Question 1
(a) State the word equation for photosynthesis. (2) (b) Complete the balanced chemical symbol equation for photosynthesis: ...CO₂ + 6H₂O → C₆H₁₂O₆ + ...O₂ (1) (c) Describe the energy conversion that takes place in photosynthesis. (1)
Show mark scheme for question 1
- carbon dioxide + water (1)
- → glucose + oxygen (1) allow light / chlorophyll written above the arrow; do not accept light as a reactant
- 6 and 6 (both needed for the mark) (1)
- light energy is converted to chemical energy (stored in glucose) (1)
Question 2
A student measured the number of bubbles of oxygen released by a water plant in one minute at different light intensities. The temperature and carbon dioxide concentration were the same each time. The results are in the table. (a) State the light intensity at which light stopped being the limiting factor. (1) (b) Name two other factors that could be limiting the rate at light intensities of 30 and above. (2)
| Light intensity (arbitrary units) | Bubbles per minute |
|---|---|
| 10 | 4 |
| 20 | 8 |
| 30 | 12 |
| 40 | 12 |
| 50 | 12 |
Show mark scheme for question 2
- 30 (arbitrary units) (1)
- carbon dioxide concentration (1)
- temperature (1) allow amount of chlorophyll; ignore water
Question 3
Explain how two features of a leaf make it well adapted for photosynthesis.
Show mark scheme for question 3
- thin / flat / large surface area (1) so light is absorbed / gases diffuse a short distance (1)
- palisade cells packed close to the upper surface with many chloroplasts (1) to absorb a lot of light / carry out most photosynthesis (1)
- air spaces in the spongy mesophyll (1) so carbon dioxide diffuses to the cells (1)
- stomata (1) allow carbon dioxide to diffuse in (1)
- transparent upper epidermis (1) lets light through to the palisade cells (1)
- Max 4. Each feature must be matched with a reason. Do not accept 'to absorb light' alone as a feature.
Question 4
Biology only (what this means)
Biology only: only in International GCSE Biology. Double Award students can skip it. What the labels meanExplain how the structure of a leaf is adapted for gas exchange.
Show mark scheme for question 4
- thin, so there is a short diffusion distance (1)
- broad / flat, so there is a large surface area (1)
- (many) stomata let carbon dioxide diffuse in and oxygen diffuse out (1) allow guard cells open and close stomata
- air spaces in the spongy mesophyll let gases diffuse (quickly) to and from the cells (1) allow large internal surface area
- moist cell surfaces, so gases dissolve before diffusing into the cells (1)
- Max 3. A feature must be linked to how it helps gas exchange.
Question 5
The leaves of a plant growing in poor soil are yellow. Name the mineral ion the plant is likely to be short of and explain why it causes this.
Show mark scheme for question 5
- magnesium (ions) (1)
- needed to make chlorophyll, so less chlorophyll is made (1)
Question 6
A student tests a leaf for starch. Describe the steps she should follow and the result that shows starch is present.
Show mark scheme for question 6
- dip the leaf in boiling water (1)
- boil the leaf in ethanol (in a hot water bath) to remove chlorophyll (1)
- rinse the leaf in water, then add iodine solution; blue-black shows starch is present (1) ignore 'purple' if blue is also given
Question 7
Describe how you could investigate the effect of light intensity on the rate of photosynthesis of a water plant. Include how you would make the investigation a fair test.
Show mark scheme for question 7
| Level | Marks | What the answer does |
|---|---|---|
| 3 | 5–6 | A detailed, logical method that includes how light intensity is changed, how the rate is measured, repeats and at least two controlled variables, each with a reason. Terms are used accurately. |
| 2 | 3–4 | A method that covers how light intensity is changed and how the rate is measured, with some attempt at controls or repeats. Some detail is missing. |
| 1 | 1–2 | A limited or partly correct method, for example naming the plant and the lamp with no measurement or no control of variables. |
Indicative content
- pondweed in water with sodium hydrogencarbonate to supply carbon dioxide
- lamp at measured distances from the plant, for example 10, 20, 30, 40, 50 cm; light intensity changes with distance
- count the bubbles of oxygen released from the cut stem in one minute, or collect the gas and measure its volume
- allow the plant to adjust at each new distance before counting
- repeat at each distance at least three times and calculate a mean
- control temperature (heat shield / water bath / LED lamp), carbon dioxide concentration (same sodium hydrogencarbonate) and the plant used
- plot rate against light intensity or distance