Cell Membranes and Membrane Transport Master Study Guide to Membrane Transport

Learning Outcomes and Course Overview

• By the end of this lecture, students should be able to:   - Outline functional properties and biological roles of cellular membranes.   - Explain the basis for differential concentrations of ions and charges across the plasma membrane and describe their specific values.   - Understand concepts including $K_{ow}$, transport kinetics, uniport, symport, and antiport.   - Understand and exemplify different ways solutes cross biomembranes.   - Describe basic principles of solute transporters using named examples.   - Explain the advantages of multiple transporters for a single substrate (e.g., glucose).   - Explain how cholera toxin causes diarrhoea and the principle of electrolyte replacement therapy.

Internal Membrane Systems and Cell Structure

• Eukaryotic cells contain a variety of internal membranes that define organelles and create specialized compartments:   - Lysosome: Involved in degradation and recycling.   - Mitochondrion: Primary site of energy (ATP) production.   - Peroxisome: Involved in oxidative reactions.   - Nuclear envelope: Protects the genetic material.   - Golgi apparatus: Involved in protein modification and sorting.   - Endoplasmic reticulum (ER): Site of protein and lipid synthesis.   - Vesicles: Facilitate transport between compartments.   - Plasma membrane: Defines the cell boundary.   - Cytosol: The fluid containing the organelles.

The Fluid Mosaic Model of Membrane Structure

• Biological membranes are structured according to the Fluid Mosaic Model, described as 'proteins floating in a sea of lipids.'

• Thickness: The bilayer is approximately $4-5 \, nm$ thick.

• Components:   - Phospholipids: Form the fundamental bilayer structure.   - Proteins: Embedded or attached to the bilayer (integral or peripheral).   - Carbohydrates: Often attached to proteins or lipids on the extracellular surface.

Biological Membranes as Selective Permeability Barriers

• Membranes act as selective barriers, blocking the passage of almost all water-soluble molecules into and out of cells/organelles.

• Simple Diffusion: Small uncharged or hydrophobic (lipid-soluble) molecules traverse the bilayer freely down concentration gradients.

• Protein-Mediated Transport: Charged or polar molecules require specialist proteins (pumps, transporters, pores) for translocation.

• Permeability Trends:   - High Permeability: Hydrophobic molecules ($O_{2}$, $N_{2}$, benzene, short chain fatty acids) and small uncharged polar molecules ($H_{2}O$, $CO_{2}$, urea, glycerol).   - Low/No Permeability (Requires Proteins):     - Ions: $H^{+}$, $Na^{+}$, $Mg^{2+}$, $HCO_{3}^{-}$, $K^{+}$, $Ca^{2+}$, $Cl^{-}$.     - Charged polar molecules: Amino acids, ATP.     - Large uncharged polar molecules: Glucose, sucrose.

Ion Concentrations and Membrane Potential

• There is a distinct difference in ion concentrations between the intracellular ([IN]) and extracellular ([OUT]) environments:   - Sodium ($Na^{+}$): $[IN] = 10 \, mM$; $[OUT] = 140 \, mM$.   - Potassium ($K^{+}$): $[IN] = 140 \, mM$; $[OUT] = 4 \, mM$.   - Chloride ($Cl^{-}$): $[IN] = 4 \, mM$; $[OUT] = 140 \, mM$.   - Calcium ($Ca^{2+}$): $[IN] = 0.0001 \, mM$; $[OUT] = 2.5 \, mM$.

• Membrane Potential ($c$): The inside of the cell is negatively charged relative to the outside, primarily due to the distribution of ions and intracellular proteins.