Photosynthesis : Definition, Equation, Formula and Process

Published on 29-Sep-2022

Initiation of the photosynthesis proces

When a photon, the unit of light energy of sunlight, falls on the green part of the plant (mainly leaves), the chlorophyll molecule in the inner chloroplast absorbs and activates it. An electron from the double bond at the head of the active chlorophyll molecule is energized and moves from the lower ring to the higher ring of the atom.

Photosynthesis begins when the primary electron acceptor accepts the electron from the higher energy ring in the atom. 

The fate of an electron entering a high-energy ring can be any of the following three types: 

1. the loss of energy from the high energy zone and returning to the low energy zone. In this case, the absorbed energy is released as thermal energy or radiated as fluorescence. This energy is not used in photosynthesis. 

2. The absorbed energy is the transfer of the electron from the higher ring to the lower energy ring by passing an electron from a neighboring pigment molecule. And in this case, energy is transferred—not the electrons. Thus, the light energy absorbed by the antenna complex is transferred to the special chlorophyll-a in the reaction center. 

3. They can easily donate a high-energy electron from a high-energy ring to a primary electron acceptor. And in this case, the high-energy electron is transferred, and the process of photosynthesis starts. 

Mechanism of Photosynthesis 

In 1905, the English physiologist Blackman (Blackman) divided the photosynthesis process into two chapters; Namely- (a) the Light-dependent section and (b) Light neutral section. Light-dependent phase: ATP and NADPH + H+ are produced in this phase. The light-dependent phase reactions take place in the thylakoid membrane. The part of the photosynthesis process in which light energy is converted into chemical energy into ATP and NADPH + H+ is called the photo-dependent part.

Light is essential for this part. Chlorophyll molecules absorb photons of light and store energy from the absorbed photons to produce high-energy ATP. Also, in the photoperiod, HO is broken down to release O2, and NADP is oxidized to form NADPH + H'. The vast amount of energy needed to generate high-energy ATP and NADPH + H' comes from sunlight.

The process of making ATP using the energy of sunlight is called Photophosphorylation. ATP and NADPH + H are called assimilatory power because the energy of ATP and NADPH + H is used to prepare sugars through CO2 assimilation. 

Process of Photophosphorylation 

The process of making ATP using light energy in photosynthesis is called Photophosphorylation. The process of attaching a phosphate to a compound is called phosphorylation, and the use of light energy to cause phosphorylation is called Photophosphorylation. Arnon and his colleagues gave the idea of Photophosphorylation in 1957. Photophosphorylation can be non-cyclic and cyclic.

They are described below in the light of current concepts: 

Acyclic photophosphorylation

In the photophosphorylation process, photosystem-1. Electrons ejected from photosystem one are called acyclic Photophosphorylation instead of returning. Both Photosystem-I and Photosystem-II participate in this process. 

(i) Chlorophyll molecules of photosystem-II (PS-II) absorb light energy. The absorbed light energy is transferred from one molecule to another and finally reaches the reaction center P680. Reactors can send energized electrons to acceptors. 

(ii) Energized 2 electrons are ejected from the orbit of P680, which are accepted by the nearest electron acceptor, pheophytin (not shown). At the same time, 2 electrons from water breakup fill the electron deficiency of P680. 

(iii) Electron 2 from pheophytin is immediately transferred to plastoquinone (PQ). PQ is a lipid and mobile carrier. 

(iv) PQ donates its electrons to cytochrome-f (Cyt. f), which again becomes ready to accept electrons from pheophytin. The energy released in this step combines inorganic phosphate with ADP to form an ATP. (Actually, ATP is produced separately in the chemiosmotic process) 

(v) Cytochrome-F donates electron 2 to plastocyanin (PC). PC is a membrane protein. (vi) Plastocyanin (PC) donates electrons to P700 of photosystem-1 (PS-I) (because the absorption of light energy by PS-I has already ejected two energized electrons from the orbit of the chlorophyll-a molecule in the P700 reaction center and there is an electron deficiency created). 

(vii) 2 electrons ejected from P 700 accept ferredoxin (Fd). (viii) NADP reductase accepts 2 electrons from Fd. NADP reductase oxidizes NADP with two electrons (ejected from the P700 reaction center) and two protons (caused by dissociation of water) to form NADPH + H+.

ATP and NADPH + H' then participate in the Calvin cycle. The electrons ejected from PS-II move to PS-I instead of returning there. Electrons from water replace the electrons lost by photosystem II. Electrons are continuously supplied from water to PS-II during the acyclic photophosphorylation process. 

Cyclic photophosphorylation

Rough different carriers (to form an ATP) and return to the photosystem is called cyclic Photophosphorylation. Only photosystem-1 (PS-1) participates in this process. Chlorophyll molecules of photosystem-I (PS-I) are energized by absorbing light photons, which transfer this energy to the reaction center (P700).

Then two energized electrons are ejected from the P700 chlorophyll- molecule. The high-energy electron goes to ferredoxin (Fd). Later electron transfer from Fd to plastoquinone (PQ). Electron Cyt from PQ. Comes to f. At this time, ADP and Pi cooperate by the free energy of electrons.

Difference between acyclic and cyclic photophosphorylation

Matter of difference

Acyclic photophosphorylation

Cyclic photophosphorylation

1. Ejected electrons

Electrons ejected from PS-II do not return to PS-II.

Electrons ejected from PS-I are carried through various carriers and returned to PS-I.

2. photosystem

Both PS-I and PS-II participate.

Only PS-I participates.

3. need water

Water is required. Because electrons and protons of water is used in this process.

There is no need for water.

4. O2 produced

The decomposition of water produces O2 which then produces no O2 as any is released into the air in the process.

Water is not used in this process.

5. Oxidation of NADP

One molecule of NADP is oxidized to form one molecule of NADPH + H+

No NADP is oxidized here.


 Limiting factor of photosynthesis 

The various environmental factors such as CO2, light, heat, water, oxygen, etc. combine to affect the rate of photosynthesis; it also brings a difference in the plants. It is difficult to single out the effect of any of the above factors on photosynthesis from other factors. Despite this, extensive research has been done on the minimum, optimum, and maximum effects of each factor affecting photosynthesis. In this regard, in 1843, Liebig proposed the Law of Minimum.
Suppose a physiological process is controlled by more than one factor. In that case, the rate of the physiological process will be controlled by the slowest factor (lowest factor), called the limiting factor. Limiting factors can change the rate of any process that is occurring. These factors can affect the different steps of the process to slow down or speed up, depending on the factors. In 1905, Blackman proposed the 'Law of limiting factor formula' or 'Limiting factor formula' based on the 'Law of minimum.'

According to this formula, when the rapidity of a physiological process is affected by several separate factors, the speed of the process will be limited by the factor with the lowest speed. In Blackman's words, "When a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the pace of the slowest factor."

Temperature, light intensity, and CO2 concentration act as three limiting factors in the photosynthesis process that affect the rate of photosynthesis. Even a slight change in the temperature, light intensity, or CO2 concentration can cause the process of photosynthesis to become slow. Three of the factors must be in the right amount for the proper process of photosynthesis.

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