PPT 11

BSC1010C: General Biology 1 - Structure and Function of Plasma Membranes

Learning Objectives

  • Understand the cell membrane fluid mosaic model.

  • Describe phospholipid, protein, and carbohydrate functions in membranes.

  • Explain why and how passive transport occurs.

  • Understand the osmosis and diffusion processes.

  • Understand how electrochemical gradients affect ions.

  • Distinguish between primary active transport and secondary active transport.

  • Describe endocytosis (phagocytosis, pinocytosis, and receptor-mediated endocytosis) and exocytosis.

Introduction to Plasma Membrane

  • The plasma membrane acts as the defining barrier of life, constructed from lipids and proteins.

  • Key components involved in movement across membranes include lipid structure and function.

  • Membrane processes under investigation include diffusion and osmosis, as well as the roles of membrane proteins.

Function of Membranes

  • Separation of Life from Nonlife:

    • The plasma membrane separates the cell’s interior from its external environment.

  • Functions of Membranes:

    • Protection: Keeps damaging materials out of the cell.

    • Nutrient Entry: Allows entry of materials essential for cellular function.

    • Facilitation of Reactions: Enables chemical reactions necessary for life.

Membrane Lipids

Phospholipid Structure

  • Characteristics:

    • Phospholipids are amphipathic molecules with hydrophilic (water-attracting) and hydrophobic (water-repelling) regions.

    • Hydrophilic Head:

    • Composed of glycerol, a negatively charged phosphate group, and a charged or polar group.

    • Hydrophobic Tail:

    • Made up of two nonpolar fatty acid or isoprene chains.

  • The main structure of membranes is a phospholipid bilayer, where polar heads face outward and hydrophobic tails face inward.

Selective Permeability of Lipid Bilayers

  • Definition: Phospholipid bilayers exhibit selective permeability.

  • Permeability Characteristics:

    • Small or nonpolar molecules (e.g., gases) cross quickly.

    • Charged or large polar substances cross slowly, if at all.

Factors Influencing Membrane Permeability

  • Several factors affect membrane behavior:

    • Number of double bonds in phospholipid tails.

    • Length of the fatty acid tails.

    • Cholesterol content within the membrane.

    • Environmental temperature.

Effects of Lipid Structure on Permeability

  • Saturation and Hydrocarbon Chain Length:

    • Double bonds introduce kinks in tails, reducing close packing and leading to increased permeability.

    • Saturated tails are denser and less permeable due to strong hydrophobic interactions.

Cholesterol's Role

  • Cholesterol impacts membrane density and fluidity:

    • Decreases membrane permeability as temperatures increase.

    • Increases membrane permeability as temperatures decrease, acting as a buffer for fluidity.

Movement through Lipid Bilayers: Diffusion and Osmosis

Diffusion

  • Definition: Movement of solutes from high concentration to low concentration regions, increasing randomness (entropy).

  • Equilibrium: Achieved when solutes are randomly distributed with no net movement.

  • Passive Transport: Substances move without energy input.

  • Dialysis: A specific type of diffusion across permeable membranes.

Osmosis

  • Definition: Special case of diffusion where water moves across selectively permeable membranes, typically from regions of low solute concentration to high solute concentration.

  • Impact of Tonicity:

    • Hypertonic Solution: Water moves out, leading to cell shrinkage.

    • Hypotonic Solution: Water moves in, leading to cell swelling and possible lysis.

    • Isotonic Solution: No net water movement, remaining stable in volume (ideal for animal cells).

Membrane Proteins and Their Functions

Protein Types

  • Plasma membranes have significant protein content alongside phospholipids.

  • Functions of Membrane Proteins:

    • Transport (active and passive).

    • Enzymatic activity.

    • Signal transduction mechanisms.

    • Cell-cell recognition.

    • Intercellular joining.

    • Attachment to the cytoskeleton.

Fluid-Mosaic Model of Membrane Structure

  • Proposes a dynamic arrangement of proteins within a phospholipid bilayer.

  • Integral Membrane Proteins: Span the membrane and have hydrophobic and hydrophilic segments.

  • Peripheral Membrane Proteins: Associated with the membrane but do not penetrate it.

Channel Proteins

  • Specialized proteins allowing ions to diffuse through membranes, influenced by electrochemical gradients.

  • Electrochemical Gradient: Establishes a charge difference across the membrane, driving ion diffusion.

Carrier Proteins

  • Facilitate solute transport via two mechanisms:

    • Facilitated Diffusion: Passive transport facilitated by protein shape change.

    • Active Transport: Movement against the concentration gradient requiring energy via pumps (primary and secondary).

Sodium-Potassium Pump

  • An example of active transport that moves sodium and potassium ions against their gradients using ATP, establishing membrane potential.

Bulk Transport Mechanisms

  • Allows the movement of large particles into and out of cells.

  • Endocytosis: Internalization via membrane invagination, including receptor-mediated endocytosis, phagocytosis, and pinocytosis.

  • Exocytosis: The process of vesicles fusing with the plasma membrane to release contents outside the cell.