Biology, Eighth Edition (Raven)

Chapter 5: Membranes

Diffusion Through Cell Membranes

How is the concentration of a substance within a cell related to the concentration of the same substance outside the cell? Many molecules can pass freely across cellular membranes. Small molecules and nonpolar molecules are the ones that can move most easily across the lipid bilayer and enter the cell. Diffusion is the process that causes these molecules to move in or out of the cell. Diffusion is driven by the random movements of individual molecules in solution. If the concentration of a molecule is greater outside a cell than inside the cell then, on average, more molecules will randomly enter the cell, than leave the cell. Molecules that are capable of diffusion across a cellular membrane will eventually reach equal concentrations inside and outside of the cell. At this point the rate at which individual molecules enter and leave will be equal.

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


1.	Particles in a fluid...

		A)	settle to the bottom of the container unless swirled.
		B)	must have energy transferred to them from an outside source.
		C)	require energy in the form of ATP to exhibit any motion.
		D)	exhibit random innate motion.
		E)	remain motionless until acted upon by an outside force.

2.	The concentration of the particles diffusing into the cell...

		A)	increases over time until there is an equal concentration of particles inside the cell and outside.
		B)	remains low due to the restrictive nature of the semipermeable membrane.
		C)	fluctuates between higher than outside to lower than outside of the cell.
		D)	is initially equal to the number of particles diffusing out.
		E)	is always greater than the number of particles diffusing out of the cell.

3.	During the time that the particles are diffusing into the cell...

		A)	the rate of this diffusion remains constant.
		B)	the rate of the diffusion begins slowly, but eventually accelerates.
		C)	the concentration of particles on the outside of the cell increases.
		D)	the concentration of particles on the outside of the cell decreases.
		E)	a point is eventually reached where equal numbers are passing into and out of the cell.

4.	Diffusion continues to occur until there is and equal concentration of particles on both sides of a semipermeable membrane.

		A)	True
		B)	False

5.	Diffusion is the spontaneous net movement of particles from an area of low concentration to an area of higher concentration.

		A)	True
		B)	False