Membrane Transport Quiz

Membrane Transport

• Membranes are selective permeability barriers that block the passage of most hydrophilic molecules.
• Small, uncharged, or hydrophobic molecules can freely pass through the membrane via simple diffusion, following their concentration gradient.
• Charged polar molecules require specialized proteins (pumps, transporters, pores) to cross the membrane.
• Examples of molecules that cross membranes readily: oxygen, nitrogen, carbon dioxide, benzene, short-chain fatty acids, water, urea, glycerol, glucose, and sucrose.
• Examples of molecules that cross with difficulty: ions (e.g., H+, Na+, Mg2+, HCO3−, K+, Ca2+, Cl−), amino acids, ATP, and other large uncharged polar molecules.

Teaching Objectives

• Describe the differences between small molecule transport by passive diffusion, facilitated diffusion, and active transport.
• Describe the structure and function of the Na+/K+-ATPase membrane pump.
• Describe the structure and function of the Na+/glucose transporter protein family.
• Describe the function of the facilitated glucose transporter protein family (GLUT).
• Identify pathologies/treatments that involve membrane transport.

Mechanisms of Transport

• Simple passive transport/Diffusion
• Facilitated diffusion
• Gated ion channels
• Primary active transport
• Secondary active transport

Passive Transport

• Solutes move down a concentration gradient across the membrane.
• At equilibrium, the concentration of solute inside and outside the cell is equal.
• The rate of diffusion depends on the partition coefficient of the solute.
• Hydrophobic solutes have higher partition coefficients and equilibrate more quickly.
• Examples: water (H₂O)

Facilitated Diffusion/Carrier-Mediated Diffusion

• Solutes move down a concentration gradient across the membrane.
• At equilibrium, the concentration of solute inside and outside the cell is equal.
• Requires membrane protein (ion channel).
• Examples: Cl-/HCO₃- channel in erythrocytes, aquaporin (water channel), and GLUT glucose transporters.

Kinetics of Transport: Passive Transport and Facilitated Diffusion

• The rate of uptake (J) increases with external concentration ([S]o) until it reaches a maximum (Jmax) for facilitated diffusion.
• Simple diffusion increases linearly with external concentration ([S]o).
• The Michaelis constant (Km) reflects the transporter's affinity for the solute. A lower Km indicates higher affinity.

Transporter Affinity

• Transporter affinity reflects how well a transporter binds to a substrate to move it across a membrane.
• Lower Km values indicate high transporter affinity, as lower concentrations of a substrate are needed to reach half-maximum transport.

Facilitate Diffusion of Glucose (GLUT)

• GLUT1: ubiquitous, high glucose uptake in many tissues, low Km / high affinity.
• GLUT2: liver, pancreatic β-cells, high Km / low affinity, high capacity.
• GLUT3: neurons, low Km / high affinity.
• GLUT4: muscle, adipocytes, low Km, (high affinity). Similar to fed-state blood glucose levels, regulated by insulin.

Insulin-Stimulated Uptake of Glucose

• Insulin stimulates glucose uptake in muscle and adipose tissue.
• Insulin increases the amount of GLUT4 in the plasma membrane.
• GLUT4 is initially on membrane-bound vesicles in the cytoplasm.
• Insulin triggers the movement of GLUT4 vesicles to the plasma membrane.
• Vesicles fuse with the plasma membrane increasing GLUT4 level on cell surfaces.
• Increased glucose transporters increases the uptake of glucose into the cell.

Recruitment of GLUT4 to the Membrane

• Insulin binding to its receptor in the cell membrane triggers the movement of GLUT4 containing vesicles to the cell membrane.
• This is a process of vesicle fusion with the plasma membrane to increase the number of glucose transporter proteins on the cell surface.

Gated Ion Channels

• Ion channels allow facilitated diffusion through the membrane, selective for specific ions (e.g., K+, Na+, Ca²⁺).
• Open or close in response to stimuli (e.g., ligand-gated channels, voltage-gated channels).

Active Transport

• Solutes move against their concentration gradient.
• Requires energy (ATP hydrolysis).
• Primary active transport: ATP hydrolysis directly drives solute movement (e.g., Na+/K+ pump).
• Secondary active transport: uses an established gradient to drive solute movement (e.g., Na+-glucose cotransporter, Na+/Ca2+ cotransporter).

Na+/K+ Pump: Function

• Maintains low intracellular Na+ and high intracellular K+ concentrations.
• Essential for nerve impulse transmission and other cellular processes.
• Uses ATP to move 3 Na+ out and 2 K+ in.

Na+/K+ Pump

• Na+/K+ pump is a tetramer (α₂β₂).
• Phosphorylation of the pump by ATP at cytoplasmic site alters the conformational state of the protein.
• The conformational change closes cytoplasmic access and opens external access.
• The pump binds K+ when hydrolysis of the phosphate group closes external access and opens cytoplasmic access, releasing K+ into the cell.

Co-transport Systems

• Pre-established gradients are used to drive transport of solutes across a membrane.
• ATP hydrolysis establishes the primary gradient.
• Symport: transport of two solutes in the same direction.
• Antiport: transport of two solutes in opposite directions.

Na+-Glucose Cotransporter (SGLUT)

• SGLUT is a symport.
• Glucose absorption from the intestine relies on this secondary active mechanism that uses the Na+ gradient established by the Na+/K+ pump.

Role of Glucose Transporters in Gut

• SGLT transporters on the apical membrane pump glucose into the cells, alongside Na+.
• GLUT2 on the basal membrane transports glucose into the bloodstream.
• The Na+/K+ pump creates the inward concentration gradient of Na+ needed for glucose absorption.

Na+/Ca2+ Cotransporter

• An antiport that transports calcium (Ca²⁺) out of a cell, coupled with Na+ moving into the cell.

Clinical Considerations: Digitoxin

• Cardiac glycosides (e.g., digitoxin) inhibit the Na+/K+ pump.
• This leads to elevated intracellular Na+ and subsequent reduced Ca²⁺ export, increasing intracellular calcium concentrations and increasing heart muscle contraction.

Clinical Considerations: Ouabain

• Cardiac glycosides (e.g., ouabain) inhibit the Na+/K+ pump.
• This leads to elevated intracellular Na+ and subsequent reduced Ca²⁺ export, increasing intracellular calcium concentrations and increasing heart muscle contraction.

Clinical Considerations: CFTR

• Cystic fibrosis involves mutations in the CFTR gene, a chloride ion channel protein.
• Mutations in CFTR disrupt Cl− secretion and can cause thick mucus build-up.
• This can lead to respiratory and digestive system problems.

Clinical Considerations: Cholera Treatment

• Cholera toxin stimulates an increase in cAMP levels, activating CFTR.
• This leads to an abnormal secretion of chloride ions into the gut lumen.
• Water follows the chloride ions, causing severe diarrhea.
• Oral rehydration therapy can re-establish hydration by using glucose to drive Na+ uptake to re-establish electrolyte balance and water balance.

Summary

• Membrane transport mechanisms include selective permeability, simple and facilitated diffusion, active transport (primary and secondary), and co-transport systems.

Test your understanding of membrane transport mechanisms including diffusion and active transport. Explore the roles of various molecules in crossing membranes and the structures involved, such as transport proteins and pumps. This quiz will help reinforce key concepts related to selective permeability in biological membranes.