photosynthesis is completed in two phasesLight reaction:

A. Light reaction:

1. It takes place in thylakoid disc/grana.

2. Also known as light phase or photochemical reactions or light dependent reactions or Hill’s reactions

3. Energy from sunlight is absorbed and converted to chemical energy (ATP and NADPH + H+)

4. Oxygen evolution due to oxidation of water

5. Involves PS1 and PSII

Splitting of Water/photolysis of water

1. The site of splitting of water is PSII.

2. The water splitting complex is found to be associated with the PSII, which itself is physically located on the inner side of the membrane of the thylakoid, thus the site of splitting of water is PSII.

3. The water is split into 2H+, [O] and electrons. This creates oxygen, one of the net products of photosynthesis.

4. It is also known as photolysis of water as light is required for splitting of the water molecule.

5. Manganese ion (Mg2+), chloride (Cl-) and calcium (Ca 2+) ion ion are required to carry out the photolysis of water.

6. The electrons released by photolysis of water enters the electron transport chain and this creates a proton gradient across the thylakoid membrane.

7. The breakdown of the proton gradient generates ATP.

                               2H2O              4H+ + O2 + 4e-           
   

Significance of photolysis of water

It is necessary to replace the electrons that were moved from photosystem II. This is achieved by electrons released due to splitting of water. The electrons needed to replace those removed from photosystem I are provided by photosystem II.

B. Dark reaction

1. Takes place in stroma of chloroplast

2. Also known as dark phase or dark reactions or light independent reactions or Blackmann reaction or biosynthetic phase

3. During dark phase sugars are synthesized from carbon dioxide using ATP and NADPH

4. Involves Carbon dioxide reduction/fixation

5. No photosystem involved

Calvin cycle (C3 cycle) is divided into three distinct phases :

A. Carbon Fixation: CO2 molecules are added to 5-carbon compound, Ribulose 1,5-bisphosphate (RuBP) to generate an unstable 6-carbon compound that quickly dissipates into two 3-carbon compounds, 3-phosphoglyceric acid (3-PGA).

B. Reduction (glycolytic reversal): ATP and NADPH from the light reactions are used to convert 3-PGA molecules into a carbohydrate precursor, glyceraldehyde-3-phosphate (G3P).

Initially, two ATP are used for the conversion of two molecules of 3-phosphoglyceric acid to 1-3 bisphosphoglyceric acid and two molecules of NADPH are required for the conversion of two molecules of 1-3 bisphosphoglyceric acid to 3 phosphoglyceraldehyde (3-PGAL). 3–phosphoglyceraldehyde is converted into Fructose 1-6 bisphosphate and finally to Glucose.

C. Regeneration of RuBP: Some G3P molecules are used to regenerate RuBP, ensuring the cycle continues. 1 ATP is used to recycle one molecule of RuBP.

So, to fix one carbon dioxide molecule, 3 ATP and 2 NADPH molecules are utilized. Six cycles of Calvin are needed to synthsise one molecule of glucose. So, to fix six carbon dioxide molecules 18 ATP and 12 NADPH are required. The Calvin cycle is also called the C3 cycle as the first stable compound formed is a 3 carbon compound, 3-phosphoglyceric acid.

Chemiosmotic hypothesis

1. Proposed by Peter Mitchell

2. The chemiosmotic hypothesis proposes that the ATP synthesis is linked to development of a proton gradient across the thylakoid membrane.

3. As electrons move through the electron transport chain, protons (H+) are transported across the thylakoid membrane leading to the formation of Proton gradient. Here, the proton accumulation is towards the inside of the membrane, i.e., in the lumen.

4. The accumulation of proton creates a gradient of both proton concentration and charge, resulting in generation of the proton motive force across the membrane.

5. Breakdown of this gradient leads to the synthesis of ATP. The ATP synthase enzyme consists of two parts: one called the CF0 is embedded in the thylakoid membrane and forms a transmembrane channel that carries out facilitated diffusion of protons across the membrane. The other portion is called CF1 and protrudes on the outer surface of the thylakoid membrane on the side

6. The gradient is broken down due to the movement of protons across the membrane to the stroma through the transmembrane channel of the CF0 of the ATP synthase.

7. The breakdown of the gradient provides enough energy to cause a conformational change in the CF1 particle of the ATP synthase, that leads to the formation of several molecules of energy packed ATP