Primary structure simply describes the order of amino acids in a polypeptide chain. Tertiary structures describe global folding of the entire chain, which may be made up of a multitude of secondary structures like alpha helices or beta sheets. Quaternary structure describes the position of numerous subunits in a protein complex comprised of two or more smaller protein. Huge multiunit proteins are ordered by supramolecular structure. 

Sucrose is one of the more common sugars, and is composed of glucose and fructose. Sucrose is also known as table sugar.

Galactose and glucose are the components of lactose. Maltose is made up of two glucose molecules. 

Both starch and cellulose are polysaccharides composed of glucose monosaccharides, but only starch can be digested by humans. The reason for this lies in the type of glycosidic linkages between glucose monomers. Starch has alpha (1-4) linkages which can be broken down, but cellulose has beta (1-4) linkages. Humans do not have the necessary enzyme to digest this type of glycosidic linkage.

A ketose is defined as any monsaccharide that has a ketone functional group while in its linear form. Fructose is a ketose, while the other three options are all aldoses.

Uracil is one of the nucleotide bases that composes RNA. It is replaced by thymine in DNA.

Uracil, thymine, and cytosine are pyrimidine residues, capable of bonding and pairing with the purines adenine and guanine via hydrogen bonding. During DNA replication, thymine matches with adenine. During transcription, uracil matches with adenine.

Nucleotides (DNA monomers) and ribonucleotides (RNA monomers) are formed from a pentose sugar, phosphate group, and nitrogenous base. Each nitrogenous base has a complement that allows it to form hydrogen bonds to the template strand. This allows for the proper sequence of genetic code in DNA replication and RNA transcription.

Purine residues will always pair with pyrimidine residues. The purines are adenine and guanine. The pyrimidines are cytosine and thymine in DNA, and cytosine and uracil in RNA. Adenine will match with thymine or uracil, forming two hydrogen bonds, while cytosine will match with guanine to form three hydrogen bonds.

Template strand cytosine and adenine are methylated in DNA replication, which allows DNA mismatch repair enzymes to distinguish between old and new DNA strands.

In contrast, histone acetylation relaxes DNA coiling and allows for the DNA to be transcribed.

You can remember that methylation makes DNA mute, and acetylation makes DNA active.

Purines are defined by their two-ring structure. A six-member ring with two amine groups and a five-member ring with two amino groups join to form each purine molecule. Addition substituents on the rings (often ketones or other amines) determine purine identity.

Glycine, aspartate, and glutamine are necessary for purine synthesis, along with phosphoribosyl pyrophosphate (PRPP). Glycine is incorporated into the final purine product structure, while glutamine is converted to glutamate and aspartate is converted to fumarate. The final purine product is used to make useful molecules, such as adenine and guanine for nucleotide synthesis.

Membrane fluidity can be buffered by cholesterol in both warm and cold environments. At high temperatures cholesterol raises the melting point, while at lower temperatures cholesterol prevents the formation of crystalline structures between phospholipids.

The correct answer is a glycerol head containing a phosphate group and two fatty acids. Phospho refers to the phosphate group and lipid refers to the glycerol backbone attached to two fatty acid chains.