Plastids

They are thought to have been originated from a group of free-living photosynthetic bacteria known as cyanobacteria that live within other host organisms.

Depending on the species of plant and algae, plastids may take up a variety of shapes ranging from discoid, spherical, dumbbell-shaped, or lens-shaped among a few others.

1. Double Membrane: Outer and inner membrane with a thickness of 7 nm each and consisting of phospholipid molecules

2. Intermembrane Space: Present within two membranes with (8-10) nm thickness

3. Grana: Stacks of membrane-bound organelle called thylakoid. Each granum contains approximately 10-20 thylakoids

4. Stroma: Cytoplasm of the organelle containing proteins, enzymes, and ribosomes

5. Peripheral Reticulum: Membranous tubules arising from the inner membrane

6. DNA: Genetic material of plastids

7. Ribosome: Organelle where protein synthesis occurs

1) Proplastids:  Colorless and immature plastids that develop into any mature plastid, depending on the organ where they function. Proplastids are always in a state of active division. They later develop into chloroplast or leucoplast.

2) Chloroplasts: Present in all the green parts of the plant, and blue-green algae. They are green in color due to the presence of the pigment chlorophyll.

3) Leucoplasts: Non-pigmented organelles that are colorless and are found in the endosperm of seed, roots, and other non-photosynthetic plant parts. Based on the kind of food they store, leucoplasts are classified into the following 3 types:

• Amyloplasts
• Proteinoplasts
• Elaioplasts

4) Etioplasts: They are chloroplasts that have not been exposed to light and are a transitional stage between the proplastid and the chloroplast during the process of greening.

5) Chromoplasts: Present in flowering plants, aging leaves, and fruits. They are colored plastids assuming different colors that develop gradually from chloroplasts with time.

6) Gerontoplasts: They are chloroplasts that are going through the aging process. Gerontoplasts are present in the old leaves that are no longer performing photosynthesis.

• Chloroplasts help in synthesizing food by the process of photosynthesis
• Chromoplasts attracts pollinators and thus helps in pollination
• Gerontoplasts helps in breaking down the photosynthetic apparatus during plant cell death
• Amyloplasts stores starch
• Proteinoplasts stores protein
• Elaioplasts stores fat and oil