Photosynthesis (as a means of autotrophic nutrition) is the formation of carbohydrates from CO2 and H2O with the help of sunlight in the green parts of plants (i.e., chlorophyll). It is an hghanabolic process.
Photosynthesis and its significance, mechanisms and the factors that Affecting it and its all Pigments are given below:
DEFINITION OF PHOTOSYNTHESIS:
All green plants are autotrophic. They can synthesise their own food. All heterotrophs directly or indirectly depend upon green plants for food. Food, here, means carbohydrates. All other nutritions such as protein, fats, etc. are synthesised later. The primary requirement of all organisms is the energy for work. Carbohydrate is the only source from where energy is liberated. This energy is used in biosynthesis of all other metabolites in plants. Synthesis of carbohydrates by plants is therefore the single most important metabolic process in the living world. The metabolic process by which the green plants synthesise carbohydrates is called photosynthesis.
Photosynthesis (photo = light, synthesis = putting together) may be defined as an anabolic process in which green plants synthesise carbohydrates from substances like carbon dioxide, water and some minerals with the help of chlorophyll, in the presence of sunlight and in which oxygen is liberated as by-product.
SIGNIFICANCE OF PHOTOSYNTHESIS:
1. Food production. All living bodies in the earth directly or indirectly depend upon carbohydrate as source of food.
2. Energy supply. Oxidation of carbohydrate generates energy which the living organisms utilize for performing various metabolic functions. Carbohydrate which is the only source of energy for hvmg organisms is produced through photosynthesis. Wood, coal, petrol which are used as sources of energy for transport, domestic and industrial purposes are the products of photosynthesis in the past. These are called fossil fuel.
3. Regulation of environment. All organisms liberate CO2 during respiration. Combustion of coal, oil; wheathering of rocks also produce C02. But CO2 balance remains the same more or less. This is because green plants utilize CO2 in photosynthesis and liberate O2 which the living organisms use in respiration. This is how the natural processes maintain gaseous balance of the atmosphere. Destruction of green belt and marine weeds may cause accumulation of CO2 which in turn may increase the world temperature with accompanying catastrophic results.
★The plants exhibit autotrophic mode of nutrition due to the process of photosynthesis.
★Ruben and Kemen (1941) proved that the source of liberated oxygen (O2) in photosynthesis is water (H2O) and not the carbon dioxide (CO2 ).
★The source of oxygen in carbohydrate produced through Photosyñthesis is carbon dioxide (CO2 ). In green plants. water (H2O) is the hydrogen donor and is oxidised to oxygen (O2), whereas in purple and green sulphur bacteria, H2S is the hydrogen donor and sulphur or sulphate is the oxidation product. Photosynthesis was first appeared in cyanobacteria.
MECHANISM OF PHOTOSYNTHESIS:
Photosynthesis occurs in two steps. The primary reactions of photosynthesis depend upon the availability of light and therefore it is called the light reactions or photochemical reactions. The second set of reactions utilize the product of the light reactions in which light is not essential. In the second step CO2 fixation takes place. Therefore the second set of reactions are called dark reactions or thermochemical reactions.
In light reactions two important products (NADPH2 and ATP) are formed. In the second step, CO2 is fixed with the help of the products of the first steps of reactions and no light is essential there. Chemically the photosynthesis is the oxidation-reduction process i,e. water is first oxidised by loss of hydrogen and NADP gets reduced by accepting hydrogen. The reduced NADP (NADPH2) then reduces the CO2 in dark reaction to produce carbohydrate.
•Pigments Involved in Photosynthesis
The most common photosynthetic pigments in higher plants and green algae are chlorophyll-a (blue-green C55H72O5N4Mg), chlorophyll-b (yellow green, (C55H70O6N4Mg) and carotenoids, i.e., orange red carotenes (C40H56) and yellow xanthophyll (C40H5602).
•factors Affecting Photosynthesis
★ Light is the energy source for Photosyñthesis . Plants obtain light energy from the sun. Different light parameters affecting Photosyñthesis are
(a) Light quality Photosyñthesis occurs in visible light spectrum (3800 Å–7600 Å wavelength). Maximum rate of Photosyñthesis is found in red light, average in blue light and minimum in green light.
(b) Light intensity Chlorophyll utilizes only about 3% of total light absorbed. Photosyñthesis may initiate at very low light intensity. High light intensity causes solarisation (disintegration of chlorophyll molecules) due to which Photosyñthesis is reduced greatly.
(c) Light duration Photosyñthesis is significantly affected by the duration of light. Long continuous periods of light increase the rate of photosynthesis.
★Atmospheric concentration of CO2 (0.03%) is sub-optimal (low), so it acts as limiting factor in nature. Rate of Photosyñthesis increases upto 0.1%. CO2 concentration become toxic to plants when it crosses to limit of 0.9%.
★However if the concentration of CO2 increases beyond the limit there is decrease in rate of Photosyñthesis. plants absorb CO2 continuously for Photosyñthesis.
★Mostly the rate of photosynthesis increases by increase in, temperature upto 40°C. Above this temperature, there is a decrease in the rate. High temperature results into the deactivation of enzymes and thus, decrease the rate of Photosyñthesis. Q10 (temperature coefficient) for Photosyñthesis is two.
★Photosynthesis occurs in a very wide range of temperature.
★Certain conifers can photosynthesize at as low temperature as – 35°C, while some blue-green algae photosynthesize at 75°C.
★At higher temperature denaturation of proteins starts.
★It is an essential raw material in Photosyñthesis. It rarely acts as limiting factor because 1% of total water absorbed by a plant is used in Photosyñthesis. Rate of Photosyñthesis decreases if plants are inadequately supplied with water.
★In a water deficient soil there is approximately 87% reduction in the rate of photosynthesis.
★The availability of water for Photosyñthesis depends upon the closure of stomata.
★Due to deficiency of water photosynthetic enzymes do not work properly.
Increase in O2 concentration decreases the rate of photosynthesis in C3 plants. This phenomenon of the inhibition of Photosyñthesis by O2 was first discovered by Warburg (1920) in green alga Chlorella and is called as Warburg’s effect. Oxygen is inhibitory because
(i) It favours rapid respiration (photorespiration).
(ii) It competes with CO2.
(iii) It destroys the activated state of chlorophyll.
★Deficiency of mineral elements like Mg,Fe, Mn, Cu, Cl reduces photosynthesis.
★Phosphorus is essential for ATP synthesis and Mn and Cl are linked to photolysis of water.
★Less availability of any of these mineral may reduce the rate of photosynthesis.
★Some gaseous and metallic pollutants like ozone, SO2, nitrogen oxides, hydrogen fluorides, H2S, HCN decrease photosynthesis.
★Dust and smoke particles present in atmosphere reduce photosynthesis by reducing light penetration.
★Chlorophyll is ‘essential for Photosyñthesis. In etiolated plants and in the non-green parts of variegated leaves in some plants, Photosyñthesis does not take place.
★Theoretically it is quite obvious that the rate of Photosyñthesis should increase With an increase in the chlorophyll content, provided the other factors also favourable.
Dimension of Photosynthesis
”The energy captured by photosynthesis is hundred times more than the energy produced by burning of all types of fuel in earth and a few thousand time more than the energy generated by hydroelectric projects of the world. This energy is produced from 0.03% of CO2 present in the atmosphere. The green plants utilise only about 1.2% of the total solar energy available to them. With these the green plants on land and oceans store nearly 200 million tons of dry matter annually. This organic matter is the source of energy for all organisms of the earth. All organisms get share of this energy as it moves through various levels of organisms. Incidentally, marine weeds store more photosynthetic products than land plants.”