The smallest living unit of organization in the body is the cell, because each cell is capable of performing any necessary functions without the aid of other cells (Figures 7-1 and 7-2, Table 7-1). Each cell has a cell membrane, cytoplasm, organelles, and inclusions. Thus, every cell is a world unto itself, like a gated small city, surrounded by a boundary, having “factories” and other “industries” that make it almost self-sufficient.

Cells also interact with one another as a city reacts with other cities. Cells with similar characteristics of form and function are grouped together to form a tissue (see Table 7-1), as countries are formed from cities for a common goal. Thus, a tissue is a collection of similarly specialized cells, most often surrounded by extracellular materials. Various tissue types are then bonded together to form an organ, a somewhat independent body part that performs a specific function or functions, similar to organizations formed from countries. Organs can function together as a system.

Cells in a tissue undergo cell division (or mitosis) to reproduce and replace the dead tissue cells. As a result of the division process, two daughter cells that are identical to each other and to the original parent cell are formed. This process consists of different phases, which are discussed later with regard to different components of the cell.

However, cells also interact with the extracellular environment in many ways. Cells can perform exocytosis, which is an active transport of material from a vesicle within the cell out into the extracellular environment. Exocytosis occurs when there is fusion of a vesicle membrane with the cell membrane and subsequent expulsion of the contained material.

The uptake of materials from the extracellular environment into the cell is endocytosis. E/>