ells require a constant exchange of solutes (ions and small molecules) with the outside of the cell. Many of these solutes undergo passive transport across the membrane. Passive transport occurs without the input of cellular energy. Some solutes are transported into the cell while others are transported out of the cell.Part A - Diffusion All molecules have energy that causes thermal motion. One result of thermal motion is diffusion: the tendency of substances to spread out evenly in the available space. Although the motion of each individual molecule is random, there can be directional motion of an entire population of molecules. Consider a chamber containing two different types of dye molecules, purple and orange. The chamber is divided into two compartments (A and B) by a membrane that is permeable to both types of dye. Initially (left image), the concentration of the orange dye is greater on side A, and the concentration of the purple dye is greater on side B. With time, the dye molecules diffuse to a final, equilibrium state (right image) where they are evenly distributed throughout the chamber. A membrane permeable to both dyes initial condition final condition (equilibrium) Adapted from Biology by Campbell and Reece © 2008 Pearson Education, Inc.
