Lecture #8 Notes

Lecture #8 – Transmembrane Transport of Ions and Small Molecules

Major Topics Covered in Lecture

  • 11.1 Overview of Transmembrane Transport

  • 11.2 Facilitated Transport of Glucose and Water

  • 11.3 ATP-Powered Pumps and the Intracellular Ionic Environment

  • 11.4 Nongated Ion Channels and the Resting Membrane Potential

  • 11.5 Cotransport by Symporters and Antiporters

  • 11.6 Transcellular Transport

Learning Objectives

  • Describe the mechanisms to transport materials across membranes:

    • Simple diffusion

    • Facilitated diffusion

    • Osmosis

    • Channel proteins

    • Active transport

  • Explain the processes involved in transcellular transport

Important Concepts to Remember about Transmembrane Transport

  • Cellular membranes regulate the traffic of molecules and ions into and out of cells and their organelles.

  • The rate of simple diffusion across the phospholipid part of a membrane is proportional to the concentration gradient and hydrophobicity of the substance.

  • Most molecules, except O₂ and CO₂, are transported by protein channels or transporters down their concentration gradients, or by other transporters and ATP-powered pumps up their concentration gradients.

The Movement of Substances Across Cell Membranes

  • Selective permeability allows for the separation and exchange of materials across the plasma membrane.

    • Net flux is the difference between influx and efflux of materials.

    • Flux can occur by passive diffusion and/or active transport.

The Energetics of Solute Movement

  • Diffusion is the spontaneous movement of material from a region of high concentration to a region of low concentration.

  • The free-energy change during diffusion of nonelectrolytes depends on the concentration gradient.

  • The free-energy change during diffusion of electrolytes depends on the electrochemical gradient.

Key Gradients

Chemical Gradient
Electrochemical Gradient

The Energetics of Solute Movement

  • Diffusion (Passive Transport):

    • Spontaneous movement from regions of high to low concentration (energetically favorable – exergonic transport).

    • Free-energy change depends on concentration gradient (nonelectrolytes) or electrochemical gradient (electrolytes).

  • Active Transport:

    • Movement from regions of low concentration to high concentration (energetically unfavorable – endergonic transport).

    • Requires external energy.

Relative Permeability of a Pure Phospholipid Bilayer to Various Molecules and Ions

  • Permeable: Many gases and small, uncharged, water-soluble (polar) molecules; require no transporters to diffuse.

  • Slightly permeable: Water, ethanol, urea.

  • Impermeable: Ions and large polar molecules.

Simple Diffusion of Molecules Across Cellular Membranes

  • Characteristics:

    • No metabolic energy expended when moving from high to low concentration down its chemical concentration gradient.

    • Rate is proportional to the concentration gradient, the size of the molecule, and the molecule's partition coefficient K (lipid solubility).

    • Higher partition coefficient results in faster movement across the bilayer.

Overview of Membrane Transport Proteins

  • Cellular membranes serve as barriers and conduits for selectively transporting molecules and ions.

  • Transport Proteins: Form pathways across the membrane for hydrophilic substances to move without contacting hydrophobic interiors.

  • Three main classes of membrane proteins:

    1. Channels: Facilitate ion or water movement down their electrochemical gradient (gated and nongated).

    2. Transporters:

    • Uniporters: Passive transporters of specific molecules down concentration gradients.

    • Cotransporters: Active transporters (symporters and antiporters).

    1. ATP-Powered Pumps: Move specific ions or small molecules against their electrochemical gradient using ATP hydrolysis as energy.

Multiple Membrane Transport Proteins in Metazoan Cells

  • Example: Lysine uptake requires:

    • Na⁺/K⁺ pump (active transporter)

    • Na⁺/lysine symporter (active transporter)

    • K⁺ channel (passive transporter)

Summary of Transport Mechanisms

Mechanism of Transport

Requires Specific Protein

Solute Transported Against Its Gradient

Coupled to ATP Hydrolysis

Driven by Movement of a Cotransported Ion Down Its Gradient

Simple Diffusion

No

No

No

No

Facilitated Transport

Yes

No

No

Yes

Active Transport

Yes

Yes

Yes

No

Cotransport

Yes

Yes

No

Yes

Important Concepts about Facilitated Transport of Glucose and Water

  • Protein-mediated transport is faster than simple diffusion, highly specific, and limited by the number of transporters (Vmax) and transporter affinity (Km).

  • Water moves by osmosis, with some membranes having aquaporin channels to increase water transport rates.

  • Uniport proteins switch between conformations to facilitate membrane diffusion.

Cellular Uptake of Glucose via GLUT Proteins

  • Characteristics:

    1. Facilitated (uniport) transport is faster and more specific than simple diffusion.

    2. GLUT uniporters are present in plasma membranes of most mammalian cells and transport glucose, the primary energy source.

    3. Transport is reversible but always down a concentration gradient and reversible according to concentration gradient changes.

    4. Vmax depends on the number of uniporters; Km represents the concentration of substrate at which transport is half Vmax.

The Human GLUT1 Uniporter Mechanism

  1. Outward-open Conformation: Binds glucose (depends on Km and glucose concentration).

  2. Occluded Conformation: Ligand bound (Km dependence).

  3. Inward Open Conformation: Releases glucose.

  4. Ligand-free Occluded Conformation: Returns to original state; if glucose concentration is higher inside the cell, glucose moves out.

The Diffusion of Water through Membranes

  • Water spontaneously diffuses through a semipermeable membrane (osmosis) from areas of lower solute concentration to areas of higher solute concentration.

  • Cells undergo changes in response to solution types: swell in hypotonic solutions, shrink in hypertonic, and remain unchanged in isotonic solutions.

Aquaporins

  • Specialized protein channels facilitate passive water movement.

Important Concepts about ATP-Powered Pumps and the Intracellular Ionic Environment

  • Four classes of transmembrane proteins couple ATP hydrolysis energy to move substances against their concentration gradients.

  • P-class ATP-powered pumps generate the typical ionic environment of animal cells.

  • ABC superfamily proteins transport diverse substrates, including toxins, drugs, phospholipids, peptides, and proteins.

Types of ATP-Powered Transport Proteins

  • P-class pumps: E.g., Na⁺/K⁺ pump; found in plasma membranes of various organisms.

  • V-class pumps: Transport protons across organelle membranes and utilize ATP hydrolysis without direct phosphorylation during transport.

  • F-class pumps: Drive ATP synthesis in mitochondria and bacteria by utilizing proton gradients.

  • ABC transporters: Coupling ATP hydrolysis with substrate movement; diverse and specific to substrates.

Xenobiotic Transporters and Drug Resistance

  • ABC transporters alter the bioavailability of drugs by exporting them and are involved in removing toxic xenobiotics from cells.

    • Examples include multidrug resistance proteins.

Operational Model of Na⁺/K⁺ P-Type ATPase

  • Tetrameric Structure: 2 alpha and 2 beta subunits; the transporter is phosphorylated and dephosphorylated.

  • Function: Transports Na⁺ out and K⁺ into the cell at a ratio of 3 Na⁺ to 2 K⁺ against their concentration gradients.

H+ Concentration Gradients and V-Class Proton Pumps

  • V-class pumps generate electric potential without significantly changing intraluminal pH.

  • Cl- channels balanced H+ movement across membranes and can affect pH levels.

Important Concepts about Nongated Ion Channels and Resting Membrane Potential

  • The resting potential is driven by the Na⁺/K⁺ pump and nongated K⁺ channels.

  • Patch-clamping techniques are used to measure ion movements through single channels.

Generation of a Transmembrane Electric Potential

  1. Na⁺ and Cl⁻ are at higher concentrations outside cells; K⁺ is higher inside cells.

  2. Uncompensated positive charges outside and negative inside the cell align across the membrane, creating a membrane potential due to charge separation.

The Resting Potential

  • Refers to the membrane potential of a nerve or muscle cell, influenced by K+ gradients maintained by Na⁺/K⁺ -ATPase.

  • Nernst equation helps calculate the voltage equivalent of concentration gradients for specific ions.

    • Negative resting potential is close to the negative Nernst potential for K+ and far from the positive Nernst potential for Na+.

The Action Potential (AP)

  • Na⁺ channels open upon stimulation leading to membrane depolarization, followed by K⁺ flow, causing repolarization.

  • Inactivation of Na⁺ channels occurs after an AP, creating a refractory period.

  • Membrane potentials exhibit all-or-nothing behavior.

Diffusion of Ions through Membranes

  • Membranes are typically impermeable to ions unless crossed by ion channels, which are selective and bidirectional, facilitating diffusion according to the electrochemical gradient.

Types of Gated Ion Channels

  • Voltage-gated channels: Open/close in response to charge potential changes.

  • Ligand-gated channels: Open via ligand binding causing conformational changes.

  • Mechano-gated channels: Open/close due to mechanical forces such as stretch or pressure.

Important Concepts about Cotransport by Symporters and Antiporters

  • Two forces create an electrochemical gradient: electric potential and ion concentration gradient.

  • Cotransporters capitalize on ion gradient energy to transport other molecules against their gradients.

Co-Transport: A Type of Active Transport

  • Coupling active transport to existing ion gradients allows transport of molecules via symports (same direction) or antiports (opposite direction).

Sodium/Glucose Symporter Mechanism

  • Na+-linked symporters enable glucose and amino acids to be imported against concentration gradients, utilizing the Na+ gradient for energy.

Chloride/Bicarbonate Antiporter

  • CO₂ is poorly soluble, converted to soluble HCO₃⁻, facilitating transport via anion antiporter AE1, exchanging Cl⁻ and HCO₃⁻ across the membrane.

Important Concepts about Transcellular Transport

  • Polarized epithelial cells have apical and basolateral regions with distinct transport proteins working together for processes like amino acid and glucose transport or stomach acid secretion.

Transcellular Transport of Glucose from Intestinal Lumen to Blood

  1. Na⁺ Gradient and Electrical Potential Generation: Na⁺/K⁺ ATPase maintains gradients; K⁺ outflow establishes a negative interior potential.

  2. Uptake of Glucose: The Na⁺ gradient drives glucose uptake via a two-Na⁺/one-glucose symporter.

  3. Transport into Blood: Glucose exits the cell through basolateral GLUT2 uniporter.

Figures and Diagrams

  • Refer to diagram explaining the transcellular transport mechanisms involved in glucose uptake and the accompanying pH changes in stomach transport.

Next Lecture

  • Topic: Cellular Energetics, Part 1

  • Reading Assignment: Chapter 12, pages 512-531

  • Quiz Availability: Available on Achieve until 8:00 AM on Thursday, September 25th.