The structure of the membrane covering the outside of every cell of the body is discussed in Chapter 2 and illustrated in Figures 1 and 2. This membrane consists almost entirely of a lipid bilayer, but it also contains large numbers of protein molecules in the lipid, many of which penetrate all the way through the membrane.

Fig1. Structure of the cell membrane, showing that it is composed mainly of a lipid bilayer of phospholipid molecules, but with large  numbers of protein molecules protruding through the layer. Also, carbohydrate moieties are attached to the protein molecules on the outside  of the membrane and to additional protein molecules on the inside. (Modified from Lodish HF, Rothman JE: The assembly of cell membranes. Sci Am 240:48, 1979. Copyright George V. Kevin.)

The lipid bilayer is not miscible with either the extra cellular fluid or the intracellular fluid. Therefore, it constitutes a barrier against movement of water molecules and water-soluble substances between the extracellular and intracellular fluid compartments. However, as demonstrated in Figure 2 by the leftmost arrow, lipid soluble substances can penetrate this lipid bilayer, diffusing directly through the lipid substance.

Fig2. Transport pathways through the cell membrane and the  basic mechanisms of transport. 

The protein molecules in the membrane have entirely different properties for transporting substances. Their molecular structures interrupt the continuity of the lipid bilayer, constituting an alternative pathway through the cell membrane. Many of these penetrating proteins can function as transport proteins. Different proteins function differently. Some proteins have watery spaces all the way through the molecule and allow free movement of water, as well as selected ions or molecules; these proteins are called channel proteins. Other proteins, called carrier proteins, bind with molecules or ions that are to be transported, and conformational changes in the protein molecules then move the substances through the interstices of the protein to the other side of the membrane. Channel proteins and carrier proteins are usually selective for the types of molecules or ions that are allowed to cross the membrane.

“Diffusion” Versus “Active Transport.” Transport through the cell membrane, either directly through the lipid bilayer or through the proteins, occurs via one of two basic processes: diffusion or active transport.

Although many variations of these basic mechanisms exist, diffusion means random molecular movement of substances molecule by molecule, either through inter molecular spaces in the membrane or in combination with a carrier protein. The energy that causes diffusion is the energy of the normal kinetic motion of matter.

In contrast, active transport means movement of ions or other substances across the membrane in com bination with a carrier protein in such a way that the carrier protein causes the substance to move against an energy gradient, such as from a low-concentration state to a high-concentration state. This movement requires an additional source of energy besides kinetic energy. A more detailed explanation of the basic physics and physical chemistry of these two processes is provided in this chapter.

مواضيع ذات صلة

Apoptosis—Programmed Cell Death

Cilia and Ciliary Movements

تركيب الخلية Structure of the cell

LYSOSOMES

السيطرة على انقسام الخلية

الظاهرة الأسموزية

طرق انتقال المواد من وإلى الخلية من خلال الغشاء

انتقال المواد الغذائية من وإلى الخلية عبر غشاء الخلية

خصائص البروتوبلازم

مكونات الخلية الكيميائية

النواة

سيتوبلازم الخلية