Types of Plastid and their Functions

The various types of plastids are discussed. Each of them have specific structure and function.

Chloroplasts

Chloroplasts are organelles found in plant cells, particularly abundant in leaves and other green tissues. The presence of DNA in chloroplasts (chloroplast DNA (cpDNA)) and different plastids is one of the principal discoveries supporting their starting point as prokaryotic (cyanobacterial) symbionts during the early advancement of life. The size of the chloroplast genome ranges between 100 and 200 kb for most plants, however bigger chloroplast genomes exist. 

Chloroplast structure have a double membrane envelope enclosing an internal space called the stroma. Within the stroma, numerous stacked thylakoid membranes are present which form grana. These thylakoid membranes contain chlorophyll pigments, responsible for capturing light energy during photosynthesis.

Also Read: Why are plants green?

Function of Chloroplast

The functions are:

• Conduct photosynthesis, capturing light energy from the sun and converting it into chemical energy in the form of glucose.
• Contain chlorophyll pigments, which absorb light energy during photosynthesis.
• Facilitate the light-dependent reactions on thylakoid membranes, generating ATP and NADPH.
• Facilitate the light-independent reactions (Calvin cycle) in the stroma, where carbon dioxide is fixed to produce sugars.
• Play a central role in providing oxygen as a byproduct of photosynthesis which is vital for sustaining life on Earth.
• Contribute to the synthesis of amino acids, lipids, and hormones within plant cells.

Chromoplasts

Chromoplasts are plastids specialized in synthesizing and storing pigments other than chlorophyll, such as carotenoids and anthocyanins. Chromoplasts strcuture contain membranous vesicles called chromoplastoglobuli, which contain the pigments responsible for the vibrant colors seen in flowers, fruits, and other plant parts. These plastids often undergo transformation from other plastid types during fruit ripening or petal development. Their main function is to impart vivid hues to plant tissues, making them more attractive and hence helping in pollination and seed dispersal.

Functions of Chromoplasts

The functions are:

• Synthesize and store pigments: Chromoplasts are specialized in synthesizing and storing pigments other than chlorophyll, including carotenoids and anthocyanins.
• Contribute to vibrant colors: The pigments stored in chromoplasts give flowers, fruits, and other plant parts their vivid shades, attracting pollinators and helping in seed dispersal.
• Enhance visual appeal: Chromoplasts play a crucial role in making plant tissues visually attractive, contributing to the aesthetic value of fruits and flowers.
• Protect against photooxidative damage: Carotenoids stored in chromoplasts act as antioxidants, protecting plant tissues from damage caused by excess light and reactive oxygen species.
• Provide nutritional benefits: Some pigments stored in chromoplasts, such as beta-carotene, have nutritional value and contribute to the health benefits of fruits and vegetables consumed by humans.

Leucoplasts

Leucoplasts are colorless plastids found in plant cells and lack pigments that impart color. They have a simple structure, consisting of a single membrane surrounding an internal matrix. Leucoplasts primarily serve as storage organelles, storing starches, lipids, and proteins. Their structure allows them to accumulate large quantities of these nutrients, especially in non-photosynthetic tissues like roots, tubers, and seeds. Leucoplasts play a crucial role in storing energy reserves and essential nutrients for the plant’s growth and development.

Functions of Leucoplasts

The functions include:

• Storage of starches: Leucoplasts serve as storage organelles for starch granules, accumulating large quantities of carbohydrates, particularly in non-photosynthetic tissues like roots, tubers, and seeds.
• Lipid storage: These plastids also store lipids and oils which provide energy reserves for seed germination and growth.
• Proteins storage: Leucoplasts store proteins in protein-rich tissues like seeds and grains, contributing to seed development and providing a source of amino acids for growth.
• Synthesis of fatty acids: Some leucoplasts, known as elaioplasts, specialize in synthesizing and storing oils and fats, essential for energy storage and membrane structure.
• Synthesis of starch: Amyloplasts, a type of leucoplast, are specifically involved in synthesizing and storing starch granules, crucial for carbohydrate storage and energy reserves in plants.

Types of Leucoplasts

There are three types of Leucoplasts. They are discussed below:

Proteinoplasts 

Proteinoplasts are a type of leucoplast found in plant cells. They are differentiated by their role in synthesizing and storing proteins. Structurally, proteinoplasts contain abundant proteinaceous matrix and lack the pigment-containing structures seen in chloroplasts and chromoplasts. These organelles typically have a granular appearance due to the presence of protein bodies.

Function of Proteinoplasts

The various functions include:

• Synthesize proteins: Responsible for the synthesis of proteins and have a role in protein metabolism within plant cells.
• Store proteins: These plastids act as storage sites for synthesized proteins, particularly in tissues where protein accumulation is essential for various physiological processes.
• Support seed development: Proteinoplasts are abundant in seeds and grains where they contribute to the accumulation of storage proteins necessary for seed development and germination.
• Facilitate nutrient reserves: By storing proteins, proteinoplasts help plants retain essential nutrients for future growth and development, particularly during periods of dormancy or environmental stress.
• Contribute to plant growth: The proteins synthesized and stored in proteinoplasts are essential for various aspects of plant growth, including cell division, elongation, and differentiation.

Amyloplasts

Amyloplasts are a type of leucoplast which are colorless plastids. Structurally, they contain dense, starch-filled granules, which give them a distinctive appearance under a microscope. These granules are surrounded by a lipid bilayer membrane, enclosing the starch molecules within the plastid.

Function of Amyloplasts

The functions include:

• Store starch reserves in plant tissues such as roots, tubers, and seeds.
• Serve as energy storage organelles, providing a readily accessible source of carbohydrates for the plant.
• Contribute to seed germination and early growth by supplying stored energy for the developing seedling.
• Play a crucial role in the storage and mobilization of carbohydrates during periods of growth, dormancy, and reproduction.
• Provide a source of energy for various metabolic processes, supporting plant growth, development, and survival.

Elaioplasts

Elaioplasts are a type of leucoplast distinguished by their role in storing oils and fats. The elaioplasts structure contain numerous lipid droplets or globules dispersed throughout their cytoplasm, giving them a characteristic appearance. These lipid droplets are composed of triglycerides, phospholipids, and other lipid molecules. Elaioplasts are particularly abundant in seeds and other oil-storing tissues, where they play a crucial role in seed germination and energy storage by storing oils and fats for future use.

Function of Elaioplasts

The functions are:

• Elaioplasts store oils and fats within plant cells.
• They contribute to seed germination by providing energy reserves.
• Elaioplasts facilitate the synthesis and storage of lipids, essential for plant growth and development.

Plastids are specialized double-membrane bound organelles in plant cells responsible for photosynthesis, pigment synthesis, and nutrient storage. Plastid was discovered by Ernst Haeckel but was named and defined better by A. F. W. Schimper. Different types of plastids perform different functions in the plant cell. In this article, we will learn about the function of plastids and their types.