BioPHYSICAL chemistry

mechanics. In general, cells from prokaryotes or bacteria tend to be simpler

than those from eukaryotes.

Cells are the structural and functional units of organisms and are designed

to carry out the required chemical reactions. The periphery of a cell is

defined by the plasma membrane whose properties are determined largely

by the presence of molecules called lipids. The interior of the cell, termed

the cytoplasm, is composed of an aqueous solution as well as specialized

organelles. Among the components are proteins that are designed to per-

form specific functions. Examples are enzymes that can expedite reactions

by many orders of magnitude, transporters that move ions and mole-

cules from one region to another, and receptors that initiate a chemical

change in response to the presence of a certain molecule. The biological

information is stored and expressed using deoxyribonucleic acid, or

DNA, and ribonucleic acid, or RNA. Since the biochemical reactions are

dictated by the performance of these cellular components, an understanding

of the molecular basis requires a detailed knowledge of the structure and

function of these molecules. Whereas such an understanding is discussed

in biochemistry texts, the fundamental aspects needed for this text are

summarized below.

Cell membranes

Membranes form the boundaries of cells and regulate the trafficking of

molecules into and out of the cell. To permit the rearrangements that occur

in processes such as membrane fusion, membranes are flexible. The com-

ponents of the membrane are not static but mobile, forming a fluid mosaic

(Figure 1.5). Phospholipids and steroids establish the bilayer in which polar

14 CHAPTER 1 BASIC THERMODYNAMIC AND BIOCHEMICAL CONCEPTS

Phospholipid

polar heads

Peripheral

protein

Integral

protein

Lipid

bilayer

Integral

protein

Peripheral

protein

Figure 1.5The fluid-mosaic model of cell membranes showing the lipids, and integral and
peripheral proteins. The lipids and proteins can move within the bilayer.