Respiration, exercise and metabolism – GCSE Biology exam questions with mark scheme

1Grade 4–54 marks

(a) Write the word equation for anaerobic respiration in muscles. [1]

(b) Give two differences between aerobic and anaerobic respiration in humans. [2]

(a) glucose → lactic acid
ignore energy as a product
do not accept carbon dioxide / water / ethanol as a product(1)
(b) any two from:
• aerobic uses oxygen, anaerobic does not
• aerobic transfers / releases much more energy (per glucose)
• aerobic produces carbon dioxide and water, anaerobic (in muscles) produces lactic acid
• aerobic is complete oxidation of glucose, anaerobic is incomplete
each difference must be a comparison (allow it implied by 'only aerobic …')(1)
(b) a second difference from the list(1)
(c) ethanol and carbon dioxide
allow alcohol for ethanol; allow CO₂(1)
Answer: (a) glucose → lactic acid (b) oxygen used/not used; more energy released in aerobic (c) ethanol and carbon dioxide
Examiner tip: Anaerobic respiration in muscles makes lactic acid only – ethanol and carbon dioxide are made by yeast and plant cells.

2Grade 6–7Higher6 marks

A student's breathing rate was 14 breaths per minute at rest and 38 breaths per minute just after a sprint.

(a) Calculate the percentage increase in breathing rate. Give your answer to 3 significant figures. [2]

(b) The student's breathing rate stayed high for several minutes after the sprint.

Explain why. [4]

(a) (38 − 14) ÷ 14 × 100(1)
(a) 171 (%)
allow 171.4 / 171.43 for the first mark only (not 3 s.f.)(1)
(b) during the sprint muscles respired anaerobically (not enough oxygen) so lactic acid built up(1)
(b) (this creates) an oxygen debt: the extra oxygen needed to react with / remove the lactic acid
allow to repay the oxygen debt(1)
(b) blood (flowing through the muscles) carries the lactic acid to the liver(1)
(b) in the liver lactic acid is converted back into glucose, which needs oxygen (so extra oxygen must be breathed in)
allow lactic acid reacts with oxygen / is oxidised (to carbon dioxide and water)(1)
Answer: (a) 171% (b) anaerobic respiration made lactic acid → oxygen debt; lactic acid carried to liver and converted to glucose using oxygen
Examiner tip: Percentage increase divides by the ORIGINAL value (14), not the new value.

3Grade 4–54 marks

(a) What is meant by metabolism? [1]

(b) Give three examples of metabolic reactions. [3]

(a) the sum of all the (chemical) reactions in a cell or the body
allow all the reactions in a cell / organism(1)
(b) any three from, one mark each:
• conversion of glucose to starch / glycogen / cellulose
• formation of lipids from glycerol and fatty acids
• use of glucose and nitrate ions to make amino acids / proteins
• respiration (aerobic or anaerobic)
• breakdown of excess proteins / amino acids to form urea
allow other correct named metabolic reactions, e.g. photosynthesis
max 1 mark for glucose → starch / glycogen / cellulose (one spec example); max 1 mark for respiration (aerobic and anaerobic count as one)(1)
(b) a second example from the list(1)
(b) a third example from the list(1)
Answer: (a) sum of all reactions in a cell/body (b) e.g. glucose → starch/glycogen/cellulose; lipid synthesis; amino acid/protein synthesis; respiration; urea formation
Examiner tip: Metabolism means ALL the reactions – 'a reaction in the body' or 'how fast you use energy' does not score.

4Grade 1–3Foundation6 marks

Respiration happens continuously in the cells of all living organisms.

(a) Complete the word equation for aerobic respiration. [2]

glucose + ____________ → carbon dioxide + ____________

(b) The symbol equation for aerobic respiration is:

$$\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O}$$

Which formula in this equation represents glucose? [1]

(d) Give two ways that organisms use the energy transferred by respiration. [2]

(a) oxygen (first gap)
allow O₂(1)
(a) water (second gap)
allow H₂O
ignore energy(1)
(b) $\mathrm{C_6H_{12}O_6}$(1)
(c) energy is transferred / released / given out to the surroundings
allow heat / thermal energy is released
do not accept 'energy is made / produced'(1)
(d) any two from:
• to build larger molecules from smaller ones / for chemical reactions such as making proteins
• for movement / muscle contraction
• to keep warm / maintain body temperature
allow other correct examples, e.g. active transport
ignore 'for energy' / 'to live'(1)
(d) a second use from the list(1)
Answer: (a) oxygen; water (b) C₆H₁₂O₆ (c) energy is transferred to the surroundings (d) e.g. building larger molecules; movement; keeping warm
Examiner tip: Never write that respiration 'makes' or 'produces' energy – energy is transferred (released), and examiners will not credit 'produces energy'.

5Grade 4–56 marks

A student measured her breathing at rest and during a fitness class.

	Breathing rate in breaths per minute	Volume of each breath in dm³
At rest	12	0.5
During exercise	32	2.5

(a) Calculate the volume of air the student breathed in per minute:

(i) at rest [1]

(ii) during exercise. [1]

(b) How many times greater was the volume of air breathed in per minute during exercise than at rest? [1]

(a)(i) 6 (dm³ per minute)(1)
(a)(ii) 80 (dm³ per minute)(1)
(b) 13.3 (times)
allow 13 / 13.33
allow ecf: their (a)(ii) ÷ their (a)(i)(1)
(c) muscles contract more / faster so they need more energy
allow muscles respire faster / more(1)
(c) (so) more oxygen is needed (for aerobic respiration)
allow more oxygen is taken into the blood / supplied to the muscles(1)
(c) (and) more carbon dioxide needs to be removed (from the blood / body)
allow to prevent / reduce anaerobic respiration / lactic acid build-up(1)
Answer: (a)(i) 6 dm³ (ii) 80 dm³ (b) 13.3 times (c) muscles respire faster for more energy; need more oxygen; more CO₂ to remove
Examiner tip: In part (c) link the extra breathing to RESPIRATION in the muscles – 'to get more oxygen' alone only scores one mark.

6Grade 4–56 marks

The table compares three types of respiration.

(a) Complete the table by putting a tick (✓) in each box where the statement is true. [4]

Statement	Aerobic respiration in humans	Anaerobic respiration in human muscle	Anaerobic respiration in yeast
Uses oxygen
Produces carbon dioxide
Produces lactic acid
Produces ethanol

(b) What name is given to anaerobic respiration in yeast cells? [1]

(a) 'uses oxygen': tick in aerobic only(1)
(a) 'produces carbon dioxide': ticks in aerobic and yeast only(1)
(a) 'produces lactic acid': tick in human muscle (anaerobic) only(1)
(a) 'produces ethanol': tick in yeast only
each row must be fully correct with no extra ticks for the mark(1)
(b) fermentation(1)
(c) glucose is only partly / incompletely oxidised (broken down)
allow the product (lactic acid / ethanol) still contains (a lot of chemical) energy
ignore 'no oxygen' on its own(1)
Answer: (a) oxygen: aerobic only; CO₂: aerobic and yeast; lactic acid: muscle only; ethanol: yeast only (b) fermentation (c) glucose is only partially oxidised
Examiner tip: Carbon dioxide is made by BOTH aerobic respiration and yeast fermentation – many students forget to tick both boxes.

7Grade 6–7Higher6 marks

An athlete ran a 400 m race. Her oxygen uptake was then measured during each minute of recovery.

Her oxygen uptake at rest is 0.3 dm³ per minute.

Minute of recovery	1	2	3	4	5	6	7	8
Oxygen uptake in dm³	2.4	1.6	1.1	0.8	0.6	0.4	0.3	0.3

(a) The oxygen debt is the extra oxygen taken in above the resting uptake. Calculate the athlete's oxygen debt. [2]

(b) The total oxygen uptake during the 8 minutes of recovery was 7.5 dm³.

Calculate the percentage of this oxygen that was used to repay the oxygen debt. [2]

(d) What is the extra oxygen used for during recovery? [1]

(a) subtracting resting uptake from each minute and adding, e.g. 2.1 + 1.3 + 0.8 + 0.5 + 0.3 + 0.1 (+ 0 + 0)
or 7.5 − (8 × 0.3)(1)
(a) 5.1 (dm³)(1)
(b) 5.1 ÷ 7.5 × 100
allow ecf from their (a)(1)
(b) 68 (%)(1)
(c) the oxygen debt had been repaid / all the lactic acid had been removed
allow oxygen uptake had returned to the resting value / only oxygen for normal (resting) respiration was needed(1)
(d) to react with / oxidise the lactic acid (to remove it from cells)
allow to convert lactic acid back into glucose (in the liver)
ignore 'to repay the oxygen debt' unqualified(1)
Answer: (a) 5.1 dm³ (b) 68% (c) oxygen debt repaid / back to resting uptake (d) to react with lactic acid / convert it to glucose in the liver
Examiner tip: Subtract the RESTING uptake each minute before adding – adding the whole column (7.5 dm³) gives the total oxygen, not the debt.

8Grade 4–56 marks

Metabolism is the sum of all the reactions in a cell or the body. Plants use the glucose made in photosynthesis in many metabolic reactions.

(a) Name the carbohydrate made from glucose that strengthens plant cell walls. [1]

(b) Many seeds store lipids. Each lipid molecule is made from one molecule of glycerol and three molecules of fatty acid.

How many fatty acid molecules are needed to make 25 lipid molecules? [1]

(d) The metabolic reactions in cells are controlled by enzymes. Which process transfers the energy needed for these reactions? [1]

(e) In humans, excess proteins are broken down. Name the waste substance that is formed. [1]

(a) cellulose(1)
(b) 75(1)
(c) nitrate ions are combined with glucose to make amino acids
allow nitrates are needed to make amino acids(1)
(c) (amino acids are used) to make proteins (needed for growth / enzymes)
allow 'cannot make protein' for this mark(1)
(d) respiration
allow aerobic respiration(1)
(e) urea
do not accept urine / uric acid(1)
Answer: (a) cellulose (b) 75 (c) nitrate + glucose → amino acids → proteins for growth (d) respiration (e) urea
Examiner tip: Write UREA, not urine – urea is the substance made in the liver; urine is the liquid the kidneys excrete.

9Grade 6–77 marks

Two runners, A and B, ran on a treadmill. The speed was increased every few minutes and a small blood sample was taken at each speed.

The graph shows the concentration of lactic acid in their blood at each speed.

(a) Describe how the blood lactic acid concentration of runner A changes as running speed increases from 6 km/h to 16 km/h. [2]

(b) Coaches use a lactic acid concentration of 4 mmol/dm³ as a guide to the fastest speed a runner can keep up.

Use the graph to calculate how much faster runner B can run than runner A before reaching this concentration. [2]

(d) Runner B has trained for several years. Suggest one reason why runner B can run at a higher speed before lactic acid builds up. [1]

(a) stays low / increases only slightly up to about 10 km/h
allow stays roughly constant (at about 1–1.4 mmol/dm³) up to 10 km/h(1)
(a) then increases rapidly / steeply (from about 10–11 km/h) to about 10 mmol/dm³ at 16 km/h
allow correct data quote, e.g. from 1.4 to 9.8 mmol/dm³ between 10 and 16 km/h (allow readings of 9.6–10 at 16 km/h)
ignore 'it increases' without reference to rate(1)
(b) A reaches 4 mmol/dm³ at 13 km/h and B at 16 km/h(1)
(b) 3 (km/h)
allow ecf: correct subtraction of their two readings(1)
(c) (at high speeds) the muscles cannot get enough oxygen / oxygen is not supplied fast enough (for aerobic respiration)
allow demand for oxygen is greater than supply(1)
(c) so muscles respire anaerobically, producing lactic acid / glucose is converted to lactic acid
do not accept 'anaerobic respiration produces ethanol'(1)
(d) runner B can deliver more oxygen to the muscles (so respires aerobically at higher speeds)
e.g. larger / stronger heart pumps more blood per beat; more blood vessels / capillaries in muscles; larger lung / breath volume
allow lactic acid removed faster (by blood / liver)
ignore 'fitter' unqualified(1)
Answer: (a) stays low (~1–1.4) to 10 km/h then rises steeply to 9.8 mmol/dm³ (b) 16 − 13 = 3 km/h (c) not enough oxygen to muscles → anaerobic respiration → lactic acid (d) e.g. stronger heart delivers more oxygen to muscles
Examiner tip: For 'describe' questions split the pattern into its two parts and quote a value from the graph – 'it goes up' scores at most one mark.

10Grade 6–76 marks

Yeast respires anaerobically to produce carbon dioxide. The diagram shows apparatus that can be used to collect the carbon dioxide.

Plan an investigation to find out how the concentration of sugar solution affects the rate of anaerobic respiration (fermentation) in yeast.

Your plan should give valid results. [6]

make up a range of sugar concentrations (at least five), e.g. 0, 2, 4, 6, 8 and 10%(1)
use the same volume of each sugar solution, e.g. 25 cm³ (measured with a measuring cylinder / pipette)(1)
add the same mass / volume of yeast (suspension) to each, e.g. 1 g / 5 cm³(1)
keep the temperature the same using the water bath, e.g. 35 °C, and check with the thermometer(1)
leave the flask in the water bath for a few minutes to reach the temperature before starting to collect gas(1)
add a layer of oil on the surface of the mixture / avoid shaking, to keep conditions anaerobic(1)
insert the bung quickly and start the stop clock(1)
record the volume of gas in the gas syringe after a fixed time, e.g. 10 minutes
allow count bubbles in a set time if apparatus adapted(1)
rate = volume of gas ÷ time (cm³ per minute)(1)
repeat for each concentration (e.g. three times) and calculate a mean; identify and ignore anomalies(1)
use the same type / batch of yeast each time(1)
plot / compare rate against sugar concentration to find the relationship(1)

Level	Marks	What it looks like
Level 3	5–6	A clear, logically sequenced plan that another student could follow to get valid results: a suitable range of sugar concentrations, a fixed time over which the gas volume is measured, at least two control variables (with values or methods of control), and repeats with means or a way of comparing rates.
Level 2	3–4	A workable plan in a mostly sensible order with the independent variable changed and gas volume measured, but with some omissions, e.g. only one control variable, no fixed time, or no repeats.
Level 1	1–2	Some relevant steps, e.g. 'use different sugar solutions and measure the gas', but the plan is unstructured and could not be followed to give valid results.
No relevant content	0	Nothing worthy of credit.

Answer: A valid plan: range of sugar concentrations; same volume of solution, mass of yeast, temperature (water bath) and time; measure CO₂ volume in gas syringe in a fixed time; calculate rate; repeat and take means.
Examiner tip: Top-band plans state the independent variable range, at least two control variables with values, exactly what is measured and over what time, and repeats – vague plans like 'see how much gas is made' stay in Level 1.

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