Basic process of photosynthesis

Photosynthesis is a process where by energy from light is harvested and used to drive synthesis of carbohydrates from carbon dioxide and water.

 

Physics of light

Quantum is the elemental unit of energy. Photon is a quantum of electromagnetic energy and it is particle of light.

 

Chloroplast structure

Chloroplast is a plant subcellular organelle where photosynthesis takes place. Outer membrane of chloroplast is permeable. Inner membrane encloses stroma. Thylakoid is flattened membrane surrounded vesicle. This is the place where light reaction occurs. In stroma, fluid surrounding the thylakoids is the place where the dark reaction occurs. Chlorophylls are the green pigments; they are present in the thylakoid membrane of chloroplast. They absorb deep blue and red light. Secondary light absorbing pigments are called accessory pigments. They are carotenoids and phycobilins. Photosystems are the light absorbing pigments of thylakoid membrane arranged in functional sets.

Light reaction and photophosphorylation

Photosystems are of two types: PS I P700, PS II P680. Components of photosystem I and II transfer the electrons from water to NADP. Light is absorbed by these photosystems and electrons are transferred from water to NADP. Photophosphorylation is a process where the light energy captured by the photosynthetic organisms is transformed into the phosphate bond energy of ATP. During the transfer of electrons, ATP is synthesized. Electron flow is cyclic. As a result oxygen is not released and ATP is formed as a result of proton gradient created by Cytochrome bf pump.

 

Dark reaction � Calvin cycle

Second step of photosynthesis is called Calvin�s cycle and it is a dark reaction. There are two stages here, I and II.  In Stage I carbon dioxide is incorporated into 5 carbon ribulose bisphosphate. In stage II ribulose bisphosphate is the regenerated.

 

Photorespiration

Photorespiration is the process wherein oxygen consumption occurs in illuminated temperature zone of plants under high oxygen and low carbon dioxide. Oxygen is consumed and carbon dioxide is released. Moreover, ATP is expended without benefit.

 

C4 pathway

C4 cycle is the pathway adopted by C4 plants to conserve the carbon dioxide released via photorespiration. It occurs in mesophyll cells. Carbon dioxide is incorporated to form 4 carbon oxaloacetate.