Plastids
பிளாஸ்டிட்கள்
Plastids are not just 'plant parts'; they are the evolutionary ancestors of endosymbiosis. If you confuse leucoplasts with chromoplasts, you are throwing away a guaranteed 4 marks. Precision is the difference between a seat in MMC and a repeat year.
The idea
Plastids are double-membrane organelles found in plant cells and euglenoids. They are classified based on pigments: Chloroplasts contain chlorophyll and carotenoids for photosynthesis. Chromoplasts contain fat-soluble carotenoid pigments like carotene and xanthophylls, providing yellow, orange, or red colors. Leucoplasts are colorless plastids that store nutrients: amyloplasts store starch, elaioplasts store oils, and aleuroplasts store proteins.
Why it matters
They allow plants to trap light energy for food production and store essential nutrients like starch, oils, and proteins.
NEET lens
NEET focuses on the storage function of leucoplasts and the pigment composition of chromoplasts. Expect matching-type questions linking plastid types to their specific storage products. Do not ignore the 'euglenoids' mention; it is a frequent trap in statement-based questions.
Clinical significance
While humans lack plastids, the ecosystem relies entirely on chloroplast-driven carbon fixation. Without this, the oxygen supply and primary productivity of the biosphere would collapse. Human survival is biologically tethered to the efficiency of these organelles.
Introduction
Plastids are double-membrane bound organelles found only in plant cells. Types: chloroplasts (green, photosynthesis), chromoplasts (colored), leucoplasts (colorless, storage).
Core concept
Chloroplast: double membrane, thylakoids (stacked into grana, site of light reactions), stroma (site of Calvin cycle), own DNA (circular) and 70S ribosomes. Chromoplasts: contain carotenoids — give colors to flowers and fruits. Leucoplasts: amyloplasts (store starch), elaioplasts (oil), aleuroplasts (protein).
Go deeper
Chloroplasts and mitochondria both have own DNA and 70S ribosomes — both originated from endosymbiosis. Chloroplast from cyanobacteria, mitochondria from proteobacteria.
From Class 10
Plants are green because of chloroplasts. But plant cells have other types of plastids too, not just chloroplasts.