| Date | May 2015 | Marks available | 2 | Reference code | 15M.3.sl.TZ1.4 |
| Level | SL | Paper | 3 | Time zone | TZ1 |
| Command term | Deduce | Question number | 4 | Adapted from | N/A |
Question
The building blocks of human proteins are the 2-amino acids with the general formula H2N–CHR–COOH, where R represents a side-chain specific to each amino acid. A list of these amino acids and their isoelectric points is given in table 19 of the data booklet.
State why they are called 2-amino acids.
Identify the amino acid with the empirical formula C3H7ON2
Deduce the structure of valine in a solution with a pH of 4.0.
Deduce the primary structures of the tripeptides formed by reacting together one molecule of each of the amino acids aspartic acid (Asp), glutamine (Gln) and histidine (His), using three-letter abbreviations to represent the amino acids.
Proteins carry out a number of important functions in the body. State the function of collagen.
Markscheme
NH2/amino group at C2 (while C of COOH/carboxyl/carboxylic acid is C1) / OWTTE;
Accept amine for NH2.
arginine/Arg;
;
Structural formulas of cation and zwitterion are required for the mark.
Accept structural formula of zwitterion alone (as it’s the dominant form).
Accept structural formula of cation alone (though lower in concentration than zwitterion based on equilibrium and pH calculations).
Accept full or condensed structural formula(s).
Asp–Gln–His;
Asp–His–Gln;
Gln–Asp–His;
Gln–His–Asp;
His–Asp–Gln;
His–Gln–Asp;
Award [2] for all six correct, [1] for five, four or three.
gives strength to tendons/bones/ligament/skin/cornea/cartilage/blood vessels / connective tissue;
Accept “elasticity” for “strength” but do not accept answers such as “protects bones” etc.
Accept just “structural”.
Examiners report
About a fifth of the candidates knew the reason why amino acids are called 2-amino acids. Some candidates attributed the ‘2’ to the position of the R group and others to the presence of two functional groups.
About half of the candidates were able to identify arginine as the amino acid with the empirical formula C3H7ON2.
The markscheme includes both the zwitterion and the cation, as both species exist in equilibrium at pH 4.0 (with the zwitterion having a higher concentration). Very few candidates gave the structure of the cation of valine. Some gave the structure of the zwitterion, however, the majority of candidates copied the structure of valine from the data booklet, a structure that does not exist in solution.
About a quarter of the candidates understood what was required by the question and gave the six tripeptide chains using three-letter abbreviations to represent the amino acids. Many candidates gave lots of detail for just one tripeptide.
About three quarters of the candidates stated the function of collagen correctly. Some candidates thought it provided the body with energy and others seemed to confuse it with keratin stating that it builds hair and nails.
