Plastids | Definition, Structure, Types & its Functions

Plastids are found in all plant cells & in euglenoids etc. These are easily observed in the microscope as they are large. Depending upon their colour & the pigments they contain, these are of three main types – leucoplasts, chromoplasts & chloroplasts.

1. Leucoplasts

These are colourless plastids which generally occur near the nucleus in non – green cells. They have variable size, form & stored nutrients. Granum is absent. There are three types of special leucoplasts :

 (i) Amyloplast : They are starch – containing leucoplasts, e.g., potato tuber, rice etc.

(ii) Elaioplasts : They store fats & oils, e.g., castor.

(iii) Aleuroplast : These leucoplasts store proteins, e.g., aleurone cells of maize.

2. Chromoplast

These plastids are yellow, orange or reddish in colour because of the presene of carotenoid pigments. Chromoplasts are formed either from leucoplasts or chloroplasts. Change of colour from green to reddish during the ripening of tomato & chilli is due to transformation of chloroplasts to chromoplasts. The orange colour of carrot roots is due to chromoplasts.

3. Chloroplasts

They are greenish plastids which possess photosynthetic pigments, chlorophylls an carotenoids and take part in the synthesis of food. Majority of the chloroplasts of the green plants are found in the mesophyll cells of the leaves. They vary in size, shape and number.

Size : They have variable thickness of 2 – 4 µm & length 5 – 10 µm.

Shape: They may be spherical, lens shaped, oval, discoid or even ribbon-shaped in some plants.

Number : Their number varies from 1 per cell in Chlamydomonas, a green alga to 20-40 per cell in the mesophyll.

Structure of Chloroplast

(i) Like mitochondria, chloplasts are also double membrane – bound  organelle having outer & inner membrane. The inner membrane is relatively less permeable to substances than outer membrane & thus, has more protein including carrier proteins. The space enclosed by the inner membrane of chloroplasts is called the stroma. Stroma contains a large number of organised flattered membranous sacs called thylakoids which are arranged in stacks like the piles of coins called grana (sing: granum) or the intergranal thylakoid.

(ii) The thylakoid of different grana are connected by flat membranous tubules called the stroma lamellae.

(iii) The stroma of the chloroplast contains enzymes required for the synthesis of carbohydrates & proteins. It contains small, double – stranded circular DNA molecules & ribosomes. The robosomes of the chloroplasts (70S) are smaller than the cytoplasmic robosomes (80S).

Functions :

The chloroplast perform various functions like;

1. Photosynthesis : Light reaction (in thylakoids), dark reaction (in stroma).
2. Storage of starch