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Secondary 3 Biology Plant Biology Quiz
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Secondary 3 Biology Quiz – Plant Biology
Answer Key & Marking Notes
Section A
| Q | Answer | Marks |
|---|---|---|
| 1 | B – Glucose and oxygen | 1 |
| 2 | C – Palisade mesophyll | 1 |
| 3 | B – Transpiration | 1 |
| 4 | B – Guard cells | 1 |
| 5 | B – Sucrose and amino acids | 1 |
Section B
6.
(a) Carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll) [1]
Accept “light” or “chlorophyll” written above the arrow.
(b) Photosynthesis produces oxygen for respiration / maintains atmospheric oxygen; it produces glucose (or organic compounds) which is the primary source of energy / food for almost all consumers / forms the base of food chains. [2]
7.
(a) Any two from: light intensity, carbon dioxide concentration, temperature. [1]
(b) Light intensity – provides energy for light‑dependent reactions; CO₂ concentration – provides carbon atoms for glucose synthesis; temperature – affects enzyme activity (e.g., RuBisCO) in the Calvin cycle. [2]
8.
(a) Diagram must show: upper epidermis, palisade mesophyll (tightly packed, columnar cells), spongy mesophyll, lower epidermis, stoma(ta) with guard cells. [2]
(b) Palisade cells are packed with chloroplasts to absorb maximum light; they are arranged vertically to minimise light loss; located near the upper surface to receive the most light. [2] – Award one mark for each adaptation explained.
9.
(a) Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through stomata. [1]
(b) Water evaporation from leaf cells creates a water potential gradient that pulls water up the xylem (transpiration pull); cohesion of water molecules keeps the column continuous; this draws water from the roots. [2]
10.
(a) Xylem – transports water and dissolved minerals; Phloem – transports sucrose and amino acids (organic nutrients). [2]
(b) Similarity: both are vascular tissues / both form continuous tubes. Difference: xylem vessels are dead (hollow, lignified), while phloem sieve tubes are living (but lack nucleus). [2]
11.
(a) Any two: long, thin projection → increases surface area for absorption; thin cell wall → short diffusion distance; many mitochondria → energy for active transport of mineral ions. [2]
(b) Mitochondria provide ATP for active uptake of mineral ions against a concentration gradient. [1]
12.
(a) The movement indicates the rate of water uptake (or transpiration rate). [1]
(b) The fan removes the layer of water vapour surrounding the leaf (increases the vapour‑pressure gradient); this increases the diffusion gradient for water, so evaporation/transpiration occurs faster, pulling more water up. [2]
13.
(a) Stomata (accept stoma). [1]
(b) Guard cells contain chloroplasts that produce sugar in the light; this lowers the water potential, causing water to enter by osmosis. The cells swell and become turgid, bending to open the pore. In the dark, solutes are metabolised / water leaves, cells become flaccid and the pore closes. [2]
14.
(a) Translocation is the transport of manufactured food (sucrose, amino acids) through the phloem from sources to sinks. [1]
(b) From leaves (source) to storage organs / roots (sink) – or bidirectional; any acceptable description. [1]
(c) Example: ringing experiment – removing a ring of bark (phloem) causes swelling above the cut, showing sugars cannot pass downwards; radioactive carbon‑14 labelling shows sugars moving from leaf to root. [1]
15.
(a) In the dark, photosynthesis stops but respiration continues, so glucose/starch reserves are broken down for energy; the plant uses up stored organic matter, causing dry mass to fall. No new organic matter is made. [2]
(b) Stomata generally close; because photosynthesis does not occur, CO₂ is not needed; closing reduces water loss. [1]
Section C
16.
(a) The rate increases with light intensity up to a point, then levels off (reaches a plateau). [1]
(b) A different factor (e.g., CO₂ concentration or temperature) has become limiting; even with abundant light, the other factor restricts the overall rate. [2]
17.
Assume typical leaf cross‑section:
A – upper epidermis [1], B – palisade mesophyll [1], C – spongy mesophyll (or lower epidermis / stoma if positioned accordingly). [1] Award marks for correct identification as per diagram.
18.
(a) The rate of water uptake (equivalent to transpiration rate under steady conditions). [1]
(b) To prevent air bubbles entering the xylem, which would block water flow and affect the reading. [1]
19.
(a) Windy, normal humidity (3.5 cm³/h). [1]
(b) Wind removes the water vapour layer around the leaf, increasing the diffusion gradient; low humidity means less water vapour in the air, so the gradient is even steeper; both factors increase evaporation, hence water loss is highest. [2]
20.
(a) The uncovered part turns blue‑black (positive for starch). [1]
(b) Light reached the exposed part, allowing photosynthesis to produce glucose, which was converted to starch for storage. The covered part had no light, so no starch was made; only the exposed area gives a positive iodine test. [2]
Total: 40 marks