| Date | May 2009 | Marks available | 3 | Reference code | 09M.2.SL.TZ2.3 |
| Level | Standard level | Paper | Paper 2 | Time zone | Time zone 2 |
| Command term | Explain | Question number | 3 | Adapted from | N/A |
Question
Explain how materials are moved across membranes of cells by active transport.
Explain the effects of pH on enzyme catalysed reactions.
Distinguish between the process of anaerobic respiration in yeast and humans.
Markscheme
transport against a concentration gradient / from low to high concentration;
through protein pumps;
uses energy/ATP;
enzymes have a pH optimum;
active site works best at this pH;
activity decreases above and below the optimum;
by interfering with H-bonding/active site structure;
denaturing by extremes of pH so enzyme activity/reaction stops;
yeast: pyruvate to ethanol and carbon dioxide;
humans: pyruvate to lactic acid;
Award [1 max] if products are appropriately linked to organisms without the mention of pyruvate.
Examiners report
Knowledge of the characteristics of active transport was generally well expressed. Many candidates understood that protein pumps, requiring energy were required as opposed to protein channels that may be used in facilitated diffusion.
This question was answered well with candidates aware of the concept of an optimal pH with activity trailing off on either side. The best answers liked this to the structure of the enzyme active site being changed by the changing pH.
Most candidates had no difficulty indicating the end products of respiration. A large number of answers indicated that pyruvate was a common source in each case of respiration, though weaker answers did not.
