Question

Explain the term semiautonomous and the developmental relationships between the various plastid types – “how they are formed, what stimulates their formation”.

Answer 1

The term semiautonomous with respect to organelles are used for those which have their own DNA. Chloroplast is also a semiautonomous organelle because it has its own DNA.

Plastids are responsible for photosynthesis, storage of products like starch, synthesis and have the ability to differentiate or redifferentiate and have various forms. Proplastids and young chloroplsts commonly divide, but this characteristic is more prominent in mature chloroplasts. Proplastids are essentially progenitor plastids, and are extensively found in young dividing cells. Undifferentiated plastids may develop into any one of the following plastids:

1. Chloroplasts for photosynthesis;
2. Chromoplasts for pigment synthesis and storage;
3. Gerontoplasts control the dismantling of the photosyntheic apparatus during senescence;
4. Leucoplasts for monoterpene synthesis; it further differentiates into:

• Amyloplasts for starch storage and detecting gravity.
• Elaioplasts for storing fat.
• Proteinoplasts for storing and modifying protein.

Plastids are thought to have originated from endosymbiotic cyanobacteria. They developed around 1500 million years ago and allowed the eukaryotes to carry out oxygenic photosynthesis. Complex plastids start by secondary endosymbiosis, when a eukaryote engulfs a red or green alga and retains the algal plastid, which is typically surrounded by more than two membranes.

Etioplasts are pre granal, immature chloroplasts but can also be chloroplasts which have been deprived of light and lack active pigment.