Summary

The cell is the fundamental structural and functional unit of every living organism. All organisms are built either from a single cell or from aggregates of many cells, and cells themselves vary widely in shape, size and the particular activities they perform. Despite this diversity, a common set of features and organelles allows us to understand cells within a single framework — the theme that runs through this entire chapter.

A basic division separates all cells into two great groups on the basis of whether they possess a membrane-bound nucleus and membrane-bound organelles. Cells that lack these are prokaryotic, while those that have a true nucleus and internal organelles are eukaryotic; organisms are correspondingly named after the kind of cell they are made of. A typical eukaryotic cell is organised into three broad parts — an outer cell membrane, a controlling nucleus, and the cytoplasm that fills the space between them. Plant cells carry an additional cell wall outside the cell membrane, giving them extra rigidity. The plasma membrane itself is selectively permeable, allowing the cell to regulate which molecules pass in and out and to carry out controlled transport.

Within the cytoplasm, several membrane-lined compartments work as a coordinated group known as the endomembrane system, which includes the endoplasmic reticulum, the Golgi complex, the lysosomes and the vacuoles. The endoplasmic reticulum is a network of tubules and flattened sacs (cisternae) and exists in two forms — rough (studded with ribosomes) and smooth — helping in the transport of substances and in the synthesis of proteins, lipoproteins and glycogen. The Golgi body, made of stacked flattened sacs, receives the cell's secretory products, packages them, and dispatches them out of the cell. Lysosomes are single-membrane structures filled with digestive enzymes capable of breaking down all kinds of macromolecules.

The cell's energy and manufacturing machinery lies in several other organelles. Ribosomes, which are not membrane-bound, are the sites of protein synthesis and occur either free in the cytoplasm or attached to the endoplasmic reticulum. Mitochondria are double-membrane organelles — with a smooth outer membrane and an inner membrane folded into cristae — that carry out oxidative phosphorylation and generate adenosine triphosphate, the cell's energy currency. Plastids, which are found only in plant cells, are pigment-containing organelles; the green chloroplasts contain chlorophyll and trap light energy for photosynthesis (their grana being the site of the light reactions and their stroma the site of the dark reactions), while other coloured chromoplasts carry pigments such as carotene and xanthophyll.

Movement and cell division depend on further specialised structures. The cytoskeleton, a network of protein filaments in the cytoplasm, provides mechanical support, enables motility and maintains cell shape. Cilia and flagella, hair-like membrane outgrowths built on a 9+2 arrangement of microtubules, bring about the movement of the cell or of the fluid around it. The centrosome and its centrioles form the basal bodies of cilia and flagella, and in animal cells the centrioles also organise the spindle apparatus during cell division. …