Lecture_08_Membrane Transport I

Membrane Transport in Cell Biology

Overview

• Authors: Alberts, Heald, Hopkin, Johnson, Morgan, Roberts, Walter

• Course: BIOL 3510: Lecture 08

• Focus: Membrane transport, principles of membrane transport, and function of transporters.

Lecture Objectives

• Describe membrane potential across the plasma membrane.

• Compare transporters and channels:

  • Types of transport: specificity, structure.

• Define osmosis and relate to:

  • Hypertonic, hypotonic, isotonic solutions.

  • Cell types resisting osmotic swelling.

• Understand structure and role of aquaporins in water transport.

• Differentiate between active vs passive transporters.

• Explore different pump systems:

  • Sodium pumps, calcium pumps, gradient-driven pumps, sodium-glucose pumps, H+-ATPase pumps.

• Define terms: symport, antiport, uniport.

Principles of Transmembrane Transport

• Selective Permeability:

  • Protein-free lipid bilayers impermeable to most water-soluble molecules.

  • Specialized transport proteins required for small, water-soluble molecules.

• Phospholipid Bilayers:

  • Hydrophobic interior prohibits ions and most organic molecules.

  • Diffusion Rate: Depends on size and solubility of solute.

Membrane Potential

• Ion Concentration Differences:

  • Disparity in ion concentrations inside vs outside of cells generates voltage (membrane potential).

Transport Proteins

Types of Membrane Transport Proteins

• Channels:

  • Form protein pores through the membrane for water/ion transport.

  • Can open/close based on conformational changes.

  • Transport depends on pore size and electric charge.

• Transporters:

  • Bind specific solutes and undergo conformational change to move solute across the membrane before reverting to original shape.

Transport Processes

Diffusion Mechanisms

• Simple Diffusion:

  • Small, nonpolar molecules cross lipid bilayer passively.

• Passive Transport:

  • Solutes move spontaneously down concentration gradients (requires transport proteins).

• Active Transport:

  • Solutes move against gradients via transport proteins and require energy input.

Electrochemical Gradient

• Definition:

  • Combination of concentration gradient and membrane potential influences solute movement.

Specialized Transport for Water

• Aquaporins:

  • Integral membrane proteins, transport water efficiently with high specificity.

  • Regulated by various mechanisms.

Osmosis and Solutions

Osmosis

• Water moves passively down concentration gradient, affecting cell volume.

• Osmolarity:

  • Total solute concentration in a solution.

• Types of Solutions:

  • Isotonic: Solute concentration inside = outside.

  • Hypotonic: Inside concentration > outside; cells swell.

  • Hypertonic: Inside concentration < outside; cells shrink.

Osmotic Balance Mechanisms

• Protozoans: Use vacuoles to expel excess water.

• Plant Cells: Have cell walls to limit expansion and vacuole regulation.

• Animal Cells: Use Na+-K+ pumps for structural integrity.

Transporter Function and Examples

Transport Mechanisms

• Active Transporters (Pumps):

  • Move solutes against gradients using energy.

• Na+ Pump:

  • Uses ATP to transport 3 Na+ out, 2 K+ in, crucial for maintaining gradients.

Summary of Gradient-Driven Transport

• Gradient-Driven Pumps:

  • Utilize solute gradients; can be symport (same direction) or antiport (opposite direction).

• Glucose-Na+ Symport:

  • Na+ gradient drives glucose transport; cooperative binding is essential.

• H+-ATPase Pumps:

  • Generate proton gradients, influencing transport across plasma membrane and organelles.

Practical Implications

Questions for Review

• Discuss Na+ Pump functions and its ATP consumption

• Strategies for glucose absorption using transporters in epithelial cells

• Explain how sports drinks aid in glucose uptake in the small intestine by replenishing lost electrolytes.