Cambridge International AS & A Level Biology 9700: Photosynthesis – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These practice questions cover Topic 13, Photosynthesis, of the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027: sections 13.1 (Photosynthesis as an energy transfer process) and 13.2 (Investigation of limiting factors). This is A Level content, examined in Paper 4, and several questions practise the planning and analysis skills used in Paper 5. Calculators are allowed in all 9700 papers. Give calculated answers to the same number of significant figures as the data, or one more.

Before you start, read the photosynthesis study guide or the photosynthesis revision notes. Course pages: Cambridge A Level Biology hub and printable checklist.

Questions

1. State the site within a chloroplast of:

(a) the light-dependent stage [1] (b) the Calvin cycle. [1]

2. A student separates the pigments in a leaf extract by paper chromatography. The solvent front is 92 mm from the origin. The centre of one spot is 58 mm from the origin. Reference Rf values for this solvent are: carotene 0.95, xanthophyll 0.71, chlorophyll a 0.63, chlorophyll b 0.52.

(a) Calculate the Rf value of the spot. [2] (b) Identify the pigment. [1]

3. (a) Compare cyclic and non-cyclic photophosphorylation. [3] (b) Explain how the flow of electrons along the electron transport chain in a thylakoid membrane leads to the synthesis of ATP. [3]

4. (a) Outline the three main stages of the Calvin cycle. [3] (b) A plant cell makes one molecule of a hexose sugar from two molecules of triose phosphate. Calculate how many molecules of carbon dioxide must be fixed, how many molecules of GP and TP are formed, and how many TP molecules are left to regenerate RuBP. Show that the carbon atoms balance. [3]

5. A suspension of algae is photosynthesising in bright light with plenty of carbon dioxide. The light is switched off. Explain the changes in the concentrations of GP and RuBP during the next minute. [4]

6. (a) Distinguish between an absorption spectrum and an action spectrum. [2] (b) The rate of photosynthesis of a plant is still high at wavelengths of about 480–500 nm, where chlorophyll a absorbs relatively little light. Suggest an explanation. [2]

7. A chloroplast suspension was mixed with blue DCPIP solution and placed at different distances from a lamp. The time for the blue colour to disappear was recorded.

Distance from lamp / cm	10	15	20	25
Time to decolourise / s	48	85	150	235

(a) Explain why the DCPIP loses its blue colour in the light. [2] (b) Calculate the rate, as 1000/t, at 20 cm. [1] (c) Explain the trend in the results. [2] (d) A tube of chloroplast suspension and DCPIP was wrapped in foil. Suggest why. [1] (e) State one variable that should be kept constant, and how. [1]

8. An aquatic plant in sodium hydrogencarbonate solution released gas into a capillary tube with a bore radius of 0.60 mm. In 5.0 minutes a bubble 18 mm long was collected.

(a) Calculate the rate of gas release in mm³ min⁻¹. [2] (b) Explain why this is lower than the true rate of oxygen production by photosynthesis. [2] (c) State how the carbon dioxide concentration could be varied in this investigation. [1]

9. The rate of photosynthesis of a plant was measured at two carbon dioxide concentrations.

Light intensity / arbitrary units	10	20	30	40	50
Rate at 0.04% CO₂ / arbitrary units	8	14	17	17	17
Rate at 0.10% CO₂ / arbitrary units	8	15	23	28	28

(a) Identify the limiting factor at a light intensity of 10 units. Give a reason. [1] (b) Explain why the rate at 0.04% CO₂ does not change between 30 and 50 units. [2] (c) Calculate the percentage increase in rate at 50 units when the CO₂ concentration is raised from 0.04% to 0.10%. [2] (d) Explain, with reference to the Calvin cycle, why raising the CO₂ concentration increases the rate. [2]

10. The rate of photosynthesis of a plant was 12.0 arbitrary units at 15 °C, 21.6 units at 25 °C and 9.0 units at 40 °C, with light and carbon dioxide not limiting.

(a) Calculate the percentage increase in rate between 15 °C and 25 °C. [2] (b) Explain the increase. [2] (c) Suggest why the rate is lower at 40 °C. [1]

11. Plan an investigation into the effect of light wavelength on the rate of the light-dependent stage, using a chloroplast suspension and DCPIP. Include a prediction with an explanation. [8]

Answers

1. (a) The thylakoids (thylakoid membranes, stacked in grana) [1]. (b) The stroma [1]. Examiner insight: “In the chloroplast” or “on chlorophyll” earns nothing: name the region.

2. (a) Rf = 58 ÷ 92 [1] = 0.63 [1]. (b) Chlorophyll a [1]. Examiner insight: Rf is a ratio with no units, so writing “0.63 mm” can cost the accuracy mark. A wrong Rf in (a) can still earn (b) by error carried forward if the pigment matches your value.

3. (a) Cyclic uses PSI only, while non-cyclic uses PSII and PSI [1]. Only non-cyclic involves the photolysis of water by the oxygen-evolving complex and releases oxygen [1]. Both make ATP, but only non-cyclic makes reduced NADP [1]. (b) Energetic electrons release energy as they pass along the chain [1]. This energy is used to transfer protons from the stroma into the thylakoid space, forming a proton gradient [1]. Protons return to the stroma by facilitated diffusion through ATP synthase, providing energy to make ATP from ADP and Pi [1]. Examiner insight: “Compare” needs both processes in each point. “Cyclic uses PSI” on its own earns nothing; add what non-cyclic does in the same sentence.

4. (a) Rubisco catalyses the fixation of CO₂ with RuBP (5C) to give two GP (3C) [1]. GP is reduced to TP using reduced NADP and ATP [1]. RuBP is regenerated from TP in reactions that use ATP [1]. (b) 6 CO₂ are fixed, forming 12 GP [1]. These give 12 TP; 2 TP (6 carbons) leave to make the hexose [1]. The other 10 TP (30 carbons) regenerate 6 RuBP (6 × 5 = 30 carbons) [1]. Examiner insight: In (a), the mark for regeneration needs ATP mentioned. In (b), numbers without the carbon check lose the last mark on a “show that”.

5. Without light, the light-dependent stage stops, so no ATP or reduced NADP is made [1]. GP cannot be reduced to TP, so GP concentration rises [1]. RuBP cannot be regenerated, because this needs TP and ATP [1]. Rubisco still combines the remaining RuBP with CO₂ for a short time, so RuBP concentration falls [1]. Examiner insight: Each compound needs both a direction and a reason. “GP rises and RuBP falls” alone usually earns at most one mark.

6. (a) An absorption spectrum shows the amount of light absorbed by a pigment at each wavelength [1]. An action spectrum shows the rate of photosynthesis at each wavelength [1]. (b) Accessory pigments (chlorophyll b, carotene, xanthophyll) absorb light at these wavelengths [1] and pass the energy to chlorophyll a, which is photoactivated [1]. Examiner insight: Name the accessory pigments or use the term. “Other pigments absorb it” without saying that the energy reaches chlorophyll a scores only the first mark.

7. (a) Light causes photoactivation of chlorophyll, and the emitted electrons from the light-dependent stage [1] reduce DCPIP, which is colourless when reduced (it accepts electrons in place of NADP) [1]. (b) 1000 ÷ 150 = 6.7 s⁻¹ [1]. (c) As distance increases, light intensity decreases [1], so there is less photoactivation of chlorophyll, fewer electrons are emitted per second, and DCPIP is reduced more slowly [1]. (d) To show that the colour change needs light and is not caused by something else in the tube [1]. (e) Temperature, using a water bath or a heat shield between lamp and tube (or volume of DCPIP and suspension, using a syringe) [1]. Examiner insight: In (a), “DCPIP is reduced” needs a source of electrons. In (e), a variable without a method, such as “keep the temperature the same”, usually earns no mark.

8. (a) Volume = π × 0.60² × 18 = 20.4 mm³ [1]. Rate = 20.4 ÷ 5.0 = 4.1 mm³ min⁻¹ [1]. (b) Some oxygen is used in respiration by the plant’s cells [1]. Some oxygen dissolves in the water and is not collected [1]. (c) Use different concentrations of sodium hydrogencarbonate solution [1]. Examiner insight: Use the radius, not the diameter, in πr². Squaring the diameter gives a volume four times too big and loses the accuracy mark, though the method mark can still be earned.

9. (a) Light intensity, because the rate is the same (8) at both CO₂ concentrations [1]. (b) Another factor is limiting: carbon dioxide concentration [1]. Evidence: at 0.10% CO₂ the rate is higher at these light intensities [1]. (c) (28 − 17) ÷ 17 × 100 [1] = 65% [1]. (d) More CO₂ is fixed with RuBP by rubisco, so more GP is made [1]. More GP is reduced to TP, using ATP and reduced NADP faster, so more organic product is made per unit time [1]. Examiner insight: In (b), naming the limiting factor needs data from the table as evidence. In (c), dividing by the new value (28) instead of the original (17) loses the accuracy mark.

10. (a) (21.6 − 12.0) ÷ 12.0 × 100 [1] = 80% [1]. (b) Enzymes of the Calvin cycle, such as rubisco, and their substrates have more kinetic energy [1], so there are more frequent successful collisions and enzyme–substrate complexes form faster [1]. (c) Enzymes such as rubisco denature: the active site changes shape, so fewer enzyme–substrate complexes form [1]. Examiner insight: Link temperature to the enzymes of the light-independent stage. Answers that only mention “the plant working faster” earn nothing in (b).

11. Grind leaves in a cold, buffered solution, filter and centrifuge to obtain a chloroplast suspension [1]. Vary wavelength with coloured filters (for example red, blue and green) [1]. Keep light intensity equal for each filter, checked with a light meter [1]. Keep temperature constant with a water bath or heat shield [1]. Use the same volumes and concentrations of suspension and DCPIP, measured with syringes [1]. Time how long DCPIP takes to decolourise, judged against a colour standard; calculate rate = 1/t and repeat for a mean [1]. Include a control tube in the dark (or without chloroplasts) [1]. Prediction: the rate is fastest in red and blue light and slowest in green, because chlorophylls absorb red and blue strongly and absorb little green [1]. Examiner insight: Planning marks reward operational detail. “Control the temperature” earns less than “place tubes in a water bath at 25 °C”, and a prediction needs its reason for the mark.

Where marks are usually lost

• Naming the whole chloroplast as a site instead of the thylakoids or the stroma.
• Quoting Rf values with units, or measuring to the edge of a spot rather than its centre.
• Saying rubisco forms one molecule of GP; each CO₂ fixed gives two.
• Forgetting that RuBP regeneration uses ATP.
• Saying cyclic photophosphorylation releases oxygen.
• Describing a plateau without naming the factor that now limits and using data as evidence.
• Giving controlled variables in a plan without saying how they are controlled.

Next steps

• Photosynthesis revision notes
• Photosynthesis study guide
• Cambridge A Level Biology hub
• Printable Biology checklist
• A Level Biology 9700 diagnostic, or all free 10-minute diagnostics
• Book a free trial class

Official syllabus

Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027, published by Cambridge Assessment International Education (Cambridge University Press & Assessment, September 2022). Topic 13, Photosynthesis, sections 13.1 and 13.2.