Lipids
Olive oil and butter, the kind you might grab for a Thanksgiving dinner, are made essentially of the same type of molecule: a triglyceride. Whether one is liquid at room temperature and the other solid depends on a tiny structural detail in their fatty acids. This section explores that diverse family of water-insoluble molecules that we call lipids. We'll come back to the oil-versus-butter question and answer it.
Triglycerides: the energy reserve
One gram of fat stores more than twice as much energy as one gram of carbohydrate (such as glycogen), and it is stored without associated water: what molecular architecture makes it possible to pack so much energy into such a small, insoluble space?
What lipids do
Lipids have three main jobs. Energy storage: triglycerides (fat in animals, oil in seeds such as peanuts) hold more than twice the energy per gram of carbohydrates. Structure: phospholipids and cholesterol build cell membranes. Signaling, insulation and protection: steroid hormones such as estrogen and testosterone carry messages, a layer of fat under the skin insulates a seal or a whale, and waxes waterproof leaves and feathers.
Name the three main functions of lipids in living organisms and give a specific example of a lipid that performs each function.
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.
What is a triglyceride? State which molecules combine to form it and what type of bond is established between them.
Fatty acids: saturated and unsaturated
At 68 °F (20 °C), olive oil flows freely in the bottle while lard stays completely solid: what difference in the geometry of their hydrocarbon chains explains such a dramatic change in melting point?
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…
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: amphipathic molecules and membranes
When lipids extracted from red blood cells are placed on the surface of water, the molecules immediately line up on their own, creating a continuous film that is impenetrable to soluble ions: what chemical duality drives them to form this spontaneous barrier?
Phospholipids are amphipathic molecules. Explain what this term means, which part of the molecule is hydrophilic and which is hydrophobic, and what structure they…
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…
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."
- 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.
- 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.
- 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.
Waxes and steroids
Name the three main functions of lipids in living organisms and give a specific example of a lipid that performs each function.
Name the three main functions of lipids in living organisms and give a specific example of a lipid that performs each function.