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Saturday 12 June 2021

12 Photosynthesis -

Part 01 - Chloroplasts

Photosynthesis :

It is the only process on earth by which solar energy is trapped by green plants and converted into food.
Defination : "Synthesis of carbohydrates ( glucose) from inorganic materials like CO2 and H2O with the help of solar energy trapped by pigments like chlorophyll."

Final light energy trapping process on which all life ultimately depends.
Most massive chemical processes going on earth.
Atmosphere contains only about 0.03 percent carbon dioxide by volume. This small percentage represents 2200 billion tons of CO2 in the atmosphere.
The oceans contain over 50 times by amount of atmospheric CO2 in the form of dissolved gas or carbonates.
From these two sources, about 70 billion tons of carbon is fixed by the green plants annually.

Chloroplasts :

Mainly located in the mesophyll cells of leaves.
The CO2 reaches them through the stomata and water reaches them through veins.
In higher plants, the chloroplasts are discoid or lens-shaped.
Each chloroplast is bounded by double membrane.
Inside the membranes is found a ground substance, the stroma.
Inside the stroma is found a system of chlorophyll bearing double membrane sacs or lamellae.
These are stacked one above the other to form grana (singular, granum).
Individual sacs in each granum are known as thylakoids.
All the pigments

chlorophylls
carotenes and
xanthophylls are located in the thylakoid membranes.

These pigments absorb light of a specific spectrum in the visible region.
The pigments are fat soluble and located in the lipid part of the membrane.
With the help of certain enzymes, they participate in the conversion of solar energy into ATP and NADPH.
The enzymes of stroma utilize ATP and NADPH to produce carbohydrates.

All photosynthetic plants have these pigments that absorb light between the red and blue region of the spectrum.

Carotenoids found mainly in higher plants absorb primarily in the violet to blue regions of the spectrum.
They not only absorb light energy and transfer it to chlorophyll but also protect the chlorophyll molecule from photo-oxidation.

Part 02 - Nature of Light

Nature of Light :

Form of energy.
Travels as stream of tiny particles called photons.
A photon contains a quantum of light.
Has different wavelengths having different colors.
Electromagnetic radiation with wavelengths ranging from 390nm to 730nm. This part of the spectrum is called the Visible light.
It lies between wavelengths of ultraviolet and infra-red.

Absorption and Action spectrum :

Pigments absorb light quanta or photons and transfer the absorbed energy to chlorophyll a.
The amount of light absorbed at each wavelength can be shown in the form of a graph.
It shows different curves at different wavelengths.
Such a curve which shows the amount of light absorbed at each wavelength is termed as Absorption spectrum.
The absorption spectrum of chlorophyll a and b clearly shows that more light energy is absorbed at

blue
violet and
red wavelengths of the visible spectrum.

The relative rate of photosynthesis at different wavelengths indicates close relationship with absorption spectrum of chlorophyll a and b.
This curve that shows the rate of photosynthesis at different wavelengths is called Action
Spectrum.
Action spectrum of photosynthesis differs from the absorption spectrum.
There is quite a lot of photosynthetic activity even in parts of the spectrum where chlorophyll a absorb little light.
This infers that the light energy absorbed by other pigments (yellow and orange carotenoids and also other forms of chlorophyll) is transferred to chlorophyll a.

Part 03 - Mechanism of Photosynthesis

Mechanism of Photosynthesis :

In 1931, Van Neil proved that bacteriaused H2S and CO2 to synthesize carbohydrates as follows :
This led Van Neil to postulate that in green plants, water is utilized in place of H2S and O2 is evolved in place of sulphur.
Ruben (in 1941) confirmed it in Chlorella.
He used water labeled with heavy oxygen (O2 having atomic mass 18 ) i.e. H2O.
The oxygen evolved contain 18O2 thereby proving Van Neil’s hypothesis that oxygen evolved in photosynthesis comes from water.
This leads to the currently accepted general equation of photosynthesis -

Hill reaction:

In 1937, R. Hill demonstrated that isolated chloroplasts evolved oxygen when they were illuminated in the presence of a suitable electron acceptor such as ferricyanide.
Ferricyanide is reduced to ferrocyanide by photolysis of water. This is called Hill reaction.
Thus Hill reaction proves that :

In photosynthesis, oxygen is released from water.
Electrons for the reduction of CO2 are obtained from water.

According to Arnon

In this process light energy is converted to chemical energy.
This energy is stored in ATP and NADPH is formed as hydrogen donor. This ATP formation is known as photophosphorylation.

In modern concept:

The process of photosynthesis is an oxidation and reduction process in which water is oxidized (to releaseO2) and CO2 is reduced to form sugar.
It consists of two successive series of reactions.

Light or Hill reaction - photochemical reaction
Dark or Blackman reaction - biochemical reaction.

Part 04 - Light reaction

Light reaction :

In light reaction, solar energy is trapped by chlorophyll and stored in the form of chemical energy as ATP and in the form of reducing power as NADPH2.
Oxygen is evolved in the light reaction by splitting of water.
When a photon is absorbed by chlorophyll molecule, an electron is boosted to higher energy level.
To boost an electron, a photon must have a certain minimum quantity of energy called quantum energy.
A molecule that has absorbed a photon is in energy rich excited state.
When the light source is turned off, the high energy electrons return rapidly to their normal low energy orbitals as the excited molecule reverts to its original stable condition, called the ground state.

Reaction centre :

The light absorbing pigments are located in the thylakoid membranes.
They are arranged in clusters of chlorophyll and accessory pigments along with special types of chlorophyll molecules P680 and P700 (the letter P stands for Pigment and 680 and 700 for the wavelengths of light at which these molecules show maximum absorbance).
P680 and P700 molecules form the Reaction centers or Photocenters.
The accessory pigments and other chlorophyll molecule harvest solar energy and pass it on the reaction centers. These are called Light harvesting or Antenna molecule.
They function to absorb light energy, which they transmit at a very high rate to the reaction center where the photochemical act occurs.

Photosystems I and II :

The thylakoid membranes of chloroplasts have two kinds of photosystems, each with its own set of light harvesting chlorophyll and carotenoid molecules.
Chlorophyll and accessory pigments help to capture light over larger area and pass it on to the photocenters.
Thus, a photon absorbed anywhere in the harvesting zone of a P680 center can pass it energy to the P680 molecule.
The cluster of pigment molecules which transfer their energy to P680 absorb at or below the wavelength 680nm.
Together with P680 they form Photosystem-II or PS-II .
Likewise, P700 forms Photosystem-I or PS-I along with pigment molecule which absorbs light at or below 700nm.

Photosystem II :

This system brings about photolysis of water and release of oxygen.
In this act, when PS-II absorbs light, electrons are released and chlorophyll molecule is oxidized.
The electrons emitted by P680 (PS-II) are ultimately trapped by P700 (PS-I).
Oxygen is given out as byproduct by the photosynthesizing plants.
Protons (H+) accumulate inside the thylakoid resulting in a Proton gradient.
The energy released by the protons when they defuse across the thylakoid membrane into the stroma against the H+ concentration gradient is used to produce ATP.

Photosystem I :

When light quanta are absorbed by photosystem I (P700), energy rich electrons are emitted from the reaction center.
These flow down a chain of electron carriers to NADP along with the proton generated by splitting of water.
This result in the formation of NADPH.
Hydrogen attached to NADPH is used for reduction of CO2 in dark reaction. This is also called Reducing power of the cell.

Part 05 - Photophosphorylation

Photophosphorylation :

Formation of ATP in the chloroplasts in presence of light is called photophosphorylation.
It takes place in the two forms.

Cyclic photophosphorylation
Non-cyclic photophosphorylation

i. Cyclic photophosphorylation :

Illumination of photosystem-I causes electrons to move continuously out of the reaction center of photosystem-I and back to it.
The cyclic electron-flow is accompanied by the photophosphorylation of ADP to yield ATP. This is termed as Cyclic photophosphorylation.
Since this process involves only pigment system I, photolysis of water and consequent evolution of oxygen does not takes place.

ii. Non-cyclic photophosphorylation :

It involves both PS-I and PS-II photosystems.
In this case, electron transport chain starts with the release of electrons from PS-II.
In this chain high energy electrons released from PS-II do not return to PS-II
But after passing through an electron transport chain, reach PS-I, which in turn donates it to reduce NADP+ to NADPH.
The reduced NADP+ (NADPH) is utilized for the reduction of CO2 in the dark reaction.
Electron-deficient PS-II brings about oxidation of water-molecule.
Due to this, protons, electrons and oxygen atom are released.
Electrons are taken up by PS-II itself to return to reduced state, protons are accepted by NADP+ where as oxygen is released.
As in this process, high energy electrons released from PS-II do not return to PS-II and it is accompanied with ATP formation, this is called Non-cyclic photophosphorylation.
Thus, during the photochemical reactions,

photolysis of water takes place
O2 is released and
ATP and NADPH are synthesized.

ATP and NADPH molecules function as vehicles for transfer of energy of sunlight into dark reaction leaving to carbon fixation.
In this reaction CO2 is reduced to carbohydrate.

The light reaction gives rise to two important products :

A reducing agent NADPH and
An energy rich compound ATP.

Both these are utilized in the dark phase of photosynthesis.

Part 06 - Dark reaction

Dark reaction :

Carbon fixation occurs in the stroma by a series of enzyme catalyzed steps.
Molecules of ATP and NADPH produced in the thylakoids (light reaction) come in the stroma where carbohydrates are synthesized.
The path of carbon fixation in dark reaction through intermediate compounds leading to the formation of sugar and starch was worked out by Calvin, Benson and their co -workers.
For this, Calvin was awarded Nobel Prize in 1961.
Path of carbon was studied with the help of radioactive tracer technique using Chlorella, a unicellular green alga and radioactive 14CO2.
With the help of radioactive carbon, it becomes possible to trace the intermediate steps of fixation of 14CO2.

Various steps in the dark reactions / Calvin cycle / C-3 pathway are as follows:

Carboxylation
Glycolytic Reversal
Regeneration of RuBP

1. Carboxylation

CO2 reduction starts with a 5-carbon sugar, ribulose-1,5-bisphosphate (RuBP).
It is a 5-carbon sugar (pentose) with two phosphate groups attached to it.
RuBP reacts with CO2 to produce a short - lived 6-carbon intermediate in the presence of an enzyme RuBP carboxylase or Rubisco and immidiately splits into 3-carbon compound, 3-phosphoglyceric acid (3-PGA).
Rubisco is a large protein molecule and comprises 16% of the chloroplast proteins.

2. Glycolytic Reversal :

Molecules of 3-PGA form 1,3-diphosphoglyceric acid utilizing ATP molecules.
These are reduced to glyceraldehyde- 3-phosphate (3-PGAL) by NADPH supplied by the light reactions of photosynthesis.
For the Calvin cycle to run continuously, there must be sufficient amount of RuBP which accepts CO2 and a regular supply of ATP and NADPH.
Out of each of 12 molecule of 3-phosphoglyceraldehyde (3-PGAL), 2 molecules are used for synthesis of one glucose molecule.
Remaining 10 molecules are used for regeneration of 6 molecules of RuBP.

3. Regeneration of RuBP :

Through a series of complex reactions, 10 molecules of 3-PGAL are used for regenration of six molecules of RuBP at the cost of 6 ATP.
For this purpose, six turns of Calvin cycle are needed to be operated so that a molecule of glucose can be synthesized.
Plants form a variety of organic compounds required for its structure and function through these complex reactions.
Thus, for every 6 molecules of CO2 and Ribulose-1, 5-biphosphate used, 12 molecules of 3-phosphoglyceraldehyde are produced.
Out of these 12 molecules, only two are utilized for the formation of a molecule of glucose.
The other 10 molecules are converted into ribulose-1, 5-biphosphate which combines with fresh CO2.
Thus, the Calvin cycle regenerates ADP and NADP required for the light reaction.

1. Light Reaction (in granum) :

2. Dark reaction (in stroma) :

Part 07 - Photorespiration

Photorespiration / PCO cycle. :

Photorespiration occurs under the conditions like

high temperature
bright light
high oxygen and
low CO2 concentration.

It is a wasteful process linked with C3-Cycle, where instead of fixation of CO2 it is given out.
It involves three organelles chloroplast peroxisomes and mitochondria and occurs in a series of cyclic reactions which is also called PCO cycle.
Enzyme Rubisco acts as oxygenase at higher concentration of O2 and photorespiration begins.
When RuBP reacts with O2 rather than CO2 to form a 3-carbon compound (PGA) and 2-carbon compound phosphologycolate.
Later is converted to glycolate which is shuttled out of the chloroplast into the peroxisomes.
In peroxisomes, enzyme glycolate oxidase converts glycolate into glyoxylate, which is converted into amino acid glycine by transamination.
In mitochondria, two molecules of glycine are converted into serine (amino acid) and CO2 is given out.
Thus, it looses 25% of photosynthetically fixed carbon.
Serine is transported back to peroxisomes and converted into glycerate.
It is shuttled back to chloroplast to undergo phosphorylation and utilized in formation of 3-PGA, which get utilized in C3 pathway.

Part 08 - C4 pathway

C4 pathway or Hatch-Slack pathway :

M. D. Hatch and C. R. Slack while working on sugarcane found four carbon compound (dicarboxylic acid) as the first stable product of photosynthesis.
It has been found to occur in tropical and sub-tropical grasses and some dicotyledons.
Some of the important plants are sugarcane, maize, Sorghum etc.
The plants in which CO2 fixation takes place by Calvin cycle are called C3 plants, because first product of CO2 fixation is a 3-carbon phosphoglyceric acid.
But in Hatch- Slack pathway, first product of CO2 fixation is a 4-carbon compound, oxaloacetic acid. Hence such plants are called C4 plants.
Anatomy of leaves of C4 plants is different from leaves of C3 plants.
C4 plants show Kranz anatomy.
In the leaves of such plants, palisade tissue is absent.
There is a bundle sheath around the vascular bundles.
The chloroplasts in the bundle - sheath cells are large and without or less developed grana.
The chloroplasts in the mesophyll cells the chloroplasts are small but with well-developed grana.

In mesophyll cell :

CO2 taken from atmosphere is accepted by a 3-carbon compound, phosphoenolpyruvic acid in the chloroplasts of mesophyll cells, leading to the formation of 4-C compound, oxaloacetic acid with the help of enzyme pepco.
It is converted to another 4-C compound, the malic acid.
It is transported to the chloroplasts of bundle sheath cells.

In bundle sheath cell :

Here, malic acid (4-C) is converted to pyruvic acid (3-C) with the release of CO2 in the cytoplasm.
Thus concentration of CO2 increases in the bundle sheath cells.
Chloroplasts of these cells contain enzymes of Calvin cycle.
Because of high concentration of CO2, RuBP carboxylase participates in Calvin cycle and not photorespiration.
Sugar formed in Calvin cycle is transported into the phloem.

In mesophyll cell again :

Pyruvic acid generated in the bundle sheath cells re-enters mesophyll cells and regenerates phosphoenolpyruvic acid by consuming one ATP.
Since this conversion results in the formation of AMP (not ADP), two ATP are required to regenerate ATP from AMP.
Thus C4 pathway needs 12 additional ATP.
The C3 pathway requires 18 ATP for the synthesis of one glucose molecule, whereas C4 pathway requires 30 ATP.
Thus C4 plants are better photosynthesizers and there is no photorespiration in these plants.

Part 09 - CAM-Crassulacean Acid Metabolism

CAM-Crassulacean Acid Metabolism:

It is one more alternative pathway of carbon fixation found in desert plants.
It was first reported in the family Crassulaceae, so called as CAM (Crassulacean Acid Metabolism).
In CAM plants, stomata are scotoactive i.e. active during night, so initial CO2 fixation occurs in night.
Thus C4 pathway fix CO2 at night and reduce CO2 in day time via the C3 pathway by using NADPH formed during the day.
PEP caboxylase and Rubisco are present in the mesophyll cell (no Kranz anatomy).
Formation of malic acid during dark is called acidification (phase I).
Malate is stored in vacuoles during the night.
Malate releases CO2 during the day for C3 pathway within the same cell is called deacidification (phase II).
Examples of CAM plants : Kalanchoe, Opuntia, Aloe etc.
The Chemical reactions of the carbon di-oxide fixation and its assimilation are similar to that of C4 plants.

Part 10 - Factors affecting Photosynthesis

Factors affecting Photosynthesis :

Like all other physiological processes, photosynthesis is also influenced by a number of factors.

A. External Factors :
Light :

It is an essential factor as it supplies the energy necessary for photosynthesis.
Both quality and intensity of light affect photosynthesis.
Highest rate of photosynthesis takes place in the red rays and then come the blue rays.
In a forest canopy the rate of photosynthesis decreases considerably in plants growing under the it.
In most of the plants, photosynthesisis maximum in bright diffused sunlight.
It decreases in strong light and again slows down in the light of very low intensity.
It has also been found that uninterrupted and continuous photosynthesis for relatively long periods of time may be sustained without any visible damage to the plant.

Carbon dioxide :

The main source of CO2 in land plants is the atmosphere, which contains only 0.3% of the gas.
Under normal conditionsof temperature and light, carbon dioxide acts as a limiting factor in photosynthesis.
An increase in concentration of CO2 increases the photosynthesis.
The increase in CO2 to about 1% is generally advantageous to most of the plants.
Higher concentration of the gas has an inhibitory effect on photosynthesis.

Temperature :

Like all other physiological processes, photosynthesis also needs a suitable temperature.
In the presence of plenty of light and carbon dioxide, photosynthesis increases with the rise of temperature till it becomes maximum.
After that there is a decrease or fall in the rate of the process.
The optimum temperature at which the photosynthesis is maximum is 25 – 30 degree Celsius.
Though in certain plants like Opuntia, photosynthesis takes place at as high as 55 degree Celsius . This is known as the maximum temperature.
The temperature at which the process just starts is the minimum temperature.

Water :

Being one of the raw material, water is also necessary for the photosynthetic process.
An increase in water content of the leaf results in the corresponding increase in the rate of photosynthesis.
Thus the limiting effect of water is not direct but indirect.
It is mainly due to the fact that it helps in maintaining the turgidity of the assimilatory cells and the proper hydration of their protoplasm.

B. Internal Factors :

Though the presence of chlorophyll is essential for photosynthesis but the rate of photosynthesis is proportional to the quantity of chlorophyll present.
It is because of the fact that chlorophyll merely acts as a biocatalyst and hence a small quantity is quite enough to maintain the large bulk of the reacting substances.
The final product in the photosynthesis reaction is sugar and its accumulation in the cells slow down the process of photosynthesis.
Factors which also affects the rate of photosynthesis are -

Thickness of cuticle and epidermis of the leaf
Size and distribution of intercellular spaces
Distribution of the stomata
Development of chlorenchyma
other tissues

Blackman’s law of limiting factors :

The Blackman’s law of limiting factors states that -when a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is controlld by the pace of the “slowest factor”.
The slowest factor is that factor which is present in the lowest or minimum concentration in relation to others.
The law of limiting factor can be explained by taking two external factors such as carbon dioxide and light.
Suppose a plant photosynthesizing at a fixed light intensity sufficient to utilize 10mg of CO2 per hour only.
On increasing the CO2 concentration, the photosynthetic rate also goes on increasing.
Now, if the CO2 concentration is further increased, no increase in the rate of photosynthesis.
Thus in this case light becomes the limiting factor.
Under such circumstances, the rate of photosynthesis can be increased only by increasing the light intensity.
This evidently shows that the photosynthetic rate responds to one factor alone at a time and there would be a sharp break in the curve and a plateau formed exactly at the point where another factor becomes limiting.
If any one of the other factors which is kept constant (say, light) is increased, the photosynthetic rate increases again reaching and optimum where again another factor become limiting.

Significance of photosynthesis :

This anabolic process uses inoganic substances and produces food for all life directly or indirectly.
This process transforms solar energy into chemical energy.
The released by product O2 is necessary not only for aerobic respiration in living organisms but also used in forming protective ozone layer around earth.
This process is also helping us in providing fossil fuels, coals, petroleum and natural gas.

Source from Internet

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