Gas exchange in flowering plants — Edexcel International GCSE Biology
Diffusion of carbon dioxide and oxygen in leaves, stomata, net gas exchange by day and night, and the hydrogen-carbonate indicator practical.
Revision notes
3 short notes, in the order of the specification. Each one in short:
Metabolism is all the chemical reactions in a cell, and two of its products are waste products in a plant. Carbon dioxide is a waste product of respiration, and oxygen is a waste product of photosynthesis. Both gases leave the plant from the leaves by diffusion through the stomata.
Net gas exchange by day and night
Biology onlyA plant respires all the time but photosynthesises only in light, so its net gas exchange changes with light intensity. In darkness and dim light it releases carbon dioxide. In bright light photosynthesis is faster than respiration, so it takes in carbon dioxide. At the compensation point the two rates are equal, so there is no net exchange.
Hydrogen-carbonate indicator shows net gas exchange in leaves: it is red/orange in normal air, turns yellow when carbon dioxide rises and purple when it falls. A leaf in bright light turns it purple because photosynthesis is faster than respiration. A leaf in darkness turns it yellow because only respiration happens. A tube with no leaf is the control.
Exam questions
6 exam-style questions (20 marks), each with its mark scheme.
Answer the questionsFlashcards
8 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of structure and functions in living organisms on one page, so you can see where this subtopic fits.
Open the mind mapPractical sheet
Method, variables, risks and a results table, with questions to answer.
Print it
Free PDFs to print or save.
Quick check questions
Biology only Name the process by which gases move into and out of a leaf.
Show answer
Diffusion.
Biology only What do guard cells do?
Show answer
They open and close the stoma.
Which gas is a waste product of photosynthesis and leaves through the stomata?
Show answer
Oxygen.
Biology only What is happening at the compensation point?
Show answer
The rate of photosynthesis equals the rate of respiration, so there is no net gas exchange.
Biology only What colour does hydrogen-carbonate indicator turn when carbon dioxide is added?
Show answer
Yellow.
Frequently asked questions
How do gases move into and out of a leaf?
Biology only Gases move into and out of a leaf by diffusion through the stomata, down their concentration gradients, so no energy is needed. In the day, carbon dioxide diffuses in because photosynthesis uses it up, lowering its concentration inside the leaf. Oxygen, a waste product of photosynthesis, diffuses out through the stomata into the air.
How do guard cells open and close stomata?
Biology only Guard cells open a stoma when they take in water and swell, and close it when they lose water and become limp. Open stomata let carbon dioxide in and oxygen out, but water vapour is also lost through them. The plant therefore closes its stomata when it is losing too much water.
Do plants respire at night and during the day?
Biology only Yes, plants respire all the time, in light and dark, but photosynthesise only when there is light. At night they release carbon dioxide and take in oxygen. In bright light photosynthesis is faster than respiration, so the carbon dioxide from respiration is used up and the plant takes in carbon dioxide overall.
What is the compensation point in plants?
Biology only The compensation point is the light intensity at which the rate of photosynthesis equals the rate of respiration, so there is no net gas exchange. The carbon dioxide made in respiration is used up by photosynthesis, and the oxygen made in photosynthesis is used up by respiration. Both processes are still happening.
Why does hydrogen-carbonate indicator turn purple in light?
Biology only Hydrogen-carbonate indicator turns purple next to a leaf in bright light because photosynthesis is faster than respiration, so the leaf takes in carbon dioxide and its concentration falls. In darkness the indicator turns yellow, because only respiration happens and carbon dioxide is released. In dim light it may stay red/orange.
Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026