Membrane Structure, Cholesterol, and Apoptosis Markers

Membrane structure and cholesterol

  • The plasma membrane provides a selective barrier, influenced by phospholipid movement.
  • Cholesterol contributes to membrane packing due to its structure:
    • It has a very small polar head group and a large hydrophobic tail.
    • This allows cholesterol to pack tightly with phospholipids in the bilayer, stabilizing the membrane.
    • Cholesterol can pack in tightly within phospholipids on both leaflets (both sides of the bilayer).

Plasma membrane asymmetry and function

  • The asymmetry of the plasma membrane is not random; it is very important for the cell’s function.
  • The membrane has two leaflets with different orientations, with parts facing the cytosol on the inner side.
  • Because of this asymmetry, researchers can use reporters to visualize specific lipid distributions.

Visualization of lipid distribution and apoptosis markers

  • Using reporters, we can visualize lipid distribution in the membrane.
  • Cells that are positive for annexin indicate they are undergoing apoptosis because phosphatidylserine (PS) becomes exposed on the outer leaflet.
  • This visualization can be done with annexin as a marker, indicating PS exposure.
  • The lecture mentions that this approach can be paired with another marker for visualization (i.e., not relying on a single marker).

Annexin and phosphatidylserine in apoptosis

  • Annexin (annexin V) binds to exposed phosphatidylserine on the outer leaflet, serving as an indicator of apoptosis.
  • PS exposure on the outer leaflet is a hallmark signal of apoptosis detectable by annexin-based assays.

Interactive class activity

  • The instructor invites students to participate in the visualization activity: “What we're gonna do is I want you guys to, like, come in.”
  • Students are encouraged to engage with the markers to visualize membrane asymmetry.

Key concepts recap

  • Selective barrier and phospholipid movement underpin membrane integrity.
  • Cholesterol’s head-tail structure drives tight packing in the bilayer.
  • Plasma membrane asymmetry is crucial for function and is exploited for visualization.
  • Phosphatidylserine exposure on the outer leaflet signals apoptosis.
  • Annexin V (annexin) is a marker for PS exposure and apoptosis; a second marker may be used for complementary visualization.