Question 23M.2.HL.TZ2.7
| Date | May 2023 | Marks available | [Maximum mark: 15] | Reference code | 23M.2.HL.TZ2.7 |
| Level | HL | Paper | 2 | Time zone | TZ2 |
| Command term | Describe, Explain, Outline | Question number | 7 | Adapted from | N/A |
Proteins are an extraordinarily diverse group of carbon compounds that have a wide range of roles in cells.
Describe the structure of proteins, including features that are common to all proteins and features that vary.
[7]
- composed of one or more polypeptides / some are single polypeptides others made of 2 or more polypeptide chains / all proteins consist of, at least, one polypeptide chain;
- a polypeptide/protein is a chain of amino acids;
- (chains of) amino acids linked by peptide bonds;
- twenty different amino acids/ amino acids have different R groups/ R groups can be hydrophilic/polar or hydrophobic/non-polar;
- primary structure is the sequence/order (and number) of amino acids in the polypeptide;
- any sequence/order of amino acids could be linked together hence many possible polypeptides/proteins;
- secondary structure is the formation of alpha/ α helices and beta/ β pleated sheets;
- secondary structure stabilized by hydrogen bonding;
- tertiary structure is the further folding of the polypeptide / tertiary structure is the three-dimensional (3-D) conformation/structure/shape of a protein;
- tertiary structure stabilized by interactions / ionic bonds/hydrogen bonds/disulfide bridges between R groups;
- fibrous or globular are the two main classes of conformation/three-dimensional/ 3-D structure/shape;
- conformation/three-dimensional (3-D) structure determined by amino acid sequence;
- quaternary structure if two or more polypeptides are linked to form a (single) protein
OR
quaternary structure if a prosthetic group is linked (to form a conjugated protein);
Accept clearly annotated diagrams.
Mpc: accept a clearly annotated diagram of the bond between 2 drawn amino acids.
Most candidates performed well in this question, many achieving the maximum number of points available. It was common to see candidates indicating the quaternary structure of a protein consisting of many "proteins" put together. Very few used the term "prosthetic group". Some answers that scored poorly did not refer to the structure of proteins, instead there were long explanations about the different functions of proteins.
Explain the roles of proteins in the movement of substances across membranes.
[5]
- pump proteins for active transport;
- movement of particles against the concentration gradient /from lower to higher concentration;
- requires ATP/energy (from ATP);
- channel proteins for facilitated diffusion;
- particles diffuse along the concentration gradient /from a higher to a lower concentration;
- channel proteins allow ions/charged/hydrophilic/polar particles to diffuse across (which would otherwise not be able to move across the hydrophobic regions of the membrane);
- aquaporins for movement of water (by osmosis);
- greater membrane permeability to water with more aquaporins;
- proteins that move substances across membranes are integral/intrinsic/transmembrane/ embedded in the phospholipid bilayer;
Do not award points for roles of proteins not involved in the movement of substances across membranes.
Accept clearly annotated diagrams.
Many candidates performed quite well in this question, describing the roles of proteins in the movement of substances across membranes, however, several students confused protein channels with pumps. A few included an annotated diagram of the cell membrane, though it was only useful to award one marking point (showing embedded / transmembrane proteins) if no other indication was made regarding the process, the concentration gradient and/or the use of ATP.
Outline how proteins can be separated by gel electrophoresis.
[3]
- gel electrophoresis involves separating molecules according to their size and charge;
- proteins differ in size (due to differing amino acids) / may be positively or negatively charged;
- place protein sample in a well in a sheet/film/block of gel;
- place the gel in an electrical field/between positive and negative electrodes / an electric current is run through the gel;
- proteins move through the gel;
- separated according to size/small proteins move faster/farther than large proteins;
- size markers/ladder used;
Accept clearly annotated diagrams.
This question about the use of gel electrophoresis to separate proteins was in many cases poorly answered as many candidates referred to DNA profiling, PCR and endonucleases, instead of focusing on the fact that gel electrophoresis involves separating molecules according to their size and charge. Many candidates were still able to get marking points when they mentioned that protein mixtures are placed in a well in a film of gel, that an electric current is applied to the gel and that proteins are separated according to their size, where small proteins travel faster/farther. Another problem observed was students mentioning that gel electrophoresis splits proteins into separate amino acids.
