Plastids

NCERT grounding

NCERT Class 11 Biology, Chapter 8 — Cell: The Unit of Life, section 8.5.5 "Plastids" — is the primary anchor. The text opens with two non-negotiable facts: plastids are found in all plant cells and in euglenoides, and they are large enough to be "easily observed under the microscope." It then groups them by pigment into chloroplasts, chromoplasts and leucoplasts, and devotes the bulk of the section to chloroplast architecture — outer and inner membranes, the stroma, thylakoids organised into grana, stroma lamellae, 70S ribosomes and circular DNA. NIOS Biology Chapter 4 §4.3.2 reinforces the same picture and pairs the chloroplast with the mitochondrion as the cell's two "energy transformers."

Types, structure & function

A plastid is a plant-cell organelle bounded by two membranes, with an internal compartment that stores or processes products of the cell's autotrophic life. The defining feature is the double envelope: an outer membrane that is freely permeable to most small solutes and an inner membrane that is selectively permeable and forms the true barrier between cytosol and plastid interior. The compartment enclosed by the inner membrane is called the stroma — a proteinaceous matrix that, in chloroplasts, also holds an internal third membrane system, the thylakoids.

All plastids in a given plant arise developmentally from proplastids — small, colourless, undifferentiated precursors present in meristematic cells. As tissues mature, proplastids follow distinct paths depending on the cell's role: a mesophyll cell coaxes them into chloroplasts, a petal cell into chromoplasts, a tuber-storage cell into amyloplasts. The same plastid lineage can also interconvert — chloroplasts redifferentiate into chromoplasts during fruit ripening, and amyloplasts can green into chloroplasts when exposed to light.

Three properties of plastids matter for NEET above all others: pigment-based classification (chloroplast vs chromoplast vs leucoplast and their sub-types), chloroplast ultrastructure (grana, stroma, stroma lamellae, thylakoid lumen) and semi-autonomy (their own circular DNA and 70S ribosomes). We work through each in turn, anchoring every claim to NCERT §8.5.5 and to the past-year questions that have actually been asked.

Three plastid types — by pigment

NCERT classifies plastids on a single criterion — the pigment they carry. Each class has a sharply different colour, location and function.

The chloroplast is named for its green colour ("chloro-" = green), which the chlorophylls produce. NCERT notes that chloroplasts also contain carotenoid pigments — but the chlorophyll dominates visually, so the organelle looks green. These pigments together are responsible for "trapping light energy essential for photosynthesis." Roughly 20 to 40 chloroplasts populate each mesophyll cell of a green leaf; in Chlamydomonas, a unicellular green alga, the entire cell carries just one.

The chromoplast ("chromo-" = colour) contains only the fat-soluble carotenoids — carotene (orange-red), xanthophyll (yellow) and similar molecules. The absence of chlorophyll makes the structure look yellow, orange or red. Chromoplasts give a ripe tomato its red, a marigold petal its orange, and a carrot root its colour. They typically develop from chloroplasts (during fruit ripening) or directly from proplastids (in flowers).

The leucoplast ("leuco-" = white) carries no pigment at all and serves as a storehouse. NCERT explicitly subdivides leucoplasts by the macromolecule they hold:

NEET 2024 Q.129 used exactly this fact, pairing "Leucoplasts" with "For storing nutrients." The three sub-types — amyloplast, elaioplast, aleuroplast — are routinely asked in match-the-column items, and the easy way to keep them straight is the mnemonic AAA: Amyloplast → Amylum (Latin for starch); Aleuroplast → Aleurone (the protein layer); Elaioplast → elaion (Greek for oil).

Chloroplast architecture — outside in

The chloroplast is the most elaborate of the three plastid types, and NCERT gives it the most space. Most chloroplasts in green plants sit in the mesophyll cells of leaves. They are lens-shaped, oval, spherical, discoid or even ribbon-like, with length 5–10 µm and width 2–4 µm. Number varies from 1 per cell in Chlamydomonas to 20–40 per cell in a mesophyll cell.

Like the mitochondrion, the chloroplast is double-membrane bound. The outer membrane is freely permeable to small solutes; the inner membrane is the gatekeeper — "the inner chloroplast membrane is relatively less permeable," NCERT states. The compartment enclosed by the inner membrane is the stroma. Floating in the stroma is a third, internal membrane system — the thylakoids — that has no counterpart in the mitochondrion.

The thylakoids are flattened membranous sacs that lie in the stroma. NCERT describes them as "organised flattened membranous sacs," and the thylakoid is the third membrane system of the chloroplast. Thylakoids are arranged "in stacks like the piles of coins called grana" (singular: granum). The flat membranous tubules connecting the thylakoids of different grana are the stroma lamellae (also called intergranal thylakoids). The membrane of each thylakoid encloses a space called the lumen.

Chlorophyll, light reactions & the Calvin cycle

Where in the chloroplast does each phase of photosynthesis happen? NCERT lays out the assignment unambiguously. Chlorophyll pigments are present in the thylakoids — therefore the light (photochemical) reactions, which need chlorophyll to harvest photons, run on the thylakoid membranes. The carbon-fixing dark reactions (the Calvin cycle) require enzymes — including RuBisCO — that NCERT places in the stroma: "The stroma of the chloroplast contains enzymes required for the synthesis of carbohydrates and proteins."

Semi-autonomy — DNA, ribosomes and binary fission

Tucked inside the stroma, NCERT records "small, double-stranded circular DNA molecules and ribosomes. … The ribosomes of the chloroplasts are smaller (70S) than the cytoplasmic ribosomes (80S)." Two consequences follow. First, the chloroplast can transcribe and translate a subset of its own proteins — including the large subunit of RuBisCO and several thylakoid proteins. Second, plastids divide by binary fission from pre-existing plastids (every plastid in a plant traces back through cell divisions to a proplastid). This combination is what NCERT calls semi-autonomous: the chloroplast (and the mitochondrion) is partially self-governing, but most of its proteins are still encoded by nuclear genes and imported from the cytosol. It cannot live independently of the cell.

NEET 2016 Q.94 tested this nuance directly. The question gave two claims: (a) mitochondria and chloroplast are semi-autonomous organelles; and (b) they are formed by division of pre-existing organelles and contain DNA but lack protein-synthesising machinery. (a) is correct; (b) is wrong precisely because both organelles do carry 70S ribosomes — their own protein-synthesising machinery. The official answer is "(a) is true but (b) is false." Knowing that 70S ribosomes sit in the chloroplast stroma is enough to solve the item.

The DNA's topology is itself a NEET datum: NEET 2024 Q.136 asked directly "The DNA present in chloroplast is:" with the correct answer "Circular, double stranded." This matches NCERT's wording in §8.5.5. Plastid DNA is not linear, not single-stranded, and not associated with histones — it resembles a bacterial chromosome, again consistent with the endosymbiotic origin of the organelle.

Worked examples

Common confusion & NEET traps