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Unit: Membranes and transport

Check your understanding of membrane structure, diffusion, osmosis, and transport!


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Learn about the amazing and versatile barrier that surrounds all of your cells: the plasma membrane!
Ever wonder why plants wilt if you don't water them? Find out here, and learn more about the basic principles of probability that underlie movement of water and other molecules in biological systems.
Sometimes it pays to go with the flow! Learn about passive transport mechanisms, in which molecules travel across the plasma membrane by moving down their concentration gradients (no energy expenditure required).
In active transport, the cell has to pay (often, in the form of ATP) to move molecules against their gradients. Learn more about the amazing protein machines that make this possible.
Macrophages (specialized immune cells) in your body can "eat" bacteria, pulling them into the cell and digesting them. Learn how they accomplish this feat, and how cells transport other large molecules across the plasma membrane.

About this unit

How do the cells in your body define their boundaries (and control what comes in or goes out)? As it turns out, cells have a sophisticated and flexible barrier, the plasma membrane, and a wide array of strategies for transporting molecules in and out. Learn more about what the membrane's made of and how different types of molecules move across it.