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.
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. For simplicity, only one ion of each type is depicted, but the sodium-potassium pump transfers 3 sodium ions and 2 potassium ions at the same time.
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.
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Director: Dennis Liu, Ph.D.
Scientific Content: Satoshi Amagai, Ph.D.
Animator: Mike Chelen