It is a mode of nutrition in which organisms build their own organic food from inorganic raw materials with the help of energy obtained from outside. The common mode of autotrophic nutrition is photosynthesis.
It is the process of formation of organic food from inorganic raw materials (CO₂, H₂O) with the help of sunlight inside chlorophyll-containing cells. The initial product is glucose from which other biochemicals are formed. Excess of glucose is stored in the form of starch, just as glycogen is stored in our body as a source of energy.
The leaf is most suitable for photosynthesis as it is flattened to receive maximum sunlight, contains abundant stomata for quick exchange of gases and a large number of vascular strands for bringing in water and taking away products of photosynthesis. A small amount of photosynthesis also occurs in young stems.
The site of photosynthesis is a green plastid called a chloroplast. Chloroplasts occur in good numbers in both palisade and spongy parenchyma cells of leaves. Chloroplasts contain the green photosynthetic pigment called chlorophyll. Chlorophyll is specialised to absorb solar energy and convert it into chemical energy.
Events. Three events occur in quick succession in photosynthesis.
(i) Absorption of light energy by chlorophyll.
(ii) Splitting of water and conversion of light energy into chemical energy.
(iii) Reduction of carbon dioxide to form carbohydrates and other biochemicals. However, most desert plants do not follow this sequence. They absorb CO₂ during the night when their stomata are open. The same is stored as an organic acid. During the day when stomata are closed, solar energy absorbed by chlorophyll is used to synthesise organic food.
Raw Materials of Photosynthesis
- Chlorophyll. It is a green coloured photosynthetic pigment that is specialised to absorb solar energy and convert it into chemical energy. Other photosynthetic pigments, called carotenoids, also take part in absorbing solar energy. They hand over the energy to chlorophyll for chemical conversion.
- Carbon Dioxide. Land plants obtain CO₂ from the air through their stomata. Stomata function as turgor-operated valves. Their guard cell curves out when they swell upon absorption of water. It creates a pore in between them. As guard cells lose water, they shrink, become straight and close the pore. When the stomata are open, a quick gaseous exchange occurs. It provides required CO₂ to photosynthetic cells. However, open stomata also pass out a lot of water vapours in transpiration. Transpiration stops as the stomata close. However, the gaseous exchange continues to occur on a smaller scale from the surface of stem, root and even leaves.
Carbon dioxide is necessary for photosynthesis.
Take two potted plants. Distarch then by keeping in perfect darkness for 2-3 days. Place the detached potted plants on glass slabs. Place a watch glass having potassium hydroxide solution near one of them. Cover both the potted plants with bell jars. Seal the edges of the bell jars with vaseline. Place the two plants in sunlight for 2-4 hours. Afterwards, pluck one leaf from each potted plant and test the same for starch (dipping in boiled water, in hot alcohol, in hot water again, in iodine solution and wiping of iodine with water). The leaf of a potted plant with potassium hydroxide does not show a positive starch test showing that it has not performed photosynthesis. The leaf of the other potted plant turns bluish-black indicating the presence of starch and hence photosynthesis. The only difference between the two potted plants is the absence of CO₂ in the first and the presence of carbon dioxide in the second. Therefore, carbon dioxide is essential for photosynthesis.
- Water. In land plants, it is obtained from the soil. The absorbed water is transported along with minerals to the photosynthetic areas through the xylem. Minerals help in the synthesis of specific biochemicals like sulphur in proteins, magnesium in chlorophyll, etc.
- Light. It is the visible part of solar radiation with a wavelength of 390-760 nm. Photosynthetically active light is 400-700 nm. Light brings about photolysis of water and excitation of chlorophyll.
Mechanism of Photosynthesis
The overall equation of photosynthesis is
6CO2 + 12H2O C6H12O6 + 6H2O +6O2
Photosynthesis has two steps, photochemical and biochemical.
- Photochemical Phase (Hill Reaction, Light Reaction). It consists of the photolysis of water and the formation of assimilatory power.
(a) Photolysis of Water In the presence of Mn, CI and Ca, light energy splits up water into its components.
2H2O 4H+ = 4 e– +O2
Oxygen evolved in photosynthesis comes from water.
(b) Formation of Assimilatory Power. The excited chlorophyll provides energy for the synthesis of ATP from ADP and inorganic phosphate.
ADP + Pi ATP energy
NDAP+ is reduced with the help of H+ and e–
H+ +2e– NADPH +H+
Both ATP and NADPH constitute assimilatory power
- Biosynthetic Phase (Blackman’s reaction, Dark reaction). In this phase, CO2 is reduced to form carbohydrates.
Energy (as ATP)
CO2 +2H2 (as NADPH) CH20 + H2O
Plants also utilise other materials obtained from the soil for the synthesis of other biochemicals, e.g., sulphur and nitrogen in protein, phosphorus and nitrogen in nucleic acids, magnesium and nitrogen in chlorophyll. Nitrogen is taken from soil in the form of nitrate or nitrite. Some bacteria convert elemental nitrogen to organic compounds for utilisation by plants.
- Food. Photosynthesis produces organic food. All the organisms depend upon this food for survival.
- Energy. Photosynthesis converts radiant energy into chemical energy.
- Oxygen. It releases oxygen. Photosynthesis is the main source of oxygen in our environment.
- Carbon Dioxide. It picks up carbon dioxide from the atmosphere and uses the same in building organic food.
Chlorophyll is necessary for photosynthesis.
Take a potted plant of Coleus, Money plant or Croton having variegated leaves. Keep it in perfect darkness for two to three days in order to destarch its leaves. Place the potted plant in sunlight for 2-6 hours. Pluck one variegated leaf and draw the outline of its green and non-green areas on rice paper. Dip the leaf in boiling water for 5-10 minutes building Dip the leaf in alcohol or spirit kept at 50°-60° C with the help of a water bath. After 30-45 minutes, the leaf will become holozoic decolourised due to the dissolution of chlorophyll. Place the decolourised leaf in hot water in order to remove alcohol and soften it. Keep the leaf in a petri dish and pour dilute iodine over it. Remove iodine and wash the leaf with water by means of a dropper. Find that the leaf has two types of patches, bluish-black and pale coloured. Compare it with the sketch drawn on rice paper. The bluish-black colour of starch appears where the leaf was green. The pale colour represents the previous non-green areas. As starch develops only after photosynthesis, the chlorophyll-containing areas perform photosynthesis. Therefore, chlorophyll is necessary for photosynthesis.
Light is necessary for photosynthesis.
Distarch a potted plant by keeping it in complete darkness for 2-3 days. Fix a strip of thick black paper on the upper surface of a leaf by means of cello tape. Expose the leaf to sunlight for 2-3 hours. Pluck the leaf and remove the black paper. Test for starch (by dipping in boiled water, then in hot alcohol, in hot water again and lastly in iodine solution). The covered part of the leaf remains pale coloured due to the absence of starch, while the rest of the leaf becomes bluish-black due to the presence of starch. As the covered part does not show photosynthesis, light is necessary for photosynthesis.