AQA GCSE Combined Science Foundation (8464), Foundation tier · Biology › Bioenergetics › Photosynthesis
Practise Photosynthetic reaction. 6 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Photosynthesis is the reaction in which plants and algae use energy transferred by light to make glucose from carbon dioxide and water. Expect recall of the word equation and the chemical formulae, 'explain why photosynthesis is endothermic', and questions that link the reaction to leaves, starch tests or gas exchange.
Key facts
carbon dioxide + water → glucose + oxygen (light above the arrow)
CO2 + H2O → C6H12O6 + O2 (recognise these formulae)
Takes place in chloroplasts; chlorophyll absorbs the light
Endothermic: energy is transferred from the environment to the chloroplasts by light
Carbon dioxide enters the leaf by diffusion through the stomata
Water is absorbed by root hair cells and carried in the xylem
Starch test: iodine solution turns from orange-brown to blue-black
Notes
The reaction
Photosynthesis takes place in the chloroplasts of plant cells and algae.
Chlorophyll, the green pigment inside chloroplasts, absorbs the light.
Word equation: carbon dioxide + water → glucose + oxygen (write 'light' above the arrow).
You need to recognise the formulae: CO2 (carbon dioxide), H2O (water), C6H12O6 (glucose) and O2 (oxygen).
Why photosynthesis is endothermic
An endothermic reaction takes in energy from its surroundings.
In photosynthesis, energy is transferred from the environment to the chloroplasts by light.
This energy ends up in the chemical energy store of the glucose. The plant, and the animals that eat it, can later transfer it by respiration.
Because they make glucose in this way, plants and algae are producers: they make the biomass that almost every food chain depends on.
Where the substances come from and go
diagram
Carbon dioxide diffuses from the air into the leaf through the stomata.
Water is absorbed from the soil by root hair cells and carried up to the leaves in the xylem.
Oxygen diffuses out of the leaf through the stomata, or is used by the plant's own cells for respiration.
Glucose is used by the plant straight away or turned into other substances, such as starch for storage.
What a leaf takes in and gives out while it photosynthesises.
How to answer each type of question
Recall: complete the equation or give a formula
1 to 3 marksGrade 3
Learn the word equation as a sentence: carbon dioxide and water make glucose and oxygen.
Use names in a word equation and formulae only when a formula is asked for.
Check the formula carefully: glucose is C6H12O6, not C6H12O2.
Example. (a) Complete the word equation for photosynthesis. .................... + water → .................... + oxygen (b) Give the chemical formula of water.
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(a) carbon dioxide (1); glucose (1) (b) H2O (1)
Explain why photosynthesis is endothermic
2 marksGrade 5
Say that energy is taken in from the environment (surroundings).
Say how it gets there: it is transferred to the chloroplasts by light.
Example. A student wrote: 'Photosynthesis gives out energy to the plant.' The student is wrong. Explain why photosynthesis is described as an endothermic reaction.
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Energy is taken in / transferred from the environment (1) to the chloroplasts by light (1).
Don’t lose marks
Writing that photosynthesis happens 'in the chlorophyll'. It happens in the chloroplasts; chlorophyll is the pigment that absorbs light.
Writing light as a reactant on the left of the equation. Light is not a substance: it transfers the energy, so write it above the arrow.
Calling photosynthesis exothermic, or saying it 'makes energy'. Energy is never made: it is transferred to the chloroplasts by light.
Saying plants get their food from the soil. Plants make their own glucose; the soil supplies only water and mineral ions.
Saying plants photosynthesise instead of respiring. Plants respire all the time, day and night, and photosynthesise only when there is light.
More tips
Memory tricks
Endo = in: in photosynthesis energy goes in, carried by light.
Chloroplast and chlorophyll both start with 'chloro' (green): the chloroplast is the organelle, chlorophyll is the green pigment inside it.
Glucose formula pattern: 6 carbon, 12 hydrogen, 6 oxygen (6-12-6).
Respiration is photosynthesis in reverse: swap the two sides of the word equation to get glucose + oxygen → carbon dioxide + water.
Check the balanced equation by counting atoms: 6 C, 12 H and 18 O on each side.
Exam technique
For 'endothermic' questions, use both key phrases: 'energy transferred from the environment' and 'to the chloroplasts by light'.
Write 'carbon dioxide' in full rather than a half-remembered formula. If you do use formulae, check every subscript.
In experiments with starch tests or indicators, write the whole chain: light → photosynthesis → glucose made (and carbon dioxide used) → the result you see.
If a question asks for a word equation, give names only; do not mix names and formulae.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Write the word equation for photosynthesiscarbon dioxide + water → glucose + oxygen, with light written above the arrow.
Grade 3
State where photosynthesis takes placeIn the chloroplasts of plant and algal cells, where chlorophyll absorbs light.
Grade 4
Recognise the formulae of the four substancesCarbon dioxide is CO2, water is H2O, glucose is C6H12O6 and oxygen is O2.
Grade 5
Explain why photosynthesis is endothermicEnergy is transferred from the environment to the chloroplasts by light, so the reaction takes in energy.
Grade 5
State where the reactants come fromCarbon dioxide diffuses into the leaf from the air through the stomata; water is absorbed by the roots and carried up in the xylem.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Complete the word equation for photosynthesis. carbon dioxide + .................... → glucose + ....................
carbon dioxide + water → glucose + oxygen
Name the green substance in chloroplasts that absorbs light.
Chlorophyll
Photosynthesis is the process by which green plants and algae make glucose. Give the word equation for photosynthesis.
carbon dioxide + water → glucose + oxygen
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy6 marks
Photosynthesis takes place in green plants and algae.
(a) Complete the word equation for photosynthesis. carbon dioxide + .................... → glucose + ....................[2]
(b) In which part of a plant cell does photosynthesis take place?[1]
(c) Which is the chemical formula of glucose? Tick (✓) one box.[1]
CO2
C6H12O6
H2O
O2
(d) Photosynthesis is an endothermic reaction. What does endothermic mean?[1]
(e) How is energy transferred to the chloroplasts for photosynthesis?[1]
Show the answer and mark scheme
(a)Answer: carbon dioxide + water → glucose + oxygen
water
oxygen
(b)Answer: Chloroplasts
chloroplast
(c)Answer: C6H12O6
(d)
energy is transferred from the surroundings / is taken in (by the reaction)
(e)Answer: By light
by light
Question 2Medium7 marks
Photosynthesis is described as an endothermic reaction.
(a) Explain why photosynthesis is described as an endothermic reaction.[2]
(b) Name the green substance in chloroplasts that absorbs light.[1]
(c) Give the chemical formulae of the two reactants in photosynthesis.[2]
(d) A student said, ‘Plants get their food from the soil.’ Use your knowledge of photosynthesis to explain why the student is wrong.[2]
Show the answer and mark scheme
(a)
energy is transferred from the environment / surroundings
to the chloroplasts by light
(b)Answer: Chlorophyll
chlorophyll
(c)Answer: CO2 and H2O
CO2
H2O
(d)
plants make their own food / glucose by photosynthesis
from carbon dioxide (from the air) and water; the soil only supplies water and mineral ions