Exercises: Lipids

Name the three main functions of lipids in living organisms and give a specific example of a lipid that performs each function.

What is a triglyceride? State which molecules combine to form it and what type of bond is established between them.

Look at the diagram of two fatty acids, A and B. State which is saturated and which is unsaturated, indicate what the kink marked with ? represents, and use the way the chains pack together to explain why vegetable oils are liquid at room temperature whereas animal fats are usually solid.

Phospholipids are amphipathic molecules. Explain what this term means, which part of the molecule is hydrophilic and which is hydrophobic, and what structure they spontaneously form in an aqueous medium.

A biotechnology team designs artificial vesicles (liposomes) to encapsulate a water-soluble antitumor drug and deliver it through the bloodstream. A team member proposes replacing the phosphoglycerides in the formulation with triacylglycerols (triglycerides), arguing that "since they are cheaper saponifiable lipids and much more insoluble in water, they will form an even more stable and impermeable bilayer."

1. a) Refute the researcher's proposal: justify, based on the chemical structure and polarity of both biomolecules, why triglycerides are unable to form biological bilayers, and describe what arrangement they will spontaneously adopt when mixed with water.
2. b) Explain the amphipathic behavior of phospholipids in water and reason why the bilayers they form tend to close spontaneously to form hollow spherical vesicles.
3. c) Predict what would happen to the integrity of the vesicle if phospholipase A2 (an enzyme that hydrolyzes one of the two fatty acids of each phospholipid, generating lysophospholipids with a single hydrocarbon tail) is added to the medium. Justify your answer based on the geometry of the resulting molecule.