Biology  by Bradford
IB Biology 2025 · Theme C · Interaction and interdependence

C1.3 Photosynthesis. Practice questions with markscheme.

50 original IB-style questions on C1.3, written from the 2025 guide: 26 multiple-choice, 15 short-answer, 4 data-based, 3 extended-response part, 2 drawing. Below is a 22-mark standard-level practice paper built from them, ready to hand out as a class quiz or homework, or to sit yourself and mark against the scheme. Print it, project it, or build a fresh one on the same topic.

What the guide asks for

8 statements at SL and HL, 11 additional higher level.

  1. C1.3.1SL / HL Transformation of light energy to chemical energy when carbon compounds are produced in photosynthesis
  2. C1.3.2SL / HL Conversion of carbon dioxide to glucose in photosynthesis using hydrogen obtained by splitting water
  3. C1.3.3SL / HL Oxygen as a by-product of photosynthesis in plants, algae and cyanobacteria
  4. C1.3.4SL / HL Separation and identification of photosynthetic pigments by chromatography
  5. C1.3.5SL / HL Absorption of specific wavelengths of light by photosynthetic pigments
  6. C1.3.6SL / HL Similarities and differences of absorption and action spectra
  7. C1.3.7SL / HL Techniques for varying concentrations of carbon dioxide, light intensity or temperature experimentally to investigate the effects of limiting factors on the rate of photosynthesis
  8. C1.3.8SL / HL Carbon dioxide enrichment experiments as a means of predicting future rates of photosynthesis and plant growth
  9. C1.3.9HL Photosystems as arrays of pigment molecules that can generate and emit excited electrons
  10. C1.3.10HL Advantages of the structured array of different types of pigment molecules in a photosystem
  11. C1.3.11HL Generation of oxygen by the photolysis of water in photosystem II
  12. C1.3.12HL ATP production by chemiosmosis in thylakoids
  13. C1.3.13HL Reduction of NADP by photosystem I
  14. C1.3.14HL Thylakoids as systems for performing the light-dependent reactions of photosynthesis
  15. C1.3.15HL Carbon fixation by Rubisco
  16. C1.3.16HL Synthesis of triose phosphate using reduced NADP and ATP
  17. C1.3.17HL Regeneration of RuBP in the Calvin cycle using ATP
  18. C1.3.18HL Synthesis of carbohydrates, amino acids and other carbon compounds using the products of the
  19. C1.3.19HL Interdependence of the light-dependent and light-independent reactions

In the bank for C1.3

  • 26 multiple-choice
  • 15 short-answer
  • 4 data-based
  • 3 extended-response part
  • 2 drawing
  • 24 higher level only

Every question is original and tagged to a guide statement. See the whole bank →

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The practice paper

Take it on screen → Build a fresh paper Paper code BbB-EAAAACAAABQAOzdf
Biology · topic quiz
Standard level · topic practice, not an exam format
35 minutes22 marks

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Covers C1.3 Photosynthesis
Name:
1.

A plant is photosynthesizing at a high rate on a bright, warm day, but the rate suddenly falls at midday when stomata close. Which factor has become limiting?

[1]
  1. Oxygen concentration
  2. Carbon dioxide supply
  3. Leaf temperature
  4. Light intensity
2.

The absorption spectrum of chlorophyll and the action spectrum of photosynthesis have peaks at similar wavelengths. What does this similarity indicate?

[1]
  1. The wavelengths absorbed by chlorophyll are the wavelengths that drive photosynthesis
  2. Chlorophyll is the only pigment involved in photosynthesis, so no other pigment contributes
  3. Green light is used more efficiently than red or blue light for photosynthesis in the chloroplast
  4. The rate of photosynthesis is independent of the wavelength of light that strikes the leaf
3.

Leaf pigments are separated by chromatography. Why do the pigments travel different distances up the paper/plate?

[1]
  1. They are of different colours
  2. Heavier pigments always travel further
  3. They differ in solubility and adsorption
  4. The solvent digests some pigments
4.

Which method allows the rate of photosynthesis of an aquatic plant to be measured directly?

[1]
  1. Counting the leaves on the plant
  2. Measuring the volume of oxygen released
  3. Measuring the mass of the soil used
  4. Weighing the whole plant once only
5.

In free-air carbon dioxide enrichment (FACE) experiments, extra CO₂ is released from rings of pipes surrounding vegetation growing outdoors. What is the purpose of these experiments?

[1]
  1. To measure the effects of raised CO₂ on plants outdoors
  2. To reduce the CO₂ concentration around crop plants
  3. To find the optimum temperature for photosynthesis
  4. To supply plants with CO₂ needed for respiration
6.

What is the overall energy conversion carried out in photosynthesis?

[1]
  1. Chemical energy to light energy
  2. Heat energy to chemical energy
  3. Chemical energy to heat energy
  4. Light energy to chemical energy
7.

Most leaves appear green. Which statement about the absorption of light by chlorophyll explains this?

[1]
  1. It absorbs red and blue light and reflects green
  2. Green light carries no usable energy
  3. It absorbs green light most strongly of all
  4. It absorbs all wavelengths of light equally
8.

Outline the role of water in photosynthesis, including the fate of the oxygen that it contains.

[3]
9.

Outline how carbon dioxide concentration, light intensity and temperature can each limit the rate of photosynthesis.

[3]
10.
NaHCO₃ concentration / %O₂ produced trial 1 / cm³Trial 2 / cm³Trial 3 / cm³
0.00.20.10.2
0.21.41.51.3
0.52.82.93.0
1.03.94.03.8
2.04.14.04.2
(a)Design an investigation to test the hypothesis that increasing CO₂ concentration increases the rate of photosynthesis up to a plateau. State the independent, dependent and controlled variables, a control, and how reliability would be improved.[5]
(b)Analyse the data to describe the relationship and identify the concentration above which CO₂ no longer limits the rate.[2]
(c)Suggest one source of error in using oxygen gas volume to measure photosynthesis, and how it could be reduced.[2]

Original practice questions © Biology by Bradford · CC BY-NC-SA 4.0 · Not affiliated with or endorsed by the International Baccalaureate Organization.
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Markscheme BbB-EAAAACAAABQAOzdf

One mark per point; / separates alternative wording within a point, OR separates alternative answers, words in brackets are not required, underlined words are essential. OWTTE = or words to that effect.

1. C1.3.7 [1]
  • B: closed stomata cut off CO₂ entry; light and temperature remain high, so CO₂ becomes the limiting factor;
2. C1.3.6 [1]
  • A: correlation between absorption and rate shows chlorophyll captures the light energy used; accessory pigments explain the differences;
3. C1.3.4 [1]
  • C: separation reflects the balance between solubility (mobile phase) and adsorption (stationary phase); Rf = distance moved by pigment ÷ distance moved by solvent;
4. C1.3.3 [1]
  • B: O₂ release (bubble counts/volume collected) per unit time gives the rate; CO₂ uptake (pH change) is an alternative;
5. C1.3.8 [1]
  • A: FACE experiments raise the CO₂ concentration around intact outdoor vegetation, to predict how rising atmospheric CO₂ will affect photosynthesis, growth and ecosystems;
6. C1.3.1 [1]
  • D: light energy is transformed into chemical energy in carbon compounds (using CO₂ and water, releasing O₂);
7. C1.3.5 [1]
  • A: the absorption spectrum of chlorophyll peaks in red and blue; green is (mostly) reflected/transmitted, so leaves look green;
8. C1.3.2 [3 max]
  • water is split by photolysis (using light energy);
  • the hydrogen (obtained from water) is used to convert/reduce carbon dioxide to glucose;
  • the oxygen is released as a waste product (diffusing out of the leaf);
  • (evidence: isotopic labelling with ¹⁸O shows the oxygen released comes from water, not from CO₂);
9. C1.3.7 [3 max]
  • CO₂ is a substrate (for carbon fixation), so low concentration limits the rate;
  • light provides the energy (for the light-dependent reactions), so low intensity limits the rate;
  • temperature affects enzyme(-catalysed) reactions, so rate falls when too cold (and above the optimum);
  • (at any moment) the factor in shortest supply / furthest below saturation determines the rate, OWTTE;
10. C1.3.3, C1.3.7
  • (a) [5]
    • independent variable = CO₂ (hydrogencarbonate) concentration, a range of at least five values;
    • dependent variable = rate of photosynthesis, measured as volume of O₂ produced per unit time (gas syringe / capillary) or bubbles per minute;
    • controlled variables (at least two): light intensity (fixed lamp distance/light meter), temperature (water bath / heat filter), same species and mass of pondweed, same time;
    • a control at 0 % hydrogencarbonate (or deionised water) to show CO₂ is required;
    • replicate each concentration and calculate a mean to improve reliability, OWTTE;

    NOS experimental design; award any 5 of the listed points.

  • (b) [2]
    • O₂ production rises steeply as concentration increases up to about 1.0 %, then levels off / plateaus;
    • above ~1.0 % (between 1.0 and 2.0 %) CO₂ is no longer limiting / another factor limits the rate, OWTTE;
  • (c) [2]
    • some O₂ dissolves in the water / gas also contains other gases / bubbles vary, so volume underestimates or misrepresents the true rate;
    • allow the system to equilibrate and collect gas over a longer measured time / use a calibrated gas syringe / repeat and average, OWTTE;

    NOS: sources of error and reliability.

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