Lipids
When a fish is fried at the corner store (colmado), the cooking oil stays liquid in the pan, while the lard or butter from that same store stays solid in the refrigerator and melts when heated. Both are lipids, and the difference depends on a tiny detail in the shape of their molecules. In this section you will discover what that detail is.
What lipids are
Lipids are a varied group of biomolecules that share one thing: they do not dissolve in water (they are hydrophobic or, at least, have a large hydrophobic part). They dissolve in organic solvents such as alcohol or ether.
They are classified using a chemical test: do they contain fatty acids joined by bonds that break with a strong base (the saponification reaction, the one used to make soap)?
| Group | Contain fatty acids | Examples |
|---|---|---|
| Saponifiable | Yes | Triglycerides (fats and oils), waxes, phospholipids |
| Non-saponifiable | No | Cholesterol and other steroids, fat-soluble vitamins |
Their main functions are three:
-
Energy reserve: the triglycerides of fatty tissue.
-
Structural: phospholipids and cholesterol are part of cell membranes.
-
Regulatory or protective: steroid hormones (made from cholesterol), fat-soluble vitamins, and waxes that waterproof.
What common characteristic do all lipids exhibit with respect to water? Explain the criterion that distinguishes saponifiable lipids from non-saponifiable lipids and…
What common characteristic do all lipids exhibit with respect to water? Explain the criterion that distinguishes saponifiable lipids from non-saponifiable lipids and give an example of each group.
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.
Triglycerides: the energy reserve
One gram of fat stores more than twice as much energy as one gram of carbohydrates. That is why the body stores its long-term reserve as fat, not as sugar.
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 room temperature in Santo Domingo, cooking oil flows in the bottle and lard is a solid paste. What is different in their molecules?
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.
During the partial industrial hydrogenation of vegetable oils rich in oleic acid (cis-9-octadecenoic) for margarine production, some of the double bonds isomerize,…
During the partial industrial hydrogenation of vegetable oils rich in oleic acid (cis-9-octadecenoic) for margarine production, some of the double bonds isomerize, giving rise to elaidic acid (trans-9-octadecenoic).
- a) Compare the spatial geometry of the hydrocarbon chains of oleic acid and elaidic acid. Explain how cis/trans isomerism affects the capacity for molecular packing through Van der Waals forces, and predict which of the two fatty acids will have a higher melting point.
- b) If the endothelial cells of blood vessels incorporate elaidic acid instead of oleic acid into the phospholipids of their plasma membrane, reasonably predict the effect on bilayer fluidity at 37 °C and justify how this alteration may contribute, together with other factors, to endothelial dysfunction and the development of atheroma plaques.
Phospholipids and membranes
If you drop a drop of oil into a glass of water, it gathers into a spot and does not mix. Phospholipids, on the other hand, do something more interesting: they organize themselves and form the barrier that surrounds every cell.
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 non-saponifiable lipids
Non-saponifiable lipids have no fatty acids. The most important are:
-
Cholesterol (a steroid): a molecule with four rings. It is found in the membrane of animal cells, where it regulates their fluidity, and it is the raw material for sex hormones, bile acids (which help digest fats), and vitamin D. It is essential; the problem is an excess in the blood.
-
Fat-soluble vitamins (A, D, E, and K): they dissolve in fats, not in water, and are absorbed together with dietary fats.
Cholesterol has a bad reputation, but it is an essential molecule. State two functions of cholesterol in the body and explain why it is classified as a non-saponifiable…
Cholesterol has a bad reputation, but it is an essential molecule. State two functions of cholesterol in the body and explain why it is classified as a non-saponifiable lipid.
The cuticular waxes of two related plant species are compared: species A, adapted to a desert environment with summer temperatures of 42 °C, and species B, adapted to a…
The cuticular waxes of two related plant species are compared: species A, adapted to a desert environment with summer temperatures of 42 °C, and species B, adapted to a temperate and humid understory at 20 °C. The waxes of species A have a melting point of 65 °C, while those of species B melt at 38 °C.
- a) Describe the general chemical composition of a wax, indicating the reacting components and the resulting type of bond. Justify why the outer cuticle is composed of waxes and not of phospholipids if the physiological objective is to minimize transpiration.
- b) Justify at a molecular level what differences in the structure of the fatty acids or alcohols of the waxes (chain length and saturation) the higher melting point of species A can be attributed to. Predict what consequence it would have for the desert plant to possess the waxes of species B during a summer day at 40 °C.
Back to the beginning: cooking oil is liquid because its unsaturated fatty acids have kinks that keep the chains from stacking; lard is solid because its saturated fatty acids, being straight, pack together compactly.