Cell — the Unit of Life: Prokaryotes, Organelles and the Membrane
This chapter carries more NEET questions than almost any other in Class 11, and it is asked in one particular way: which structure has which property. Not "what does a mitochondrion do" but "which organelles contain DNA", "which are double-membraned", "which are not bounded by a membrane at all". So the profitable way to learn it is not organelle by organelle but property by property — build four short lists (double membrane, own DNA and ribosomes, no membrane, plant-only) and most of the chapter answers itself. One organising idea runs underneath: an organelle's structure is a solution to a logistics problem. A mitochondrion needs surface area, so its inner membrane folds. A lysosome needs to digest without digesting the cell, so it is a sealed acidic bag. Read for the problem being solved and the structures stop being arbitrary.
The Plasma Membrane and Transport
The accepted model is the fluid mosaic model of Singer and Nicolson (1972). A bilayer of phospholipids arranges itself with the polar heads outward and the hydrophobic tails inward — an arrangement that forms spontaneously in water and needs no machinery. Protein molecules sit in it as a mosaic: integral proteins buried partly or wholly in the bilayer, and peripheral proteins on the surface. The crucial property is the quasi-fluid nature of the lipid, which allows lateral movement within the layer, and that fluidity is what makes membrane growth, cell division, secretion and endocytosis possible at all.
| Route | Needs energy? | Direction and carrier |
|---|---|---|
| Simple diffusion | No — passive | Down the concentration gradient, no carrier. Only neutral, non-polar solutes |
| Osmosis (water) | No | Water moves towards the more concentrated solution |
| Facilitated transport | No | Down the gradient but through a carrier protein — how polar molecules get across |
| Active transport | Yes — ATP | Against the gradient, via a pump protein — the Na⁺/K⁺ pump is the standard example |
🎯 Why a polar molecule cannot simply diffuse through, and what that costs the cell
The interior of the bilayer is a layer of hydrocarbon tails — chemically it is oil. A non-polar molecule such as oxygen or carbon dioxide dissolves in oil and crosses freely, which is why gas exchange needs no protein at all. A polar or charged particle — a sugar, an amino acid, a sodium ion — cannot enter that oily interior, and no amount of concentration difference changes that. So the cell must pay for every polar molecule it moves: with a carrier protein if the movement is downhill, and with carrier plus ATP if it is uphill. Two consequences worth carrying forward. The membrane is therefore selectively permeable rather than semi-permeable — it does not merely sort by size, it sorts by chemistry, and this distinction is asked directly. And a large share of a cell's entire energy budget goes on active transport, which is why the Na⁺/K⁺ pump reappears in the neural chapter as the reason a resting neuron must keep spending ATP simply to stay ready.The Organelles, Grouped by Property
| Property | Members | Why it matters |
|---|---|---|
| Double membrane | Mitochondrion, chloroplast (and all plastids), nucleus | The nucleus is the one candidates forget when listing double-membraned structures |
| Own DNA + 70S ribosomes | Mitochondrion and chloroplast only — semi-autonomous, divide by fission | The basis of the endosymbiotic origin argument |
| No membrane at all | Ribosome, nucleolus, centriole, cytoskeleton, bacterial inclusion bodies | A guaranteed question. The nucleolus is inside the nucleus but has no membrane of its own |
| Plant cells only | Cell wall, plastids, a large central vacuole (up to 90% of volume); centrioles absent in higher plants | Animal cells have centrioles and prominent lysosomes instead |
| Organelle | Structure | Function and the examinable detail |
|---|---|---|
| Mitochondrion | Double membrane; the inner one folds into cristae; matrix inside | Aerobic respiration and ATP — the powerhouse. Cristae exist to multiply surface area for the ATP-synthesising complexes |
| Chloroplast | Thylakoids stacked into grana, plus stroma and stroma lamellae | Light reactions on the thylakoid, dark reactions in the stroma — the split reappears in photosynthesis |
| Endoplasmic reticulum | Rough ER bears ribosomes; smooth ER does not | RER → protein synthesis; SER → lipid and steroid synthesis. The one-word distinction is the whole question |
| Golgi apparatus | Flattened cisternae; convex cis (forming) face near the ER, concave trans (maturing) face | Packaging and dispatch; glycosylation of proteins and lipids happens here |
| Lysosome | Single membrane, acidic interior, full of hydrolytic enzymes | Intracellular digestion — the "suicide bag"; the enzymes work only at low pH, which is the cell's safety catch |
| Ribosome | Two subunits of RNA and protein; no membrane | Protein synthesis. Several on one mRNA make a polysome. Discovered by Palade |
| Nucleus | Double envelope with pores; nucleoplasm, nucleolus, chromatin | The nucleolus is the site of ribosomal RNA synthesis. Euchromatin is loose and active; heterochromatin dense and inactive |
| Centriole and cilia | Centriole: cartwheel of nine triplet fibrils, no central one. Cilium: 9+2 axoneme | Centriole → spindle fibres and basal bodies. Note 9 triplets in a centriole against 9 doublets + 2 singlets in a cilium |
🧠 The plastid family, and the leucoplast trio
Plastids come in three kinds and the naming is transparent once you see it. Chloroplasts are green and photosynthesise. Chromoplasts carry other pigments — the carotenoids that make a carrot orange and a tomato red. Leucoplasts are colourless storage plastids, and they split three ways by what they store: amyloplasts store starch, elaioplasts store oils and fats, aleuroplasts store proteins. That trio is asked as a match-the-pair and the roots give it away — amylo for starch as in amylase, elaio for oil, aleuro for the protein-rich aleurone layer we met in the monocot seed. One more contrast worth fixing: a vacuole is bounded by a single membrane called the tonoplast and in a plant cell occupies up to 90% of the volume, while in Amoeba a contractile vacuole handles excretion and osmoregulation instead of storage.ℹ️ Chromosome shapes, named by where the centromere sits
A chromosome carries a primary constriction, the centromere, on which sit the disc-shaped kinetochores that spindle fibres attach to. The centromere's position defines four shapes, and drawing them once fixes them for good. Metacentric — centromere in the middle, two equal arms, so the chromosome looks like a V at anaphase. Sub-metacentric — slightly off centre, one arm shorter, an L shape. Acrocentric — very near one end, one arm extremely short; these often carry a satellite, a small fragment beyond a secondary constriction. Telocentric — centromere at the very tip, a single straight rod. Questions usually give you the arm ratio or the anaphase shape and ask for the name, so the mapping middle-V, off-centre-L, near-end, at-end is the whole answer.Key takeaways
- Virchow completed cell theory: cells arise only from pre-existing cells. Hooke saw cells first; Mycoplasma is the smallest.
- Double-membraned: mitochondrion, plastids and the NUCLEUS. Only mitochondria and chloroplasts have their own DNA and 70S ribosomes.
- No membrane at all: ribosome, nucleolus, centriole, cytoskeleton, bacterial inclusion bodies.
- Prokaryotic ribosomes are 70S, eukaryotic 80S — but 70S inside mitochondria and chloroplasts.
- Fluid mosaic model (Singer & Nicolson): polar heads out, tails in; lipid fluidity enables division, secretion and endocytosis.
- Non-polar solutes diffuse freely; polar ones need a carrier; uphill movement needs a carrier plus ATP. The membrane is selectively permeable, not merely semi-permeable.
- RER makes protein, SER makes lipid and steroid. Golgi does glycosylation and packaging; the nucleolus makes ribosomal RNA.
- Centriole has 9 triplet fibrils and no central fibril; a cilium has the 9+2 axoneme. Leucoplasts: amyloplast starch, elaioplast oil, aleuroplast protein.
Practice questions (8)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
Which of the following is NOT bounded by a membrane?
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Answer: A — Nucleolus
The nucleolus sits inside the nucleus but has no membrane of its own — it is a dense region where ribosomal RNA is synthesised. Its position inside a double-membraned organelle is exactly what makes it tempting to call membrane-bound; lysosomes, vacuoles and peroxisomes all have a single membrane.In a prokaryotic cell, the respiratory enzymes are located on:
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Answer: A — Mesosomes — infoldings of the plasma membrane
Prokaryotes have no mitochondria at all, so the respiratory machinery sits on mesosomes — infoldings of the plasma membrane that increase surface area, doing structurally what cristae do in a eukaryote. Inclusion bodies are reserve stores lying free in the cytoplasm, and the nucleoid is the genetic material.A polar molecule moving down its concentration gradient with the help of a membrane protein, and without ATP, is an example of:
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Answer: A — Facilitated transport
Facilitated transport is passive — no ATP — but needs a carrier, because a polar molecule cannot cross the oily interior of the bilayer unaided. Active transport would require ATP and would move the solute against the gradient, while simple diffusion needs no carrier and is available only to non-polar solutes.Which pair of organelles contains its own DNA and 70S ribosomes?
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Answer: A — Mitochondrion and chloroplast
Only these two are semi-autonomous — they carry circular DNA, make some of their own proteins on 70S ribosomes and divide by fission, which is the evidence behind the endosymbiotic account of their origin. The nucleus has DNA but 80S ribosomes are in the cytoplasm, not inside it, so the second option fails on the ribosome half.The centriole differs from the axoneme of a cilium in that the centriole has:
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Answer: A — Nine triplet fibrils and no central fibril
A centriole is a cartwheel of nine evenly spaced peripheral triplet fibrils with a proteinaceous hub and radial spokes, and crucially no central fibril; the cilium has the 9+2 pattern instead. Option two is precisely the axoneme description, which is why the two structures are almost always set against each other.Elaioplasts are leucoplasts specialised for storing:
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Answer: A — Oils and fats
Elaioplasts store oils and fats, amyloplasts store starch and aleuroplasts store proteins — the three colourless leucoplasts. Carotenoids belong to chromoplasts, which are a different class of plastid altogether, so that option is testing whether the leucoplast/chromoplast boundary is clear.The statement "Omnis cellula-e cellula" — cells arise from pre-existing cells — was given by:
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Answer: A — Rudolf Virchow
Schleiden and Schwann proposed that all organisms are made of cells but could not explain how new cells appear; Virchow supplied that in 1855 and completed the theory. Hooke merely observed and named cells, and Singer and Nicolson gave the fluid mosaic model of the membrane over a century later.A chromosome with the centromere slightly away from the middle, giving one shorter and one longer arm, is described as:
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Answer: A — Sub-metacentric
Sub-metacentric gives unequal arms and an L shape at anaphase. Metacentric has the centromere exactly in the middle with two equal arms; acrocentric places it very near one end with one extremely short arm, often carrying a satellite; and telocentric has it at the very tip.