CELLMOL: Lecture-6-Membrane-Lipids--Function

Chapter 1: Introduction

  • Cell membranes are fluid and their fluidity changes with temperature

  • Membrane fluidity is measured by the transition temperature, which is the temperature at which the membrane becomes a fluid

  • Below the transition temperature, membrane functions that rely on fluidity are disrupted

  • Increasing temperature increases membrane fluidity up to a point where the membrane breaks apart

  • Membrane fluidity is influenced by external factors like temperature and intrinsic factors like lipid composition

  • Membranes need to be fluid in order to properly carry out cellular functions

Number of carbons in fatty acids

  • Longer fatty acid tails result in a less fluid membrane

  • Longer tails lead to stronger hydrophobic interactions, restricting the movement of phospholipids

Degree of unsaturation or number of double bonds

  • More double bonds result in a more fluid membrane

  • Double bonds introduce kinks in the fatty acid chain, preventing close packing and weakening hydrophobic interactions

Chapter 2: Most Plasma Membrane

  • The number of double bonds and their configuration influence membrane fluidity

  • More cis double bonds lead to a more fluid membrane

  • Trans fats should be avoided as they pack together like saturated fats and decrease membrane fluidity

  • Membrane fluidity is also influenced by sterols, such as cholesterol

  • Cholesterol decreases membrane fluidity and prevents tight packing of phospholipid hydrocarbon chains

  • Cholesterol acts as a fluidity buffer, regulating membrane fluidity in response to temperature

  • Cholesterol decreases membrane permeability to ions and small polar molecules by filling spaces between phospholipid hydrocarbon chains

Chapter 3: Fluidity Of Membrane

  • Cholesterol molecule is slightly bent and twisted in 3D representation

    • Allows cholesterol to decrease packing efficiency among phospholipids at lower temperatures

    • Acts as a hydrophobic molecule

  • Cholesterol holds phospholipids together at higher temperatures

    • Prevents membrane disassembly

  • Cholesterol is incorporated in the lipid bilayer via noncovalent interactions

    • Hydroxyl moiety of cholesterol interacts with carbonyl carbon of phospholipid fatty acid

    • Maximizes hydrophobic interaction

  • Organisms regulate membrane fluidity by varying lipid composition

    • Poikilotherms use homeoviscous adaptation to compensate for temperature changes

    • Desaturase enzyme introduces double bonds into fatty acids as needed

    • Varying chain length and number of double bonds regulates membrane fluidity

Chapter 4: Fluidity Of Membrane

  • Thermophilic archaea use ethers with long hydrocarbon tails in phospholipids

    • Ether bonds are more stable than ester bonds

    • Long hydrocarbon tails reduce membrane fluidity at high temperatures

  • Membrane asymmetry is established during synthesis and remains relatively unchanged

  • Lipid rafts are microdomains in the membrane with high concentration of proteins and biomolecules

    • Involved in cell signaling, transport, defense, etc.

  • Caveolae are invaginations of the plasma membrane enriched in cholesterol and sphingolipids

    • Involved in endocytosis, exocytosis, redox sensing, regulation of airway function

  • Membrane fluidity is important for invagination formation and membrane integrity

Chapter 5: Inner Membrane

  • Caveolae formation mediated by caveolin protein

    • Synthesized in rough ER, undergoes post-translational modification

    • Phosphorylation activates the protein, dephosphorylation deactivates it

  • Caveolin attaches to inner membrane of plasma membrane

  • Caveolin complex recruits cavins to form caveol