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Principles of Transmembrane Transport

• Transport of small molecules across cell membranes involves various mechanisms, including:

  • Uniporters: Transport a single molecule down its concentration gradient.

  • Symporters: Co-transport an ion and a molecule in the same direction.

  • Antiporters: Transport an ion and a molecule in opposite directions.

• These transport mechanisms can be either passive or active, depending on whether they require energy input.

Lipid Bilayers and Membrane Transport

• The lipid bilayer is hydrophobic, restricting the passage of polar molecules and ions.

• Cells use membrane transport proteins to facilitate the movement of water-soluble molecules and ions across the membrane.

• Membrane transport proteins constitute approximately 15-30% of a cell's membrane proteins.

Transmembrane Transport

• A membrane with transport proteins can transfer particular substances across the membrane.

• The discussion primarily focuses on plasma membranes, but similar mechanisms exist in other eukaryotic membranes.

Principles of Transmembrane Transport

• Lipid Bilayers: Impermeable to ions and most uncharged polar molecules.

• Ion Concentrations: Significantly different inside the cell compared to outside.

• Membrane Potential: Differences in inorganic ion concentrations across the membrane create an electrical potential.

• Membrane Transport Proteins:

  • Transporters: Active or passive transport.

  • Channels: Passive transport.

• Solutes Cross Membranes by either

  • Passive Transport

  • Active Transport

Lipid Bilayer Permeability

• The rate at which a solute crosses a protein-free lipid bilayer depends on:

  • Size

  • Solubility

• Smaller and less water-associated molecules diffuse more rapidly.

• Lipid bilayers are essentially impermeable to ions, regardless of size.

Ion Concentrations Inside and Outside the Cell

• $Na^+$ is the most plentiful positively charged ion outside the cell.

• $K^+$ is the most abundant positively charged ion inside the cell.

• Positive and negative charges must balance both inside the cell and in the surrounding fluid to avoid electrical imbalances.

Ion Balance in Animal Cells

• Outside the cell, $Cl^-$ primarily balances the high $Na^+$.

• Inside the cell, negatively charged inorganic ions and metabolites balance the high $K^+$.

Membrane Potential

• In animal cells, the resting membrane potential typically ranges from -20 to -200 mV.

• "Resting" implies a steady-state potential, but not necessarily zero.

• The membrane potential is generated by electrical imbalances across the membrane, making the inside of the cell more negative than the outside.

Significance of Membrane Potential

• Allows cells to power transport of specific metabolites.

• Enables excitable cells to communicate with neighboring cells.

• Membrane transport proteins establish and maintain the characteristic membrane potential.

Membrane Transport Proteins

• Present in all membranes.

• Most allow only one type of molecule through.

• Polypeptide chains traverse the bilayer multiple times.

Transporter Proteins vs. Channel Proteins

• Transporter Proteins:

  • Alternate between two conformations to transfer solutes.

  • Solute-binding site is sequentially accessible on either side of the bilayer.

  • Exhibit high specificity, similar to enzyme-substrate binding.

• Channel Proteins:

  • Form a narrow pore for solutes to passively diffuse.

  • Discriminate primarily based on size and electric charge.

  • Primarily allow water and small inorganic ions to cross the membrane.

Transport Speed

• Transport through channels is much faster than transport mediated by transporters.

Passive vs. Active Transport

• Passive Transport:

  • Allows solutes to move down their concentration gradients.

  • Occurs spontaneously.

• Active Transport:

  • Moves solutes against their concentration gradients.

  • Requires energy input.

  • Only transporters can carry out active transport (pumps).

Passive Transport Details

• Channels and many transporters facilitate passive transport ("downhill").

• For a single, uncharged molecule, the concentration gradient drives passive transport from high to low concentration.

• For charged solutes, the electrochemical gradient (combination of concentration and electrical gradients) determines the net driving force.

Electrochemical Gradient

• Electrical potential favors the entry of positively charged ions and opposes the entry of negatively charged ions.

• Almost all plasma membranes have an electrical potential with the inside negative relative to the outside.

Active Transport and Energy Coupling

• Transporters can couple to an energy source to catalyze active transport.

• Electrochemical gradients store potential energy, which drive transport processes, convey electrical signals, and produce ATP.

Transmembrane Movement Summary

• Mediated by transporters (active or passive) or channels (always passive).

• Simple diffusion, channel-mediated, and transporter-mediated pathways exist for passive transport.

• Active transport requires energy and utilizes pumps.

Active vs Passive Transport

• Active transport requires energy, passive doesn't.

Transporters: Active Transport

• ATP-driven $Na^+$ pump.

• ATP-driven $Ca^{2+}$ pumps.

• $H^+$ pumps.

• Symports/Antiports.

Transporters: Passive Transport

• Uniports.

Transporter Specificity

• Each cell membrane has its own characteristic set of transporters.

• Transporters enable each membrane to carry out its unique functions.

Transporters and Their Roles

• Responsible for the movement of most small, water-soluble, organic molecules and a few inorganic ions across membranes.

• Examples:

  • Plasma membrane: Nucleotides, sugars, amino acids ($Na^+$.

  • Lysosome membrane: $H^+$ to acidify the interior.

  • Mitochondrial inner membrane: Pyruvate import for ATP generation and ATP export.

Active Transport Mechanisms

• Cells actively pump solutes across membranes against electrochemical gradients.

• Pumping activity is directional and coupled to an energy source, such as:

  • Ion gradient

  • ATP hydrolysis

  • Light

Transmembrane Pumps

• Gradient-driven pumps: Link uphill transport of one solute to downhill transport of another.

• ATP-driven pumps: Use energy released by ATP hydrolysis.

• Light-driven pumps: Use energy from sunlight (e.g., bacteriorhodopsin).

• Pumps often act together; for example, the ATP-driven $Na^+$ pump transports $Na^+$ out, and then $Na^+$ flows back in through other gradient-driven pumps.

ATP-driven $Na^+$ Pump

• Key pump in animal cells.

• Accounts for approximately 30% of ATP consumption.

• Pumps $Na^+$ out and carries $K^+$ in.

• Also known as $Na^+-K^+$ ATPase or $Na^+-K^+$ pump.

• Cycle time is approximately 10 milliseconds.

• Energy from ATP hydrolysis drives conformational changes in the transporter protein.

• A phosphate group from ATP is transferred to the pump itself.

Function of the $Na^+$ pump

• Transport of three $Na^+$ (out) and two $K^+$ (in), contributing to electrochemical gradients.

Inhibition of the $Na^+$ pump

• Ouabain: Inhibits the pump by preventing $K^+$ binding.

• Digitalis: Inhibits by blocking dephosphorylation.

Clinical Insight

• Digitalis, from the foxglove plant, strengthens heart contractions and is used to treat heart disease.

• It inhibits the $Na^+$ pump, leading to a higher intracellular $Na^+$ concentration.

• This reduces the $Na^+$ gradient, diminishing the activity of the $Na^+/Ca^{2+}$ antiporter.

• Consequently, $Ca^{2+}$ ions remain in the cell longer, enhancing heart muscle contraction.

ATP-Driven $Na^+$ Pump Importance

• Keeps $Na^+$ concentration in cytosol 10-30 times lower than in extracellular fluid.

• Keeps $K^+$ concentration 10-30 times higher inside the cell.

• $Na^+$ concentration gradient and membrane potential create a large $Na^+$ electrochemical gradient.

ATP-driven $Ca^{2+}$ Pumps

• Maintain low cytoplasmic $Ca^{2+}$ concentrations.

• In muscle cells, the $Ca^{2+}$ pump in the sarcoplasmic reticulum pumps $Ca^{2+}$ back into the reticulum to allow muscle relaxation after contraction.

• The pump phosphorylates itself using ATP, inducing conformational changes.

• $Ca^{2+}$ binding sites are eliminated when open to the lumen of the sarcoplasmic reticulum, ejecting two $Ca^{2+}$ ions into the organelle.

• Sudden rise in cytosolic $Ca^{2+}$ concentration – initiates muscle contraction.

$H^+$ Pumps

• Rely on the electrochemical gradient of $H^+$ to import solutes into the cell.

• Pump $H^+$ out of the cell (some are ATP-driven; others are light-driven), creating an acidic pH outside the cell.

• In plant cells, bacteria, and fungi (which do not have $Na^+$ pumps), $H^+$ pumps transport $H^+$ out of the cytosol and into organelles to maintain their acidity.

Symports and Antiports Driven by Ion Gradients

• In animal cells, the electrochemical $Na^+$ gradient drives symports to import solutes.

• Lysosomes contain $H^+$ pumps to maintain an acidic internal environment.

• In plant and fungal cells, an electrochemical gradient of $H^+$ is often used by symports.

• Vacuoles contain a similar $H^+$ pump to maintain an acidic internal environment.

Electrochemical Gradient Driven Transporters

• variety of Symports and Antiports are driven by the electrochemical gradients.

• Symports: A variety of symports in animal plasma membranes specifically import related sugars or amino acids.

• Antiports Examples:

  • $Na^+ /H^+$ exchanger: Uses downhill influx of $Na^+$ to pump $H^+$ out of the cell, controlling cytosolic pH.

  • $Na^+/Ca^{2+}$ antiporter: Another mechanism to maintain low cytoplasmic $Ca^{2+}$ concentrations.

Glucose-$Na^+$ Symport

• Electrochemical $Na^+$ gradient drives active import of glucose across the plasma membrane of animal cells.

Glucose Uptake

• Intestinal epithelial cells utilize both a glucose symporter (active) and a glucose uniporter (passive) for glucose uptake.

• The diet provides ample $Na^+$ in the gut lumen to drive the $Na^+$ gradient-driven glucose symport.

• Glucose is transferred from the gut lumen to the extracellular fluid via the uniporter.

• $Na^+$ is pumped out by $Na^+$ pumps in the basal and lateral plasma membranes, maintaining a low cytosolic $[Na^+]$ and a steep $Na^+$ electrochemical gradient.

Transcellular Transport and Tight Junctions

• Adjacent cells are connected by impermeable tight junctions, which:

  • Act as diffusion barriers, confining transporters to respective membrane domains.

  • Prevent solutes from crossing the epithelium between cells.

  • Allow glucose concentration gradients to be maintained across the cell sheet.

Glucose Uniport

• Passive Transporters Move a Solute Along Its
  Electrochemical Gradient

• The glucose uniport facilitates the movement of glucose down its concentration gradient.

Active vs. Passive Transporters

• Gradient-driven pumps: Transfer solutes either in the same direction (symports) or opposite directions (antiports).

• Uniports: Facilitate movement of a solute down its concentration gradient.

• Because uniports do not require an additional energy source, they are not pumps.