Almost every question in NEET's cell biology block — and a surprising number in genetics, plant physiology and biotechnology — traces back to knowing exactly which structure does what, and which organism has which structure. This lesson builds that map from scratch, starting with the idea of the cell itself and ending inside the nucleus.
The Cell as a Unit of Life: Theory and Diversity
The recognition that living matter is built from cells came in stages. Robert Hooke, looking at a slice of cork under a crude compound microscope in 1665, saw a honeycomb of empty compartments and borrowed the Latin word for "little room" — he was actually seeing dead cell walls, not living cells. Living cells were first seen by Anton van Leeuwenhoek. Nearly two centuries later, Matthias Schleiden (studying plants) and Theodor Schwann (studying animals) independently concluded that organisms are cellular; Schwann further proposed that cells secrete their own boundary layer. Their combined generalisation became the cell theory, which Rudolf Virchow completed in 1855 with the crucial addition that cells arise only from pre-existing cells — no spontaneous generation.
The modern statement of cell theory has three parts:
- All living organisms are made of one or more cells.
- The cell is the basic structural and functional unit of life.
- Every cell originates from a pre-existing cell by division.
Cells vary enormously in size and shape, and these extremes are favourite one-mark questions:
- Smallest cell: Mycoplasma, roughly 0.3 µm across — the smallest known living cell.
- Bacteria: typically 3–5 µm long.
- Largest single cell: the ostrich egg.
- Longest cells: nerve cells (in animals); among plants, elongated fibre cells.
- Largest human cell: the ovum (about 100 µm); smallest, the RBC (~7 µm).
Shape follows function: red blood cells are biconcave discs for gas exchange, guard cells in a leaf are bean-shaped so they can bow apart, and cork cells are boxy for tight packing.
Prokaryotic Cells and the Cell Envelope
Prokaryotes — bacteria, cyanobacteria, mycoplasma and actinomycetes — are defined by the absence of a true, membrane-bound nucleus. Their genetic material is a single circular DNA molecule lying naked in the cytoplasm in a region called the nucleoid. Extra, smaller circular DNA pieces called plasmids may be present; these carry genes such as those for antibiotic resistance and are the workhorses of genetic engineering. Prokaryotes are generally smaller but multiply much faster than eukaryotes, and their cytoplasm contains ribosomes but no membranous organelles — no mitochondria, no plastids, no ER, no Golgi.
Outside the cytoplasm lies a three-layered cell envelope: the outermost glycocalyx, then the cell wall, then the plasma membrane. When the glycocalyx is a loose, easily washed-off sheath it is a slime layer; when it is a thick, tough coat it is a capsule. Gram staining, developed by Christian Gram, divides bacteria into Gram-positive and Gram-negative types on the basis of envelope differences.
Other prokaryotic features worth memorising:
- Ribosomes are 70S, made of a 50S and a 30S subunit. Several ribosomes strung along one mRNA form a polysome (polyribosome).
- Mesosomes are infoldings of the plasma membrane forming vesicles, tubules and lamellae; they increase surface area for enzymes, aiding respiration and secretion.
- Chromatophores are pigment-containing membrane extensions in cyanobacteria and some photosynthetic bacteria.
- Inclusion bodies (gas vacuoles, glycogen granules, phosphate granules, cyanophycean granules) store reserve material and lie free in the cytoplasm without any membrane.
- Flagella of bacteria consist of three parts — filament, hook and basal body — and are used for movement. Pili and fimbriae are surface appendages for attachment, not motility.
Plant Cell Wall, Plasma Membrane and Vacuole
The plasma membrane is described by the fluid mosaic model of Singer and Nicolson: a bilayer of phospholipids (polar heads facing outward, hydrophobic tails inward) studded with proteins that can drift laterally. This lateral mobility — the "fluid" part — is essential for endocytosis, cell growth and cell division. Earlier, Gorter and Grendel had proposed the simple bimolecular lipid layer. Membranes also contain cholesterol (animals) and, on the outer face, carbohydrates.
Transport across the membrane comes in two broad kinds:
- Passive transport — no energy needed; the substance moves down its concentration gradient. Non-polar molecules cross the lipid part easily (simple diffusion); water movement across a membrane is osmosis.
- Active transport — energy from ATP is spent to pump substances against the gradient, as in the Na⁺/K⁺ pump.
The plant cell wall is a non-living, rigid outer layer that gives shape, resists mechanical injury and infection, and permits cell-to-cell interaction. In algae it is built of cellulose, galactans, mannans and calcium carbonate; in higher plants of cellulose, hemicellulose, pectin and proteins. A young cell first lays down a thin primary wall, which may be followed by an inner secondary wall as the cell matures. The middle lamella, largely calcium pectate, cements adjacent cells together. Cytoplasmic bridges called plasmodesmata pass through wall pits, linking the cytoplasm of neighbouring cells.
The vacuole in a mature plant cell can occupy up to 90% of the volume, pushing the cytoplasm into a thin peripheral layer. Its single membrane is the tonoplast, which actively accumulates ions so that the vacuole's sap is far more concentrated than the cytoplasm — the resulting water influx generates turgor pressure, the reason a well-watered herb stands upright. In Amoeba the contractile vacuole handles excretion, and in many protists food vacuoles carry out digestion.
The Double-Membraned Organelles: Mitochondria and Plastids
Mitochondria are the sites of aerobic respiration and are hence called the powerhouse of the cell, since they generate ATP. Each is bounded by two membranes. The outer membrane is smooth and continuous; the inner membrane folds inward into cristae, greatly enlarging the surface available for the respiratory enzyme complexes. The two membranes enclose two compartments: the narrow outer (intermembrane) space and the inner matrix, which is dense and packed with enzymes of the Krebs cycle. Mitochondria have their own circular DNA and 70S ribosomes, synthesise some of their proteins, and divide by fission — the basis of the endosymbiotic idea.
Plastids are found only in plant cells and some protists and are classified by pigment:
- Chloroplasts — contain chlorophyll and carotenoids; carry out photosynthesis.
- Chromoplasts — carry other carotenoid pigments (fat-soluble), giving flowers and fruits yellow, orange or red colours.
- Leucoplasts — colourless storage plastids, subdivided into amyloplasts (starch), elaioplasts (oils and fats) and aleuroplasts (proteins).
A chloroplast in a mesophyll cell is a lens-shaped body 5–10 µm long, with 20–40 per cell being typical. Inside its double membrane is the stroma, containing the enzymes of the Calvin cycle (dark reactions), plus DNA and 70S ribosomes. Suspended in the stroma is a system of flattened sacs, the thylakoids, stacked like coins into grana; some thylakoids extend between grana as stroma lamellae. Chlorophyll sits in the thylakoid membranes, so light reactions occur in the thylakoids and carbon fixation in the stroma — a distinction examiners love.
The Endomembrane System and Ribosomes
Several organelles work as one coordinated network — the endomembrane system: endoplasmic reticulum, Golgi complex, lysosomes and vacuoles. (Mitochondria, chloroplasts and peroxisomes are excluded because their functions are not coordinated with this system.)
The endoplasmic reticulum is a network of tubules and flattened cisternae dividing the cytoplasm into a luminal and an extra-luminal compartment. Where ribosomes are attached to its surface it is rough ER (RER), abundant in cells actively exporting protein; where ribosomes are absent it is smooth ER (SER), the site of lipid and steroid synthesis. Sarcoplasmic reticulum in muscle is a specialised SER.
The Golgi complex (described by Camillo Golgi) consists of stacks of flattened, concentrically arranged cisternae held near the nucleus. It is polarised: the convex cis or forming face lies close to the ER, the concave trans or maturing face away from it. Materials received from the ER are chemically modified — notably glycosylation of proteins and lipids — then packaged and dispatched. The Golgi is thus the cell's principal packaging and dispatch centre, and it also forms lysosomes.
Lysosomes are single-membraned vesicles rich in hydrolases — lipases, proteases, carbohydrases — that work best in an acidic medium and can digest carbohydrates, proteins, lipids and nucleic acids. Peroxisomes and glyoxysomes are separate microbodies containing oxidative enzymes.
Ribosomes, discovered as dense particles by George Palade, are made of RNA and protein and possess no membrane. They are the sites of protein synthesis. Eukaryotic cytoplasmic ribosomes are 80S (60S + 40S subunits), while prokaryotic ones, and those inside mitochondria and chloroplasts, are 70S. The "S" stands for Svedberg unit, a measure of sedimentation rate — which is why 50S + 30S gives 70S rather than 80S.
Nucleus, Cytoskeleton, Cilia and Flagella
The nucleus was first described by Robert Brown, and the stainable material within it was named chromatin by Flemming. Its envelope is made of two membranes, the outer one often continuous with the ER, separated by a perinuclear space of 10–50 nm. The envelope is interrupted at intervals by nuclear pores, through which RNA and proteins move between nucleoplasm and cytoplasm. Inside lie the nucleoplasm, chromatin and the nucleolus — a non-membranous, spherical site of ribosomal RNA synthesis, larger in metabolically active cells.
Interphase chromatin condenses at division into chromosomes, each with a centromere bearing disc-shaped kinetochores to which spindle fibres attach. Chromosome shape depends on centromere position:
- Metacentric — centromere in the middle, two equal arms.
- Sub-metacentric — slightly off-centre, one arm shorter.
- Acrocentric — near one end, one very long and one very short arm.
- Telocentric — terminal, with the centromere at one extreme end so only a single arm is visible.
Beyond the chromosomes, the cytoplasm is criss-crossed by a cytoskeleton of microtubules, microfilaments and intermediate filaments, which gives the cell its shape, anchors organelles in place and drives movements such as cytoplasmic streaming and the pulling apart of chromosomes at division (the spindle itself is built of microtubules). Closely related to microtubules are centrioles — a pair of cylindrical structures lying at right angles to each other near the nucleus in animal cells, each built of nine peripheral triplets of microtubules in a cartwheel-like array; centrioles organise the spindle during division and give rise to the basal body of cilia and flagella. Plant cells (apart from some motile gametes) lack centrioles altogether.
Cilia and flagella are both motile, hair-like projections of the plasma membrane, differing chiefly in number and beat pattern: cilia are numerous and short, beating like oars to move fluid over a stationary cell or to move a cell through fluid, while flagella are typically one or two per cell, much longer, and propel the cell with whip-like undulations. Internally both share the same 9 + 2 axoneme — nine peripheral doublets of microtubules surrounding one central pair, enclosed by the plasma membrane and anchored below by a basal body structurally identical to a centriole. Motility comes from the motor protein dynein, whose ATP-driven sliding of adjacent doublets bends the axoneme.
Common Mistakes and Exam Traps
- 70S vs 80S confusion. Cytoplasmic ribosomes of eukaryotes are 80S, but ribosomes inside mitochondria and chloroplasts — like all prokaryotic ribosomes — are 70S; NEET often tests exactly this "odd one out".
- Mixing up chromoplasts and leucoplasts. Chromoplasts carry colour pigments (carotenoids); leucoplasts are colourless and purely for storage. Amyloplasts, elaioplasts and aleuroplasts are subtypes of leucoplasts, not separate categories of plastid.
- Overcrowding the endomembrane system. Mitochondria, chloroplasts and peroxisomes are commonly (and wrongly) lumped into the endomembrane system; only the ER, Golgi complex, lysosomes and vacuoles qualify, since only these coordinate through vesicle transfer.
- Blaming the wall for selective permeability. The cell wall is freely permeable and purely mechanical; it is the plasma membrane that is selectively permeable and controls transport, and plasmolysis results from water loss across the membrane, not from any property of the wall.