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Figure

(1) A membrane permeable to water and sugar molecules separates a container into two compartments. Compartment A contains both types of molecules, while compartment B contains only water molecules. (2) As a result of molecular motions, sugar molecules tend to diffuse from compartment A into compartment B. Water molecules tend to diffuse from compartment B into compartment A. (3) Eventually, equilibrium is reached.

Cell membrane

Figure

Cell membrane lire 3.23

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Some substances move into or out of cells by facilitated diffusion, transported by carrier molecules from a region of higher concentration to one of lower concentration.

because oxygen is used up in metabolic reactions. Extracellular (eksctrah-selcu-lar) oxygen is maintained high by the respiratory and cardiovascular systems, thus providing a gradient for oxygen to diffuse continuously into body cells.

In general, diffusion of substances into or out of cells can occur if (1) the cell membrane is permeable to that substance and (2) if a concentration gradient for that substance exists, such that it is at a higher concentration either outside or inside of the cell.

Some of the previous examples considered imaginary membranes with specific permeabilities. For the cell membrane, the issue of permeability is somewhat more complex because of its selective nature. Lipid-soluble substances, such as oxygen, carbon dioxide, steroids, and general anesthetics, freely cross the cell membrane by simple diffusion. Small solutes that are not lipid-soluble, such as ions of sodium, potassium and chloride, may diffuse through protein channels in the membrane, described earlier. (Water molecules may also diffuse through similar channels, called pores.) Because this type of movement uses membrane proteins as "helpers," it is considered to be a form of another type of diffusion, called facilitated diffusion (fah-silci-tatced di-fuczhun). Facilitated diffusion is very important not only for ions, but for larger water-soluble molecules, such as glucose and amino acids.

A number of factors influence the diffusion rate, but those most important in the body are distance, the concentration gradient, and temperature. In general, diffusion is more rapid over shorter distances, larger concentration gradients, and at higher temperatures. Homeostasis maintains all three of these factors at optimum levels.

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.

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