Photosynthesis and respiration comparedIB MYP Biology: Revision notes

Section 1

Two opposite processes

Photosynthesis and respiration are opposite processes. Photosynthesis stores energy from light in glucose. Respiration releases energy from glucose.

Photosynthesis: carbon dioxide + water → glucose + oxygen, using light energy and chlorophyll.
Respiration: glucose + oxygen → carbon dioxide + water, releasing energy.

$6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$ (photosynthesis)

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$ (respiration)

The products of one are the reactants of the other.

Key termsphotosynthesisrespiration

Section 2

Comparing them

Location: photosynthesis in chloroplasts; respiration in mitochondria.
Energy change: photosynthesis takes in light energy (endothermic); respiration releases energy (exothermic).
Which cells: photosynthesis only in green cells with chlorophyll, in the light; respiration in all living cells, all the time.
Organisms: photosynthesis in plants and algae; respiration in plants, animals and microorganisms.

Key termschloroplastendothermic

Common mistake

Plants respire as well as photosynthesise. They do not only 'breathe in carbon dioxide and breathe out oxygen': in the dark they take in oxygen and give out carbon dioxide.

Section 3

The balance of gases

Animals respire, using oxygen and releasing carbon dioxide. Plants and algae photosynthesise in the light, using carbon dioxide and releasing oxygen. They also respire all the time.

In bright light, a plant photosynthesises faster than it respires, so overall it takes in carbon dioxide and gives out oxygen. In the dark, only respiration happens, so it takes in oxygen and gives out carbon dioxide.

In a balanced ecosystem, the oxygen used in respiration is replaced by photosynthesis, and the carbon dioxide released is taken in again.

Section 4

Effects on the atmosphere

Over a very long time, photosynthesis and respiration have kept the atmosphere at about 21% oxygen and a very small percentage of carbon dioxide.

Human activity upsets this balance. Clearing and burning forests removes organisms that carry out photosynthesis and releases carbon dioxide, so the carbon dioxide level in the atmosphere rises. Planting trees and protecting forests helps to remove carbon dioxide from the air.

Key termsatmosphere

Section 5

Investigating respiration

Limewater: air from living organisms is bubbled through limewater. It turns cloudy (milky) if carbon dioxide is present.
Hydrogencarbonate indicator: red in normal air, yellow when carbon dioxide rises, purple when it falls. Living organisms in a sealed tube make it turn yellow.
Respirometer: living organisms (such as germinating seeds or woodlice) are sealed in a tube with soda lime, which absorbs the carbon dioxide they make. As they use up oxygen, the gas pressure falls and a drop of coloured liquid in a capillary tube moves towards the organisms. The distance moved shows how much oxygen was used.

A good investigation has a control, such as boiled (dead) seeds, to show that the change is caused by living organisms.

Key termslimewaterhydrogencarbonate indicatorrespirometer

Exam tip

In a respirometer, the soda lime absorbs carbon dioxide, so any change in gas volume is because of the oxygen used.

Must know

Photosynthesis: carbon dioxide + water → glucose + oxygen, in chloroplasts, takes in light energy
Respiration: glucose + oxygen → carbon dioxide + water, in mitochondria, releases energy
Respiration happens in all living cells all the time
The two processes balance oxygen and carbon dioxide in the atmosphere
Limewater turns cloudy; indicator turns yellow; respirometer liquid moves towards the organisms

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Photosynthesis and respiration compared

A sealed glass bottle garden in a flat in Singapore contains soil, several small green plants and a few snails. It has been kept in a bright room for two years without ever being opened, and everything in it is still alive.
Explain how the gases in the sealed bottle stay balanced so that the plants and snails stay alive.2 marks
Large areas of tropical rainforest are being cleared for farmland, and the trees are often burned. Scientists are measuring the effect on the gases in the atmosphere. Every living organism uses oxygen in respiration, yet the oxygen level in the atmosphere has stayed at about 21% for a very long time.
Explain why the oxygen level in the atmosphere stays roughly constant even though all living organisms use oxygen.2 marks
A student investigates respiration in bean seeds using hydrogencarbonate indicator, which is red in normal air, turns yellow when the carbon dioxide level rises and turns purple when it falls. She puts 10 germinating bean seeds on a gauze platform above 10 cm³ of indicator in tube 1, and 10 boiled bean seeds on a gauze platform above 10 cm³ of indicator in tube 2. She seals both tubes with bungs and keeps them in a dark cupboard at 20 °C for four hours. Afterwards the indicator in tube 1 is yellow and the indicator in tube 2 is still red.
Explain the colour change of the indicator in tube 1.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).