BCOR 011 Lecture 10 Membrane Transport Sept 21, 2005 Membrane 1. Permeability Transport 2. Diffusion 3. Role of transport proteins - facilitated Channel proteins Carrier proteins

4. Active vs passive transport

1. Lipid bilayers are selectively permeable The Permeability of the Lipid •small,nonpolar Bilayer •small uncharged, polar • Hydrophobic molecules – Are lipid soluble and can pass through •larger Decreasing the membrane rapidly uncharged, polar permeability molecules • Polar molecules – Do not cross membrane rapidly •ions • Ions – Do not cross the membrane at all Size – polarity - ions Transport processes Simple Solutes – dissolved ions and small organic molecules Diffusion: i.e., Na+,K+, H+, Ca++, Cl,- sugars, amino acids, nucleotides •Tendancy of a material to spread out •Always moves toward equilibrium Three transport processes: a. Simple diffusion – directly thru membrane Req b. Facilitated diffusion (passive transport) Carrierc. Active transport – requires energy prot Net diffusion Net diffusion Equilibrium Figure 7.11 B Net diffusion Equilibrium Equilibrium Net diffusion Net diffusion Net diffusion

H2O transport: diffusion from area with low simple diffusion example: [solute] to one with high [solute] Lower Higher Oxygen crossing red cell membrane concentration concentration Same concentration of solute (sugar) of sugar of sugar

HIGH -> low - HCO3 O O 2 O22 CO2 Osmosis Lungs CO 2 2 Diffusion of water Selectively Water molecules permeable mem- cluster around brane: sugar mole- sugar molecules cules cannot pass O2 through pores, but Tissues CO HCO - water molecules can Impermeable 2 3 - O HCO3 More free water 2 Fewer free water molecules (higher CO molecules (lower concentration) 2 concentration) Solutes

Osmosis Driving force: concentration gradient • Water moves from an area of higher Figure 7.12 free water concentration to an area Trying to even out concentration of lower free water concentration Animal cells – pump out ions Plants, bacteria – cell walls …but most things are too large or too Hypotonic solution Isotonic solution Hypertonic solution (a) Animal cell. An polar to cross at reasonable rates using animal cell fares best simple diffusion in an isotonic environ- H O H O H2O 2 H2O ment unless it has 2 special adaptations to offset the osmotic uptake or loss of water.

Figure 7.13 Lysed Normal Shriveled Facilitated diffusion:

(b) Plant cell. Plant cells protein–mediated movement down a are turgid (firm) and H O H O generally healthiest in H O H2O 2 2 a hypotonic environ- 2 gradient ment, where the uptake of water is eventually balanced by the elastic wall pushing back on the cell. Transmembrane transport proteins Turgid (normal) Flaccid Plasmolyzed

Transmembrane transport proteins Transmembrane transport proteins allow selective transport of hydrophilic molecules & ions allow selective transport of hydrophilic molecules & ions

1. carrier protein aqueous channel 2. channel protein Bind solute, hydrophilic pore conformational change, very rapid EXTRACELLULAR release selective –size/chargeFLUID Selective binding “trap door”

“turnstile”

Solute Channel protein Carrier protein Solute CYTOPLASM (b) A carrier protein alternates between two conformations, moving a (a) A channel protein (purple) has a channel through which solute across the membrane as the shape of the protein changes. water molecules or a specific solute can pass. The protein can transport the solute in either direction, with the net Figure 7.15 movement being down the concentration gradient of the solute. Figure 7.15 Kinetics of simple vs facilitated Gets For CHARGED solutes (ions): net driving force Diffusion “saturated” is the electrochemical gradient Maximum •has both a concentration + charge component; rate •Ion gradients can create an electrical voltage gradient across the membrane (membrane potential) + + + + + + + + + + + + + + + + +++ ++++ ------Does Not Get ------+++ +++ v “saturated” -60 mVolts +++

(solute concentration gradient) ->

Channel Proteins: Ligand-gated ion channel facilitate passive transport Ion channels: move ions down an “Wastebasket model” – step on pedal & lid opens electrochemical gradient; gated

“keys”

Voltage Ligand Mechanosensitive Ligand-gated example: ligand-gated ion channel Voltage-gated channels

“Key” - acetylcholine

+ + - + + + + + + -

- - - - + - - - - - +

Note: channels are passive, facilitated transport systems

Example of voltage-gated ion channel

Protein -requireion channels: a membrane protein -typically movefacilitated ions very transport rapidly fromsystems an area -are passive, of HIGH concentration to one of lower concentration Example: Glucose transporter GluT1 : Carrier proteins: carrier-mediated facilitated diffusion Transport solute across membrane Glucoseout (HIGH)->glucose in (low) outside cell by binding it on one side, 2. Conformational change 3. Glucose undergoing a conformational change T 1. Glucose binds 2 Released- and then releasing it to the other side Conformational shift 2. 1.

T1 inside cell T1 3.

Glucose + ATP Æ glucose-6-phosphate + ADP hexokinase

Carrier proteins: three types Carrier Proteins can mediate either:

1. Passive transport driving force -> concentration/electrochemical gradient OR (a) Uniport (b) Co-transport Uniport – one solute transported 2. Active transport against a gradient; unfavorable [Symport – two solutes in the same direction requires energy input Antiport – two solutes in opposite directions Note: channel proteins mediate only passive transport Active • Active transport • Active transport transport: – Carrier protein moves solute Na+K+ Pump AGAINST its concentration gradient (Na+K+ATPase) – Requires energy, usually in the form of ATP hydorlysis P

– Or a favorable gradient established P P by use of ATP 3 Na+ out P 2 K+ in

ATP!

1 2 Cytoplasmic Na+ binds to [Na+] high Na+ binding stimulates the sodium-potassium pump. [K+] low phosphorylation by ATP. Na+ Na+ + + Na+ + The Na /K Pump: Na Na+ Na+ + ATP + [Na ] low P + Na [K+] high CYTOPLASM ADP Na + The sodium “bilge pump” Na + EXTRACELLULAR -potassium FLUID Na Creates an electrochemical + Na+ Na pump Na+ + Na+ gradient (high external [Na ])

3 K+ 4 K+ is released and Na+ Phosphorylation causes the sites are receptive again; protein to change its conformation, expelling Na+ to + the cycle repeats. the outside. + K+ P potential energy Na Na – like “storing water behind a dam” Na+ Na+

K+

P P i K+

+ uses ~1/3 of cell’s ATP!! Figure 7.16 K K+ 5 6 Loss of the phosphate Extracellular K+ binds to the restores the protein’s protein, triggering release of the original conformation. Phosphate group. + H + H + H + + H H EXTRACELLULAR FLUID + + + + + + transport Indirect active *Transport driven by cotransport of ions Proton pump

ion gradient was – – – – –

active transport + H ATP CYTOPLASM Direct active transport Transport coupled to Exergonic rxn, i.e. ATP hydrolysis across a membrane Figure 7.18 – Is a transport protein that generates the voltage

*note that the favorable established by direct • electrogenic An pump + H + H + + + H H Diffusion of H + H Sucrose + H + + + + + + + Sucrose-H cotransporter Proton pump – – – – – – + H ATP Coupled transport gradient drives other transport glucose symport + + Figure 7.19 concentration gradient Glucose Gradient • Cotransport: active transport driven by a Na Na Example of indirect active transport: Summary: ….Each membrane has its own Passive transport characteristic set of transporters Simple diffusion Facilitated diffusion Active transport

No protein channel carrier protein protein carrier protein HIGH to low conc HIGH to low conc low to HIGH conc favorable energy

favorable

ATP Unfavorable Add Figure 7.17