RV+MEDS1001_2024_L2

Control of the Cellular Environment

• Human Biology Lecture 2 2024

  • Instructor: Professor Rob Vandenberg

  • Institution: School of Medical Sciences, Faculty of Medicine and Health

The Story So Far

• Key Concept: The cell is a self-contained ecosystem.

• Question Raised: How does “stuff” get in and out of the cell?

The Plasma Membrane

• Structure: Phospholipid bilayer

  • Hydrophobic and Hydrophilic Regions

Permeability of the Plasma Membrane

• Impermeable to Most Essential Molecules and Ions:

  • Ions: K+, Na+, Ca2+, Cl-, HCO3-

  • Small water-soluble molecules: glucose

  • Large molecules: proteins and RNA

• Permeable Substances:

  • Water molecules, small uncharged molecules like oxygen and carbon dioxide

Movement of Molecules

• Diffusion:

  • Molecules distribute evenly over time due to random motion (eliminates concentration gradients).

  • Example: Ink drop in water.

Osmosis

• Definition: Diffusion of water through a semipermeable membrane.

• Key Relationships:

  • Higher solute concentration = lower water concentration.

  • Water moves towards lower water concentration to reach equilibrium.

• Characteristics of Plasma Membrane: Semi-permeable.

Tonicity and Osmotic Pressure

• Examples of Solutions:

  • 1 M glucose: 180 g/L

  • 1 M lactose: 342 g/L

  • 0.1 M glucose: 18 g/L

• Tonicity Types:

  • Isotonic, Hypotonic, Hypertonic, based on solute concentration ratios.

Example of Sucrose Concentration

• 1 M Sucrose vs. 0.3 M Sucrose:

  • Higher concentration of sucrose leads to a lower concentration of water.

  • Water moves to equalize concentration.

Characteristics of Cell Membranes

• Non-rigid nature of cell membranes.

• Osmosis Concept: "Salt sucks" due to osmotic pressure gradients.

Red Blood Cells (RBC)

• Hypertonic, Isotonic, Hypotonic environments:

  • Effects on RBC appearance and functionality.

Ion Transport Across Membranes

• Methods of Ion Passage:

  • Gases can cross the lipid bilayer but ions cannot without assistance.

Transmembrane Proteins

• Functions:

  • Integral membrane proteins create pathways for substances to cross the membrane.

  • Types of proteins:

    • Channel proteins

    • Transporters (facilitated diffusion or active transport)

Aquaporins

• Function: Water channels that facilitate rapid water transport.

  • Importance in regulating water flow in organs like the kidney and gut.

  • Recognition: Peter Agre, Nobel Prize for Chemistry 2004.

Transport Mechanisms

• Facilitated Diffusion: Movement of ions and nutrients through proteins without energy expenditure.

• Active Transport: Requires energy (ATP), movement against the concentration gradient.

Differences Between Active and Passive Transport

• Active Transport:

  • Moves substances against the concentration gradient using energy (ATP).

• Passive Transport:

  • Moves substances along the concentration gradient without energy.

Saturation of Channels

• Channels can become saturated, influencing the rate of substance movement.

Ion Channels and Electrical Potential

• Critical in cellular function: Ions conducting charge and establishing electrochemical potential differences.

Patch-Clamping Technique

• Nobel Prize in Physiology or Medicine 1991:

  • Based on discoveries concerning single ion channels in cells.

Ion Concentrations in Cells

• Understanding concentrations in blood and their implications for cell function.

Sodium-Potassium Pump (Na-K ATPase)

• Mechanism:

  • Moves Na+ out and K+ into the cell.

  • Functions to maintain electrochemical gradients critical for cell life.

Consequences of Na-K ATPase Action

• Electrochemical Balance:

  • Creates a negative membrane potential necessary for action potentials.

Membrane Potential Dynamics

• Action Potential in Nerves:

  • Phases: Depolarization, Repolarization, and Hyperpolarization.

Facilitated Diffusion of Nutrients

• Primary Roles of Glucose and Amino Acids:

  • Often use sodium gradient as a driving force.

Endocytosis and Exocytosis

• Definitions:

  • Endocytosis: Molecule movement into cells.

  • Exocytosis: Release of molecules from cells.

Clathrin-Mediated Endocytosis (CME)

• Role:

  • Involves nutrient uptake, signal transduction, and recycling of synaptic vesicles.

Types of Endocytosis

• Examples: Phagocytosis for engulfing particles.

Learning Objectives Overview

• Key Concepts to Understand:

  • Survival and function of a single cell in isolation.

  • Structures and functions of a cell and its organelles.

  • Regulation of substance flow across cell membranes.

  • Role of aquaporins and Na-K ATPase in maintaining gradients.

  • Mechanisms for transport of larger molecules.