| Date | November 2009 | Marks available | 3 | Reference code | 09N.2.SL.TZ0.3 |
| Level | Standard level | Paper | Paper 2 | Time zone | TZ0 |
| Command term | Explain | Question number | 3 | Adapted from | N/A |
Question
Researchers extracted an enzyme from the human digestive system and tested its activity at different pH values on proteins extracted from the blood of cows. The results are shown in the graph below.
Deduce from where in the human digestive system this enzyme was extracted.
Outline the need for enzymes in the digestive system.
State one function of the large intestine.
Explain how the structure of the villus is adapted for absorption.
Markscheme
stomach
enzymes speed up the digestive processes;
(chemical) break down of food/food particles/large molecules;
make soluble products/molecules small enough to be absorbed;
to (re)absorb water/vitamins(s) (e.g. K and B12) / temporary storage of feces
N.B. for each marking point, function should accompany structure.
shape of villus has large surface area to improve absorption / microvilli increase surface area to improve absorption;
thin walls/epithelium to allow fast diffusion;
capillaries/rich blood supply (nearby) to absorb digested food products/ maintain concentration gradient;
lacteal in villus to absorb fatty acids/fats (and carry them away from small intestine);
protein pumps in membrane to carry on active transport / channel proteins in membrane to facilitate diffusion;
large number of mitochondria provide ATP for active transport;
Examiners report
The stomach was correctly mentioned by most students, but a few mentioned the pancreas and even the liver.
Many candidates gave generic descriptions of enzyme properties without any application to digestion, such as speeding up the process. Acceptable answers frequently included the breakdown of food particles/molecules and the notion that the products had to be soluble or small enough to be absorbed.
Not answered particularly well. For those who were correct, absorption of water/vitamins was usually cited. A few mentioned temporary storage of feces.
This question required an explanation involving structure and function. The problem for a number of candidates was that they gave accurate structural details, but neglected to include any reasoning, that is explaining how the structural feature aided absorption. For example, the pencil shape of the villus provides a high surface area to volume ratio which makes absorption easier/faster. For those candidates who supported their answers with reasoning, maximum credit was easily obtained.
