Info

- 0 to 20 mg/dl

- 200 mg/dl

Figure 4-1

Chemical compositions of extracellular and intracellular fluids.

Carrier proteins

Carrier proteins

Energy

Simple diffusion

Energy

Facilitated diffusion

Diffusion

Active transport

Figure 4-2

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

protein. 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

Figure 4-3

Diffusion of a fluid molecule during a thousandth of a second.

the basic physics and physical chemistry of these two processes.

Diffusion

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 diffusion.

Ions diffuse in the same manner as whole molecules, and even suspended colloid particles diffuse in a similar manner, except that the colloids diffuse far less rapidly than molecular substances because of their large size.

Diffusion Through the Cell Membrane

Diffusion through the cell membrane is divided into two subtypes called simple diffusion and facilitated diffusion. Simple diffusion means that kinetic movement 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 molecules 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 membrane by two pathways: (1) through the interstices of the lipid bilayer if the diffusing substance is lipid soluble, and (2) through watery channels that penetrate 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 insoluble in the membrane lipids, it readily passes through channels 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.

Diffusion Through Protein Channels, and "Gating" of These Channels

Computerized three-dimensional reconstructions of protein channels have demonstrated tubular pathways all the way from the extracellular to the intracellu-lar fluid. Therefore, substances can move by simple diffusion directly along these channels from one side of the membrane to the other. The protein channels are distinguished by two important characteristics: (1) they are often selectively permeable to certain substances, and (2) many of the channels can be opened or closed by gates.

Selective Permeability of Protein Channels. Many of the protein channels are highly selective for transport of one or more specific ions or molecules. This results from the characteristics of the channel itself, such as its diameter, its shape, and the nature of the electrical charges and chemical bonds along its inside surfaces. To give an example, one of the most important of the protein channels, the so-called sodium channel, is only 0.3 by 0.5 nanometer in diameter, but more important, the inner surfaces of this channel are strongly negatively charged, as shown by the negative signs inside the channel proteins in the top panel of Figure 4-4. These strong negative charges can pull small dehydrated sodium ions into these channels, actually pulling the sodium ions away from their hydrating water molecules. Once in the channel, the sodium ions diffuse in either direction according to the usual laws of diffusion. Thus, the sodium channel is specifically selective for passage of sodium ions.

Conversely, another set of protein channels is selective for potassium transport, shown in the lower panel of Figure 4-4. These channels are slightly smaller than the sodium channels, only 0.3 by 0.3 nanometer, but they are not negatively charged, and their chemical bonds are different. Therefore, no strong attractive force is pulling ions into the channels, and the potassium ions are not pulled away from the water

Outside ooooooc ooooooo

Inside

Outside

0000000

Inside

Gate closed

Gate closed

Outside ooooooc ooooooo

Inside

[ Gate open

Gate I closed K+

Gate open

Gate I closed K+

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment