All living things get the energy they need to live from a chemical reaction called respiration. This process needs glucose as a starting point. First we'll see how respiration works, and then look at how plants use respiration in combination with photosynthesis.
Living cells respire. Aerobic respiration is the chemical reaction used to release energy from glucose. It is called aerobic because oxygen from the air is also needed.
Here is the word equation for aerobic respiration. Energy is put in brackets because it not a substance:
glucose + oxygen → carbon dioxide + water (+ energy)
Notice that the word equation for respiration is the reverse of the word equation for photosynthesis. Check back if you are not sure of this.
Plant cells respire, just as animal cells do. If they stop respiring, they will die. Remember that respiration is not the same as breathing, so take care - plants do not breathe.
As we can see from the word equations respiration and photosynthesis are opposites. Respiration uses oxygen and produces carbon dioxide. Photosynthesis uses carbon dioxide and produces oxygen.
So what happens to a plant overall? This depends on whether it is in the dark or the light, and how bright the light is.
Plants respire all the time, whether it is dark or light. They photosynthesise only when they are in the light.
|Conditions||Photosynthesis v respiration||Overall result|
Oxygen taken in
|Dim light||Photosynthesis rate equals respiration rate||Neither gas is taken in or given out, as each cancels the other out|
|Bright light||Photosynthesis rate greater than respiration rate||
Carbon dioxide taken in
Photosynthesis usually results in a net food gain (net glucose gain), once respiration has been accounted for. This means that there is an increase in the biomass of the plant.
Plants that lose their leaves in winter store food produced during the summer by photosynthesis. They store enough food to last them over winter, and to provide energy reserves for new growth in the spring.
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