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BioFlix: Membrane Transport

by Pearson
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We get all our energy and organic molecules from food. Before we can use the molecules we eat, they have to enter our cells, starting with the cells lining the small intestine. Let’s zoom in to the surface of a cell. The plasma membrane is selectively permeable -- some molecules can move across it, while others cannot. How do materials enter and leave cells? Lipids, such as these yellow molecules, can dissolve in the lipid bilayer. Notice how they move down their concentration gradient -- from where they are more concentrated to where they are less concentrated. This is an example of diffusion. Diffusion is a form of passive transport -- it does not require energy from the cell. Most molecules can’t cross the lipid bilayer. Here, the sugar fructose moves into intestinal cells by facilitated diffusion, moving down its concentration gradient through a transport protein. Facilitated diffusion doesn’t require energy from the cell, so it’s also a form of passive transport. Water crosses the plasma membrane by facilitated diffusion or by diffusing across the lipid bilayer directly. The diffusion of water across a membrane is called osmosis. The sodium-potassium pump moves ions against their concentration gradient, from where they are less concentrated to where they are more concentrated. This requires energy from the cell and is known as active transport. Energy from ATP is used to move sodium ions out of the cell and potassium ions in. Another type of active transport is cotransport. Here, both sodium ions and glucose move into the cell through a cotransporter protein. Sodium ions move down the concentration gradient created by the sodium-potassium pump, and glucose moves against its concentration gradient. Now let’s move to the other side of our intestinal cell. Materials can be exported in vesicles that fuse with the plasma membrane and release their contents outside the cell. This process is called exocytosis. In endocytosis, the plasma membrane pinches in, forming a vesicle that contains material from outside the cell. On this side of the cell, we can also see oxygen and carbon dioxide diffusing across the lipid bilayer. Cells use all these processes to get what we need. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
We get all our energy and organic molecules from food. Before we can use the molecules we eat, they have to enter our cells, starting with the cells lining the small intestine. Let’s zoom in to the surface of a cell. The plasma membrane is selectively permeable -- some molecules can move across it, while others cannot. How do materials enter and leave cells? Lipids, such as these yellow molecules, can dissolve in the lipid bilayer. Notice how they move down their concentration gradient -- from where they are more concentrated to where they are less concentrated. This is an example of diffusion. Diffusion is a form of passive transport -- it does not require energy from the cell. Most molecules can’t cross the lipid bilayer. Here, the sugar fructose moves into intestinal cells by facilitated diffusion, moving down its concentration gradient through a transport protein. Facilitated diffusion doesn’t require energy from the cell, so it’s also a form of passive transport. Water crosses the plasma membrane by facilitated diffusion or by diffusing across the lipid bilayer directly. The diffusion of water across a membrane is called osmosis. The sodium-potassium pump moves ions against their concentration gradient, from where they are less concentrated to where they are more concentrated. This requires energy from the cell and is known as active transport. Energy from ATP is used to move sodium ions out of the cell and potassium ions in. Another type of active transport is cotransport. Here, both sodium ions and glucose move into the cell through a cotransporter protein. Sodium ions move down the concentration gradient created by the sodium-potassium pump, and glucose moves against its concentration gradient. Now let’s move to the other side of our intestinal cell. Materials can be exported in vesicles that fuse with the plasma membrane and release their contents outside the cell. This process is called exocytosis. In endocytosis, the plasma membrane pinches in, forming a vesicle that contains material from outside the cell. On this side of the cell, we can also see oxygen and carbon dioxide diffusing across the lipid bilayer. Cells use all these processes to get what we need. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings