Thus, photosynthesis (photo means light, synthesis means combination) may be defined as a biochemical process by which living cells of plants containing chlorophyll manufacture their own food (glucose and starch) using carbon dioxide and water as raw materials in the presence of sunlight. Oxygen is released as a by-product of photosynthesis.
Photosynthesis is an important activity that occurs in all green plants, whether flowering or non- flowering.
Photosynthesis is the only process by which solar energy is converted into chemical energy. All living beings depend on photosynthesis for two reasons one for oxygen released as a by-product, and second for food prepared by plants.
Image Source: remotesensing.forestry.oregonstate.edu
Significance of Photosynthesis:
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i. Photosynthesis provides food for all. The process of photosynthesis occurs in green plants which are the primary producers in a food chain.
ii. Photosynthesis is essential for sustaining life. It is the ultimate source of oxygen and energy for all living organisms.
iii. Photosynthesis helps in growth and development of plants.
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iv. It is necessary for synthesis of organic compounds from inorganic compounds
v. It converts atmospheric carbon dioxide (given out during respiration and other activities) back to oxygen.
Essential Raw Materials for Photosynthesis:
To perform photosynthesis, plants require carbon dioxide (C02), water (H20), light energy and chlorophyll. C02 and H20 serve as raw materials and sunlight serves as a source of energy. The process of photosynthesis takes place in chloroplasts.
Carbon Dioxide:
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The main source of C02 for land plants is the atmosphere, which contains 0.03-0.04 per cent of C02. Water plants use C02 dissolved in water. Two main processes, photosynthesis and respiration, take place side by side but photosynthesis does not take place in the absence of light whereas respiration continues throughout the day and night.
During the day when the light intensity is greater, the rate of C02 consumption for photosynthesis is higher than that of C02 liberation by respiration; hence, C02 is continuously absorbed from the atmosphere through stomata. During morning and evening hours, the intensity of light is usually low.
At this time, a stage may come when C02 liberated during respiration is equal to C02 used in photosynthesis. At this stage, no exchange of C02 takes place between the environment and plants. This stage is known as compensation point.
Water:
Plants absorb water from the soil by their root hair. This water is then transported up to the stem and leaves through the xylem vessels. Many minerals which are dissolved in water are also absorbed by the plants. Aquatic plants absorb water through their general body surface because they have poor root system. Water rarely serves as a limiting factor in photosynthesis because less than 1 per cent of the water absorbed by a plant is used in photosynthesis.
Light:
Light is very important for photosynthesis. In photosynthesis, light energy of sun is converted into chemical energy. Sun is the main source of light energy. Artificial light is also effective in photosynthesis but this light should be of a required intensity.
The rate of photosynthesis is affected both by the quality as well as the quantity (intensity and duration) of light. Too much light intensity may destroy chlorophyll. In red-coloured light, the rate of photosynthesis is maximum whereas in green- coloured light photosynthesis does not occur.
Leaves the Organs to Trap Solar Energy:
Leaves contain chlorophyll which is a photoreceptor molecule. Chlorophyll absorbs photons unit of sunlight. It is present in the chloroplast. Chloroplast organelles are plastids containing chlorophyll pigments and are mostly present in leaves.
That is why leaves are called photosynthetic organs. Chloroplasts are also present in young stems and fruits. The green colour of plants is due to the chlorophyll. Chlorophyll is mainly of nine types. Of these, chlorophyll a and chlorophyll b are most important as they receive energy from the sun (solar energy) to bring about splitting of water.
Chlorophyll pigment is mainly present in the chloroplasts (plastids). The chloroplasts are 4-6 (μm in diameter and can be seen easily under a light microscope. Chloroplasts are double- membraned structures. The inner membrane lines the lumen of the chloroplast called matrix or stroma.
In higher plants, chloroplasts contain stacks of lamellar structures called grana (singular granum). If you look into the cross section of the grana, you will find sac-like structures formed of lamellae. These structures are called thylakoids. The thylakoids of one granum are connected to other granum by a membrane called stroma lamellae.
The chlorophyll pigment is contained in the walls of the thylakoids. Chloroplasts are mainly located in the mesophyll cells between upper and lower epidermis (palisade and spongy cells) of leaves. Guard cells of stomata also contain chloroplasts.
Water passes into the palisade cells of mesophyll tissue by osmosis from the xylem and carbon dioxide diffuses in from the atmosphere. Sunlight is absorbed by the chlorophyll. By using this energy, carbon dioxide and water are combined in the chloroplast with the help of a number of enzymes to yield sugar which is readily converted into a storable form of food, starch. The oxygen formed in the reaction diffuses out of the cells and is released into the atmosphere through the stomata.
Opening and Closing of Stomata:
Stomata (singular stoma) are minute pores present either on the lower or both the surfaces of the leaf to facilitate exchange of gases between the leaf and the atmosphere. Aquatic plants use carbon dioxide dissolved in water to prepare their food through photosynthesis.
Each stoma consists of a stomatal aperture and two surrounding guard cells. The guard cells are kidney-shaped and contain chloroplasts. The inner wall of each guard cell is thick and outer wall is thin.
There are two theories for opening and closing of stomata, namely sugar concentration theory and K+ ion concentration theory.
1. Sugar Concentration Theory:
During the daytime, the cell-sap concentration becomes high due to accumulation of sugar. This leads to endosmosis and water is withdrawn inside guard cells from neighbouring cells. This makes guard cells turgid so that their thin outer walls get stretched out widening the stomatal pore and stomata open. The pressure developed in guard cells is turgor pressure.
During the night, there is no photosynthesis; carbon dioxide gets accumulated in guard cells. This carbon dioxide then combines with water to form carbonic acid which has a pH of 5.0. It promotes the conversion of sugar into starch which is insoluble in water. As a result, exosmosis takes place and guard cells become flaccid or lose turgidity. Thus, the slit like stomatal pore gets narrowed down and closes.
2. K+ ion Concentration Theory:
Recently a new theory of opening and closing of stomata has come into existence. According to this theory, opening and closing of stomata depends on the generation of potassium ion (K+) gradient. Opening of stomata: During daytime, photosynthesis takes place and chloroplasts in the guard cells help in production of ATP.
This ATP helps in pumping of potassium ions (K+) of the adjacent cells into the guard cells. As a result, the concentration of K+ ions in the guard cells increase making them hypertonic. Thus, more water from the adjacent cells move into the guard cells making them turgid leading to opening of stomata.
Closing of stomata: During night, reverse process of daytime takes place. The ATP formation stops during night as no photosynthesis is taking place. Thus K+ ions move out of guard cells leading to hypotonic condition. The water moves out of the guard cells and they become flaccid, leading to closing of stomata.
How are the leaves adapted for photosynthesis?
1. Large surface area:
Leaves have large surface area for maximum absorption of light.
2. Large number of stomata:
The leaves have large number of stomata to allow rapid exchange of oxygen and C02 gases.
3. Arrangement of leaves:
The leaves are arranged at right angles to light source so as to trap maximum light.
4. Concentration of chloroplasts:
The chloroplasts are more concentrated on the upper epidermis of leaf so as to obtain maximum light energy.
5. Extensive vein system:
The vein system is extensively developed for rapid transport of water to and from mesophyll cells.