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