Chloroplasts

The most commons of the plastids are the chloroplasts. They are very important for the plant, because photosynthesis takes place in them.

Shape. Chloroplasts may be spherical may be spherical, ovoid or disc-shaped. The chloroplasts of some algae are in the form of stellate plates or spiral bands.

Size. Chloroplasts are commonly 4 to 6 μ m in diameter, and are about 1 μ m thick. Those of shade plants are larger than those of sun plants. Polyploid cells contain larger chloroplasts than diploid cells.

Number. In some algal cell there is only single chloroplast. In higher plants the number is larger. There are usually 20 to 40 chloroplasts in each cell. Chloroplasts increase by division when their number becomes too much.

Distribution. Chloroplasts may be evenly distributed throughout the cytoplasm. Frequently they are found packed near the cell wall or the nucleus. The distribution may change with the amount of light.

Movement. Chloroplasts of higher plants generally move passively with streaming movements (cyclosis) of the cytoplasm. Independent active movements (cyclosis) of the cytoplasm. Independent active movements in the form of contraction, and amoeboid movements have also been observed. In the presence of light, chloroplasts change shape and their volume decrease.

Chemical composition. Chloroplasts of spinach leaf cells contain about 56% protein, 32% lipid and 8% chlorophyll. Chlorophyll has a tetraphyrrole structure and consists of a hydrophilic head and a hydrophobic tail. The head is made up of a magnesium atom surrounded by four pyrrolic nuclei (tetraphyrrole structure). The long tail consists of a phytol chain. Green plants have two types of chlorophyll a and chlorophyll b. In chlorophyll b one of the mythyl (-CHa)  groups of chlorophyll a is replaced by a formyl (-CHO) group.