chapter 7

Membrane Structure and Function

Overview of Plasma Membrane

  • Definition: The plasma membrane serves as the boundary that separates the living cell from its surroundings.
  • Functionality: It is described as selectively permeable, controlling the entry and exit of substances.

Concept 7.1: Fluid Mosaic Model of Membranes

  • Fluid Mosaic Model Definition: The membrane is conceptualized as a mosaic of protein molecules floating in a fluid bilayer of phospholipids.
  • Membrane Composition:
    • Lipids: Phospholipids that form the bilayer, possessing hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.
    • Diagram: Water molecules interact with the hydrophilic heads of phospholipids while the hydrophobic tails avoid water, leading to a bilayer arrangement.
    • Proteins:
    • Peripheral Proteins: Loosely bound to the surface of the membrane.
    • Integral Proteins: Span the hydrophobic core; those that cross the membrane entirely are termed transmembrane proteins.
  • Interactions: Membranes are mainly held together by weak hydrophobic interactions, allowing molecules like lipids and some proteins to move laterally within the layer.

Functions of Membrane Proteins

  • Categories of Membrane Proteins:
    • Transport: Assist in the movement of substances across the membrane.
    • Enzymatic Activity: Enzymes embedded in the membrane catalyze reactions.
    • Signal Transduction: Receive signals from outside the cell and trigger responses.
    • Cell-Cell Recognition: Glycoproteins play critical roles in identifying cells.
    • Intercellular Joining: Linking cells together for tissue formation.
    • Attachment to Cytoskeleton and ECM: Anchoring proteins connect to cytoskeleton components or extracellular matrix for maintaining structure.

Concept 7.2: Selective Permeability of Membranes

  • Definition of Selective Permeability: A membrane's ability to regulate the passage of materials, allowing only specific molecules to enter or leave the cell.
  • Mechanisms: The plasma membrane exchanges materials with the environment, controlling the flow through its selective properties.

Permeability of the Lipid Bilayer

  • Hydrophobic Molecules: Nonpolar molecules (e.g., hydrocarbons) can dissolve in the lipid bilayer and easily pass through the membrane.
  • Hydrophilic Molecules: Polar and ionic substances do not cross the membrane readily due to their hydrophilic nature.

Transport Proteins

  • Function: Allow hydrophilic substances to traverse the lipid bilayer.
  • Types of Transport Proteins:
    • Channel Proteins: Form pores that allow specific ions or molecules to pass.
    • Carrier Proteins: Bind to specific substances, change shape to shuttle them across the membrane, facilitating the transport.

Concept 7.3: Passive Transport

  • Definition: Passive transport is defined as the diffusion of substances across a membrane without the use of cellular energy.
  • Process of Diffusion:
    • Description: Molecules tend to spread out evenly in the available space due to random movements.
    • Directional Movement: Although individual molecules move randomly, the collective behavior may show a directional trend from an area of higher concentration to one of lower concentration.

Osmosis and Water Balance

  • Definition of Osmosis: The diffusion of water across a selectively permeable membrane.
  • Process: Water moves from areas of lower solute concentration to areas of higher solute concentration until equilibrium is achieved.

Effects of Osmosis on Cells

  • Hypotonic Solution: A solution with lower solute concentration compared to the cell's interior, leading to swelling.
  • Isotonic Solution: Equal solute concentration between the cell and its surroundings, maintaining normal cell shape.
  • Hypertonic Solution: A solution with higher solute concentration, causing cell shrinkage.

Facilitated Diffusion

  • Definition: Passive transport facilitated by proteins (channel and carrier proteins).
  • Key Feature: Allows movement down concentration gradients without expending energy.

Concept 7.4: Active Transport

  • Definition: Active transport utilizes energy (ATP) to move solutes against their concentration gradients.
  • Energy Requirement: Active transport necessitates energy input to function, primarily from ATP hydrolysis.

Mechanisms of Active Transport

  • Example: Sodium-Potassium Pump
    • Process: ATP donates energy to pump sodium (Na extsuperscript{+}) out of the cell while bringing potassium (K extsuperscript{+}) into the cell against their gradients.
  • Process Diagram: Includes the cellular movement of ions and the energy investment involved.

Concept 7.5: Bulk Transport

  • Definition: Method by which large substances cross the membrane via vesicular transport.
  • Processes:
    • Exocytosis: Expelling materials from the cell via vesicles.
    • Endocytosis: Intake of substances where the plasma membrane engulfs materials forming vesicles.
    • Types of Endocytosis:
    • Phagocytosis: Engulfing large particles.
    • Pinocytosis: Engulfing small particles and liquids.
    • Receptor-Mediated Endocytosis: Specific intake via receptor binding and vesicle formation.