Facilitated diffusion Flashcards
What is the definition of facilitated diffusion?
The passive transfer of polar molecules or charged ions down a concentration gradient, across a membrane, by channel or carrier proteins in the membrane
When is facilitated diffusion used?
To transport ions and large polar molecules, such as Na+, glucose and amino acids that are relatively insoluble in phospholipids
Does facilitated diffusion require ATP?
No
Why is facilitated diffusion more rapid that simple diffusion?
For these molecules as going through a protein is easier than directly through the bilayer
What four factors affect the rate of facilitated diffusion?
Temperature
Concentration gradient
Number of transport proteins
Type of molecule
How does the number of transport proteins affect the rate of facilitated diffusion?
As more proteins are present, more sites are available
This means the higher the diffusion capacity and diffusion will be faster
What are the two types of transport proteins?
Channel proteins
Carrier proteins
What do channel proteins do?
Protein molecules with water-filled hydrophilic pores
Ions, being water soluble, can pass through because the pores are hydrophilic
Each channel is specific for one type of ion
Channels open and close depending on the needs of the cell
What do carrier proteins do?
Allow diffusion of larger polar molecules such as glucose and amino acids
Polar molecule attaches to a binding site on the carrier protein
Causes the protein to change shape
Releases the molecule through to the other side of the membrane
What are gated protein channels?
Can be opened and closed to allow ions through
E.g. sodium ion gated channels that open during initiation of the nerve impulse
What is co-transport?
A type of facilitated diffusion
Brings molecules and ions into cells together on the same protein transport molecule
Is co-transport a passive process?
Yes
Explain the process of sodium-glucose co-transport
There is a concentration gradient for sodium ions to move into the cell
Sodium ions and glucose bind to the carrier protein
The carrier protein changes shape, and the sodium ions and glucose are transported to the other side of the membrane
Process can move glucose against its concentration gradient without the use of ATP - known as secondary active transport
Give two examples of processes that involve uptake of sodium ions and glucose molecules by co-transport
Absorption in the small intestine (ileum) - active transport
Selective reabsorption of solutes in the kidney tubes (nephrons) - active transport/secondary active transport
