Which of these is not a lipid?

a) RNA.

b) Phospholipid.

c) Steroids.

d) Cholesterol.

e) Wax.

Diagram of an RNA nucleotide
Diagram of an RNA nucleotide

The correct answer is a) RNA

Lipids are organic molecules that contain fatty acids that are bonded to a glycerol molecule. They often consist of large chains of carbon and hydrogen atoms. There are many examples of fats found in cells and tissues of living organisms.

Examples of molecules that are fats include wax, steroids, and cholesterol. Phospholipids also have lipids in their structure, which are attached to phosphate.

RNA is not a fat molecule, but in fact is a nucleic acid. These molecules are made of basic units known as nucleotides. Each of these units is comprised of a nitrogenous base linked to a sugar which in turn is linked to a phosphate group.

There are three types of nucleotides: DNA, RNA, and ATP. The RNA is ribonucleic acid and has a similar structure to a DNA nucleotide.

The RNA though has a ribose sugar that is bonded to a nitrogen base rather than deoxyribose sugar. Protein synthesis is the function of this nucleic acid in which the code from DNA is copied and translated to form a polypeptide.

ATP is the energy molecule of the cell which contains stored energy in the phosphate bonds. This molecule provides energy for the cell and can be reformed during cellular respiration in the cell.

Biological molecules

There are four biological macromolecules that make up living organisms. These molecules are all organic, that is, they all contain carbon in the molecular structure.  These macromolecules are carbohydrates, proteins, lipids, and nucleic acids.

Carbohydrates are the sugars and starches that provide energy by being broken down during respiration. Lipids are fats which have a number of functions from being a source of stored energy to being important as signaling molecules.

Proteins have a range of functions including as catalysts of biological reactions. Nucleic acids carry the genetic material, help proteins to be made, or serve as energy carriers for cells.


The carbohydrates all have oxygen, hydrogen and carbon atoms in their structure. The ratio of hydrogen to oxygen atoms tends to be similar in all carbs.

These molecules are important in providing energy and forming structural elements of organisms. For instance, cellulose is a carbohydrate that forms the cell walls of plant cells while chitin forms the walls of fungal cells.


The basic unit of proteins is the amino acid which is a molecule that always has an amino group present. Polypeptides can fold and form complex structures and have numerous functions in the cell. They form receptors at the surfaces of cells to which other molecules can attach, triggering responses in the cell.

In addition, some form protein channels which allow the transport of substances into and out of a cell. Others are enzymes which speed up cellular reactions necessary for cells to stay alive and function.

Each amino acid has a different functional group attached which gives it specific properties. These acids are carried to ribosomes by transfer RNA where they line up during protein synthesis. In this way, the genetic code is expressed as a protein.

Nucleic acids

These are all molecules which have some type of nucleotide structure that is linked together.  ATP is the energy molecule of the cell that carries energy in the three phosphate bonds and is being continually used and regenerated in cells.

This molecule adenosine triphosphate (ATP) actually can be broken down to form ADP which has only two phosphates present. When this happens, energy is released. The cell relies on many phosphorylation and dephosphorylation reactions for both cell signaling and for cellular metabolism.

RNA is the biological macromolecule that works to translate strands of DNA into a protein. That is they act to express the code that is found on DNA. The RNA is very similar in structure to DNA but has a different sugar and one of the nitrogen bases is different.

Deoxyribonucleic acid nucleotides all have one of four possible nitrogenous bases: adenine, thymine, cytosine, and guanine. In comparison, ribonucleic acid has adenine, cytosine, and guanine, but instead of thymine, it has uracil.

This molecule also has a pentose sugar and a very reactive hydroxyl group enabling reactions of hydrolysis to happen easily. Compared with the DNA molecule, this molecule does not form a double helix but is a single polynucleotide strand.


These are complex organic compounds that consist of fatty acids and a glycerol. The fatty acid consists of long chains of hydrocarbons which link to a carboxyl group.

The hydrocarbons are really carbons and hydrogen atoms that are bonded together. In some cases, there are single bonds, in others, double bonds.

Fats are very important, and in fact, most of the plasma membrane of cells consists of lipids that are bonded to phosphates. These fatty acid tails are hydrophobic and face inwards, and have phosphate heads that point outwards.

Waxes and steroids are also lipid molecules. The wax molecules are made of fatty acids linked to alcohol compounds. The cuticle of a plant is a good example of a wax, which provides a protective layer and prevents water loss from the epidermal cells.

Sterols such as cholesterol have rings in the structure to which are attached a hydroxyl group and hydrocarbon tail. We often think of the sterol molecule, cholesterol as being bad for us, but the reality is that it is an important molecule that is needed by the cell membrane.

In fact, it is found in the cell membrane where it provides a degree of flexibility to the structure. Some cholesterol is also actually good for you and helps to take fats to the liver.

The steroids all have 17 carbons in their structure and are important in making up the hormones such as estradiol and testosterone.


  1. Y Wan, K Chatterjee (2018). RNA. Retrieved from Encyclopedia Britannica.
  2. EA Davidson (2018). Carbohydrate. Retrieved from Encyclopedia Britannica.
  3. F Haurowitz, DE Koshland (2018). Protein. Retrieved from Encyclopedia Britannica.
  4. TE Thompson (2018). Lipid. Retrieved from Encyclopedia Britannica.
  5. RH Kluger, RB Clayton (2018). Steroid. Retrieved from Encyclopedia Britannica.


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