Photosynthesis (Significance and mechanism)
Although literary meaning of photosynthesis is synthesis with the help of light, this term is usually applied to a very important vital process by which green plants synthesize organic matter in the presence of light.
Photosynthesis is sometimes called carbon assimilation. It is represented by the following traditional equation:
Thus, during photosynthesis, light energy is converted into chemical energy and stored in the form of carbohydrates. One molecule of glucose (C6H12O6) contains about 686 K cal. of energy. CO2, and H20 constitute raw materials for this process.
Significance of Photosynthesis to Mankind:
(i) It maintains equilibrium of O2 in the atmosphere.
(ii) It provides food either directly as vegetables or indirectly as meat or milk of animals which feed on plants.
(iii) It provides vast reserves of energy in the form of fuel such as coal, oil, peat, wood and dung.
Chloroplasts in green plants constitute the photo- synthetic apparatus. Internally, the chloroplast is filled with hydrophilic matrix known as stroma in which are embedded grana. Each grana encloses a space called the loculus. Chlorophyll and other photosynthetic pigments are confined to grana. Thus, grana are the sites of primary photochemical reactions.
Photosynthetic pigments are of three types: (i) Chlorophylls (ii) Carotenoids and (iii) Phycobillins
Stomata are primarily meant for the diffusion of CO2, which is to be used for photosynthesis. But when CO2, diffuses in, at the same time, a large amount plants have to pay the price of photosynthesis in the term of transpiration.
Absorption and Utilization of Sunlight by Photosynthetic Pigments
(i) The chief source of light energy for photosynthesis is the sun.
(ii) Photosynthetic pigments absorb light energy only in the visible part of the spectrum.
(iii) Only about 1% of the total solar energy received by the earth is absorbed by the pigments and is utilized in photosynthesis.
(iv) Chlorophyll chiefly absorbs violet-blue and red part of the light specitrum.
(v) Carotenoids absorb light energy in blue, blue- green and green parts of the spectrum.
Mechanism of Photosynthesis
The process of photosynthesis is a complicated oxidation-reduction process resulting in the oxidation of water and reduction of CO2 The mechanism of photosynthesis consists of two parts:
I. Light reaction (hill’s reaction):
Light reaction is also known as primary photochemical reaction and always takes place in the presence of light in the grana portion of chloroplast. It can be studied under the following heads:
(i) Absorption of Light Energy:
Different chloroplast pigments absorb light in different regions of the visible part of the spectrum.
(ii) Transfer of Light Energy:
The light energy absorbed by chloroplast pigment is transferred to chlorophyll-which alone can take part in light reaction.
(iii) Activation of Chlorophyll-Molecules:
When a chlorophyll-a moleculexeceives a photon of light, it becomes an excited molecule with more energy. This excited chlorophyll-a molecule expels its energy along with an electron and a positive charge comes on it which then becomes oxidized.
2. Dark Reaction (Biosynthetic Phase):
The electron expelled by chlorophyll-a (step (iii)) after travelling through a number of electron carriers is either cycled back or is consumed in reducing NADP+ (Nicotinamide Adenine Dinucleotide Phosphate) to NADPH + H+. The extra light energy carried by the electron is utilized in the formation of ATP molecules at certain places during its transport. This process of the formation of ATP from ADP in photosynthesis is called the path of carbon and is purely enzymatic and slower than light reaction.
It takes place in the stroma portion of chloroplast and is independent of light, i.e., it can occur in the presence as well as absence of light. The conversion of CO2 to carbohydrate in dark reaction has been analysed by the use of radioactive isotope of carbon Cu (half-life 5720 years). The main credit for investigation of dark reaction in photosynthesis goes to Melvin Calvin who was awarded Nobel Prize in 1961. Dark reaction can be studied in the form of a cycle known as Calvin Cycle (C3-Pathway).
Dark reaction simply means that it does not require light energy. Light and dark reactions occur almost simultaneously. The time gap between the two is less than one thousandth of a second.
(i) CO2 is accepted by ribulose 1, 5-diphosphate (RUDP) and a 6-carbon compound is formed which is unstable and soon gets converted into 2 molecules of 3-phosphoglyceric acid.
(ii) 3-phosphoglyceric acid is reduced to 3- phosphoglyceraldehyde.
(iii) Some of the molecules of 3-phosphoglyceral- dehyde isomerise into dihydroxyacetone phosphate, both of which then unite to form fructose 1, 6-diphosphate.
(iv) Fructose 1, 6-diphosphate is converted into fructose 6-phosphate.
(v) Some of the fructose 6-phosphate is trapped off and is converted into glucose, sucrose and starch.
(vi) Some of the 3-phosphoglyceraldehyde molecules produced in step (ii) are diverted to regenerate ribulose 1, 5-diphosphate in the system as follows:
(a) 3-phosphoglyceraldehyde reacts with fructose 6-phosphate to form erythrose- 4-phosphate and xylulose-5 phosphate.
(b) Erythrose-4-phosphate combines with dihydroxyacetone phosphate to form sedoheptulose 1, 7-diphosphate.
(c) Sedoheptulose 1, 7-diphosphate loses one phosphate group to form sedoheptulose 7-phosphate.
(d) Sedoheptulose-7-phosphate reacts with 3-phosphoglyceraldehyde to form xylulose-5-phosphate and ribose-5- phosphate.
(e) Xylulose-5-phosphate is converted into another 5-C atom sugar, ribulose-5- phosphate.
(f) Ribose-5-phosphate is also converted into ribulose-5-phosphate.
(g) Ribulose-5-phosphate is finally converted into ribulose 1, 5-diphosphate and ATP, thus completing the Calvin Cycle. Since the first visible product of this cycle is 3- phosphoglyceric acid which is a 3-C compound, Calvin Cycle is also known as C-3 pathway.