Nutrition In Plants

Nutrition In Plants

Photosynthesis:
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 :
6CO₂ + 12H₂O   →  C₆H₁₂O₆ + 6H₂O + 6O₂
Carbon   Water        Glucose     Water    Oxygen
dioxide

Conditions Necessary for Photosynthesis
The general conditions necessary for photosynthesis are:
•  The presence of a green pigment called chlorophyll.
•  The presence of light.
•  The availability of carbon dioxide.
•  The availability of water.

Photosynthetic Pigment:
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

Light 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.

Water: The root system of plants enables them to obtain water from the soil. The root system consists of a main or primary root, rootlets or secondary roots, and root hair. Root hair increase the surface area of roots, and are found in great numbers near the tips of roots. Water and soluble nutrients travel through the root system and reach the leaves.
Water and soluble nutrients are transported from roots to other parts of the plant through structures called xylem. The starch formed as a result of photosynthesis is transported to the various parts of the plant by structures called phloem.

Major Step:
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.

Dark Reaction:
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.

Activity 1

Aim: To find out if light is necessary for photosynthesis
Materials needed: Green plant, black paper, scissors, and clip
Method: 

1. Cover a part of a leaf using a strip of black paper.
2. Place the plant in a dark corner for three days.
3. Pluck a leaf from the plant and test it for starch to confirm whether it is fully starch-free.
4. Now place the plant in sunlight for at least six hours.
5. Pluck the leaf covered with the strip of black paper, remove the strip, and test the leaf for starch.

Observation: The covered portion of the leaf shows no change when iodine solution is added.
Conclusion: There was no starch formation in the covered part, so this part did not turn blue-black. This shows that light is necessary for photosynthesis.

Activity 2

Aim: To demonstrate that carbon dioxide is needed for photosynthesis

Materials needed: A potted plant, potassium hydroxide solution, a conical flask with a split cork, iodine solution, and a dropper
Method:

1. Keep the potted plant inside a dark room for a few hours. Water it.
2. Pour potassium hydroxide solution into a conical flask. The solution absorbs carbon dioxide from the air.
3. Place one of the leaves (without breaking it from the plant) inside the flask and cork it.
4. Now, keep the entire arrangement in sunlight.
5. After a few hours, test the leaf in the flask and another leaf for starch, using iodine solution.

Observation: When iodine solution is added, the leaf clasped inside the conical flask shows no change, whereas the other leaf turns blue-black.
Conclusion: The leaf inside the conical flask did not carry out photosynthesis because there was no carbon dioxide available to it.