Photosynthesis takes place in the chloroplasts of higher photosynthetic organisms like a tree. Some Bacteria however have no chloroplasts but photosynthetic membranes within their cells.
Photosynthesis requires light energy which is captured by special pigments, most notably chlorophyll. This energy is used to create energy-rich carbohydrate molecules (sugar and starch) out of carbon dioxide (CO2) and water (H2O). During this process oxygen (O2) is released as a by-product. All the oxygen in the Earth's atmosphere which is used by animals and plants for respiration, originates as a by-product of photosynthesis.
The process may be summed up in the following formula : CO2 + 2H2O >> CH2O + O2 + H2O
Photosynthesis may be subdivided into two major processes:
The light dependent reactions require sunlight so as to utilise its energy to produce the energy-rich molecules ATP and NADP. This phase of photosynthesis is also known as the light phase as it requires light energy.
The light independent reactions utilise the energy-rich molecules ATP and NADP which were produced in the light phase. The ATP and NADP are used to produce energy-rich carbohydrate molecules (simple sugars) which can be stored and used as a basis for all other forms of organic molecules. This phase of photosynthesis is also known as the Calvin cycle or dark phase as it does not require light energy.