Chapter 5

CHAPTER 5: PLASMA MEMBRANES, TRANSPORT, AND ENZYMES

Introduction

  • Grand Central Station serves as an analogy for cell function; organized movement reflects cellular processes across the plasma membrane.
  • Plasma Membrane (Cell Membrane): Defines the cell's borders, maintaining functionality.
    • Selective Permeability: Allows certain materials to enter or exit freely, while others require specific transport mechanisms.

5.1 | Components and Structure

Objectives
  • Understand the Fluid Mosaic Model of the cell membrane.
  • Describe the functions of phospholipids, proteins, and carbohydrates in membranes.
  • Discuss membrane fluidity and its implications.
Structure of the Plasma Membrane
  • Functionality: Defines boundaries, controls substance movement, and facilitates communication between cells.
  • Flexibility: Important for the shape changes in cells, such as red and white blood cells.
  • Recognition Markers: The surface of the membrane has markers necessary for cell recognition, crucial in immune response and tissue formation.
Fluid Mosaic Model
  • Description of the plasma membrane as a combination of components:
    • Phospholipids
    • Cholesterol
    • Proteins
    • Glycolipids
  • Membrane Thickness: Ranges from 5 to 10 nm.
  • Carbohydrates (Glycoproteins and Glycolipids): Extend from the surface, playing roles in recognition and signaling.
Components of the Plasma Membrane

  • Phospholipids: Composed of a glycerol, two fatty acids, and a phosphate group.

    • Amphipathic Nature: Hydrophilic heads and hydrophobic tails contribute to bilayer formation in aqueous environments.
    • Arrangement: In water, they form a lipid bilayer with hydrophilic heads outward and hydrophobic tails inward.

  • Proteins: Integral (spanning the membrane) and peripheral (on the membrane's surface).

    • Integral Proteins: Can be transmembrane (cross the membrane completely) or associated with one layer only.
    • Peripheral Proteins: Attached to the exterior/interior surfaces and involved in various cellular functions including enzymatic activity and cytoskeletal attachment.

  • Cholesterol: Modulates membrane fluidity; maintains structure at varying temperatures by preventing phase transitions in phospholipids.

  • Carbohydrates (Glycocalyx): Function in cellular recognition and attraction of water, important for cell-signaling and identification.

    • Compositions of glycocalyx help immune cells distinguish between self and non-self cells.
Membrane Fluidity
  • Fluidity Factors: Temperature, lipid composition, and the presence of cholesterol impact the fluidity of membranes.
  • Proteins and lipids can move within the bilayer, allowing the membrane to self-seal and maintain integrity.

5.2 | Passive Transport

Objectives
  • Explain the mechanisms of passive transport.
  • Understand osmosis and diffusion processes.
  • Define tonicity relative to cell movement.
Passive Transport Overview
  • Definition: Natural movement of materials across cell membranes without energy use, moving down the concentration gradient.
  • Diffusion: Movement from high to low concentration until equilibrium is achieved.
  • Osmosis: Water movement through semipermeable membranes driven by solute concentration gradients.
Selective Permeability
  • Plasma membranes facilitate specific substances' passage depending on molecular polarity, charge, and size.
    • Nonpolar and Lipid-Soluble Molecules: Easily diffuse through the bilayer.
    • Polar Molecules and Ions: Require specific membrane proteins to aid transport.
Factors Affecting Diffusion Rates
  1. Concentration Gradient: Greater differences drive faster diffusion.
  2. Molecule Mass: Lighter molecules diffuse more quickly.
  3. Temperature: Higher temperatures increase diffusion rates.
  4. Solubility: Nonpolar substances pass through membranes faster.
  5. Surface Area/Thickness of Membrane: Larger surface areas promote faster diffusion; thicker membranes slow down the process.
  6. Distance Travelled: Greater distances reduce diffusion rates.
Facilitated Diffusion
  • Uses transport proteins (channels or carriers) to assist polar molecules and ions across the membrane.
  • Aquaporins: Specific channel proteins facilitating rapid water movement.
  • Carrier proteins undergo conformational changes when binding substances to propel them across the membrane.

5.3 | Active Transport

Objectives
  • Understand how electrochemical gradients affect ions.
  • Distinguish between primary and secondary active transport mechanisms.
Overview of Active Transport
  • Definition: Movement against concentration gradients requiring energy, often from ATP.
  • Active Transport Mechanisms: Pumps that maintain ion concentration by moving substances against their gradient.
Primary Active Transport
  • Example: Sodium-Potassium Pump (Na+-K+ ATPase)
    • Pumps sodium ions out of the cell and potassium ions in (3 Na+ for every 2 K+), both against their concentration gradients, powered by ATP.
Secondary Active Transport
  • Utilizes the energy generated from primary active transport to move other substances.
  • Relies on the established concentration gradients created by primary active transport to drive the movement of new substances.

5.4 | Bulk Transport

Objectives
  • Describe endocytosis (including phagocytosis, pinocytosis, and receptor-mediated endocytosis).
  • Understand exocytosis processes.
Endocytosis
  • Process for large material transport into cells via vesicle formation.
  • Types:
    • Phagocytosis: “Cell eating”; engulfing of large particles or cells (e.g., macrophages targeting pathogens).
    • Pinocytosis: “Cell drinking”; uptake of fluids and smaller molecules.
    • Receptor-Mediated Endocytosis: Specific uptake of target molecules after binding to membrane receptors.
Exocytosis
  • Process where materials are expelled from the cell by fusing vesicles with the plasma membrane.
  • Key processes include neurotransmitter release and protein secretion.

5.5 | Enzymes

Objectives
  • Describe exergonic and endergonic reactions.
  • Explain enzyme efficiencies as molecular catalysts.
  • Discuss enzyme regulation by various factors.
Enzyme Functionality
  • Catalysts: Enzymes speed up chemical reactions by lowering activation energies.
    • Enzymatic Action: Bind to substrates forming complexes that facilitate product formation.
  • Substrate Binding: Occurs at the active site, specific to substrates which ensure reaction specificity.
Activation Energy
  • Energy required to initiate a reaction; enzymes reduce this energy barrier.
  • Distinction between exergonic (energy-releasing) and endergonic (energy-absorbing) reactions.
Enzyme Regulation
  • Enzymes can be inhibited or activated, impacting metabolic pathways:
    • Competitive Inhibition: Inhibitors compete with the substrate for the active site.
    • Noncompetitive Inhibition: Inhibitors bind elsewhere, altering active site shape.
    • Allosteric Regulation: Modulators change enzyme shape to increase or decrease substrate binding affinity.
Enzyme Compartmentalization
  • Eukaryotic cells compartmentalize enzymes into organelles, enhancing reaction efficiency.
Feedback Inhibition in Metabolic Pathways
  • End products of pathways can inhibit upstream reactions, regulating enzymatic activity based on necessity.

Key Terms

  • Activation Energy: Energy necessary for reactions.
  • Active Site: Specific enzymatic binding location.
  • Active Transport: Energy-requiring material movement.
  • Adenosine Triphosphate (ATP): Primary energy currency in cells.
  • Endocytosis: Active transport for moving substances into a cell.
  • Exocytosis: Active process for expelling materials from a cell.
  • Facilitated Diffusion: Passage through membrane via proteins