Unit 6 - Biological Membrane Flashcards
Membrane Functions
- Provides selectively permeable barrier
- Transports solutes
- Responds to external signals
- Energy Transduction
- Compartmentalization
- Scaffold for biochem. activities
Membrane Phospholipids
- polar head group
- phosphate group
- glycerol
- fatty acid chains/tails
Amphipathic
-molecule with both hydrophobic & hydrophilic parts
Movement of Phospholipids within Membrane
- constantly moving
- spinning in place
- travelling laterally within leaflet
- occasionally ‘flipped’ to opposite leaflet during membrane synthesis
Membrane Fluidity
-how easily lipid molecules move rotationally and laterally within a membrane leaflet
Membrane Fluidity is affected by…
- Temperature
2. Changes in lipid composition which affects alignment of phospholipid tails
Transition Temperature (Tm)
- temp. at which membrane transitions between fluid and gel phase
- above Tm –> membrane ‘melts’
- below Tm –> membrane gels, incompatible with life
Tm is affected by
- altering length of fatty acid chains: longer chains –> less fluid
- altering degree of saturation of fatty acids: more double bonds –> more fluid
- altering amount of sterol: cholesterol acts as ‘buffer’, inhibits phase transitions when temp. changes
Homeo-viscous Adaptation
-maintaining membrane fluidity at temps low enough to cause membrane to enter gel phase, through altering the membrane lipid composition
Low Temperatures
-enzymes which cut fatty acid chain length
Difference between Membrane phospholipids in eukaryotic cells & archaebacteria
- Eukaryotic: fatty acid chains ester-linked to D-glycerol
2. Archaebacteria: fatty acid chains ether-linked to L-glycerol
Role of ER in Membrane Assembly
- Free fatty acids in cytosol are catalyzed by enzymes bound to cytosolic side of ER
- new phospholipids added to cytosolic side
- Scramblases transfers random phospholipids to other leaflet
- membrans with ‘scrambled’ phospholipids emerge from ER
Role of Golgi in MA
- Membranes with evenly distributed phospholipids arrive from ER
- Flippases move PS and PE to cytosolic leaftlet
- membrane asymmetry maintained from this point on
Why is the Human RBC a Model Organism for the plasma membrane?
- best understood plasma membrane
- cells inexpensive and available in large numbers
- already present in single-cell suspension
- simple, pure preps of plasma membrane
Types of Membrane Proteins (TARE)
- Transport
- Anchors
- Receptors
- Enzymes
polypeptide chains usually cross as…
alpha helices
hydrophilic channels are formed via…
several alpha helices
proteins folded into pleated sheets can form…
pores!!
Diffusion
- dissolved solutes in constant, random motion
- solutes ‘spread out’ until conc. in all regions are equal
- water moves down concentration gradient, if solute cannot move, water will move to even out solute conc.
Osmosis
- diffusion of water across semi-permeable membrane down conc. gradient
- once water conc. equal on both sides, no net movement of water
Hypotonic Solution
- net water gain
- cell swells
- Animal: bursts (lyses)
- Plant: normal state, turgid
Isotonic Solution
- no net loss or gain
- Animal: normal state
- Plant: shrivels
Hypertonic Solution
- net water loss
- cell shrinks
- Animal: shrivels
- Plant: plasmolysis
Osmoconformers
-adjustment of internal salt conc. to match seawater/outside environement
Osmoregulators
-single-celled eukaryotes have contractile vacuoles that periodically pump out water
Turgor
-water pulled into cell, presses membrane out to cell wall
Order to Molecules’ ability to pass directly through membranes?
- O2,CO2,N2, steroid hormones
- H20, ethanol, glycerol
- amino acids, glucose, nucleosides
- ions
Ion Channel Functions
- allow movement of ions down conc. gradient
- critical in many cell activities
Ion Channel Features
- discriminate on both charge and size
- usually highly selective
- much faster than carriers
- bidirectional
- electrochemical gradient
What determines whether channels are open or closed?
- Voltage
- Ligand (extracellular/intracellular)
- mechanical
Carrier Proteins
- mediate facilitated diffusion
- binding solute at specific site
- solute moves down conc. gradient
GLUT1
-gluose transporter on mammalian cells
Features of Membrane Carrier Proteins
- Specificity
- Passive
- Saturable
- Can be inhibited/blocked
Active Transport
- transport against a gradient
- using energy (directly/indirectly) to move ions
- transport closely coupled to hydrolysis of ATP, absorption of light, and movement of electrons
Sodium-Potassium ATPase
- first pump discovered
- only present in animal cells
- move 3 NA+ out for every 2 K+ in
- coupled to hydrolysis of ATP
Hydrogen-Potassium ATPase
- on parietal cells lining stomach
- enables cells to secrete high conc. of H+
Coupled Transport
- membrane carrier proteins use driving force of ion moving down conc. gradient to move a solute across membrane against solute gradient
- indirect active transport
