Biology, Eighth Edition (Raven)

Chapter 5: Membranes

How Diffusion Works

How are molecules distributed in the environment? As explained in this animation, the random movement of molecules in a gas or a liquid will cause the molecules to become evenly distributed in the environment. If we look at the environments in which cells exist, for example inside a body or in a pond, we do not see an even distribution of molecules associated with life. Cells prevent, control, and even work counter to diffusion to create concentration gradients, to exclude waste or harmful materials, and to store needed materials. Life can be described as a process of controlling the diffusion of molecules.

View the animation below, then complete the quiz to test your knowledge of the concept.


Simple diffusion is defined as the movement of
A)molecules from areas of higher concentration to areas of lower concentration.
B)molecules from areas of lower concentration to areas of higher concentration.
C)water molecules across a membrane.
D)gas molecules across a membrane.
E)gas or water molecules across a membrane.

When sugar is mixed with water, equilibrium is reached when
A)molecules of sugar stop moving.
B)water and sugar molecules are moving at the same speed.
C)the dissolved sugar molecules are evenly distributed throughout the solution.
D)there are the same number of water molecules as dissolved sugar molecules.
E)two tablespoons of coffee are added.

The rate of diffusion is affected by which of the following?
B)size of molecules
C)steepness of the concentration gradient
D)A and B
E)A, B and C

The molecules in a solid lump of sugar do not move.

Diffusion is one of the processes whereby materials are exchanged between a cell and its environment.
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