Cell Biology and Signalling Block: Membrane Transport

Questions and Answers

Which of the following transporters has a low affinity for glucose?

GLUT1

GLUT2 (correct)

GLUT4

GLUT3

In which tissues is GLUT1 abundant and GLUT4 scarce?

Neurones and adipocytes

Liver and pancreatic ß-cells

Erythrocytes and skeletal muscle (correct)

Muscle and erythrocytes

What is the primary function of GLUT2 in pancreatic ß-cells?

Regulate blood glucose levels

Transport glucose into pancreatic ß-cells when blood glucose is high (correct)

Transport glucose out of pancreatic ß-cells

Increase glucose uptake in response to insulin

What is the effect of insulin on GLUT4?

Increase the amount of GLUT4 in the plasma membrane Signup and view all the answers

What is the primary mechanism of ion channel gating?

Change in membrane potential Signup and view all the answers

What is the effect of insulin on glucose uptake in muscle and adipose tissue?

Increase glucose uptake Signup and view all the answers

What is the primary distinguishing feature of small-molecule transfer across membranes by active transport?

Requires energy input to pump molecules against their concentration gradient Signup and view all the answers

Which of the following molecules can freely cross the membrane by simple diffusion?

O2 Signup and view all the answers

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

To maintain the electrochemical gradient across the membrane Signup and view all the answers

Which type of transport requires a membrane protein, such as an ion channel?

Facilitated diffusion Signup and view all the answers

What is the relationship between the Partition Coefficient of a solute and its rate of diffusion?

Solute with a higher Partition Coefficient equilibrate more quickly Signup and view all the answers

What is the function of the GLUT family of glucose transporters?

To transport glucose into the cell Signup and view all the answers

What is the significance of the Km in transporter affinity?

It represents the external concentration at which the transport rate is half-maximal Signup and view all the answers

What is the primary difference between facilitated diffusion and active transport?

Facilitated diffusion moves solutes down their concentration gradient, while active transport moves solutes against their concentration gradient Signup and view all the answers

Which of the following is an example of secondary active transport?

Na+/glucose transporter Signup and view all the answers

What is the term for the selective permeability of membranes to certain molecules?

Selective permeability Signup and view all the answers

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

To maintain the ion gradient of sodium and potassium across the plasma membrane Signup and view all the answers

What type of transport occurs when two solutes move in the same direction across a membrane?

Symport Signup and view all the answers

What is the result of inhibiting the Na+/K+ pump in heart muscle cells?

Increased contraction of heart muscle cells Signup and view all the answers

What is the function of the CFTR protein in cells responsible for producing mucus, sweat, saliva, and tears?

To facilitate the movement of chloride ions out of the cell Signup and view all the answers

What is the purpose of oral rehydration therapy in treating cholera?

To drive the uptake of glucose into cells via SGLUT Signup and view all the answers

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

Primary active transport Signup and view all the answers

What is the result of the Na+/Ca2+ cotransporter in muscle cells?

Export of calcium ions from muscle cells Signup and view all the answers

What is the role of the phosphorylation step in the Na+/K+ pump?

To cause a conformational change in the pump Signup and view all the answers

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

It inhibits the dephosphorylation step of the pump Signup and view all the answers

What is the mechanism by which SGLUT transports glucose into cells?

Symport with sodium ions down a concentration gradient 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

Hydrophobic molecules that can freely cross the membrane include O2, N2, CO2, benzene, and short-chain fatty acids.
Small uncharged polar molecules that can cross the membrane include H2O, urea, and glycerol.
Large uncharged polar molecules that cannot cross the membrane include glucose, sucrose, and ions like H+, Na+, Mg2+, HCO3-, K+, Ca2+, and Cl-.
Charged polar molecules that cannot cross the membrane include amino acids and ATP.

Mechanisms of Transport

Passive transport: solutes move down a concentration gradient, crossing the membrane.
Facilitated diffusion: solutes move down a concentration gradient, crossing the membrane with the help of a transport protein.
Gated ion channels: ion channels that allow facilitated diffusion, selective for different ions, open or close in response to a stimulus.
Primary active transport: solutes move against a concentration gradient, requiring energy from ATP hydrolysis.
Secondary active transport: solutes move against a concentration gradient, using energy from the electrochemical gradient established by primary active transport.

Passive Transport

Solutes move down a concentration gradient, crossing the membrane.
At equilibrium, the concentration inside the cell equals the concentration outside the cell.
The 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 with the help of a transport protein.
Examples include Cl-/HCO3- channel in erythrocytes, aquaporin (water channel), and GLUT glucose transporters.
The kinetics of transport for passive transport and facilitated diffusion show that Jmax (rate of uptake) is dependent on the external concentration.

Transporter Affinity

Transporter affinity for solutes is given by the Km (Michaelis constant).
A lower Km indicates a higher affinity for the solute.

Glucose Transporters (GLUT)

A family of related glucose transporters with different locations and functions.
GLUT1: ubiquitous, low Km (high affinity), mediates constitutive glucose uptake in many tissues.
GLUT2: liver, pancreatic β-cells, high Km (low affinity), large Jmax (high capacity), transports glucose into hepatocytes and β-cells.
GLUT3: neurons, low Km (high affinity).
GLUT4: muscle, adipocytes, Km similar to fed-state blood glucose concentration, regulated by insulin.

Insulin-Stimulated Glucose Uptake

Insulin stimulates glucose uptake in muscle and adipose tissue by increasing the amount of GLUT4 in the plasma membrane.
Insulin triggers the movement of vesicles containing GLUT4 to the plasma membrane, increasing the uptake of glucose into the cell.

Gated Ion Channels

Ion channels that allow facilitated diffusion, selective for different ions, open or close in response to a stimulus.
Examples include ligand-gated channels (e.g., acetylcholine receptor) and voltage-gated channels (e.g., Na+ and K+ channels in axons involved in nerve transmission).

Active Transport

Solutes move against a concentration gradient, requiring energy from ATP hydrolysis.
Primary active transport: ATP hydrolysis directly causes the movement of solutes (uniport).
Example: Na+/K+ pump (Na+/K+ ATPase) in plasma membrane.

Na+/K+ Pump (Na+/K+ ATPase)

A tetramer (α2β2) that pumps Na+ out of the cell and K+ into the cell, establishing an electrochemical gradient.
The pump consists of an open cytoplasmic access, phosphorylation from ATP at the cytoplasmic site, conformational change, and hydrolysis of the phosphate group.

Co-Transport Systems

A 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.
Symport: transport of two solutes in the same direction.
Antiport: transport of two solutes in opposite directions.

Na+/Glucose Cotransporter (SGLUT)

A symport that uses the Na+ gradient established by the Na+/K+ pump to drive the uptake of glucose into cells.
Secondary active transport.

Clinical Considerations

Digitoxin: inhibits Na+/K+ pump, leading to increased [Na+] and [Ca2+] inside heart muscle, and increased contraction.
Ouabain: inhibits Na+/K+ pump, leading to increased [Na+] and [Ca2+] inside heart muscle, and increased contraction.
Cystic fibrosis: caused by mutations in the cystic fibrosis transmembrane conductance regulator protein (CFTR), leading to reduced chloride transport and thick mucus production.
Cholera treatment: oral rehydration therapy includes high glucose concentration to drive Na+ uptake into cells via SGLUT, reducing electrolyte and fluid secretion.

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Description

Learn about the mechanisms of small-molecule transport across membranes, including passive diffusion, facilitated transport, and active transport. Understand the structure and function of the Na+/K+-ATPase membrane pump.