Passive diffusion through the membrane

Molecules and ions are in constant motion and the velocity of their motion is proportional to their temperature. This passive movement of molecules and ions from one place to another is referred to as diffusion. When a molecule is unevenly distributed across a permeable membrane with a higher concentration on one side and a lower concentration on the opposite side, there is said to be a concentration gradient or a concentration difference. Although all of the molecules are in motion, the tendency is for a greater number of molecules to move from the area of high concentration toward the area of low concentration. This uneven movement of molecules is referred to as net

Table 2.1 Factors Influencing Rate of Diffusion of a Substance


Rate of diffusion

î Concentration gradient î Permeability of membrane î Surface area of membrane î Molecular weight of substance î Thickness of membrane

diffusion. The net diffusion of molecules continues until the concentrations of the substance on both sides of the membrane are equal and the subsequent movement of molecules through the membrane is in a dynamic equilibrium. In other words, the number of molecules moving in one direction across the membrane is equal to the number of molecules moving in the opposite direction. At this point, although the diffusion of molecules continues, no further net diffusion takes place.

The rate of diffusion of a substance is influenced by several factors (see Table 2.1). It is proportional to the concentration gradient; the permeability of the membrane; and the surface area of the membrane. For example, as the permeability of the membrane increases, the rate of diffusion increases. It is inversely proportional to the molecular weight of the substance and the thickness of the membrane. Larger molecules diffuse more slowly.

The movement of ions, in particular, depends not only on a concentration gradient but also on an electrical gradient. Positively charged ions (cations) are attracted to a negatively charged area and negatively charged ions (anions) are attracted to a positively charged area. Ions of a similar charge tend to repel each other and oppose diffusion.

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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