lipids

What do lipids consist of?

C, H O, with a much smaller proportion of O as compared to H and C

Describe the solubility of lipids

Insoluble in water but soluble in organic solvents

How is a triglyceride formed?

• 3 fatty acid + 1 glycerol molecule via an ester linkage (fatty acid is later known as hydrocarbon chain)

• OH group of a glycerol molecule and COOH group of a fatty acid chain via condensation involving the loss of a water molecule per fatty acid (3 total)

What is the structure of glycerol?

1 OH group per carbon

What are the properties of glycerol?

• Polar molecule → can form hydrogen bonds with water molecules → soluble in water

• Viscous

• Higher density than water

What is the structure of a fatty acid?

• Long, non-polar hydrophobic hydrocarbon chains

• Carboxyl group at the end of the change is the functional group

• Saturated or unsaturated

What is the difference between a saturated and unsaturated fatty acid?

Saturated — C-C single bonds and C-H bonds only → Pack closely to one another

Unsaturated — 1 or more C=C double bonds → “Kink” in structure wherever a cis double bond occurs → Prevents chains from packing too close

What are the properties of fatty acids?

• Generally non-polar ← Numerous non-polar C-H bonds in hydrocarbon chains → Hydrophobic → Cannot form H bonds with water → Insoluble in water

• Weak acids ← COOH acid can dissociate to a small extent to form H+ in aqueous solutions

What are the properties of triglycerides?

Non polar ← No uneven charge distribution within the molecule → Do not form H bonds with water molecules → Hydrophobic → Insoluble in water

What is the difference between fats and oils?

Fats = Solids at RTP

Oils = Liquids at RTP

What is the main function of triglycerides?

Compact energy storage

• ATP released upon oxidation

• Stores more energy than carbohydrates as they are less oxidized and hence yield significantly more energy upon oxidation

• Higher proportion of C-H bonds and a lower proportion of O atoms

• Long non-polar hydrophobic hydrocarbon chain is insoluble in water → Does not affect WP of cell → Cannot be easily transported out of the cell → good storage molecule

Why do animals need fat for energy storage while plants don’t?

• Fat = compact energy store

• Animals are mobile and need to carry energy stores

• Plants are relatively immobile so they can function with the bulkier starch (but oils can be found in seeds)

What are other functions of triglycerides (other than the main one)? (5)

• Production of metabolic water → O supplied during oxidation + H → H2O

  • Particularly important for desert animals where water supply is limiting

• Protects internal organs → Cushions organs from internal damage

• Thermal insulation → Subcutaneous fat, especially thick in marine mammals and seals (protection from cold environment)

• Improves buoyancy → Especially in marine animals, lipids are less dense than water

• Storage of fat soluble vitamins → A, D, K

What is the structure of phospholipids?

• 2 non-polar hydrophobic hydrocarbon tails (from 2 fatty acids) + 1 glycerol backbone

• Third hydroxyl group of the backbone joined to a phosphate group with negative electric charge

• Additional small molecules, usually charged or polar e.g. choline can be linked to the phosphate group to form a variety of phospholipids

How are phospholipids formed?

• Condensation

• Ester linkage between the COOH group of the fatty acid and the OH group of the glycerol

• Phosphoester linkage between OH group of the glycerol and phosphoric acid

What are the properties of phospholipids?

• Amphipathic

  • Hydrophobic non-polar hydrocarbon tails

  • Hydrophilic negatively-charged phosphate head → interacts with water molecules

• Self-assemble to the phospholipid bilayer / micelle with a hydrophobic core in water or aqueous solutions

What is the function of phospholipids in relation to cells and organelles?

Major component of cell and organelle membranes ← Phospholipid bilayer

• Boundary between the intracellular and extracellular aq environment → Compartmentalization → Formation of unique environments in a cell for specialized processes

• Selectively permeable barrier ← Hydrophobic core only permeable to small hydrophobic solutes, not polar and charged molecules/ions

• Fluid membrane ← Phospholipids can move laterally in the membrane → Movement of incorporated proteins within the bilayer

What are liposomes?

• Artificial, lab-produced lipid spheres with an aqueous core to transport synthetic DNA to alter genetic make up of a cell

• Vesicles surrounded by a phospholipid bilayer that can fuse with the phospholipid bilayer of the target cell

What is the structure of steroids?

Four fused ring structure

What are the properties of steroids e.g. cholesterol?

• Slightly amphipathic

  • Hydrophilic OH group → Interacts with phosphate heads of phospholipids via hydrophilic interactions

  • Hydrophobic ring structure → Interacts with hydrocarbon tails of phospholipids via hydrophobic interactions

• Rigid structure → provides mechanical stability to the fluid lipid layer

What are the functions of steroids e.g. cholesterol?

• Regulates membrane fluidity

  • Lower temperature → lower kinetic energy → cholesterol prevents close packing of phospholipids → prevents solidification or crystallization (from being overly firm)

  • Higher temperature → higher kinetic energy → cholesterol restricts phospholipid movement (from being overly fluid) through interactions with phospholipids

• Anchors proteins in the membrane

How do we test for fats?

Ethanol emulsion test

• Lipids dissolve in organic solvents like ethanol but not water

• A white emulsion is seen when lipids are shaken vigorously with water, if there are no lipids, the solution stays clear

• Add ethanol to test sample → Mix and stand → Decant the ethanol into a test tube with water