Chloroplast Structure & Ribosomes — NEET Biology
✅ Asked in NEET 2026Majority of the chloroplasts of the green plants are found in the mesophyll cells of the leaves. These are lens-shaped, oval, spherical, discoid or even ribbon-like organelles having variable length (5-10μm) and width (2-4μm). Their number varies from 1 per cell of the Chlamydomonas, a green alga to 20-40 per cell in the mesophyll. Like mitochondria, the chloroplasts are also double membrane bound. Of the two, the inner chloroplast membrane is relatively less permeable. The space limited by the inner membrane of the chloroplast is called the stroma. A number of organised flattened membranous sacs called the thylakoids, are present in the stroma. Thylakoids are arranged in stacks like the piles of coins called grana (singular: granum) or the intergranal thylakoids. In addition, there are flat membranous tubules called the stroma lamellae connecting the thylakoids of the different grana. The membrane of the thylakoids enclose a space called a lumen. The stroma of the chloroplast contains enzymes required for the synthesis of carbohydrates and proteins. It also contains small, double-stranded circular DNA molecules and ribosomes. Chlorophyll pigments are present in the thylakoids. The ribosomes of the chloroplasts are smaller (70S) than the cytoplasmic ribosomes (80S).
NTA focuses on chloroplast ultrastructure, particularly the distinction between chloroplast ribosomes (70S) and cytoplasmic ribosomes (80S). This tests understanding that chloroplasts, like mitochondria, are semi-autonomous organelles with their own protein-synthesizing machinery. Students commonly confuse ribosome sizes or forget that chloroplasts contain 70S ribosomes (prokaryotic-type) rather than 80S (eukaryotic-type), indicating their endosymbiotic origin. Remember: chloroplasts have 70S ribosomes in their stroma because they originated from prokaryotes. Also recall that chloroplasts contain their own circular DNA and can synthesize some proteins independently—key evidence for the endosymbiotic theory tested frequently in NEET.
Non-membrane bound cell organelles found in both prokaryotic and eukaryotic cells are
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