Plastids are a characteristic component of plant cells. They produce and store food materials in algae and plant cells. All plastids develop from proplastids and depending on the specific function a cell is going to perform, they mature into various specialised mature plastids. Mature plastids can even convert from one form to another.
There are three types of plastids found in plants cells:
- Chloroplast: Contains chlorophyll pigment and carotenoids and performs photosynthesis
- Chromoplast: Contains carotene and xanthophylls. They impart a specific colour to flowers and fruits and help in pollination and dispersal of seeds
- Leucoplast: They are colourless and store various food products, e.g. amyloplasts- store starch, proteinoplasts or aleuroplasts- store proteins, elaioplasts- store fat
- Chromoplasts are plastids and contain carotenoids
- They lack chlorophyll
- 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. Chlorophyll and thylakoids are degraded
- The DNA of plastid 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
- Chromoplasts also derive from leucoplasts or proplastids, e.g. in watermelon, papaya, carrot, mango, etc.
- 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 colour
- In xanthophylls, oxygen is present, e.g. fucoxanthin, lutein, etc.
- Carotenes only contain carbon and hydrogen, e.g. ꞵ-carotene, lycopene, etc.
Types of Chromoplasts
Chromoplasts are heterogenous plastids. Based on the carotenoid bearing components, they are classified into four types:
1. Globular Chromoplasts: It consists of plastoglobuli, which bear carotenoid pigments. Often they are concentrated in the peripheral stroma.
Examples: Ranunculus repens– petals, Capsicum- yellow fruits, Citrus fruit- perianth, etc.
2. Membranous Chromoplasts: Carotene pigments are present in about 20 concentric membranes.
Examples: Citrus sinensis– petals, daffodils, tulip
3. Tubular Chromoplasts: Carotenoids are present in lipoprotein tubules.
Examples: Red fruits of capsicum, hypanthium and rose
4. 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 are antioxidants
- ꞵ-carotene found in carrot is a precursor of vitamin A
- Dietary carotenoids reduce the risk of cardiovascular diseases and cancer
- Fucoxanthin has shown to have antidiabetic and anti-obesity effects
Difference between Chloroplast, Chromoplast and Leucoplast
|They are pigmented plastid other than green in colour, mostly yellow or red||It is a plastid, typically green in colour||They are colourless plastids|
|They contain carotenoids and xanthophylls. They lack chlorophyll||They contain chlorophyll and other carotenoids||They do not contain pigments|
|They provide distinct colour to the plant parts and have a role in pollination and seed dispersal||They perform photosynthesis||They are used to store starch, fat and proteins|
|Do not contain lamellar system and only remnants of thylakoids are present||They contain lamellar system, ribosomes and thylakoids||They have several nucleoids and uniform granular stroma|