Diffusion Across Membranes
This animation describes two different ways by which chemicals migrate through membranes: passive diffusion and active transport.
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Diffusion Across Membranes Background
The basic structure of almost all biological membranes is a lipid bilayer.
As the name implies, the bilayer is made up of two layers of lipid molecules.
The fatty acid tails of the lipids face each other, forming the middle of the
bilayer, and the hydrophilic heads line both surfaces. Most membranes also have
proteins attached or embedded in the lipid bilayer. The actual composition of
proteins varies depending on the cell type and the subcellular location.
Biological membranes are selectively permeable barriers; in other words, they
will let some molecules through but not others. This animation discusses two
types of transport across cell membranes: active and passive transport.
In the kidneys, water and salt move in and out of the nephron through
the membranes of the cells that make up the tube. The cell membrane must
prevent important chemicals from leaking out and undesirable chemicals
from leaking in, as well as allow movement of material such as oxygen
and waste products in and out of the cell. How is this accomplished? We'll
describe two different ways by which chemicals move through the membrane:
passive diffusion and active transport.
Active transport across membranes can move molecules regardless of the
concentration gradient (see passive diffusion below). It requires energy
(hence the term "active"), which is provided by ATP molecules. One well-studied
active transport mechanism, shown in this animation, is the sodium-potassium
pump, which pumps potassium ions into the cell while pumping sodium ions
out. In the animation, the purple tube that spans through the lipid bilayer
is the pump.
To start the pumping process, a sodium ion attaches to a protein. Next,
an ATP molecule splits (yellow explosion), yielding a phosphate group
and an ADP molecule. The sodium is released outside the cell, and a potassium
ion enters the pump to be released inside the cell after the phosphate
group is released.
Passive diffusion is the process by which molecules space themselves
apart into the surrounding area. For example, if you were in a gym with
a large group of people running in place, you wouldn't huddle in a corner;
instead, you would all spread out. Molecules thus move from a more concentrated
area to a less concentrated one down the concentration gradient.
A cell membrane is selectively permeable; some molecules such as oxygen
and carbon dioxide, can pass through unimpeded. Water, represented by
a green molecule in the animation, is also one such molecule. The animation
shows water passing through the lipid bilayer from the inside (top) to
the outside (bottom) of the cell.
Diffusion Across Membranes Animation Tips
Want to download this animation for classroom use? The files are in QuickTime format and are available for nonprofit
educational use. Howard Hughes Medical Institute holds the copyright for all of these materials and should be credited.
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Diffusion Across Membranes Credits
Director: Dennis Liu, Ph.D.
Scientific Content: Satoshi Amagai, Ph.D.
Animator: Mike Chelen