Facilitated Diffusion

Facilitated Diffusion

Facilitated diffusionPassive movement of molecules from areas of high to low concentration. is movementA fundamental property of life involving motion of the body or its parts. down a concentration gradientA difference in the concentration of a substance across a space. from high to low. It requires a protein to cross the plasma membraneThe outer boundary of a cell that controls what enters and exits..  Water and other polar moleculesGroups of atoms bonded together. are not able to cross the lipid bilayer. They get repelled by the hydrophobic region created by the nonpolar tails of the phospholipids.  Therefore, a transport protein acts like a bridge spanning the lipid bilayer and covering the hydro phobic region.  Now the polar molecules are free to move down their concentration gradient.  Water crosses into an out of cell membranes using a protein called an aquaporin.  When you drink a diuretic like coffee or alcohol, aquaporins are removed from the urine-generating tubules in your kidney. As a result, you are incapable of conserving waterThe universal solvent essential for life..  This is commonly known as breaking the seal which is an undeniable urge to urinate.  Also, the urineThe liquid waste excreted by the kidneys. and will be incredibly dilute and flooded with the water that you were unable to conserve.

Protein Channels/Carriers

There are different types of protein carrier that complete facilitated diffusion.  One kind is a channel that is always open.  We call those leak channelsProtein passages in the cell membrane that allow specific molecules to pass through.. You can see one of these channels on the left side of this diagram in purple.  This channel is open for travel.  However, channels aren’t a free-for-all.  Channels have specific shapes that allow one item to move through them.  So, potassium(K⁺): Major ICF cation; essential for muscle and nerve function. can’t move through the sodium(Na⁺): Major ECF cation; important for fluid balance, nerve function. channels and vice versa.  There are other proteinsLarge molecules made of amino acids with various functions in the body. embedded in a cell membrane. They also complete facilitated diffusion, but these proteins are called carriersMembrane proteins that transport substances across a cell membrane..  The term carrier indicates that the protein has to change shape for its molecule to pass through.  This still doesn’t require energyThe capacity to do work or cause change..  The protein changes shape and the molecule passes down the gradientMovement from an area of higher concentration to lower concentration., keeping this process a passive process requiring no energy.  Carriers usually have a limit to how many items it can “carry” across in one shape change.  In the diagram in this slide, you can see a carrier on the right in green.  Notice how the carrier is open on one end, the bottom end, and closed on the top end.  Yes.  You try to carry all the items from the fridge at the same time, needed to make a great sandwich. First, you load the carrier. Then you try to walk across the kitchen holding the hummus, avocado, muenster, sprouts, tomato, and knife.  I appreciate a good vege sandwich now and then.

Conformational Change

I somewhat hate this term “conformational.”  Right now, everyone who is listening, raise your hand if you know what this word means.  Yeah, just as I thought, no one raised their hand.  Wait, I must remind you of the glossary of your book. It actually does not contain this word.  I just don’t want you to forget that the glossary is a great resource.  I dislike this term. It’s a term that textbook authors use. They assume that students know this word.  It just means shape.  The title of this slide could be Shape Change.  Channels don’t change shape. They are just pores in the cell membrane. If you can fit, you can use the channel.  Carriers have specific shapes too, but they also have to change shape to bring a molecule across the cell membrane.  Carriers are still using the gradient, just as channels use the gradient.