Lecture Outline: Cell Membranes and Transmembrane Transport

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  1. Plasma Membrane Structure
    1. Main Infrastructure: Phospholipid Bilayer
      1. Forms spontaneously because phospholipids are amphipathic
      2. Each phospholipid has:
        1. Polar head: Contains phosphate, is hydrophilic (water-loving), faces water
        2. Non-polar tails: Made of hydrocarbons (fatty acids), are hydrophobic (water-fearing), form the interior of the bilayer
    2. Fluid Mosaic Model
      1. Membrane components (phospholipids, proteins) are not chemically bonded to each other and can move independently
      2. Lateral movement of phospholipids within a layer is very rapid (~10 million times per second)
      3. Flip-flop (movement between layers) is rare due to energy required to move polar heads through non-polar tails
    3. Membrane Fluidity
      1. Cells maintain correct fluidity based on conditions (e.g., temperature)
      2. Adjusted by changing types of phospholipids in the membrane:
        1. Saturated fatty acids: Have only single bonds between carbons, are straight, pack tightly, used to tighten membrane (e.g., in warm temperatures)
        2. Unsaturated fatty acids: Used to make membrane looser (e.g., in cold temperatures)
      3. Cholesterol: Also adjusts membrane fluidity and permeability by filling gaps
    4. Membrane Polarity
      1. The two sides of the membrane are different from each other
      2. Proteins are more associated with the inner layer than the outer layer
  2. Membrane Proteins
    1. Integration and Movement
      1. Proteins are crucial for substances that cannot pass directly through the phospholipid bilayer
      2. Proteins are held in place by hydrophobic/hydrophilic interactions with phospholipid tails and water
      3. Inserted into the membrane via vesicles from the Rough ER and Golgi apparatus
    2. Categories of Membrane Proteins
      1. Peripheral proteins: Associated with only one surface (monolayer) of the bilayer
      2. Integral proteins: Stick at least partially through the thickness, associated with the hydrophobic tails
        1. Transmembrane proteins: A special type of integral protein that sticks all the way through the thickness of the bilayer
    3. Major Functions of Membrane Proteins
      1. Transport: Facilitate movement of specific substances across the membrane
        1. Channel proteins: Form hollow tunnels that allow specific particles to pass through
        2. Carrier proteins: Temporarily bind to a particle and change shape to move it across the membrane
      2. Enzymatic Activity: Catalyze reactions while bound to the membrane
      3. Signal Transduction: Act as receptors for chemical signals (ligands)
        1. Signal binding causes a conformational change in the protein, relaying the message inside the cell without the signal entering
        2. Requires transmembrane proteins to relay signals to the cell interior
      4. Cell-to-Cell Recognition: Proteins (often with attached sugars) on the cell surface allow cells to recognize each other
      5. Intercellular Joining: Proteins connect adjacent cells together (e.g., in desmosomes)
      6. Attachment to Cytoskeleton and Extracellular Matrix (ECM): Anchor the membrane and provide structural support
  3. Transport Processes Across the Membrane
    1. Three Major Categories
      1. Passive Transport
      2. Active Transport
      3. Vesicular Transport
    2. Passive Transport (Diffusion)
      1. Does not require additional energy; energy is already built into the system in the form of a gradient
      2. Moves particles from high concentration to low concentration (down the gradient)
      3. Based on random movement (Brownian motion) but results in net directional movement
      4. Requirements for direct diffusion through the phospholipid bilayer:
        1. Particle must be small enough
        2. Particle must be non-polar enough (e.g., oxygen)
      5. Types of Diffusion:
        1. Simple Diffusion: Particles move directly through the phospholipid bilayer (small, non-polar substances)
        2. Facilitated Diffusion: Requires the help of a transport protein (channel or carrier) for particles that are too large or too polar; still passive and moves down the gradient
    3. Active Transport
      1. Requires additional energy (e.g., ATP)
      2. Moves substances against their gradient (from low concentration to high concentration)
      3. Always requires a carrier protein (cannot use a channel protein)
      4. Often referred to as a pump
      5. Types of Active Transport:
        1. Primary Active Transport: Energy (e.g., ATP) is directly spent to power the pump and establish a gradient
          1. Example: Sodium-potassium exchange pump: Pumps sodium ions out and potassium ions in, both against their gradients
          2. Example: Proton pump: Pumps hydrogen ions out, creating a proton gradient (stored energy)
        2. Secondary Active Transport (Co-transport): Uses the energy stored in a pre-existing gradient (established by primary active transport) to move another substance against its gradient
          1. Example: Sucrose-proton co-transporter: Protons diffuse down their gradient, dragging sucrose against its gradient without direct ATP consumption for sucrose
        3. Co-transporters: Transport two different types of particles
          1. Symporters: Move both particles in the same direction
          2. Antiporters: Move particles in opposite directions (e.g., sodium-potassium pump)
    4. Vesicular Transport
      1. Involves the use of membrane-bound vesicles
      2. Types based on direction:
        1. Endocytosis: Inward vesicular transport
          1. Phagocytosis: Cell eating (taking in solid particles)
          2. Pinocytosis: Cell drinking (taking in liquid samples)
          3. Receptor-mediated endocytosis: Vesicles form only when specific particles bind to receptors
        2. Exocytosis: Outward vesicular transport
  4. Osmosis and Tonicity
    1. Osmosis
      1. A special case of diffusion
      2. Requirements for osmosis:
        1. Movement of a solvent (always water in biology)
        2. Movement through a selectively permeable membrane (allows water but not solutes)
      3. Water moves from a place of high water concentration (low solute concentration) to a place of low water concentration (high solute concentration), down its gradient
      4. Continues until equilibrium or balancing forces (e.g., gravity) are achieved
    2. Tonicity
      1. Refers to the tendency for osmosis to occur through a cell membrane
      2. These terms describe the cell's environment/surroundings, NOT the cell itself
      3. Degrees of Tonicity:
        1. Hypertonic environment:
          1. Has a higher solute concentration than the cell (less watery)
          2. Cell will lose water by osmosis
          3. Animal cells: Shrivel (crenation), can be fatal
          4. Plant cells: Plasma membrane collapses (plasmolysis), fatal
        2. Hypotonic environment:
          1. Has a lower solute concentration than the cell (more watery)
          2. Cell will gain water by osmosis
          3. Animal cells: Swell and burst (lysis), can be fatal (e.g., red blood cells; aquatic protists use contractile vacuoles to prevent this)
          4. Plant cells: Swell and become turgid (pressurized), ideal condition for plant support (erection)
        3. Isotonic environment:
          1. Has the same solute concentration as the cell (same water concentration)
          2. No net movement of water; water enters and leaves at the same rate
          3. Ideal for animal cells (e.g., extracellular fluid homeostasis)
          4. Plant cells: Can survive but are flaccid (limp), not ideal