The term photosynthesis was first proposed by Barnes in 1898. The word has been derived from photo (light) and synthesis (building up). Photosynthesis is a process by which green plants are able to synthesize their food in the presence of sunlight and chlorophyll by taking carbon dioxide from the atmosphere and water from the soil. Food is synthesized in the form of simple carbohydrate like sugar.
Photosynthesis is one of the most important biochemical activities of green plants. In this process, solar energy is converted into utilizable forms of energy. Almost all energy found in the organic molecules is the result of photosynthesis. All the basic necessities of our life viz., food, fuel (wood, coal, petroleum), clothes and oxygen are the result of photosynthesis either directly or indirectly. Oxygen is the bi-product of photosynthesis and is released into the atmosphere. Oxygen forms 21 percent of our atmospheric air. Before the evolution of green plants on this earth, oxygen was absent in the atmosphere. After the origin of green plants the amount of oxygen in the air increased gradually. This event is started on the earth about 2.8 billion years ago.
The main photosynthesis product is glucose which may be either used by the photosynthesis cells for liberation of energy, or it may be polymerized into starch and stores as such. Some of the glucose may be polymerized into cellulose, or may be used to generate precursors of protein, fats and other useful forms. These organic compounds are required by the plants for metabolic activities.
Site and process of photosynthesis
The green parts of plants are able to perform photosynthesis. Leaves are most suitable organs for this process. They arise at the nodes of stem and appear green, flat and thin. They are exposed to the sun and are specialized for photosynthesis and transpiration.
Anatomically, the leaf has outermost epidermal layers (on both the surfaces) which are covered by waxy cuticle. In the internal layers, palisade and spongy parenchyma are found which contain green plastids, known as chloroplasts. Palisade and spongy parenchyma together form mesophyll tissue. The cells of palisade layer are elongated, with more chloroplasts, while spongy parenchyma contains almost spherical cells with less number of chloroplasts.
The leaf is admirably adapted to carry out the process of photosynthesis. The adaptations are as follows.
1. They have broad, wide and flat surface to absorb light and carbon dioxide.
2. Leaves bear minute pores, known as stomata on both the surfaces to facilitate exchange of gases between the leaf and the atmosphere.
3. Palisade tissue absorbs maximum sunlight because of its being situated just below the epidermis and also because of the fact that it contains abundant chloroplast.
4. There is a continuous supply of water and minerals in every cell through the extensive network of veins within the leaf lamina. It also helps in the translocation of prepared food to other parts of the plant.
5. The diffusion of carbon dioxide becomes very easy in each and every cell because of the presence of large intercellular spaces inside the leaf.
Each palisade cell has more than 300 chloroplasts. Water coming from the vein enters the cell by osmosis. Carbon dioxide from the atmosphere also diffuses into these cells. Chlorophyll pigments found inside chloroplast absorb sunlight. The oxygen is released into the atmosphere through the stomata.
Mechanism of Exchange of Gases
Stomata are the actual sites for the free exchange of gases. Each stomatal consists of a stomatal aperture and two surrounding guard cells. The guard cells are living and contain chloroplasts. The inner wall of guard cells is thick and the outer wall is thin. Stomata open towards the inner side of the leaf which is surrounded by spongy tissues.
Opening and Closing of Stomata
During daytime, there is intense photosynthetic activity which leads to an accumulation of glucose phosphate in the guard cells.
Glucose phosphate has the ability to retain water. In order to meet the demand, water is absorbed by the guard cells from the surrounding cells. This causes swelling of the guard cells. As a result, the inner wall of the guard cell, which lies towards the stomata, is pulled in. this widens the stomatal opening.
At night when there is no photosynthesis, sugars are converted into starch which is not soluble in water. Starch does not retain water. Therefore, whatever water is accumulated by glucose phosphate has to leave the cell. As a result, the cells become somewhat deflated and the outer stretched walls regain their original position. The inner cell walls are also relaxed and the stomatal opening is closed.
Requirements for Photosynthesis
Photosynthesis requires chlorophyll pigments, carbon dioxide, water and sunlight.
Three types of pigments namely chlorophylls, carotenes and xanthophylls are found in the chloroplasts of leaf cells. The colours of other pigments are masked by the green colour of chlorophylls. Chlorophylls are the main pigments which trap light energy. There are five types of chlorophylls- chlorophyll a, b, c, d and e. chlorophyll a is found in all photosynthetic plants. All the pigments found in chloroplasts absorb light energy, but ultimately they transfer the absorbed energy to chlorophyll a which converts it into chemical energy. Thus they act as photoreceptors of light energy.
In plants, chlorophyll is mainly found in leaves. That is why; the leaves are called photosynthetic organs. In lower plants like algae, the whole plant is green and takes part in photosynthesis. Young stems and fruits may also have chlorophyll.
2. Carbon dioxide:
Air contains about 0.03% of carbon dioxide which is released by expiration, combustion of wood, petroleum, coal, etc., and microbial decomposition. Terrestrial plants use atmospheric carbon dioxide in photosynthesis, while aquatic plants get it from water in the form of bicarbonate ions.
Photosynthesis and respiration are two different processes but they take place simultaneously in green plants. During daytime the rate of photosynthesis is greater than the rate of respiration. This means that the rate of utilization of carbon dioxide in photosynthesis is much more than the rate of liberation of carbon dioxide in expiration. Therefore, carbon dioxide is continuously absorbed from the atmosphere through stomata. At night plants do not take in carbon dioxide from the atmosphere as there is no photosynthesis. But respiration takes place all the time which results in the liberation of carbon dioxide in the atmosphere. During respiratory activity, the moist spongy cells absorb carbon dioxide from the atmosphere which combines with water in the following manner.
The carbon dioxide in the form of bicarbonate ions (HCO3) diffuses into the moist surface cells of the respiratory cavity and from there it passes to other cells by the process of diffusion. Oxygen and water vapours are also released in the atmosphere through the stomata.
The rate of photosynthesis is not the same throughout the day. It increases with the increase of light available to the plants. During early morning and evening hours, when the photosynthetic rate becomes equal to the rate of respiration, no exchange of gases takes place between plant and environment. This is known as compensation point. The increase in concentration of carbon dioxide in the environment of a plant increases the rate of photosynthesis up to a certain limit. After that this increase is inhibited because of other limiting factors.
Water is an important raw material for photosynthesis. Plants absorb water from the soil through their roots and root hairs. The water is then translocated up to the leaves through the stem. Xylem tissues present in roots and stems are responsible for translocation of water. Aquatic plants absorb water and minerals from their general surface. Minerals absorbed along with water contribute to the process of photosynthesis besides general development of the plant. For example, sulphates, nitrates and phosphates are required by the plants for converting the carbohydrates into proteins.
In photosynthesis, water molecule is split in the presence of sunlight and chlorophyll. This reaction takes place in the leaves of the plant. The use of radioactive isotopes of oxygen has proved that the oxygen released during photosynthesis comes from water and not from carbon dioxide. If a photosynthesizing plant is supplied with water containing 18O instead of normal 16O, the oxygen released will be 18O type. However, if supply carbon dioxide in which oxygen is of 18O type, no oxygen of 18O type is release.
When a photon, a particle of light, is absorbed by a molecule of chlorophyll, the energy level of the electrons in the molecule is increased and it reaches the excited state. This energy is used in splitting water molecule (H2O->H + (OH)). The splitting of water in the presence of light is called photolysis. The electrons of the above reaction are used for production of adenosine tiphosphate (ATP). Energy needed for the formation of glucose comes from ATP.
The rate of photosynthesis is dependent on both quality and quantity of light. The rate increases with the increase in light intensity. In blue and red lights, the rate of photosynthsis is maximum, while in green light the rate is minimum. Very high light intensity has adverse effect on the protoplasm of the leaf.
Conversion of Light Energy into Chemical Energy
White sunlight consists of a range of wavelengths of light representing the colours violet, indigo, blue, green, yellow, orange and red (these colours can be remembered by the acronym VIBGYOR). During photosynthesis chlorophyll does not absorb all colours of sunlight. The green portion of sunlight is not absorbed but is reflected and, therefore, chlorophyll appears green. Green plants are not capable of absorbing green light. Solar energy or light energy trapped by green plants is converted into chemical energy. It is stored in the molecules of carbohydrates.
Importance of Photosynthesis
Photosynthesis is a very important process because of the following reason:
1. Source of Energy:
One of the products of photosynthesis becomes the source of energy for all living organisms- both plants and animals. The oxidation of food produced by plants generates energy.
2. Production of Food:
On the earth, only plants are capable of producing food which is directly or indirectly consumed by all animals and human beings. It also helps in the growth and development of plants.
3. Balancing of Atmospheric Carbon Dioxide and Oxygen:
The combustion of coal, oil and other fuels releases carbon dioxide in the atmosphere. Expiration by living organisms also releases carbon dioxide in the atmosphere. But photosynthesis requires carbon dioxide which is taken up from the atmosphere. Consequently, the amount of carbon dioxide in the atmosphere does not increase too much. Carbon dioxide in the atmosphere is able to absorb solar heat. This heat is retained and is not allowed to escape into the atmosphere. This is known as the greenhouse effect. As a result of this effect there would be considerable warming of the atmosphere.
Photosynthesis enriches the atmosphere by supplying oxygen. This oxygen is used by all living begins for respiration.
Factors affecting Photosynthesis
Temperature, light intensity, water and carbon dioxide concentration in air are the important external factors. Internal factors that influence photosynthesis include chlorophyll content, accumulation of end products and the physio-chemical state of the cell.