Transport of Substances Through the Cell Membrane
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
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