Transport of Substances Through the Cell Membrane

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The Lipid Barrier of the Cell Membrane,
and Cell Membrane Transport Proteins
The structure of the membrane covering the outside of every cell of the body is discussed in Chapter 2 and illustrated in Figures 2–3 and 4–2.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, as shown in Figure 4–2.

The lipid bilayer is not miscible with either the extracellular 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 4–2 by the leftmost arrow, a few substances can penetrate this lipid bilayer, diffusing directly through the lipid substance itself; this is true mainly of lipid-soluble substances, as described later.

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. Most of these penetrating proteins, therefore, can function as transport proteins. Different proteins function differently. Some have watery spaces all the way through the molecule and allow free movement of water as well as selected ions or molecules; these are called channel proteins. Others, called carrier proteins, bind with molecules or ions that are to be transported; conformational changes in the protein molecules then move the substances through the interstices of the protein to the other side of the membrane. Both the channel proteins and the carrier proteins are usually highly selective in 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 by one of two basic processes: diffusion or active transport.

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

By contrast, active transport means movement of
ions or other substances across the membrane in combination
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. Following is a more detailed explanation of
the basic physics and physical chemistry of these two
All molecules and ions in the body fluids, including
water molecules and dissolved substances, are in constant
motion, each particle moving its own separate
way. Motion of these particles is what physicists call
“heat”—the greater the motion, the higher the temperature— and the motion never ceases under any
condition except at absolute zero temperature.When
a moving molecule, A, approaches a stationary molecule,
B, the electrostatic and other nuclear forces of
molecule A repel molecule B, transferring some of the
energy of motion of molecule A to molecule B. Consequently,
molecule B gains kinetic energy of motion,
while molecule A slows down, losing some of its
kinetic energy. Thus, as shown in Figure 4–3, a single
molecule in a solution bounces among the other
molecules first in one direction, then another, then
another, and so forth, randomly bouncing thousands of
times each second. This continual movement of molecules
among one another in liquids or in gases is called
Ions diffuse in the same manner as whole molecules,
and even suspended colloid particles...
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