Gas Cam

Gas Cam: respiration, photosynthesis and gas exchange from noon to sunrise

This free game is part of our Pearson Edexcel iGCSE Biology (4BI1) course for exams in EXAM YEARS 2026-2029.

Every living thing respires, all the time

Respiration releases energy from glucose to make ATP, which provides energy for cells. Because cells need energy all the time, every living organism respires all day and all night: plants, animals, fungi and microorganisms. Aerobic respiration takes in oxygen and gives out carbon dioxide:

• Word equation: glucose + oxygen → carbon dioxide + water
• Balanced equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Respiration also releases heat. That is why a compost heap full of bacteria and fungi steams on a cold evening, and why germinating seeds in a vacuum flask warm up (spec point 2.39).

Day, night and net gas exchange in plants

Plants also photosynthesise, but only in the light. Photosynthesis does the opposite of respiration: it takes in carbon dioxide and gives out oxygen. What a plant exchanges overall (its net gas exchange) depends on the light intensity (spec points 2.41B and 2.44B):

• Bright light: photosynthesis is faster than respiration, so the plant takes in CO₂ and gives out O₂ overall.
• Dim light: at the compensation point the two rates are equal, so there is no net gas exchange.
• Darkness: there is no photosynthesis, so the plant only respires. It takes in O₂ and gives out CO₂, just like an animal.

Gases move in and out of a leaf by diffusion, through pores called stomata. Animals, fungi, microorganisms and seeds that have no green leaves yet cannot photosynthesise, so they always give out carbon dioxide.

Hydrogen-carbonate indicator

Hydrogen-carbonate indicator shows changes in carbon dioxide level, which is why it is used for the light and net gas exchange practical (spec point 2.45B):

• Purple: less CO₂ than normal. A leaf in bright light has taken it in.
• Red/orange: normal CO₂. Nothing has changed, as in a control tube with no leaf, or a leaf under thin gauze at its compensation point.
• Yellow: more CO₂ than normal. A leaf wrapped in foil, or a snail, has only been respiring.

How to play Gas Cam

You are the camera operator for a nature show at Lemon Marsh. The camera rolls round a boardwalk by a pond, and Kemi, the director, gives you a shot list such as “Snap 2 organisms taking in carbon dioxide”. Aim at an organism and click (or tap) to take the photo.

• Four shoots: noon, dusk, midnight and sunrise. The same plant swaps from taking in CO₂ to giving it out as the light changes, so you have to think about the light every time.
• Indicator Vision: hold SPACE (or the eye button) to see every living thing glow in hydrogen-carbonate indicator colours, with oxygen and carbon dioxide molecules drifting in and out. Snap without it for double points.
• Wrong shots cost 10% of your score, and Kemi explains why that organism didn't fit the shot.
• Lab check: after each shoot, answer quick questions on the indicator experiment, the compensation point, respiration equations and the germinating seeds practical.

Exam tips

• Never write that plants respire only at night. They respire all the time. In bright light their respiration is just hidden by faster photosynthesis.
• Explain colour changes using CO₂ levels: “the leaf photosynthesised faster than it respired, so it took in carbon dioxide and the indicator turned purple”.
• Always mention the control: a tube with no leaf, or a flask of boiled (dead) seeds, shows that the living organism caused the change.
• The compensation point is a light intensity where the rates of photosynthesis and respiration are equal, not a time when respiration stops.