Cell — The Unit of Life — NEET Notes

Cell theory — who built it?

The idea that life is cellular did not arrive in a single insight; it emerged piece by piece across two centuries of patient microscopy. The invention of the microscope, and its later improvement leading to the electron microscope, revealed all the structural details of the cell that earlier generations could only guess at. Antonie van Leeuwenhoek, the Dutch draper turned microscopist, was the first to see and describe a live cell in the late seventeenth century — he peered through hand-ground lenses at pond water, blood, and his own dental plaque, and called what he saw "animalcules". Robert Brown followed in 1831 by discovering the nucleus — the dense body inside every plant cell he examined. But it was two German scientists, working independently on opposite sides of the plant/animal divide, who turned scattered observations into a unifying theory.

In 1838, Matthias Schleiden, a German botanist, examined a large number of plants and observed that all plants are composed of different kinds of cells which form the tissues of the plant. About a year later, in 1839, Theodor Schwann, a German zoologist, studied different types of animal cells and reported that cells have a thin outer layer — today known as the plasma membrane — and that the presence of a cell wall is a unique character of plant cells. On the basis of these studies, Schwann proposed the hypothesis that the bodies of animals and plants are composed of cells and products of cells. Schleiden and Schwann together formulated the cell theory — a single sentence that unified zoology and botany under one principle.

The theory as Schleiden and Schwann left it had one obvious gap: it did not explain how new cells were formed. The prevailing view was that cells could appear spontaneously in fluids, a kind of cellular abiogenesis. Rudolf Virchow closed this gap in 1855 with the famous dictum Omnis cellula-e cellula — every cell from a pre-existing cell. Virchow's contribution was not merely an addendum; it was the decisive blow against spontaneous generation at the cellular level, and it transformed cell theory from a structural claim into a developmental one. NEET has tested this attribution directly: NEET 2019 asked who proposed "Omnis cellula-e cellula" and the answer is Virchow, not Schleiden, not Schwann, not Aristotle (the latter being a particularly mischievous distractor). With Virchow's amendment, the modern cell theory took its final two-clause form, and that is the form NCERT and NEET expect you to reproduce.

(i) All living organisms are composed of cells and products of cells. (ii) All cells arise from pre-existing cells.

The cell theory in its modern form — Schleiden, Schwann, Virchow

An overview of the cell

Before drilling into individual organelles, NCERT asks you to picture a typical onion cell and a human cheek cell — the two specimens every Class 11 student has seen under a microscope. The onion cell, a typical plant cell, has a distinct cell wall as its outer boundary and just within it the cell membrane. The cheek cell has only an outer membrane as its delimiting structure. Inside each cell sits a dense, membrane-bound body — the nucleus — and within the nucleus, chromosomes carrying DNA. This minimal anatomy already separates living from non-living: a cell is the smallest unit that satisfies the criterion of independent existence and performs all the essential functions of life. Anything less than a complete cell — a stray ribosome, a free chromosome, an isolated mitochondrion — cannot live on its own.

Cells that have a membrane-bound nucleus are called eukaryotic; cells that lack one are prokaryotic. In both, a semi-fluid matrix called the cytoplasm fills the volume of the cell and serves as the main arena of cellular activity. Various chemical reactions occur in the cytoplasm to keep the cell in the "living state". Beyond the nucleus, eukaryotic cells contain other membrane-bound organelles — endoplasmic reticulum, Golgi complex, lysosomes, mitochondria, microbodies, vacuoles — that prokaryotes lack. Ribosomes are the one organelle found in both: non-membrane-bound granules of protein and RNA that synthesise protein in every living cell. In eukaryotes, ribosomes appear free in the cytoplasm, attached to the rough endoplasmic reticulum, and inside two organelles — mitochondria and chloroplasts in plants. Animal cells additionally have another non-membrane-bound organelle, the centrosome, which assists in cell division.

Cells differ greatly in size, shape and activity. Mycoplasmas, the smallest cells, are only 0.3 µm long. Typical bacteria are 3–5 µm. Viruses, although not cells themselves, are even smaller at 0.02–0.2 µm. A typical eukaryotic cell is around 10–20 µm. Human red blood cells are about 7 µm in diameter, biconcave and disc-shaped; white blood cells are amoeboid and branched. Nerve cells are some of the longest cells, reaching a metre or more in tall animals. The largest isolated single cell is the egg of an ostrich. Shape can be disc-like, polygonal, columnar, cuboid, thread-like, or irregular — and it almost always tracks the cell's function. Columnar epithelial cells of the gut are long and narrow because they line a tube; tracheids are elongated because they conduct water; biconcave red blood cells flex through narrow capillaries.

Prokaryotic cells

The prokaryotes are represented by bacteria, blue-green algae (cyanobacteria), mycoplasma, and PPLO (Pleuro-Pneumonia-Like Organisms). They are generally smaller and multiply more rapidly than eukaryotic cells, and they come in four classic shapes: bacillus (rod), coccus (sphere), vibrio (comma), and spirillum (spiral). Despite the diversity in shape, their internal organisation is fundamentally similar.

All prokaryotes have a cell wall outside their plasma membrane — except mycoplasma, which has only the membrane. The cytoplasm fills the cell as a semi-fluid matrix. There is no membrane-bound nucleus: the genetic material is naked, a single circular chromosome of DNA lying free in the cytoplasm. Many bacteria carry additional small circular DNA molecules called plasmids outside the main genome. Plasmids confer phenotypic traits like antibiotic resistance, and in later classes you will see plasmids used as vectors to introduce foreign DNA into bacteria.

No membrane-bound organelles exist in prokaryotic cells. Their only organelle is the ribosome. They do, however, possess a peculiar membrane structure — the mesosome — which is an infolding of the plasma membrane. They also possess inclusion bodies, non-membrane-bound reserves of stored material.

Cell envelope: glycocalyx, cell wall, plasma membrane

Most prokaryotic cells, particularly bacterial cells, have a chemically complex cell envelope built as a tightly bound three-layered structure. Although each layer of the envelope performs a distinct function, the three act together as a single protective unit. The outermost layer is the glycocalyx, which differs in composition and thickness among different bacteria. In some, it is a loose sheath called the slime layer; in others, it is thick and tough and is called the capsule, important for resisting host immune attack. Below it is the cell wall, which determines the shape of the cell and provides strong structural support to prevent the bacterium from bursting or collapsing. Innermost is the plasma membrane, selectively permeable in nature and structurally similar to the eukaryotic plasma membrane — it is here that the bacterium interacts with the outside world. Bacteria are sorted into Gram positive (those that take up the Gram stain) and Gram negative (those that do not) on the basis of differences in this envelope, particularly the thickness of the peptidoglycan layer in the cell wall. The Gram staining procedure, developed by Hans Christian Gram, remains a cornerstone of bacterial classification.

Mesosome — the prokaryote's membrane folding

The mesosome is the most prokaryote-specific structure NEET expects you to name. It is formed by extensions of the plasma membrane into the cell as vesicles, tubules, and lamellae. Its functions are remarkably broad: it helps in cell wall formation, DNA replication and distribution to daughter cells, respiration, secretion, and it increases the surface area and enzymatic content of the plasma membrane. In some prokaryotes like cyanobacteria, additional membrane extensions called chromatophores carry photosynthetic pigments.

Flagella, pili, fimbriae — surface structures

Bacterial flagella are thin filamentous extensions from the cell wall responsible for motility. A bacterial flagellum has three parts: filament, hook, and basal body — structurally completely different from the eukaryotic 9+2 flagellum. Pili are elongated tubular structures of a special protein involved in conjugation (not motility). Fimbriae are small bristle-like fibres that help bacteria attach to rocks in streams or to host tissues.

Ribosomes and inclusion bodies

Prokaryotic ribosomes are 70S, made of two subunits — 50S and 30S — that come together to translate mRNA. They are about 15 nm × 20 nm and often associated with the plasma membrane. Several ribosomes may attach to a single mRNA to form a polysome, translating the same message into multiple protein copies. Inclusion bodies store reserve material — phosphate granules, cyanophycean granules, glycogen granules — and lie free in the cytoplasm without any bounding membrane. Gas vacuoles are found in blue-green, purple, and green photosynthetic bacteria.

Eukaryotic cells — the big tent

Eukaryotic cells include all protists, plants, animals, and fungi. The defining feature is extensive compartmentalisation of the cytoplasm by membrane-bound organelles, plus an organised nucleus enclosed by a nuclear envelope. This compartmentalisation is more than tidy housekeeping — it lets a eukaryotic cell run multiple biochemistries side by side, each at its own pH and ion concentration, each isolated from enzymes that would otherwise scramble it. Eukaryotic cells also carry an elaborate cytoskeleton and complex locomotory structures. Their genetic material is organised into chromosomes — DNA wrapped around histones and other basic proteins, condensed dramatically during cell division and unspooled during interphase.

Not all eukaryotic cells are alike. Plant cells and animal cells diverge on three definitive features. Plant cells possess a rigid cellulosic cell wall outside the plasma membrane, plastids (chloroplasts, chromoplasts, leucoplasts) that carry pigments and store food, and a large central vacuole that can occupy up to 90% of the cell's volume. Animal cells lack all three of those features but possess centrioles (housed in the centrosome) which are absent in almost all plant cells. NEET routinely tests this divergence both ways: questions about a feature "absent in animal cells" usually point at cell wall, plastids, or large vacuole, while questions about a feature "absent in plant cells" point at centrioles. Both kinds of cell still share the universal eukaryotic toolkit — nucleus, ER, Golgi, mitochondria, ribosomes, lysosomes, peroxisomes, cytoskeleton — so a question that lists "mitochondria" as a point of difference is wrong.

Plasma membrane — fluid mosaic model

The detailed structure of the plasma membrane was deduced only after the electron microscope arrived in the 1950s, though chemical studies on human red blood cells in the early twentieth century had already hinted at the answer. Those chemical studies revealed that the membrane is mainly composed of lipids and proteins. The major lipids are phospholipids arranged in a bilayer with polar heads pointing outward toward the aqueous environment and non-polar hydrocarbon tails pointing inward toward each other. This orientation protects the hydrophobic tails of saturated hydrocarbons from contact with water — the same physical principle that makes oil and water separate. The bilayer also contains cholesterol, which interleaves between phospholipids and stabilises fluidity across the body's temperature range. In human erythrocyte membranes, the lipid–protein ratio is roughly 40 : 52 by mass, but the actual ratio varies considerably between cell types — myelin sheaths are lipid-heavy, mitochondrial inner membranes are protein-heavy.

Membrane proteins are classified by ease of extraction. Peripheral proteins lie loosely on the surface of the membrane and can be detached with mild salt washes. Integral proteins are partially or totally buried in the bilayer — many of them span it completely as transmembrane proteins — and require detergents to extract. The membrane also carries carbohydrates: short oligosaccharide chains attached to lipids (glycolipids) and proteins (glycoproteins) on the outer face, forming the glycocalyx that mediates cell–cell recognition.

The fluid mosaic model, proposed by Singer and Nicolson in 1972, gave the membrane its modern picture and remains the model NEET asks about. According to this model, the lipid bilayer is quasi-fluid, allowing lateral movement of proteins within it like islands floating in a two-dimensional sea. The ability of a protein to move within the membrane is measured as the membrane's fluidity. This fluidity is not decorative — it is essential for cell growth, formation of intercellular junctions, secretion, endocytosis, and cell division. A membrane that cannot move cannot fuse, cannot bud off vesicles, and cannot rearrange receptors. A frozen membrane is a dead membrane.

Transport across the plasma membrane

One of the most important functions of the plasma membrane is the regulated transport of molecules across it. The membrane is selectively permeable: some molecules pass freely, others are blocked, others require active machinery. Molecules cross it by several routes, classified by whether energy is consumed.

Passive transport requires no ATP. Many molecules move briefly across the membrane without any requirement of energy — this is the case for neutral, non-polar solutes, which diffuse along their concentration gradient by simple diffusion, from higher to lower concentration. Water moves across the membrane by osmosis, again from higher water potential to lower. Polar molecules — sugars, amino acids, ions — cannot pass through the non-polar lipid bilayer on their own and require a carrier protein to facilitate their movement; this carrier-assisted but still energy-free passage is called facilitated diffusion.

Active transport moves ions and molecules against their concentration gradient — from lower to higher concentration — and is therefore energy-dependent. ATP is hydrolysed to power the carrier. The textbook example is the Na⁺/K⁺ pump, which expels three sodium ions and imports two potassium ions per ATP, maintaining the resting membrane potential of every animal cell. NEET 2023 tested this distinction directly — any question that says "against a concentration gradient" has only one answer: active transport.

Cell wall — the non-living outer coat

A non-living rigid structure called the cell wall forms an outer covering for the plasma membrane in fungi and plants. It performs four functions which together explain its evolutionary importance: it gives shape to the cell, protects the cell from mechanical damage and infection, mediates cell-to-cell interaction, and acts as a barrier to undesirable macromolecules. Without a cell wall, a plant cell placed in a hypotonic solution would simply burst from osmotic pressure; with one, it merely becomes turgid and the wall absorbs the strain. Composition varies with the lineage. In algae, the wall is made of cellulose, galactans, mannans, and minerals like calcium carbonate. In other plants, it consists of cellulose, hemicellulose, pectins, and proteins. Fungal walls are made not of cellulose but of chitin, the same polymer that hardens insect exoskeletons.

The cell wall is not deposited all at once. The young plant cell has a primary wall, thin, elastic, and capable of growth — the wall must stretch as the cell expands. As the cell matures, growth diminishes and a secondary wall is laid down on the inner side of the primary wall, toward the membrane. Secondary walls may carry lignin, suberin, or cutin depending on the tissue type, which is why xylem cells become rigid conduits while epidermal cells become waterproofed. Neighbouring cells are glued together by a layer of calcium pectate called the middle lamella, the very first layer deposited between two daughter cells after division.

Both the cell wall and the middle lamella are traversed by plasmodesmata — narrow cytoplasmic strands lined by plasma membrane that connect the cytoplasm of adjacent plant cells into one continuous network called the symplast. Through these channels, ions, small molecules, and even some proteins and RNAs can move directly from cell to cell without crossing a membrane. Plasmodesmata are the plant cell's equivalent of an animal cell's gap junctions, and they are the reason a plant tissue behaves as an integrated unit rather than a collection of isolated boxes.

Endomembrane system

The cytoplasm of a eukaryotic cell is not a homogeneous broth — it is divided into compartments by membranes. Each of these membranous organelles is distinct in terms of its structure and function, but four of them work as an integrated logistics network whose functions are coordinated, so NCERT groups them under one umbrella: the endomembrane system. It consists of the endoplasmic reticulum (ER), Golgi complex, lysosomes, and vacuoles. Material enters this system at the ER, passes through the Golgi, and either becomes a lysosome or a secretory vesicle that fuses with the plasma membrane. Mitochondria, chloroplasts, and peroxisomes are also membrane-bound organelles, but because their functions are not coordinated with the above four — they do their own thing, generate their own energy, and have their own DNA — they are not part of the endomembrane system. NEET 2021 tested this list verbatim, and a wrong choice that includes mitochondria, chloroplasts, or ribosomes is always a trap.

Endoplasmic reticulum — RER and SER

Electron microscopic studies of eukaryotic cells reveal a network or reticulum of tiny tubular structures scattered through the cytoplasm — the endoplasmic reticulum (ER). The ER membrane divides the intracellular space into two distinct compartments: the luminal compartment (inside the ER) and the extra-luminal compartment (the cytoplasm proper). When ribosomes are attached to the outer (cytoplasm-facing) surface, the ER appears bumpy under an electron microscope and is called rough endoplasmic reticulum (RER). When ribosomes are absent, the surface appears smooth — smooth endoplasmic reticulum (SER). RER is found extensively in cells actively engaged in protein synthesis and secretion — plasma cells, pancreatic acinar cells, hepatocytes — and is continuous with the outer membrane of the nuclear envelope. SER is the main site of lipid synthesis, and in animal cells it makes lipid-like steroidal hormones. NEET 2022 used this distinction to set a trap: prokaryotes have no ER at all, neither RER nor SER. NEET 2018 also tested it from the opposite angle — phospholipid synthesis happens in SER, not RER.

Golgi apparatus — packaging and dispatch

Camillo Golgi first observed densely-stained reticular structures near the nucleus in 1898; they were named Golgi bodies after him. They consist of many flat, disc-shaped sacs or cisternae of 0.5–1.0 µm diameter, stacked parallel to each other near the nucleus. The number of cisternae per stack varies. Each Golgi stack has two faces. The cis face, also called the forming face, is convex and faces the ER — it receives vesicles arriving from there. The trans face, or maturing face, is concave and faces the plasma membrane — it releases finished vesicles. The two faces are entirely different in composition but interconnected by the cisternae between them.

The Golgi apparatus principally performs the function of packaging materials, to be delivered either to intra-cellular targets or secreted outside the cell. Materials packaged in the form of vesicles fuse with the cis face, are modified as they move through the cisternae, and exit at the trans face. This explains why the Golgi apparatus remains in close physical association with the endoplasmic reticulum — they form an assembly line. A number of proteins synthesised by RER ribosomes are modified in the Golgi cisternae before release. The Golgi is the important site of formation of glycoproteins (sugars added to proteins) and glycolipids (sugars added to lipids) — NEET 2020 tested this exact fact, and NEET 2018 asked about the Golgi's role in forming secretory vesicles.

Lysosomes — the cell's recycling depot

Lysosomes are membrane-bound vesicular structures formed by the process of packaging in the Golgi apparatus. They bud off the trans face. NEET 2019 used this as a trap: a tempting wrong option says "lysosomes are formed by packaging in the endoplasmic reticulum" — they are not. Isolated lysosomal vesicles are very rich in almost all types of hydrolytic enzymes (hydrolases — lipases, proteases, carbohydrases, nucleases), all of which are optimally active at acidic pH. These enzymes can digest carbohydrates, proteins, lipids, and nucleic acids. Lysosomes break down worn-out organelles (autophagy), engulfed bacteria (heterophagy), and the whole cell during programmed cell death. Their single membrane keeps the destructive enzymes safely bottled up — a leaking lysosome would digest the cell from within, which is why they are sometimes called the "suicide bags" of the cell.

Vacuoles — the storage compartments

The vacuole is the membrane-bound space found in the cytoplasm. It contains water, sap, excretory products, and other materials not useful (or even harmful) to the cell. The vacuole is bound by a single membrane called the tonoplast. In plant cells, the vacuoles can occupy up to 90 percent of the volume of the cell — a single large central vacuole pushes the rest of the cytoplasm into a thin layer against the cell wall, an arrangement that minimises the metabolic cost of building cytoplasm. The tonoplast facilitates the transport of a number of ions and other materials against concentration gradients into the vacuole, so their concentration is significantly higher inside the vacuole than in the cytoplasm. Water-soluble pigments like anthocyanins, which give red and purple colours to many flowers and fruits, are stored in the vacuolar sap — NEET 2016 tested exactly this. In Amoeba, the contractile vacuole is important for osmoregulation and excretion; in many protists, food vacuoles are formed by engulfing food particles.

Protein secretion — the canonical route

A secretory protein takes a fixed path through the endomembrane system. It is translated on ribosomes attached to the rough ER, threaded into the ER lumen, packaged into transport vesicles, ferried to the cis face of the Golgi, modified through the cisternae (glycosylation among other steps), released from the trans face inside secretory vesicles, and finally exocytosed across the plasma membrane. This route — RER → Golgi → vesicle → plasma membrane — is the most-tested pathway in this chapter.

Mitochondria — power house of the cell

Mitochondria are usually invisible without specific staining, which is one reason they were among the last major organelles to be characterised. Their number per cell varies with physiological activity — muscle cells and neurons carry hundreds, mature red blood cells carry none — and so do their shape and size. A typical mitochondrion is sausage-shaped or cylindrical, 0.2–1.0 µm in diameter (average 0.5 µm) and 1.0–4.1 µm long. Each is a double-membrane-bound organelle, and this double-membrane architecture is the single most important structural fact about them.

The outer membrane is smooth and forms the continuous limiting boundary of the organelle. It is highly permeable to small molecules and ions because it carries large porin channels. The inner membrane is much more selective, and it folds inward into ridges called cristae that project into the matrix. The cristae dramatically increase the surface area of the inner membrane — by some estimates fivefold — and this is where the electron transport chain enzymes and ATP synthase live. A NEET 2019 trap that catches many students: the enzymes of the electron transport chain are embedded in the inner membrane, not the outer one. The two membranes divide the mitochondrion's lumen into two aqueous compartments: the intermembrane space (outer compartment, between the two membranes) and the matrix (inner compartment, enclosed by the inner membrane), a dense homogeneous fluid.

The matrix is more than a filler. It contains a single circular DNA molecule, a few RNA molecules, 70S ribosomes, and the components required for protein synthesis. Mitochondria divide by fission, like bacteria, from pre-existing mitochondria — they are never made de novo. These features make mitochondria semi-autonomous organelles: they can produce some of their own proteins but rely on the nucleus for most. NEET 2016 tested this label directly, and the same NEET 2016 question included a deliberate distractor claiming mitochondria "lack protein synthesising machinery" — they do not; the 70S ribosomes inside the matrix are exactly that machinery. The bacterial-style DNA and 70S ribosomes are powerful evidence for the endosymbiotic theory: that mitochondria evolved from free-living alpha-proteobacteria that were engulfed by an ancestral eukaryotic cell about two billion years ago.

Functionally, mitochondria are the sites of aerobic respiration — the Krebs cycle runs in the matrix and oxidative phosphorylation runs on the inner membrane. They couple electron transport to chemiosmosis and produce cellular energy in the form of ATP, hence the textbook nickname power house of the cell. NEET 2017 used those exact words. The amount of ATP a mitochondrion can churn out per second is staggering — a single liver cell produces and consumes roughly its own body weight in ATP each day, and almost all of that traffic runs through the mitochondrial cristae.

Plastids — chloroplast, chromoplast, leucoplast

Plastids are found in plant cells and in Euglenoides. They are large enough to be seen under the light microscope and carry specific pigments that give plant tissues their colour. Based on pigment content, plastids fall into three groups.

Most chloroplasts of green plants are found in the mesophyll cells of leaves, where photosynthesis happens at industrial scale. They are lens-shaped, oval, spherical, discoid, or even ribbon-like, with a variable length of 5–10 µm and width of 2–4 µm. Their number varies dramatically — from 1 per cell in the alga Chlamydomonas to 20–40 per mesophyll cell in a higher plant. Like mitochondria, chloroplasts are double-membrane bound, and the inner chloroplast membrane is relatively less permeable than the outer. The space enclosed by the inner membrane is the stroma — host to Calvin-cycle enzymes for carbohydrate and protein synthesis, double-stranded circular DNA molecules, and 70S ribosomes (smaller than the 80S ribosomes of the surrounding cytoplasm).

Suspended in the stroma are organised flattened membranous sacs called thylakoids, stacked like piles of coins into grana (singular: granum), with each granum holding ten to a hundred thylakoid discs. The grana are linked together by flat membranous tubules called the stroma lamellae or intergranal thylakoids. The space enclosed by a thylakoid membrane is called the lumen, and the chlorophyll pigments are embedded in the thylakoid membrane itself — this is where the light reactions of photosynthesis happen. Like mitochondria, chloroplasts have their own DNA, their own ribosomes, and divide by fission, making them semi-autonomous organelles. NEET 2016 tested this concept with a paired statement: both mitochondria and chloroplasts are semi-autonomous and contain protein-synthesising machinery, so the claim that they "lack protein synthesising machinery" is false.

NEET 2021 tested the cristae–thylakoid–cisternae match — three deceptively similar terms that map to three different organelles. Cristae are infoldings of the inner mitochondrial membrane; thylakoids are flattened sacs in chloroplast stroma; cisternae are disc-shaped sacs in the Golgi apparatus. Get these three straight and a whole class of questions becomes free marks. The same kind of vocabulary trap surfaces with chromosome anatomy: the centromere is the primary constriction in a chromosome, often confused with cristae and thylakoids in poorly read paragraphs.

Ribosomes — 70S and 80S

Ribosomes were first observed as dense granular particles under the electron microscope by George Palade in 1953, who described them as the "dark granules of the cytoplasm" before their function was understood. They are composed of ribonucleic acid (RNA) and proteins — specifically ribosomal RNA (rRNA) and ribosomal proteins — and they are not bounded by any membrane. They are the universal site of protein synthesis. Every living cell has them, with no exception: free in the cytoplasm, bound to rough ER, inside mitochondria, and inside chloroplasts of plant cells. NCERT lists this distribution explicitly, and NEET frequently uses it in statement-correct questions.

Two sizes exist, and the distinction is crucial. Prokaryotic ribosomes are 70S, with a 50S large subunit and a 30S small subunit. Eukaryotic cytoplasmic ribosomes are 80S, with a 60S large subunit and a 40S small subunit. Each ribosome has two subunits, and the two subunits associate only when ready to translate — between rounds of translation, the subunits dissociate. The crucial NCERT note that often catches students: the ribosomes inside mitochondria and chloroplasts of eukaryotes are 70S, not 80S — exactly like prokaryotic ribosomes. This is one of the strongest pieces of evidence for the endosymbiotic origin of these two organelles. The "S" is the Svedberg unit (named after Theodor Svedberg, who invented the ultracentrifuge), and it is indirectly a measure of density and size — specifically, the rate at which a particle sediments in a centrifugal field. It is not a unit of mass, which is why the subunit numbers do not add neatly: 50S + 30S gives 70S, not 80S, because sedimentation depends on density and shape, not mass alone. Several ribosomes may attach to a single mRNA simultaneously to form a chain called a polyribosome or polysome; each ribosome translates the same mRNA independently, producing multiple copies of the same protein in parallel.

Cytoskeleton

An elaborate network of filamentous proteinaceous structures running through the cytoplasm is collectively called the cytoskeleton. It is built of three classes of filaments. Microtubules, the largest, are hollow tubes made of the protein tubulin; they serve as tracks for motor proteins, form the mitotic spindle, and are the structural core of cilia, flagella, and centrioles. Microfilaments, the thinnest, are solid threads of the protein actin; they generate the forces that drive cell crawling, cytokinesis, and muscle contraction. Intermediate filaments are made of a variety of fibrous proteins — keratins in epithelial cells, vimentin in connective tissue cells, lamins in the nuclear envelope — and provide mechanical strength. Cytoskeletal functions are largely mechanical: support, motility, maintenance of the shape of the cell, anchoring of organelles, and intracellular transport. NEET 2016 tested microtubules specifically — the correct answer identified spindle fibres, centrioles, and cilia as the structures made of microtubules. Chromatin, nucleosomes, and peroxisomes are not.

Cilia and flagella

Cilia (singular: cilium) and flagella (singular: flagellum) are hair-like outgrowths of the cell membrane. Cilia are small structures which work like oars, causing the movement of either the cell itself or the surrounding fluid. The ciliated cells lining the human trachea sweep mucus upward; the ciliated epithelium of the fallopian tube moves the ovum toward the uterus. Flagella are comparatively longer and primarily responsible for cell movement — the sperm cell is the most familiar example. Both cilia and flagella in eukaryotes are covered with an extension of the plasma membrane, and their structural core is called the axoneme.

The axoneme possesses a number of microtubules running parallel to the long axis, arranged in the classic 9 + 2 array: nine doublets of microtubules around the periphery and a pair of single, centrally located microtubules. The central pair is connected by bridges and is enclosed by a central sheath, which is connected to one tubule of each peripheral doublet by a radial spoke — making nine spokes in all. The peripheral doublets are additionally interconnected by linkers. Both cilia and flagella emerge from a centriole-like structure at their base called the basal body, and the basal body is what nucleates the microtubules of the axoneme as it grows. The eukaryotic 9 + 2 flagellum is structurally completely different from the bacterial flagellum, which has three parts — filament, hook, and basal body — and is built from the protein flagellin, not tubulin. NEET distractor questions exploit this similarity in name despite the difference in structure; do not confuse them.

Centrosome and centrioles

The centrosome is an organelle found typically in animal cells. It contains two cylindrical structures called centrioles surrounded by amorphous pericentriolar material. The two centrioles in a centrosome lie perpendicular to each other, each organised like a cartwheel. Each centriole is made of nine evenly spaced peripheral fibrils of tubulin protein, and each peripheral fibril is a triplet — three microtubules linked together as a unit. Adjacent triplets are also linked. The central part of the proximal region of the centriole is also proteinaceous and is called the hub, which is connected to the peripheral triplets by radial spokes made of protein. There are no central microtubules — the centriole's arrangement is therefore described as 9 + 0, in contrast to the 9 + 2 of cilia and flagella. This pair (9 + 0 versus 9 + 2) is a classic NEET distractor — keep them separate.

Centrioles serve two key functions during the life of the cell. First, they form the basal body of cilia and flagella, nucleating the axoneme. Second, during cell division in animal cells, they organise the spindle apparatus — the two centrosomes migrate to opposite poles of the dividing cell, and microtubules grow out from them to form the spindle that pulls the chromosomes apart. Almost all plant cells lack centrioles entirely, and yet they manage to form a spindle from other microtubule-organising centres — a long-standing source of confusion that NEET occasionally tests by asking which organelle is "absent in plant cells".

Nucleus

The nucleus, as a cell organelle, was first described by Robert Brown in 1831. The deeply staining material inside, observed later by Walther Flemming, was named chromatin. The interphase nucleus — that is, the nucleus of a cell when it is not dividing — contains highly extended and elaborate nucleoprotein fibres (the chromatin), a nuclear matrix (also called nucleoplasm), and one or more spherical bodies called nucleoli. Together these components organise the genome and orchestrate gene expression for the entire cell.

The nuclear envelope, revealed by electron microscopy, consists of two parallel membranes with a space between them of 10–50 nm called the perinuclear space. It forms a barrier between the materials present inside the nucleus and those of the cytoplasm. The outer membrane is usually continuous with the endoplasmic reticulum and even bears ribosomes on its cytoplasmic face — a beautiful structural connection that ties the nucleus directly to the endomembrane traffic. At a number of places, the nuclear envelope is interrupted by minute pores, formed by the fusion of its two membranes. These nuclear pores are the passages through which movement of RNA and protein molecules takes place in both directions between the nucleus and the cytoplasm. Without them, the genome would be inaccessible — every transcript made inside the nucleus must exit through a pore to reach a ribosome in the cytoplasm.

Normally there is only one nucleus per cell, but variations in the number of nuclei are frequently observed. Multinucleate cells include the syncytial cells of skeletal muscle and the latex-bearing cells of plants like Calotropis. Some mature cells even lack a nucleus altogether — for example, the erythrocytes of many mammals (which extrude their nucleus on maturation to maximise haemoglobin space) and the sieve tube cells of vascular plants (which lose theirs but remain functional under the control of companion cells). Whether these enucleated cells are "living" in the strict sense is a question NCERT poses but does not settle — they remain metabolically active but cannot divide or make new proteins.

The nuclear matrix or nucleoplasm contains the nucleolus and the chromatin. The nucleolus is a spherical structure present in the nucleoplasm. It is not membrane-bound — its contents are continuous with the rest of the nucleoplasm. It is the site of active ribosomal RNA synthesis, where rRNA is transcribed from clusters of rRNA genes and assembled with imported ribosomal proteins into the 60S and 40S subunits before being exported to the cytoplasm. Larger and more numerous nucleoli are present in cells actively carrying out protein synthesis — a fact NEET 2018 tested exactly. A trap option in that question claimed nucleoli are membrane-bound; they are not.

Chromosomes — chromatin condensed

During cell division, the diffuse interphase chromatin condenses into discrete chromosomes. Chromatin contains DNA, basic histone proteins, non-histone proteins, and RNA. A single human cell holds about two metres of DNA distributed among 46 chromosomes (23 pairs). Every chromosome carries a primary constriction called the centromere, with disc-shaped kinetochores on either side that anchor spindle fibres during division. Some chromosomes additionally carry a secondary constriction, marking off a small fragment called the satellite.

Based on the position of the centromere, chromosomes are classified into four types:

• Metacentric: centromere in the middle — two equal arms.
• Sub-metacentric: centromere slightly off-centre — one shorter arm, one longer arm.
• Acrocentric: centromere very close to one end — one extremely short and one very long arm.
• Telocentric: centromere at the terminal end of the chromosome.

The shorter arm is conventionally called the p-arm; the longer arm is the q-arm (NEET 2019). NEET 2022 tested the four chromosome-types match directly.

Microbodies

Many membrane-bound minute vesicles called microbodies are present in both plant and animal cells. They contain various enzymes and are the catch-all category for small organelles that do not fit elsewhere. The most familiar examples — though NCERT names them only in passing — are peroxisomes and glyoxysomes. Peroxisomes carry catalase and other oxidative enzymes that break down hydrogen peroxide, a toxic by-product of metabolism, into water and oxygen. They also participate in photorespiration in plants, where they handle one of the intermediates produced when RuBisCO accidentally fixes O₂ instead of CO₂. Glyoxysomes are found in fat-storing seeds and house the enzymes of the glyoxylate cycle, which converts stored fats into sugars during germination — the energy source that powers a seedling before it can photosynthesise. Although NCERT keeps the treatment of microbodies brief, NEET expects you to know they exist, that they are single-membrane vesicles, and that peroxisomes do not form part of the endomembrane system because their functions are not coordinated with the ER–Golgi–lysosome traffic.

That completes the tour of the cell. From the cell theory of Schleiden, Schwann, and Virchow to the microbodies tucked into corners of the cytoplasm, every component you have just read about is a target for NEET questions — and the chapter is reliable territory because the facts are stable, the diagrams are standardised, and the language NCERT uses is the language the question paper uses. Drill the structure–function pairings, keep the lookalike vocabulary (cristae, thylakoids, cisternae, centrioles) straight in your head, and the four marks per year this chapter pays out will be yours.