Photosynthesis: Provider of Food for All — ICSE Class 10 Biology Important Questions

Answer: (b) Water.

During the light reaction, water is split by photolysis ($2H_2O\rightarrow 4H^+ + 4e^- + O_2$2H_2O→ 4H^+ + 4e^- + O_2), 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 $CO_2$CO_2 occurs in the stroma.

Answer: (b) Absorbs carbon dioxide.

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

Answer: (c) Carbon dioxide concentration or temperature.

Once light is no longer in short supply the curve plateaus, so some other factor in shortest supply (usually $CO_2$CO_2 or temperature) becomes the limiting factor, as per Blackman's law of limiting factors.

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$6CO_2 + 6H_2O [chlorophyll]light C_6H_12O_6 + 6O_2

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 $CO_2$CO_2 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.

Raw materials and sources:

1. Carbon dioxide — from the atmosphere, entering the leaf through stomata.

2. Water — absorbed by roots from the soil and carried to the leaf by the xylem.

3. Light energy — from the Sun, trapped by chlorophyll.

4. Chlorophyll — the green pigment in chloroplasts (acts as a catalyst, not consumed).

By-product: Oxygen is released; excess glucose is stored as starch.

(i) Light intensity: As intensity increases the rate rises, then reaches a plateau when another factor becomes limiting. Very high intensity can bleach chlorophyll.

(ii) Carbon dioxide concentration: $CO_2$CO_2 is usually the chief limiting factor. Increasing it (up to about 0.1%) raises the rate; beyond an optimum the rate stops rising.

(iii) Temperature: The rate increases with temperature up to an optimum (about $25\text{–}35^\circ C$25–35^ C) because enzymes work faster; above this the enzymes are denatured and the rate falls sharply.

Adaptations of the leaf for photosynthesis:

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

2. Thin leaf — a short diffusion path lets $CO_2$CO_2 reach mesophyll cells and $O_2$O_2 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 $CO_2$CO_2 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 $CO_2$CO_2, serves as the control; both halves receive equal light, water and chlorophyll, so $CO_2$CO_2 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 $CO_2$CO_2.

Conclusion: Since only the half deprived of $CO_2$CO_2 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) Light intensity falls with the square of the distance ($I\propto\dfrac{1}{d^2}$I1/d^2). Doubling the distance from 10 cm to 20 cm cuts the intensity to about one-quarter, so the leaf receives far less light energy and the count drops steeply from 60 to 30 and lower. Here light is the limiting factor.

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