Question
What are plastids? Describe the three types — chloroplasts, chromoplasts, and leucoplasts — with their structure, pigments, and functions.
Solution — Step by Step
Plastids are double-membrane-bound organelles found only in plant cells and some protists. They are classified by the pigments they contain (or lack). Like mitochondria, plastids have their own DNA and 70S ribosomes — supporting the endosymbiotic theory.
Pigments: Chlorophyll a, chlorophyll b, carotenoids, xanthophylls.
Structure: Double membrane. Internal membrane forms thylakoids stacked into grana, connected by stroma lamellae. The fluid matrix is called stroma.
Function: Photosynthesis — light reactions occur on thylakoid membranes (ATP, NADPH production). Dark reactions (Calvin cycle) occur in the stroma (CO₂ fixation to glucose).
Location: Green parts of the plant — mainly leaves, young stems.
Pigments: Carotenoids (orange, yellow), xanthophylls (yellow), lycopene (red). No chlorophyll (or very little).
Function: Impart colour to flowers and fruits to attract pollinators and seed dispersers. Tomatoes turn red due to lycopene in chromoplasts. Carrots are orange due to carotene.
Note: Chloroplasts can convert to chromoplasts (green tomato turning red = chloroplasts → chromoplasts).
Pigments: None (colourless).
Function: Storage of nutrients. Three subtypes:
- Amyloplasts — store starch (potato tubers)
- Elaioplasts (oleoplasts) — store oils and fats
- Proteinoplasts (aleuroplasts) — store proteins
Location: Non-green, storage parts — roots, tubers, seeds.
flowchart TD
A[Plastids - Plant cell organelles] --> B[Chloroplasts]
A --> C[Chromoplasts]
A --> D[Leucoplasts]
B --> B1[Green - chlorophyll]
B --> B2[Photosynthesis]
C --> C1[Coloured - carotenoids]
C --> C2[Attract pollinators]
D --> D1[Colourless - no pigment]
D --> D2[Storage: starch, oil, protein]
B -->|Can convert to| C
D -->|Can convert to| BWhy This Works
The classification is based on pigment content: green (chloroplast), coloured non-green (chromoplast), colourless (leucoplast). All three types can interconvert — leucoplasts in a potato exposed to light develop chlorophyll and become chloroplasts (potato turning green).
Common Mistake
Students forget that plastids can interconvert. Green tomatoes have chloroplasts that convert to chromoplasts (with lycopene) as the fruit ripens. A potato stored in light develops green patches — leucoplasts converting to chloroplasts. This interconversion is a common NEET question.
Both chloroplasts and mitochondria have their own DNA and ribosomes (70S) — they are semi-autonomous organelles. This is evidence that they were once free-living prokaryotes (endosymbiotic theory). NEET loves this connection.