Untitled Quiz

Cell Membranes

• Membranes enclose the contents of a cell, separating internal components from the external environment.
• This separation allows internal conditions to be regulated.
• Cell membranes have two key properties:
  • Semi-permeability: Only certain materials can pass through.
  • Selectivity: Membranes control the movement of materials.
• Primarily composed of two biomolecules:
  • Phospholipids (non-polar barrier).
  • Proteins (selectivity).

Phospholipids

• Structure:
  • Polar head (glycerol and phosphate).
  • Two non-polar tails (fatty acids).
• The head is hydrophilic (attracted to water).
• The tails are hydrophobic (repelled by water).
• Phospholipids are amphipathic molecules.
• Arrangement in membrane:
  • In water, phospholipids form a spontaneous bilayer.
  • Tails face inwards, heads face outwards.

Phospholipid Bilayer

• Properties:
  • Held together by weak hydrophobic interactions between the fatty acid tails.
  • A central hydrophobic layer restricts the passage of most water-soluble substances.
  • Individual phospholipids move within the bilayer, contributing to membrane fluidity and flexibility.
  • Fluidity allows for spontaneous breaking and reforming of membranes (endocytosis/exocytosis).

Membrane Proteins

• Embedded within the phospholipid bilayer.
• Can be classified as integral or peripheral.
• Integral Proteins:
  • Permanently attached to the phospholipid bilayer.
  • Require detergents to remove (transmembrane).
• Peripheral Proteins:
  • Temporarily associated with membrane periphery.
  • Can be removed with salts or pH changes.

Protein Functions

• Membrane proteins have a variety of functions:
  • Junctions: Connect cells.
  • Enzymes: Localize reactions.
  • Transport: Facilitated diffusion and active transport.
  • Recognition: Function as markers for "self."
  • Anchorage: Attachment points for the cytoskeleton and extracellular matrix (ECM).
  • Transduction: Function as receptors for peptide hormones.

Glycosylation

• Phospholipids and proteins can have carbohydrate chains attached (glycolipids or glycoproteins).
• The carbohydrate extends from the extracellular side of the membrane.
  • Functions in adhesion and recognition.
    • Can serve as attachment points (e.g., sperm binding).
    • Can act as recognition points (e.g., ABO blood groups).
• Plays a role in anchoring cells to the extracellular matrix (ECM).

Fluid Mosaic Model

• A model describing the structure of cell membranes.
• Phospholipid bilayer with proteins embedded and carbohydrates attached.
• Proteins and phospholipids can move laterally within the membrane.

Topic Connections

• Carbohydrates: Glycoproteins in cell-cell recognition.
• Cell Theory: Freeze-fracturing visualized membrane proteins.
• Cell Respiration: Membrane-bound enzymes in aerobic respiration.
• Cell Signaling: Integral proteins as transmembrane hormone receptors.