Photosynthesis : mechanism

Photosynthesis : mechanism

The process of photosynthesis is divided in two phases:

Dark phase (photochemical reaction)

Light phase (chemical reaction)

Light phase: this reaction phase require light therefore also called photochemical reaction.

First the light energy is absorbed by chlorophyll. This takes place in the grana region. The chlorophyll a pigment traps the visible light in form of quanta or photons (energy). When the chlorophyll molecules are exposed to light they gets exited and emits electrons. Photons are the energy packets emitted by light. This energy is inversely proportional to its wavelength, shorter the wavelength of light higher is the energy of photon.

Light energy is converted into chemical energy and splits water molecule into hydrogen and oxygen. emitted electrons are channelled through electron transport chain in chloroplast. The photons absorbed by chlorophyll  then carries out three function:

i.                    Formation of ATP (Adenosine triphosphate) . ATP is the source of energy used during the during the dark phase of photosynthesis. ATP is formed when ADP (Adenosine diphosphate)  joins a phosphate group.

ADT + Phosphate Gr ----------à ATP

ii.                  Photolysis of water. Decomposition of water with the help of light energy  into hydrogen and oxygen is called photolysis of water

Water + energy -------à Hydrogen + oxygen

This oxygen is by product of photo synthesis.

iii.                Synthesis of NADPH (reduced nicotinamide adenine dinucleotide phosphate). Hydrogen ions released from water  joins with NADP and reduces it to NAPH. This is also used in dark reaction.

Dark phase: this is the light independent phase. It does not need light energy and thus it is completely chemical process. This reaction takes place in stroma region of chloroplast

Carbon-dioxide is reduced to carbohydrate. Carbon dioxide is reduced to glucose by hydrogen in NADPH and energy in ATP. This reaction is also called clavin cycle. Melvin clavin explained this process using radioactive carbon.

ATP and NADPH molecules together are called as assimilatory power.

RuBP  + CO₂  + NADPH  + ATP  ------à   RuBP  +  C₆H₁₂O₆    +  ADP  +  NADP  + Po₄

RuBP : ribulose biphosphate enzyme

RuBP combines with  CO₂ to make carbohydrate and regenerating RuBP.

Diagrammatic representation of photosynthesis mechanism.

  

Significance of photosynthesis

1.     Food . food is synthesized from raw material like carbon-dioxide and water.

2.     Oxygen . oxygen required for respiration and combustion processes are formed as a  by-product of photosynthesis.

3.     Fuels. Fossil fuel lie coal, oil natural gas are forms of stored solar energy synthesized by photosynthesis.