Question
What are the three types of plastids — chloroplasts, chromoplasts, and leucoplasts — and what functions does each serve in plant cells?
Solution — Step by Step
Plastids are double-membrane-bound organelles found only in plant cells (and some protists). They have their own DNA and ribosomes (similar to mitochondria — both are believed to have originated through endosymbiosis).
All plastids develop from proplastids (colourless precursors found in meristematic cells). Depending on the pigments they accumulate, they differentiate into three types.
Colour: Green (due to chlorophyll a and b)
Function: Site of photosynthesis — convert light energy to chemical energy.
Structure:
- Outer and inner membranes
- Thylakoids: flattened membrane sacs where light reactions occur
- Grana: stacks of thylakoids (plural: grana; singular: granum)
- Stroma: fluid matrix where the Calvin cycle (dark reactions) occurs
- Contains its own DNA, 70S ribosomes
Chloroplasts are found mainly in mesophyll cells of leaves.
Colour: Yellow, orange, or red (due to carotenoid pigments — carotene, xanthophyll, lycopene)
Function: Provide colour to flowers, fruits, and autumn leaves to attract pollinators and seed dispersers.
Examples:
- Tomato (red) — lycopene
- Carrot (orange) — beta-carotene
- Marigold petals (yellow) — xanthophyll
Chloroplasts can convert into chromoplasts. This is why green tomatoes turn red as they ripen — their chloroplasts transform into chromoplasts.
Colour: Colourless (no pigment)
Function: Storage of nutrients in non-photosynthetic parts (roots, tubers, seeds).
Three subtypes based on what they store:
- Amyloplasts: store starch (e.g., potato tubers)
- Elaioplasts (oleoplasts): store fats and oils (e.g., castor seeds)
- Proteinoplasts (aleuroplasts): store proteins (e.g., pulses, maize endosperm)
flowchart TD
A["Plastids in Plant Cells"] --> B["Proplastids — undifferentiated"]
B --> C["Chloroplasts — green"]
B --> D["Chromoplasts — coloured"]
B --> E["Leucoplasts — colourless"]
C --> F["Function: Photosynthesis"]
C --> G["Pigment: Chlorophyll"]
D --> H["Function: Colour for attraction"]
D --> I["Pigment: Carotenoids"]
E --> J["Function: Storage"]
E --> K["Amyloplasts / Elaioplasts / Proteinoplasts"]
C -->|"Ripening"| D
E -->|"Light exposure"| CWhy This Works
The three types of plastids represent a division of labour within plant cells. Green tissues need chloroplasts for energy production. Colourful flowers and fruits need chromoplasts for ecological signalling. Storage organs need leucoplasts to stockpile nutrients. All three arise from the same precursor (proplastid), and they can interconvert — showing remarkable plasticity (pun intended) in plant cell function.
The interconversion ability is biologically significant: when a potato is exposed to light, its leucoplasts (amyloplasts) develop into chloroplasts, which is why potatoes turn green when left in sunlight.
Alternative Method
For quick NEET recall, use the mnemonic: C-C-L = Chloro-Chromo-Leuco = Green-Coloured-Colourless = Photosynthesis-Attraction-Storage. Three letters, three colours, three functions.
Common Mistake
Students often say “chromoplasts perform photosynthesis because they contain pigments.” Wrong — chromoplasts contain carotenoid pigments, not chlorophyll. Carotenoids can absorb light but do not carry out the light reactions of photosynthesis (they serve as accessory pigments in chloroplasts, but chromoplasts lack the thylakoid machinery for photosynthesis). Only chloroplasts are the sites of photosynthesis. NEET has tested this distinction multiple times.