Year 8 Science · Unit 1 · Lesson 12
Challenge Worksheet
Learning Goals
Find the errors
This paragraph has exactly four errors. Underline each one and correct it below.
A student’s summary
Your kidneys filter your blood and remove urea. The urine they make travels to the bladder through the urethra. The bladder then makes more urine and stores it. The lungs play no part in removing waste, because they only take oxygen in. Sweat from the skin carries some water and salt out of the body. Plants do not need to remove any wastes, because they make their own food.
1. Write the four corrections.
Fact file: a day in the kidneys
Each day, an adult’s kidneys filter a large amount of fluid out of the blood. Most of it is taken back into the blood. What is left becomes urine.
| Substance | In the fluid filtered from the blood | In the urine |
|---|---|---|
| Water (litres per day) | 180 | 1.5 |
| Urea (grams in every 100 mL) | 0.03 | 2.0 |
| Glucose (grams in every 100 mL) | 0.1 | 0 |
Source: approximate, rounded values for a healthy adult; they vary from person to person.
2. (a) How many litres of filtered fluid are taken back into the blood each day? (b) What percentage of the filtered fluid is this? Give your answer to one decimal place. (c) How many times more concentrated is urea in the urine than in the filtered fluid?
(a) L
(b) %
(c) times
3. Glucose is filtered out of the blood but none is in the urine, while urea ends up far more concentrated. Explain what this shows about how the kidneys treat useful substances and wastes, and why this matters for the body’s balance.
Evaluate the claim
A wellness post claims
“Sweating is the body’s main way of getting rid of waste. So sitting in a sauna and sweating as much as you can is the best way to clean out your blood.”
4. Identify one part of the claim that is supported by science. Then identify two errors and explain the correct science.
5. A brain cell needs a steady supply of glucose and oxygen, and it makes wastes such as carbon dioxide. Explain how the digestive, respiratory, circulatory and excretory systems work together to keep this cell working. Then explain why the cell would still be harmed if only the excretory system failed.
Find the errors
1. (i) Urine reaches the bladder through the ureters; the urethra carries it out. (ii) The bladder only stores urine; the kidneys make it. (iii) The lungs do remove waste: carbon dioxide. (iv) Plants do make wastes (such as carbon dioxide and oxygen) and release them through their stomata. [4 marks: 1 per correction]
Your Turn
2. (a) 180 − 1.5 = 178.5 L. (b) 178.5 ÷ 180 × 100 = 99.2%. (c) 2.0 ÷ 0.03 = 66.7, so about 67 times. [4 marks: 1 (a); 1 working for (b); 1 (b) to one decimal place; 1 (c)]
3. The kidneys take useful glucose (and most of the water) back into the blood but leave urea in a small volume of urine. Wastes are removed without losing what cells need, which keeps conditions in the body within useful limits (balance). [3 marks: useful glucose returned; urea removed in urine; link to balance]
Evaluate the claim
4. Supported: sweat does carry some water, salt and a little urea out. Errors, any two: sweat is not the main route (the kidneys remove most urea and the lungs remove carbon dioxide); heavy sweating loses water and salt the body needs, upsetting its balance; the kidneys, not sweat, filter the blood. [3 marks: 1 supported part; 1 per explained error, up to 2]
Show What You Know
5. Marking guide (points expected): 1 digestive (input): digests food and absorbs glucose into the blood; 1 respiratory (input and output): takes in oxygen, removes carbon dioxide; 1 circulatory (transport): carries glucose and oxygen to the cell and wastes away; 1 excretory (output): kidneys remove urea, extra water and salt as urine; 1 if it failed, wastes would build up even though inputs still arrived; 1 the body would lose balance and the cell could not work properly. [6 marks]
Common error to watch for: in Q3, saying the kidneys “filter out only waste”. They filter out a lot of fluid, then take the useful parts back.