UnitC 3

• Cell membranes


  Act as selective barriers
• Plasma membrane


  • Separates inner cellular content from the external environment
  • Bacteria have a single membrane surrounding the cell; eukaryotes contain both the plasma membrane and internal membranes that enclose individual organelles

  
• Plasma membrane functions


  • Formation of intracellular endosomes and extracellular vesicles
  • Cell signaling
  • Presentation of surface proteins
  • Protein secretion
  • Ion exchange
  • Import/export of small molecules
  • Motility

  
• Plasma membrane


  • Composed of both lipids and proteins
  • Serves as permeability barrier to most water-soluble molecules, depending on their size

  
• Lipid bilayers


  Behave as two-dimensional fluids in which individual molecules (both lipids and proteins) are free to rotate and move in lateral directions

  
• Lipid


  Composed of a hydrophilic "head" and hydrophobic "tail"

  
• Most abundant lipids in membranes

  • Phospholipids

  
• Phospholipid


  Composed of a phosphate-containing head linked to hydrophobic tails

  
• Phosphotidylcholine


  One of the most abundant phospholipids present in cell membranes
• Amphipathic


  Molecules that have both hydrophilic and hydrophobic characteristics
• Different phospholipids


  • Cholesterol
  • Phosphatidylserine
  • Glycolipids

  
• Amphipathic lipid bilayer formation

  1. Hydrophilic head attracted with H2O

  2. Hydrophobic tail aggregation with one another

  3. Lipids position to form a lipid bilayer

  4. Tears in the bilayer are energetically unfavorable, thus membrane re-seals

  
• Lipid bilayer


  • Flexible and able to bend
  • Behaves as 2D fluid

  
• Bilayer fluidity


  • Depends on hydrocarbon tails: length and number of double bonds (C=C)
  • Shorter tails: increased fluidity
  • Longer tails: decreased fluidity due to interaction (van der Waals)
  • Unsaturated tails (with C=C) = more fluid, saturated tails (without C=C) = less fluid
• Cholesterol


  In animal cells, moderates membrane fluidity
• Importance of membrane fluidity

  • Enables rapid diffusion of cellular proteins
  • Enables interaction of extra/intra cellular proteins with bilayer proteins (receptors)
  • Permits membrane proteins to diffuse from the membrane to other sites in the cell
  • Ensures membrane proteins are evenly distributed between daughter cells following cell division
  • Allows for multiple processes including cell signaling and movement to take place

  
• Membrane assembly in the endoplasmic reticulum

  1. Phospholipids are made by enzymes bound to cytosolic surface of the ER

  2. Scramblase transporter protein moves random phospholipids from one monolayer to the other, resulting in symmetric growth of bilayer

  
• Generating membrane asymmetry


  1. Newly synthesized membrane is delivered to the Golgi apparatus

  2. Flippase protein transporter uses ATP hydrolysis to move specific phospholipids to cytosolic monolayer

  3. Membrane asymmetry is highly preserved throughout cellular division or budding

  
• Membrane proteins


  • Transmembrane proteins extend through bilayer and are amphipathic
  • Cytosolic membrane proteins are associated with monolayer of membrane via amphipathic helix
  • Lipid-anchored membrane proteins are Glycosylphosphatidylinositol (GPI)-anchored
  • Membrane protein-protein anchoring/interactions

  
• Single-pass transmembrane proteins


  1. Peptide backbone is highly hydrophilic, thus H-bonds tend to form between atoms of the backbone itself

  2. Formation of 𝝰-helix favors maximum H-bonding

  3. Hydrophobic amino acid side chains are positioned on the outside of the helix, favoring interactions with hydrocarbon tails
• Multi-pass transmembrane proteins (ionic channels)

  1. Formation of aqueous pores (channels) consists of a series of 𝝰-helices that cross bilayer multiple times

  2. Most of these proteins are amphipathic whose hydrophobic aa side chains interact with hydrocarbon tails

  
• Solubilizing membrane proteins in detergents

  1. Detergents (Triton X-100, SDS) contain a single hydrophobic carbon tail, allowing them to form micelles

  2. Detergent molecules interact with hydrophobic aa side chains of transmembrane proteins and hydrophobic tails of phospholipids, drawing the membrane proteins into aqueous solution

  
• Cell cortex


  • In animals, a network of proteins that supports and is attached to the underside of the cell membrane
  • In plants, bacteria and yeast, the cell wall supports the plasma membrane

  
• Spectrin


  Rod-like protein (100nm) that forms a dimer linked to specific transmembrane proteins, forming a lattice-like structure of support

  
• Fluorescence Recovery after Photobleaching (FRAP)


  1. Membrane proteins are labeled with fluorescent marker (GFP)

  2. A laser beam irradiates ("bleaches") a portion of the membrane, decreasing the fluorescent signal

  3. Labelled neighboring proteins then diffuse to the patch area, and fluorescence is recovered

  4. Diffusion coefficient is calculated to determine how fast membrane recovered
• Studying specific membrane proteins in lipid bilayer

  1. Cell membrane is treated with detergents to purify protein of interest

  2. Protein is reconstituted with artificial phospholipids and position can be confirmed within the artificial phospholipid bilayer

  
• Cell membranes


  Act as selective barriers
• Plasma membrane


  • Separates inner cellular content from the external environment
  • Bacteria have a single membrane surrounding the cell; eukaryotes contain both the plasma membrane and internal membranes that enclose individual organelles

  
• Plasma membrane functions


  • Formation of intracellular endosomes and extracellular vesicles
  • Cell signaling
  • Presentation of surface proteins
  • Protein secretion
  • Ion exchange
  • Import/export of small molecules
  • Motility

  
• Plasma membrane


  • Composed of both lipids and proteins
  • Serves as permeability barrier to most water-soluble molecules, depending on their size

  
• Lipid bilayers


  Behave as two-dimensional fluids in which individual molecules (both lipids and proteins) are free to rotate and move in lateral directions

  
• Lipid


  Composed of a hydrophilic "head" and hydrophobic "tail"

  
• Most abundant lipids in membranes

  • Phospholipids

  
• Phospholipid


  Composed of a phosphate-containing head linked to hydrophobic tails

  
• Phosphotidylcholine


  One of the most abundant phospholipids present in cell membranes

  
• Amphipathic


  Molecules that have both hydrophilic and hydrophobic characteristics
• Different membrane lipids


  • Cholesterol
  • Phosphatidylserine
  • Glycolipids

  
• Amphipathic properties


  • Hydrophilic head attracted with H2O
  • Hydrophobic tail aggregation with one another
  • Lipids position to form a lipid bilayer
  • Tears in the bilayer are energetically unfavorable, thus membrane re-seals

  
• Lipid bilayer


  • Flexible two-dimensional fluid
  • Able to bend
  • Critical in forming smaller compartments such as liposomes
  • Energetically favorable

  
• Bilayer fluidity


  • Depends on hydrocarbon tails: length and number of double bonds (C=C)
  • Shorter tails: increased fluidity
  • Longer tails: decreased fluidity due to interaction (van der Waals)
  • Unsaturated tails (with C=C) = more fluid
  • Saturated tails (without many C=C) = less fluid