Photosynthesis and respiration

All life on this planet depends on one thing – sunlight. All the sugars which provide energy for every life form originate from the sun. Only three types of organism (plants, algae and some bacteria) are able to harness the energy from the sunlight to produce sugars. So without them, none of us would be here.

The process by which plants convert sunlight into sugars is called photosynthesis. Plants then use the sugars to power their own metabolic processes by converting them to energy through a process called respiration. Below is a brief overview of both these processes.


Key to photosynthesis is the presence of chlorophyll, a pigment which resides in the chloroplasts within plant cells. Chlorophyll is green, which is why most leaves are green (other pigments within the leaf are red, orange or yellow and these are sometimes more numerous, giving a different leaf colour, or show through when the chlorophyll pigments break down in autumn, hence the leaves changing colour). The primary sites for photosynthesis are on the leaves, although chloroplasts are generally also quite numerous in external stem cells.

The process of photosynthesis uses light, water and carbon dioxide in a series of two reactions; light reaction (which can only happen when light is available) and carbon fixation (which doesn’t require light). The net result of photosynthesis is the production of sugars (either in the form of glucose to be used, sucrose to be transported to other parts of the plant or starch to be stored) and the release of oxygen.

A summary of the inputs and outputs of photosynthesis is:

3CO2 + 6H2O + light → C3H6O3 + 3O2 + 3H2O


This process uses the sugars produced by photosynthesis and converts them into energy which the plant can use (this energy is contained within molecules called ATP – adenosine triphosphate). Oxygen is used in these reactions and, as well as charged up ATP molecules, the process releases water and carbon dioxide.

A summary of the inputs and outputs of respiration is:

C6H12O6 + 6O2 → 6CO2 + 6H2O + energy