Introduction

In order to survive, all organisms need to move molecules in and out of their cells. Molecules
such as gases (e.g., O₂, CO₂), water, food, and wastes pass across the cell membrane. There are
two ways that the molecules move through the membrane: passive transport and active
transport. While active transport requires that the cell uses chemical energy to move substances
through the cell membrane, passive transport does not require such energy expenditures. Passive
transport occurs spontaneously, using heat energy from the cell’s environment.

Diffusion is the movement of molecules by passive transport from a region in which they are
highly concentrated to a region in which they are less concentrated. Diffusion continues until the
molecules are randomly distributed throughout the system. Osmosis, the movement of water
across a membrane, is a special case of diffusion. Water molecules are small and can easily pass
through the membrane. Other molecules, such as proteins, DNA, RNA, and sugars are too large
to diffuse through the cell membrane. The membrane is said to be semipermeable, since it allows
some molecules to diffuse through but not others.

If the concentration of water on one side of the membrane is different than on the other side,
water will move through the membrane seeking to equalize the concentration of water on both
sides. When water concentration outside a cell is greater than inside, the water moves into the
cell faster than it leaves, and the cell swells. The cell membrane acts somewhat like a balloon. If
too much water enters the cell, the cell can burst, killing the cell. Cells usually have some
mechanism for preventing too much water from entering, such as pumping excess water out of
the cell or making a tough outer coat that will not rupture. When the concentration of water
inside of a cell is greater than outside, water moves out of the cell faster than it enters, and the
cell shrinks. If a cell becomes too dehydrated, it may not be able to survive. Under ideal
conditions, the water concentration outside is nearly identical to that inside.

Objectives