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Learning Objective

Identify the characteristics of the cell.

The cell is the smallest and most numerous structural unit that possesses and exhibits the basic characteristics of living matter. A typical cell is made up of the plasma membrane, the nucleus, and the cytoplasm. Each cell is surrounded by a membrane called the plasma membrane which is a selectively permeable. It controls the exchange of materials between the cell and its environment by physical and chemical means. Gases (such as oxygen) and solids (such as proteins, carbohydrates, and mineral salts) pass through the plasma membrane through a process known as diffusion; a process during which elements achieve equilibrium by moving from an area of higher concentration to an area of lower concentration.

The nucleus is a small, dense, usually spherical body that controls the chemical reactions occurring in the cell. The substance contained in the nucleus is called nucleoplasm. The nucleus is also important in the cell's reproduction, due to the storage of genetic information. Every human cell contains 46 chromosomes, and each chromosome has thousands of genes that determine the cell's function.

Cells are composed largely of a gel-like substance, called cytoplasm, upon which depend all the vital functions of nutrition, secretion, growth, circulation, reproduction, excitability, and movement. The cytoplasm is a gelatinous substance surrounding the nucleus and is contained by the plasma membrane. The cytoplasm is made of various organelles and molecules suspended in a watery fluid called cytosol, or intracellular fluid (Fig. 1).

Figure 1.óTypical, or composite, cell. An Artistís interpretation of cell structure. Note, too, the innumerable dots bordering the endoplasmic reticulum. These are ribosomes, the cellís "protein factories."

Image reprinted from: Thibedeau, G. A., & Patton, K. T. (2006). Anatomy & Physiology (6th ed.). St. Louis: Elsevier Health Sciences.

The simplest living organism consists of a single cell. The amoeba (Fig. 2a) is a unicellular animal. The single cell of a one-celled organism must be able to carry on all processes necessary for life. This cell is called a simple or undifferentiated cell, one that has not acquired distinguishing characteristics.



Figure 2.ó Common cells. (a) Amoeba. (b) Stained cells under a microscope.

In multi-cellular organisms, cells vary in size, shape, and number of nuclei. When stained (Fig. 2b), the various cell structures can be more readily recognized under a microscope. Many cells are highly specialized to perform special functions (e.g., muscle cells which contract and epithelial cells which protect the skin).

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David L. Heiserman, Editor

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Revised: June 06, 2015