lec 1

• What defines the cellular structure of cells?

  • Cells are compartmentalised
  • Compartments are surrounded by biological membranes
• Which lipids are included in diverse lipid structures?

  Fatty acids, phospholipids, mono-, di-, & triglycerides, waxes, sterols, fat-soluble vitamins (A, D, E, K)
• What are the general properties of lipids?

  • Generally insoluble in water (hydrophobic)
  • Most contain C, H, O (phospholipids also contain P, N)
  • Building blocks of biological membranes
• What is the general structure of a fatty acid?

  $CH_3$(CH$_2$)_nCOOH,</latex> where $n$ = 14-18 (ie 16-20 C in total)
• What are the hydrophobic and hydrophilic properties of fatty acids?

  Hydrophobic: hydrocarbon chains

  Hydrophilic: polar carboxyl group
• What is an amphipathic molecule?

  A molecule with both hydrophobic and hydrophilic properties
• How are fatty acids primarily found in organisms?

  • Phospholipids (membranes)
  • Triglycerides (body fat)
  • Cholesterol esters (storage/transport)
• What types of fatty acids are commonly found?

  Saturated and unsaturated fatty acids
• What are the types of lipids found in biological membranes?

  • Glycolipids
  • Cholesterol
  • Phospholipids
• What hydrophobic, amphipathic, and hydrophilic properties characterize phospholipids?

  Hydrophobic: non-polar tails

  Amphipathic: both hydrophobic and hydrophilic regions

  Hydrophilic: polar heads
• What structural backbone is commonly found in glycolipids?

  Sphingosine
• What properties does cholesterol possess in membranes?

  Hydrophobic, amphipathic, and hydrophilic
• How do hydrophobic molecules interact with water?

  • Do not interact with water
  • Cluster together to exclude water
  • Form micelles
• Why do hydrophobic molecules readily form micelles in water?

  The polar heads interact with water while the hydrophobic tails cluster inward, excluding water.
• How do amphipathic phospholipids self-assemble in aqueous solutions?

  • Form lipid bilayers
  • Bilayers are ~4 nm thick
  • Hydrophobic core
  • Hydrophilic surface
• What are liposomes?

  • Phospholipid bilayers that form spherical vesicles
  • Separate compartments
  • Mimic cell membranes of biological cells
• What determines the permeability of a lipid bilayer?

  • Semi-permeable
  • Hydrophobic and small uncharged polar molecules can diffuse
  • Large polar and charged molecules cannot diffuse
• How is the permeability of lipid bilayers measured?

  By the membrane permeability coefficient
• Is membrane permeability an all-or-nothing property?

  No, it's a graded property where substances considered impermeable can still cross with low probability.
• Besides lipids, what other component constitutes biological membranes?

  Proteins, up to 75% of membrane composition.
• What functions do membrane proteins carry out?

  Many important functions including structural support and transport.
• How can membrane proteins be classified according to their topology?

  • Integral membrane proteins (1-4)
  • Lipid-anchored membrane proteins (5-6)
  • Peripheral membrane proteins (7-8)
• What are glycoproteins and glycolipids?

  • Glycoproteins: proteins with attached carbohydrates
  • Glycolipids: lipids with attached carbohydrates
• Who proposed the fluid mosaic model of membrane structure and when?

  Singer & Nicholson in 1972.
• Describe the fluid mosaic model.

  The model describes membranes as a fluid mixture of lipids and proteins, where proteins move freely in a phospholipid sea, with some proteins anchored by cytoskeletal proteins.
• What are lateral diffusion and flip-flop in membrane lipids?

  • Lateral diffusion: movement of lipids in the plane of the membrane
  • Flip-flop: movement of lipids from one leaflet to the other (very rare)
• Has flip-flop of protein molecules been observed?

  No, it has never been observed.
• How does FRAP provide evidence for the fluid mosaic model?

  • Label membrane components with fluorescent molecules
  • Bleach fluorescence in a small area
  • Observe recovery of fluorescence
  • Recovery indicates mobility of labelled molecules
• What are the diffusion coefficients for phospholipids, rhodopsin, and fibronectin?

  • Phospholipids: ~1 $\mu m^2 s^{-1}$
  • Rhodopsin: 0.4 $\mu m^2 s^{-1}$
  • Fibronectin: $10^{-4}$ $\mu m^2 s^{-1}$
• What was the result of the cell fusion experiment to support the fluid mosaic model?

  • Mouse and human cells labelled with coloured antibodies against cell surface proteins
  • Cells fused to create heterokaryon
  • No initial mixing of proteins
  • Proteins mixed after several hours
• How does temperature affect membrane fluidity?

  At higher temperatures, membrane lipids are in a disordered state, leading to higher fluidity; at lower temperatures, lipids become ordered, resulting in lower fluidity.
• How do fatty acid chain length and saturation affect membrane fluidity?

  • Long fatty acid chains: stronger interactions, lower fluidity
  • Double bonds in unsaturated chains: prevent close packing, higher fluidity
• How does cholesterol influence membrane fluidity at high and low temperatures?

  At high temperatures, cholesterol stabilizes the membrane, reducing fluidity; at low temperatures, it prevents lipids from clustering, increasing fluidity.
• What are the key properties of biological membranes?

  • Composed of amphipathic lipids and proteins
  • Approximately 2 molecules thick
  • Held together by non-covalent interactions
  • Fluid structures where components diffuse readily