Lipids in Biochemistry II Trivia Quiz

Answer: Generation of heat

Brown fat is abundant in newborns and is important in keeping them warm. The reason for the brown colouration is an abundance of mitochondria in the brown adipose cells. With almost all other cell types, protons leave the mitochondrial matrix during oxidative phosphorylation, and re-enter via the ATP synthase protein, which generates ATP.

In brown adipose cells, however, there is an abundance of a protein called thermogenin in the mitochondria, which provides an alternate entry route into the matrix for the protons. Protons may therefore bypass the ATP synthase, and the energy released is converted to heat, rather than chemical, energy.

Answer: A mostly hydrophobic molecule with four fused hydrocarbon rings

Cholesterol belongs to the sterol family of lipids and is the main sterol in animal tissues. It is made up of four fused hydrocarbon rings (three six-carbon rings and one five-carbon ring), with an alkyl side chain at one end and a hydroxyl group at the other.

The hydroxyl group is polar, but only makes up a tiny proportion of the cholesterol molecule. Cholesterol is an important constituent of cell membranes, and can be used to synthesise a variety of hormones.

Answer: They are packaged in proteins known as lipoproteins

Lipoproteins form a kind of hydrophilic shell around the hydrophobic lipid core, and therefore provide an energetically stable vehicle for lipid transport around the body. The size of this entity, as well as the proteins involved and the type of lipid carried, defines the lipoprotein. Dietary lipids are transported in what are known as chylomicrons, which are taken to the liver for further processing. Lipids can also be carried as very low-density lipoproteins (VLDL) or low-density lipoproteins (LDL), together known as "bad cholesterol", or as high-density lipoproteins (HDL), or "good cholesterol".

Answer: Familial hyperlipidaemia

ApoE is an example of a lipoprotein that assists lipid transport in the blood (its full name is apolipoprotein E). It is commonly found combined with very-low density lipoprotein in the blood, and can bind to LDL receptors in the liver, which mediate the uptake of "bad cholesterol". The liver can then metabolise these lipids to yield a range of important compounds.

ApoE exists in several isoforms and performs a range of activities in the body. ApoE has been linked to the immune response in mammals. It was shown in mice that lack this gene that they are more susceptible to bacterial infection.

One isoform of ApoE, ApoE4, also shows interesting links with Alzheimer's disease. People who inherit two copies of the gene coding for this isoform are sixteen times more likely to develop this neurological disease than the average, and this usually sets in before the age of 70. In contrast, the average age of onset for people who possess two copies of the ApoE3 gene is over 90.

Answer: Ectopic lipids

In adipocytes, lipids are stored as triglycerides - a fairly stable and inert form. High-fat diets may result in hyperlipidaemia and therefore overwhelm the body's ability to store fat properly. As a result, lipids can become deposited in muscle, where they interfere with normal functionality and may even cause apoptosis (programmed cell death). Fatty deposits can also build up in blood vessels, causing atherosclerosis. Atherosclerotic plaques can form when lipids invade the vessel wall.

This leads to inflammation and the deposition of a fibrous cap on top of this lesion. If the fibrous cap is too thick, it can occlude blood flow and cause ischemia.

However, if the cap is too thin, the plaque is prone to rupture (called a thromboembolism) which is also detrimental.

Answer: Lipases

There are several species of lipase, which vary in the specific lipid they act on, the site at which they hydrolyse bonds, and the products they make. Lipases are found in the pancreas and the small intestine, as well as other sites such as the lysosomes of cells.
One important aspect of lipid digestion is the emulsification of said lipids. This is done by bile, which is produced in the liver, and is stored and secreted by the gall bladder. Interestingly, the bile salts (which are the second largest component of bile, behind water) are derived from cholesterol - a lipid.

Answer: Flippases

The difference between phospholipids is their head-groups. They can have a choline group, an ethanolamine group, or a number of other groups attached to their central glycerol base. Different head-groups are added at different stages, meaning that all of one particular phospholipid may only be found on one side of the membrane (membranes are bilayers and possess two "leaflets"). Proteins in the membrane are therefore used to change the distribution of phospholipids. Movement of phospholipids from one leaflet to another is energetically unfavourable and so flippases utilise ATP as an energy source.

Answer: Sphingolipids

Rather than having a glycerol group, sphingolipids have a sphingosine group. This already contains a long hydrocarbon chain at carbon 3 (analogous to the fatty acid chain in glycerophospholipids). The carbon 2 of sphingosine is capable of forming an amide linkage to a second hydrocarbon chain. Carbon 1 of sphingosine can then be linked to a variety of groups, which defines whether the sphingolipid is a sphingomyelin (which is enriched in the myelin which insulates axons), glycosphingolipids (which contain at least one sugar), or gangliosides (which contain oligosaccharides with at least one sialic acid at the chain's terminus).

Answer: A long chain alcohol

The long chain fatty acid - which may be saturated (containing only carbon-carbon single bonds) or unsaturated (containing at least one carbon-carbon double bond) - is joined to the long chain alcohol by an ester bond. Due to the length of these hydrocarbon chains (usually 20-30 carbons long), waxes are highly hydrophobic, hence their use in waterproofing in biology.

For example, waxes are often found on leaves and are secreted by the preen gland of birds, which they then spread over their feathers.

In plankton, waxes serve as the main storage form of metabolic fuel.

Answer: Peroxisome

Peroxisomes are found in both animal and plants cells, and while animals predominantly use the mitochondria for beta-oxidation of fatty acids, the plants prefer peroxisomes. The biochemical pathways are very similar between the two organelles, but are not identical. Peroxisomes are more important for the beta-oxidation of very long chain fatty acids, such as those found in dairy products or animal fats. Patients who suffer from Zellweger syndrome cannot make peroxisomes and so very long chain fatty acids build up in the body and can impair the function of various organs.