Photosynthesis

Photosynthesis is an anabolic process in which light energy is used to convert carbon dioxide and water into an organic molecule (sugar). 6CO2 + 6H2O → C6H12O6 + 6O2

The organisms that use light energy to synthesise food by photosynthesis are known as photoautotrophs. Some autotrophs synthesise food, but do not use light energy. Instead, they use chemicals and so are known as chemoautotrophs.
These take place with the help of the organelle chloroplast that contains photosynthetic pigments. The chloroplasts trap the light energy.
Other than plants, various bacteria and protists (algae) also use photosynthesis to synthesise food.

Process:
The photosynthesis involves two types of reactions: The light-dependent reactions and the light-independent reactions.
The photosynthetic pigments are arranged in the form of photosystems in the chloroplast. In each photosystem, a primary photosynthetic pigment is surrounded by hundreds of accessory pigments. The photosystems are of two (2) types: Photosystem I and photosystem II. The chlorophyll a is the primary pigment and chlorophyll b and carotenoids are accessory pigments. The primary pigments are known as reaction centres and the photosystem I have p700 reaction centre, while photosystem II has p680 reaction centre. These are present in the thylakoid membranes (Fig.1).

Light-dependent reaction:

It involves two steps:
1. Photolysis of water and give hydrogen ions
2. Synthesis of ATP by phosphorylation

The light-independent reaction involves the Calvin cycle in which carbon dioxide is converted into sugar.

Excitation of chlorophyll and phosphorylation:

When a photon of light is absorbed by the pigments, an electron from the ground step in the chlorophyll pigment of photosystem I is excited and moves to a higher level. It is unstable at the higher level so, it immediately loses this energy in the form of heat and falls back to the ground state. This electron is captured by electron acceptor and is sent back to a chlorophyll molecule. This is known as cyclic phosphorylation. During this process, energy is released that is used to convert ADP into ATP by chemiosmosis.

In non-cyclic photophosphorylation, electrons are excited from both the photosystems that is, I and II. The electron from photosystem I go to NAD+ (an electron acceptor), while the electron from photosystem II is captured by electron acceptors and is sent to the photosystem I. Photosystem II receives electrons again when electrons are produced by photolysis of water.
Photolysis of water:
An enzyme that helps in the splitting of water is present in the photosystem II.
H2O → 2H+ + 2e− + 1/2 O2
The electron goes to the photosystem II. The hydrogen ion goes to the photosystem I to bind with NADP+ and forms NADPH.

Light-independent reactions

In this step, sugar is formed with the help of ATP and NADPH produced in light reaction and carbon dioxide.
It occurs in the stroma of the chloroplast as the enzyme RuBP carboxylase-oxygenase, or rubisco is present here.

Remember: Rubisco is the most abundant enzyme in the world.

Phase I: Fixation of carbon: Carbon dioxide is combined with RuBP (ribulose bisphosphate, which is a five-carbon sugar. This is done by the enzyme RuBP. This causes the formation of two molecules of glycerate 3-phosphate (GP) which is a three-carbon compound. Two molecules of GP are formed by one molecule of carbon dioxide.

Phase II: Reduction: A phosphate group from ATP is added to the GP that forms 1,3-bisphosphoglycerate. Then, 1,3-bisphosphoglycerate is reduced with the help of a pair of electrons from NADPH to form glyceraldehyde-3-phosphate (G3P).
Phase III: Regeneration of RUBP: To form one sugar molecules, three carbon dioxide molecules and 5 RUBP molecules are used. One six-carbon sugar is formed from 1 G3P and the remaining 5 G3P is used to recycle the RUBP molecules. These form three molecules of RUBP. Three ATP molecules are used in this regeneration.
This cycle is known as the Calvin cycle and is seen in C3 plants. These are known as C3 as the first molecule formed in the reaction is a three-carbon molecule. These plants have chlorophyll present in their mesophyll cells (Fig.).

Other mechanisms of carbon fixation:

1. In C4 plants, special cells known as bundle sheath cell are present along with the mesophyll cells. It is seen in plants like sugarcane and corn. The bundle sheath cells are arranged tightly packed and in between them loosely arranges mesophyll cells are present.
Step 1: It takes place in the mesophyll cells in which carbon dioxide is added to phosphoenolpyruvate (PEP) with the help of the enzyme PEP carboxylase. Oxaloacetate is formed, which is a four-carbon molecule. This type of carbon fixation is preferred when oxygen is absent in conditions such as hot or dry when stomata remain close. The Oxaloactete molecules are sent to the bundle sheath cells from the mesophyll cell for the next step.
Step 2: The carbon dioxide is released from oxaloacetate, and it is fixed by the Calvin cycle like in the C3 plants with the help of RUBP. Pyruvate us also generate which is sent to the mesophyll cells to generate PEP that can accept the carbon dioxide molecule again and the cycle can go on. ATP is used to convert pyruvate to PEP.
The mesophyll cells transport carbon dioxide in the bundle sheath cells to keep the level of carbon dioxide level high in the bundle sheath cells. This is necessary to avoid binding of RUBP with oxygen.
2. Other types of plants, known as CAM plants, are present in arid (extremely dry) conditions. They need to keep their stomata close to prevent water loss by transpiration. They open stomata during the night when sunlight is absent. They take in carbon dioxide during the night and fix it into organic acids. This process is known as CAM or Crassulacean Acid Metabolism. During the day, the light energy is used to form ATP and NADPH by the light reaction, and the carbon dioxide fixed the night before is released. This results in the formation of sugar by the Calvin cycle. This is seen in plants like Pineapple.

Factors affecting photosynthesis:

1. Light:
Photosynthesis rate increases with an increase in the intensity of the light. Excess light can inhibit it too as excess loss of water by transpiration results in the closing of the stomata. Thus, less oxygen is taken in.
2. Carbon dioxide:
The rate of photosynthesis increases with the amount of carbon dioxide taken in by plants. Excessive accumulation can reduce the photosynthesis too.
3. Temperature:
Photosynthesis is at peak at a certain temperature known as the optimum temperature. When the temperature increase, the enzymes can denature and the rate of photosynthesis will reduce.
4. Water:
Water is also necessary so, increased water can increase the rate of photosynthesis, but to a certain extent.