Plastids

Plastids

Plastids are double membrane bound organelles found in plants and some algae. They are responsible for preparation and storage of food. There are three types of plastids.

Chloroplast -        green coloured plastids

Chromoplast        -  yellow, red, orange coloured Plastids

Leucoplast  -        colourless plastids

1. Structure of Chloroplast

Chloroplasts are green plastids containing green pigment called chlorophyll. Chloroplasts are oval shaped organelles having a diameter of 2-10 micrometer and a thickness of 1-2 micrometer.

·                 Envelope: Chloroplast envelope has outer and inner membranes which is seperated by intermembrane space.

·                 Stroma: Matrix present inside to the membrane is called stroma. It contains DNA, 70 S ribosomes and other molecules required for protein synthesis.

·                 Thylakoids: It consists of thylakoid membrane that encloses thylakoid lumen. Thylakoids forms a stack of disc like structures called a grana (singular-granum).

·                 Grana: Some of the thylakoids are arranged in the form of discs stacked one above the other. These stacks are termed as grana, they are interconnected to each other by membranous lamellae called Fret channels.

2. Functions of Chloroplast

1. Photosynthesis 2. Storage of starch 3. Synthesis of fatty acids 4. Storage of lipids 5. Formation of chloroplasts

3. Photosynthesis

Photosynthesis (Photo light; synthesis = to build) is a process by which autotrophic organisms like green plants, algae and chlorophyllcontaining bacteria utilize the energy from sunlight to synthesize their own food. In this process, carbon dioxide combines with water in the presence of sunlight and chlorophyll to form carbohydrates. During this process oxygen is released as a byproduct.

4. Where does photosynthesis occur?

Photosynthesis occurs in green parts of the plant such as leaves, stems and floral buds.

5. Photosynthetic Pigments

Pigments involved in photosynthesis are called Photosynthetic pigments. Photosynthetic pigments are of two classes namely, the primary pigments and accessory pigments. Chlorophyll a is the primary pigment that traps solar energy and converts it into electrical and chemical energy. Thus it is called the reaction centre. Other pigments such as chlorophyll b and carotenoids are called accessory pigments as they pass on the absorbed energy to chlorophyll a (Chl.a) molecule. Reaction centres (Chl. a) and the accessory pigments (harvesting centre) together are called photosystems.

6. Role of Sunlight in Photosynthesis

The entire process of photosynthesis takes place inside the chloroplast. The structure of chloroplast is such that the light dependent (Light reaction) and light independent (Dark reaction) take place at different sites in the organelle

1. Light dependent photosynthesis (Hill reaction \ Light reaction)

This was discovered by Robin Hill (1939). This reaction takes place in the presence of light energy in thylakoid membranes (grana) of the chloroplasts. Photosynthetic pigments absorb the light energy and convert it into chemical energy ATP and NADPH₂. These products of light reaction move out from the thylakoid to the stroma of the chloroplast.

2. Light independent reactions (Biosynthetic phase)

The second steps (dark reaction or biosynthetic pathway) is carried out in the stroma. During this reaction CO₂ is reduced into carbohydrates with the help of light generated ATP and NADPH₂. This is also called as Calvin cycle and is carried out in the absence of light.

In Calvin cycle the inputs are CO₂ from the atmosphere and the ATP and NADPH₂ produced from light reaction.

7. Factors Affecting Photosynthesis

a) Internal Factors:

Pigments ii) Leaf age iii) Accumulation of carbohydrates iv) Hormones

b) External Factors:

i) Light ii) Carbon dioxide iii) Temperature iv) Water v) Mineral elements