Cell Membranes Flashcards
What are the functions of the plasma membrane?
- Physical barrier, structural support & protection
- Transport - selective permeability
- Organelle delineation and individuality
- Cell recognition - intercellular communication - signal transduction
What is the basic structure of phospholipids?
- Backbone molecule
- 1 or 2 fatty acid chains (hydrophobic)
- Phosphate group
- Mar or may not have alcohol containing group
- They are amphipathic molecules
What is the structure of Glycerophosopholipids?
- Glycerol backbone
- 2 fatty acids at C1 & C2 - can be saturated or unsaturated
- Phosphate group at C3
- No alcohol containing group (But serine, choline or ethanolamine can be added to phosphate group)
What are Phospholipases (PLs)?
- Enzyme that cleaves bonds in glycerophospholipids
- PLC, PLD hydrolyse either side of the polar head (releasing glycerol backbone and OH- containing group respectively)
- PL(A1), PL(A2) hydrolyse FA from C1 and C2 respectively
- PLs are poisonous signals in venom: Phospholipase (A2) in venom causes relate of FA at C2 ultimately breaks down phospholipids (i.e. the membrane)
What is the structure of sphingolipids?
- Sphingosine backbone
- Single fatty acid chain attached at amino group
- MIGHT have phosphate and/or alcohol group
- Ceramides are simplest sphingolipids (1 FA linked at amide terminal)
What is the structure of Sphingosine?
- The backbone of sphingolipids
- Has amino group at C2
- Unsaturated 18C hydrocarbon chain
What is sphingomyelin?
- Sphingolipid found in the membrane bilayer of nerve cells
- Phosphate + choline group (Phosphocholine) attached to C of amide group
What are glycolipids?
- Part of sphingolipid family
- Ceramide bound to 1 or more sugar residue in glycosidic linkage
- Found in outer portion of cell membrane bilayer
Asymmetry and heterogeneity of membrane bilayer
2 leaflets (outside and inside) of membrane bilayer are NOT structurally or functionally identical.
Transverse Asymmetry of the membrane bilayer
Refers to different lipid and/or protein compositions in the 2 leaflets of a bilayer membrane
Later Heterogeneity of the membrane bilayer
Refers to different lipid and/or protein compositions in the plane of one of the leaflets of the membrane
3 Process that modulate membrane lipid movement
- Lateral Diffusion:
a. Lipid changes orientation within same plane of membrane
b. Heads still associating with heads etc.
c. Isn’t a lot of resistance to movement through the plane, therefore quite fast - Rotation:
a. Phospholipid rotates on axis to associate with neighbors
b. Stays in same spot so not a lot of resistance so quite fast - Transverse diffusion: (flip-flop)
a. From 1 leaflet to the other
b. Entire polar head and hydrophobic tail need to go through the membrane to get to the other side
c. Lots of resistance so its quite slow
3 Lipid-translocator proteins
- ATP-dependent flippases:
a. Transport lipids from outer leaflet to inner leaflet
b. E.g. (lipids that are transported)phosphatidylserine and phosphatidylethanolamine.
c. Needs to be input of energy (ATP) to break through resistance.
- ATP-floppases:
a. Transport lipids from inner leaflet to outer leaflet
b. E.g. (lipids that are transported) cholesterol, phosphatidylcholine and sphingomyelin. - ATP-independent bi-directional scramblases:
a. Moves lipids between bilayers.
b. Calcium driven
c. E.g. any lipid
What are Lipid rafts
- Subdomains of the plasma membrane that contain high conc. of lipids (esp. cholesterol and mostly saturated phospholipids) and proteins.
- Ordered and tightly packed, reduced fluidity.
- Function:
o Serves to concentrate molecules to aid in cellular processes such as cell signaling, signal transduction, protein trafficking.
Functions of membrane proteins
- Transport
- Enzymatic activity
- Signal transduction
- Cell-cell recognition
- Intercellular joining (synapses between neurons)
- Attachment
What are the 3 classes of membrane proteins?
- Integral membrane proteins – span entire width (or just 1 side) of membrane
- Peripheral protein – only on surface of membrane bilayer
- Lipid-anchored protein – protein attached to lipid, lipid inserted into membrane bilayer.
What are integral polytropic proteins?
Transmembrane proteins that span the entire width of the membrane
What are integral mono tropic proteins?
Integral proteins that are attached to 1 side of the bilayer but don’t span the entire width
What are alpha-helices?
Single stranded hydrophobic amino acids that for a helix and cross the membrane
Need 21-25 amino acids to cross the membrane
How do alpha-helices affect the fluidity of the membrane?
- α – helix kinks due to proline provides fluidity
- Membrane helix distortions are kinks at proline residues.
- Proline amino acid distorts the helix
- Proline can’t form H-bonds, this creates steric hindrance which also affects its neighbor blocking it from creating H-bonds.
What is a Hydropathy (Hydrophobicity) plot?
- Used to determine if an unknown protein is a membrane protein or not
What is glycophorin?
A glycoprotein on red blood cells made of a single alpha-helical TM-segment protein.
What is the glucose transporter?
A 12 spiral transmembrane alpha-helical segment which functions in transporting glucose across the plasma membrane
What are beta-sheets?
- found in membranes of bacterial cells
- Arranged in anti-parallel fashion
- Strands form zig-zag (3 strands)
- Multiple B-sheets in membrane come together to make a barrel. Need at least 9-11 amino acids per sheet to cross the membrane
- Hydrophobic amino acids orient toward the exterior (where they contact the surrounding lipids In membrane), and hydrophilic amino acids orient toward the aqueous interior pore.