Cellular structure, function and the plasma membrane

All living things are made of cells, and the cell is the smallest unit that can carry out all the processes of life: obtaining energy, growing, responding to the environment and reproducing. The cell theory summarises this idea. All organisms are made of one or more cells, the cell is the basic structural and functional unit of life, and all new cells arise from existing cells.

Single-celled organisms such as bacteria and amoebas must perform every life function inside one cell, while a multicellular organism such as a human has trillions of cells that are specialised for different jobs.

Cells fall into two major types. Prokaryotic cells are small and simple. Their genetic material (a single circular chromosome of DNA) lies free in a region of the cytoplasm called the nucleoid, because there is no nucleus. They have no membrane-bound organelles, but they do have ribosomes, a plasma membrane, cytoplasm and usually a cell wall. Bacteria and archaea are prokaryotes. Eukaryotic cells are generally larger and more complex.

Their DNA is linear, packaged with proteins into several chromosomes and enclosed by a double membrane to form a nucleus. They also contain many membrane-bound organelles such as mitochondria, the endoplasmic reticulum and the Golgi apparatus. Animals, plants, fungi and protists are eukaryotes.

Both cell types share some essential features. Each is surrounded by a plasma membrane that controls what enters and leaves, each contains cytoplasm in which chemical reactions occur, each has DNA as its genetic material, and each has ribosomes that build proteins. These shared features are evidence that all cells have a common origin.

When you are asked to distinguish the two types in an exam, state a difference for the same feature in both cells (for example, DNA free in the cytoplasm versus DNA inside a nucleus) rather than listing a feature of one type only.