These eight questions go from leaf structure to data questions. Write each answer on paper before you open the explanation.
The lessons are in leaves and photosynthesis. Note your slips in the mistake log and paper-error review.
Questions
1. Name the layer of the leaf that contains the most chloroplasts and state its position.
Answer
The palisade mesophyll. It lies just below the upper epidermis, where the most light arrives.
2. Explain why the upper epidermis of a leaf is transparent.
Answer
The upper epidermis has no chloroplasts and is one cell thick, so light passes through it to the palisade cells below. If it were opaque, light would not reach the cells that carry out photosynthesis.
3. Describe how the guard cells open a stoma.
Answer
Water enters the guard cells by osmosis, so they become turgid. The inner wall is thicker and stretches less than the outer wall, so the cells bend apart and the pore opens.
4. On a hot, dry afternoon a plant closes its stomata. State one advantage and one disadvantage for the plant.
Answer
Advantage: less water vapour escapes by transpiration, so the plant is less likely to wilt. Disadvantage: less carbon dioxide enters the leaf, so the rate of photosynthesis falls.
5. A graph shows the rate of photosynthesis rising with light intensity from 0 to 5 units, then staying flat from 5 to 10 units. Name the limiting factor in each section, assuming temperature and carbon dioxide are constant.
Answer
From 0 to 5 units, light intensity limits the rate because the rate increases as light increases. From 5 to 10 units, light is no longer limiting, so carbon dioxide or temperature limits the rate.
6. A plant is grown at 25 °C with enough light. Its rate is 40 units with normal carbon dioxide and 64 units with extra carbon dioxide. Calculate the increase and state what it shows.
Answer
The increase is 64 − 40 = 24 units. Adding carbon dioxide raised the rate while light and temperature stayed the same, so carbon dioxide was the limiting factor.
7. State where the light-dependent and light-independent stages take place, and name one product of each.
Answer
The light-dependent stage takes place in the grana and releases oxygen. The light-independent stage takes place in the stroma and produces glucose.
8. Pondweed in a beaker produces 12 bubbles per minute with a lamp 20 cm away. The lamp is moved to 10 cm and the rate becomes 30 bubbles per minute, then stays at 30 bubbles per minute at 5 cm. Explain each change and suggest why it levels off.
Answer
Moving the lamp closer raises light intensity, so the light-dependent stage makes more energy-carrying molecules. The rate rises from 12 to 30 bubbles per minute, an increase of 18.
At 5 cm the rate does not rise again, so light is no longer limiting. Carbon dioxide concentration or temperature now sets the maximum rate. To test which one, change one factor and keep the other constant.
If you got these wrong
- Questions 1 and 2: read relating leaf structure to photosynthesis.
- Questions 3 and 4: read explaining stomatal function.
- Questions 5, 6 and 8: read interpreting limiting-factor graphs.
- Question 7: read comparing light-dependent and light-independent processes.
To practise against the clock, use the timed original practice session builder. If you want a teacher to read your answers and sharpen the wording, see online one-to-one Biology tuition or the one-hour trial class (from RM50).