Plasma Membrane

Lipid Bilayer Components

Phospholipids Structure

  • Heads: Polar (hydrophilic)
  • Tails: Nonpolar (hydrophobic)
  • Amphipathic: Molecules that possess both polar and nonpolar domains, allowing them to arrange in a bilayer in aqueous environments.

Glycolipids

  • Composition: Contain carbohydrates.
  • Location: Found on the outer leaflet of the lipid bilayer.
  • Function: Sugars form a protective sugar coat on the cell surface.
  • Synthesis: Added to the cell surface by fusing with the membrane (secretory pathway) and are modified in the Golgi apparatus.

Cholesterol

  • Composition: A sterol, which is primarily a lipid.
  • Function: Regulates membrane fluidity by altering the fluidity of the membrane.
    • More cholesterol: reduces fluidity, making the membrane more rigid.
    • Less cholesterol: increases fluidity.
  • Position: Fills up the space between phospholipids, affecting overall membrane stability.

Proteins in the Membrane

  • Four Major Phospholipids (structure not required to know):
    1. Phosphatidylcholine: Located on the outer leaflet of the cell.
    2. Phosphatidylethanolamine: Located on the inner leaflet.
    3. Phosphatidylserine: Found on the inside of the cell membrane.
    4. Sphingomyelin: Present on the outer leaflet.
Asymmetry in Membrane
  • Types of lipids in the membrane include the four main phospholipids.
  • Orientation of membrane proteins: Glycolipids face outside of the cell.

Plasma Membrane

Protection and Control

  • Function: Protects the internal environment of the cell from the external environment.
  • Controls Movement: Regulates which substances move in and out of the cell.
  • Structure: Consists of a lipid bilayer that is hydrophilic (polar) on the outside and hydrophobic (nonpolar) on the inside, making it impermeable to water-soluble molecules in an aqueous environment.

Hydrocarbon Tails

  • Unsaturated Hydrocarbon Tails:
    • Contain double bonds (C=C) which create kinks, preventing tight packing and enhancing fluidity.
    • Cholesterol assists in filling gaps created by unsaturated tails.
  • Saturated Hydrocarbon Tails:
    • No double bonds, resulting in a straight, tightly packed configuration, increasing overall membrane fluidity.

Lipid Rafts

  • Definition: Specific structures in the plasma membrane where cholesterol and sphingolipids cluster together.
  • Characteristics:
    • Form small, semi-solid patches known as lipid rafts that make the plasma membrane stiffer and more organized.

Fluid Mosaic Model

  • Concept: The membrane structure is dynamic and fluid, with various proteins embedded or associated with the lipid bilayer, resembling a mosaic.

Main Types of Membrane Proteins

Peripheral Membrane Proteins

  • Bind to other membrane proteins without penetrating the lipid bilayer.

Integral Membrane Proteins

  • Have portions that are embedded within the membrane. They may or may not span the entire membrane.
Transmembrane Proteins (Integral)
  • Integral proteins that span the membrane allowing parts to be exposed on both the extracellular and intracellular sides.
  • Functions: Carry out a wide range of functionalities including transport and signaling.

Membrane Spanning Domain

  • Usually consists of alpha helices, which help stabilize the insertion of proteins into the lipid bilayer.

Protein Anchors

  • Glycosylphosphatidylinositol (GPI) Anchors: These modifications occur inside the endoplasmic reticulum (ER) lumen and help anchor proteins to the cell’s outer surface.
  • Lipid Modification: Involves a lipid component and another molecule that anchors the protein in the membrane.

Glycocalyx (Intestines)

  • Definition: A carbohydrate layer that covers the cell surface.
  • Function: Serves protective purposes as well as plays roles in cell recognition and interaction.

Polarized Cells

Definition and Structure

  • Polarized Cells: Cells that exhibit two different layers or domains.
  • Many epithelial cells show polarization characterized by differing functions and structures in their layers.
Domains of Polarized Cells

  1. Apical Domain:

    • Location: Faces the external environment such as the inside of lungs and intestines.
    • Function: Primarily involved in digestion and absorption in the intestine.

  2. Basolateral Domain:

    • Location: The bottom layer attached to the underlying tissue.

Lipid Rafts Revisited

  • Characteristics:
    • Enriched in cholesterol, making them thicker.
    • Integrate specialized proteins, with some proteins designed exclusively for lipid rafts, performing specific functions reliant on the raft environment.