Diffusion Through the Cell Membrane
Diffusion through the cell membrane is divided into two subtypes called simple diffusion and facilitateddiffusion. Simple diffusion means that kinetic move-ment of molecules or ions occurs through a membrane opening or through intermolecular spaces without any interaction with carrier proteins in the membrane. The rate of diffusion is determined by the amount of substance available, the velocity of kinetic motion, and the number and sizes of openings in the membrane through which the molecules or ions can move.
Facilitated diffusion requires interaction of a carrier protein. The carrier protein aids passage of the mole-cules or ions through the membrane by binding chemically with them and shuttling them through the membrane in this form.
Simple diffusion can occur through the cell mem-brane by two pathways: (1) through the interstices of the lipid bilayer if the diffusing substance is lipid soluble, and (2) through watery channels that pene-trate all the way through some of the large transport proteins, as shown to the left in Figure 4–2.
Diffusion of Lipid-Soluble Substances Through the Lipid Bilayer.
One of the most important factors that determines how rapidly a substance diffuses through the lipid bilayer is the lipid solubility of the substance. For instance, the lipid solubilities of oxygen, nitrogen, carbon dioxide, and alcohols are high, so that all these can dissolve directly in the lipid bilayer and diffuse through the cell membrane in the same manner that diffusion of water solutes occurs in a watery solution. For obvious reasons, the rate of diffusion of each of these substances through the membrane is directly proportional to its lipid solubility. Especially large amounts of oxygen can be transported in this way; therefore, oxygen can be delivered to the interior of the cell almost as though the cell membrane did not exist.
Diffusion of Water and Other Lipid-Insoluble Molecules Through Protein Channels. Even though water is highly insolublein the membrane lipids, it readily passes through chan-nels in protein molecules that penetrate all the way through the membrane. The rapidity with which water molecules can move through most cell membranes is astounding. As an example, the total amount of water that diffuses in each direction through the red cell membrane during each second is about 100 times as great as the volume of the red cell itself.
Other lipid-insoluble molecules can pass through the protein pore channels in the same way as water molecules if they are water soluble and small enough. However, as they become larger, their penetration falls off rapidly. For instance, the diameter of the urea molecule is only 20 per cent greater than that of water, yet its penetration through the cell membrane pores is about 1000 times less than that of water. Even so, given the astonishing rate of water penetration, this amount of urea penetration still allows rapid transport of urea through the membrane within minutes.
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