Adult human is composed of about 100 trillions cells, out of which 25 trillions are erythrocytes only (therefore, most numerous cells of body). Erythrocytes mainly perform oxygen transport function while rest of body functions are performed by remaining 75% of body cells. Although all body cells markedly differ from each other in structure and functions, yet their basic structure and functions are alike. For example all cells oxidises glucose to carbon dioxide and water using oxygen from blood. All cells synthesize proteins from amino acids and lipids from fatty acids and glycerol etc.
In human beings and other multicellular organisms, the cells tend to be organized in specific ways. A group of like cells performing a particular function is referred to as a tissue. An organ is a discrete structure composed of several different tissues together. A system is a group of organs together performing an overall function. An example of a system is the digestive system which comprises of several organs like stomach, intestines, pancreas etc. The individual organism is the combination of all of such systems as a discrete and separate entity.
Organization of a typical cell
The individual cell is the unit of structure of all living things. Cell is the smallest unit of life that can carry out all the functions of a living thing and is often called the building block of life. It was discovered by Robert Hooke. All organisms are made of one or more cells. An entire organism may consist of a single cell (unicellular) or many cells (multicellular). A typical human cell measures 5-10 microns (µm, one micron equals 10-6 meters) in diameter.