Green plants prepare their own organic food (starch) from inorganic sources by the process of photosynthesis. They are autotrophic in nature
Photosynthesis: photo – light; synthesis = putting together. It is the process of conversion light energy into chemical energy in which the inorganic components like carbon dioxide and water (CO2 and H2O) are utilised for the production of carbohydrates (food) and oxygen gas in the presence of light and chlorophyll pigment. Chemical equation for photosynthesis:
Chlorophyll: These are the pigments present in the chloroplast of the leaves. These pigments are photoreceptors that absorb visible light which is more effective for the photosynthesis process.
Site of photosynthesis: Photosynthesis does not occur in all the cells of green plants, it occurs only in the cells which possess chloroplast. Mesophyll cells contain abundant chloroplasts. These chloroplasts are green plastids. Chloroplast consists of two membranes an outer membrane and an inner membrane. The fluid present inside the matrix of chloroplast is called stroma. Stroma consists of membrane like structures called thylakoids. The bunch of thylakoids is called as grana. Each grana are joined by the structures called lamellae. Chloroplast contains its own DNA.
The events occur during photosynthesis process are:
(a) Absorption of light energy by chlorophyll.
(b) Conversion of light energy into chemical energy and splitting of water molecules into hydrogen and oxygen.
(c) Reduction of carbon dioxide into carbohydrates.
CO2 and H2O are the raw materials for photosynthesis process. Leaves contain tiny pores called stomata which helps in the absorption of atmospheric CO2. Guard cells regulate the opening and closing of stomata.
Photosynthetic events are broadly classified into two phases:
(1) Light reaction (2) dark reaction
(1) Light reaction: The light reactions occur in the thylakoids or grana of the chloroplast. In the first step, photosynthetic pigments (chlorophyll and accessory pigments) absorb visible light that results in the breakdown of water and gives hydrogen protons (H+), electrons (e–) and molecular oxygen (O2). This process is called photolysis of water.
Oxygen is released into atmosphere. The protons and electrons are utilized for the production of energy rich compounds NADPH (Nicotinamide adenine di – nucleotide phosphate) and ATP (adenosine triphosphate). The end products of light reaction/ photolysis are – Molecular oxygen, NADPH, ATP.
(2) Dark reaction:
Dark reactions occur in the stroma of chloroplast. They do not depend on the light but depends on the product of the light reactions. The energy rich compounds ATP, NADPH are utilized in the dark reactions for fixing the CO2 and to produce the carbohydrates. These reactions do not require light, so that they are called as dark reactions. The steps of dark reactions were discovered in detail by Calvin, Benson and Bassam. These biochemical reactions occur in a cyclic process which require different enzymes to produce carbohydrates(C6H12O6) from CO2 molecules. This cycle is also called as Calvin cycle.
Overall equation of Calvin cycle:
RUBP – Ribulose biphosphate
Plants produce carbohydrates (glucose) and utilize them for the energy. Excess glucose molecules are stored in the form of starch. In case of animals, excess glucose molecules are stored in the form of glycogen.
Plants obtain other nutrients like nitrogen, phosphorus, iron and magnesium from the soil. Nitrogen is essential for the synthesis of proteins. Atmospheric N2 is not assimilated by plants, they utilize N2 in the form of nitrates and nitrites from the soil. Some of the plants take N2 in the form of ammonia which is produced by some of the microorganisms like rhizobium.
FACTORS AFFECTING PHOTOSYNTHESIS:
The rate of photosynthesis is affected by several external and internal factors. They are:
Plants absorb solar radiation. During low light intensity, the rate of photosynthesis is low. Increase in light intensity causes increase in the rate of photosynthesis up to a certain limit. Blue and red light are more efficient for photosynthesis whereas photosynthesis does not occur in green light.
The rate of photosynthesis increases by increase in temperature up to 40˚C. Above this temperature enzymes of dark reaction gets deactivated and results in the decrease of photosynthesis.
(c) Carbon dioxide:
The rate of photosynthesis increases by increase in the CO2 concentration up to a limit, very high CO2 concentration is toxic to plants and inhibits photosynthesis.
Water rarely acts as the limiting factor because less than 1% of the water absorbed by a plant is used in photosynthesis.