Human cells are a combination of water, proteins, carbohydrates, nucleic acids and fats contained in a lipid-based membrane. The precise composition of human cells varies widely, depending on a given cell's function in the body.
Almost all human cells have a common structure consisting of a cell membrane, composed of two layers of lipids, and a nucleus that contains the DNA that codes for the proteins that compose the cell. Exceptions to this rule include mature red blood cells, which lose their nuclei during their maturation process. The cell membrane itself contains a number of embedded proteins and carbohydrate-protein complexes that allow molecules to move in and out of the cell and act as recognition signals for nearby cells. Most cells also contain RNA-based structures known as ribosomes that assemble proteins and mitochondria that process energy from food molecules for the cell; these have their own membranes and DNA separate from that of the primary cell.
The specific structures in a given cell vary widely from cell type to cell type, as do the sizes of cells. For example, typical skin cells are microscopic and oval in shape, while peripheral nerve cells can have long cellular processes that may stretch for long distances. The proteins produced by a particular cell type also vary widely, though many proteins essential to cellular maintenance and reproduction are common to most cell types.