The cell is the basic biological unit of all known living organisms (Figure 1). All cells consist of a cytoplasm contained within a cell membrane, sometimes called the plasma membrane. Beyond this, however, they can differ significantly, with major differences between prokaryotic (bacteria and archaea) and eukaryotic (plants, animals, and fungi) cells, in their structure and in the organelles they contain. Even cells within an organism can differ greatly facilitating all the different physiological functions required for life.

  • Cell membrane: The cell membrane surrounds the cytoplasm of the cell which serves to separate and protect the cell from its environment. The membrane is composed of a double layer of phospholipids making it very flexible. It is able to host various proteins and semi or selectively permeable.

  • Cytoplasm: The cytoplasm contains all of the material in the cell excluding the cell nucleus. Comprised of the cytosol, a gel-like substance which is enclosed by the cell membrane, and all the other organelles.

  • Nucleus: The nucleus is one of the many organelles found within a cell. Cells typically contain one nucleus each, although certain specialized cells may contain many, for example muscle cells, with others, such as red blood cells, containing none. The nucleus contains most of a cell’s DNA molecules organized as multiple linear DNA molecules known as chromosomes. The nucleus is kept separate from the cytoplasm by the nuclear envelope, another double layer of phospholipids, although this membrane is punctuated by nuclear pores.

  • Mitochondria: Mitochondria are the powerhouses of the mammalian cell generating a supply of adenosine triphosphate (ATP), for use as a source of chemical energy. The number of mitochondria present in a cell gives an idea as to how much energy it requires, for example red blood cells have none whereas cells in the liver can contain thousands.

  • Rough endoplasmic reticulum: The rough endoplasmic reticulum is studded with protein-producing ribosomes and is the major source of protein translation in the cell.

  • Golgi apparatus: The golgi apparatus, or golgi body is an organelle found in most cells and is a continuation of the endomembrane system and functions to package proteins for dispersal throughout the cell, or even to the outside of the cell via secretory vesicles.

  • Lysosome and peroxisome: The lysosome and peroxisome can be thought of as the recycling centers of the cell. Both rich in enzymes they are responsible for breaking down many kinds of biomolecules into their constituents parts for later reuse. Peroxisomes can be thought of as hazardous waste recycling centers as a major function is to reduce the damaging reactive oxygen species into harmless waste products.

Cross section of a generic eukaryotic cell with the most important organelles and cellular structures - nucleoplasm, nucleus, nuclear envelope, nuclear pore, golgi apparatus, secretory vesicles, mitochondrion, peroxisome lysosome, cytoplasm and membrane.

Figure 1. Generic animal cell structure.

Animal cells differ massively in size, appearance and function but some factors are conserved. All cells are comprised of a cytoplasm surrounded by a cell membrane. Most cells also contain a nucleus which contains a complete copy of an individual's DNA as well as other structures such as the energy-producing mitochondria and protein-producing rough endoplasmic reticulum.