Plant cells possess a characteristic component called plastids. They primarily function to produce and store food materials in algae and plant cells. All plastids develop from proplastids in the plant cell and mature into respective matured plastids based on the specific function a cell is going to perform. Mature plastids also show the ability to convert from one form to another.
Plastids are a distinctive feature that is possessed by plant cells. Plastids are organelles that have double-membrane-bound. Their main purpose is to make and store food. They often have different types of pigmentations, which causes them to change the colors of the cells.
Plastids are membrane-bound heterogenous organelles. Chromoplasts are a type of plastids that lack chlorophyll. Chromoplasts are seen in fruits, flowers, aging leaves of the plants, etc. They are also responsible for imparting distinctive colors to the plants.
We will learn in detail about chromoplast, it’s characteristics and functions in this article.
Types of Plastids
The three types of plastids that are found in algae or plant cells are:
These are specialized organelles containing chlorophyll pigment and carotenoids found in most plant cells. They contain a pigment known as chlorophyll. Chlorophyll is responsible for the plant’s green color and ability to absorb energy from sunlight. This energy is used to convert water and atmospheric carbon dioxide into sugars. That can be metabolized through the biochemical process of photosynthesis.
These are plastids that produce and store pigments. They contain carotene and xanthophylls. Chromoplasts are responsible for different colors found in leaves, fruits, flowers and vegetables. They impart colors other than green. These are present in petals and ripened fruits. Chromoplasts also help in the pollination and dispersal of seeds.
These plastids are non-pigmented, in distinction to other plastids such as chloroplast. Leucoplast varies in shape. They are found in underground plants and stored in food.
They are colorless organelles that store various food products, For example, amyloplasts store starch, proteinoplasts store proteins, elaioplasts store fat and so on.
Characteristics of Chromoplasts
Chromoplasts are plastids containing carotenoids.
They lack chlorophyll but synthesize various other coloured pigments.
Carotenoid pigments are responsible for different colors like yellow, orange and red color imparted to fruits, flowers, old leaves, roots, etc.
Chromoplasts may develop from green chloroplasts. Chlorophyll and thylakoid membranes disappear and carotenoids are accumulated, e.g. during ripening of fruits.
The transition of chloroplasts to chromoplasts is much evident in fruit ripening. There is an extensive synthesis of carotenoids. Thus, chlorophyll and thylakoids are degraded.
The DNA of plastids remains unchanged during transformation but ribosomes and rRNA disappear.
Some chromoplasts are capable of differentiating back to chloroplasts, e.g. chromoplasts of carrot root and citrus fruit, pumpkin, cucumber, etc. They lose carotene pigment and develop photosynthetic apparatus consisting of chlorophyll and thylakoid system.
Redifferentiation to chloroplasts is promoted by gibberellin and nitrates.
Chromoplast development is irreversible in plants like pepper and tomato.
In some plants like papaya, carrot, mango, watermelon chromoplasts are derived from the proplastids or leucoplasts.
Light, temperature and nutrients are important factors in the formation of chromoplasts.
They have plastoglobules and carotenoid-lipoprotein substructures, which store specific carotenoid pigments and specific lipoprotein fibrils.
Carotenoids are divided into two classes: carotene and xanthophylls. Carotene is orange in color and xanthophyll is yellow.
In xanthophylls, oxygen is present, e.g. fucoxanthin, lutein, etc.
Carotenes only contain carbon and hydrogen, e.g: carotene, lycopene, etc.
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Types of Chromoplasts
Chromoplasts are basically heterogenous plastids. Based on the carotenoid bearing components, they are classified into four types:
Globular Chromoplasts: They consist of plastoglobuli that bear carotenoid pigments. Often, they are concentrated in the peripheral stroma of the plant cell. Examples: Ranunculus repens– petals, Capsicum- yellow fruits, Citrus fruit- perianth, etc.
Membranous Chromoplasts: Carotene pigments are present in about 20 concentric membranes. Examples: Citrus sinensis– petals, daffodils, tulips.
Tubular Chromoplasts: Carotenoids are present in lipoprotein tubules. Examples: Red fruits of capsicum, hypanthium and rose.
Crystalline Chromoplasts: Pure carotene is embedded as crystals. Examples: Carrot roots- ꞵ-carotene, Tomato fruits- lycopene.
Functions of Chromoplast
They play an essential role in cross-pollination and dispersal of seeds by attracting animals and insects.
Carotenoids act as antioxidants in plants.
Carotene found in carrots is a precursor of vitamin A.
Dietary carotenoids reduce the risk of cardiovascular diseases and cancer.
Fucoxanthin has shown to have anti-diabetic effects and it is also known to work against obesity.
In some plants and algae, chromoplasts act as organelles within the cells that serve as a site for photosynthesis.