Green plants are autotrophic. They synthesize their own food by the process of photosynthesis.
Autotrophic plants are able to produce food so they are known as producers.
Photosynthesis is the primary mode of food production in green plant. ” The process by which green plants synthesize food from simple substances carbon dioxide and water in the presence of sunlight is called Photosynthesis”
The process of photosynthesis can be represented in the form of chemical reaction, as given below :
The light energy must be absorbed by a suitable pigment i.e. chlorophyll (green pigment).
Chlorophyll is green colour pigment.
Mechanism of Photosynthesis :
Photosynthesis is divided in 2 main steps
(A) light reaction (B) dark reaction
- It is also called Hill Reaction.
- It occurs in grana of thylakoids.
- It is named as light reaction as it occurs only in presence of light.
- Absorption of light by chlorophyll.
- Photolysis of water.
- Reduction of CO2 to Carbohydrates.
- In this process ADP changes to ATP & inorganic phosphate.
- Release of oxygen into atmosphere.
Opening and Closing Stomata
- The opening and closing of stomata depend upon the turgid or flaccid state of the guard
cells. When guard cells are in turgid state the stomatal aperture opens and when guard cells are in flaccid state the stomatal aperture closes. The inner wall of guard cells (towards pore) is thick and outer wall (towards other epidermal cells) is thin. When the turgor pressure of the guard cells is increased the outer thinner wall of the guard cell is pushed out (towards the periphery) due to which a tension is created on the inner thicker wall thus pulling the inner thicker wall towards the periphery thus leading to the opening of stomatal aperture. On the contrary when the guard cells are in a flaccid state the outer thinner wall of guard cells returns to original position (moves towards pore) due to which tension on the inner wall is released which also returns to its original position and stomatal aperture gets closed again.
- This reaction is not dependent on light. It is also known as calvin – Benson Cycle or C3 cycle as first stable product is phosphogliceric acid (PGA) a 3 carbon compound.
C4 Cycle or Hatch & Slakcycle
- 4-C compound i.e. oxaloacetic acid (OAA).
- This cycle is found in many other tropical & subtropical monocots eg : Maize, Sorghum , Wheat, Oat, Pearl, millet etc.
- In dicots also many such plants are known eg : Amaranthus, Chenopodium, Atriplex, Euphorbia etc. In some families of dicots Compositae, Portulaceae, Nyctaginaceae.
Crassulacean Acid Metabolism (CAM)
- Certain plants, especially succulents which grow under extremely xeric (dry) condition, fix atmospheric CO2 in dark.
- Since the process was first observed in the plants belonging to family crassulaceae (eg. Bryophyllum, kalanchoe etc.) It was termed crassulacean acid metabolism (CAM).
- The most characteristic feature of these plants is that their stomata remain open at night (in dark) but closed during the day (in light).
- Thus, CAM is a kind of adaptation in succulents to carry out photosynthesis without much loss of water.