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.
