Plastids

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 su***ce 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.

More on Plastids:
http://sln.fi.edu/qa97/biology/cells/cell4.html
Plastids on Wiki:
http://en.wikipedia.org/wiki/Plastid