MEMBRANE STRUCTURE CLASS 3A LEARNING OBJECTIVES To appreciate the main functions of the plasma membrane and intracellular membranes All cells have a plasma membrane which acts as a selective barrier. Functions: Cell communication Import (nutrients such as glucose) and export of molecules (waste) Cell growth Motility Provides basic cell structure preventing contents of cell mixing with surrounding medium Proteins mediate other functions and give the membrane functional characteristics (channels and pumps) Create internal compartments of eukaryotic cells which are chemically separate Prokaryotic (bacterial) cells lack internal membranes. Internal membranes occur on many organelles in eukaryotic cells (nucleus, mitochondria). Vesicles join the plasma membrane, constantly adding to the membrane. To understand the structure and characteristics of phospholipid bilayer General structure of lipid molecules with hydrophilic head pointing outwards and hydrophobic tails pointing inwards creating a bilayer. Most abundant lipids are phospholipids in which the hydrophilic head group is linked to the rest of the molecule through a phosphate group. Phospholipid bilayers form compartments which are energetically favourable such as naturally forming a sphere so hydrophobic areas arent exposed. To be familiar with the structure and properties of different membrane phospholipids Kink is due to unsaturated conformation and the double bond, essential to keep fluidity. If there was no kink lipids would solidify as they could closely stack together. There are 4 major phospholipids in mammalian membranes: To appreciate the concept of amphipathic molecules Amphipathic: having both hydrophilic and hydrophobic parts. Subjected to 2 conflicting forces. Lipid molecules are amphipathic as they have a hydrophilic polar head and a hydrophobic nonpolar tail. The lipid bilayer resolves these forces and is the energetically most favourable situation. To understand the concept of membrane fluidity Lipid bilayer behaves as a 2D fluid. Phospholipids can move within the plane of the lipid bilayer. The fluidity of the lipid bilayer protects against mechanical stress and pressure. A flip flop when lipid molecules change sides of the bilayer maintains asymmetry of the membrane. Rotation of the hydrocarbon tails or flexion (kinks) can also occur.