BIOL2021: Chapter 11

What does diffusion across the membrane depend on?

Size and hydrophobicity

How well do hydrophobic molecules diffuse through the cell membrane?

Pass through very easily

How easily do small, uncharged polar molecules diffuse through the membrane?

Require facilitation to permeate

How easily do large, uncharged polar molecules diffuse through the membrane?

Cannot diffuse and require transport

How easily do ions diffuse through the membrane?

Cannot diffuse, require transporters

What do membrane transport proteins create to allow solutes through?

A hydrophilic pathway

What are the 2 main classes of membrane transport proteins and what differentiates them?

• Transporters/carriers/permeases; move solute via conformational change and have specific binding sites

• Channels; forms a pore, does not change conformation to move solute but can open/close (minimal solute interaction)

What type of transporters use diffusion passive transport?

All channels and some transporters

What do concentration gradients provide for diffusion?

Energy

How high are typical Na+ concentrations in the cytoplasm?

Low

How high are typical K+ concentrations in the cytoplasm?

High

How high are Cl^- concentrations in the cytoplasm

Low

What is membrane potential/potential difference?

The difference between the inside and outside charges (always a negative value)

What does charged solute (ion) movement depend on?

• Concentration gradient

• Electrical/membrane potential

What is the electrochemical gradient?

The combination of ion concentration gradient and electrical/membrane potential

What do cells use electrochemical gradients for?

• Transport

• Signaling (action potentials)\

• ATP production

• other (ex. stomata open/close)

What is the typical animal cell electrical potential?

-20mV to -120mV

What type of transport do transporters use?

Passive or active

What is a uniporter?

A transporter that transports 1 solute

How do transporters and channels kinetics compare?

• Transporters have similar rates to an enzyme-substrate reaction

• Channels rates are directly proportional to solute concentration

What is Vmax?

The point at which all solute bonding sites are occupied; Max rate of a single transporter

What is Km?

The binding constant; Affinity for the solute

What are GLUT transporters?

• A family of passive transporters for glucose

• GLUT 1: RBC’s and other tissues

• GLUT 2: Liver, gut epithelium

Which direction does solute move in active transport?

Against the concentration gradient

Where does the energy for active transport come from?

• Ion gradient

• ATP

• Light

• Redox reactions

What do coupled transporters do?

Couple uphill transport of 1 solute to downhill transport of another solute (ion)

What is a symporter?

Transporter that transports molecules in the same direction

What is an antiporter

Transporter that transports molecules in the opposite direction

How do Na⁺-glucose cotransporters work?

• Symporter

• Moves glucose against concentration gradient

• uses Na⁺ gradient as energy

How do Gut epithelial cells transport glucose?

• transport glucose from lumen to extracellular space

• Gut → cell; Na⁺ glucose transporter

• Cell → extracellular space; GLUT2 transporter

What regulate the pH of cytosol?

• Coupled-transporters

• Sodium-proton exchanger moves Na⁺ in and H⁺ out

• Na gradient puts NaHCO₃ in and HCL out

What are ATP-driven transporters?

Pumps that couple transport of solute against the concentration gradient of ATP hydrolysis

What are the 3 classes of ATP driven pumps?

• P-type

• ABC

• V-type

What are the key features of P-type ATP pumps?

• Phosphorylated by ATP during pumping cycle

• Phosphorylation and dephosphorylation drives conformational change

• Moves ions against concentration gradient

• Primary transport

What are the main roles of P-type ATP pumps?

• maintains low Ca²⁺ levels in cytoplasm

• Na⁺/K⁺ gradients in animal cells

• H⁺ gradient in plants and fungi

• Maintain trace metal ions

• Drug targets

What is SR Ca⁺⁺ P-type ATPase?

• Transporter in muscle cells

• Triggers muscle contraction

• Releases Ca⁺⁺ from sarcoplasmic reticulum (modified ER)

• Na⁺ channels respond to action potential propagates

• composed of 10 transmembrane alpha-helices (binds to ions) and 3 cytosolic domains

What does the Na⁺- K⁺ pump do?

• Moves 3 Na⁺ and 2 K⁺ against the concentration gradient

• Forms electrochemical gradient in animal cells

What does the H⁺-ATPase (proton pump) do?

• Creates gradients across cell membranes

• In plant, fungi, protozoal, archaeal, and bacterial

What does the Gastric H⁺/K⁺ ATPase in the stomach lining do?

• Acidifies the stomach during digestion

What are pantoprazole, omeprazole, esomeprazole, and lansoprazole examples of?

Proton pump inhibitors

What is the largest family of membrane transporters?

ABC (ATP-Binding Cassette) transporters

What do ABC transporters do?

Pump small molecules across cell membranes

What are the 2 highly conserved domains in ATPase called and what side of the cell are they on?

• ATP binding cassettes

• On the cytoplasmic side

How does the ABC transporter mechanism work?

• Solute binds to the transporter

• 2 ATP bind, causing a conformational change

• solute is released to the other side

• ATP is hydrolyzed and ADP + Pi is removed

• Transporter assumes original conformational change

What medical relevance do ABC transporters have?

• Pump drugs out of cells

• ex. Cancer cells pump out cancer drugs

• ex. Antimalarial drug resistance

• ex. Bacterial antibiotic resistance

What are F-type ATPases, what pump type are they related to, and what do they use for energy?

• ATP synthases

• Structurally related to V-type proton pumps

• Uses H⁺ gradient for ATP synthesis