Membrane Transport

In order of most likely to pass through to least likely, what kind of molecules pass through the cell membrane?

Hydrophobic molecules (O2, CO2, N2, steroid hormones), small uncharged polar molecules (H2O, urea, glycerol, NH3), large uncharged polar molecules (glucose, sucrose), ions

Give examples of amino acids that can pass through the membrane in comparison to ones that can’t pass through.

• more likely to pass:

  • nonpolar amino acids (GLAMP TIP CV)

    • cysteine and methionine less likely

• slightly less likely to pass:

  • uncharged polar amino acids (GASST)

    • especially threonine, tyrosine, serine

• very unlikely to pass:

  • charged polar amino acids (GA and HAL)

How do you find the flow of solute of the membrane?

multiply the concentration difference, across a membrane (mol/cm³) by the permeability coefficient (cm/s) to get flow of solute (mol/cm²s)

What is the direction of these concentration gradients in the cell cytosol: Na+, K+, Ca²+, H+, Cl-

• Na+: into the cell

• K+: out of the cell

• Ca²+: into the cell (less concentration in cell cytosol)

• H+: out of the cell (lower pH inside cell)

• Cl-: into the cell (pairs with Na+ most of the time)

What are the different ways solutes cross the membrane?

simple diffusion, facilitated diffusion, and active transport

Discuss which solutes and how solutes cross the membrane via simple diffusion.

• Simple diffusion: unassisted net movement of solute from higher to lower concentration (for small and large nonpolar molecules or very small polar molecules like water, glycerol, or ethanol)

  • net diffusion moves solute toward equilibrium

  • net diffusion tends toward minimum free energy

  • always an exergonic process

• net rate of transport is proportional to solute concentration gradient and permeability of the membrane

  • Vinward= PΔ[S]

    • Δ[S]= [S]outside - [S]inside

• when solute is charged, its concentration gradient and electrical gradient must be taken into account (more negative inside cell)

Discuss which solutes cross the membrane via facilitated diffusion.

small polar (H2O, glycerol), large polar (glucose), ions

Discuss which solutes cross the membrane via active transport.

large polar (glucose), ions

What is osmosis and what drives it?

Osmosis is the net flow of water molecules driven by solute concentrations. a type of simple diffusion

is two solutions are separated by semipermeable membrane and differ is solute concentrations, water moves by osmosis to make solute concentrations equal

What is osmolarity and tonicity?

• osmolarity: concentration of solute particles in osmoles/liter (osmotically active solute: solute CANT pass through membrane)

• tonicity: driver of net movement into or out of cell

  • hypertonic: impermeant solute concentration higher out of cell; water moves out of cell

  • hypotonic: impermeant solute concentration lower out of cell; water moves into cell

  • isotonic: impermeant solute concentration same out of cell; no net movement of water

What are the main classes of membrane transport proteins in facilitated diffusion?

• channels: continuous hydrophilic pores through membrane; transport through channel is always passive

• transporters: bind specific solute then undergoes conformational change to move solute across membrane; transporters can be passive or active

  • passive transport: moves molecules down gradient

    • when solute concentration is saturated, transport rate cannot increase with solute concentration

    • V= (Vmax[S]) / (Km+[S])

  • requires ATP to move against gradient

What is active transport? What are active transports?

transporters couple transport to energy source=active transport

• ATP hydrolysis: primary active transport

• existing electrochemical gradient: secondary active transport

What is Keq for uncharged solutes? What is inward solute transport?

• Keq will always be 1 for uncharged solutes. Keq= [S]inside / [S]outside

• inward solute transport: [S]outside → [S]inside

  • if the concentration outside is greater than inside ΔG(inward) is negative and exergonic

  • if concentration inside is greater than outside ΔG(inward) is positive and endergonic

    • if inward transportation endergonic, transporter needed

  • if concentration inside and outside same, ΔG is zero

What does the ΔG of a charged solute take into account?

the concentration gradient AND the electrochemical gradient of a solute (charge of ion and membrane potential)

What are three types of ATP-driven pumps and their process of transport?

• P-type pump: transport ions out of cytosol by phosphorylating using one ATP (ATP → ADP)

  • phosphorylated at an intermediate step in transport

• ABC transporter: transport small molecules out of cytosol by using two ATP (2 ATP → 2 ADP + 2 Pi)

  • the largest family and are structurally distinct from P-type pumps

• V-type proton pumps: transport H+ out of cytosol using one ATP (ATP → ADP + Pi)

  • large, turbine like machines that pump H+ ions into organelles such as lysosomes

Discuss two examples of P-type ATPase pumps and their general structure.

• P-type ATPase pumps Ca²+ into sarcoplasmic reticulum in muscle cells

• Na+-K+ pump (P-type ATPase in plasma membrane) main cause of Na+ and K+ electrochemical gradients (Na+ electrochemical gradient into cell, K+ electrochemical gradient out of cell)

  • pump is electrogenic, making contribution to membrane potential

• P-type pumps have 10 transmembrane a-helices and 3 cytosolic domains

Discuss ABC Transporters and their structure.

• ABC (ATP-binding cassette) transporters: can transport large range of molecules - ions to proteins

• ABC transporters have two 6-helix transmembrane domains and two cytosolic ATP binding domains

  • ATP binded → hydrolysis → release of ADP and Pi

Discuss secondary active transport.

Transporters use energy stored in electrochemical gradient to transport solute against gradient

• coupled transport: transported molecule drives transport of co-transported molecule against gradient into or out of cell at the same time

  • Na+ (in animal cell plasma membrane) or H+ usually driving ion

    • ex. Na+ coupled with glucose into cell (Na+ moving with electrochemical gradient, glucose moving against concentration gradient) uptake of glucose in gut and kidney (SGLT-1 and -2)

Discuss what regulates membranes’ permeability to water.

aquaporins, water channels, regulate membrane’s permeability to water

• tetramers with aqueous pores;

  • one side of pore lines with carbonyl groups that can hydrogen bond to water, other side of pore is hydrophobic and limits orientation to single line of water molecules

  • two lining asparagines limit H3O passage and make channel specific to water

• aquaporins are always passive; if water needed in cell ions are pumped and water follows through osmosis through aquaporins

What are three mechanisms that regulate channels?

• voltage gating

• ligand gating (also called chemical gating)

• mechanical gating

Describe one example of a mechanical gated channel (bacterial K+ channel)

Bacterial K+ channel is a tetramer of subunits with two transmembrane a-helices

its selectivity is lined with carbonyl oxygens, which coordinate K+ and non-energetically strips its hydration shell but not Na+ because its too small

Describe one example of a voltage gated channel (Na+ channel)

voltage gated Na+ channel in animals is single polypeptide of a domain repeated 4 times. each major domain has 6 membrane spanning helices (voltage sensing domains have large number of arginine) and an inactivation loop

• channel is closed at rest but opened by change in membrane potential (depolarization). (membrane at rest) → channel opens (membrane depolarization), inactivation loop closes (membrane refractory)

Describe one example of a ligand gated channel.

Nicotinic Acetylcholine receptors are ligand gated cation channels responsible for signal transmission at many synapses. made of 5 subunits. its pores have negatively charged amino acids at each end that only allow cations to pass

receptor is closed by default (by leucines) but opens to Na+, K+, or Ca²+ when acetylcholine binds