Plastids – Definition, Classification, Structure, Functions

Plant cells are eukaryotic cells. They perform the photosynthesis process and synthesize food. The characteristic features of a plant cell include primary cell walls containing, plastids, a large vacuole, absence of flagella or centrioles (except in gametes and phragmoplast formation during cell division)

Plastids

Plastids are a double membrane diverse group of physiologically, phylogenetically, and genetically related eukaryotic organelles that play important roles in plant metabolism through different processes of like –

• Photosynthesis  
• Nitrogen, and sulfur assimilation 
• Amino acid synthesis

These plastids are found in plant cells and algae. Hence, they differentiate plants and animal cells. The term plastid was given by Schimper in 1885. Plastid contains  a double-stranded DNA molecule that is circular and resembles the circular chromosome of cells of prokaryotes

Classification 

Proplastids (undifferentiated plastids) may undergo differentiation into many forms, depending upon their specialized functions in the cell. They may develop as  following variants

• Chloroplasts 
• Chromoplasts 
• Leucoplasts 
• Gerontoplasts

Chloroplasts 

Chloroplasts (green plastids) are 5 to 10 μm long organelles.  They are surrounded by a double membrane known as the chloroplast envelope. In addition to the double membrane, chloroplasts have a third internal membrane system, known as the thylakoid membrane. They are located in the mesophyll of plant cells. They perform the process of photosynthesis and synthesize their own food.

Other functions carried out by plant cells include

• Synthesis of  fatty acid
• Synthesis of amino acids, and 
• Plays important role in regulating the immune response in plants.

Structure of Green plastids – The chloroplasts

The majority of chloroplasts of green plants exist in the leaf mesophyll cells. Their shape resembles a lens and is discoid, oval, spherical, or ribbon-like organelles. Their length varies from 5-10 µm and their width varies from 2-4 µm.

These organelles are encircled by double membrane-like mitochondria. The inner membrane of chloroplast is comparatively less permeable. Stroma is the space surrounded by the internal membrane of the chloroplast. This internal space is filled with fluid that surrounds the grana, thylakoids.  It also contains ribosomes, starch, and DNA

A number of flattened membranous sacs are organized and are called the thylakoid. These are located in the stroma. Light-dependent reactions of photosynthesis occur in these thylakoids. There are packets of chlorophyll pigments in the thylakoid membrane, these are termed quantasomes. There are 230-250 chlorophyll molecules present in every single quantasome.

Space surrounded by the membrane of the thylakoids is called a lumen. This lumen is the site for the phosphorylation process of photosynthesis. During light reactions of photosynthesis, the pH of the lumen becomes acidic due to the inward pumping of protons.

These Thylakoids are arranged in a bunch that resembles the heaps of coins known as grana. A single chloroplast contains 10-100 grana enclosed in it.

Along with these grana, there are membranous flattened tubules called the stroma lamellae. These stroma lamellae interlink the thylakoids of the various grana.

Stroma

Chloroplast contains essential enzymes. These enzymes play a key role in the synthesis of carbohydrates and proteins. Stoma also contains other structures like ribosomes, and small, double-stranded circular DNA molecules. Thylakoids also contain chlorophyll pigments. The ribosomes resent in the chloroplasts are smaller (70S type) than the ribosomes of cytoplasm (80S).

Etioplasts

1. These are precursors of chloroplasts.  
2. These arise from proplastids that are not exposed to sunlight.
3. Upon exposure to sunlight, these etioplasts are converted into chloroplasts.
4. They usually exist in leaf tissue and stem angiosperms that grow in extremely low-light conditions and darkness.

Chromoplasts

These are colored plastids. Their main function includes pigment synthesis and storage. They exist in flowering plants, fruit roots (carrots and sweet potato), aging leaves i.e. senescence, and are responsible for their color. Their distinctive color formation is due to carotenoids.

Chromoplasts synthesize and store pigments such as xanthophylls (yellow), carotene (orange), and many other red pigments.

Through these distinctive colors, they attract pollinators to the plants.

Leucoplasts

These are unpigmented i.e. colorless plastids. They are found in non-photosynthetic roots, bulbs, tubers, and seeds.

Leucoplast exists in four types

• Amyloplasts – These are double enveloped organelles. For the plant,   they store and synthesize starch through glucose polymerization
• Proteinoplasts – They are known as proteoplasts or aleuroplasts, or aleuronaplasts. They store proteins and exist in seeds (pulses).
• Elaioplasts -Elaioplasts is also known as oleosomes. They store fats and oils that are needed for the plant. They exist in seeds.
• Tannosomes: They are found in vascular plants. They synthesize polyphenols and tannins.

Gerontoplasts 
They arise from a chloroplast during the aging of plant foliage. Structural changes in the thylakoid membrane cause the formation of gerontoplasts.

Inheritance of plastids 

Plastid inheritance in many green algae and land plants is from a single parent.  Generally, plastids are inherited from the maternal parent in oogamous species. Even If plastid inheritance is from both parents, it is generally predominated by maternal plastids. 
In conifers, plastid inheritance is from the paternal side. Even though there is strong maternal influence, there exist some species in which plastid inheritance shifts from completely maternal to absolutely paternal nature.

Conceptual Questions

Question 1: Lack of which plastids may affect the process of pollination?

Answer: 

Chromoplasts: These are colored plastids. Their main function includes pigment synthesis and storage. They exist in flowering plants, fruit roots (carrots and sweet potato), aging leaves i.e. senescence, and are responsible for their color. Their distinctive color formation is due to carotenoids. Chromoplasts synthesize and store pigments such as xanthophylls (yellow), carotene (orange), and many other red pigments. Through these distinctive colors, they attract pollinators to the plants.

Question 2: Which type of leucoplasts synthesize polyphenols and tannins? 

Answer:  

Tannosomes: They are found in vascular plants. They synthesize polyphenols and tannins.

Functions of other leucoplasts are 

• Amyloplasts – These are double enveloped organelles. For the plant,   they store and synthesize starch through glucose polymerization 
• Proteinoplasts – They are known as proteoplasts or aleuroplasts, or aleuronaplasts. They store proteins and exist in seeds (pulses). 
• Elaioplasts -Elaioplasts is also known as oleosomes. They store fats and oils that are needed for the plant. They exist in seeds.

Question 3: What are Etioplasts and their function?

Answer:

Etioplasts- These are precursors of chloroplasts.  These arise from proplastids that are not exposed to sunlight. Upon exposure to sunlight, these etioplasts are converted into chloroplasts. They usually exist in leaf tissue and stem angiosperms that grow in extremely low-light conditions and darkness. They are used to store oil synthesis.

Question 4: Which organelle arises from a chloroplast during the aging of plant foliage?

Answer: 

Gerontoplast– They arise from a chloroplast during the aging of plant foliage. Structural changes in the thylakoid membrane cause the formation of gerontoplasts.

Question 5: Colored plastids present in carrots and sweet potatoes are?

Answer:

Chromoplasts– These are colored plastids. Their main function includes pigment synthesis and storage. They exist in flowering plants, fruit roots (carrots and sweet potato), aging leaves i.e. senescence, and are responsible for their color. Their distinctive color formation is due to carotenoids.

Question 6: What genetic material present in a plastid is?

Answer:  

Double-stranded DNA- Plastid contains  a double-stranded DNA molecule that is circular and resembles circular chromosome of cells of prokaryotes