Animals eat plants, and live on the energy that plants capture by Photosynthesis. If you have not read the photosynthesis section, turn to it now, as it will help you to understand this page on respiration.

     The process by which animals make use of food is called respiration. By this process the animals free the energy captured by plants and other photosynthesizing organisms. They use this energy for maintaining their bodies, breathing, digesting, moving, growing, and reproducing.

      In the photosynthesis section we saw how plants build themselves by capturing the carbon in carbon dioxide.  They use the carbon to serve as a building material. Plants take in carbon dioxide gas, which is made up of molecules containing one carbon atom and two oxygen atoms. The plants make use of the carbon atoms to make a sugar, glucose, and let the oxygen molecules, O2 , escape into the air. Plant tissues are classified as carbohydrates because they are made up of a good proportion of various sugar molecules containing carbon atoms.

However, animals are made up of mostly of proteins. Proteins also contain carbon, but they are made up of molecules of amino acids. When animals eat plants, they take in more carbon atoms than they need to build themselves. So what do they do with the extra carbon? They burn it! Yes!

Adding oxygen to carbon very quickly is what happens when wood or paper or grass burns in a fire. Animals combine carbon and oxygen more slowly, and at a lower temperature, so they do not burst into flames.

Now we see why animals breathe in oxygen (to combine with the extra carbon atoms) and breathe out carbon dioxide. The carbon dioxide molecules go out into the atmosphere -- and guess what! The plants use them for photosynthesis! It all works together!

Here is a diagram of the respiration process.

The atoms are:

Does this look familiar? It looks like the photosynthesis diagram, but the reaction is going in the other direction now. The animal eats the plant, and digestion frees up the glucose molecule. The animal has been drinking water, and so has the 6 water molecules, H₂0 , available. By breathing, the animal has oxygen in its bloodstream.

Here we see an animal respiring. You can see that it has an enlarged picture of the glucose molecule in its stomach.

On the left you see the 6 water molecules that the animal needs: they are red and yellow in the picture.

On the right you can see the 6 oxygen molecules that it needs -- the blood brings them from the lungs.

The animal puts together the glucose molecule, the water molecules, and the oxygen molecules.

Energy is released!

You can see the animal breathing out the 6 carbon dioxide molecules and the 12 water molecules that are the results of the chemical processes in respiration. In real life the animal probably uses some of the water molecules in its body instead of exhaling them.

The picture does not show the energy that is released and used by the animal. We know that it is there, though, because the animal is alive and looking pretty frisky!

Things to Remember:

Plants use carbon dioxide, water, and sunlight to make glucose.

Plants release water and oxygen that animals breathe in.

Plants build themselves with carbon from the carbon dioxide in the air.

Animals use oxygen, water, and glucose to release the stored energy in plants.

Animals breathe out water and carbon dioxide molecules that the plants use.

The system is dependent on solar energy to power photosynthesis.