Photosynthesis Class 10 ICSE Important Questions with Answers

Answer: (b) Water.

During the light reaction, water is split by photolysis ($2H_2O\rightarrow 4H^+ + 4e^- + O_2$2H₂O→ 4H^+ + 4e^- + O₂), so the evolved oxygen originates from water, not from carbon dioxide.

Answer: (b) Stroma of the chloroplast.

The light reaction occurs in the grana (thylakoid membranes) while the light-independent fixation of $CO_2$CO₂ occurs in the stroma.

Answer: (b) Absorbs carbon dioxide.

KOH absorbs the $CO_2$CO₂ inside the flask. Deprived of a raw material, that leaf cannot photosynthesise and forms no starch, proving $CO_2$CO₂ is essential.

Answer: (d) Glucose.

In the photochemical phase, light activates chlorophyll, water is split (photolysis) releasing oxygen, and the energy is stored as ATP and NADPH. Glucose is made later, in the biosynthetic (dark) phase, when hydrogen from NADPH combines with carbon dioxide.

Answer: (a) Only the green (chlorophyll-containing) regions make starch, so only they turn blue-black with iodine; the white regions lack chlorophyll and stay brown. R correctly explains A.

$6CO_2 + 6H_2O \xrightarrow[\text{chlorophyll}]{\text{light}} C_6H_{12}O_6 + 6O_2$6CO₂ + 6H₂O [chlorophyll]light C₆H₁₂O₆ + 6O₂

Photosynthesis occurs in the chloroplasts, chiefly in the mesophyll cells of the leaf.

1. Site: The light reaction occurs in the grana (thylakoids); the dark reaction occurs in the stroma of the chloroplast.

2. Requirement of light: The light reaction needs light and carries out photolysis of water; the dark reaction does not need light directly and uses $ATP$ATP and $NADPH$NADPH to fix $CO_2$CO₂ into glucose.

1. Boil the leaf in water — kills the cells and stops all activity, making the leaf permeable.

2. Boil in alcohol (over a water bath, not a naked flame as alcohol is inflammable) — removes chlorophyll so the colour change is visible; the leaf becomes brittle.

3. Dip in warm water — softens the leaf.

4. Add dilute iodine solution — starch, if present, turns the leaf blue-black, proving photosynthesis has occurred; a brown colour means no starch.

1. Destarch a potted plant by keeping it in the dark for $48$48 hours, so that its leaves use up the starch already present.

2. Cover part of one leaf on both sides with a strip of black paper (or a stencil) held by clips, and place the plant in bright sunlight for $4$4 to $6$6 hours.

3. Pluck the leaf, remove the paper and do the starch test (boil in water, boil in alcohol, wash, add iodine).

Observation: the uncovered part turns blue-black; the covered part stays brown (no starch). Conclusion: starch was made only where light reached the leaf, so light is necessary for photosynthesis.

Destarching is needed so that any starch found at the end must have been made during the experiment; otherwise old starch would give a false positive.

In light (stomata open): potassium ions ($K^+$K^+) move from the neighbouring cells into the guard cells. This makes the cell sap of the guard cells more concentrated, so water enters them by osmosis. The guard cells become turgid; their thick inner walls curve outwards and the pore (stoma) opens.

In darkness (stomata close): $K^+$K^+ ions move out of the guard cells, water leaves them by osmosis, and they become flaccid. The guard cells straighten and the stoma closes.

Open stomata let carbon dioxide enter the leaf for photosynthesis during the day.

Carbon moves between the air and living things. Photosynthesis is the only major process that takes carbon dioxide out of the air: green plants fix it into glucose and other food. This carbon passes to animals through food, and returns to the air as $CO_2$CO₂ through respiration, decay of dead bodies by decomposers, and burning of fossil fuels and wood. Photosynthesis therefore balances the $CO_2$CO₂ released by the other processes and keeps the level of carbon dioxide in the air roughly steady.

Adaptations of the leaf for photosynthesis:

1. Broad, flat lamina — provides a large surface area to trap maximum sunlight and absorb $CO_2$CO₂.

2. Thin leaf — a short diffusion path lets $CO_2$CO₂ reach mesophyll cells and $O_2$O₂ leave quickly.

3. Palisade tissue — column-shaped cells packed just below the upper epidermis contain the most chloroplasts and are ideally placed to receive light.

4. Numerous stomata and air spaces — stomata (guarded by guard cells) allow gaseous exchange; the spongy mesophyll's air spaces allow rapid diffusion of gases.

5. Network of veins (vascular bundles) — the xylem supplies water and the phloem removes manufactured sugars.

6. Transparent, waxy cuticle and epidermis — protect the leaf yet let light pass through to the mesophyll.

Aim: To show that $CO_2$CO₂ is necessary for photosynthesis.

Method:

1. Destarch a potted plant by keeping it in the dark for 24–48 hours.
2. Insert one half of a leaf (across the midrib) into a wide-mouthed bottle containing potassium hydroxide (KOH), sealed with a split cork and vaseline. The other half remains outside in the air.
3. Place the plant in bright sunlight for a few hours.

Control: The outer half of the same leaf, exposed to atmospheric $CO_2$CO₂, serves as the control; both halves receive equal light, water and chlorophyll, so $CO_2$CO₂ is the only variable.

Observation: On the starch test, the outer half turns blue-black (starch present) while the half inside the KOH bottle stays brown (no starch), because KOH absorbed the $CO_2$CO₂.

Conclusion: Since only the half deprived of $CO_2$CO₂ failed to form starch, carbon dioxide is essential for photosynthesis.

(i) The process is photosynthesis; the bubbles are oxygen, released as a by-product of the light reaction.

(ii) As the distance of the lamp increases, the rate of photosynthesis (bubbles per minute) decreases — they are inversely related.

(iii) Counting bubbles is only a rough measure, since bubbles differ in size. Collecting the gas in an inverted test tube shows how much is given off, and it can be identified: a glowing splinter bursts into flame in it, proving it is oxygen.

(iv) Sodium bicarbonate ($NaHCO_3$NaHCO₃) releases $CO_2$CO₂ into the water, raising the $CO_2$CO₂ concentration. Since $CO_2$CO₂ is a raw material (and was limiting), the rate of photosynthesis rises.