Cell

Microscopic examination of any organism reveals that it is composed of membrane-enclosed structures called cells. The enclosing membrane is called the cell membrane or the plasma membrane. Cells vary enormously in size and shape, but even the largest cells would have to be much larger to be visible to the naked eye. Within this tiny object thousands of chemical reactions are taking place, all regulated, all designed to serve a specific function. Collectively these reactions serve the function of maintaining the cell and permitting to replicate when the time is right.
Perhaps the most amazing thing about the living cell is that so much organized activity takes place in such a small space. Sometimes the cells are connected into long chains. In multicellular organisms the cells associate to form specialized tissues.
The plasma membrane is a delicate, semipermeable, sheetlike covering the entire cell. Forming an enclosure prevents gross loss of intracellular contents. In many plant and bacterial cells a cell wall encompasses the plasma membrane. The cell wall is a more porous structure than the plasma membrane, but it is mechanically stronger because it is constructed of a covalently cross-linked, three dimensional network. The cell wall maintains a cell's three dimensional form when it is under stress.
The contents enclosed by the plasma membrane constitute the cytoplasm. The purely liquid portion of the cytoplasm is called cytosol. Within the cytoplasm are a number of macromolecules and larger structures, many of which can be seen by high-power light microscopy or by electron microscopy. Some of the structures are membranous and are called organelles. Organelles commonly found in plant and animal cells including the nucleus, mitochondria, the endoplasmic reticulum, the Golgi apparatus and the Lysosomes.