Membrane Structure and Transport

Chapter 5: Membrane Structure and Transport

Overview

  • Focus on the structure and function of biological membranes

  • Key Concepts:

    • Membrane Structure

    • Fluidity of Membranes

    • Membrane Transport

    • Transport Proteins

    • Exocytosis and Endocytosis

5.1 Membrane Structure

  • Phospholipid Bilayer: Framework of the membrane.

    • Phospholipids: Amphipathic molecules with a hydrophobic region facing inward and a hydrophilic region facing outward.

    • Asymmetry of Leaflets: The two halves of the bilayer have different compositions, leading to functional differences.

  • Fluid-Mosaic Model:

    • Describes the membrane structure as a mosaic consisting of various lipid, protein, and carbohydrate molecules.

    • Membrane displays fluid characteristics, allowing lipids and proteins to move relative to each other.

  • Components of Membranes:

    • Proteins: Integral (transmembrane and lipid-anchored) and Peripheral membrane proteins.

    • Carbohydrates: Often attached to proteins (glycoproteins) or lipids (glycolipids).

    • Cholesterol: Present in animal cells, influences fluidity and stability.

5.2 Fluidity of Membranes

  • Semifluid Nature:

    • Lipids can rotate and move laterally within their leaflet; flip-flop movement between leaflets does not occur spontaneously.

    • Flippase: An ATP-requiring enzyme that facilitates lipid movement between leaflets.

  • Factors Affecting Fluidity:

    • Length of Fatty Acid Tails: Shorter tails increase fluidity due to decreased interactions.

    • Presence of Double Bonds: Kinks in tails from double bonds prevent tight packing, enhancing fluidity.

    • Cholesterol: Stabilizes membranes, effects vary with temperature.

  • Movement of Integral Membrane Proteins:

    • Many membrane proteins can move laterally; however, 10–70% may be restricted due to attachment to cytoskeletal components or extracellular matrix.

5.4 Membrane Transport

  • Selective Permeability: Plasma membranes allow certain molecules to pass while excluding others.

    • Essential molecules can enter, intermediates remain inside, and waste products can exit.

Types of Movement Across Membranes

  1. Passive Transport: No energy required, involves movement down a concentration gradient.

    • Simple Diffusion: Movement of a solute through the membrane without assistance.

    • Facilitated Diffusion: Requires a transport protein for the solute to cross the membrane.

  2. Active Transport: Requires energy, moves solute against its concentration gradient.

    • Utilizes specific transport proteins.

Factors Influencing Diffusion Rate

  • Barrier characteristics of the phospholipid bilayer affect solutes based on polarity and concentration.

  • Example: Diethylurea diffuses 50 times faster than urea due to nonpolar ethyl groups.

  • Permeability Scale:

    • Very Low: Charged polar molecules (e.g., ATP, proteins)

    • Low: Sugars, ions (e.g., Na⁺, K⁺)

    • Moderate: Water, urea

    • High: Gases (e.g., O2, CO2) and other small uncharged molecules.

Gradients in Cells

  • Transmembrane Gradient: Concentration differences across the membrane.

  • Electrochemical Gradient: Combines concentration and electrical gradients (e.g., for Na⁺ ions).

Tonicity

  • Isotonic: Equal concentrations of solutes on both sides.

  • Hypertonic: Higher solute concentration outside—can cause cell crenation.

  • Hypotonic: Lower solute concentration outside—can cause osmotic lysis.

Osmosis

  • Water moves from high water concentration to low water concentration.

    • Osmotic Pressure: Force driving water movement across membranes, important for cell size maintenance.

    • Animal Cells: Balance is crucial to avoid crenation (shrinking) in hypertonic environments or lysis (swelling) in hypotonic environments.

Plant Cells

  • Have a cell wall that prevents drastic size changes.

    • Turgor Pressure: Pressure exerted by the fluid in the vacuole against the cell wall.

    • Plasmolysis: Condition of a wilting plant cell as water exits.

Freshwater Protists

  • Organisms like Paramecium use contractile vacuoles to expel excess water in hypotonic environments, maintaining constant cell volume.

5.5 Transport Proteins

  • Transmembrane Proteins: Provide pathways for ion and hydrophilic molecule transport.

  • Two Classes: Channels (open passageways) and Transporters (conformational changes to move the solute).

Channels

  • Facilitate direct diffusion of ions and molecules; often gated (Example: Aquaporins).

Transporters

  • Types of Transporters:

    • Uniporter: Transports a single solute.

    • Symporter: Transports two or more solutes in the same direction.

    • Antiporter: Transports two or more solutes in opposite directions.

5.6 Exocytosis and Endocytosis

  • Exocytosis: Process of expelling materials from a cell via vesicles formed from the Golgi apparatus.

    • General Steps:

    1. Vesicle forms around cargo in the Golgi apparatus.

    2. Vesicle fuses with the plasma membrane.

    3. Cargo is released into the extracellular environment.

  • Endocytosis: Involves the plasma membrane folding inward to form vesicles, bringing substances into the cell.

    • Types:

    • Receptor-Mediated Endocytosis: Specific uptake facilitated by receptor binding.

    • Pinocytosis (cell drinking): Uptake of liquid and small molecules.

    • Phagocytosis (cell eating): Uptake of larger particles or cells.