All cells are composed of the same kinds of molecular building blocks and share some common features. Despite these common features, cellular diversity is enormous, both between different types of organisms and within individual multicellular organisms (organisms that contain lots of different cell types – like human beings).
Some common properties of cells are that they use the same kinds of carbon based macromolecules as basic components such as proteins, lipids, carbohydrates and nucleic acids. They also use DNA as their genetic material, which they decode to make proteins. Finally, they are enclosed by a membrane and require a constant supply of energy. You might also be wondering whether another common property of cells is the ability to grow and divide but note that within adult multicellular organisms some individual specialised cells have lost their ability to divide; for example, mature nerve cells (also known as neurons) are in fact unable to divide.
Animal and human cells have a number of specialised organelles (sub-cellular compartments) each enclosed by its own intracellular membrane. The DNA is separated from the cell cytoplasm because it is enclosed by a nuclear membrane, forming a large organelle called the nucleus. Another kind of organelle found in these types of cells is mitochondria. Mitochondria play a vital role in cell function and survival because they generate most of the cell’s supply of a substance called Adenosine 5’-triphosphate (ATP). ATP moves around the cell and provides the energy that is needed to drive biochemical reactions that construct essential molecules from smaller units (for example, building a protein from amino acids).
The different types of cells in multicellular organisms are specialised to perform different functions, such as movement or secretion. Different molecules, particularly but not exclusively proteins, play an important role in these specialised functions. In animals and humans the muscle cells synthesise specific proteins that enable them to contract, while non-contractile cells, such as skin cells, do not synthesise these proteins. The differential expression of proteins is therefore fundamental to the characteristic properties of specialised cells.
In addition to the differences in biochemical properties of the various cell types in multicellular organisms, the shapes of different cell types also vary. In humans, for example, red blood cells are small and disc shaped, whereas nerve cells (neurons) are have long processes, called axons, some of which extend long distances, for example from the spinal cord to the muscles of the toes. The structure, or form (i.e. the shape and appearance) of cells is known as ‘cell morphology’ and plays an important role in cell function.
In multicellular organisms different cell types tend to be organised into distinct groups or tissues according to their function. The most complex organisms, like humans, have evolved highly organised arrangements of different types of cells and tissues into organs and organ systems that perform specific functions. Examples include the cardiovascular and digestive system.
All living cells are continuously active. In addition to the obvious examples of physical activity exhibited by muscle cells and motile cells such as sperm cells, at the molecular level all cells are highly dynamic. They continuously take up nutrients from their environment and use these as a source of energy and raw materials for synthesising new molecules; they transport the molecules to different locations within the cell and eliminate waste molecules; and if conditions are right, many cells grow and divide. Some can change their shape, and all cells respond to changes in the environment and interact in various ways with each other, by processes known as cellular communication or cell signalling. All these different processes require the constant movement of molecules, multiple coordinated biochemical events and in some cases major structural re-arrangements within the cell. As well as these ‘housekeeping processes’ that take place within all cells, some specialised reactions occur only in particular cell types.