Lipids in Biochemistry I Trivia Quiz

Answer: Triacylglycerides

Triacylglycerides (triglycerides) are stored in adipose cells. Adipose tissue is what makes us appear "fat" in the eyes of others. It is commonly told that we have a set number of adipose cells, and that fluctuations in our body fat are due to variation in the amount of fat stored in each cell, rather than their number.

These stores are used as energy reserves (more on this later), for warmth, and for protection (fat is a good shock absorber and lines vital organs). Lipids can perform more specialised functions also - for example, the waxy covering of some leaves makes them largely waterproof.

Answer: Acetyl Co-enzyme A

Acetyl Co-enzyme A (acetyl CoA) is an important molecule in biochemistry and is a product of glucose metabolism. Indeed, the amount of acetyl CoA in a cell acts as an indicator of the cell's own metabolic state. Lots of acetyl CoA is suggestive of lots of glucose metabolism and, hence, plentiful glucose.

A molecule known as malonyl CoA, which is derived from acetyl CoA and which is the direct precursor of fatty acid chains, directly inhibits fatty acid oxidation. In doing this, the use of lipid as a fuel is inhibited when glucose is abundant. Conversely, when glucose (and therefore acetyl CoA) is scarce, fatty acids are utilised as a fuel source.

Answer: Mitochondria

In the cytosol, fatty acids (also known as acyl chains) are "activated" by having a coenzyme A attached to them. This makes the fatty acid (now referred to as acyl CoA) a suitable candidate for the carnatine shuttle system, which transports it from the cytosol to the mitochondria.

This acyl CoA is here converted to acetyl CoA - this is beta oxidation - which can then enter the citric acid cycle, eventually yielding energy.

Answer: Amphipathicity

Life may have first been thought to spring to life (for want of a better word) after molecules arose that were capable of replication. This refers to nucleic acid (e.g. RNA and DNA). A dichotomy exists in that nucleic acids are fragile and cannot exist without the protective cell.

However, the cell is made up of components that only exist because the DNA codes for them. How can this paradox be resolved? Lipids are not coded for by DNA, and pre-existed nucleic acid. Amphipathic lipids spontaneously form bilayers (or single-layered entities called micelles) due to the hydrophobic effect.

It was these pre-existing (non-living) lipid shells that may have been hijacked by nucleic acids, eventually morphing into what we recognise today as cells.

Answer: Cylindrical

A phospholipid is a glycerol molecule which has two fatty acids chains ligated to it. A phosphate group is also ligated to the glycerol, and it is this phosphate group that is modified. Variations include phosphatidylcholine, phosphatidylserine, and phosphatidylethanolamine. Mercifully, these are usually denoted by PC, PS, and PE, respectively.

Their shape is important in generating the correct type of curvature in the membrane. Additionally, the headgroups may be chemically interesting in that they can be charged (positive or negative), involved in signalling, or be pro-apoptotic (i.e. induce cell death).

Answer: A phosphatase

While phospahtases remove phosphate groups, kinases add them. In the case of PI, PI3K (PI-3 kinase) is the major kinase, and PTEN is the major phosphatase. The activities of PI3K and PTEN are tightly regulated and dysregulation in this system is often implicated in cancer. PIP3 binds proteins with a so-called plecstrin homology domain.

This includes protein kinase B, which has a plethora of targets within the cell. For example, protein kinase B inhibits proteins which induce apoptosis and so promotes cell survival.

Answer: Vesicular budding

Vesicular budding is a major form of transport within the cell. It packages proteins and other substances into membrane-bound vesicles which can then fuse with other compartments of the cell, releasing the cargo. Dynamin is a GTPase - it hydrolyses GTP, which is similar to ATP - and binds to highly curved membrane regions that possess PIP3 (discussed in the II section of the previous question). Specifically, dynamin is involved in vesicular budding that is promoted by the molecule clathrin. Other types of vesicular budding (for example those mediated by COPI or COPII proteins) do not tend to use dynamin.

Answer: Lipid deposits kill pancreatic cells, reducing insulin production

Lipids play an essential and subtle role in insulin secretion and, therefore, in the pathology of diabetes. Certain lipids such as free fatty acids actually induce insulin secretion from pancreatic cells in normal circumstances, but an excess of these molecules can induce apoptosis of the pancreatic cells and so reduce the ability to produce insulin. For more information on the molecular basis of diabetes, please play my quiz "Keeping an Eye on Your Blood Sugar".

Answer: A lipid anchor

Prenylation is a term used to describe the attachment of hydrophobic groups to proteins, such as the attachment of a farnesyl or geranyl group (both lipids), which can then link the associated protein to the plasma membrane. The benefit of this localisation is that certain proteins are in close proximity to one another and so can communicate rapidly upon receiving the appropriate stimulus.

Answer: Cholesterol

Cholesterol is usually referred to as either good (high density) or bad (low or very low density). Despite this, it is generally seen as a bad thing, since its levels are elevated in many human beings. This stigma is undeserved however, as cholesterol is an important component of our cell membranes and we would quickly die without it.

It forms the basis for the synthesis of several hormones such as testosterone and estrogen. Estrogen in particular has been shown in recent years to contribute to wound repair and its reduction following menopause is believed to be the primary cause of related diseases such as osteoporosis.