Exam 3- Transport of Small Molecules Flashcards
Which molecules are able to cross freely through the plasma membrane? Why?
Gases : O2 and CO2
Lipids / hydrophobic
Steroid Hormones
Small Polar Molecules: H2O, Ethanol
Which molecules are unable to cross freely through the plasma membrane? Why?
Glucose and sugars
Amino Acids
Ions: Na+ K+ Cl- H+ Ca2+ / other small charged ions
nucleosides
What’s the “force” that drives Diffusion (both, passive and facilitated)?
A concentration gradient across the membrane.
What’s a “concentration gradient”?
Unequal distribution of ions across the cell membrane
How does the Glucose Transporter work? What’s the mechanism behind its activity?
has 12 alpha-helical transmembrane domains made of hydrophobic amino acids but have polar residues that can bind glucose.
When glucose binds a conformational change occurs moving the binding site into the cell and releasing glucose. `
what is facilitated diffusion?
uses no energy, molecules move relative to [gradients] or electric potentials.
what does facilitated diffusion require?
carrier protein or channel protein
what are the molecules that require facilitated diffusion?
Polar, charged, sugars, AA, nucleosides, ions
what is a carrier protein?
binds specifically
changes conformation to allow molecule to travel
what molecules need a carrier protein?
Sugars, AA, nucleosides
What is a channel protein?
A pore that only allows the correct size and charge to pass freely with the concentration gradient
two examples of channel proteins
ion channels
aquaporins
3 features of ion channels?
- Fast! more than 1M ions per second
- very selective to m/z
- most are closed and must be opened
What are the two mechanisms that regulate the opening of ion channels?
Voltage- gated and Ligand- gated
How is specificity achieved for the Na+ channel?
the Na channel is 10x more permeable to Na than K.
The specificity is based on Na’s small size
How is specificity achieved for the K+ channel?
more than 1000x more permeable to K than to Na.
Uses selectivity filter
Lined with carbonyl groups that allow dehydrated K+ to pass but not hydrated Na+
K+ will interact with carbonyl since it is so large and this displaces H2O
What generates ionic potentials at the plasma membrane?
The concerted action of ion pumps and ion channels.
which ions have higher concentration inside the cell?
K+
what ions have higher concentration outside the cell?
Na+
Cl-
