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. It was first seen by Robert Hooke (1665, in cork) and the living cell by Leeuwenhoek.

The cell theory, given by Schleiden (plants) and Schwann (animals) and extended by Rudolf Virchow, states that: (1) all living organisms are made of cells and their products; (2) the cell is the basic unit of life; and (3) all cells arise from pre-existing cells (Omnis cellula-e cellula).

Cells are of two fundamental types:

• Prokaryotic cells — found in bacteria, cyanobacteria, mycoplasma. They are smaller and simpler, with no true (membrane-bound) nucleus (the genetic material lies free in a region called the nucleoid) and no membrane-bound organelles. They have a cell wall, plasma membrane, cytoplasm with 70S ribosomes, and may have extra DNA called plasmids and a glycocalyx, flagella and pili.
• Eukaryotic cells — found in protists, plants, fungi and animals. They are larger and complex, with a true nucleus bounded by a nuclear membrane and membrane-bound organelles (ER, Golgi, mitochondria, etc.) and 80S ribosomes.

Plant cells differ from animal cells in having a cell wall, large central vacuoles and plastids (e.g. chloroplasts); animal cells lack these but have a centrosome and small vacuoles.

The plasma membrane is the outer boundary of the cell. It is made mainly of lipids (a phospholipid bilayer) and proteins. The accepted fluid mosaic model (Singer and Nicolson) describes it as a fluid lipid bilayer in which proteins float like a mosaic. The membrane is selectively permeable, controlling what enters and leaves: by passive transport (diffusion, osmosis — no energy) and active transport (against the gradient, using ATP).

The cell wall is a non-living, rigid outer covering present in plant cells, fungi and bacteria. In plants it is made of cellulose; it gives shape, support and protection, and prevents the cell from bursting.

Several membrane-bound organelles work together as the endomembrane system:

• Endoplasmic reticulum (ER) — a network of tubules: rough ER (bears ribosomes; makes proteins) and smooth ER (no ribosomes; makes lipids).
• Golgi apparatus — stacked flattened sacs that modify, pack and dispatch materials (the cell's 'post office'); also forms lysosomes.
• Lysosomes — membrane sacs full of digestive enzymes; they digest worn-out parts and foreign material (the 'suicide bags' of the cell).
• Vacuoles — membrane-bound spaces for storage of water, food and wastes; large and central in plant cells (bounded by the tonoplast).

Some organelles produce energy or organise the cell:

• Mitochondria — the 'powerhouses of the cell'; double-membraned, with the inner membrane folded into cristae. They carry out aerobic respiration to produce ATP. They have their own DNA and 70S ribosomes (semi-autonomous).
• Plastids (in plant cells) — chloroplasts (green, contain chlorophyll, site of photosynthesis; have stacks of thylakoids called grana in a stroma), chromoplasts (coloured, give colour to flowers/fruits) and leucoplasts (colourless, store food). Chloroplasts too have their own DNA and 70S ribosomes.
• Ribosomes — non-membrane organelles made of RNA and protein; the site of protein synthesis (70S in prokaryotes, 80S in eukaryotes).
• Centrosome (in animal cells) — contains two centrioles; helps form the spindle during cell division.
• Cytoskeleton — a network of protein filaments (microtubules, microfilaments) giving shape, support and helping movement.

The nucleus is the control centre of the cell. It is bounded by a double nuclear membrane with pores, and contains a jelly-like nucleoplasm, one or more nucleoli (make ribosomes), and the genetic material as chromatin. During cell division the chromatin condenses into rod-like chromosomes, which carry the genes (DNA) responsible for heredity. The nucleus controls all cell activities and inheritance.