Describe the molecular structures of a triglyceride and a phospholipid. #
Triglyceride structure
- Triglycerides consist of a glycerol backbone bonded to three fatty acid chains.
- Glycerol is an alcohol with three carbon atoms, each bearing a hydroxyl (-OH) group.
- Fatty acids are long hydrocarbon chains containing a carboxyl (-COOH) end group. Common fatty acids include palmitic, stearic, oleic, and linoleic acid.
- The fatty acids form ester bonds with the hydroxyl groups on glycerol, releasing water. This joins the hydrocarbon tails of fatty acids to the glycerol backbone.
- The resulting molecule has a polar, hydrophilic glycerol head and nonpolar, hydrophobic fatty acid tails. This makes triglycerides insoluble in water.
- In triglycerides found in animal fats, the fatty acid tails are usually 14-18 carbons long and saturated or monounsaturated. Plant oils contain higher proportion of unsaturated 18-carbon fatty acids.
Phospholipid structure
- Phospholipids are similar to triglycerides but have only two fatty acid tails attached to glycerol. The third hydroxyl group on glycerol is esterified to a phosphate group.
- A phospholipid is thus made up of a phosphate group, two fatty acid tails, and a polar head group attached to phosphate. Common head groups are choline, serine, inositol, and ethanolamine.
- The dual hydrophobic fatty acid tails and hydrophilic phosphate-head group give phospholipids an amphipathic structure. The head is polar while the hydrocarbon tails are nonpolar.
- When placed in water, phospholipids form bilayer membranes with the nonpolar tails facing each other in a hydrophobic interactions while the phosphate heads face outwards on both sides interacting favorably with water.
- Phospholipids are thus the major building blocks of cell membranes and lipoprotein complexes that transport lipids in the body. Their amphipathic nature allows these important biological roles.
Explain how the molecular structure of triglycerides is related to their functions.[8] #
- possess hydrophobic tails of fatty acids;
- which cause the molecule to be insoluble in water;
- they are not so easily dissolved out of the cell;
- this functions to provide the properties of the phospholipid bilayer in cell membranes;
- acts as energy store for the cell;
- due to their higher proportion of hydrogen compared to carbohydrates;
- as a result the breakdown of triglycerides yields ore energy;
- due to the lower proportion of oxygen to carbon that requires more oxygen for complete oxidation to occur;
- triglycerides also float in water due to their lighter density;
- this enables them to aid in the buoyancy of aquatic animals;
Describe with examples the roles of lipids in an organism. [6] #
- Used as high energy stores e.g triglycerides (fat or oil)
- Used in waterproofing coverings e.g. in wax cuticles found in insects and leaves to prevent water loss while sea birds prune their feathers with oil
- Form an insulating layer against heat loss
- Phospholipids are major components of cell membranes
- Steroids such as cholesterol is an important component of cell membranes and nerve fibres. Steroids form the basis of many hormones
- Lipids form the basis of scents that attract insects for pollination
Relate the structures of triglycerides and phospholipids to their functions in living organisms. #
Triglyceride structure and function #
- The long hydrophobic fatty acid tails of triglycerides allows them to be packed together forming hydrophobic cores that are energy-dense. This enables triglycerides to serve as efficient energy storage molecules.
- Triglycerides group together into spherical droplets with hydrophobic cores and surface monolayers. This reduces surface area exposed to water allowing triglyceride droplets to serve as waterproof energy reserves.
- The non-polar nature of triglycerides also enables them to serve as insulation and padding for organs. Adipose tissue filled with triglyceride droplets provide cushioning and insulation in the body.
- Variations in fatty acid saturation influence melting point of triglycerides. Fats with higher saturated fatty acids have higher melting points than unsaturated oils. This adapts triglycerides for functions in different environments.
Phospholipid structure and function
- The amphipathic structure with hydrophobic tails and hydrophilic head allows phospholipids to spontaneously form bilayer membranes in water. This makes them ideal to serve as building blocks of cell membranes.
- Phospholipid bilayers forms closed compartments allowing cells to establish concentration gradients, regulate transport, and create internal environments. This enables phospholipids to serve as structural matrices supporting living systems.
- The fluidity of phospholipid membranes can be adapted by variations in phospholipid structure. Changes in tail length, degree of saturation, and headgroup alter the fluidity to maintain optimal membrane function.
- Phospholipids form micelles and liposomes in water that serve as transport vehicles for fats and lipid-soluble substances in aqueous environments within organisms.
- Phospholipids complexed with proteins act as identity markers on cell surfaces and facilitate cell-cell communication critical for multicellular existence.
The amphipathic structures of triglycerides and phospholipids relate directly to their biological roles in energy storage, thermogenesis, membrane formation, transport, and cellular communication.
