-Carbon dioxide is taken in by plants in order to carry out photosynthesis and make food.
-The carbon from the carbon dioxide is used to make carbohydrates, fats and proteins that make up the plant body. For example the cell wall of plants is made from cellulose - a carbohydrate.
-Some of this carbon is returned into the atmosphere as carbon dioxide when the plant respires. When these plants are eaten by other animals this carbon becomes part of the proteins and the fats that make up their bodies.
-When these animals respire some of this carbon becomes carbon dioxide and is released back into the atmosphere.
-When plants and animals die micro-organisms feed on their body. In order to do so these micro-organisms have to respire to produce energy. As a result they give out carbon dioxide as they respire
-Also the combustion of fossil fuels releases carbon dioxide back into the atmosphere as they contain locked up carbon compounds.
You could be asked for the equation for photosynthesis:
Carbon Dioxide + Water ------> Glucose + Oxygen
OR the equation (balanced) for respiration
Glucose + Oxygen ------> Carbon dioxide + water + energy
c6h12o2 + 6o2 -------> 6co2 + 6H20 + energy
ALSO why is more carbon dioxide been given out to the atmosphere than taken in? As a result of deforestation less carbon dioxide is being taken in by plants for photosynthesis for food. Also due to the combustion of fossil fuels more and more carbon dioxide as given out and there are less trees available to remove this from the atmosphere due to deforestation.