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Chapter: Biology: Physiological Activities in Plants Nutrition

Photosynthesis: Process, Organ, Factors Affecting, Importance

Photosynthesis is a complex and lengthy process. In 1905 A.D. Blackman divides the process into two phases namely: 1. Light Phase and 2. Dark Phase.

Photosynthesis and Respiration


Photosynthesis is a biochemical process. In this process green parts (specially leaves) of plants manufacture carbohydrate food by CO2 and water with the help of energy obtained from sunlight. Here light energy is converted to chemical energy. Plants absorb CO2 and release O2 as bi-product in the process of photosynthesis. The components required for this process are: - (i) Light, (ii) Chlorophyll, (iii) Water and, (iv) Carbon dioxide (CO2).


Chlorophyll pigments are present in the chloroplasts of plant cells. Water absorbed by roots reaches to the mesophyll cells in the leaves. The process of photosynthesis is shown by the following reaction

''Photosynthesis is such a biochemical reaction by which chloroplast, in the plant cell, uses the energy, received from the photon of sunlight, manufactures carbohydrate food by CO2 of the atmosphere and water and other organic substances in the CO2 cell.


Process of Photosynthesis:


Photosynthesis is a complex and lengthy process. In 1905 A.D. Blackman divides the process into two phases namely: 1. Light Phase and 2. Dark Phase. Both the phases are described here: -


1. Light Phage: Light is essential for this phage. In this process water breaksdown to O2, e- (electron) and Hydrgen (proton = H+) by the help of energy obtained from sunlight. Such break down is called as Photolysis or Photolysisof water. Large amount of light energy is assimilated in chemical compoundnamed ATP (Adenocine Tri Phosphate) and NADPH2 (Nicotinamide Adenocine Dinucleotide Hydrogen Phosphate) as chemical energy. ATP and NADPH2  are called assimilatory Power. This process of formation  of  ATP  by  using  light  energy  is  called Photophosphorilation. Photophosphorilation  may be  of  two types -  Cyclic and Non-cyclic. Two pigment systems (Pigment system-1 and Pigment system-2) control the light phase of photosynthesis

2. Dark Phase: Light is not required in this process. ATP a NADPH2,produced in light phase, are utilised in this process to manufacture carbohydrates. The process of manufacture of carbohydrate is as follows: - CO2, from atmosphere enters inside the leaf thus in the cell, through stomata. CO2 combined with Ribulose 1,5 Diphosphate, present in the ccll produces an unstable compound named Keto acid. This Keto acid later on break down into two molecules named a 3 -Carbon compound named 3-Phosphoglyceric acid (first stable compound). This Phosphoglyceric acid then converted to 3-Phosphoglyceraldehyde and Dihydroxy Acetone Phosphate utilising ATP and NADPH2, produced earlier in light phase. By several successive reactions these 3-Phosphoglyceric acid and Dihydroxi Aceton Phosphate ultimately produce Carbohydrate (Sucrose-Sugar) in one side and Ribulose 1,5, Diphosphate on the other side.


Prof. Calvin and Bassham of California University discovered this cylce of producing carbohydrate by utilising CO2 of the atmosphere. Thus this cycle is named as Calvin and Bassham cycle. First stable compound of this cycle 3-Carbon compound thus these types of plants are called C3 plants.

Another cycle of producing carbohydrate has discovered. It is called Hatch and Slack Pathway. In this process the first stable compound is a 4- Carbon compound, named oxalo-acetic acid. These types of plants are called C4 plants. Sugarcane, Maize, Cyprus grass etc. are examples of C4 plants. You will know more detailed in higher classes.


Organ of Photosynthesis:


Photosynthesis occurs in green parts in leaves. Various types of tissues are present in the leaf, such as upper epidermis, lower epidermis, vascular tissues and mesophyll tissue. Mesophyll tissue again are of two types-compactly, arranged elongated cells at the upper epidermis are the palisade parenchyma tissue and loosely arranged cells at the lower epidermis is the spongy par-enchyma tissue.


Usually photosynthesis occurs in palisade parenchyma cells of the leaf. Palisade parenchyma contains innumerous chloroplasts. The process of photosynthesis completed in these chloroplasts. The portion of palisade parenchyma in a dorsiventral leaf is shown in diagram (Fig: 9.8).

Factors Affecting Photosynthesis:


Factors involved in photosynthesis are of two types - External and Internal factors.

External Factors :


0. Sunlight: Sunlight is essential for photosynthesis. The high-energycompound - ATP and NADPH2 are formed utilising the Sunlight which help carbon reduction in dark phage. High intensity of light reduces the rate of Photosynthesis.


0. Carbon dioxide: Manufacture of carbohydrate is not possible withoutcarbon dioxide. Thus if the amount of CO2 reduces in air (in air there is 0.03% of CO) the rate of photosynthesis reduces.


0. Water: Water makes the guard cells turgid and the stomata opens. on theother hand water is utilised as a raw material in dark phase of the process.


0. Temperature: Photosynthesis seized near 0¡C and above 45¡ of temperature.Optimum temperature for photosynthesis is 22¡C- 35¡C. Below 22¡C and above 35¡C rate of photosynthesis decreases.


0. Oxygen: If density of oxygen increases in atmosphere the rat ofphotosynthesis will decrease.


Importance of Photosynthesis:


The importance of photosynthesis is unlimited. A short description is given regarding this: -


0. Manufacture of plant food: To live food is essential. Green plantsmanufacture food by the process of photosynthesis. If the green plant would not be able to manufacture food, then no plant could live in this world.


0. Supply of food for Animal Kingdom: Only green plants manufactures foodby the process of photosynthesis. No animal, even man can manufacture their food. Rice, Bread, Fruits Fish Meat, Milk etc. whichever we eat all we get directly or indirectly from plants. So the Animal Kingdom is completely dependent on green plants, and the green plant manufacture these food by the process photosynthesis. So it can be said, the process of photosynthesis prepares of all the foods for both plants and animals.

0. Source of Energy: Much energy is exhausted in our works and movements.Source of this energy is food. Green plants manufacture food by the process of photosynthesis. Energy comes to food from the Sun light energy trapped as chemical energy in foods manufacture by photosynthesis. Food breaks down in every living cell producing energy. Source of our all energy is food and source of energy of food is the sun. So Sun is the source of all energy. Energy that needs in metabolic activities in plants and animals come from food too.


0. Ecological Balance: In maintaining Ecological Balance particularlybalancing the ratio of O2 and CO2, photosynthesis plays a special role. The amount of oxygen gas in the airs 20.946% and that of carbon dioxide is 0.033%. This is the normal amount of oxygen and carbon dioxide gas in the atmosphere. For normal growth and living of plants and animals in this world, amount of these gases should be within normal range. On variation of the normal ratio the atmosphere will be imbalanced. We know respiration goes on in every living being (plants and animals). In respiration O2 gas is taken in and CO2 gas is driven out. Plants and animals both are taking O2 and releasing CO2 during respiration. If this system goes on successively there would have shortage of O2 and excess of CO2. But as green plants absorb CO2 and release O2 during Photosynthesis, so the ratio of O2 and CO2in the atmosphere is still all right. Due to excessive deforestation the ratio of these two gases in the atmosphere is being destroyed. Therefore, we should have to plant trees more and more.


0. Contribution of Photosynthesis in Human Civilization: Men survive onlybecause photosynthesis is present in green plants. But apart from the survival, the contribution of photosynthesis is unlimited in the arena of human civilisation. Coal, Petrol, Rayon, Cellophane, Paper, Rubber and Medicines etc. all are the gift of plant's.

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