Unit 4 Flashcards
Membrane functions
- Compartmentalization (euks): create separate environments for different activities
- Selectively permeable barrier: prevent unrestricted molecule exchange
- Transport solutes: molecule exchange across membrane
- Energy transduction: convert from one form to another
- Respond to external signals: signals travelling from distance/nearby cells
- Scaffold for biochemical activities
What are membrane phospholipids composed of?
Polar head group, phosphate, glycerol, 2 fatty acid chains
Major polar head groups of phospholipids
Used for naming
- Phosphatidyl… choline (PC), serine (PS), ethanolamine (PE), inositol (PI)
Amphipathic
Have both hydrophilic and hydrophobic parts
e.g. phospholipid, cholesterol (sterol), glycolipid
Formation of lipid bilayers
- Hydrophobic molecules exclude water, clustering together to minimize energy cost of organizing water molecules
- Amphipathic molecules –> conflicting forces –> solved by formation of bilayer (energetically most favourable, spontaneous)
- Lipid bilayers are: closed, self-sealing
- Sealed compartment formed by phospholipid bilayer –> energetically favourable vs planar phospholipid bilayers (edges exposed to water)
Movement of phospholipids within membrane
- Phospholipids –> constantly moving within leaflet: lateral diffusion (swapping places within leaflet), twisting, turning, rotation
- Intentional movement: flipping to opposite leaflet during membrane synthesis (rarely flip back)
Allows membrane fluidity
Factors that affect membrane fluidity
- Temperature
- Change in lipid composition that affect alignment of phospholipid tails
Tightly packed tails –> more viscous, less fluid
Freely moving tails –> higher fluidity
Interaction between temperature + lipid composition
Membrane fluidity will change if:
- temp changes, lipid composition stays constant
- lipid composition changes, temp stays constant
Transition temperature (Tm)
Temp at which a membrane transitions b/w fluid phase + gel phase
If temp above Tm
Membrane ‘melts’ –> lipids move more freely, rotationally, laterally within leaflets
If temp below Tm
Hydrophobic tails pack together (as if cold, huddling together) –> membrane gels –> incompatible with life
Tm + membrane fluidity
Cells must maintain fluidity within a relatively narrow range even w/ changes in environmental temp
Usually deal with too cold temps
Factors that affect Tm (fatty acids)
- Altering length of fatty acid chains
- Longer chains –> more interactions b/w fatty acid tails –> tighter packing –> less fluid at a given temp, higher Tm, higher temp to melt - Altering saturation of fatty acids –> # of db
- More db (more unsat) –> less packing –> more fluid at given temp, lower Tm, lower temp to melt
- Membrane phospholipids typically have 1 sat fatty acid and 1 w/ 1+ db (unsat)
Factor that affects Tm (sterols)
- Altering amount of sterol (e.g. cholestrol) - (can be up to 50% of membrane lipid in animal cells)
- Cholesterol acts as “buffer”, inhibiting phase transitions when temp changes
- Higher cholesterol at cool temps –> more fluid
- Higher cholesterol at warm temps –> less fluid
Examples: variability of membrane fluidity between organisms (within life compatible range)
- Organisms from warm climates –> membranes near melt
- Organisms from cold climates –> membranes near gel
Mechanisms of membrane fluidity regulation in living cells
Homeo viscous adaptation (whole organism level): maintaining fluidity at temps low enough for potential gelling by altering membrane composition
Dealing with low temps:
- Shorter fatty acid chain length (e.g. enzymes cut C18 –> C16)
- Increase # db (= decrease sat): (e.g. desaturase enzymes triggered by low temps)
Membranes of archaea vs eubacteria + eukaryotic
Both: glycerol, phosphate, 2 HC chains
Archaea: branched ISOPRENE chains (instead of fatty acids) ETHER-linked to L-glycerol
- Allow extremophile archaebacteria to not suffer from membrane breakdown
Eubacteria/Euk: fatty acid chains ESTER-linked to D-glycerol
Ways proteins can associate w/ membranes
Integral:
- Transmembrane (across entire membrane, both leaflets, external parts outside membrane: alpha-helix, beta sheet
- Monolayer-associated: 1 leaflet
- Lipid-linked: linked to membrane by a lipid
Peripheral:
- Protein-linked: fully outside membrane, linked by integral protein
Integral protein association w/ membrane
- Hydrophobic R groups allow protein to be located in hydrophobic environment (hold orientation in membrane) since the backbone of protein is always hydrophilic (polar)
- Polypeptide chains usually cross membrane as alpha-helices
Formation of hydrophilic channels
- From several alpha-helices
- Hydrophobic parts hold protein in membrane
- Hydrophobic parts (very polar) form pore that water and small soluble molecules can flow through
Formation of Porins
- From beta pleated sheets
- Common in outer membranes of gram-negative bacteria + endosymbiont derived organelles
- Not common in animals
- Very common in bacteria
Movement restriction of membrane proteins
Restriction of movement by:
- Cytosolic protein
- Extracellular protein
- Identical/non-identical proteins of separate cells interact
- Large membrane protein prevents flow of membrane proteins (e.g. tight junction)
Membrane protein distribution in epithelium
- Tight junctions create 2 different domains in the membrane, prevent protein movement
- Epithelial cells: line tubular + spherical organs (can bring things in/move things out)
Apical domain
Domain on top section of cell