2-5. STRUCTURAL COMPONENTS OF A CELL
The cell was once viewed as a bag of fluid, enzymes, and chemicals. Now, we
understand that the cell is an extremely complex living the entity. With the advent of
electron microscopy in the early 1940's, several distinct cellular structures called
organelles were clearly recognized. A typical animal cell contains several types of
these organelles (Figure 2-1). Each organelle has an important role in the functioning of
the cell. It is important for you to become familiar with these organelles.
a. Cell Membrane. (Animal cells do not have cell walls; they have cell
membranes only. Plant cells have both cell walls and cell membranes.)
(1) Practically all the structures within the cell, as well as the cell itself, are
lined with a porous, elastic membrane. The cell membrane is composed primarily of
lipids (fats) and proteins that are arranged in layers at right angles to each other
Figure 2-1. Diagram of a cell membrane.
(2) The lipids of the cell wall are composed of two portions: a long
hydrocarbon chain (that is insoluble in water) and a glycerol-phosphate head (that is
soluble in water). The long chains are in the center of the protein and the glycerol-
phosphate group is attached to the end of the protein.
(3) The cell membrane contains many pores. It is through these pores that
lipid-insoluble particles, such as water and urea, pass between the interior and the
exterior of the cell. Diffusion experiments have shown that particles up to approximately
8-Angstrom units in diameter pass through the pores freely.
(4) The main function of the cell membrane is to regulate the flow of
substances into and out of the cell. This regulation of flow is accomplished by the
membrane's selective permeability. That is, only certain substances may pass through