Page 27 - 11-Bio-4 The Cell

4. The Cell eLearn.Punjab

Chloroplasts vary in their shape and size with a diameter of about 4-6 nm. Under light microscope
they appear to be heterogeneous structures with small granules known as grana embedded in the
matrix. Under electron microscope, a chloroplast shows three main components, the envelope,
the stroma and the thylakoid. The envelope is formed by a double membrane, while stroma covers
most of the volume of the chloroplast. Stroma is a fluid which surrounds the thylakoids. It contains
proteins, some ribosomes and a small circular DNA. It is in this part of the chloroplast where CO2
is fixed to manufacture sugars. Some proteins are also synthesized in this part. Thylakoids are the
flattened vesicles which arrange themselves to form grana and intergrana. A granum appears to be
a pile of thylakoids stacked on each other like coins. On an average, there are 50 or more thylakoids
piled to form one granum. On the layers of thylakoids chlorophyll molecules are arranged and that
is why granum appears to be green (Fig.4.14). Each granum is interconnected with others by the
non-green part called intergranum. Membranes of the grana are sites where sun light energy is
trapped and where ATP is formed. Chloroplasts are self-replicating organelles.

2. Chromoplasts

They impart colours to the plants other than green. They are present in the petals of the flower and
in the ripened fruit. They help in pollination and dispersal of seeds.

3. Leucoplasts

They are colourless. They are triangular, tubular or of some other shape. They are found in the
underground parts of the plant and store food.

Fig. 4.14: Diagram of chloroplast showing the main structural components. V: 1.1

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