This course provides an overview of the basic structural components of living cells.

 The fundamental unit of structure in biology is the living cell. Regardless of whether an organism is single celled or multicellular, it must carry out the same basic functions in order to survive, both as an individual and as a species. These functions include acquisition of nutrients and energy sources, disposal of unusable and toxic materials, reproduction, locomotion, and interaction with components in the environment. All of these functions require coordination both for short-range activities, such as sensation, and for long-range activities, such as growth. A comparison of organisms across phylogenetic lines reveals similarities, which have led to a formulation of the unity principle as applied to biology

 All living organisms are composed of cells. A cell is a small, membrane-bound compartment that contains all the chemicals and molecules that help support an organism's life. An understanding of the structure of cells is one of the first steps in comprehending the complex cellular interactions that direct and produce life.

 Cells can be thought of as building blocks of organisms. Some organisms are composed of a single cell. Others, like ourselves, are composed of millions of cells that work together to perform the more complex functions that make us different from bacteria. It is difficult to imagine that humans are descendants of a single cell, but this is a common belief in the scientific world. Before we can understand how multiple cells can work together to create complex biological functions, it is necessary to understand what biological functions single cells are capable of performing on their own to sustain life.

 There are different types of cells with individuated structures. Single-celled organisms have different cell structure than multi-celled organisms and plant cells have different structures from animal cells. These differences reflect differences in the functions that each of these classes of cells is required to perform. While the focus of this guide will be on the structures that compose complex multi-cellular organisms, we will begin our discussion of cell structure with a structure that is universal to all cells, membranes.