Cell Biology and Signalling: Membrane Transport

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Questions and Answers

What is the characteristic of GLUT2 in terms of glucose uptake?

Low Km and low Jmax

High Km and low Jmax

Low Km and high Jmax

High Km and high Jmax (correct)

In which type of cells is GLUT3 primarily found?

Muscle cells

Neurones (correct)

Adipocytes

Hepatocytes

What is the effect of insulin on GLUT4 in muscle and adipose cells?

Insulin has no effect on GLUT4

Insulin increases the amount of GLUT4 in the plasma membrane (correct)

Insulin stimulates the degradation of GLUT4

Insulin decreases the amount of GLUT4 in the plasma membrane

What is the primary function of ligand-gated ion channels?

To open or close in response to a stimulus Signup and view all the answers

What is the characteristic of GLUT4 in terms of its Km?

Low Km, similar to fed state blood glucose concentration Signup and view all the answers

What is the location of GLUT4 in the absence of insulin?

On the membrane-bound vesicles in the cytoplasm Signup and view all the answers

What is the primary characteristic of a membrane that allows it to be selective in regulating the passage of molecules?

Its selective permeability to certain molecules Signup and view all the answers

Which of the following molecules would be more likely to cross the membrane by simple diffusion?

O2 Signup and view all the answers

What is the name of the process by which solutes move down a concentration gradient across a membrane, but require a membrane protein (ion channel) to facilitate transport?

Facilitated diffusion Signup and view all the answers

What is the term for the maximum rate of uptake of a solute by a transporter?

Jmax Signup and view all the answers

Which of the following is an example of a facilitated diffusion transporter?

Aquaporin Signup and view all the answers

What is the name of the glucose transporter that has a high affinity for glucose, with a Km of approximately 1.8mM?

GLUT1 Signup and view all the answers

What is the primary function of the Na+/K+-ATPase membrane pump?

To maintain the ion balance across the membrane Signup and view all the answers

What is the term for the concentration of solute at which the rate of uptake is half the maximum rate?

Km Signup and view all the answers

Which of the following types of transport requires the input of energy to move solutes across the membrane?

Active transport Signup and view all the answers

What is the term for the ability of a transporter to bind to a solute?

Transporter affinity Signup and view all the answers

What is the primary function of the Na+/K+ pump in nerve transmission?

To establish a concentration gradient of Na+ and K+ across the plasma membrane Signup and view all the answers

Which type of transport is facilitated by the Na+/K+ pump?

Primary active transport Signup and view all the answers

What is the role of the sodium-calcium exchanger in muscle cells?

To export Ca2+ ions from the cell Signup and view all the answers

What is the effect of digitoxin on the Na+/K+ pump?

It inhibits the pump by blocking the dephosphorylation step Signup and view all the answers

What is the type of transport facilitated by the Na+/glucose cotransporter (SGLUT)?

Secondary active transport Signup and view all the answers

What is the function of the cystic fibrosis transmembrane conductance regulator protein (CFTR)?

To regulate the movement of chloride ions in the cell Signup and view all the answers

What is the effect of cholera toxin on the CFTR channel?

It activates the channel, increasing chloride ion secretion Signup and view all the answers

What is the role of the Na+ gradient in the uptake of glucose into cells via SGLUT?

It drives the uptake of glucose into cells Signup and view all the answers

What is the type of transport facilitated by the GLUT protein?

Facilitated diffusion Signup and view all the answers

What is the effect of ouabain on the Na+/K+ pump?

It inhibits the pump by blocking the binding of K+ Signup and view all the answers

Study Notes

Membrane Transport

Membranes are selective permeability barriers, blocking the passage of almost all hydrophilic molecules into cells and organelles.
Small uncharged or hydrophobic molecules can freely cross the membrane by simple diffusion along their concentration gradients.
Charged polar molecules require specialist proteins (pumps, transporters, pores) to allow them to cross the membrane.

Molecules Crossing Membranes

Lipid bilayer permeability is higher for molecules that are uncharged, non-polar, and small.
Examples of molecules that can cross the membrane:
- Hydrophobic molecules: O2, N2, CO2, benzene, short chain fatty acids.
- Small uncharged polar molecules: H2O, urea, glycerol.
- Large uncharged polar molecules: glucose, sucrose.
- Ions: H+, Na+, Mg2+, HCO3-, K+, Ca2+, Cl-.
- Charged polar molecules: amino acids, ATP.

Mechanisms of Transport

Simple passive transport / Diffusion: solutes move down a concentration gradient, crossing the membrane.
Facilitated diffusion: solutes move down a concentration gradient, crossing the membrane, requiring a membrane protein (ion channel).
Gated ion channels: allow facilitated diffusion, selective for different ions, and open or close in response to a stimulus.
Primary active transport: solutes move against a concentration gradient, requiring a membrane protein and energy from ATP hydrolysis.
Secondary active transport: uses a pre-established gradient to drive transport of solutes across the membrane against a gradient.

Passive Transport

Solutes move down a concentration gradient, crossing the membrane, at equilibrium [inside cell] = [outside cell].
Rate of diffusion depends on the Partition Coefficient of the solute.
Solutes that are more hydrophobic have a higher Partition Coefficient and equilibrate more quickly.

Facilitated Diffusion

Solutes move down a concentration gradient, crossing the membrane, at equilibrium [inside cell] = [outside cell].
Requires a membrane protein (ion channel).
Examples:
- Cl-/HCO3- channel in erythrocytes.
- Aquaporin: water channel.
- GLUT glucose transporters.

Transporter Affinity

Transporter affinity for solutes is given by the Km (Michaelis constant).
The lower the Km, the higher the affinity.

GLUT (Glucose Transporter) Family

Mediates constitutive glucose uptake in many tissues.
Examples:
- GLUT1: ubiquitous, low Km (high affinity), abundant in erythrocytes and low in skeletal muscle.
- GLUT2: liver, pancreatic ß-cells, high Km (low affinity) and large Jmax (high capacity).
- GLUT3: neurons, low Km (high affinity).
- GLUT4: muscle, adipocytes, Km similar to fed state blood glucose concentration, regulated by insulin.

Insulin and GLUT4

Insulin stimulates uptake of glucose in muscle and adipose tissue.
Insulin increases the amount of GLUT4 in the plasma membrane.
GLUT4 is present on membrane-bound vesicles in the cytoplasm.
Insulin triggers the movement of vesicles to the plasma membrane, increasing the level of GLUT4 on the cell surface.
Increased glucose transporters increase the uptake of glucose into the cell.

Gated Ion Channels

Ion channels that allow facilitated diffusion, selective for different ions.
Examples:
- Ligand-gated: e.g., acetylcholine and acetylcholine-gated Na+/K+ channel.
- Voltage-gated: e.g., Na+ and K+ channels in axons involved in nerve transduction.

Active Transport

Solutes move against a concentration gradient, requiring a membrane protein and energy from ATP hydrolysis.
Examples:
- Na+/K+ pump (Na+/K+ ATPase) in the plasma membrane.
- Part of co-transport systems to drive solute movement.

Na+/K+ Pump

Consists of a tetramer (α2β2).
Na+ enters the open cytoplasmic access, and phosphorylation from ATP at the cytoplasmic site causes a conformational change.
Conformational change closes the cytoplasmic access and opens the external access.
Conformational change means that the pump binds K+ and releases Na+ outside the cell.
Hydrolysis of the phosphate group closes the external access, opens the cytoplasmic access, and releases K+ into the cell.

Co-Transport Systems

Pre-established gradient is used to drive transport of solutes across the membrane against a gradient.
ATP hydrolysis is used to establish the primary gradient.
Examples:
- Symport: transport of two solutes in the same direction.
- Antiport: transport of two solutes in opposite directions.

Na+/Glucose Cotransporter (SGLUT)

Glucose absorption from the intestine against a gradient.
SGLUT is a symport.
Na+ gradient established by the Na+/K+ pump and ATP hydrolysis is used to drive the uptake of glucose into cells.
Secondary active transport.

Clinical Considerations

Digitoxin: cardiac glycosides that inhibit the Na+/K+ pump, increasing [Na+] inside heart muscle and contraction.
Ouabain: cardiac glycosides that inhibit the Na+/K+ pump, increasing [Na+] inside heart muscle and contraction.
CFTR (Cystic Fibrosis Transmembrane Conductance Regulator): a chloride ion channel in cells responsible for producing mucus, sweat, saliva, and tears.
Cholera toxin: stimulates an increase in cAMP level, activating CFTR and secretion of chloride ions, followed by Na+ and water into the lumen of the gut.
Oral rehydration therapy: includes high glucose concentration to drive Na+ uptake into cells via SGLUT, treating cholera.

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Description

This quiz covers the fundamentals of membrane transport, including passive diffusion, facilitated transport, and active transport. It also explores the structure and principle of action of the Na+/K+-ATPase membrane pump.