Function of membranes in cells
- serve as barriers to keep things in the cell and to keep things out of the cell (a mode of protection)
- Serve as areas for localization of proteins
- Receive information via localized areas of communication
- Import & export of small molecules via protein mediated means
- Movement and expansion
- interaction w/ neighbors requires: adhesion, movement to new place or movement to stay in the same place
In animal cells, is composed of 40% lipid, 50% protein, and 10% carbohydrate.
-Contains hydrophobic and hydrophilic regions
Lipid bilayer structure
Very common lipid, most common is phophotil choline
- Contains a polar (hydrophilic) head and a fatty acid tail.
- is fluid due to unsaturation
Phospholipids
Double bonds; liquid at room temp.
unsaturation
Single bonds; solid at room temp.
saturation
Bilayer formation
-is most energetically favorable because of H2O interactions (H2O makes a “cage” around the hydrophobic molecule)
Formation of sealed compartments: lipid bilayer
- Energetically favorable: Sealed compartment formed by phospholipid bilayer (will self seal if penetrated)
- Energetically unfavorable: planar phophokipid bilayer with edges exposed to H2O
Fluidity of the membrane
- lateral diffusion
- flexion (bending)
- rotation
- Flip-flop only occurs with the aide of enzymes (hydrophobic vs. hydrophilic)
Fluidity of the membrane depends on composition
- If fatty acids are shorter, the membrane will move more fluidly
- If fatty acids are longer, the membrane will be more viscous
1. bonds
2. length of fatty acids
3. amount of cholesterol
- Acts as a “buffer” to balance membrane fluidity
- Makes less fluid in unsaturated regions and more fluid in saturated regions
- Can fit within the membrane because of its small size
Cholesterol
- Specific region(s) on a membrane that keep signaling molecules together.
- Has different properties than the membrane surrounding it.
- one way to make membrane asym. (membrane is usually asym)
Lipid raft(s)
Synthesis of membranes
- ER synthesizes and adds phospholipids to the cytosolic half of ER bilayer
- Scramblase catalyzes transfer of random phospholipids from one monolayer to another (moves from one leaflet to another)
- symmetric growth of both halves of bilayer
Membrane asymmetry
- Cell exterior: asym. lipid bilayer of plasma membrane
- Cytosol:
- delivery of new membrane from ER to golgi membrane
- flippase catalyzes transfer of specific phospholipids to cytosolic monolayer of golgi
- golgi moves membrane to plasma membrane
Organelle responsible for packing and shipping material; also adding carbohydrates
Golgi
SEE NOTES FOR DIAGRAM OF PHOSPHOLIPIDS
Functions of membrane proteins
- Transporters and Channels: Include ions (K+, Na+, glucose, amino acids, and metabolites)
- Anchors provide movement and adhesion for overall cell or specific lipids (actin, matrix)
- Receptors bind to hormones, neurotransmitters; act as cell surface markers.
