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Mechanism of Photosynthesis

Updated on May 08, 2014
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I am a high school teacher in Mathematics & Science.I was appointed by GOVT of TS in INDIA. My qualifications are M.Sc.,B.Ed. My Exp:20years

Did you ever heard of Match fixing in a cricket? We know that it is wrong. But if the process of photosynthesis is viewed as a game, we see that a match fixing takes place between the factors of photosynthesis to produce the end product called the glucose/ starch, which gives life to the world of living organisms. The factors which influence the process of photosynthesis are light, water, CO2 and Chlorophyll of the leaf. Let’s take above at "the mechanism of photosynthesis," which takes place in two phases, i.e., light reaction and dark reaction. Lets watch this day, night match worth the life of the Biosphere.
Do you know that the biosphere contains more than 3,50,000 species of plants including algae, fungi, moses and higher forms of plants. All the essential requisites of life are supplied by the biosphere. These requisites include light, heat, water, food and living space.
Solar energy comprising 99.98% of the total energy supply of the biosphere keeps the biosphere going.

Photo Chemical Process:

The process by which Solar energy is transferred to molecules, where electrons of a molecule get excited by the sunlight and gets kicked out by it, and pairs with other electrons of neighbouring molecule forming electron pair bonds, which inturn create new molecules.
The most important photochemical activity is photosynthesis in plants. The light absorbed by chlorophyll molecules and by other pigments in plants is transferred to electrons in such a way as to create strong oxidents, i.e. molecules that readily remove electrons from other molecules (oxidise them) or reductants, i.e. molecules that rapidly supply electrons to other molecules (reduce them).
These oxidents and reductants that assist plants in producing carbohydrates and oxygen from molecules of CO2 and H2O. Plants release oxygen and retain carbohydrates. These carbohydrates are converted to energy and stored in the form of ATP.
ATP is the basic energy currency of all living cells. High energy phosphate bonds of ATP (Adenosine tri phosphate) contain 12,000 calories and release 7,500 calories when broken.
Now we came to know that all organic matter originates from the process of Photosynthesis.

The over all equation of photosynthesis:

6 CO2 + 12 H2O-------------> C6H12O6 + 6 O2 + 6 H2O

  • Oxygen is derived from H2O through photolysis of water.
  • Carbohydrate is formed from the carbon of CO2.

The Process of photosynthesis takes place in two phases

Name of the Phase
Other Name
Takes Place in
Light Reaction (First Phase)
Photochemical Reaction
Grana of Chloroplasts
Dark Reaction (Second Phase)
Thermo Chemical Reaction
Stroma of Chloroplasts

Photosynthesis: It is a metabolic process where light energy is converted to chemical energy. It is the source of all living matter and biological energy.
Chlorophyll and other accessory pigments are situated in the thylakoid membranes of the chloroplast. These membranes organise to form Reaction Centres called Photosystem - I and Photosystem - II.

Light Reaction:This reaction takes place in the presence of light, in the Grana region of the chloroplast. It is a photochemical reaction. Light energy is absorbed by all the pigments of the chloroplast. (Like Chlorophyll, Xanthophyll, Carotene etc). But as the other pigments cannot use this energy, they transfer the absorbed energy to the pigment chlorophyll, which is the reaction centre.
When the chlorophyll molecules absorb light energy, its electrons get excited and are displaced from their normal orbit and in a very short span of time (10-10 seconds) of this state, this energy is utilised to form ATP and NADPH2. Then the electron deficient chlorophyll performs the photolysis of water, spiltting water molecule into H+ + OH- ions. Both the systems PS I and PS II play an important role in light reaction which involves release of O2 from H2O and formation of energy rich molecules (ATP and NADPH2). This process happens due to cyclic and Non-Cyclic Photophosphorylation.
Photophosphorylation: The energy obtained in the movement of protons is used to produce ATP. Cyclic photophosphorylation, when PS I is activated by light, electrons in chlorophyll get energised and go through different steps to form ATP molecule.

Non-Cyclic Photophosphorylation: It results in the photolysis of water releasing O2
and in the formation of ATP and NADPH2 molecules, involving both PS I and PS II.
2 H2O → 2 H+ + 2 OH- 2 H+ Protons reduce NADP to form NADPH2.
2 OH- → 2 (OH) + 2 e- (these electrons are transferred to PS II)
2 OH → H2O + O
At the end of light reaction, energy is trapped in ATP and NADPH2; which are essential for reduction of carbon dioxide in the dark reaction.
The photosystems represents an aggregate of a large number of pigment molecules which are connected together by an electron transport system.

Dark Reaction: It takes place in the presence of light as well as in the absence of light; in the stroma region of the chloroplast. It is a thermo - chemical reaction. Light is not absorbed in this reaction, thus called as dark reaction. CO2 enters into the process of photosynthesis in this phase. It is used to synthesise sugars. It is a reduction reaction the ATP and NADPH2 molecules formed at the end of the light reaction are utilised in the dark reaction.
Here, CO2 -------> complex organic substances.

One molecule of CO2 is transferred to a five carbon sugar with two Phosphates Attached to it called Ribulose - 1, 5 - diphosphate which results in the formation of a six carbon sugar phosphate (it is highly unstable), which splits into 2 PGA molecules.

CO2 + RuDP → 6 Carbon compound---> (Splits)2 Molecules of 3 - Phospho glyceric acid (PGA)

This 3 - Phosphoglyceric acid undergoes a series of reactions and is converted into glyceraldehyde-3-phosphate, which is later converted into glucose.

  • 6 turns of the cycle are required to fix 6 molecules of CO2 for the net production of one glucose molecule.

Melvin Calvin

Scientist: Melvin Calvin: (April 8, 1911- January 8, 1997) He was American Chemist, most famed for discovering the Calvin Cycle. His full name is Melvin Cllis Calvin. He worked along with Andrew Benson and James Basshan. He was awarded the 1961 Nobel Prize in Chemistry.
Using the carbon-14 isotope as a tracer, Calvin and his team mapped the complete route that carbon travels through a plant during Photosynthesis.


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