Cell: The Unit of Life

The cell is the basic structural and functional unit of all living organisms — the smallest unit capable of independent existence. The history is a frequent NEET one-marker. Robert Hooke in 1665 first saw and named 'cells' while looking at dead cork under his microscope, but these were only empty walls. Anton von Leeuwenhoek was the first to see and describe a living cell. Later, Robert Brown discovered the nucleus.

The cell theory was formulated by two scientists: Matthias Schleiden (1838), who concluded that all plants are composed of cells, and Theodor Schwann (1839), who proposed the same for animals and noted that cells have a thin outer layer (the plasma membrane). Their combined statement — that all living organisms are made of cells and their products — was incomplete because it did not explain how new cells form. Rudolf Virchow (1855) supplied the missing piece with 'omnis cellula-e cellula': all cells arise from pre-existing cells. The modern cell theory therefore has two parts: (1) all organisms are composed of cells, and (2) all cells come from pre-existing cells.

Cells fall into two fundamental categories. Prokaryotic cells (bacteria, cyanobacteria, mycoplasma and PPLO) lack a true membrane-bound nucleus and membrane-bound organelles, and are generally smaller and multiply faster. Eukaryotic cells (protists, fungi, plants and animals) have a true nucleus enclosed by a nuclear envelope and possess membrane-bound organelles. Recognising which organisms are prokaryotic versus eukaryotic is essential.

Although every cell shares some features — a plasma membrane, cytoplasm and genetic material — cells vary enormously in size, shape and function. The smallest cells are mycoplasmas (about 0.3 µm), bacteria are a few micrometres, while the largest isolated single cell is the egg of an ostrich. Cell shape ranges from the disc-like red blood cell to the long branched nerve cell, each shape suited to its job. This diversity, built on a common plan, is the theme that the rest of the chapter develops.

Prokaryotic cells represent the bacteria, cyanobacteria, mycoplasma and PPLO. They are the most abundant micro-organisms and, despite their simple appearance, are metabolically very diverse. By shape, bacteria are bacillus (rod), coccus (sphere), vibrio (comma) and spirillum (spiral). The crucial point — heavily tested — is that a prokaryote has no membrane-bound nucleus and no membrane-bound organelles; its genetic material is a single, naked, circular DNA lying in a region called the nucleoid, often with extra small circular DNA called plasmids.

The prokaryotic cell is bounded by a three-layered cell envelope: an outer sticky glycocalyx (a slime layer or a firmer capsule), a rigid cell wall made of peptidoglycan (murein) that gives shape and prevents bursting, and an inner plasma membrane that is selectively permeable. Differences in the wall make bacteria Gram-positive or Gram-negative in the Gram stain, a distinction with clinical importance.

Because prokaryotes lack organelles, several functions are carried out by membrane specialisations. Mesosomes — infoldings of the plasma membrane — help in respiration, secretion, DNA replication and cell division. In cyanobacteria, membranous extensions called chromatophores contain pigments for photosynthesis. Ribosomes here are the smaller 70S type (made of 50S and 30S subunits) and are the site of protein synthesis. Reserve materials are stored in non-membrane-bound inclusion bodies such as gas vacuoles, glycogen and phosphate granules.

Many prokaryotes are motile or have surface appendages. The bacterial flagellum — composed of a filament, hook and basal body — produces movement; short bristle-like fimbriae help cells attach to surfaces, and pili are involved in attachment and the transfer of genetic material between cells. For NEET, the must-knows are: nucleoid (not a true nucleus), peptidoglycan wall, mesosomes, 70S ribosomes, inclusion bodies (non-membranous) and the flagellar structure.

Every cell is bounded by the plasma (cell) membrane, whose accepted structure is the fluid-mosaic model proposed by Singer and Nicolson (1972). The membrane is a bilayer of phospholipids with their polar (hydrophilic) heads facing outward and their non-polar (hydrophobic) tails facing inward, together with proteins that are either embedded through the bilayer (integral) or attached to its surface (peripheral). The 'fluid' nature allows lateral movement of lipids and proteins, which is important for functions like growth, endocytosis and cell division.

The membrane is selectively permeable, controlling what enters and leaves. Movement of substances is passive when it needs no energy and occurs down a concentration gradient — by diffusion (and the diffusion of water, osmosis) — and active when it uses ATP to pump substances against their gradient (e.g. the Na⁺/K⁺ pump). Passive vs active transport is a standard NEET contrast.

Plant cells, fungi and many protists have, outside the membrane, a cell wall that gives shape and protection and prevents bursting. In plants it is made mainly of cellulose (hemicellulose, pectin and proteins), in fungi of chitin. The wall has a middle lamella (calcium pectate) cementing adjacent cells, a primary wall and often a thicker secondary wall; adjacent plant cells stay connected by cytoplasmic bridges called plasmodesmata.

Within the cytoplasm, several organelles whose functions are coordinated form the endomembrane system: the endoplasmic reticulum, Golgi apparatus, lysosomes and vacuoles. The endoplasmic reticulum (ER) is a network of tubules; rough ER bears ribosomes and synthesises and transports proteins, while smooth ER lacks ribosomes and makes lipids and steroids. The Golgi apparatus is a stack of flattened sacs (cisternae) that modifies, packages and dispatches materials (its cis face receives from the ER and its trans face ships out). Lysosomes are membrane sacs full of hydrolytic (digestive) enzymes — the cell's 'suicidal bags' for intracellular digestion — and vacuoles are membrane-bound spaces (the membrane is the tonoplast) for storage; the large central vacuole of a plant cell maintains turgor.

The remaining organelles handle energy, expression and architecture. Mitochondria are the 'power houses' of the cell: each is bounded by a double membrane whose inner membrane folds inward into cristae that increase surface area, enclosing a matrix. They are the site of aerobic respiration, producing ATP. Crucially, mitochondria are semi-autonomous — they contain their own circular DNA and 70S ribosomes, so they make some of their own proteins and divide by fission.

Plastids are found only in plant cells and certain protists. Chloroplasts carry out photosynthesis; within their double membrane lies the stroma containing stacks of membranous thylakoids (a stack is a granum) that hold the chlorophyll, and like mitochondria they have their own DNA and 70S ribosomes (also semi-autonomous). Chromoplasts contain coloured carotenoid pigments (giving flowers and fruits their colour), while leucoplasts are colourless storage plastids — amyloplasts (starch), elaioplasts (oils) and aleuroplasts (proteins).

Ribosomes are the granular, non-membrane-bound sites of protein synthesis; in the eukaryotic cytoplasm they are the larger 80S type (60S + 40S subunits), in contrast to the 70S ribosomes of prokaryotes (and of mitochondria/chloroplasts). The cell's shape and internal movements are maintained by the cytoskeleton of microtubules, microfilaments and intermediate filaments. Cilia and flagella are membrane-bound projections with a core (axoneme) of microtubules in a 9 + 2 arrangement, arising from a basal body; the centrosome contains two centrioles made of microtubules in a 9 + 0 pattern that organise the spindle during division.

The control centre is the nucleus, bounded by a double-membraned nuclear envelope perforated by nuclear pores for exchange with the cytoplasm. Inside, the nucleoplasm holds one or more nucleoli (sites of ribosomal-RNA synthesis and ribosome assembly) and the chromatin — a complex of DNA and histone proteins that condenses into chromosomes during division. Finally, microbodies such as peroxisomes and glyoxysomes are tiny membrane-bound vesicles containing specialised enzymes. The high-yield NEET facts here are: mitochondria/chloroplasts are double-membraned and semi-autonomous (own DNA + 70S), ribosome 80S vs 70S, cilia/flagella 9+2 vs centriole 9+0, and the nucleolus makes rRNA.