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school. “Paper 42” means Paper 4, variant 2.
Practice questions
Original questions written by HomiLabs in the style of the exam, with answers and mark schemes. They are not
Cambridge past paper questions and have not been reviewed by Cambridge. Try each one before opening the answer.
Q1Blood cells, haemoglobin & gas transport · Core · Paper 1 style · 1 mark
(a)What is the role of carbonic anhydrase in red blood cells?[1]
Show answer
(a) B It catalyses the reaction between carbon dioxide and water to form carbonic acid.
B: carbonic anhydrase catalyses CO₂ + H₂O ⇌ H₂CO₃, which dissociates into H⁺ and HCO₃⁻. D is wrong: carbaminohaemoglobin forms by CO₂ combining with amine groups of haemoglobin without this enzyme. C is done by a membrane transport protein (in exchange for Cl⁻).
Q2Blood cells, haemoglobin & gas transport · Core · Paper 2 style · 6 marks
A stained blood smear is viewed with a light microscope. Four cells are described:A – about 7 µm across, no nucleus, pale centre, red/pink.B – about 12 µm across, nucleus with three lobes, granular cytoplasm.C – about 8 µm across, large round nucleus filling almost the whole cell, a thin rim of cytoplasm.D – about 18 µm across, large bean-shaped nucleus, plenty of cytoplasm.
(a)Identify cells A, B, C and D.[4]
(b)State one way in which the shape of cell A is related to its function.[1]
(c)Cell D can leave the blood and develop into a macrophage. State the function of macrophages.[1]
Show answer
(a) A red blood cell (erythrocyte); B neutrophil; C lymphocyte; D monocyte.
B1 each.
(b) Biconcave disc gives a large surface area to volume ratio for fast diffusion of oxygen (also: flexible to squeeze through capillaries; no nucleus leaves more room for haemoglobin).
B1 any one valid structure–function link.
(c) Phagocytosis of pathogens / cell debris, and presenting antigens to lymphocytes.
B1
Q3Blood cells, haemoglobin & gas transport · Standard · Paper 1 style · 1 mark
(a)[1]
Show answer
(a) C The curve shifts to the right and more oxygen is released.
C: the Bohr shift – higher CO₂ partial pressure lowers the pH (more H⁺), reducing haemoglobin's affinity for oxygen, so the curve moves right and oxyhaemoglobin unloads more oxygen at the same partial pressure. D wrongly links a right shift with less unloading.
Q4Blood cells, haemoglobin & gas transport · Standard · Paper 2 style · 7 marks
(a)Describe how most carbon dioxide produced by respiring cells is carried in the blood.[4]
(b)Explain how the hydrogen ions formed in this process cause oxygen to be released from haemoglobin.[2]
(c)State two other ways in which carbon dioxide is carried in the blood.[1]
Show answer
(a) CO₂ diffuses into red blood cells; carbonic anhydrase catalyses CO₂ + H₂O → H₂CO₃; carbonic acid dissociates into H⁺ and HCO₃⁻; HCO₃⁻ diffuses out of the red cell into the plasma (and is carried as hydrogencarbonate in the plasma); Cl⁻ moves into the cell to balance the charge (chloride shift).
1 mark each for any four of: CO₂ enters red blood cells; carbonic anhydrase; carbonic acid formed; dissociates to H⁺ and HCO₃⁻; HCO₃⁻ diffuses out into plasma; chloride ions move in (chloride shift).
(b) H⁺ bind to haemoglobin forming haemoglobinic acid; this changes the shape (tertiary/quaternary structure) of haemoglobin, lowering its affinity for oxygen, so oxyhaemoglobin releases oxygen.
B1 H⁺ combine with haemoglobin / haemoglobinic acid formed. B1 reduces affinity for O₂ so oxygen unloaded (Bohr effect). (Also accept: H⁺ buffered, preventing a fall in blood pH.)
(c) Combined with haemoglobin as carbaminohaemoglobin; dissolved in plasma.
B1 both needed.
Q5Blood cells, haemoglobin & gas transport · Standard · Paper 2 style · 6 marks
When fully saturated, 100 cm³ of blood carries 20.0 cm³ of oxygen. Percentage saturation of haemoglobin:Lungs (pO₂ 13 kPa): 97 %Resting tissue (pO₂ 5 kPa): 70 %Exercising muscle (pO₂ 2.5 kPa): 35 % on the normal curve; 20 % when the high CO₂ in the muscle is taken into account.
(a)Calculate the volume of oxygen released to resting tissues by 100 cm³ of blood.[2]
(b)Calculate how much more oxygen is released by 100 cm³ of blood in exercising muscle, when the Bohr shift is taken into account, than in resting tissue.[2]
(c)Explain why the saturation at 13 kPa is hardly affected by the Bohr shift, but the saturation at 2.5 kPa is reduced from 35 % to 20 %.[2]
(c) At 13 kPa the curve is flat (plateau), so a shift to the right makes little difference and haemoglobin still loads fully in the lungs; at low partial pressures the curve is steep, so the lower affinity causes a large fall in saturation and much more unloading.
B1 lungs: plateau / high pO₂, loading still nearly complete. B1 tissues: steep part of curve, small change in affinity gives large change in saturation.
Q6Blood cells, haemoglobin & gas transport · Challenge · Paper 2 style · 6 marks
(a)Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin. State where its dissociation curve lies compared with the adult curve and explain why this is important for the fetus.[3]
(b)Mature red blood cells have no nucleus and no mitochondria. Suggest how each of these features is an advantage for the transport of oxygen.[3]
Show answer
(a) To the left of the adult curve; at the low partial pressures in the placenta, fetal haemoglobin is more saturated than maternal haemoglobin; so oxygen moves from the mother's oxyhaemoglobin to the fetal haemoglobin.
B1 left. B1 placenta has low pO₂ / mother's haemoglobin unloads there. B1 fetal haemoglobin loads oxygen released by maternal haemoglobin.
(b) No nucleus: more space inside the cell for haemoglobin, so each cell carries more oxygen (and the cell can be a flexible biconcave disc). No mitochondria: the cell does not respire aerobically (it makes ATP by anaerobic respiration / glycolysis), so it does not use any of the oxygen it carries, and all of it can be delivered to the tissues.
B1 no nucleus → more room for haemoglobin. B1 so more oxygen carried per cell (or: flexible / biconcave shape allowed). B1 no mitochondria → no aerobic respiration, so the oxygen carried is not used up by the red cell itself.
Q7Blood cells, haemoglobin & gas transport · Challenge · Paper 2 style · 6 marks
In winter, some families in Quetta heat closed rooms with gas or charcoal heaters. Incomplete combustion releases carbon monoxide. Hospitals treat patients who arrive with headaches and dizziness.
(a)Explain why breathing carbon monoxide reduces the oxygen supplied to tissues.[3]
(b)Suggest why the first symptoms are headache and dizziness.[1]
(c)Suggest why the patients were given pure oxygen to breathe.[2]
Show answer
(a) Carbon monoxide combines with haemoglobin (at the haem/iron sites) to form carboxyhaemoglobin; it binds much more strongly than oxygen and is not easily released; so fewer sites are available to carry oxygen and less oxygen reaches respiring tissues (it also makes the remaining haemoglobin release oxygen less readily).
1 mark each for any three of: forms carboxyhaemoglobin; binds to haem / same site as oxygen; binds more strongly / almost irreversibly; less haemoglobin available for oxygen transport; less oxygen delivered / less aerobic respiration.
(b) The brain has a high demand for oxygen for aerobic respiration, so it is affected first by reduced oxygen supply.
B1
(c) A high partial pressure of oxygen in the alveoli competes with carbon monoxide for haemoglobin, speeding the replacement of CO; and more oxygen dissolves in plasma, raising oxygen supply to tissues.
B1 high pO₂ helps displace CO from haemoglobin. B1 more oxygen carried (dissolved / bound) to tissues.
Q8Heart & cardiac cycle · Core · Paper 1 style · 1 mark
(a)Which valves are closed during ventricular systole, while blood is being pumped into the arteries?[1]
Show answer
(a) A atrioventricular valves only
A: when ventricular pressure exceeds atrial pressure the atrioventricular (tricuspid and bicuspid) valves close; when it exceeds arterial pressure the semilunar valves open so blood leaves. C is true only during the brief isovolumetric phase before ejection.
Q9Heart & cardiac cycle · Core · Paper 2 style · 6 marks
(a)State where the sinoatrial node (SAN) is found.[1]
(b)Describe how the heartbeat is initiated and coordinated after the SAN produces a wave of excitation.[4]
(c)Explain why there is a delay at the AVN.[1]
Show answer
(a) In the wall of the right atrium.
B1
(b) Excitation spreads across both atria, which contract (atrial systole); a band of non-conducting tissue stops it spreading directly to the ventricles; the atrioventricular node (AVN) delays the impulse (about 0.1 s); excitation passes down the bundle of His / Purkyne tissue in the septum to the apex; the ventricles contract from the apex upwards.
1 mark each for any four of: spreads over atrial walls → atria contract; non-conducting (fibrous) tissue between atria and ventricles; AVN; delay at AVN; Purkyne tissue / bundle of His down septum; to the apex, so the ventricles contract from the apex upwards.
(c) So the atria finish contracting and emptying blood into the ventricles before the ventricles contract.
B1
Q10Heart & cardiac cycle · Core · Paper 2 style · 6 marks
(a)Name the blood vessel that carries blood from the right ventricle and the vessel that brings blood into the right atrium from the body.[2]
(b)State the function of the tendons (chordae tendineae) attached to the atrioventricular valves.[1]
(c)Mammals have a closed double circulation. Explain what is meant by a double circulation and why it is an advantage.[3]
(b) They stop the valves turning inside out (being pushed into the atria) when the ventricles contract, preventing backflow of blood.
B1
(c) Blood passes through the heart twice in each complete circuit of the body – the pulmonary circuit (heart → lungs → heart) and the systemic circuit (heart → body → heart); blood returning from the lungs is pumped again, so it can be sent to the body at high pressure; low pressure to the lungs avoids damage, and oxygen is delivered quickly to tissues with high metabolic rates.
B1 passes through heart twice per circuit / pulmonary and systemic circuits. B1 blood repressurised after lungs / high pressure to body. B1 lower pressure to lungs / rapid delivery of oxygen to supply high metabolic rate.
Q11Heart & cardiac cycle · Standard · Paper 2 style · 7 marks
Pressures in the left side of the heart during one cardiac cycle:Time / s: 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8Left atrium / kPa: 0.5, 1.2, 0.6, 0.4, 0.6, 0.8, 0.9, 0.6, 0.5Left ventricle / kPa: 0.4, 1.0, 8.0, 15.5, 14.8, 2.0, 0.5, 0.4, 0.4Aorta / kPa: 10.5, 10.0, 9.8, 15.0, 14.5, 11.5, 11.0, 10.7, 10.5
(a)Between which two times does the bicuspid (left atrioventricular) valve close? Explain your answer.[2]
(b)Between which two times does the aortic semilunar valve open, and between which two times does it close?[2]
(c)The cycle shown lasts 0.8 s. Calculate the heart rate in beats per minute.[1]
(d)Explain why the aortic pressure does not fall to near zero between 0.5 s and 0.8 s.[2]
Show answer
(a) Between 0.1 s and 0.2 s; ventricular pressure rises above atrial pressure (1.0 → 8.0 kPa while atrial pressure is 0.6 kPa).
B1 0.1–0.2 s. B1 ventricular pressure becomes higher than atrial pressure.
(b) Opens between 0.2 s and 0.3 s; closes between 0.4 s and 0.5 s.
B1 opens 0.2–0.3 s (ventricle > aorta). B1 closes 0.4–0.5 s (aorta > ventricle).
(c) 75 beats per minute
60 ÷ 0.8 = 75
(d) The elastic tissue in the aorta wall is stretched during systole; it recoils during diastole, pushing blood onward and maintaining pressure (and the closed semilunar valve stops backflow).
Q12Heart & cardiac cycle · Standard · Paper 2 style · 5 marks
Hamza, a student, has a resting stroke volume of 70 cm³ and a cardiac cycle lasting 0.8 s. Bilal, a long-distance runner from Abbottabad, has a resting stroke volume of 95 cm³ and a resting heart rate of 52 beats per minute.
(a)Calculate the cardiac output of each person in dm³ per minute.[2]
(b)Suggest how training has led to Bilal's larger stroke volume and explain how this allows a lower resting heart rate.[3]
(b) Training increases the size/thickness of cardiac muscle in the ventricle walls (and chamber volume); so more blood is ejected per beat; the same cardiac output needed at rest can be supplied with fewer beats per minute.
B1 larger / stronger ventricular muscle. B1 more blood per contraction. B1 similar output needed at rest so fewer beats needed.
Q13Heart & cardiac cycle · Challenge · Paper 2 style · 6 marks
(a)Explain why the wall of the left ventricle is thicker than the wall of the right ventricle.[2]
(b)Explain why the atrial walls are much thinner than the ventricle walls.[1]
(c)A baby is born with a hole in the septum between the two ventricles. Suggest and explain the effect on the oxygen supplied to the baby's tissues.[3]
Show answer
(a) The left ventricle pumps blood around the whole body (systemic circulation), which needs a much higher pressure to overcome resistance; the right ventricle pumps only to the nearby lungs and a high pressure would damage the lung capillaries.
B1 left ventricle produces higher pressure for systemic circulation / long distance / more resistance. B1 right only to lungs, lower pressure needed / avoids damaging capillaries / tissue fluid build-up in lungs.
(b) Atria only pump blood a short distance into the ventricles, at low pressure.
B1
(c) The pressure is higher in the left ventricle, so during systole some oxygenated blood is pushed through the hole into the right ventricle and is pumped back to the lungs instead of to the body; less blood (and so less oxygen) leaves in the aorta per beat; the heart must beat faster / work harder to supply enough oxygen, and pressure in the pulmonary circulation rises. (If the hole is large, or blood flows right → left, deoxygenated blood mixes into the aorta and the oxygen concentration of systemic blood falls.)
1 mark each for any three of: blood flows left → right because pressure is higher in the left ventricle; oxygenated blood returns to the lungs / mixes with blood in the right ventricle; smaller volume of oxygenated blood leaves in the aorta / less oxygen delivered to tissues per beat; heart works harder / faster rate / tiredness or breathlessness; raised pressure in pulmonary circulation; (if right → left flow) deoxygenated blood enters the aorta, lowering its oxygen concentration.
Q14Blood vessels & circulation · Core · Paper 1 style · 1 mark
(a)Which feature of the wall of an artery allows it to stretch when blood enters and then recoil to keep blood flowing smoothly?[1]
Show answer
(a) B a thick layer containing elastic fibres
B: elastic fibres in the tunica media stretch as each pulse of blood enters and recoil between beats, evening out pressure. Valves are found in veins, not arteries. Collagen gives strength to resist bursting but does not provide recoil.
Q15Blood vessels & circulation · Core · Paper 2 style · 6 marks
(a)Describe two differences between the structure of an artery and the structure of a vein.[2]
(b)Explain how the structure of a capillary is related to its function.[3]
(c)Explain why veins have valves.[1]
Show answer
(a) Artery has a thicker wall (more smooth muscle and elastic fibres); artery has a narrower lumen relative to its wall; veins have valves (arteries do not, except at the base of the aorta and pulmonary artery); artery endothelium is folded.
1 mark each for any two comparative differences.
(b) Wall is one cell thick (endothelium only), giving a short diffusion distance; pores / gaps between endothelial cells allow plasma (and white cells) to leave and form tissue fluid; very narrow lumen (about 7 µm) slows red blood cells and brings them close to the wall; large numbers give a large surface area for exchange.
1 mark each for any three structure–function points.
(c) Blood pressure in veins is low, so valves prevent blood flowing backwards (and help return blood to the heart when muscles squeeze the veins).
B1
Q16Blood vessels & circulation · Standard · Paper 2 style · 6 marks
A student examined three blood vessels in a thin slice of sheep tissue from a butcher in Rawalpindi.Vessel P: wall thickness 1.2 mm, lumen diameter 4.0 mm, wall contains many elastic fibres.Vessel Q: wall thickness 0.4 mm, lumen diameter 6.0 mm, flaps of tissue seen inside.Vessel R: seen only with the microscope; wall one cell thick, lumen just wide enough for one red blood cell.
(a)Calculate the ratio of wall thickness to lumen diameter for vessel P and for vessel Q. Give each answer to 2 decimal places.[2]
(b)Identify vessels P and Q and give a reason for each.[2]
(c)Suggest why vessel R cannot be measured with a ruler and identify it.[2]
Show answer
(a) P: 0.30; Q: 0.07
B1 1.2 ÷ 4.0 = 0.30. B1 0.4 ÷ 6.0 = 0.07.
(b) P is an artery – thick wall relative to lumen, with much elastic tissue; Q is a vein – thin wall, wide lumen, with valves.
B1 P artery with reason. B1 Q vein with reason.
(c) R is a capillary; its diameter (about 8 µm) is too small to measure with a ruler – it needs a microscope with an eyepiece graticule.
Q17Blood vessels & circulation · Standard · Paper 2 style · 5 marks
(a)Explain how arterioles can change the amount of blood flowing to a particular organ.[2]
(b)During a cricket match, blood flow to leg muscles increases but blood flow to the gut decreases. Suggest why.[2]
(c)Explain why large arteries contain a lot of collagen in their outer layer.[1]
Show answer
(a) Arterioles have smooth muscle in their walls; contraction narrows the lumen (vasoconstriction), reducing flow to the organ; relaxation widens it (vasodilation), increasing flow.
(b) Leg muscles respire faster and need more oxygen and glucose (and removal of CO₂/heat), so their arterioles dilate; digestion is less urgent, so gut arterioles constrict and blood is redirected.
B1 muscles need more O₂ / glucose / remove CO₂. B1 blood redistributed / limited cardiac output shared.
(c) Collagen is strong and resists the high pressure, so the artery does not burst / overstretch.
B1
Q18Blood vessels & circulation · Challenge · Paper 2 style · 6 marks
(a)Describe the changes in blood pressure as blood flows from the aorta to the vena cava.[2]
(b)The speed of blood flow is lowest in the capillaries, although each capillary is very narrow. Explain why, and why this is an advantage.[3]
(c)Suggest why the speed of blood increases again in the veins.[1]
Show answer
(a) Pressure is highest and fluctuates (pulses) in the aorta and large arteries; it falls through the arterioles (largest fall) and the fluctuations disappear; it continues to fall through capillaries and venules, and is lowest (near zero) in the vena cava.
B1 high and fluctuating in arteries. B1 large drop in arterioles, continues falling to lowest in veins / fluctuations lost.
(b) There are so many capillaries that their total cross-sectional area is much larger than that of the arteries; the same volume per second flows through a larger total area, so speed falls; slow flow gives more time for exchange of substances (oxygen, glucose, CO₂) with tissues.
B1 very large total cross-sectional area. B1 same volume flow spread over larger area → lower velocity. B1 more time for exchange / diffusion.
(c) Capillaries join into fewer, larger vessels, so the total cross-sectional area decreases.
B1
Q19Tissue fluid · Standard · Paper 2 style · 6 marks
(a)Explain how tissue fluid is formed at the arterial end of a capillary.[3]
(b)Explain how much of this fluid returns to the blood at the venous end.[2]
(c)State what happens to the tissue fluid that does not return to the capillaries.[1]
Show answer
(a) Hydrostatic (blood) pressure is high at the arterial end; it is greater than the tendency of water to enter by osmosis (plasma has a lower water potential due to proteins); so water and small solutes (glucose, ions, oxygen) are forced out through gaps in the capillary wall; large plasma proteins and red cells stay in the blood.
1 mark each for any three of: high hydrostatic pressure; greater than the opposing water potential / osmotic effect; fluid forced out through gaps / pores in endothelium; small molecules leave (e.g. glucose, amino acids, ions); proteins / red blood cells remain.
(b) Hydrostatic pressure has fallen at the venous end; the plasma still has a lower water potential than tissue fluid because of the plasma proteins, so water moves back in by osmosis.
B1 lower hydrostatic pressure. B1 water re-enters by osmosis down water potential gradient (due to plasma proteins).
(c) It drains into lymph capillaries / lymph vessels and is returned to the blood (in veins near the heart).
B1
Q20Tissue fluid · Challenge · Paper 2 style · 5 marks
Some children in areas affected by famine have diets very low in protein. Their blood plasma contains less protein than normal, and their legs and abdomen become swollen with fluid (oedema).
(a)Explain why a low concentration of protein in blood plasma causes tissue fluid to build up.[3]
(b)Suggest why people with very high blood pressure can also develop swelling of the ankles.[2]
Show answer
(a) With less protein, the water potential of the plasma is higher (less negative); the water potential gradient from tissue fluid to plasma at the venous end is smaller; so less water returns to the capillaries by osmosis, and more fluid stays in the tissues than the lymph can remove.
B1 plasma water potential higher / less negative. B1 less water re-enters by osmosis at venous end. B1 more tissue fluid remains / lymph system cannot drain it all.
(b) Higher hydrostatic pressure in capillaries forces more fluid out (and less returns at the venous end); in the legs gravity also raises pressure in the veins/capillaries, so fluid accumulates in the ankles.
B1 more fluid forced out by higher hydrostatic pressure. B1 less reabsorbed / gravity effect in legs.
Q21Skill: planning a method · Standard · Paper 5 style · 8 marks
Students at a school in Peshawar want to find out how the intensity of exercise affects heart rate immediately after exercise.
(a)Describe a method to investigate the effect of exercise intensity on heart rate. Your method should allow reliable results to be collected.[6]
(b)Suggest one ethical consideration when planning this investigation.[1]
(c)State the dependent variable and suggest one variable that cannot easily be controlled.[1]
Show answer
(a) Use step-ups on a bench of fixed height at different rates set by a metronome (e.g. 10, 15, 20, 25, 30 steps per minute) for a fixed time (e.g. 3 min); measure resting pulse first (count at the wrist for 30 s, ×2) and wait until it returns to resting between trials; measure pulse immediately after each bout; use the same person (or a group matched for age, sex, fitness); repeat each intensity three times on different days or after full recovery and calculate means; record results in a table and plot heart rate against step rate.
1 mark each for any six of: at least five intensities set precisely (metronome / treadmill speed); fixed duration of exercise; same step height; resting heart rate measured first; pulse counted for a stated time with a stated method (or pulse meter); measured immediately after exercise; allow return to resting rate between trials; same subject or matched subjects (age / sex / fitness / no caffeine); repeats and means; safety – stop if unwell / subjects in good health.
(b) Volunteers must give informed consent and may withdraw at any time / people with heart or breathing problems should not take part.
B1
(c) Dependent: heart rate (beats per minute); uncontrolled: e.g. emotional state / time since last meal / sleep.
B1 both.
Q22Skill: planning a method · Challenge · Paper 5 style · 8 marks
Daphnia (water fleas) are transparent, so their heart can be seen beating under a microscope. A student wants to test the hypothesis that higher concentrations of caffeine increase the heart rate of Daphnia.
(a)Describe a method the student could use to investigate the effect of caffeine concentration on the heart rate of Daphnia.[6]
(b)Daphnia are small invertebrates. Suggest two ways the student can act ethically in this investigation.[2]
Show answer
(a) Prepare caffeine solutions by dilution of a stock, e.g. 0, 0.25, 0.5, 1.0, 2.0 mg cm⁻³, using pond water; place one Daphnia on a cavity slide with a small drop of pond water, remove excess water, view under low power; let it acclimatise for 2–3 minutes and count the resting heart rate (e.g. beats in 20 s, ×3); replace the water with a fixed volume of caffeine solution, wait a fixed time (e.g. 5 min) and count again; keep temperature constant (e.g. temperature of room checked / avoid heat from lamp – use LED or limit time under light); use different Daphnia of similar size for each concentration, at least 5 per concentration, and calculate the mean change.
1 mark each for any six of: at least five concentrations including 0 (control) made by dilution; same volume of solution; acclimatisation / fixed exposure time; heart rate counted for a fixed period with method (tally / counter / video slowed down); resting rate measured for each animal before treatment (or calculate % change); temperature controlled (lamp heat avoided, water bath for solutions); similar size / age of Daphnia; fresh Daphnia for each trial; replicates and mean.
(b) Use the minimum number of animals; keep exposure short and return them to pond water afterwards; avoid overheating under the microscope lamp; handle gently with a wide pipette.
1 mark each for any two.
Q23Skill: variables & controls · Core · Paper 5 style · 6 marks
A student counts the heartbeats of Daphnia placed in pond water at 10 °C, 15 °C, 20 °C, 25 °C and 30 °C.
(a)State the independent variable and the dependent variable.[2]
(b)Describe how the student could control the temperature of the water around the Daphnia.[2]
(c)State two other variables that should be standardised.[2]
Show answer
(a) Independent: temperature of the water. Dependent: heart rate of Daphnia (beats per minute).
B1 each.
(b) Keep the pond water samples in thermostatic water baths at each temperature; check the temperature of the drop on the slide with a thermometer/temperature probe; use a cold light source so the lamp does not heat the slide; count quickly.
B1 water baths at set temperatures. B1 avoid lamp heating / measure temperature on slide.
(c) Size/age of Daphnia; time allowed to acclimatise before counting; duration of count; volume of water on the slide; source of Daphnia.
1 mark each for any two.
Q24Skill: errors, reliability & improvements · Standard · Paper 5 style · 7 marks
Students measured pulse rate after a 2-minute run by finding the pulse at the wrist, counting beats for 15 s and multiplying by 4. Sometimes counting started up to 20 s after the run ended.
(a)Identify three sources of error in this method.[3]
(b)Suggest improvements to reduce the errors you described.[2]
(c)Explain why repeating the readings and calculating a mean improves the reliability but not the accuracy of a reading that is always taken 20 s late.[2]
Show answer
(a) Counting for only 15 s then ×4 multiplies any counting error by 4; a beat may be missed or counted twice at the start/end of the count; heart rate falls quickly after exercise, so a delay before counting lowers the reading; pulse may be hard to find; reaction time starting/stopping the timer.
1 mark each for any three.
(b) Use a digital heart-rate monitor / pulse oximeter that reads continuously; count for 30 s or 60 s; start counting immediately (or record continuously and read the value at a fixed time after exercise); a partner times while the subject counts.
1 mark each for any two improvements matched to errors.
(c) Repeats reduce the effect of random errors (variation between readings); a delay of 20 s every time is a systematic error, so every reading is lower by a similar amount and the mean is still too low.
B1 repeats reduce random error. B1 systematic error not removed by averaging.
Q25Skill: conclusions & interpreting data · Standard · Paper 5 style · 6 marks
Mean heart rate (beats per minute) of 8 trained athletes and 8 untrained students from the same college, before and after a 3-minute run:Time: rest, 0 min after, 1 min, 2 min, 3 min, 4 minTrained: 60, 150, 112, 84, 68, 61Untrained: 78, 172, 150, 128, 110, 96
(a)Compare the recovery of heart rate in the two groups.[3]
(b)A student concludes that training causes faster recovery. Discuss whether this conclusion is valid.[3]
Show answer
(a) Trained students had a lower resting rate (60 v 78) and a lower peak (150 v 172); trained students' heart rate fell faster – back to resting within 4 min, untrained still 18 beats above resting (96) after 4 min; both groups decreased most in the first minute.
1 mark each for any three comparative points with data.
(b) The data are consistent with the conclusion (trained group recovers faster); but this is a comparison of two existing groups, so other differences (genetics, diet, body mass, sex) may explain it; only means are given with no spread/statistical test, and group sizes are small; a valid test would measure the same students before and after a training programme.
B1 data support. B1 other variables / correlation not causation. B1 need spread / statistical test / sample size / before-and-after design.
Q26Skill: calculations & data processing · Core · Paper 5 style · 4 marks
A student counted 47 heartbeats of a Daphnia in 20 s. After caffeine solution was added she counted 58 beats in 20 s.
(a)Calculate the heart rate in beats per minute before and after adding caffeine.[2]
(b)Calculate the percentage increase in heart rate. Give your answer to 1 decimal place.[1]
(c)The caffeine solution was made by dissolving 0.40 g of caffeine in 250 cm³ of pond water. Calculate its concentration in mg cm⁻³.[1]
Q27Skill: graphs & charts · Standard · Paper 5 style · 6 marks
Percentage saturation of human haemoglobin at different partial pressures of oxygen (pO₂):pO₂ / kPa: 1, 2, 3, 4, 5, 6, 8, 10, 12Saturation / %: 10, 25, 45, 60, 72, 80, 90, 95, 97
(a)State the type of graph you would draw and describe the shape of the curve you would expect.[2]
(b)Estimate, by linear interpolation between the readings, the partial pressure of oxygen at which haemoglobin is 50 % saturated. Give your answer to 1 decimal place.[2]
(c)Explain why the curve is steepest between 2 kPa and 4 kPa.[2]
Show answer
(a) Line graph with pO₂ on the x-axis and percentage saturation on the y-axis, points joined by a smooth curve; S-shaped (sigmoid) curve.
B1 line graph, pO₂ on x-axis. B1 sigmoid / S-shaped smooth curve.
(c) Once one oxygen molecule has bound, the haemoglobin changes shape, making it easier for further oxygen molecules to bind (cooperative binding); so a small rise in pO₂ gives a large rise in saturation.
B1 binding of first O₂ changes shape of haemoglobin. B1 increases affinity for further O₂ / cooperative binding.
Q28Skill: hypotheses & predictions · Core · Paper 5 style · 4 marks
A student plans to compare the resting heart rates of 15 hockey players and 15 students who do no regular exercise.
(a)Write a hypothesis for this investigation.[1]
(b)State the null hypothesis that would be tested with a t-test.[1]
(c)Explain why a t-test is suitable for these data and Spearman's rank correlation is not.[2]
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(a) Athletes have a lower mean resting heart rate than non-athletes (because training increases stroke volume).
B1 directional, testable statement about the difference.
(b) There is no significant difference between the mean resting heart rate of athletes and non-athletes.
B1
(c) The investigation compares the means of two groups / two independent samples of continuous data (approximately normally distributed); Spearman's rank tests for a correlation between two variables measured on the same individuals, which is not what is being asked.
B1 t-test compares means of two groups (normal, continuous data). B1 Spearman's is for correlation / relationship between two variables.
Q29Skill: explaining results with biology · Challenge · Paper 5 style · 4 marks
Mean red blood cell counts of healthy adults:Karachi (sea level): 5.0 million per mm³Villages in Hunza living at about 2,500 m for many years: 5.7 million per mm³
(a)Explain the difference in red blood cell count between the two groups.[3]
(b)Suggest a disadvantage of having a very high red blood cell count.[1]
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(a) The partial pressure of oxygen in the air is lower at high altitude; so haemoglobin is less saturated and less oxygen reaches tissues; over time the body responds by producing more red blood cells in the bone marrow, increasing the oxygen-carrying capacity of the blood.
B1 lower pO₂ at altitude. B1 less oxygen loaded / tissues receive less. B1 more red blood cells made (more haemoglobin), so more oxygen carried.
(b) The blood becomes more viscous, so the heart has to work harder / greater risk of blood clots.
B1
Q30Skill: statistics (SD, SE, CI, t-test, Spearman, chi-squared) · Challenge · Paper 5 style · 6 marks
Resting heart rates (beats per minute) of 10 students who train for football and 10 who do not:Trained: 62, 58, 66, 60, 64, 59, 63, 61, 67, 60Untrained: 68, 72, 65, 70, 74, 66, 71, 69, 75, 67s = √(Σ(x − x̄)² ÷ (n − 1)); t = |x̄₁ − x̄₂| ÷ √(s₁²/n₁ + s₂²/n₂)
(a)Calculate the standard deviation for each group. Give your answers to 2 decimal places.[2]
(b)Calculate the value of t. Give your answer to 2 decimal places.[2]
(c)The critical value of t at p = 0.05 with 18 degrees of freedom is 2.10. State what you can conclude.[2]
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(a) Trained: s = 2.98; untrained: s = 3.33
Trained: mean 62.0, Σ(x − x̄)² = 80, s = √(80 ÷ 9) = 2.98. Untrained: mean 69.7, Σ(x − x̄)² = 100.1, s = √(100.1 ÷ 9) = 3.33. B1 each.
(b) t = 5.44
M1 √(2.98²/10 + 3.33²/10) = 1.415. A1 t = 7.7 ÷ that = 5.44.
(c) 5.44 is greater than 2.10, so the null hypothesis is rejected: there is a significant difference between the mean resting heart rates (probability of the difference being due to chance is less than 5 %).
B1 t > critical value so reject null hypothesis. B1 difference is significant / not due to chance at p < 0.05.
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Blood cells, haemoglobin & gas transportBanker70 questions · 9/9 sessions
