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A pore of a cell membrane
Passive Transport
Last time, we talked about how the phospholipid bilayer that forms the cell membrane serves as a protective barrier between the cell and the outside environment. And that is really good, because we wouldn't want anything getting into our cell! But what about molecules that need to get in and out? Well, the cell membrane has a solution...
The cell membrane has more than simply a bilayer of phospholipids, it also has protiens that are found within the bilayer, called integral membrane proteins. The cell uses these proteins in a variety of ways to move substances in and out of the cell. There are two types of cell membrane transportation - passive transport and active transport. First, we will talk about passive transport because it is the simplest.
Passive transport is when molecules move from one side of the cell membrane to the other by themselves, WITHOUT any energy from the cell. BUT, diffusion only takes place from an area of high concentration to an area of low concentration, which means the molecules will only move passively towards the direction of the lowest concentration of those molecules. When we first introduced the cell membrane, we said that hydrophobic molecules can move through the cell membrane on their own through diffusion.
Hint: look at this webpage for a refresher!
So, we know that hydrophobic molecules can move across the cell membrane by diffusion, but we still don't know how other molecues get across since they can't pass through the hydrophobic lipids! This is where the cell membrane uses the integral proteins. The simplest type of transportation using a protein is through a protein channel or pore. Molecules use these proteins as a channel to move through the cell membrane, but since the cell isn't using any energy, the molecules still can only move from high concentration to low. This is called facilitated diffusion, which just means the proteins are "facilitating" or "helping" the molecule diffuse across the membrane. Look at the simulation of facilitated diffusion below...
Diffusion through a membrane pore
Now continue to Cell Membrane, Page 3...
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<font face="Century Gothic">Do you remember what the diffusion of water is
called?</font>
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<font face="Century Gothic">Hydrophobic molecules can move across a cell
membrane through _________, which is a type of __________. This means that
they can ________________.</font>
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Protein Diffusion/ Passive Transport/ Move wherever they want, whenever they want.
Diffusion/ Active Transport/ move from an area of high concentration to an area of low concentration
Osmosis/ Active Transport/ Move wherever they want, whenever they want
Diffusion/ Passive Transport/move from an area of high concentration to an area of low concentration
Facilitated Diffusion/ Passive Transport/ not move across the membrane by themselves
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http://en.wikipedia.org/wiki/Diffusion16Century Gothic
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<font face="Century Gothic">Once the first white molecules move through
the protein channel by facilitated diffusion, do you think they could move
back across to the inside of the cell? Explain your answer.</font>
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<font face="Century Gothic">CHALLENGE QUESTION: Can you name all three
types of facilitated diffusion?</font>
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