Region pH mouth 7 stomach 2 duodenum 8 ileum 8 The conditions inside the alimentary canal are not the same all the way along it. Each region has its own pH,...
Question 1 Report
| Region | pH |
|---|---|
| mouth | 7 |
| stomach | 2 |
| duodenum | 8 |
| ileum | 8 |
The conditions inside the alimentary canal are not the same all the way along it. Each region has its own pH, which suits the enzymes that work there. A student used a pH probe to measure the pH of the contents of four different regions of the canal a short time after a person had eaten a meal. The results are shown in Table 9.1.
(a) State the region shown in the table that has the lowest pH. [1]
(b) Name an enzyme that works best in the conditions found in the stomach. [1]
(c) Explain how the low pH of the stomach both aids digestion and helps to protect the body. [3]
(d) Describe how the pH changes between the stomach and the duodenum, and explain how this change is brought about. [3]
(e) Name the alkaline secretion that helps to raise the pH in the duodenum. [1]
(f) Explain why enzymes that act in the duodenum would not work well at pH 2. [3]
(g) State the general term for the breakdown of large food molecules into small soluble ones. [1]
Answer Details
This question tests how pH varies along the alimentary canal and why each region suits its own enzymes. A pH probe reading shows that conditions change sharply from the acidic stomach to the alkaline duodenum.
(a) The region with the lowest pH is the stomach [1].
(b) An enzyme that works best in stomach conditions is pepsin [1].
(c) The low pH of the stomach both aids digestion and protects the body: the acid provides the optimum pH for pepsin, so protein digestion is fast [1]; it kills bacteria / pathogens in the food [1]; thereby protecting the body from infection [1].
(d) Between the stomach and duodenum the pH rises / becomes alkaline (from about 2 to 8) [1]; because alkaline bile and pancreatic juice enter the duodenum [1]; and these neutralise the acid from the stomach [1]. The change is a deliberate switch of conditions to suit intestinal enzymes.
(e) The alkaline secretion that raises the pH is bile / pancreatic juice [1].
(f) Duodenal enzymes would not work at pH 2 because their optimum pH is alkaline (about 8) [1]; at pH 2 the active site changes shape / the enzyme is denatured [1]; so substrate no longer fits and digestion stops [1].
(g) The breakdown of large food molecules into small soluble ones is called digestion [1].
Exam tip: connect each region's pH to the enzymes released there - acid for pepsin in the stomach, alkali for intestinal enzymes - and remember that neutralisation by bile and pancreatic juice is what drives the pH up.
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