Photosynthesis | Plant Physiology

Photosynthesis- Equation, Mechanism and Types

What is Photo-synthesis

Conversion of the light energy in the chemical energy is known as Photosynthesis.


Formation of the simple organic compound (glucose) by the simple inorganic compounds (H2O and CO2), in the presence of light and pigment, and oxygen (O2) is released in this process.

Chemical reaction-

Photosynthesis Reaction
Photosynthesis- Redox Reaction

Photosynthesis is a-

  • Redox reaction.
  • Anabolic (formative) reaction, as food is formed.
  • Endergonic reaction, as energy is stored in the form of bond energy (chemical energy).

Note1st visible product in photosynthesis is Starch.

Types of Photosynthesis

Based on the release of oxygen (O2) it is of 2 types-


O2 is not released.

Water is not used as the hydrogen donor, instead of water other chemical compound is used like Hydrogen Sulphide (H2S).

This type of photosynthesis was first to appear on the earth.

Note- Non-oxygenic photosynthesis should not be misunderstood as the chemosynthesis (formation of food with the use of chemicals).

Since, in chemosynthesis source of energy, is the energy released by the breaking the chemical compounds. Ex- Nitrifying bacteria.

While, in non-oxygenic photosynthesis, light energy is used as the energy source, and the chemicals are just used as the hydrogen donor. Ex-Blue and green Sulphur bacteria.

Oxygenic Photosynthesis

O2 is released, as the water (H2O) is used as the hydrogen donor.

The first organism which appeared as the Oxygenic Photosynthate was the Cyanobacteria (BGA).

So, Blue-green algae (BGA) was the first organism that evolved the free oxygen (O2) in our environment.

History of the Photosynthesis-

Joseph Priestley (in 1770) performed a series of experiments and revealed that the air plays an essential for photosynthesis. 

But he was unable to identify that the gas was CO2 (as we know now).

Later, Ingenhousz experimented with the aquatic plant and observed that in the presence of light the bubbles of air are formed from the green part of the plant, which were absent during the night.

Later these bubbles were identified as oxygen (O2).

Also, it was identified that it was the green part that releases the oxygen (O2).

An interesting experiment was done by the T.W.Engelmann using a prism and the Cladophora (green algae).

Green Algae
Green algae magnified to show clear thallus.

He split the light into its spectral components and then the light was passed to the Cladophora, which were place in the suspension of the aerobic bacteria.

Why aerobic bacteria were used?

The growth of aerobic bacteria will be more where photosynthesis in the Cladophora is more, as more the photosynthesis more will be the oxygen evolution and that favor the growth of bacteria.

He found that the growth of the aerobic bacteria was more in the blue and red spectrum of light, hence he concluded that carbon fixation is maximum in Blue and Red light.

Hence, the first action spectrum of photosynthesis was thus described, which is necessarily Blue-Red.

A milestone contribution to understanding the process was done by Cornelius Van Neil.

He based on his studies on the purple and green Sulphur bacteria, he first identified that –

In the presence of light, hydrogen from the suitable oxidizable compound (reducing agent), reduces carbon dioxide in the carbohydrate.

Photosynthesis empirical reaction
Empirical reaction of Photosynthesis

NoteIn green plants this hydrogen donor is water.

In case of the purple and green Sulphur bacteria where H2S is used as the H2 donor, then instead of O2, sulphur or sulphate are formed


  • Light, pigment (Chlorophyll in plants), and carbon dioxide are essential for photosynthesis.
  • Green plants release oxygen (O2), during photosynthesis.
  • Photosynthesis is done in the green part of the plant (today is known as Chloroplast).
  • A hydrogen donor is required to reduce the CO2 in the carbohydrate.
  • Overall photosynthesis is a 2 step process in which-
    1. Hydrogen is released by a hydrogen donor
    2. Hydrogen reduces the CO2 in the carbohydrate.

Mechanism of the photosynthesis-

Till now we have learned what is essential for photosynthesis and what happens in it? 

But now let’s see how and where it takes place.

Photosynthesis occurs in the Chloroplast, further chloroplast is divided into 2 parts Stroma (the liquid part of chloroplast) and Grana (which absorb light energy).

Chloroplast structure
Chloroplast– labelled Grana and Stroma.


  1. Photo-oxidation of the chlorophyll molecule– Chlorophyll release the electron by absorbing the energy of the photon (light).
  2. Photolysis of water– water (H2O) hydrolises (break) in the presence of light, into O2, H+(proton), and e(electron).
  3. Formation of ATP and NADPH2.
  4. Formation of Glucose – by reduction of CO2.

The first three steps can be observed under the series of process during the presence of light, called Light reaction.

Which are followed by the physio-chemical (enzymatic) reaction called dark reaction. (the 4th step)

Points to remember-

1. The light reaction is dependent on the light.

2. It should not be understood that the dark reaction only happens in the dark that’s why it is called dark.

It is called dark because it is not directly dependent on the light, while it is dependent on the product of the light reaction, and light reaction occurs in presence of light.

Dark reaction could occur irrespective on the presence of light.
The dark reaction will be continued until the products of the light reaction are not over.

3. Oxygen formed in the light reaction is not used in the dark reaction and is released in the environment.

4. Water is not essential for photosynthesis, instead of water other compounds can also act as the hydrogen donor.

5. in green plants and oxygenic photosynthates (like BGA), water is essential.

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