Site of Photosynthesis and Its Role in Plant Life

Photosynthesis takes place in the green plastid (Chloroplast or Kitchen of the cell) present in tissues inside leaves and young stem.

When we observe the cross section of a leaf under a microscope, we can see the mesophyll cells full of green dots. Mesophyll cells are of two types – cylindrical palisade cells and irregular spongy cells. These green dots are chloroplasts containing chlorophyll pigments.

Thylakoid membranes have chlorophyll pigment (which is a magnesium containing pigment) to capture sunlight. Maximum absorption of light occurs in the violet, blue and red colour spectrum of VIBGYOR.

1.0Mechanism of Photosynthesis

During photosynthesis, following events occur:

(i) Absorption of light energy by chlorophylls.

(ii) Conversion of light energy into chemical energy and splitting of water molecules into H₂ and O₂.

Photolysis is the splitting of water molecules in the presence of light.

(iii) Hydrogen reduces CO₂ and synthesis of glucose (reduction of CO₂ into carbohydrates) occurs.

The events of photosynthesis can be categorized into two main events: light reaction and dark reaction.

2.0Differences between Light and Dark reaction

Adaptations in Leaves for Photosynthesis 

(1) Large surface area: Leaves have large surface area for maximum absorption of light.

(2) Large number of stomata: The leaves have a large number of stomata to allow rapid exchange of oxygen and CO₂ gases.

(3) Arrangement of leaves: The leaves are arranged at right angles to the light source so that the maximum surface area of leaves is exposed to sunlight.

(4) Concentration of chloroplasts: The chloroplasts are more concentrated on the upper epidermis of the leaf so as to obtain maximum light energy.

(5) Extensive vein system: The vein system is extensively developed for rapid transport of water to mesophyll cells.

3.0Factors Affecting Photosynthesis

(a) Light

(b) Water

(c) Temperature

(d) CO₂

(e) Oxygen

Out of several factors which affect photosynthesis, three are the most important: carbon dioxide, light and warm temperature (but not higher than 40°C because it slows down and even stops the enzyme action). All the three factors are inter-dependent and each of these factors limits the other. For example, if CO₂ is less and the other two are more, there will be less photosynthesis. Similarly, less light will slow down the process even if there is lot of CO₂ and warm temperature. Similarly, if the temperature is low, there will be less photosynthesis even if CO₂ and sunlight are plenty.

Too much light beyond a certain limit destroys chlorophyll and the process of photosynthesis is hampered.

Importance of photosynthesis

(i) Photosynthesis is an anabolic process in nature for providing food supply to the living organisms.

(ii) It purifies the atmospheric air, by utilizing CO₂ and evolving oxygen.

(iii) It converts light energy into chemical energy.

4.0Heterotrophic Nutrition

Mode of nutrition in which the organisms derive their nutrition by taking ready-made food, from other dead or living plants and animals. E.g., Animals, fungi and most bacteria. The survival of heterotrophs depends directly or indirectly on autotrophs.

Heterotrophic mode of nutrition is classified into three types on the basis of methods of obtaining food.

(i) Holozoic Nutrition - Ingestive nutrition

Mode of nutrition in which the animals take complex solid food or whole material and break down (digest) it inside the body. They may be herbivores (cow), carnivores (lion) and omnivores (rat).

In Paramecium, the cell has a definite shape and a specific spot for ingestion of food. The food is moved to this spot by the movement of cilia which cover the entire surface of the cell.

(ii) Saprotrophic Nutrition

In this type of nutrition, the organisms obtain their food from dead and decaying organic substances. They are called saprotrophs. They break down the food material outside the body and then absorb the digested food. E.g., Bacteria, Fungi (bread moulds, yeasts and mushrooms).

(iii) Parasitic nutrition

Mode of nutrition in which organisms (parasite) derive nutrition from other plants and animals (host) without killing them. e.g., Cuscuta (Amarbel), ticks, lice, leeches and tapeworms. Cuscuta is a total stem parasite while Mistletoe is partial parasite.

5.0Also Read