IB Biology SL C1.3 Photosynthesis Questions

Practise IB Biology SL C1.3 by explaining light energy conversion, pigment evidence, wavelength spectra and limiting-factor experiments.

62 questions in this syllabus scope, with question parts counted together.

Exam analysis

SL · 2010–2025

Papers with this topic62 / 141in the analysed sample

Latest appearanceNovember 2025

Tagged question parts92

Paper types5

Most tested objectives

C1.3.7—Limiting factors investigation29 papers
C1.3.5—Absorption of specific wavelengths9 papers
C1.3.1—Light energy → chemical energy8 papers
C1.3.3—Oxygen as by-product7 papers
C1.3.4—Photosynthetic pigment separation7 papers

Common question formats

Calculation
Comparison
Data analysis
Diagram interpretation
Experimental design
Graph interpretation
Process explanation
Structured response
Definition or recall

Recent exam appearances

November 2025Paper 1A SL · TZ3Q5[ 1 ]

November 2025Paper 1A SL · TZ1Q5[ 1 ]

May 2025Paper 1A SL · TZ1Q4[ 1 ]

141 papers analysed · Tagging incomplete

Exam points

Analyse pigment and chloroplast evidence to explain how light energy is captured and converted in photosynthesis.
Trace carbon dioxide reduction using water-derived hydrogen and light-derived energy to form glucose and storage carbohydrates.
Explain that water is split by light-driven photolysis and is the source of oxygen released during photosynthesis.
Separate photosynthetic pigments by chromatography, describe the procedure and calculate or interpret Rf values to identify pigments.
Interpret chlorophyll and accessory-pigment absorption spectra across wavelengths, including reflected light.
Compare absorption and action spectra to explain how pigment wavelength absorption relates to photosynthetic rate.
Interpret limiting-factor graphs, calculate photosynthesis rates and evaluate controlled experiments using plotted evidence.
Interpret carbon-dioxide and temperature data to separate effects on uptake and judge predicted plant-growth changes.

Covered syllabus points

C1.3.1—Light energy → chemical energy C1.3.2—CO₂ → glucose C1.3.3—Oxygen as by-product C1.3.4—Photosynthetic pigment separation C1.3.5—Absorption of specific wavelengths C1.3.6—Absorption vs. action spectra C1.3.7—Limiting factors investigation C1.3.8—CO₂ enrichment experiments

Exam points

Explain how chlorophyll absorbs light and photosynthesis converts its energy into chemical energy in carbon compounds.
Identify chloroplasts or cyanobacterial membranes as sites where photoautotrophs use chlorophyll for photosynthesis.

[Maximum number: 5]

Life is based on carbon compounds.

Outline how carbon compounds are produced in cells using light energy.

Exam points

Trace how carbon dioxide is reduced using hydrogen from water and light-derived energy to form glucose or other carbohydrates in photosynthesis.
Identify glucose as the main stored photosynthetic product and distinguish its conversion into carbohydrate storage products.

[Maximum number: 4]

Outline how photosynthesis produces glucose.

a. solar/light energy is converted to chemical energy

b. energy needed to produce glucose

c. only specific wavelengths are absorbed by chlorophyll OR

red and blue absorbed most strongly.

OR

chlorophyll is the pigment that absorbs light energy

d. $\mathrm{H}^{(+)} /$ electrons from water are used to reduce compounds

e $\quad \mathrm{CO}_{2}$ is absorbed/used/reduced to produce carbohydrates

f. correct word/balanced symbol equation of photosynthesis

Marking guidance:

Accept correct reference to NADPH/ATP from AHL.

Exam points

Explain that light-driven photolysis of water releases oxygen during photosynthesis; the oxygen does not come directly from carbon dioxide.
Identify plants, algae and cyanobacteria as organisms that carry out oxygenic photosynthesis.

[Maximum number: 1]

What is light energy used for in photosynthesis?

To absorb carbon dioxide.

To split water molecules.

To oxidize organic compounds.

To make oxygen and hydrogen react.

Exam points

Explain how pigment solubility or affinity for the mobile and stationary phases separates photosynthetic pigments into distinct bands.
Calculate or interpret Rf as pigment distance divided by solvent-front distance, then compare values or colours with standards to identify pigments.
Describe the extraction and chromatographic procedure, including a suitable solvent, baseline spotting and development of separated bands.

[Maximum number: 5]

The diagram represents the pigment fractions in a thin layer chromatography of the alga Gambierdiscus toxicus.

(a)

Identify the pigment that is most soluble in the solvent used in the chromatography.

[ 1 ]

( $\beta$ -) carotene;

(b)

Describe the process used to obtain this chromatogram.

[ 3 ]

(c)

State the equation used to obtain an $R_{f}$ value.

[ 1 ]

Exam points

Compare chlorophyll and accessory-pigment absorption across red, blue and green wavelengths, including which light is reflected.
Interpret absorption-spectrum evidence to explain how absorbed wavelengths excite photosynthetic pigments and support photosynthesis.

[Maximum number: 1]

Chromatography is a technique used to separate the components of a mixture. The photosynthetic pigments from three organisms were separated by thin layer chromatography: spinach (Spinacia oleracea), a red alga (Porphyra) and a brown alga (Fucus).

Spinacia oleracea

Porphyra

Fucus

Porphyra also contains phycoerythrin, which is a red pigment.

Phycoerythrin appears red under white light. Which colour of visible light is predominantly reflected or transmitted to produce this appearance?

Exam points

Compare absorption spectra with action spectra to explain how pigment wavelength absorption relates to photosynthetic rate.
Construct or interpret action-spectrum graphs, including wavelength peaks, troughs and differences among photosynthetic organisms.

[Maximum number: 1]

Absorption spectra of two photosynthetic pigments are shown. Phycoerythrin is a red pigment found in many marine red algae, while chlorophyll a is the major pigment in green plants and algae.

What do the absorption spectra show about the pigments?

Phycoerythrin absorbs mostly red light.

Chlorophyll a reflects more green light than phycoerythrin.

Chlorophyll a photosynthesizes more at 550 nm than phycoerythrin.

Phycoerythrin does not absorb blue light.

Exam points

Interpret photosynthesis data or graphs to identify whether light intensity, carbon dioxide concentration or temperature is limiting the rate.
Describe controlled investigations that vary one factor and estimate photosynthesis rate from oxygen production or carbon dioxide uptake.

[Maximum number: 1]

An experiment investigated the effect of temperature on the rate of photosynthesis of a plant. Results are shown for temperatures between $5^{\circ} \mathrm{C}$ and $25^{\circ} \mathrm{C}$ .

How would further increases in temperature, up to $70^{\circ} \mathrm{C}$ , affect the rate of photosynthesis?

It would decrease between $25^{\circ} \mathrm{C}$ and $35^{\circ} \mathrm{C}$ and then increase between $35^{\circ} \mathrm{C}$ and $70^{\circ} \mathrm{C}$ .

It would continue increasing as temperature increases.

It would increase to a maximum of between $30^{\circ} \mathrm{C}$ and $40^{\circ} \mathrm{C}$ and then decrease.

It would peak above $25^{\circ} \mathrm{C}$ and then remain constant.

Exam points

Interpret data showing how increased carbon dioxide concentration affects CO₂ uptake or photosynthesis.
Use crop-growth data to assess whether rising atmospheric CO₂ increases growth, distinguishing its effect from temperature.

[Maximum number: 2]

In a study carried out at the University of Arizona, the effects of increased CO2 concentration on the rate of photosynthesis in spring wheat, Triticum aestivum, were investigated over the course of an entire growing season (from the beginning of February to the end of April). The rate of photosynthesis was measured as the rate of $\mathrm{CO}_{2}$ uptake from the time of emergence from the seed to maturity. The control plants were grown at a normal air $\mathrm{CO}_{2}$ concentration while the test plants were grown at an elevated $\mathrm{CO}_{2}$ concentration.

Outline the effect of increased carbon dioxide concentration on $\mathrm{CO}_{2}$ uptake.

a. increased $\mathrm{CO}_{2}$ leads to greater (rate of) photosynthesis;
b. greatest effect on March 30th;
c. smallest effect on April 28th;
d. effect is not constant / difference varies;

a	crush/blend algae with organic solvent/alcohol/other valid solvent;
b	place drop of extracted algal pigments/ obtained liquid on thin layer OR mark the origin;
c	place slide (with pigments) in solvent ensuring the pigment spot does not touch the solvent OR solvent moves up carrying pigments OR different pigments move at different rates/distances (so can be distinguished);

Exam analysis

Most tested objectives

Common question formats

Recent exam appearances

Covered syllabus points

Question 1

Question 1

Question 1

Question 1

Question (a)

Question (b)

Question (c)

Question 1

Question 1

Question 1

Question 1