Ch: 7

Chapter 7: Membrane Structure and Function

  • Learning Outcomes

    • Describe plasma membrane structure and function

    • Understand diffusion and osmosis in solute and water movement

    • Explain processes for material transport across cell membranes

Structure and Fluidity of Biological Membranes

  • Membranes are modeled by the fluid mosaic model.

  • Phospholipids create a bilayer; proteins/carbohydrates float within it.

  • Membranes possess both polar and nonpolar regions (~ amphipathic).

  • Compartmentalization of cellular contents.

  • Membrane properties influenced by lipids, proteins, carbohydrates.

Membrane Proteins

  • Membranes contain varied proteins.

    • Integral proteins: Span entire membrane.

    • Peripheral proteins: Loosely attached to surface.

Functions of Plasma Membrane Proteins

  • Major functions include:

    1. Transport

    2. Enzymatic activity

    3. Signal transduction

    4. Cell-cell recognition

    5. Intercellular joining

    6. Attachment to cytoskeleton and ECM

Selective Permeability

  • Selective permeability determines which substances can cross membranes.

  • Nonpolar molecules (e.g., hydrocarbons, CO2, O2) cross easily.

  • Polar molecules and ions have difficulty crossing.

Types of Membrane Transport

  • Passive transport:

    • No energy required; moves down concentration gradient.

    • Includes simple diffusion and facilitated diffusion.

  • Active transport:

    • Requires energy; moves against concentration gradient.

    • Example: endocytosis.

Diffusion and Osmosis

  • Diffusion: Movement from high to low concentration until equilibrium.

  • Osmosis: Movement of water from high to low free water concentration.

Tonicity and Water Balance

  • Tonicity: Ability of a surrounding solution to affect cell water content.

    • Hypertonic: Higher solute concentration.

    • Isotonic: Equal solute concentrations.

    • Hypotonic: Lower solute concentration.

  • Water balance is crucial for cell integrity and function.

Facilitated Diffusion and Active Transport

  • Facilitated diffusion: Polar molecules/ions cross via transport proteins.

  • Active transport: Requires ATP to move solutes against gradients.

Endocytosis

  • Large molecules enter cells through vesicles:

    • Phagocytosis: Cellular eating.

    • Pinocytosis: Cellular drinking.

    • Receptor-mediated endocytosis.

Comparison of Transport Mechanisms

  • Passive transport vs. Active transport:

    • Passive does not require ATP, while Active does.

    • Transport mechanisms differ in energy requirements and movement direction (high to low vs. low to high).

Study Guide Review Questions

  • Define key terminology from Chapter 7.

  • Describe the structure and functions of cell membranes.

  • Relate phospholipid structure to membrane properties.

  • Define diffusion and passive transport.

  • Explain osmosis and tonicity.

  • Discuss cellular changes in hypertonic vs. hypotonic solutions.

  • Compare active transport to facilitated diffusion.