Year 8 Science · Unit 1 · Lesson 11
Challenge Worksheet
Learning Goals
Find the errors
This paragraph has exactly four errors. Underline each one and correct it below.
A student’s summary
When you eat a slice of bread, the starch in it passes straight into your blood without being changed. Chewing is not really part of digestion, because no chemicals are involved. In the stomach, acid and enzymes break the food down further. Most absorption also happens in the stomach, where food waits until the blood is ready to collect it. The large intestine takes back water, and what is left leaves the body as faeces. The digestive system and the circulatory system work completely separately.
1. Write the four corrections.
Investigation: a model gut
Visking tubing is a thin skin with microscopic holes, used here to model the wall of the small intestine. Three bags of tubing were tied shut and each put in a beaker of 100 mL of water at 37 °C. Bag A held starch and glucose. Bag B held starch only. Bag C held starch and amylase (the enzyme in saliva). Every 10 minutes the water outside each bag was tested for sugar and for starch.
| Time (min) | Bag A: starch + glucose | Bag B: starch only | Bag C: starch + amylase |
|---|---|---|---|
| 0 | 0 | 0 | 0 |
| 10 | 0.6 | 0 | 0.1 |
| 20 | 1.1 | 0 | 0.4 |
| 30 | 1.5 | 0 | 0.8 |
Starch was never found in the water outside any bag. Source: sample class results.
2. (a) Calculate the mean rate at which sugar appeared outside bag A over the 30 minutes, in grams per litre per minute. (b) At 30 minutes, how many times as much sugar was outside bag A as outside bag C? Give your answer to one decimal place.
(a) g/L per min
(b) times
3. Explain why starch never got out of any bag, and why sugar appeared outside bag C even though no sugar was put into it. Use the words digestion and absorption.
4. Which bag is the control, and why is it needed? What part of the body does the water outside the bags represent?
Evaluate the claim
Someone claims
“Sports drinks contain glucose, which is already small enough to be absorbed. So a person could live on sports drinks alone and would not need a digestive system at all.”
5. Identify one part of the claim that is supported, using evidence from Table 1. Then identify two errors in the claim and explain the correct science.
6. After an operation that removed a large part of his small intestine, Mr Nguyen eats normal meals but keeps losing weight. A visitor says: “That can’t be his gut’s fault. His stomach still digests food perfectly well.” Evaluate the visitor’s claim.
Find the errors
1. (i) Starch must first be digested into small sugar particles before it can enter the blood. (ii) Chewing is digestion: it breaks food into smaller pieces (and saliva starts breaking down starch). (iii) Most absorption happens in the small intestine, through the villi. (iv) The systems work together: absorbed nutrients enter the blood, which carries them to cells. [4 marks: 1 per correction]
Your Turn
2. (a) 1.5 ÷ 30 = 0.05 g/L per minute. (b) 1.5 ÷ 0.8 = 1.875, so about 1.9 times as much. [3 marks: 1 (a); 1 working (b); 1 (b) to 1 d.p.]
3. Starch particles are too large to pass through the tiny holes, just as undigested starch cannot be absorbed through the gut wall. In bag C, amylase digested starch into small sugar particles that could pass out (be “absorbed”); sugar appeared more slowly than in A because the starch had to be digested first. [4 marks: 1 per idea]
4. Bag B: it shows starch alone gives no sugar outside, so the sugar outside C must come from amylase digesting starch. The water represents the blood. [2 marks: 1 control with reason, 1 blood]
Evaluate the claim
5. Supported: glucose needs no digestion and can be absorbed as it is (it left bag A with no enzyme). Errors, any two: absorption is itself a job of the digestive system (villi of the small intestine); the body needs many other nutrients (for example for growth and repair) from foods that must be digested; the gut is still needed to take in water and pass out waste. [3 marks: 1 supported part with evidence; 1 per explained error, up to 2]
Show What You Know
6. Marking guide (points expected): 1 digestion and absorption are different steps; 1 correct part: his stomach still breaks food down; 1 most absorption happens through the villi of the small intestine; 1 with much of it removed there are fewer villi and less surface, so fewer nutrients are absorbed and many pass out as waste; 1 his cells get fewer nutrients, so his body uses its stores and he loses weight; 1 judgement: the visitor is wrong, because digested food that is not absorbed cannot help his cells. [6 marks]
Common error to watch for: treating digestion as the finish line. Food only helps cells once it is absorbed into the blood.