What is Photosynthesis ?

Photosynthesis is the method by which plants trap radiant energy from the sun and convert the energy into a biochemical form. This biochemical energy is then used to build internal structures and organs, and is also available to organisms which may consume the plants. Photosynthesis also takes up carbon dioxide waste produced respiration and releases oxygen. The chemical processes of photosynthesis can be summarized by the following reaction:

6 CO₂ +12 H₂O + sunlight and chlorophyll --> C₆H₁₂O₆ + 6 H₂O + 6 O₂

The energy-building reactions of photosynthesis that depend upon photons absorbed from sunlight are the light reactions. A separate pathway of endergonic processes called the dark reactions uses high-energy compounds and H+ produced by the light reactions to synthesize glucose. These reactions occur in a cycle called the Calvin cycle, named after the botanist Melvin Calvin. Dark reactions can occur in the light, but do not require light to proceed.

Photosynthesis in plant cells takes place in organelles called chloroplasts, which contain pigments that absorb light energy. Chlorophyll, one of the most important pigments in plants, absorbs light most strongly in the red and blue parts of the light spectrum, producing more photosynthetic activity in those colors (wavelengths). Different types of chlorophyll absorb light of different wavelengths or colors of sunlight, an adaptation to the plant's environment. There are four main types of chlorophyll, named a, b, c, and d, in higher organisms, and yet another type in prokaryotes. The a and b types are the most common in higher plants.

Plants also contain accessory pigments that enable them to absorb a wider spectrum of light. Phycobilins, found in blue-green bacteria and red algae, absorb short wavelengths of light that penetrate water and so enable plants to survive at greater depths. Carotenoid pigments such as b-carotene also absorb photons in blue and blue-green wavelengths, extending the photosynthetic energy-trapping capacity of plants.

Like mitochondria, chloroplasts contain electron transport systems and use chemiosmosis to produce chemical energy in the form of ATP. However, chloroplasts convert light energy from the sun into chemical energy, while mitochondria simply transfer chemical energy from food.