Becker's World of the Cell - Chapter 8 - Transport Across Membranes

Simple Diffusion

Direct, unaided movement of solute molecules through the lipid bilayer along the concentration gradient

Facilitated Diffusion

Aided movement of solute molecules down their free energy gradient in the direction of thermodynamic equilibrium by a transport protein

Direct Active Transport

Accumulation of solute is directly coupled to an exergonic reaction that is most likely the hydrolysis of ATP

Indirect Active Transport

Accumulation of solute is coupled directly to an endergonic chemical reaction

Concentration Gradient

Magnitude of difference in concentration of a substance on opposite sides of a molecule

Membrane Potential

The voltage difference across a plasma membrane

Erythrocyte Plasma Membrane Example

- O2, CO2, & Bicarbonate ions & glucose must be transported across this membrane

- Membrane potential must be regulated by pumping K+ in and Na+ out

- Aquaporins allow H20 to rapidly enter and exit the cell in response to its needs

Hypertonic Solution

Higher solute concentration outside of the cell

Hypotonic Solution

Lower solute concentration outside of the cell

Isotonic Solution

Same solute concentration both inside and outside of the cell

Turgor Pressure

The force directed against a plant cell wall after the influx of water and swelling of the cell due to osmosis

Phosphate Buffered Saline

Buffer solution commonly administered to patients in the hospital

Liposome

A minute spherical sac of phospholipid molecules enclosing a water droplet, especially as formed artificially to carry drugs or other substances into the tissues

Factors Affecting Diffusion Of Solutes

Solute Size - Smallest molecules diffuse easily

Solute Polarity - Nonpolar molecules diffuse easily

Solute Charge - Ions cannot cross easily because water forms a shell of hydration around them

Expression For Velocity Of Solute Inward Diffusion Through Membrane

V Inward = Permeability Coefficient x Solute Concentration

Factors Affecting Permeability Coefficient

Thickness and viscosity of membrane

Size, shape, and polarity of solutes

Equilibrium distribution of solutes

Carrier Protein

A protein that forms hydrophilic channels through the membrane and allows passage of solutes without major conformational change

Channel Protein

A protein that binds to one or more solute molecules on one side of the membrane and undergoes conformational change that transfers the solute to the other side of the membrane

Alternating Conformational Model

States that a carrier protein is an allosteric protein that alternates between two conformational states, such that the solute binding site is open or accessible to one side of the membrane first and then to the other

Permeases

Another name for carrier proteins

Kinetics Of Carrier Protein Function

Exhibits saturation kinetics: Has an upper limiting Vmax and a constant Km corresponding to concentration of transportable solute needed to achieve 1/2 of the max rate of transport

Uniport

Transfer of only one solute molecule

Coupled Transport

Transfer of two solute molecules

Porins

Proteins that allow the passage of certain ions and small polar molecules through membranes

Aquaporins

Proteins that facilitate the transport of water across at a much higher rate than simple diffusion. All are integral membrane proteins with six helical transmembrane segments

Types Of Gated Channels

Ligand Gated

Voltage Gated

Mechanically Gated

P-Type ATPases

Large family of transporters for ions and phospholipids that are reversibly phosphorylated

V-Type ATPases

Pump protons into organelles such as vesicles, lysosomes, endosomes and Golgi

F-Type ATPases

Multisubunit complexes that can use the energy from ATP hydrolysis to pump protons against the electrochemical gradient

Also facilitate the exergonic flow of protons down their concentration gradient to synthesize ATP

ABC-Type ATPases

Multisubunit complexes that have 4 protein domains, 2 of which are highly hydrophobic and embedded in the membrane, and 2 of which are peripheral and associate with the cytoplasmic side of the membrane

Proton Gradient

The difference in proton concentration on either side of a membrane.

Proton Cotransport

The simultaneous transport of two protons across a membrane with the assistance of a protein or a protein complex

Sodium Potassium Pump

Membrane carrier protein that couples ATP hydrolysis to the inward transport of potassium ions and the outward transport of sodium ions to maintain the plasma membrane Na+ and K+ potential

Plasmolysis

A phenomenon in walled cells in which the cytoplasm shrivels and the plasma membrane pulls away from the cell wall due to the presence of a hypertonic solution

Symport

Transport of two solute molecules in the same direction

Antiport

Transport of two solute molecules in the opposite direction

Sodium Potassium Pump Mechanism

1. Intracellular sodium binds to E1 allosteric site

2. Alpha subunit is autophosphorylated using ATP

3. Conformational change of E1 to E2 exports sodium outside the cell

4. Extracellular sodium binds to E2 allosteric site

5. Alpha subunit is dephosphorlyated

6. Conformational change of E2 to E1 expels sodium back inside the cell

Sodium Glucose Symporter Mechanism

1. Extracellular sodium binds to symporter

2. Glucose binds and elicits a conformational change

3. Symporter opens to inside

4. Sodium is released inside but is continually extruded back out by the sodium potassium pump

5. Glucose is released inside the cell

6. Empty symporter returns to initial state

Bacteriorhodopsin Proton Pump

Proton pump found in halophilic archaea that uses energy derived from photons of light to drive active transport

General Reaction For Transport Of Molecules

∆G Inward = ∆G° + RTln([S]Inside/[S]Outside)

Equilibrium Constant For Transport Of Uncharged Solute At Equilibrium

Keq = [S]Inside/[S]Outside = 1

∆G° therefore is always 0

Simplified General Reaction For Transport Of Molecules

∆G Inward = RTln ([S]Inward/[S]Outward)

Dependence Of Free Energy Of Transport On Electrochemical Potential

Electrochemical potential is determined by concentration gradient and membrane potential

Because membrane potential is negative, favors inward movement of cations

Free Energy For Transport Of Ions

∆G Inward = RTln ([S]Inside/[S]Outside) + zFVm