Cells, Cell Theory and the Diversity of Life

Why is a cell the unit of life?

A tree, a bacterium and a human differ enormously in appearance, yet all depend on cells as the minimum organised units of living activity. A complete living cell has a boundary separating its contents from the environment, genetic information, machinery for making molecules and processes that capture or use energy. In a unicellular organism, one cell performs essential tasks such as exchange, metabolism, response and reproduction. In a multicellular organism, specialised cells share these tasks; the organism is not simply a bag of identical cells.

A smaller structure, such as a ribosome or a mitochondrion, can perform important cellular functions but ordinarily cannot maintain all essential life activities independently. Therefore the cell is the fundamental structural and functional unit. Structural means it makes up the bodies and tissues of organisms; functional means cellular processes are the basis of the organism's life activities. This does not imply that every mature specialised cell carries out every possible function or that all cells have exactly the same parts.

How the cell theory developed

Microscopes made cellular organisation visible. Robert Hooke used the term cell when observing chambers in cork; Antonie van Leeuwenhoek later described living microscopic cells. Robert Brown described the nucleus in plant cells in 1831. Matthias Schleiden concluded in 1838 that plants are composed of cells, while Theodor Schwann extended the cellular idea to animals in 1839. Their combined theory unified two major groups of living organisms under one structural principle.

An important omission remained: how do new cells form? Rudolf Virchow argued in 1855 that new cells arise from pre-existing cells, summarised as 'Omnis cellula e cellula'. Modern cell theory therefore includes both cellular composition and cellular continuity. These statements are not the same: the first answers what organisms are built from; the second explains where new units come from. Microscopy could show cell boundaries, whereas repeated observations of division revealed their origins.

A look inside a cell

Plant epidermal cells viewed in onion peel have a cell wall outside a plasma membrane. Human cheek cells lack the rigid wall and are delimited by the plasma membrane. Both types contain cytoplasm, and many mature eukaryotic cells contain a membrane-bound nucleus that houses chromosomes made of DNA and associated proteins. The nucleus does not hold all the cell's DNA: mitochondria and, in photosynthetic plant cells, chloroplasts have genetic material too. Cytoplasm contains many coordinated reactions and cellular components.

Prokaryotic cells lack a membrane-enclosed nucleus; their DNA typically occupies a nucleoid region in the cytoplasm. Eukaryotic cells have a true nuclear envelope and generally possess several membrane-bound compartments. Ribosomes occur in both cell types because both must synthesise proteins. The cellular distinction is about internal organisation, not about whether a particular organism is 'higher' or more biologically successful.

Different forms suit different functions

Cell sizes span enormous ranges. Mycoplasmas can be about 0.3 µm across and many bacteria are only a few micrometres long, whereas an ostrich egg is an exceptionally large single cell. A typical human red blood cell is about 7–8 µm in diameter; some nerve cells extend long distances through their projections. White blood cells may change shape to move through tissues, columnar epithelial cells form ordered linings, and tracheids are elongated for water transport. These are examples of structure supporting a particular role.

A cell's apparent shape depends on how it is viewed: a three-dimensional biconcave disc can look circular from above and thin from the side. Microscope drawings must therefore state whether the view is a whole cell, a surface view or a section. Shape alone is rarely sufficient for classifying an organism; internal membranes, hereditary structures and developmental history are also important.

Quick Quiz

Model Answers

1. It performs energy-related reactions but depends on the cell for most proteins, materials and regulatory processes; it cannot independently perform all requirements of life.

2. Leeuwenhoek observed living cells; Schleiden described plants as cellular (1838); Schwann extended this to animals (1839); Virchow explained that cells arise from pre-existing cells (1855).

3. A unicellular organism performs many activities in one cell, whereas cells in multicellular organisms differentiate into specialised forms that cooperate to sustain the whole organism.

4. A biconcave erythrocyte supports gas exchange and flexibility; a long neuron conducts signals across distance; an elongated tracheid contributes to conducting water and supporting plant tissue.