| Date | May 2015 | Marks available | 1 | Reference code | 15M.3.HL.TZ1.7 |
| Level | Higher level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Suggest | Question number | 7 | Adapted from | N/A |
Question
Over a thousand bacterial species occupy the human gut. The gut bacteria show much larger genetic diversity than the host cells. Gut bacteria are vital to proper food digestion and vitamin synthesis. Fecal samples were collected from people in various locations so the genomes of their gut bacteria could be analysed. Bacteria with the same unique DNA sequences were identified as species. The graph shows the number of bacterial species in the digestive tract of people in three different parts of the world.
Identify the age and ethnic group of the individual with the highest diversity of gut bacterial species.
Outline the trends in the number of bacterial species in the digestive tracts of Amerindians.
Distinguish between the trends seen in the three populations.
Suggest two reasons for how the different environments of the three human populations affect the number of bacterial species in their digestive tracts after the age of four.
A century ago, it was discovered that each person belonged to one of four blood types. Now some researchers are reporting that human gut ecosystems fall into three distinct types, each involving a great number of similar bacterial species.
Suggest one medical application based on the knowledge that humans could be typed according to their gut ecosystem.
Markscheme
34 (years old) and Amerindian
Allow answers in the range 33–35.
a. rapid increase in diversity early in life/before age four;
b. (from age four into adulthood) bacterial diversity tends to level off/stay within same (broad) range of diversity/great variation;
a. Amerindians reach highest plateau / Malawians and US reach a lower plateau than the Amerindians;
b. US reach lowest plateau / US reach a lower plateau than the Malawians and Amerindians;
a. US population use disinfectants/antiseptics / pasteurise/sterilise/irradiate food more than populations in Malawi or Amazon;
b. different diets support different populations of bacteria;
c. different soil/water/local animal bacteria;
d. different use of antibiotics;
e. contact with farm/wild animals by rural populations;
a. diets could be tailored to a particular gut ecosystem to maximize digestion / personal health/weight control;
b. antibiotics could be prescribed with minimal effect on gut bacteria/reduce diarrhoea;
c. fecal transplants; (accept other reasonable answers)
Examiners report
The majority of candidates were able to identify the point required from the graph.
Candidates struggled to outline the trend for Amerindians in (i), perhaps due to the variation in the data. There was seldom reference to a plateau.
Likewise, only the better candidates could clearly distinguish the trends in the 3 populations in (ii).
Usually only 1 of the 2 marks available was awarded, often for different food sources that would provide different bacteria. Many candidates did not mention environmental differences but individual habits.
A good number of candidates were able to make reasonable suggestions of how the knowledge of human gut flora could be applied despite this being a novel idea to most.
