lipids

• lipids are macromolecules which include

  • triglycerides (structure)
  • phospholipids (component of membranes)
  • glycolipids (component of membranes)
  • cholesterol (component of membranes)
  • steroid hormones
  • lipid based vitamins

  macromolecule:

  • a large organic molecule (often a polymer made up of monomers but not in the case of lipids)
• Functions of triglycerides in living organisms:

  • source of energy: can be used in respiration to provide twice as much energy per gram than carbs or proteins
  • energy storage: for the equivalent amount of energy, lipids have half the mass of carbs and proteins. As lipids don't mix with water they don't affect the water potential of the cell and large amounts can be stored
  • protection: vital organs have fat padding: cushion and absorb impact
  • (reducing water loss: waxy cuticle)
• Functions of triglycerides in living organisms (part 2):

  • buoyancy : lipids are less dense than water
  • thermal insulation: bad conductor of heat. lipids also provide electrical insulation in the form of myelin sheaths around axons of nerve cells (speeds up electrical impulse)
  • aquatic mammals have large amounts of subcutaneous fat
• triglycerides:

  • composed of 1 glycerol and 3 fatty acids
  • fatty acids: (hydrophobic): contain methyl group, -CH3, long chain hydrocarbon (CH2)n, carboxyl group: -COOH
• there are two types of fatty acids (in triglycerides):

  1. saturated fatty acid: every carbon in the hydrocarbon chain has the maximum number of hydrogens. All C-C bonds are single bonds. Palmitic acid: formula: CH15H31COOH

  • general formula for saturated fatty acids: CnH2n+1COOH

  2. unsaturated fatty acids: there are carbon=carbon double bonds so fewer hydrocarbons in the hydrocarbon chain, e.g. Oleic acid, formula C17H33COOH

  • general formula for unsaturated fatty acids: CnH2n-1COOH
• Triglyceride:

  • formed by 3 condensation reactions between 1 glycerol and 3 fatty acids
  • (hydrolysis (when triglyceride is broken back down into glycerol and fatty acids) lipase enzyme: pH of solution will fall as fatty acids are released)
• Phospholipids:

  • one fatty acid is replaced by a phosphate group
  • membranes are made of a phospholipid bilayer
  • polar/hydrophilic bond- in contact with water (hydrophobic tails in the middle - not in contact with water)
• Glycolipids:

  • membrane lipid with a carb attached
  • often used for cell-cell recognition
  • the carb is present on the extra-cellular side of the plasma membrane
• Cholesterol:

  • important precursor for:
  • steroid hormones - lipid soluble hormones
  • fat soluble vitamins, vitamin D series
  • bile salts - help emulsify fats in the small intestine
  • found in the Phospholipid bilayer of the membrane: important for both membrane stability and membrane fluidity
• LDLs (lower-density lipoproteins): a lower density molecule made of proteins and lipids that is used to transport cholesterol around the body to different tissues which can cause cholesterol buildup in blood vessels
• pulmonary surfactant: a complex mixture of lipids and proteins that covers the alveolar surface of the lungs. (surface active agents - surfactants)

  • prevents alveolar collapse at low lung volume
  • reducing alveolar surface tension, which promotes efficient gas exchange