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 membrane-bound heterogenous organelles. Chromoplasts are a type of plastids that lack chlorophyll. Chromoplasts are seen in fruits, flowers, ageing leaves of the plants, etc. They are also responsible for imparting distinctive colours to the plants.
We will learn in detail about chromoplast, it’s characteristics and functions in this article.
Types of Plastids: There are three kinds of plastids that are found in plant or algal cells:
Chloroplast: Contains chlorophyll pigment and carotenoids and is involved in photosynthesis.
Chromoplast: Contains carotene and xanthophylls. They impart a specific colour to flowers and fruits and are also involved in pollination and dispersal of seeds.
Leucoplast: They are colourless organelles that store various food products, For example, amyloplasts store starch, proteinoplasts store proteins, elaioplasts store fat and so on.
Chromoplasts are plastids containing carotenoids.
They lack chlorophyll but synthesize various other coloured pigments.
Carotenoid pigments are responsible for different colours like yellow, orange and red colour 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 colour and xanthophyll is yellow.
In xanthophylls, oxygen is present, e.g. fucoxanthin, lutein, etc.
Carotenes only contain carbon and hydrogen, e.g. ꞵ-carotene, lycopene, etc.
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Chromoplasts are 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, tulip.
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.
They play an essential role in cross-pollination and dispersal of seeds by attracting animals and insects.
Carotenoids act as antioxidants in plants.
ꞵ-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.
Q1. What are Chromoplasts?
Ans: Chromoplasts are pigmented plastids that are not green in colour. Chromoplasts are usually yellow, red and appear in fruits, flowers or ageing leaves of the plant parts. Their primary function is to provide distinctive colours to the plant parts and they are non-photosynthetic pigments that lack chlorophyll.
Q2. What are the Types of Chromoplasts?
Ans: Chromoplasts are 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.
Membranous Chromoplasts: Carotene pigments are present in about 20 concentric membranes.
Tubular Chromoplasts: Carotenoids are present in lipoprotein tubules.
Crystalline Chromoplasts: Pure carotene is embedded as crystals.
Q3. What is the difference between Chromoplasts and Leucoplasts?
Ans: The differences between chromoplasts and leucoplasts are listed below:
They are pigmented plastid (other than green in colour) mostly yellow or red.
Leucoplasts are colourless plastids.
They contain carotenoids and xanthophylls and lack the pigment chlorophyll.
They do not contain any sort of pigments.
They provide distinct colour to the plant parts and have a role in pollination and seed dispersal.
They are used to store starch, fat and proteins in the plant body.
They do not contain lamellar system and only remnants of thylakoids are present.
They have several nucleoids and uniform granular stroma.
Q4. What is the difference between Chromoplast and Chloroplast?
Ans: The basic differences between chromoplasts and chloroplasts are as follows:
Chromoplasts are plastids other than green in colour while chloroplasts are green-coloured plastids. This is because chromoplasts lack chlorophyll but chloroplasts contain chlorophyll and other carotenoids.
Chromoplasts are usually not involved in photosynthesis but they play an important role in pollination by providing distinct colour to plant parts. Chlorophyll, on the other hand, performs photosynthesis.
Chromoplasts contain lamellar systems and only remnants of thylakoids and chloroplasts contain lamellar systems, ribosomes and thylakoids.