L36: membrane proteins and membrane fluidity

• What are membranes primarily composed of?

  A mosaic of proteins embedded in the phospholipid bilayer
• Why is membrane fluidity important?

  It allows lipids and proteins to diffuse laterally and facilitates signal transmission and transport.
• How can the rate of lateral diffusion in membranes be measured?

  By using membranes with fluorophores and measuring the rate of diffusion after bleaching.
• What is the typical rate of lateral diffusion in membranes?

  Approximately $2 \, \mu m$ per second.
• What happens to membrane fluidity when it is too fluid or too solid?

  Too fluid leads to disorder and high permeability, while too solid slows down movement excessively.
• How does temperature affect membrane fluidity?

  Fluidity increases with temperature as lipid molecules move faster, and decreases as temperature drops.
• What role do unsaturated lipids play in membrane fluidity?

  Unsaturated lipids increase fluidity by introducing kinks in the fatty acid chains.
• How do organisms adapt their lipid composition to temperature changes?

  They increase short unsaturated fatty acids at low temperatures and long saturated fatty acids at high temperatures.
• How does cholesterol affect membrane fluidity?


  Less fluid at warm temperatures BC structure restricts phospholipid movement. More fluid at cooler temperatures
• What is one toxic effect of alcohol on membranes?

  Alcohol increases membrane fluidity, which can disrupt cellular functions.
• What is the significance of transverse diffusion in membranes?

  Transverse diffusion, or flip-flop, is rare and occurs approximately once every 3 days.
• How does the endoplasmic reticulum (ER) influence membrane composition?

  The ER synthesizes and modifies lipids, proteins, and carbohydrates, determining their asymmetric distribution in membranes.
• What are the two main types of membrane proteins?

  Integral and peripheral membrane proteins.
• What are the types of integral membrane proteins?

  • Single span hydrophobic α-helix
  • Multi-spanning α-helices
  • β-barrel proteins forming a pore
• What maintains the topology of integral membrane proteins?


  Hydrophobic and electrostatic interactions maintain the arrangement relative to the membrane.
• How do the environments differ for extra-cellular and cytoplasmic parts of membrane proteins?

  They differ in glycosylation and the presence of disulfide bonds.
• What is the role of ICAM in the immune system?

  ICAM is involved in cell-cell adhesion and is upregulated during inflammation.
• What is the function of bacteriorhodopsin in photosynthesis?

  Bacteriorhodopsin pumps protons from the cytosol to the extracellular space, creating a proton gradient.
• What is the structure and function of OmpA in E. coli?


  OmpA is a multifunctional protein that forms a barrel-shaped structure with a pore, aiding in adhesion and invasion.
• How do peripheral membrane proteins interact with membranes?

  They interact non-covalently with lipid head groups and integral membrane proteins.
• What is the role of ankyrin and spectrin in the plasma membrane?

  They maintain plasma membrane integrity via the spectrin-actin based cytoskeletal structure.
• What is the glycocalyx and its function?

  The glycocalyx is a network of glycoproteins that serves as a physical barrier and plays roles in mechanosensing and cell shape.
• What is the difference between glycoproteins and glycolipids?

  Glycoproteins usually have oligosaccharide chains, while glycolipids usually have single sugar residues.
• What are the functions of membrane carbohydrates?

  They are involved in cell-cell recognition, communication, and adhesion, especially in immune responses.
• How do blood group antigens relate to carbohydrates?

  Different carbohydrates determine blood group antigens, influenced by the enzymes we possess.
• What are the key messages regarding membrane fluidity and proteins?

  • Membrane fluidity is crucial for cell function.
  • Membrane proteins have diverse structures and can be integral or peripheral.
  • Glycans are located on the outside of membranes, attached to proteins or lipids.
• What are glycans important for?

  Recognition and signalling
• What is the recovery rate?

  The rate of diffusion of fluorophores
• • Biological membranes have constant movement within the bilayer. Rotation and flexion occur at a high rate. Proteins are similar but generally move slower
• Does the membrane become more or less permeable at high temperatures?

  More permeable because there is increased membrane fluidity
• Fluidity and permeability go hand in hand
• Increases fluidity: unsaturated lipids give kinks;short chains allow fewer interactions between lipids;high temperature
• Decreases fluidity: saturated chains; long chains; low temperature
• • Integral proteins do not flip-flop. They cross the membrane a least once. Either the C- or N-terminal can be intracellular
• What is membrane topology?


  Arrangement relative to the membrane (does not change)
• Describe a porin


  • Forms a barrel shape structure with a pore in the centre
  • Hydrophobic exterior, hydrophilic interior
• Name some features of peripheral membrane proteins.

  • Do not interact with the hydrophobic core of the membrane
  • Can be cytoplasmic/exoplasmic
  • Interact with lipid head groups and integral membrane proteins
  • Non-covalent interactions
  • Electrostatic interactions, H-bonds, vdWs bonds
• What is the shape of the cell dependent on and why?

  Ankyrin -glue dots. Binds to several integral membrane proteins and to spectrin

  Spectrin- net mesh/scaffold. Cytoskeleton protein creating a scaffold on the intra-cellular side of membrane