Plastids in Plant Cell

Plastids are cell organelles that store specific things found only in plant cell but absent in animal cells.In plant cell they are found in the cytoplasm. Plastids are spherical or ovoid in shape. They are involved in manufacture and storage of certain important chemical compounds.

The term plastids was coined by Schimper in 1885 and was derived from a Greek word 'plastikas' which means formed or moulded.

Plastids in plants include chloroplasts, chromoplasts, leucoplasts, amyloplast, elaioplast and proteinoplast/aleuronoplast depending on the function they play.

Chloroplasts

The word chloroplast is derived from the Greek word chloros meaning green and plast meaning form or entity. It is the most important plastid as they are involved in photosynthesis. The chloroplasts are situated near the surface of the cell and in parts where there is sufficient reception of sunlight. The shape of the cholorplast varies, it may be spheroid or ovoid or discoid.

For a given cell type the size of plastid is constant but it differs from species to species.It is about 4-5 microns in length and 1-3 microns in thickness. The number of chloroplast may be 20 to 40 per cell may be upto 1000, the number varies from species to species but is constant for a plant.

Structure

Chloroplasts are disc-shaped and are enclosed by a double membrane.

Within the inner membrane is a protein-rich substance known as stroma, it is embedded in a membrane system. This membrane system forms membrane bound vesicles called thylakoids.

The thylakoids lie in stacks called grana. This contains the photosynthetic pigments - chlorophyll a and b and carotenoids. Lamellae are tubular membranes which interconnect the grana.

Functions

Photosynthesis is carried out in the chloroplast.

The enzymes and co-enzymes necessary for photosynthesis is present.

Chromoplast

Chromo means color; plast means living. Chromoplasts are colored plastids and they contain various pigments like yellow,orange and red.

They are found commonly in flowers and fruits. The color is due ot pigement, carotenes and xanthophylls.

Functions

In flowers the main function is attract agents for pollination.

In fruits it is to attract agents for dispersal.

Leucoplasts

These are colorless plastids and occur in parts of plants that are not exposed to light like roots and seeds.

The absence of color is due to the lack of pigments.

Functions

Starch grain formations are seen in leucoplast.

Oils and proteins are synthesized here.

Date: 2016-04-22; view: 970