Membrane Structure and Function Flashcards
List and describe the main molecules that form biological membranes
50% lipid and 50% proteins.
1) Phospholipids: They are the most abundant membrane lipid. Have properties that make them ideal for forming lipid bilayers. They have two fatty acid tails that orient towards the interior of the lipid bilayer. Also have a polar head that orients towards water exterior to repel the water from the bilayer. If the charged polar head is removed than the -OH that is left will be able to be attacked by the water and that would allow water to get into the membrane and destroy the phospholipid and the bilayer.
- Phosphoglycerides: Contain a glycerol backbone
- Sphingolipids: Contain a sphingosine
3) Cholesterol: It is present in large amounts. Can reach a 1:1 ratio of cholesterol to phospholipids. Cholesterol increases the rigidity of the membrane and decreases the permeability of the membrane to water-soluble molecules. Cholesterol has a small polar head and nonpolar hydrocarbon tail. Also has a rigid steroid ring that imparts the rigidity
Sphingosine
Has a double bond in its fatty acid chain. Also contains a nitrogen in the other fatty acid chain.
Glycerol
CH2OH-CHOH-CH2OH. 2 OHs will bind the fatty acid, one the polar head
What are the main physical forces that underlie the stability of biological membranes
Phospholipids self-aggregate without external biochemical energy. That is because this self-aggregation is energetically favorable because organizing water around individual hydrophobic molecules is energetically expensive. The main forces are hydrophobic forces via Van Der Waals interactions via the fatty acid tails. The hydrophobic interactions provide tremendous plasticity and thus the membrane can undergo a tremendous amount of strain.
Define membrane fluidity and explain what molecular characteristics of lipids determine that property.
Fluidity is defined as the ability of the phospholipids to diffuse laterally or the level of mobility of lipids. It is increased membrane dynamics.
5 Things alter fluidity:
1) Shorter fatty acid chains: less hydrophobic interactions
2) Double bonds: More double bonds introduce more kinks or disruptions in the hydrophobic interactions so more fluid with more double bonds
3) Cholesterol increases fluidity
4) Lipid composition
5) Temperature: colder=less fluidity
As fluidity increases the transition temperature decreases meaning it will have to be a lower temperature to freeze!
Define Transition Temperature in biological membranes and explain what properties are responsible for it
The transition temperature is the characteristic temperature at which a lipid freezes. The more fluid the membrane, the lower the transition temperature (freezes at an even lower temperature)
more double bonds give a lower transition temperature as well as shorter fatty acid tails
Define the effects of cholesterol on lipid membranes
Cholesterol makes the membrane physically more rigid but it has the weird property of making it more fluid as well. Also, cholesterol inhibits small polar things (K+, Na+, etc.) from diffusing through the membrane because it “fills in the gaps” between the phospholipids, especially those that are kinked.
Cardiovascular problems are NOT related to the fluidity of membranes
Describe the structure of sphingolipids and glycolipids
Glycolipids: These are lipids with sugars and thus are facing OUTSIDE the cell. They are generated in the ER lumen. They function in cell recognition. When sugars that are supposed to be on the outside of the cell end up on the inside, it usually indicates something has gone wrong. They associate themselves via Van Der Waals interactions in the membrane and with H-bonds between head groups. Cholera and Tay-Sachs play a role with these. They also protect the membrane.
Sphingolipids have a double bond in one chain and are made up of sphingosine. They also have a nitrogen. Overall increase membrane fluidity
Phosphatidyl-Serine
It is usually on the inner leaflet of the cell but when a scramblase flips it to the outside it signals for macrophages to eat the cell.
Describe Raft formations
Rafts have a higher density of (sphingolipids + cholesterol) in the membrane. Membrane proteins prefer specific membrane patches with whom they can interact with better electrostatically. They are formed by the phospholipids and cholesterol spontaneously self associating
Describe cell signaling via phosphotidylinositol.
Phospholipases cleaves the inositol phospholipids to generate a relay signal and a scaffold for protein Kinase C. The polar head group could also be cleaved off to generate a relay signal.
Physical Properties of Membranes
1) The amphiphilic properties of phopholipids make them spontaneously form lipid bilayers, a critical component of life on earth
2) Because of these properties of lipid bilayers, membranes are fluid, deformable, and self-sealing allowing for compartmentalization of biological material
3) Cholesterol is a critical component of eukaryotic membranes. Increasing the amount of cholesterol increases the rigidity and fluidity.
4) Some phospholipids and cholesterol spontaneously self-associate to form membrane microdomains, such as lipid rafts
5) The leaflets of the membrane are asymmetric
Describe the distinct association of proteins with lipid membranes
Proteins can associate with membranes in numerous ways via hydrophobic regions usually. Long stretches of hydrophobic regions suggests a transmembrane protein. There are also polar amino acids in the membrane that make up the “core” of an ion channel for example. Tryptophan can interact with the polar and nonpolar regions making it anchor transmembrane proteins.
1) Transmembrane proteins: they pass through the lipid bilayer. Can also be multipass
2) Beta-Barrels: Form a pore to allow things through
3) Proteins spanning one side of the membrane. The protein is amphipathic so the polar region is in contact with the cytosol, the hydrophobic region in contact with the membrane
4) Lipid modifications: Can add a protein to the lipid
5) GPI linked proteins: can link to a phosphate
Also, it is a common mechanism to associate a protein with a lipid bilayer that is not a transmembrane protein
Transmembrane Proteins
They are often alpha-helices and the amino acids in the bilayer are hydrophobic. They often organize themselves to maximize H-bond interactions.
Explain the functional significance of the hydropathy plot
It can show hydrophobic patches indicative of a transmembrane protein. ~30% of proteins are likely membrane proteins.
