What does Photosynthesis cover in IB Biology?

This topic explains photosynthesis as the transformation of light energy into chemical energy to produce carbon compounds. The overall word equation is: carbon dioxide + water xrightarrowlight glucose + oxygen. Oxygen released during photosynthesis is a by-product of water splitting.

Is Photosynthesis SL or HL?

Both. SL and HL students study Photosynthesis, and HL goes further: C1.3.9 Photosystems as arrays of pigment molecules that generate and emit excited electrons. Photosystems are always located in membranes and occur in cyanobacteria and in the chloroplasts of photosynthetic eukaryotes. They are molecular arrays of chlorophyll and accessory pigments with a special chlorophyll as the reaction centre from which an excited electron is emitted.

How does FourtyFive help me practise Photosynthesis?

FourtyFive has 8 Photosynthesis questions. Every answer you write is marked mark by mark, IB-style, and you see where each mark was won or lost. Every part has a hint, the AI tutor helps you through the step you are stuck on, and your Study Profile picks what to practise next.

Is FourtyFive free for Photosynthesis practice?

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Can I handwrite Photosynthesis answers on an iPad?

Yes. In the FourtyFive iPad app you write your working by hand with Apple Pencil, the way you would on paper, and it is marked the same way.

Topic C.3 · SL and HL

Photosynthesis: notes and practice questions

Q: How do I revise Photosynthesis for IB Biology?

Start from the core idea: this topic explains photosynthesis as the transformation of light energy into chemical energy to produce carbon compounds. In the exam: limiting factor graphs are the most examined part of this topic at SL and turn up in Paper 1A, 1B and 2. Answers must name which factor is limiting in which region of the curve. Then practise exam-style questions, easiest first, writing out every step of your working before you check it.

8 min read Jump to 8 practice questions

Summary

This topic explains photosynthesis as the transformation of light energy into chemical energy to produce carbon compounds.
The overall word equation is: carbon dioxide + water $\xrightarrow{\text{light}}$ glucose + oxygen.
Oxygen released during photosynthesis is a by-product of water splitting.
Photosynthetic pigments absorb specific light wavelengths, which can be analyzed using absorption and action spectra.
The rate of photosynthesis is influenced by limiting factors such as carbon dioxide concentration, light intensity, and temperature.
Skills include separating pigments by chromatography and determining photosynthesis rates from experimental data.

How it is examined

Limiting factor graphs are the most examined part of this topic at SL and turn up in Paper 1A, 1B and 2. Answers must name which factor is limiting in which region of the curve. Rf calculations are 1 to 2 marks and need working. At HL, the interdependence point in C1.3.19 is a favourite explain item, and the mark for "no CO2 means the Calvin cycle stops consuming NADPH and ATP, so photosystem II backs up" needs the chain spelled out.

Key ideas

C1.3.1 Transformation of light energy to chemical energy when carbon compounds are produced in photosynthesis. This supplies most of the chemical energy needed for life processes in ecosystems.
C1.3.2 Conversion of carbon dioxide to glucose using hydrogen obtained by splitting water. Students should be able to write a simple word equation for photosynthesis with glucose as the product.
C1.3.3 Oxygen as a by-product of photosynthesis in plants, algae and cyanobacteria. Students should know the simple word equation and that the oxygen produced comes from the splitting of water.
C1.3.4 Separation and identification of photosynthetic pigments by chromatography. Application of skills: calculate Rf values from a chromatographic separation and identify pigments by colour and by Rf value. Thin-layer or paper chromatography can be used.

At HL

C1.3.9 Photosystems as arrays of pigment molecules that generate and emit excited electrons. Photosystems are always located in membranes and occur in cyanobacteria and in the chloroplasts of photosynthetic eukaryotes. They are molecular arrays of chlorophyll and accessory pigments with a special chlorophyll as the reaction centre from which an excited electron is emitted.
C1.3.10 Advantages of the structured array of different pigment types. A single molecule of chlorophyll or any other pigment could not perform any part of photosynthesis.
C1.3.11 Generation of oxygen by photolysis of water in photosystem II. The protons and electrons from photolysis are used in photosynthesis but oxygen is a waste product, with immense consequences for organisms and geology once it appeared.
C1.3.12 ATP production by chemiosmosis in thylakoids. Include the proton gradient, ATP synthase, and proton pumping by the chain of electron carriers. Electrons are sourced either from photosystem I in cyclic photophosphorylation or from photosystem II in non-cyclic photophosphorylation.

Guiding questions

How is energy from sunlight absorbed and used in photosynthesis?
How do abiotic factors interact with photosynthesis?

Linking questions

What are the consequences of photosynthesis for ecosystems?
What are the functions of pigments in living organisms?

Practice questions

8 questions · 6 medium · 2 hard

Showing 8 of 8

Question 1

MediumPaper 1B · calculator6 marks

(a)

An experiment was conducted to investigate the effect of light intensity and temperature on the rate of photosynthesis in the aquatic plant Cabomba caroliniana. The rate of photosynthesis was measured by the volume of oxygen produced per minute. The results are shown in the graph below.

Graph showing the rate of photosynthesis (mm³ O₂ min⁻¹) on the y-axis from 0 to 10, and light intensity (arbitrary units) on the x-axis from 0 to 80. There are three curves representing different temperatures: 15 °C (circles), 25 °C (squares), and 35 °C (triangles). All curves start at the origin and increase with light intensity before plateauing. The 15 °C curve plateaus at the lowest rate (around 4.0 mm³ O₂ min⁻¹). The 35 °C curve plateaus at a higher rate (around 8.2 mm³ O₂ min⁻¹). The 25 °C curve plateaus at the highest rate (around 9.2 mm³ O₂ min⁻¹).

(a) State the rate of photosynthesis at a light intensity of 40 arbitrary units and a temperature of 25 °C.

[1]

(b)

(b) Identify the limiting factor for photosynthesis at a light intensity of 20 arbitrary units.

[1]

(c)

(c) Explain the effect of increasing the temperature from 15 °C to 25 °C on the rate of photosynthesis at a light intensity of 70 arbitrary units.

[2]

(d)

(d) The experiment was repeated at 45 °C. Predict, with a reason, the effect on the rate of photosynthesis at high light intensity compared to the rate at 35 °C.

[2]

Question 2

HardPaper 1A · calculator8 marks

(a)

A student investigated the photosynthetic pigments in two species of algae: Chlorella vulgaris, a green alga found in freshwater, and Porphyra umbilicalis, a red alga found in the intertidal zone. The pigments were extracted from each alga and separated using paper chromatography. The resulting chromatogram is shown below. The solvent front moved 9.0 cm from the origin line.

A paper chromatogram with an origin line at the bottom and a solvent front line at the top, 9.0 cm apart. There are two lanes. Lane 1, for Chlorella vulgaris, shows four spots from bottom to top: Spot A (yellowish-green) at 2.7 cm, Spot B (blue-green) at 3.6 cm, Spot C (yellow) at 6.3 cm, and Spot D (orange) at 8.6 cm. Lane 2, for Porphyra umbilicalis, shows three spots: Spot E (blue-green) at 3.6 cm, Spot F (reddish-purple) at 4.5 cm, and Spot G (orange) at 8.6 cm.

A table of known photosynthetic pigments and their properties is provided.

Pigment	Colour	$R_f$ value in this solvent
Carotene	Orange	0.95
Phaeophytin	Grey-brown	0.81
Xanthophyll	Yellow	0.70
Chlorophyll a	Blue-green	0.40
Chlorophyll b	Yellowish-green	0.30
Phycoerythrin	Reddish-purple	0.50

(a) Calculate the $R_f$ value for spot B.

[2]

(b)

(b) Using the data provided, identify the pigments represented by spots A, C, and D in Chlorella vulgaris.

[3]

(c)

(c) The red alga Porphyra often lives in deeper water than the green alga Chlorella. Explain how the pigments present in Porphyra are an adaptation to this environment.

[3]

Question 3

MediumPaper 1B · calculator6 marks

(a)

A student is studying a cross-section of a leaf from a sunflower (Helianthus annuus) using a light microscope. They focus on a single palisade mesophyll cell. The actual length of this cell is 30 µm. The student draws the cell with a length of 6 cm in their notebook.

(a) Calculate the magnification of the student's drawing.

[2]

(b)

(b) Distinguish between the magnification and the resolution of a microscopic image.

[2]

(c)

(c) The student wants to observe the detailed internal structure of a chloroplast from the palisade cell, including the thylakoid stacks. State the type of microscope required for this task.

[1]

(d)

(d) State one reason for the high density of chloroplasts in palisade mesophyll cells.

[1]

Question 4

HardPaper 1B · calculator6 marks

(a)

The graph shows data-logging of three variables in a coastal rock pool containing the brown alga Fucus vesiculosus over a 48-hour period.

$A graph showing changes over 48 hours in a rock pool. The x-axis is time from 0 to 48 hours. There are three y-axes: pH (ranging from 7.4 to 8.6), light intensity in $\text{W m}^{-2}$ (ranging from 0 to 1000), and temperature in °C (ranging from 10 to 30). On day 1, light intensity peaks at $1000\text{ W m}^{-2}$ at 12 hours, temperature peaks at 25 °C at 14 hours, and pH peaks at 8.5 at 13 hours. During the night (18 to 30 hours), light is 0, temperature drops to 15 °C, and pH drops to 7.5. On day 2, at 36 hours, light intensity drops sharply to $200\text{ W m}^{-2}$ due to fog, but temperature remains high at 24 °C. During this fog period, the pH drops to 7.7.$

Explain the causes of the rises and falls in the pH of the rock pool water.

[2]

(b)(i)

(b.i) State the relationship shown in the graph between light intensity and dissolved $\text{CO}_2$ concentration.

[1]

(b)(ii)

(b.ii) State the relationship shown in the graph between temperature and dissolved $\text{CO}_2$ concentration.

[1]

(c)

(c) From the graph, deduce whether light intensity or temperature has a greater effect on the dissolved $\text{CO}_2$ concentration.

[2]

Question 5

MediumPaper 2 · calculator5 marks

(a)(i)

The image shows a transmission electron micrograph (TEM) of a chloroplast.

Transmission electron micrograph of a chloroplast showing double membrane, grana which are stacks of thylakoids, and stroma.

(a)

(i) Name a type of cell in a flowering plant that does not contain chloroplasts.

[1]

(a)(ii)

(a) (ii) State two products of the overall process of photosynthesis.

[2]

(b)

(b) Chloroplasts contain their own circular DNA and 70S ribosomes. Explain how these features provide evidence for the endosymbiotic theory.

[2]

Question 6

MediumPaper 2 · calculator5 marks

Explain how predicted changes in the global climate and atmospheric composition could affect future rates of photosynthesis in terrestrial ecosystems.

Question 7

MediumPaper 2 · calculator10 marks

(a)

(HL only) A researcher treats isolated chloroplasts with a chemical that makes the thylakoid membrane highly permeable to protons ( $\text{H}^+$ ).

Explain the effect of this chemical on the synthesis of ATP in the chloroplast.

[2]

(b)

Explain how the failure to produce ATP affects the light-independent reactions of photosynthesis.

[4]

(c)

Even though ATP synthesis is inhibited, oxygen continues to be produced for a short time. Explain the process by which oxygen is produced in the chloroplast.

[4]

Question 8

MediumPaper 1A · calculator1 mark

Hydrogen carbonate indicator is red at atmospheric $\text{CO}_2$ levels. It turns yellow when $\text{CO}_2$ levels increase and purple when $\text{CO}_2$ levels decrease. Tests were carried out in sealed tubes containing this indicator, varying the presence of an aquatic plant (Cabomba caroliniana) and a freshwater snail (Lymnaea stagnalis).

Which condition would result in the indicator turning purple?

A. Plant kept in the light without a snail

B. Plant kept in the dark without a snail

C. Snail kept in the light without a plant

D. Plant and snail kept in the dark

Every Photosynthesis question, marked for you

Every answer is marked mark by mark, IB-style, and the AI tutor helps when you are stuck.

Where marks are lost

Command terms are consistently under-read. Students answer describe when the question said explain, so they give an account with no reasons and cap at half marks. The reverse also happens on outline, where a student writes an essay for a 2-mark summary and runs out of time.

Photosynthesis: notes and practice questions

How it is examined

Guiding questions

Linking questions