(a) A student investigated plant growth responses in roots and shoots. They used this method:
- Damp cotton wool was placed in two Petri dishes.
- Three bean seedlings were attached to the cotton wool in each Petri dish.
- Each seedling was orientated so that the roots pointed in a different direction in each Petri dish.
- Petri dish 1 was kept on its side in a fixed position.
- Petri dish 2 was kept on its side and rotated constantly.
- Both Petri dishes were kept in the dark.
- Both Petri dishes were kept in these conditions for two days.
- After two days the seedlings were observed.
Fig. 6.1 is a diagram of the apparatus.
Fig. 6.2 shows the seedlings after two days.
(i) Describe the pattern of growth of the bean roots and shoots in Petri dish 1 shown in Fig. 6.2.
(ii) State the name of the growth responses observed in the bean roots and shoots.
(iii) Explain how auxin causes the difference in the pattern of growth shown by the shoots of seedlings B and E shown in Fig. 6.2.
(b) Seeds require oxygen and water to germinate.
(i) State one other environmental condition that affects germination.
(ii) Suggest why oxygen and water are required for germination.
▶️ Answer/Explanation
(a)(i) In Petri dish 1, all roots grew downward (regardless of their initial orientation) showing positive gravitropism, while all shoots grew upward showing negative gravitropism.
Explanation: The fixed position allowed gravity to consistently influence growth. Roots naturally grow toward gravity (positive gravitropism) to anchor the plant and access water/minerals, while shoots grow away from gravity (negative gravitropism) to reach light for photosynthesis.
(a)(ii) Gravitropism (or geotropism).
Explanation: Gravitropism is the growth response of plant parts to gravity. The term “geotropism” is also acceptable though less commonly used in modern biology. This directional growth is crucial for plant survival and orientation.
(a)(iii) Auxin is produced in the shoot tip and moves by diffusion. In seedling B (fixed position), auxin accumulates on the lower side due to gravity, causing more cell elongation on that side and making the shoot curve upward. In seedling E (rotating dish), auxin distribution remains equal on all sides as the rotation counters gravity’s effect, resulting in straight growth.
Detailed explanation:
1. Auxin is synthesized in the shoot apical meristem and moves downward through the plant.
2. In a stationary plant (B), gravity causes auxin to accumulate on the lower side of the shoot.
3. Higher auxin concentration promotes cell elongation on the lower side, causing upward curvature.
4. In the rotating dish (E), constant movement prevents sustained auxin accumulation on any one side.
5. Equal auxin distribution leads to uniform cell elongation and straight growth.
6. This demonstrates how plants use auxin distribution to respond to environmental stimuli like gravity.
(b)(i) Temperature.
Explanation: Temperature affects enzyme activity during germination. Most seeds have an optimal temperature range (typically 20-30°C) where germination occurs most efficiently. Too low temperatures slow metabolic processes, while too high can damage cellular components.
(b)(ii) Oxygen is needed for aerobic respiration to provide energy for growth, while water is required to activate enzymes, soften the seed coat, and serve as a medium for metabolic reactions.
Detailed explanation:
1. Oxygen: Essential for aerobic respiration which produces ATP needed for cell division and growth during germination. The emerging seedling requires energy before it can photosynthesize.
2. Water:
– Rehydrates dry seed tissues, activating metabolic processes
– Softens the seed coat to allow radicle emergence
– Acts as solvent for enzyme reactions and transport of nutrients
– Provides turgor pressure for cell expansion
– Helps break down stored food reserves into usable molecules
(a) Fig. 3.1 shows a growing plant. The plant is growing in the dark.
(i) State the name of the growth response shown by the plant shoot in Fig. 3.1.
(ii) Explain how the growth of the plant shoot in Fig. 3.1 is controlled.
(b) Plant roots growing towards water is another growth response. State two uses of water in a plant.
(c) A student measured the root length of a bean seedling over a period of 120 hours. Fig. 3.2 shows the results.
Calculate the percentage increase in root length between 48 hours and 96 hours. Give your answer to two decimal places.
▶️ Answer/Explanation
(a)(i) The growth response shown is gravitropism (growth in response to gravity).
(a)(ii) The growth is controlled by auxin:
- Auxin diffuses from the shoot tip.
- It accumulates on the lower side of the shoot.
- This stimulates cell elongation, causing the shoot to bend upwards.
(b) Two uses of water in plants:
- Photosynthesis: Water is a raw material for photosynthesis (\(6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2\)).
- Support: Water maintains turgor pressure, keeping cells rigid.
(c) Percentage increase in root length: \[ \text{Initial length (48 hours)} = 9 \, \text{mm} \\ \text{Final length (96 hours)} = 25 \, \text{mm} \\ \text{Increase} = 25 – 9 = 16 \, \text{mm} \\ \text{Percentage increase} = \left(\frac{16}{9}\right) \times 100 = 177.78\% \] Final answer: 177.78% (to two decimal places).