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13+ · Biology

CE.S-B5 Biology: Respiration, Digestion and Gas Exchange

ISEB COMMON ENTRANCE ISEB Common Entrance Biology (13+) · Calculator allowed · about 80 minutes

Total Marks

Name: _______________________________ Date: ____ / ____ / ______

Answer ALL questions. Show all your working.

Breathing and respiration are related but different processes.

(a)Define 'breathing'.(1)

(b)Define 'respiration'.(1)

(c)For each statement below, state whether it describes 'breathing' or 'respiration'.

(i) The diaphragm contracts and moves downward.
(ii) Glucose is broken down inside mitochondria to release energy.(2)

(Total for Question 1 is 4 marks)

Aerobic respiration takes place continuously inside almost every living cell in the human body, not only during exercise.

(a)Complete the word equation for aerobic respiration.

____ + ____ -> carbon dioxide + ____ (+ energy released)(2)

(b)Name the organelle inside a cell where most aerobic respiration takes place.(1)

(c)A pupil says: 'My cells only respire when I am exercising, and stop again once I sit down and rest.' Explain why this statement is incorrect.(2)

(Total for Question 2 is 5 marks)

During very vigorous exercise, a runner's muscles cannot always get enough oxygen from the blood to respire fully aerobically.

(a)Complete the word equation for anaerobic respiration in human muscle cells.

glucose -> ____ (+ energy released)(1)

(b)Explain why anaerobic respiration releases much less energy per glucose molecule than aerobic respiration.(2)

(c)Define the term 'oxygen debt', and state how it is repaid after exercise finishes.(2)

(Total for Question 3 is 5 marks)

Yeast is a single-celled fungus that can respire anaerobically. This process is used by bakers to make bread rise.

(a)Complete the word equation for anaerobic respiration in yeast (fermentation).

glucose -> ____ + ____ (+ energy released)(2)

(b)Explain why yeast is added to bread dough.(1)

(c)State one difference between the products of anaerobic respiration in yeast and in human muscle cells.(1)

(d)Bread dough does not taste noticeably of alcohol once it has been baked, even though the yeast produces ethanol. Suggest why.(1)

(Total for Question 4 is 5 marks)

A student investigates the rate of respiration of germinating pea seeds using a respirometer. The apparatus contains soda lime, which absorbs the carbon dioxide the seeds produce. As the seeds use up oxygen, a coloured liquid drop in a capillary tube moves along a scale towards the seeds. The student also sets up an identical control tube containing the same volume of seeds that have been boiled (killed) and cooled.

At 20 deg C, the liquid drop in the tube with live seeds moved 24 mm in 10 minutes.

(a)Explain why the control tube of dead, boiled seeds is needed.(2)

(b)Explain why soda lime must be present in the respirometer for this investigation to correctly measure the rate of respiration.(1)

(c)Calculate the rate of respiration of the seeds, in mm of drop movement per minute.(2)

(d)Predict and explain what would happen to the rate of respiration if the investigation were repeated at 70 deg C instead of 20 deg C.(1)

(Total for Question 5 is 6 marks)

Digestion breaks food down using both physical and chemical methods.

(a)Distinguish between mechanical digestion and chemical digestion, giving one example of each.(2)

(b)Match each type of tooth to its main function.

Teeth: incisors, canines, molars/premolars
Functions: tearing food; cutting/biting food; grinding/chewing food(3)

(Total for Question 6 is 5 marks)

Food is moved along the gut (digestive tract) by a process called peristalsis.

(a)Define peristalsis.(1)

(b)The wall of the oesophagus contains circular muscle and longitudinal muscle. Explain how these two sets of muscles work together to push a ball of food (bolus) along.(2)

(c)Explain why peristalsis allows food to travel down the oesophagus into the stomach even if a person is lying down flat, and not relying on gravity.(1)

(Total for Question 7 is 4 marks)

Digestive enzymes are described as being 'specific' to the substrate they break down.

(a)Describe the 'lock and key' model of how an enzyme works.(2)

(b)Using the lock and key model, explain why amylase can break down starch but cannot break down protein.(2)

(Total for Question 8 is 4 marks)

The table shows the approximate pH in different regions of the human digestive system.

Region: mouth (saliva), stomach, small intestine
pH: 7, 2, 8

Figure (to be drawn): Table with columns: Region (mouth/saliva, stomach, small intestine) and pH (7, 2, 8 respectively).

(a)Identify which region has the lowest pH, and give one reason why this acidity is useful.(2)

(b)Explain how the acidic contents leaving the stomach are neutralised as they enter the small intestine, where the pH rises to about 8.(2)

(c)Explain why a digestive enzyme, such as protease in the stomach, would stop working properly if the pH moved too far away from its optimum.(2)

(Total for Question 9 is 6 marks)

The wall of the small intestine is folded into millions of tiny, finger-like projections called villi, which increase the efficiency of absorption of digested food into the blood.

(a)Describe four ways in which villi are adapted for efficient absorption.(4)

(b)A person with coeliac disease has villi that become damaged and flattened. Explain why this can cause the person to lose weight or become malnourished.(2)

(Total for Question 10 is 6 marks)

A student carries out four food tests on an unknown food sample, X, and records the results.

Test: iodine test - Result: solution stays orange/brown (no colour change)
Test: Benedict's test (heated) - Result: solution turns brick-red precipitate
Test: biuret test - Result: solution stays blue (no colour change)
Test: ethanol emulsion test - Result: no cloudy white layer forms

Figure (to be drawn): Table with columns Test | Result, listing the iodine test, Benedict's test, biuret test and ethanol emulsion test results for food sample X, as given in the question text.

(a)For each test, name the food substance it is used to detect and state the expected positive result (colour change).(4)

(b)Using the results table, identify which nutrient(s) are present in food sample X, and explain your reasoning.(2)

(Total for Question 11 is 6 marks)

The diagram shows a simplified cross-section through the human thorax (chest), including the ribcage, intercostal muscles, lungs and diaphragm.

Figure (to be drawn): Cross-section diagram of the human thorax: the ribcage curves around the outside, with intercostal muscles shown between the ribs; two lungs fill most of the chest cavity; the diaphragm is shown as a dome-shaped sheet of muscle forming the floor of the thorax, below the lungs.

(a)State what happens to the diaphragm and to the intercostal muscles/ribcage during inhalation (breathing in).(2)

(b)Explain, in terms of the volume and pressure inside the thorax, why these movements cause air to flow into the lungs.(2)

(c)State what happens to the diaphragm and to the intercostal muscles/ribcage during exhalation (breathing out).(1)

(d)Explain, in terms of the volume and pressure inside the thorax, why these movements cause air to flow out of the lungs.(2)

(Total for Question 12 is 7 marks)

The graph below is a spirometer trace for a resting adult over one minute. The vertical axis shows lung volume (litres) and the horizontal axis shows time (seconds).

For the first 30 seconds, the trace shows regular, evenly-spaced normal breathing cycles between a lung volume of 2.5 litres and 3.0 litres; there are 6 complete breathing cycles in this 30 second period. At 40 seconds, the person then takes one maximal breath in, reaching a peak lung volume of 6.0 litres, before breathing out fully to a minimum lung volume of 1.5 litres. Normal breathing then resumes.

Figure (to be drawn): Line graph titled 'Spirometer trace': x-axis time in seconds (0 to 60), y-axis lung volume in litres (0 to 6.5). A regular zig-zag trace between 2.5 and 3.0 litres runs from 0 to 30 seconds (6 full cycles), continues normally to 40 seconds, then rises steeply to a peak of 6.0 litres and falls steeply to a trough of 1.5 litres (the maximal breath), before the regular zig-zag pattern resumes.

(a)Define 'tidal volume', and state its value from the trace during normal resting breathing.(2)

(b)Define 'vital capacity', and calculate its value from the maximal breath shown on the trace.(2)

(c)Calculate the person's breathing rate at rest, in breaths per minute, using the number of cycles shown in the first 30 seconds of the trace.(2)

(d)Suggest why a well-trained athlete would be expected to have a larger vital capacity than a non-athlete of a similar size.(1)

(Total for Question 13 is 7 marks)

The table shows the approximate percentage composition of inhaled air and exhaled air.

Gas: oxygen - Inhaled air: 21%, Exhaled air: 16%
Gas: carbon dioxide - Inhaled air: 0.04%, Exhaled air: 4%
Gas: nitrogen - Inhaled air: 78%, Exhaled air: 78%

Figure (to be drawn): Table with columns Gas | Inhaled air (%) | Exhaled air (%), for oxygen, carbon dioxide and nitrogen, with the values given in the question text.

(a)Explain why the percentage of oxygen is lower, and the percentage of carbon dioxide is higher, in exhaled air compared with inhaled air.(2)

(b)Explain why the percentage of nitrogen is approximately the same in inhaled air and exhaled air.(1)

(c)Describe how limewater could be used to compare the carbon dioxide content of inhaled and exhaled air, and state the result you would expect.(3)

(Total for Question 14 is 6 marks)

A student compares two people: one is a trained athlete and the other does not exercise regularly ('untrained').

Person: trained athlete - Resting heart rate: 48 bpm, Time to recover to resting heart rate after a standard exercise: 1.5 minutes
Person: untrained person - Resting heart rate: 78 bpm, Time to recover to resting heart rate after the same exercise: 4.0 minutes

Figure (to be drawn): Table with columns Person | Resting heart rate (bpm) | Time to recover to resting heart rate after a standard exercise (minutes), for the trained athlete and the untrained person, with the values given in the question text.

(a)Describe the differences shown by the data between the trained athlete and the untrained person.(2)

(b)Suggest an explanation, in terms of the heart, for why the trained athlete has a lower resting heart rate.(2)

(c)Suggest one variable that should be controlled to make this comparison of recovery time a fair test.(1)

(d)Suggest one limitation of this comparison, given that data was collected from only one person in each group.(1)

(Total for Question 15 is 6 marks)

The graph shows a person's rate of oxygen consumption (litres per minute) before, during and after a short period of vigorous exercise. Resting oxygen consumption is 0.3 litres per minute. During exercise, oxygen consumption rises to and stays at 2.0 litres per minute. Exercise stops at the 5 minute mark; oxygen consumption then falls gradually, only returning to the resting rate of 0.3 litres per minute at the 9 minute mark.

Figure (to be drawn): Line graph titled 'Oxygen consumption during and after exercise': x-axis time in minutes (0 to 10), y-axis oxygen consumption in litres per minute (0 to 2.5). The line is flat at 0.3 from 0 to 2 minutes (rest), rises sharply and is flat at 2.0 from about 2.5 to 5 minutes (exercise), then falls in a curve from 5 minutes, levelling back out at 0.3 by the 9 minute mark (recovery).

(a)Define 'oxygen debt'.(1)

(b)Explain what happens to the lactic acid built up in the muscles during the recovery period, to repay the oxygen debt.(2)

(c)Using the graph, state the time taken for oxygen consumption to return to the resting rate after exercise stopped.(2)

(Total for Question 16 is 5 marks)

Carbon monoxide in cigarette smoke combines with haemoglobin in red blood cells to form carboxyhaemoglobin. On average, about 1% of a non-smoker's haemoglobin is combined this way (from normal background sources); in a heavy smoker, this rises to about 6% of their haemoglobin.

(a)Explain what carboxyhaemoglobin is, and why its formation reduces how much oxygen the blood can carry.(2)

(b)Calculate how many times greater the percentage of carboxyhaemoglobin is in the heavy smoker's blood compared with the non-smoker's.(2)

(c)Suggest, using ideas about oxygen supply, why smoking during pregnancy can restrict the growth of an unborn baby.(2)

(Total for Question 17 is 6 marks)

The table shows how average lung capacity (as a percentage of a healthy young adult's expected value) changes with age, for a group of long-term smokers and a group of non-smokers.

Age (years): 20, 40, 60, 70
Non-smokers (%): 100, 96, 90, 85
Smokers (%): 100, 92, 68, 55

Figure (to be drawn): Table with columns Age (years) | Non-smokers (%) | Smokers (%), for ages 20, 40, 60 and 70, with the values given in the question text.

(a)Describe the difference between the pattern shown for smokers and the pattern shown for non-smokers.(2)

(b)Calculate the difference in lung capacity between smokers and non-smokers at age 70.(1)

(c)Suggest a reason, referring to the effects of tar on the alveoli, why long-term smokers' lung capacity declines more than non-smokers'.(2)

(Total for Question 18 is 5 marks)

Using your knowledge of digestion, gas exchange and respiration, explain how the body ensures that an athlete's leg muscles have enough glucose and oxygen to respire aerobically during a long run, and explain what happens if the muscles' demand for oxygen becomes greater than the supply.

(Total for Question 19 is 6 marks)

Mark scheme · CE.S-B5 Biology: Respiration, Digestion and Gas Exchange

Question 1

(a) B1 the mechanical (muscular) process of moving air into and out of the lungs (ventilation) oe
(a) Answer: The mechanical process of moving air into and out of the lungs (ventilation).
(b) B1 the chemical process, occurring continuously inside cells, that releases energy from glucose oe
(b) Answer: The chemical process inside cells that releases energy from glucose.
(c) B1 (i) breathing
(c) B1 (ii) respiration
(c) Answer: (i) breathing (ii) respiration

Question 2

(a) B1 glucose and oxygen on the left-hand side (both needed for the mark)
(a) B1 water on the right-hand side
(a) Answer: glucose + oxygen -> carbon dioxide + water (+ energy released)
(b) B1 mitochondria (mitochondrion) cao
(b) Answer: Mitochondria.
(c) B1 cells respire all the time (continuously), even at rest, to release the energy needed for processes such as keeping warm, cell repair and the activity of internal organs oe
(c) B1 exercise increases the rate of respiration (more energy is needed for muscle contraction), it does not switch respiration on and off oe
(c) Answer: Respiration happens continuously in all cells, even at rest, to release energy for essential processes; exercise only increases its rate, it does not start or stop it.

Question 3

(a) B1 lactic acid cao
(a) Answer: glucose -> lactic acid (+ energy released)
(b) B1 anaerobic respiration only partially (incompletely) breaks down the glucose molecule, into lactic acid, whereas aerobic respiration breaks it down fully oe
(b) B1 because glucose is not fully broken down, most of the energy stored in it remains locked inside the lactic acid, so much less is released oe
(b) Answer: Anaerobic respiration only partially breaks glucose down into lactic acid, so most of the energy stored in the glucose is not released.
(c) B1 oxygen debt is the extra oxygen the body needs after exercise, above the normal resting amount, to deal with the lactic acid built up during anaerobic respiration oe
(c) B1 it is repaid by continuing to breathe faster and deeper after exercise stops, supplying the extra oxygen needed to break down the lactic acid oe
(c) Answer: Oxygen debt is the extra oxygen needed after exercise to deal with lactic acid build-up; it is repaid by continued fast, deep breathing after exercise stops.

Question 4

(a) B1 ethanol (alcohol)
(a) B1 carbon dioxide
(a) Answer: glucose -> ethanol + carbon dioxide (+ energy released)
(b) B1 the yeast respires anaerobically and produces carbon dioxide gas, which forms bubbles that get trapped in the dough and make it rise oe
(b) Answer: The yeast produces carbon dioxide gas as it respires, which gets trapped in the dough and makes it rise.
(c) B1 yeast produces ethanol and carbon dioxide, whereas human muscle cells produce (only) lactic acid oe
(c) Answer: Yeast produces ethanol and carbon dioxide; human muscle cells produce lactic acid instead.
(d) B1 ethanol has a low boiling point, so it evaporates out of the dough during the heat of baking oe
(d) Answer: The ethanol evaporates out of the dough during baking, because of the heat.

Question 5

(a) B1 it shows whether factors other than respiration, such as changes in temperature or air pressure during the experiment, could also cause the liquid drop to move oe
(a) B1 if the drop in the control tube does not move (or barely moves), this confirms that the movement seen in the live-seed tube is caused by the seeds' respiration oe
(a) Answer: It checks that any movement of the liquid drop is caused by the seeds' respiration, and not by other factors such as temperature or pressure changes.
(b) B1 soda lime absorbs the carbon dioxide produced by the seeds, so the only change in gas volume detected is due to the oxygen being used up oe (otherwise the carbon dioxide produced would roughly replace the oxygen used, and the drop would not move)
(b) Answer: Soda lime absorbs the carbon dioxide produced, so the drop's movement is caused only by the oxygen used up, not by any gas produced.
(c) M1 24 / 10 oe
(c) A1 2.4 mm per minute cao
(c) Answer: 2.4 mm per minute
(d) B1 the rate would fall to (near) zero, because the high temperature denatures the enzymes needed for respiration, so respiration can no longer take place oe
(d) Answer: The rate would fall to near zero, because 70 deg C would denature the enzymes needed for respiration.

Question 6

(a) B1 mechanical digestion is the physical breakdown of food into smaller pieces without changing it chemically, e.g. chewing by the teeth, or churning by the stomach
(a) B1 chemical digestion is the breakdown of large molecules into smaller, soluble molecules using enzymes, e.g. amylase breaking down starch
(a) Answer: Mechanical digestion physically breaks food into smaller pieces (e.g. chewing); chemical digestion uses enzymes to break large molecules into smaller ones (e.g. amylase acting on starch).
(b) B1 incisors: cutting/biting food
(b) B1 canines: tearing food
(b) B1 molars/premolars: grinding/chewing food
(b) Answer: Incisors: cutting/biting. Canines: tearing. Molars/premolars: grinding/chewing.

Question 7

(a) B1 the wave-like squeezing movement of the muscular walls of the gut that pushes food along oe
(a) Answer: The wave-like squeezing movement of the gut wall that pushes food along.
(b) B1 circular muscle behind the bolus contracts, squeezing/narrowing the tube and pushing the bolus forward
(b) B1 longitudinal muscle ahead of the bolus relaxes (or contracts to shorten that section), widening the tube ahead so the bolus can move into it oe
(b) Answer: Circular muscle behind the food contracts to squeeze it forwards, while the muscle ahead relaxes to widen the tube so the food can move into it.
(c) B1 peristalsis actively squeezes the food along using muscle contractions, so it does not depend on gravity to move the food downward oe
(c) Answer: Peristalsis actively pushes food along using muscle contractions, so it works without needing gravity.

Question 8

(a) B1 an enzyme has an active site with a specific shape, which fits (like a lock) only one particular substrate (like a key) oe
(a) B1 the substrate binds to the active site, allowing the enzyme to catalyse (speed up) the breakdown of that substrate oe
(a) Answer: An enzyme's active site has a specific shape that only fits one substrate, like a lock and key; the substrate binds there so the enzyme can catalyse its breakdown.
(b) B1 amylase's active site has a shape that is complementary to (fits) starch molecules only oe
(b) B1 a protein molecule has a different shape, so it does not fit/bind to amylase's active site, meaning amylase cannot catalyse its breakdown oe
(b) Answer: Amylase's active site shape only fits starch molecules; protein has a different shape that does not fit the active site, so amylase cannot break it down.

Question 9

(a) B1 the stomach
(a) B1 the (hydrochloric) acid kills harmful bacteria/pathogens in food, and/or gives the optimum pH for the enzyme protease (pepsin) to work oe
(a) Answer: The stomach; its acid kills harmful bacteria and gives protease its optimum pH.
(b) B1 bile, produced by the liver and stored in the gall bladder, is released into the small intestine and is alkaline oe
(b) B1 bile neutralises the acid from the stomach, raising the pH so digestion can continue at the higher, alkaline pH shown in the table oe
(b) Answer: Alkaline bile, from the liver (stored in the gall bladder), is released into the small intestine and neutralises the stomach acid, raising the pH.
(c) B1 each enzyme has an optimum pH at which its active site has the correct shape to bind its substrate oe
(c) B1 at a pH far from the optimum, the active site's shape is altered (the enzyme denatures), so the substrate can no longer bind and the reaction slows or stops oe
(c) Answer: Away from its optimum pH, an enzyme's active site changes shape (denatures), so its substrate can no longer bind and the reaction stops.

Question 10

(a) B1 large (total) surface area, due to their number and folded, finger-like shape
(a) B1 wall (epithelium) is one cell thick, giving a short diffusion distance
(a) B1 good (capillary) blood supply, which maintains a steep concentration gradient by continuously carrying absorbed nutrients away
(a) B1 each villus contains a lacteal (a small lymphatic vessel), which absorbs digested fats
(a) Answer: Large surface area; wall one cell thick; good blood supply; each villus has a lacteal for absorbing fats.
(b) B1 flattened, damaged villi have a much smaller surface area than healthy villi oe
(b) B1 so less digested food (fewer nutrients) can be absorbed into the blood, meaning the body receives less energy and fewer nutrients than it needs oe
(b) Answer: Damaged, flattened villi have a smaller surface area, so less digested food can be absorbed, and the body receives less energy and fewer nutrients than it needs.

Question 11

(a) B1 iodine test detects starch; positive result = solution turns blue-black
(a) B1 Benedict's test detects reducing sugar; positive result = solution turns from blue to a (brick-)red/orange precipitate on heating
(a) B1 biuret test detects protein; positive result = solution turns from blue to purple/lilac
(a) B1 ethanol emulsion test detects fat (lipid); positive result = a cloudy white layer/emulsion forms
(a) Answer: Iodine: starch, blue-black. Benedict's: reducing sugar, brick-red precipitate. Biuret: protein, purple/lilac. Ethanol emulsion: fat, cloudy white layer.
(b) B1 reducing sugar is present, since the Benedict's test gave a positive (brick-red precipitate) result
(b) B1 starch, protein and fat are all absent, since each of those three tests gave a negative result (no blue-black, no purple/lilac, no cloudy layer) oe
(b) Answer: Only reducing sugar is present; the other three tests were all negative, so starch, protein and fat are absent.

Question 12

(a) B1 the diaphragm contracts and flattens (moves downward)
(a) B1 the intercostal muscles contract, moving the ribcage upward and outward
(a) Answer: The diaphragm contracts and flattens; the intercostal muscles contract, moving the ribcage up and out.
(b) B1 these movements increase the volume of the thorax (chest cavity)
(b) B1 this decreases the air pressure inside the lungs to below atmospheric (outside) pressure, so air flows in from the higher pressure outside to the lower pressure inside oe
(b) Answer: The thorax volume increases, decreasing pressure inside the lungs below atmospheric pressure, so air flows in from the higher outside pressure.
(c) B1 the diaphragm relaxes and moves upward (domes upward), and the intercostal muscles relax, moving the ribcage downward and inward
(c) Answer: The diaphragm relaxes and domes upward; the intercostal muscles relax, moving the ribcage down and in.
(d) B1 these movements decrease the volume of the thorax (chest cavity)
(d) B1 this increases the air pressure inside the lungs above atmospheric pressure, so air flows out from the higher pressure inside to the lower pressure outside oe
(d) Answer: The thorax volume decreases, increasing pressure inside the lungs above atmospheric pressure, so air flows out to the lower outside pressure.

Question 13

(a) B1 tidal volume is the volume of air breathed in or out in one normal (resting) breath oe
(a) B1 0.5 litres (3.0 - 2.5) cao
(a) Answer: Tidal volume is the volume of air moved in one normal breath; from the trace, this is 0.5 litres.
(b) B1 vital capacity is the maximum volume of air that can be breathed out after taking the deepest possible breath in oe
(b) B1 4.5 litres (6.0 - 1.5) cao
(b) Answer: Vital capacity is the maximum volume of air that can be forced out after a maximal breath in; from the trace, this is 4.5 litres.
(c) M1 6 (cycles in 30 s) x 2 oe
(c) A1 12 breaths per minute cao
(c) Answer: 12 breaths per minute
(d) B1 regular (aerobic) training strengthens the breathing muscles (diaphragm and intercostal muscles) and increases lung development, allowing a greater maximum volume of air to be moved in and out oe
(d) Answer: Regular training strengthens the breathing muscles and develops the lungs, allowing a greater maximum volume of air to be moved.

Question 14

(a) B1 oxygen diffuses from the alveoli into the blood, to be used by body cells for respiration, so exhaled air contains less oxygen
(a) B1 carbon dioxide, a waste product of respiration, diffuses from the blood into the alveoli, so exhaled air contains more carbon dioxide
(a) Answer: Oxygen diffuses out of the alveoli into the blood for respiration (so less remains); carbon dioxide, a waste product of respiration, diffuses into the alveoli (so more is present).
(b) B1 nitrogen is not used or produced by the body (it is not involved in gas exchange/respiration), so it passes through the lungs largely unchanged oe
(b) Answer: Nitrogen is not used or produced by the body, so it passes through the lungs unchanged.
(c) B1 bubble (or shake) separate samples of inhaled air and exhaled air through separate samples of clear limewater
(c) B1 the limewater will turn cloudy (milky) faster, or more, when exhaled air is passed through it than when inhaled air is passed through it
(c) B1 because exhaled air contains a much higher percentage (concentration) of carbon dioxide than inhaled air, as shown in the table oe
(c) Answer: Bubble each air sample through separate limewater samples; the limewater turns cloudy faster with exhaled air, because it contains far more carbon dioxide.

Question 15

(a) B1 the trained athlete has a much lower resting heart rate (48 bpm) than the untrained person (78 bpm)
(a) B1 the trained athlete also recovers (returns to resting heart rate) much faster after exercise (1.5 minutes) than the untrained person (4.0 minutes)
(a) Answer: The athlete has a lower resting heart rate (48 vs 78 bpm) and recovers much faster after exercise (1.5 vs 4.0 minutes).
(b) B1 regular training increases the size and strength of the heart muscle, increasing its stroke volume (the volume of blood pumped out per beat) oe
(b) B1 because more blood is pumped with each beat, fewer beats per minute are needed to supply the body with the blood/oxygen it needs at rest oe
(b) Answer: Training strengthens the heart, increasing the volume of blood pumped per beat, so fewer beats per minute are needed at rest.
(c) B1 both people should do the same type, intensity and duration of exercise (or be tested under similar conditions, e.g. similar age/time of day) oe
(c) Answer: Both people should complete the same type, intensity and duration of exercise.
(d) B1 individual differences other than training (e.g. age, natural fitness, health) could also explain the difference, so a larger sample of people would be needed in each group before a reliable conclusion could be drawn oe
(d) Answer: With only one person per group, individual differences other than training could explain the result; a larger sample is needed for a reliable conclusion.

Question 16

(a) B1 the extra oxygen needed by the body after exercise, above the normal resting amount, to deal with the lactic acid built up during anaerobic respiration oe
(a) Answer: The extra oxygen needed after exercise to deal with the lactic acid built up during anaerobic respiration.
(b) B1 the lactic acid is transported, in the blood, to the liver oe
(b) B1 in the liver it is oxidised (broken down using oxygen) into carbon dioxide and water oe
(b) Answer: Lactic acid is carried in the blood to the liver, where it is oxidised (broken down using oxygen) into carbon dioxide and water.
(c) M1 identifies exercise stopped at the 5 minute mark and the resting rate was reached again at the 9 minute mark
(c) A1 4 minutes cao
(c) Answer: 4 minutes

Question 17

(a) B1 carbon monoxide combines very strongly (almost permanently) with haemoglobin in red blood cells, forming carboxyhaemoglobin oe
(a) B1 because the binding is so strong, that haemoglobin can no longer combine with (carry) oxygen, reducing the oxygen-carrying capacity of the blood oe
(a) Answer: Carboxyhaemoglobin forms when carbon monoxide binds very strongly to haemoglobin, permanently blocking that haemoglobin from carrying oxygen, so the blood's oxygen-carrying capacity falls.
(b) M1 6 / 1 oe
(b) A1 6 times (x6) cao
(b) Answer: 6 times (x6)
(c) B1 the mother's blood carries less oxygen (because more of her haemoglobin is bound to carbon monoxide instead), so less oxygen diffuses across the placenta to the fetus oe
(c) B1 with less oxygen available, the fetus's cells cannot respire (aerobically) as fast, so less energy is available for growth, restricting how much the fetus grows oe
(c) Answer: The mother's blood carries less oxygen, so less reaches the fetus through the placenta; with less oxygen for respiration, the fetus has less energy available for growth.

Question 18

(a) B1 both groups' lung capacity decreases with age, but the non-smokers decline only slowly/gradually (100% to 85%, a fall of 15 percentage points, by age 70)
(a) B1 the smokers decline only slightly up to age 40, then decline much more steeply after this, falling to 55% by age 70, a much larger overall fall than the non-smokers
(a) Answer: Both groups' lung capacity falls with age, but non-smokers decline only slowly and steadily, while smokers decline steeply after age 40, ending much lower (55% vs 85% at age 70).
(b) B1 30 percentage points (85 - 55) cao
(b) Answer: 30 percentage points
(c) B1 tar in cigarette smoke gradually damages the walls of the alveoli, causing them to break down oe
(c) B1 this reduces the surface area of the lungs available for gas exchange, which reduces (usable) lung capacity/function over time oe
(c) Answer: Tar gradually damages and breaks down the alveoli walls, reducing the surface area available for gas exchange and so reducing lung capacity over time.

Question 19

Level 1 (1-2): Basic, isolated statement(s), with little or no reference to digestion, gas exchange, respiration or exercise linked together. Limited use of scientific terminology.
Level 2 (3-4): Some relevant ideas from more than one body system (e.g. digestion and breathing) are described, with a partial explanation of how they supply the muscles; the link to anaerobic respiration under high demand may be incomplete or not clearly explained. Some scientific terminology used.
Level 3 (5-6): A clear, logically linked explanation covering digestion and absorption of glucose, increased breathing supplying oxygen via the alveoli, increased heart rate transporting glucose and oxygen to the muscles, and a correct explanation of anaerobic respiration and oxygen debt occurring when oxygen demand exceeds supply. Correct scientific terminology used throughout.
Indicative content:
- digestion breaks down carbohydrate in food into glucose, which is absorbed into the blood across the villi of the small intestine
- during exercise, breathing rate and depth increase, bringing more air (and oxygen) into the alveoli
- the alveoli's adaptations (large surface area, thin walls, good blood supply) allow oxygen to diffuse quickly into the blood
- heart rate also increases during exercise, so blood carrying glucose and oxygen is transported to the muscles faster
- as long as enough oxygen reaches the muscles, they respire aerobically, releasing energy for contraction, with carbon dioxide and water as the only waste products
- if the muscles' demand for oxygen becomes greater than the blood can supply, the muscles begin to respire anaerobically as well
- anaerobic respiration in muscles produces lactic acid instead of carbon dioxide and water, and creates an oxygen debt
- this oxygen debt must be repaid after exercise, when extra oxygen is used to break down the lactic acid that has built up

Biology: Respiration, Digestion and Gas Exchange - Worksheets, Questions and Revision