Chloroplasts
The chloroplasts (Greek: chlörós — green and plastós — formation,
creation, reproduction, modellation) are the most spread plastids in living
nature. They are obligatory organelles in all photosynthesizing cells. In the
different types of cells they vary in shape, number and size.
Also there exist other kinds of plastids — colourless (proplastids,
leucoplastids and ethioplastids) and coloured (chromoplasts). They are less
spread. The leucoplastids are colourless, with indefinite shape in most
cases. They are present in the cells of colourless parts of the plants,
epidermis, tubers, etc. They participate in the secondary synthesis of
starch, reserve proteins and lipids. The chloroplasts are coloured in yellow,
orange, red or brown. They are found in ripe fruits, leaves, algae, etc. They
contain different pigments. Phycoerythrin and phycocyanin are more known
of them. For example, the pigment lycopene from carotenoid group
determines the red colour of tomatoes.
It is known that after a complex reorganization of the inner
membranes, leucoplasts can turn into chloroplasts (turning green of potato
tubers), and chloroplasts — into chromoplasts (root fruits of carrots, the
leaves of trees in autumn, etc.). For a long time these frequently observed
facts have posed the question of the genetic relationship of the three types
of plastids, capable to change from one kind into another.
Usually chloroplasts are elliptic small bodies, 5—10 μm in length, with
a diameter of 2—3 μm. In one cell from a plant leaves one can find 15—20
or more chloroplasts, and in some algae — 1—2 giant chloroplasts of
different shape (Figs. 2–67, 2–68).
image
Figure 2–67. Electron micrograph of a mesophilic cell from a pea leaf (Pisum sativum). Bar 0.5 μm (Courtesy of S. Doncheva and V. Vassileva, Institute of Plant Physiology, Sofia). e — outer membrane; m — intermembrane space; i — inner membrane; g — gran; o — osmophilic globule; S — stroma; TS — thylakoids of the stroma.