelled diagram of a nephron and explain the process of urine formation. [5-marks — CBSE 2019]

Model Answer:

Labelled diagram (description):

The nephron should be drawn with these labelled parts:

1. Bowman's capsule — cup-shaped structure at the top.
2. Glomerulus — capillary cluster inside Bowman's capsule.
3. Afferent arteriole — wider; brings blood IN to glomerulus.
4. Efferent arteriole — narrower; takes blood OUT.
5. Proximal Convoluted Tubule (PCT) — coiled tubule after Bowman's capsule.
6. Loop of Henle — descending and ascending limbs (U-shaped, dips into medulla).
7. Distal Convoluted Tubule (DCT) — second coiled section.
8. Collecting Duct — into which multiple nephrons drain.
9. Peritubular capillaries — wrapping around the tubule.

See section 8 image for reference.

Process of urine formation — three steps

Step 1: Glomerular Filtration (Ultrafiltration)

• Site: Glomerulus → Bowman's capsule.
• The afferent arteriole is wider than the efferent → high pressure builds in the glomerulus.
• This pressure forces water and small dissolved molecules OUT of the blood into the Bowman's capsule.
• Filtered: water, glucose, amino acids, salts, urea, vitamins.
• NOT filtered: blood cells, plasma proteins (too large).
• Volume: ~180 L of filtrate per day.

Step 2: Tubular Reabsorption (Selective Reabsorption)

• Site: PCT, Loop of Henle, DCT.
• As filtrate flows through the long tubule, useful substances are reabsorbed back into peritubular capillaries.
• Reabsorbed: 100% glucose, 100% amino acids, ~99% water, most useful salts.
• Uses: active transport (ATP) + passive diffusion/osmosis.
• Result: 180 L of filtrate reduces to ~1.5 L.

Step 3: Tubular Secretion

• Site: Mainly DCT.
• Additional wastes are actively secreted FROM blood INTO the tubule.
• Secreted: excess H⁺ (for pH control), excess K⁺, drugs, creatinine.

Final urine path

$$\text{Collecting Duct} \to \text{Renal Pelvis} \to \text{Ureter} \to \text{Urinary Bladder} \to \text{Urethra} \to \text{Out}$$

Summary table

Step	Site	Direction	Substances
1. Filtration	Glomerulus → Bowman's capsule	Blood → Tubule	Water, urea, small molecules
2. Reabsorption	PCT, Loop, DCT	Tubule → Blood	Glucose, amino acids, water, salts
3. Secretion	DCT	Blood → Tubule	H⁺, K⁺, drugs

Mark scheme:

• Labelled diagram — 2 marks.
• Three steps with sites and substances — 3 marks.

[Board Important] 5-mark Board staple. MUST have both diagram AND three-step explanation.

PYQ 15: With the help of a labelled diagram of the human heart, explain the process of double circulation. Why is it called 'double circulation'? [5-marks — CBSE 2022]

Model Answer:

Labelled diagram (description):

The heart should be drawn with all FOUR chambers and FOUR valves labelled:

1. Right atrium — top right, receives deoxygenated blood.
2. Right ventricle — bottom right, pumps to lungs.
3. Left atrium — top left, receives oxygenated blood from lungs.
4. Left ventricle — bottom left, pumps to body (thickest wall).
5. Tricuspid valve — between right atrium and right ventricle.
6. Bicuspid (mitral) valve — between left atrium and left ventricle.
7. Pulmonary semilunar valve — at exit of right ventricle.
8. Aortic semilunar valve — at exit of left ventricle.
9. Septum — wall separating left and right sides.
10. Major vessels: vena cavae (in), pulmonary artery (out to lungs), pulmonary veins (in from lungs), aorta (out to body).

See section 6 image for reference.

Process of double circulation

Double circulation = blood passes through the heart TWICE in one complete cycle.

1. Pulmonary circulation (Heart → Lungs → Heart)

$$\text{Body tissues (deox.)} \to \text{Vena cavae} \to \text{Right atrium}$$ $$\to \text{Right ventricle (via tricuspid valve)}$$ $$\to \text{Pulmonary artery (via pulmonary semilunar valve)} \to \text{Lungs}$$

At lungs: CO₂ removed, O₂ added. Blood becomes oxygenated.

$$\to \text{Pulmonary veins} \to \text{Left atrium}$$

Purpose: Pick up O₂, drop off CO₂.

2. Systemic circulation (Heart → Body → Heart)

$$\text{Left atrium} \to \text{Left ventricle (via bicuspid valve)}$$ $$\to \text{Aorta (via aortic semilunar valve)} \to \text{Body tissues}$$

At body tissues: O₂ delivered to cells, CO₂ picked up. Blood becomes deoxygenated.

$$\to \text{Vena cavae} \to \text{Right atrium (back to start)}$$

Purpose: Deliver O₂ and nutrients to body cells; pick up CO₂ and wastes.

Why is it called 'double circulation'?

It is called 'double' circulation because blood passes through the heart TWICE in one complete loop:

• Once on the RIGHT side (for pulmonary circulation — from body to lungs).
• Once on the LEFT side (for systemic circulation — from lungs to body).

Why is it essential in humans?

• Mammals and birds are warm-blooded with high metabolic rates → need maximum O₂ at all times.
• Double circulation keeps oxygenated and deoxygenated blood completely separate — no mixing → every cell gets maximum-oxygenated blood.
• Fish have single circulation (only 2 chambers); they manage because they are cold-blooded with lower energy demand.

Mark scheme:

• Labelled diagram — 2 marks.
• Two loops with paths — 2 marks.
• Why 'double' + significance — 1 mark.

[Board Important] 5-mark Board staple — MUST have diagram + both loops + significance.

Section 10 Wrap-Up

You've now worked through 15 real CBSE Board PYQs covering:

• 1-mark recall questions (4 — quick definitions).
• 2-mark concept questions (4 — short explanations).
• 3-mark analytical questions (4 — comparisons and mechanisms).
• 5-mark comprehensive questions (3 — diagrams + detailed explanations).

Patterns that recur every year

Almost every Board exam of Class 10 Science has at least one question on:

• Photosynthesis equation (1 or 2 mark).
• Heart diagram or function (3 or 5 mark).
• Nephron and urine formation (5 mark).
• Xylem vs phloem or artery vs vein (3 mark).
• Aerobic vs anaerobic respiration (2 or 3 mark).
• Plant excretion methods (2 or 3 mark).

Memorise these high-yield areas.

Last-minute revision strategy

1. The morning of the exam: Quickly revise Section 11 (Summary). Don't study new material.
2. In the exam hall: Read all questions first; allocate time per question by marks (1.5 min/mark).
3. For 5-mark questions: Draw diagram FIRST, then label, then explain.
4. For comparison questions: ALWAYS use a table.
5. End every answer: With a concluding sentence that explicitly answers the question.
6. NCERT phrases = guaranteed marks. Don't paraphrase if you can use them verbatim.

What's Next

Section 11 is the chapter's master cheat-sheet — designed for last-minute revision the night before the exam. Use it!

You've completed the toughest part of this chapter. Section 11 is the cherry on top.