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.

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