what are plastids? explain its function with a diagram.

In most plant cells structures called plastids are found. They are found in the cytoplasmic matrix of plant cells only. These structures are generally spherical or ovoid in shape and they are clearly visible in living cells. We will discuss 3 types of plastids found in plant cells:

Chromoplasts

Chromoplasts are red, yellow or orange in colour and are found in petals of flowers and in fruit. Their colour is due to two pigments, carotene and xanthophyll.

Functions

• the primary function in the cells of flowers is to attract agents of pollination,
• and in fruit to attract agents of dispersal.

Leucoplasts

Leucoplasts are colourless plastids and occur in plant cells not exposed to light, such as roots and seeds. They are colourless due the absent of pigments.

Functions

• leucoplasts are the centers of starch grain formation;
• they are also involved in the synthesis of oils and proteins.

Chloroplasts

Chloroplasts are probably the most important among the plastids since they are directly involved in photosynthesis. They are usually situated near the surface of the cell and occur in those parts that receive sufficient light, e.g. the palisade cells of leaves. The green colour of chloroplasts is caused by the green pigment chlorophyll.

Structure

Chloroplasts are usually disc-shaped and surrounded by a double membrane. Inside the inner membrane there is a watery protein-rich ground substance or stroma in which is embedded a continuous membrane system, the granal network. This network forms a three- dimensional arrangement of membrane-bound vesicles called thylakoids. The thylakoids usually lie in stacks called grana and contain the photosynthetic pigments - green chlorophyll a and b and the yellow to red carotenoids. The grana are interconnected by tubular membranes called the intergranal frets or lamellae.

Functions

• chloroplasts are the sites for photosynthesis;
• they contain enzymes and co-enzymes necessary for the process of photosynthesis.

f

Plastids are double membraned organelles which are found in plant cells only. They are usually spherical or discoldal in shape and their average size is 4 to 6 um. A plastid shows tow distinct regions-grana and stroma Grana are stacks of membrane-bound, flattened, discold sacs containing chlorophyll molecules. These molecules are responsible for the production of food by the process of photosynthesis.

They are, therefore, called Kitchen of the cell. They are the main functional units of the chloroplast. The homogenous matrix in which grana are embedded is known as stroma. A variety of photosynthetic enzymesw and starch grains are present in the stroma. The stroma is colourless, whereas the grana contain the pigments. Plastids are living and multiply by division of the pre-existing plastids called proplastids.

1. Leucoplasts are colorless plastids. They store the food of the plant body in the form of starch, protein and lipids. They occur most commonly in the storage cells of roots and underground stems.

2. Chloroplasts are green plastids because of the presence of chlorophyll. Chloroplasts occur abundantly in green leaves, and also to some extent in green parts of the shoot.

3. Chromoplasts are variously colored plastids. They are mostly present tin flowers and fruits.

One form of plastid can change into another. For example, leucoplasts can change into chloroplasts when the former are exposed to light for a long period.

1. By trapping solar energy, green plastids manufacture food through photosynthesis

2. Chromoplasts provide colored to various flowering parts.

3. Leucoplasts help in storage of protein, starch and oil