Significance of Photosynthesis : It is a physico-bio chemical process of the world which is responsible for existence of life on the earth.
Photosynthesis : Plants with chlorophyll convert the solar energy into chemical energy. Plants synthesise carbohydrates using raw materials got from the surroundings. They store that chemical in the form of potential energy in carbohydrates and lead autotrophic life. This is called photosynthesis.
Anabolic Process
(a) Photosynthesis is a anabolic process.
(b) It is a constructive process.
(c) It increases the dry weight of a plant.
(d) Energy is stored as potential energy.
(e) Water, carbondioxide, etc. produce substances like glucose.
Chlroplasts
These are green plastids which help in the synthesis of organic food. Chloroplasts are found in leaves. These are ellipsoid or disc shaped. The size is from 3-10 micron in diameter.
Each chloroplast is enclosed by a double membraned envelope—two unit membranes made up of lipoproteins. The space between membranes is called periplastidal space.
Stroma is the inner membrane of the chloroplast surrounded by a large central space.
Chloroplast contains all the enzymes needed for photosynthesis.
The Quantasomes
These are also called photosynthetic units. Pigments are found in organa thylakoids. They are arranged as specific photo reaction centres. This has a number of pigment molecules required to affect a photo chemical act.
The cells of the mesophyll tissue are the main site of photosynthesis. Photosynthesis occurs in the green part of a growing stem. Plant which does not have chlorophyll cannot do photosynthesis. Chlorophyll is a green pigment.
Bacterial photosynthesis : Photosynthetic bacteria using light and chemical compound as hydrogen donor performs photosynthesis. Carbohydrate is synthesized through the process of photosynthesis.
Some Photosynthetic Bacteria
1. Green sulphur bacteria—(containing photosynthetic pigment bacterioviridin).
2. Purple sulphur bacteria—(containing photosynthetic pigment bacteriochlorophyll).
3. Purple non-sulphur bacteria—(containing pigment bacterio-chlorophyll)
Photosynthetic reaction shown by green sulphur bacteria (Chlorobium) :
6CO2 + 12H2S
(Hydrogen sulphide)
C6H12O6 + 12S + 6H2O + energy.
(glucose)
Organelle of photosynthesis—Chioroplastids : It is double membrane bound organelle present in the plant cell cytoplasm of plant.
Adaptation of Leaves for Photosynthesis
1. Leaves being flat, they present a large surface area to the light.
2. The midrib and veins of leaves are surrounded by tissues which give strength to the leaf and maintain original shape.
3. A thick cuticle is present on the upper surface and a thin cuticle on the lower surface to prevent excessive water loss from the surface area of the leaves.
4. Stomatal openings are present through which exchange of gases takes place with the atmosphere.
5. A network of veins supply water to the photosynthetic cells.
6. Intercellular air spaces allow easy diffusion of gases throughbout the whole leaf.
7. Presence of maximum number of organelle of photosynthesis.
Carbon dioxide—This gas goes through the stomata of leaves which later on enters the mesophyll tissue of leaves. CO2 also enters a plant body by means of lenticle and cuticle.
In cases of submerged aquatic plants (e.g. Hydrilla) CO2 enters the plant body by means of diffusion through the plant surface.
Atmospheric CO2 is about 0.03-0.04 percent. In the mesophyll tissue of leaf water is present. This water along with carbon dioxide form carbonic acid (CO2 + H2O = H2CO3). In the presence of sunlight the carbonic acid again forms CO2 and H2O. This CO2 is used up in photosynthetic mechanism.
Water : Plant body absorbs water by the method of osmosis from the soil.
About 1% of the total amount of water is needed during photosyn-thesis. The solar energy is transformed into chemical energy by chlorophyll molecules. The electrons of chlorophyll molecules gets excited by the sunrays and water molecule breaks up into H+ ion and OH– ion by means of light activated chlorophyll. From OH– ions oxygen is formed, later on which is also liberated out. The method of splitting up of water by means of light activated chlorophyll is called photolysis.
NADP+ (Nicotinamide Adenine Dinucleotide Phosphate) is converted into NADPH by taking H+ ion.
Chlorophyll
It is a green pigment present in called Chlorophyll in the cells of mesophyll tissue of leaves. Photosynthesis occurs within the chloroplasts.
Light : Energy required to perform photosynthesis by plants is obtained from the sun.
(i) Cyclic photophosphorylation: It takes place independent of the photolysis mechanism of water and is not accompanied by evolution of oxygen.
(ii) Non cyclic Photophos-phorylation : It is linked to both the formation of reduced pyridine nuclotide and photolysis of water. NADP+ + ADP + Pi + H2O
NADPH + ATP + ½O2 + H+
Full names of NADP, ADP, ATP, RuDP PGA, PGAId
NADP = Nicotinamide Adenine Dinucleotide Phosphate.
ADP = Adenosine diphosphate.
ATP = Adenosine triphosphate.
RuDP = Ribulose 1, 5, diphosphate.
PGA = Phosphoglyceric acid.
PGAId = Phosphoglyceral-dehyde.
(i) The water, absorbed by the root hairs, is conducted to the mesophyll cells of leaves. From these cells water diffuses into the cell organelles, chloroplasts.
(ii) The chlorophyll molecules present within the chloroplasts are activated by absorbing photons.
(iii) When water comes in contact with the excited chlorophyll molecule, it splits up into H+ and OH– ions. Splitting of H2O into H+ ion and OH– ion by means of light activated chlorophyll is called photolysis of water.
(iv) The extra energy produced by the charged chlorophyll molecules is transferred as chemical energy to another organic compound ADP (Adenosinde diphosphate) to perform the reactions of the dark phase. This step is called photophosphorylation.
ADP + Pi—ATP
(v) The H+ ion formed due to photolysis of water, is taken up by an hydrogen acceptor molecule NADP (Nicotinamide Adenine Dinucleotide Phosphate), which is then reduced to NADPH a compound rich in energy.
(vi) The NADPH is next used up in the dark phase for the reduction of carbon dioxide and finally to glucose.
(vii) The OH– ion formed during photolysis of water is converted into OH radical, which next gives rise to H2O (water) and O2 (oxygen). This oxygen (O2) is liberated in the environment and the water (H2O) formed is utilized by the plant.
Assimilatory Power of Dark Reaction
Since no light energy is involved in this process, here the energy for the performance of the several steps of reactions initiated by enzymes, comes from the two energy rich compounds produced during light reaction, i.e., ATP—Adenosine tri-phosphate and NADPH2—Reduced nicotinamide adenine dinucleotide phosphate.
The overall light reaction is—
4H2O + ADP + Pi + 2NADP—2H2O + O2 + ATP + 2NADPH + 2H+ sunlight chlorophyll.
Site of dark phase : The sight is stroma of chloroplastid.
Dark Reaction
Dark phase of photosynthesis is also called Calvin cycle, after the name of the scientist Melvin Calvin, who investigated the sequences of dark reaction and stated that RuDP the CO2 acceptor is regenerated in this cycle.
(i) The carbon dioxide gas enters into the mesophyll cells of the leaves by means of diffusion through the stomata and finally diffuses into the chloroplast.
(ii) Carbon dioxide gas is next picked up by RuDP (Ribulose 1, 5, diphosphate-a 5—Carbon compound) an acceptor molecule to form an unstable six carbon compound (6C), involving carboxylation. This compound next breaks down into 2 molecules of Phosphoglyceric acid (PGA). It is a three carbon (3C) compound, and is the first product of photosynthesis.
(iii) Phosphoglyceric acid (PGA) reacts with ATP to form 1-3, diphosphoglyceric acid which in the next step is reduced by NADPH2 to form 3—PGA (3 phosphogly-ceraldehyde) by taking hydrogen (H).
(iv) 3PGA through different intermediate products in the Calvin cycle finally reduced to glucose (C6H12O6). This reduction of atmospheric CO2 to glucose is also called Blackman’s reaction.
The complete dark reaction is—
CO2 + RuDP + NADPH + ATP ®
Glucose + NADP + ADP + Pi
(carbohydrate)
The Complete equation for photosynthesis is—
6CO2 + 12H2 O
C6H12O6 + 6O2 + 6H2O
In what form glucose is stored in the plant body?
Glucose is stored in the form of starch in the plant body.
Photosynthesis is called a redox reaction because both oxidation & reduction reactions take place.
Sources of the different elements of glucose molecule :
(Formed during photosynthesis)
Glucose (C6H12O6) molecule contain three elements, such as—Carbon, hydrogen and oxygen.
(i) Source of Carbon : Carbon of glucose is from the CO2 (Carbon dioxide) taken from air.
(ii) Source of Hydrogen : Hydrogen of glucose is from the component of H2O (water) taken from soil.
(iii) Source of Oxygen : Oxygen of glucose is from the oxygen of CO2 (Carbon dioxide) taken from air.
Storage of photosynthetic products : Photosynthetic products are stored in storage parts of the plant, such as roots, stems, leaves, fruits and seeds. The stored food is transported to different regions of plant when needed.
Transformation of glucose to starch : When glucose saturates the cells of mesophyll tissue, it has to be replaced in order to ensure the continuation of photosynthetic process.
Stored starch : Starch is stored during night. Starch formed from glucose in the mesophyll tissues of leaf is converted to soluble glucose during night and conducted to different storage regions through phloem vessel.
Solarization
The inhibition of photosynthesis due to high light intensity of light which slows down its rate is called solarization.
Photosynthesis may occur between 0OC to 45OC but the optimum temperature ranges from 25OC to 35OC. With the increase of temperature the rate of photosynthesis increases.
This is one of the most important raw materials used in photosynthesis. Air contains 0.03%—0.04% of CO2. An increase in CO2 concentration upto 0.1% approx. increases the rate of photosynthesis. Higher concentration of CO2 is, however, toxic to plants and it slows down the rate of photosynthesis.
Rate of photosynthesis is decreased with increasing concentration of oxygen.
Water is one of the raw materials utilised in photosynthesis. The rate of photosynthesis decreases in drought condition.
Chlorophyll is green pigment and becomes activated in the presence of light and splits water subsequently. Light energy captured by them is converted into chemical energy within the glucose molecule which is formed as a result of photosynthesis.
Through stomata some amount of CO2 enters the leaves. Light regulates the opening and closing stomatal aperture.
As a result of photosynthesis glucose is formed. This is transformed into starch, that is why potassium is necessary.
Photosynthesis depends on the structure and proper functioning of mesophyll tissue cells, guard cells, etc.
Protoplasm contains enzymes, coenzymes, cofactors which directly influence the rate of photosynthesis.
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