Cells and Cell Structure Cells and Cell Structure Cells are the principle means for study in the subject of Cellular and Molecular Biology. They are the fundamental structural units of all living organisms. A cell is very small, usually invisible to the human eye. Cells can also move, duplicate, and alternate their biomedical reactions. In the most structured organisms, different tasks are divided by groups of specialized cells, whereas in bacteria and protozoa, each cell is independent and capable of carrying out all activities of life.
Cells vary greatly in size.
For example, small bacteria are barely visible in the light microscope, with a cell diameter of close to .5 micrometer, as compared to the single cell that makes up a hen's egg. The cytoplasmic region makes proteins according to directions given to it by the nucleus, and is also responsible for converting fuel substances into energy that can be used by the cell. Both the nuclear region and the cytoplasm collectively are called protoplasm. Cells also consist of thin layers of proteins and lipid molecules called membranes. Membranes create a boundary that separates the internal contents of the cell from the exterior, and in many cases membranes are also responsible for subdividing the interior into distinct compartments within the cell itself.