Lecture 2: Membrane Transport Flashcards
Efflux and Influx
Efflux=net movement from inside to outside
Influx=net movement from outside to inside
Movement of a substance across cell membranes depends on:
- Driving Force
- Properties of the molecule
- Properties of the membrane
Driving Force and Direction of Movement
-Concentration and electrical gradients (electrochemical gradients) provide energy to drive molecules and ions across membranes, aka passive transport
Properties of the molecule and direction of movement
- Gases and hydrophobic molecules/lipid soluble or Non polar molecules diffuse easily across lipid bilayer
- Polar molecules that are uncharged and small diffuse fairly well across L.B.
- Large uncharged polar molecules and ions rarely, if ever, diffuse
Properties of membrane: Membrane Fluidity
- No covalent bonds btw phospholipids, instead stabilized by electrostatic and hydrogen bonding interactions of polar heads with water and
- Van der Waals attractions btw non-polar tails
- these forces are weak and thus allow lateral movement of lipids and proteins within
Membrane fluidity cont.
- Lipid bilayer is in constant motion
- Membrane fluidity is the ability of lipids and proteins to travels through the L.B., and is determined by the L.B.’s lipids
Types of movement in phospholipids
- lateral diffusion, which occurs fast and often
- rotation, occurs fast and often
- transverse diffusion/”flip-flop”, rare
Fluidity and Permeability
The more fluid the membrane, the more permeable it is to small polar molecules
membrane composition and permeability
- saturated/”unkinked” fatty acids are more compacted
- unsaturated/”kinked” fatty acids have more space and are more fluid; allows small polar molecules to diffuse more easily and increases overall permeability
- sometimes cholesterol fills the gaps betwen unsaturated fatty acids, reducing permeability
Membrane Transport Proteins
- used to move molecules that cannot be defused
- channels (fastest) and permease allow facilitated diffusion
- other proteins have an ATP-powered pump for active transportation
Facilitated diffusion
- via ion channels and mermeases molecules can move across membrane, powered by gradient
- passive=no energy/ATP required, move down their electrochem. gradient
Permeases
- Chemical specific
- Inhibitable
- Exhibits saturation kinetics: has a maximum uptake rate, dependant on the number of permeases and the external concentration of the specific chem.
Channels
- allow either small polar molecules or ions to move down thir gradient
- ions=ion channels : small polar molecules=porins
Ion Channels: Structure
- have protein domains~4 arranged in a ring to form a pore
- wide area of pore=vestibule
- narrow aread of pore=selectivity filter; larger molecules cannot get through
- sometimes made of one large pore-forming subunit with 4 internal repeats
- some have four seperate pore-forming subunits
Ion Channel: Selectivity
selectivity=allow some molecules but not others
- discriminate by size, charge, and charge density
- subunits are lined with animo acids of a specific charge, so negative lining repels -ve charged anions and attracts +ve charged cations
Ion Channel: Charge Density
- charge density=discrimiates between ions of similar size and charge, but diff. charge density, ie. the diff. btw Na+ and K+
- occurs via selective filter of -ve charged amino acids which acts as a lock and key and removes the hydration shell of K+
- cannot do so for Na+ because it is slightly smaller, and does not fit the selectivity filter properly, so water outcompetes the amino acids at binding to Na+
Gated Ion Channels
- can be opened or closed
- opened by changes in voltage, chemicals binding to receptors (ligands), or changes in mechanical force
Active Transport
Moves substances across membrane against their electrochemical gradient, but requires input of energy; either directly or indirectly from ATP
Primary and Secondary Active Transport
-Primary A.T. generates an electrochem. gradients that acts as potential energy that drives secondary A.T.
Na+/K+ ATPase/pump
- P-type ATPase
- Powered by hydrolysis of 1 ATP molecule
- pumps 3 Na+ out and 2 K+ in for each ATP molecule
- establishes an electrochem. gradient/difference in distribution of Na+ and K+; therefore called an electrogenic pump
