Also on this page: The carbon Cycle

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All living organisms, animals and plants,  carry out respiration
Respiration is a chemical reaction that provides all life with energy

There are two kinds of respiration: aerobic respiration which requires oxygen and anaerobic respiration which does not..

Aerobic respiration

Normal (aerobic) respiration requires oxygen and glucose (a kind of sugar)

Aerobic respiration is a chemical reaction that takes place in the mitochondria of all living cells and provides the cells with the energy needed for all of the processes of life.

The sugar reacts with the oxygen releasing energy.
Carbon dioxide and water which are released as waste products

Word equation:
glucose + oxygen  → carbon dioxide + water + energy

Respiration is the exact reverse of photosynthesis.

Respiration and photosynthesis are both chemical reactions and have similar equations.
Learn them both carefully but DO NOT MUDDLE THEM UP.
See “Words and facts often muddled

Anaerobic respiration

Anaerobic respiration does not need oxygen. It takes place when there is not enough oxygen for normal  aerobic respiration.
Anaerobic respiration does not release as much energy as aerobic respiration

In animals it takes place when energy is used very rapidly.
In humans it might be when the muscles are exerting themselves a lot more than usual (sprinting or weight lifting)
In other animals when they are escaping from a predator.

Word equation for anaerobic respiration in animals
glucose  → lactic acid + energy

Anaerobic respiration also takes place in some plant cells and micro-organisms such as yeast.

Word equation for anaerobic respiration in yeast
glucose  → ethanol + carbon dioxide + energy

The process we call fermentation is really just the process of anaerobic respiration and happens in the making of beer or wine.
The sugars in the grape are turned into alcohol (ethanol) by the yeast carrying out anaerobic respiration.

The balance between respiration and photosynthesis in plants

All living organisms contain carbon. Most of the molecules in our body are based on carbon atoms.

Most of the food we eat contain carbon.


Important reactions involving carbon and carbon dioxide:

Uses up oxygen. Releases carbon dioxide into the atmosphere
glucose + oxygen → carbon dioxide + water

Uses up Carbon dioxide. Releases oxygen into the atmosphere
carbon dioxide + water → glucose + oxygen


Plants Overall effect
In the light Photosynthesis is carried out at a faster rate than  respiration Carbon dioxide is taken out of the atmosphere.
Dark Respiration but no photosynthesis Carbon dioxide added to the atmosphere


Animals carry out respiration all the time adding carbon dioxide to the atmosphere.

The effect of humans
Burning fossil fuels (coal, oil and gas)  add carbon dioxide to the atmosphere.
carbon +  oxygen → carbon dioxide

Burning forests has two effects: it adds carbon dioxide to the atmosphere and reduces the number of plants which can carry out photosynthesis
The diagram below shows how easy it is to produce a lot more carbon dioxide than is good for the planet.
The reactions above can all be summarized in a diagram called the carbon cycle