Photosynthesis and limiting factors (RP6) – GCSE Biology exam questions with mark scheme

1Grade 1–33 marks

(a) Complete the word equation for photosynthesis. [2]

______ + water → glucose + ______

(b) Photosynthesis is an endothermic reaction. What does endothermic mean? [1]

(a) carbon dioxide
allow CO₂
ignore light / chlorophyll written above the arrow(1)
(a) oxygen
allow O₂
do not accept energy as a product(1)
(b) energy is transferred from the environment (to the chloroplasts by light)
allow energy is taken in / absorbed (from the surroundings)
do not accept energy is given out / released(1)
Answer: (a) carbon dioxide; oxygen (b) takes in energy from the surroundings
Examiner tip: Light goes above the arrow: it is not a reactant, and neither energy nor chlorophyll is a product.

2Grade 4–54 marks

Give four ways in which plants use the glucose produced in photosynthesis. [4]

any four from, one mark each:
• for respiration (allow to release / transfer energy; do not accept to make / produce energy)
• converted to (insoluble) starch for storage (ignore 'stored' alone)
• converted to fats / oils / lipids for storage
• to make cellulose (for cell walls)
• to make amino acids / proteins (allow with nitrate ions)(1)
a second use from the list(1)
a third use from the list(1)
a fourth use from the list(1)
Answer: Respiration; starch for storage; cellulose for cell walls; fats/oils for storage; amino acids for proteins.
Examiner tip: Say what the glucose is converted INTO and why — 'stored' alone does not score; 'stored as starch' does.

3Grade 6–7Higher5 marks

A student investigated the effect of light intensity on the rate of photosynthesis in pondweed. She counted the bubbles produced per minute with a lamp placed at different distances.

The table shows her results for the lamp at 20 cm (number of bubbles per minute).

Distance in cm	Trial 1	Trial 2	Trial 3	Trial 4
20	28	31	14	30

(a) Calculate the mean number of bubbles per minute at 20 cm. Do not include the anomalous result. Give your answer to 3 significant figures. [2]

(b) Light intensity is inversely proportional to the square of the distance. How many times greater is the light intensity at 10 cm than at 30 cm? [1]

(a) identifies 14 (trial 3) as anomalous and excludes it: (28 + 31 + 30) ÷ 3(1)
(a) 29.7 (bubbles per minute)
allow 29.67 for the first mark only
25.8 (anomaly included) scores 0(1)
(b) 9 (times)
allow 9×(1)
(c) place a (glass) tank / beaker of water (heat shield) between the lamp and the pondweed
allow use an LED / cold lamp; keep the pondweed in a water bath; monitor the temperature with a thermometer and keep it constant(1)
(c) because temperature affects the rate of photosynthesis (so it must be controlled / so only light intensity changes)(1)
Answer: (a) 29.7 (b) 9 times (c) heat shield/LED/water bath because temperature affects rate
Examiner tip: Spot and leave out the anomalous result before calculating the mean: including 14 gives 25.8, which scores 0.

4Grade 6–7Higher5 marks

The graph shows how light intensity affects the rate of photosynthesis at two carbon dioxide concentrations.

(a) Which factor limits the rate of photosynthesis at point X? [1]

(b) Which factor limits the rate of photosynthesis at point Y? Give a reason for your answer using the graph. [2]

(a) light intensity(1)
(b) carbon dioxide (concentration)
allow CO₂(1)
(b) (because) at the same light intensity the rate is higher at 0.10% CO₂ / increasing the CO₂ concentration increases the rate
allow (because) increasing light intensity no longer increases the rate and the 0.10% curve is higher
no mark for the reason if the factor named is wrong(1)
(c) burning paraffin releases carbon dioxide and/or heat (raises the temperature)(1)
(c) (so) the rate of photosynthesis increases, so more glucose for growth / to make proteins, cellulose, starch
allow carbon dioxide / temperature was limiting the rate(1)
Answer: (a) light intensity (b) CO₂ concentration – the rate is higher at 0.10% CO₂ at the same light intensity (c) releases CO₂ and heat, so faster photosynthesis and more glucose for growth
Examiner tip: At a plateau, the limiting factor is NOT the one on the x-axis — look at which other curve is higher.

5Grade 1–3Foundation5 marks

Photosynthesis takes place in the chloroplasts of plant cells.

(a) Which substance in chloroplasts absorbs light? Tick (✓) one box. [1]

☐ cellulose ☐ chlorophyll ☐ glucose ☐ starch

The symbol equation for photosynthesis is:

$$6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$$

(b) Name the substance with the formula $\text{C}_6\text{H}_{12}\text{O}_6$. [1]

(d) Light intensity is one factor that can limit the rate of photosynthesis.

Give one other factor that can limit the rate of photosynthesis. [1]

(e) Plants use glucose and nitrate ions to make amino acids.

Where does a plant get nitrate ions from? [1]

(a) chlorophyll (only box ticked)
if more than one box is ticked, 0 marks(1)
(b) glucose
do not accept sugar alone / starch / glycogen / sucrose(1)
(c) H₂O
the formula is asked for, so ‘water’ alone does not score
do not accept H²O / HO₂ / CO₂(1)
(d) any one from:
• temperature
• carbon dioxide (concentration)
• amount of chlorophyll
allow CO₂ / number of chloroplasts
ignore water / oxygen / minerals / light (colour / wavelength)(1)
(e) (from) the soil
allow (absorbed by) the roots / root hair cells from the soil (water)
ignore from the air / fertiliser alone(1)
Answer: (a) chlorophyll (b) glucose (c) H₂O (d) temperature / CO₂ concentration / amount of chlorophyll (e) the soil
Examiner tip: Read whether the question asks for a name or a formula: in (c) the formula H₂O is needed, and in (b) the name glucose (not just ‘sugar’).

6Grade 4–55 marks

A student measured the rate of photosynthesis of a plant at different temperatures. All other conditions were kept the same.

The graph shows the results.

(a) Give the optimum temperature for photosynthesis in this plant. [1]

(b) Calculate the percentage decrease in the rate of photosynthesis when the temperature increases from 30 °C to 40 °C. [2]

(a) 30 (°C)(1)
(b) (8.0 − 2.4) ÷ 8.0 (× 100)
allow 5.6 ÷ 8.0
allow a reading of 2.3–2.5 at 40 °C(1)
(b) 70 (%)
allow 69–71 (%) from a reading of 2.3–2.5
correct answer with no working scores 2 marks
0.7 scores 1 mark only(1)
(c) (the) enzymes (that control photosynthesis) are denatured
allow enzymes damaged / change shape
do not accept enzymes are killed(1)
(c) (so) the shape of the active site changes and the substrate can no longer fit / bind
allow fewer enzyme–substrate complexes form(1)
Answer: (a) 30 °C (b) 70% (c) enzymes are denatured, so the active site changes shape and the substrate no longer fits
Examiner tip: ‘Enzymes are killed’ never scores — enzymes are not alive; write ‘denatured’ and say what happens to the active site for the second mark.

7Grade 6–7Higher6 marks

A grower produces tomatoes in a heated greenhouse. She tested four different temperature settings over several weeks.

The table shows her results.

Temperature setting in °C	Cost of heating per week in £	Value of tomatoes sold per week in £
16	120	900
19	210	1150
22	330	1300
25	480	1340

(a) Which temperature setting should the grower choose to make the most profit? Use calculations from the table to support your answer. [2]

(b) Suggest why the value of tomatoes sold increases only slightly when the temperature is raised from 22 °C to 25 °C. [2]

Explain why this might not be worth the cost. [2]

(a) calculates value − cost for at least two settings: 780, 940, 970, 860 (£)
allow profit for 22 °C and one other setting correct(1)
(a) 22 (°C) as the choice supported by the correct profits
no mark for 22 °C without any calculation(1)
(b) temperature is no longer the (main) limiting factor(1)
(b) (another factor such as) light intensity / carbon dioxide concentration / amount of chlorophyll is limiting the rate (of photosynthesis)
allow temperature is close to the optimum (for the enzymes)(1)
(c) (in winter) light intensity is low / days are short so light is the limiting factor
allow another factor (e.g. light) limits the rate, not carbon dioxide(1)
(c) so adding carbon dioxide would not increase the rate of photosynthesis (much) so the extra yield / income would be less than the cost
allow the cost of the CO₂ would be greater than the value of any extra tomatoes(1)
Answer: (a) 22 °C – profit £970 compared with £780, £940 and £860 (b) temperature is no longer limiting; light or CO₂ now limits the rate (c) light is limiting in winter, so extra CO₂ gives little extra growth and the extra income would not cover the cost
Examiner tip: Economics questions want a link to money: say that the extra yield would be worth less than the cost, not just that photosynthesis would not speed up.

8Grade 8–9Higher7 marks

A student investigated the effect of light intensity on photosynthesis in pondweed. She collected the gas given off in an upturned measuring cylinder filled with water.

Distance of lamp from pondweed in cm	Volume of gas collected in 5 minutes in cm³
10	4.20
40	1.05

(a) Calculate the rate of gas production at 10 cm in cm³ per hour. [2]

(b) Light intensity is inversely proportional to the square of the distance from the lamp.

Calculate the light intensity at 40 cm as a fraction of the light intensity at 10 cm. [2]

Suggest why the rate did not decrease by the same factor as the light intensity. [2]

(d) The volume of gas collected is lower than the volume of oxygen produced by photosynthesis.

Give one reason why. [1]

(a) 4.20 ÷ 5 (= 0.84 cm³ per minute) or 4.20 × 12(1)
(a) 50.4 (cm³ per hour)
allow 50 (cm³ per hour)
0.84 alone scores 1 mark(1)
(b) $\frac{1}{40^2} \div \frac{1}{10^2}$ or $\frac{10^2}{40^2}$ or $\frac{100}{1600}$
allow (40 ÷ 10)² = 16(1)
(b) $\frac{1}{16}$
allow 0.0625 / 6.25% / 6.3%
an answer of $\frac{1}{4}$ scores 0(1)
(c) at 10 cm (high light intensity) light was not the (only) limiting factor
allow at 10 cm the rate was close to its maximum / on the plateau(1)
(c) another factor (e.g. carbon dioxide concentration / temperature) was limiting the rate at 10 cm
ignore rate is not proportional to light intensity (restates the question)
allow for 1 mark only: light from the room added to the lamp light, so the light intensity at 40 cm was more than 1/16 of that at 10 cm(1)
(d) any one from:
• some oxygen is used (by the pondweed) for (aerobic) respiration
• some oxygen dissolves in the water
allow some gas stays trapped on the pondweed / in the tubing
ignore human / reading error(1)
Answer: (a) 50.4 cm³ per hour (b) 1/16 (c) at 10 cm another factor such as CO₂ concentration was limiting, so the rate was not proportional to light intensity (d) oxygen is used in respiration / dissolves in the water
Examiner tip: In (b) square the distance ratio — 40 cm is 4 times further away, so the light intensity is 4² = 16 times lower, not 4 times.

9Grade 6–76 marks

A student wants to investigate how light intensity affects the rate of photosynthesis in pondweed.

She has: pondweed, a boiling tube, a lamp, a metre rule, a stop clock and sodium hydrogencarbonate solution (which provides carbon dioxide).

Describe a method the student could use. Your answer should include how she would make sure her results are valid. [6]

place the pondweed (cut end up) in a boiling tube of sodium hydrogencarbonate solution(1)
place the lamp at a measured distance (e.g. 10 cm) from the pondweed, measured with the metre rule(1)
leave the pondweed to adjust (acclimatise) for a few minutes before taking readings(1)
count the number of bubbles produced in a set time (e.g. 1 minute) or collect the gas (in an upturned measuring cylinder / gas syringe) and measure its volume in a set time(1)
repeat with the lamp at different distances (e.g. 20, 30, 40, 50 cm)(1)
repeat each distance (at least three times) and calculate a mean(1)
control variable: keep the temperature constant, e.g. a beaker / tank of water as a heat shield between lamp and tube, or use an LED lamp, and check with a thermometer(1)
control variable: use the same concentration / volume of sodium hydrogencarbonate solution(1)
control variable: use the same piece / same length / same species of pondweed(1)
carry out in a darkened room so the only light is from the lamp(1)
the rate is the number of bubbles (or volume of gas) divided by the time(1)

Level	Marks	What it looks like
Level 3	5–6	A coherent, logically ordered method that could be followed to give valid results: how light intensity is changed and measured, what is measured and over what time, repeats with a mean, and at least two relevant control variables including how temperature is kept constant.
Level 2	3–4	A method with the main steps present (changing lamp distance and measuring bubbles or gas volume), though some detail or order is missing, with at least one control variable or repeat.
Level 1	1–2	Simple relevant statements, e.g. move the lamp and count the bubbles, or a control variable named. The method could not easily be followed.
No marks	0	No relevant content.

Answer: Pondweed in a boiling tube of sodium hydrogencarbonate solution; lamp at measured distances (e.g. 10–50 cm); allow to acclimatise; count bubbles / measure gas volume in a fixed time; repeat each distance and take a mean; keep temperature constant (heat shield / LED), same CO₂ supply, same pondweed, darkened room.
Examiner tip: Many answers list control variables but never say what is measured — top-level answers must state the dependent variable (bubbles or gas volume in a fixed time) and how distance is changed.

10Grade 6–7Higher7 marks

Scientists measured the rate of photosynthesis of a plant at different carbon dioxide concentrations. They did this under three sets of conditions, A, B and C.

The graph shows the results.

(a) Which factor limits the rate of photosynthesis at point P? [1]

(b) At a carbon dioxide concentration of 0.16%, which factor is limiting the rate for curve B? Use information from the graph to give a reason. [2]

(c) At a carbon dioxide concentration of 0.16%, which factor is limiting the rate for curve C? Use information from the graph to give a reason. [2]

(d) Calculate the percentage increase in the rate of photosynthesis at 0.16% carbon dioxide when the temperature is raised from 15 °C to 25 °C at high light intensity. [2]

(a) carbon dioxide (concentration)
allow CO₂(1)
(b) temperature(1)
(b) at the same light intensity (and CO₂ concentration) the rate is higher at 25 °C (curve A) than at 15 °C (curve B)
allow curve B has levelled off / increasing CO₂ no longer increases the rate and curve A is higher
no mark for the reason if the factor is wrong(1)
(c) light intensity(1)
(c) at the same temperature (25 °C) the rate is higher at high light intensity (curve A) than at low light intensity (curve C)
allow curve C has levelled off and curve A (high light) is higher
no mark for the reason if the factor is wrong(1)
(d) (8 − 5) ÷ 5 (× 100)
allow 3 ÷ 5(1)
(d) 60 (%)
correct answer with no working scores 2 marks
37.5 (dividing by 8) scores 0(1)
Answer: (a) carbon dioxide concentration (b) temperature – at the same light intensity, A (25 °C) is higher than B (15 °C) (c) light intensity – at the same temperature, A (high light) is higher than C (low light) (d) 60%
Examiner tip: Compare two curves that differ in only one condition: A and B differ only in temperature; A and C differ only in light intensity.

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