Passive Transport and Water

Diffusion is defined as the movement of a molecule from an area of high concentration to an area of low concentration. For example, if you placed a crystal of potassium permanganate into a beaker of water, the dye will eventually spread throughout the water and the water color will be similar at any location because the dye molecules will be evenly distributed in the water.

There are several important facts to remember. First, each substance diffuses along its own specific concentration gradient independent of the concentration gradient of other substances including water. While the dye molecules are moving in the beaker, so are the water molecules. Water also moves from an area of high concentration (where there is a localized dilute dye solution) to an area of low concentration (where there is a localized concentrated dye solution). Second, when diffusion occurs, there is no expenditure of energy.

Passive transport is a special type of diffusion because it refers to the diffusion of a substance across a biological membrane. Not all substances can move across a membrane because membranes are usually selectively permeable. Factors that affect the movement of molecules include their electrical charge, molecular size and the need for transport proteins.

• Rank these chemicals by how easily they diffuse across a biological membrane: sodium, chloride, glucose, ethanol, and water. Provide an explanation of the factor(s) that affect the ability of these substances to diffuse across a membrane.
• How can a cell membrane be selectively permeable?
• What does water have to do with cell size?

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