The plasma membrane Flashcards
Describe the plasma membrane and its constituents
Creates a membrane for most water soluble molecules. Lipid-protein interactions are non covalent and both and move in the bilayer.
There are four classes of membrane lipids:
1. phosphoglycerides - two fatty acids attached to a glycerol
- sphingolipids - sphingosine backbone, sphingomyelin
- glycolipids - often sphingosine backbone and always a carbohydrate on the external side
- Sterols.
Most are amphipathic molecules, and so it is energetically favorable to form a liposome in vitro or surfaces of organelles in vivo.
The fluidity allows for lateral transfer, and rotation but rarely flipping.
Describe phosphoglyceride.
Its a phosphate, glycerol, then two fatty acid chains.
One chain ins nonsaturated and has a kink in it which inhibits dense packing and increases fluidity or it would solidify at room temperature. Bacteria can regulate the number of kinks.
The phosphate can then bind to a ethanolamine, serine or choline.
These are all major constituents with sphingomyelin (amide group and single fatty acid chain rather than glycerol and two) and a phosphotidyl choline head.
Describe sterols and glycolypids
Sterols have a polar OH group which is located near the membrane surfaces on both sides.
The rigid sterol ring will stiffen region of the membrane in the vicinity.
Alipathic regions will keep fatty chains apart.
Glycolipids are derived from sphingosine with sugar instead of phosphotidyl groups. (normally found in plasma membrane and myelin)
Describe the asymmetric distribution of lipids between internal and external.
Inner leaflet: phosphotidylethanolamine, phosphotidylserine (negative)
Outer: phosphotidylcholine, spingolipids, glycolipids (exclusively!).
Cholesterol is pretty much equal
What are lipid rafts?
Non random lipid distribution on the inner or outer membrane leaflet. They are enriched with cholesterol and sphingomyelin. They can sequester a specific subset or membrane proteins by excluding other ones because that region is thicker.
glycolipids are not found here.
Describe membrane proteins
They can be integral - by being incorporated into the membrane by hydrophobic regions (alpha helix or beta barrel)
Some integral proteins have a covalent lipid attached post translationally which can reversibly bind to the membrane.
-these lipids are known as anchors and you can competitively inhibit the process of integral proteins binding to the anchor.
Peripheral proteins will associate with the membrane from charge.
How do small ions and small molecules cross the membrane? (include properties)
A pump: uses ATP, goes against the gradient, and rate is the slowest. Is the most specific
Carrier (enzyme like proteins, no chemical change) : doesn’t use ATP, goes down the gradient, rate is in the middle, and is intermediate specificity.
Channel: doesn’t use ATP, goes down the gradient, rate is the fastest, a lack of specificity.
-only dependent on charge or size.
Curare - antagonist at the Ach binding site.
Describe carrier type transporter and the sodium glucose symport.
Uniport- when the solute, ligand binds, the transporter will random switch between the states which allows the opening to the inside, allowing for faster than diffusion transport.
Glucose is going against the gradient (high internally) When sodium is bound, this increases affinity for the glucose. High specificity, when both are bound, there is a conformational change opening the two into the cell.
Cell polarity is established by nonrandom distribution of membrane proteins with tight junctions limiting them. This is important in epithelium of the small intestines.
Two steps:
- Glucose is brought in with sodium from the small intestines.
- uniporters on the basal side of the cell will transport glucose down its concentration gradient out of the epithelial cell.
PS: there is a sodium potassium pump also on the basal side which gets sodium out so there is a low intracellular soidum concetration.
Calcium is released out of the ER after contraction. How is the calcium reestablished despite a still intracellularly high calcium level in the ER?
Via P-type transporters - I
They will autophosphorylate themselves via intrinsic ATPase.
There is a cavity in nonphosphorylated transporter which binds calcium.
Then ATP binds and hydrolysis allow phosphorylation of the aspartic acid subunit
Conformational change occurs and channel opens to lumen of the muscle ER and releases calcium .
Describe a P-type transporter II.
The Na+/K+ ATPase. Sodium binds in the cell.
ATP binds gets hydrolyzed and pump is phosphorylated leading to conformational change.
Sodium is released out of the cell.
When potassium binds, dephosphorylation occurs and induces a nother conformational return and K+ enters the cytoplasm.
Describe ABC transporters.
They are pumps and are unique in that they bind small molecules not ions and there is no phosphorylation of the pump.
Bacteria = import, Eukaryotic = export
There are two ATPase domains on each protein. The small molecule binds on the intracellular side (eukaryotes) only when ATP is not bound. Then ATP binds and allows the ATPase domains to dimerize, causing a conformational change that exposes the substrate to the opposite side of the membrane.
ATP hydrolysis will release the substrate and return the protein to its original conformation.
Describe three clinical examples of ABC transportes
For cancer and drug resistance, high levels of a specific ABC transporter can arise in tumor cells. IT allows more drug to be removed from the cytoplasm so their effects are reduced.
IN malaria, they also synthesize an ABC transporter encoded in their DNA which leads to chloroquine resistance.
ATP binding to CFTR drives opening and closing of the Cl- channel.
What does cholera toxin bind to?
Cholera toxin binds to glycolipid GM1 ganglioside.
A defect in glycolipids would likely decrease binding and entry of the toxin.
