Plant Physiology — Photosynthesis, Transpiration and Transport — Practice

Question 1

Write the balanced equation for photosynthesis. Name the site and the two raw materials.

Accepted Answer

Step 1 — Write the equation.
  6CO₂ + 12H₂O →(Sunlight + Chlorophyll)→ C₆H₁₂O₆ + 6O₂ + 6H₂O

Step 2 — Site: CHLOROPLASTS (in mesophyll cells of leaves).

Step 3 — Two raw materials:
  (1) Carbon dioxide (CO₂): from atmosphere via stomata.
  (2) Water (H₂O): from soil via roots and xylem.

✦ Answer: Equation above. Site: chloroplasts. Raw materials: CO₂ and H₂O.

Question 2

Why is transpiration called a 'necessary evil'? Name two factors that increase its rate.

Accepted Answer

Step 1 — Why 'necessary':
  Transpiration creates TRANSPIRATION PULL — the main force that pulls water from roots through xylem to leaves. Without it, plants cannot transport water or minerals. It also COOLS the plant (evaporative cooling).

Step 2 — Why 'evil':
  It causes significant WATER LOSS. In dry conditions (like Rayalaseema), this water loss can stress the plant.

Step 3 — Two factors that increase transpiration rate:
  (1) High temperature — increases evaporation from leaf cells.
  (2) Low humidity (dry air) — increases the concentration gradient for water vapour to escape.
  (Others: wind, high light intensity — stomata open.)

✦ Answer: Necessary (drives water transport + cooling) but evil (wastes water).

Question 3

Describe the experiment to prove that CO₂ is necessary for photosynthesis. What control is used?

Accepted Answer

Step 1 — AIM: Prove CO₂ is necessary for photosynthesis.

Step 2 — SETUP:
  Take two potted plants of the same species. Place one in a bell jar with KOH solution (potassium hydroxide absorbs CO₂). Place the other in a bell jar without KOH. Keep BOTH in adequate sunlight for 24 hours.

Step 3 — PROCEDURE:
  After 24 hours, take leaves from both plants. Kill cells (boil in water). Remove chlorophyll (boil in alcohol — leaf turns white). Wash with water. Apply IODINE SOLUTION.

Step 4 — OBSERVATION:
  Control (no KOH) leaf: turns BLUE-BLACK → starch present → photosynthesis occurred.
  KOH jar leaf: remains BROWN → no starch → no photosynthesis.

Step 5 — CONCLUSION:
  In the absence of CO₂ (absorbed by KOH), photosynthesis did NOT occur, even with adequate sunlight and water. Therefore, CO₂ is NECESSARY for photosynthesis.

✦ Answer: KOH removes CO₂ → no starch formed → CO₂ is necessary.

Question 4

Compare xylem and phloem under four headings: material transported, direction, cell type, driving force.

Accepted Answer

Step 1 — Comparison table.

Feature | XYLEM | PHLOEM
  --------|-------|-------
  Material transported | WATER + dissolved minerals | FOOD (sucrose, organic compounds)
  Direction | ONE-WAY: roots → shoots (upward) | TWO-WAY: source (leaves) → sink (roots, fruits, growing tips)
  Cell type | DEAD at maturity (tracheids, xylem vessels) | LIVING (sieve tubes — no nucleus; companion cells — have nucleus, control sieve tubes)
  Driving force | TRANSPIRATION PULL — passive, no ATP | ACTIVE TRANSPORT — pressure flow, requires ATP

Step 2 — Why xylem cells are dead:
  Dead cells form unobstructed hollow tubes. Rigid lignified walls prevent collapse under the negative pressure created by transpiration pull.

✦ Answer: See table. Key: xylem = dead/passive, phloem = living/active.

Question 5

Describe photosynthesis completely: equation, site, raw materials, light reaction vs dark reaction, three classic experiments, and four factors that affect the rate. Also include the AP agricultural context.

Accepted Answer

Step 1 — EQUATION AND SITE:
  6CO₂ + 12H₂O →(Sunlight + Chlorophyll)→ C₆H₁₂O₆ + 6O₂ + 6H₂O.
  Site: CHLOROPLASTS (mesophyll cells of leaves).

Step 2 — LIGHT REACTION (thylakoid membranes):
  Requires LIGHT. Chlorophyll absorbs light energy. Photolysis of water: 2H₂O → 4H⁺ + O₂ (O₂ released). Produces ATP and NADPH (energy carriers). These drive the dark reaction.

Step 3 — DARK REACTION / CALVIN CYCLE (stroma):
  Does NOT directly require light (but depends on products of light reaction). CO₂ is fixed: CO₂ + RuBP → PGA → using ATP + NADPH → PGAL → Glucose. RuBP is regenerated.

Step 4 — THREE CLASSIC EXPERIMENTS:
  (1) STARCH TEST (variegated leaf): Green parts → blue-black (starch). White parts → brown. Proves chlorophyll is necessary.
  (2) CO₂ NECESSITY: KOH absorbs CO₂ → no starch formed. Proves CO₂ is needed.
  (3) O₂ RELEASE: Hydrilla in water + sunlight → bubbles. Glowing splint rekindles = O₂. Proves O₂ is a product.

Step 5 — FOUR FACTORS:
  (1) Light intensity (more light → faster rate, up to saturation point).
  (2) CO₂ concentration (more CO₂ → faster rate).
  (3) Temperature (optimal ~30°C; above 40°C → enzymes denature → rate falls).
  (4) Water availability (drought → stomata close → CO₂ entry stops → rate falls).

Step 6 — AP AGRICULTURAL CONTEXT:
  Rice (C₃ plant): dominant crop in Krishna-Godavari delta. Uses photosynthesis efficiently in warm, wet conditions.
  Sugarcane (C₄ plant): grown in coastal AP. More efficient in hot, sunny conditions.
  In Rayalaseema drought: WATER is the limiting factor — even with abundant sunlight, photosynthesis stops when stomata close to conserve water.

✦ Answer: Complete treatment as above.

Practice — Plant Physiology — Photosynthesis, Transpiration and Transport

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