DP Biology HL B3.1 Gas Exchange

DP Biology HL B3.1 Gas Exchange - Full notes

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DP Biology HL | B3.1 Gas Exchange

DP BIOLOGY · THEME B: FORM AND FUNCTION

B3.1

Gas Exchange

Full notes for HL · IB-style

Contents

1	Gas exchange and the properties of exchange surfaces
2	The human lungs and ventilation
3	Adaptations of alveoli and capillaries
4	Measuring lung volumes
5	Gas exchange in leaves
6	Measuring the rate of transpiration
7	Haemoglobin and oxygen dissociation curves (HL only)
8	Carbon dioxide transport and the Bohr shift (HL only)
9	Properties of gas exchange surfaces
10	Fick's law
11	Maintaining concentration gradients
12	The human gas exchange system
13	Adaptations of alveoli
14	Ventilation: inhalation and exhalation
15	Lung volumes and spirometry
16	Gas exchange in leaves
17	Stomata and transpiration
18	Measuring stomatal density
19	Gas exchange in other organisms
20	Lung disease
21	Mixed review: worked problems
22	Additional exam-style practice
23	Oxygen dissociation curves (HL)
24	Ventilation and exercise
25	High altitude adaptation
26	Common misconceptions and links
27	Exam-style question with markscheme
28	Check your understanding
29	Summary and topic map

These are full teaching notes. Read each section in order, try the Think about it question before you read on, and do every Quick check before moving to the next section.

Before you start

Rate yourself on each statement now, and again after you've worked through the notes. Shade R (not yet), A (getting there) or G (confident).

I can…	R	A	G
explain why gas exchange is needed and the properties of efficient exchange surfaces
explain the mechanism of ventilation
explain the adaptations of alveoli and capillaries
define tidal volume, vital capacity and inspiratory and expiratory reserve volumes
explain the adaptations of leaves for gas exchange
explain how the rate of transpiration can be measured and the factors that affect it
explain the oxygen dissociation curve and cooperative binding
explain the Bohr shift and the transport of carbon dioxide
compare fetal and adult haemoglobin

Sections marked HL only (Sections 7 and 8) are Additional Higher Level content. Every other section is common to SL and HL.

Guiding questions

How are organisms adapted to exchange gases efficiently with their environment? How do structures as different as lungs and leaves solve the same problem?

How this topic is examined

B3.1 is examined through diagrams of the lungs, alveoli and leaf, ventilation and lung volume data (spirometer traces), and transpiration experiments. At HL, oxygen dissociation curves are a frequent data question.

Link each adaptation to the property of exchange surfaces it improves: area, thinness, concentration gradient, moisture.

Command term	What the examiner wants
State / Identify	a brief answer with no explanation needed
Outline	a brief account or summary
Describe	a detailed account, including trends or features, without reasons
Explain	a detailed account including causes, reasons or mechanisms
Distinguish / Compare and contrast	give the differences (and similarities), point by point
Calculate	a numerical answer with working and units
Deduce / Suggest	reach a conclusion from data; propose a reasonable explanation
Evaluate / Discuss	weigh strengths and limitations and reach a judgement

How to use these notes

Each section teaches one part of the syllabus in full. You'll see these boxes throughout; each has one job:

Box	What it's for
Think about it	a question to get you thinking before a section; the answer comes in the section
Key point	the one idea you must not forget
Common mistake	an error examiners see every year, and how to avoid it
In the real world	where the biology shows up outside the classroom
Try this	something quick you can do yourself to test the idea
Worked example	full solutions with IB-style marks (M1 method, A1 answer, R1 reasoning)
Quick check	short questions to test yourself at the end of each section

1 Gas exchange and the properties of exchange surfaces

Every exchange surface solves the same physical problem.

Think about it

A flatworm absorbs oxygen through its skin. Why can't a human do the same?

Cells need oxygen for aerobic respiration and must remove carbon dioxide; photosynthesizing cells need carbon dioxide and release oxygen. These gases move by diffusion, which is fast only over very short distances. Small, flat organisms can exchange gases across their body surface, but large, active organisms have a small surface area to volume ratio (B2.3) and need specialized gas exchange surfaces.

Efficient gas exchange surfaces share four properties: large surface area; thin walls, giving a short diffusion distance; moist, so gases can dissolve before diffusing; and permeable. In addition, a steep concentration gradient must be maintained, by ventilation (bringing fresh air or water) and by a good blood supply (carrying gases away). Fick's law summarizes this: rate of diffusion ∝ surface area × concentration difference ÷ thickness.

Property	Effect	Example in lungs
Large surface area	more gas can diffuse at once	about 300 million alveoli, 70 m²
Thin	short diffusion distance	alveolar and capillary walls one cell thick
Moist	gases dissolve first	film of moisture lining alveoli
Steep gradient	fast diffusion	ventilation and blood flow

Quick check

1. Name four properties of gas exchange surfaces.

2. How is the concentration gradient maintained in the lungs?

Answers: 1. large area, thin, moist, permeable 2. ventilation and continuous blood flow

2 The human lungs and ventilation

Pressure changes move air in and out.

Air enters through the nose and mouth, passes down the trachea, which divides into two bronchi, then into smaller bronchioles, ending in clusters of air sacs called alveoli. The trachea and bronchi are supported by rings of cartilage that keep them open.

Ventilation is the movement of air into and out of the lungs. It works by changing the volume of the thorax, which changes the pressure inside it; air always moves from higher to lower pressure.

Inspiration (breathing in): the external intercostal muscles contract, moving the ribcage up and out; the diaphragm contracts and flattens. Thorax volume increases, pressure in the lungs falls below atmospheric pressure, and air flows in.

Expiration (breathing out): at rest it is mainly passive: the external intercostals and diaphragm relax, the ribcage moves down and in, the diaphragm domes upwards, and elastic recoil of the lungs reduces volume. Pressure rises above atmospheric, and air flows out. During forced expiration (exercise), the internal intercostal muscles and abdominal muscles contract.

	Inspiration	Expiration (at rest)
External intercostals	contract	relax
Diaphragm	contracts, flattens	relaxes, domes up
Ribcage	up and out	down and in
Thorax volume	increases	decreases
Lung pressure	falls below atmospheric	rises above atmospheric
Air	flows in	flows out

Common mistake

Saying 'the lungs inflate, so air rushes in'. The muscles increase thorax volume, which lowers pressure; air then flows in because of the pressure difference.

Quick check

1. What happens to the diaphragm during inspiration?

2. Why does air enter the lungs?

Answers: 1. contracts and flattens 2. pressure in the lungs falls below atmospheric pressure

3 Adaptations of alveoli and capillaries

Millions of tiny sacs, each wrapped in capillaries.

Each alveolus is a tiny sac about 0.2 mm across. Its wall is made mostly of very thin type I pneumocytes, with type II pneumocytes secreting surfactant, which reduces surface tension and stops alveoli collapsing. The alveoli are surrounded by a dense network of capillaries, whose walls are also one cell thick (endothelium).

Oxygen diffuses from air in the alveolus, through the alveolar wall and capillary wall, into the blood, where it binds to haemoglobin in red blood cells. Carbon dioxide diffuses the other way. The total diffusion distance is less than 1 µm.

Adaptations: huge total surface area; very short diffusion distance; moist lining; dense capillary network with continuous blood flow maintaining the gradient; and ventilation bringing fresh air. Elastic fibres allow alveoli to stretch and recoil.

In the real world

Emphysema, usually caused by smoking, destroys the walls between alveoli, merging them into larger spaces with a much smaller total surface area. Patients become breathless because gas exchange is far less efficient.

Quick check

1. What is the function of surfactant?

2. Give three adaptations of alveoli for gas exchange.

Answers: 1. reduces surface tension, preventing collapse 2. large area, thin walls, moist, capillary network

4 Measuring lung volumes

A spirometer records how much air moves.

A spirometer measures the volumes of air breathed in and out. Tidal volume is the volume of air breathed in or out in a normal breath at rest (about 0.5 dm³). Inspiratory reserve volume is the extra air that can be breathed in by maximum effort after a normal inspiration. Expiratory reserve volume is the extra air that can be forced out after a normal expiration. Vital capacity is the maximum volume of air that can be exhaled after a maximum inhalation: tidal volume + inspiratory reserve + expiratory reserve. Some air (the residual volume) always remains in the lungs.

Ventilation rate is the number of breaths per minute; minute volume = tidal volume × ventilation rate. During exercise, both increase.

Figure 1. A spirometer trace showing normal breaths and one maximum breath.

WORKED EXAMPLE 1 Lung volume calculations

A student has a tidal volume of 0.5 dm³, an inspiratory reserve volume of 2.5 dm³ and an expiratory reserve volume of 1.2 dm³, breathing 14 times per minute. Calculate vital capacity and minute volume.

Solution

Vital capacity = 0.5 + 2.5 + 1.2 = 4.2 dm³A1

Minute volume = 0.5 × 14 = 7.0 dm³ min⁻¹A1

Quick check

1. Define vital capacity.

2. Define tidal volume.

Answers: 1. maximum volume exhaled after maximum inhalation 2. volume breathed in or out in a normal breath

5 Gas exchange in leaves

Leaves exchange gases through stomata and air spaces.

In leaves, cells need carbon dioxide for photosynthesis and release oxygen; at night, respiration reverses the net flow. Leaves are adapted for gas exchange: they are thin and flat, giving a large surface area; the spongy mesophyll has large air spaces so gases reach every cell; mesophyll cell walls are moist; and stomata, pores mainly in the lower epidermis, allow gases in and out.

Each stoma is surrounded by two guard cells that control its opening. When guard cells take up water and become turgid, the stoma opens; when they lose water, it closes. The waxy cuticle reduces water loss from the epidermis.

Gas exchange inevitably causes water loss: water evaporates from moist mesophyll cell walls into the air spaces and diffuses out through open stomata. This loss of water vapour from the leaves is transpiration. Plants balance the need for CO₂ uptake with the need to conserve water, closing stomata in hot, dry conditions or when water is scarce (the hormone abscisic acid triggers closure).

Leaf tissue	Adaptation for gas exchange
Waxy cuticle	reduces water loss
Upper epidermis	transparent, lets light through
Palisade mesophyll	many chloroplasts; photosynthesis
Spongy mesophyll	air spaces for diffusion; moist surfaces
Lower epidermis	stomata with guard cells
Vascular bundles	xylem brings water; phloem removes sugars

Quick check

1. Where are most stomata found?

2. What causes stomata to open?

3. Why does gas exchange cause water loss?

Answers: 1. lower epidermis 2. guard cells become turgid 3. water evaporates from moist cell surfaces and diffuses out through stomata

6 Measuring the rate of transpiration

Measure water uptake as a proxy for water loss.

A potometer measures the rate of water uptake by a cut shoot, which is approximately equal to the rate of transpiration. A shoot is cut underwater (to stop air entering the xylem) and fitted into a tube with a scale. An air bubble introduced into the capillary tube moves as the shoot takes up water; the distance moved per minute gives the rate.

Transpiration rate is increased by higher temperature (faster evaporation and diffusion), lower humidity (steeper water vapour gradient), air movement (wind removes humid air near the leaf), and higher light intensity (stomata open for photosynthesis). It is reduced when stomata close.

Stomatal density (number per mm²) can be measured by painting nail varnish on the leaf surface, peeling it off to make an impression, and counting stomata under a microscope.

Figure 2. Potometer results under different conditions.

WORKED EXAMPLE 2 Calculating volume of water uptake

The bubble in a potometer capillary tube of radius 0.5 mm moves 30 mm in 5 minutes. Calculate the rate of water uptake in mm³ per minute.

Solution

Volume = πr² × distance = 3.14 × 0.5² × 30 = 23.6 mm³M1

Rate = 23.6 ÷ 5 = 4.7 mm³ min⁻¹A1

Quick check

1. What does a potometer actually measure?

2. Give three factors that increase transpiration.

Answers: 1. rate of water uptake 2. temperature, low humidity, wind, light

7 Haemoglobin and oxygen dissociation curves (HL only)

Haemoglobin loads oxygen in the lungs and unloads it in tissues.

HL only

This section is Additional Higher Level content. SL students do not need it.

Haemoglobin is a conjugated protein with four polypeptide subunits, each with a haem group containing an iron ion that binds one O₂ molecule. Each haemoglobin can therefore carry four O₂.

Binding is cooperative: when one O₂ binds, the shape of the protein changes slightly, making it easier for the next O₂ to bind (an allosteric effect). Releasing one O₂ makes it easier to release the others. This produces an S-shaped (sigmoid) oxygen dissociation curve, which plots percentage saturation of haemoglobin against the partial pressure of oxygen (pO₂).

Significance of the S-shape. In the lungs, pO₂ is high (about 13 kPa), so haemoglobin becomes almost fully saturated. In respiring tissues, pO₂ is low (about 2 to 5 kPa); here the curve is steep, so a small fall in pO₂ causes a large release of oxygen exactly where it is needed.

Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin: its curve lies to the left. At the low pO₂ of the placenta, fetal haemoglobin can take oxygen from the mother's haemoglobin.

Figure 3. Oxygen dissociation curves: fetal haemoglobin lies to the left; high CO₂ shifts the adult curve to the right.

Quick check

1. Why is the dissociation curve S-shaped?

2. Why does fetal haemoglobin have a higher affinity for oxygen?

Answers: 1. cooperative binding changes affinity as O₂ binds 2. to take oxygen from maternal haemoglobin in the placenta

8 Carbon dioxide transport and the Bohr shift (HL only)

More CO₂ means oxygen is released more readily.

HL only

This section is Additional Higher Level content. SL students do not need it.

Carbon dioxide is carried in three ways: about 5% dissolved in plasma; about 10% bound to haemoglobin (carbaminohaemoglobin); and about 85% as hydrogen carbonate ions (HCO₃⁻). In red blood cells, the enzyme carbonic anhydrase catalyses CO₂ + H₂O → H₂CO₃, which dissociates to H⁺ + HCO₃⁻. HCO₃⁻ diffuses into the plasma (balanced by Cl⁻ moving in, the chloride shift).

The H⁺ ions bind to haemoglobin, reducing its affinity for oxygen. So in actively respiring tissues, where CO₂ is high, haemoglobin releases more oxygen. This is the Bohr shift: high CO₂ shifts the dissociation curve to the right. In the lungs, where CO₂ is low, the reverse happens and oxygen is loaded readily.

Quick check

1. What enzyme converts CO₂ to carbonic acid?

2. What is the Bohr shift?

Answers: 1. carbonic anhydrase 2. high CO₂ shifts the curve right, releasing more O₂ in tissues

9 Properties of gas exchange surfaces

Four key features.

Efficient gas exchange surfaces have: a large surface area; a short diffusion distance (thin); a moist surface (gases dissolve before diffusing); and a steep concentration gradient (maintained by ventilation and blood flow).

WORKED EXAMPLE 3 Features (standard)

List four properties of an efficient gas exchange surface.

Solution

Large surface area; thinA1

Moist; steep concentration gradientA1

Quick check

1. Why must gas exchange surfaces be moist?

Answers: 1. gases dissolve in water before diffusing through cells

10 Fick's law

Factors affecting diffusion rate.

Rate of diffusion ∝ (surface area × concentration difference)/thickness of the exchange surface. Doubling the surface area doubles the rate; doubling the thickness halves it.

WORKED EXAMPLE 4 Applying Fick's law (standard)

Emphysema reduces alveolar surface area by 40%. Predict the effect on diffusion rate.

Solution

Rate ∝ surface areaM1

Rate falls to 60% of normal (other factors constant)A1

11 Maintaining concentration gradients

Ventilation and blood flow.

In the lungs, ventilation brings fresh air (high O2, low CO2) to the alveoli, and blood flow carries oxygenated blood away and deoxygenated blood in. Both keep the gradients steep.

WORKED EXAMPLE 5 Gradients (exam level)

Explain how ventilation and blood flow maintain gradients in the alveoli.

Solution

Ventilation replaces alveolar air: keeps O2 high and CO2 lowR1

Blood flow removes O2 and brings CO2: keeps gradients steepR1

12 The human gas exchange system

From trachea to alveoli.

Air passes through the trachea (with C-shaped cartilage rings), bronchi, bronchioles and into alveoli. The airways are lined with ciliated epithelium and mucus-secreting goblet cells that trap and remove particles. There are about 480 million alveoli with a total area of about 70 m2.

WORKED EXAMPLE 6 Cartilage (standard)

Why does the trachea have cartilage rings?

Solution

Keeps the airway open during pressure changes in breathingR1

13 Adaptations of alveoli

Designed for diffusion.

Alveoli have walls one cell thick (type I pneumocytes), a dense capillary network, surfactant from type II pneumocytes, and a large total surface area. The alveolar and capillary walls together are less than 1 μm thick.

WORKED EXAMPLE 7 Adaptation (exam level)

Explain three adaptations of alveoli for gas exchange.

Solution

Thin walls (one cell): short diffusion distanceR1

Large surface area: many alveoliR1

Dense capillaries: maintain gradientR1

14 Ventilation: inhalation and exhalation

Pressure changes.

Inhalation: external intercostal muscles contract (ribs up and out), diaphragm contracts (flattens); thoracic volume increases, pressure falls below atmospheric, air flows in. Exhalation (at rest) is passive: muscles relax, elastic recoil decreases volume, pressure rises, air flows out. Forced exhalation uses internal intercostals and abdominal muscles.

WORKED EXAMPLE 8 Inhalation (standard)

Explain how contraction of the diaphragm causes inhalation.

Solution

Diaphragm flattens, increasing thoracic volumeR1

Pressure in lungs falls below atmospheric; air moves inR1

Common mistake

Saying the lungs 'suck in air'. Air flows in because the pressure inside the lungs falls below atmospheric pressure.

Quick check

1. Why is quiet exhalation passive?

Answers: 1. it relies on elastic recoil of the lungs and muscle relaxation

15 Lung volumes and spirometry

Measuring breathing.

A spirometer measures lung volumes. Tidal volume: volume per normal breath (about 0.5 dm3). Vital capacity: maximum volume exhaled after maximum inhalation (about 4–5 dm3). Inspiratory and expiratory reserves: extra volumes beyond tidal. Ventilation rate = tidal volume × breaths per minute.

WORKED EXAMPLE 9 Ventilation rate (standard)

Tidal volume is 0.48 dm3 at 15 breaths per minute. Find the pulmonary ventilation.

Solution

0.48 × 15M1

= 7.2 dm3 min−1A1

Quick check

1. Vital capacity is 4.6 dm3, tidal volume 0.5, inspiratory reserve 2.8. Find the expiratory reserve.

Answers: 1. 4.6 − 0.5 − 2.8 = 1.3 dm3

16 Gas exchange in leaves

Plants exchange gases too.

Leaves exchange CO2 and O2 through stomata. The spongy mesophyll has air spaces that give a large internal surface area; cells are moist. Guard cells control stomatal opening, balancing CO2 uptake against water loss.

WORKED EXAMPLE 10 Leaf structure (standard)

Explain how the spongy mesophyll is adapted for gas exchange.

Solution

Air spaces give large surface areaR1

Moist cell walls; gases diffuse quickly through airR1

17 Stomata and transpiration

Trade-off between CO2 and water.

Open stomata allow CO2 in for photosynthesis but also allow water vapour out (transpiration). Stomata close in darkness, drought (abscisic acid) or high temperature. Stomatal density varies between species and environments.

WORKED EXAMPLE 11 Trade-off (exam level)

Explain why xerophytes often have fewer, sunken stomata.

Solution

Reduces water loss by transpirationR1

Humid air trapped near the stomata reduces the gradientR1

Quick check

1. Name the hormone that causes stomatal closure during drought.

Answers: 1. abscisic acid

18 Measuring stomatal density

A practical technique.

Paint clear nail varnish on the leaf surface, peel it off when dry using tape, and examine under a microscope. Count stomata in the field of view and divide by its area to find density (stomata mm−2).

WORKED EXAMPLE 12 Calculation (standard)

28 stomata are counted in a field of view of diameter 0.40 mm. Calculate the density.

Solution

Area = π(0.20)2 = 0.126 mm2M1

28/0.126 = 223 mm−2A1

Try this

Compare stomatal density on the upper and lower surfaces of a leaf using nail-varnish peels.

19 Gas exchange in other organisms

Gills, tracheae and skin.

Fish gills have filaments and lamellae with a large surface area and use countercurrent flow (water and blood flow in opposite directions) to maintain a gradient along the whole lamella. Insects use tracheae that deliver air directly to tissues. Earthworms and amphibians exchange gases through moist skin.

WORKED EXAMPLE 13 Countercurrent (exam level)

Explain why countercurrent flow in gills is more efficient than parallel flow.

Solution

A concentration gradient is maintained along the entire lengthR1

Blood can absorb a higher proportion of oxygen from waterR1

20 Lung disease

Effects on gas exchange.

Emphysema destroys alveolar walls, reducing surface area. Lung cancer and fibrosis thicken or block tissue. Smoking damages cilia and causes inflammation. Asthma narrows airways.

WORKED EXAMPLE 14 Emphysema (standard)

Explain why people with emphysema are breathless.

Solution

Alveolar walls broken down: reduced surface areaR1

Less oxygen diffuses into the bloodR1

21 Mixed review: worked problems

Bringing the ideas together.

WORKED EXAMPLE 15 Mixed problem (exam level)

Compare gas exchange in a human lung and a leaf.

Solution

Both: large surface area, moist surfaces, diffusion down gradientsR1

Lungs: ventilation and blood flow maintain gradientsR1

Leaves: stomata and air spaces; no active ventilationR1

22 Additional exam-style practice

Paper 1 and Paper 2.

WORKED EXAMPLE 16 Paper 1 question (standard)

Which muscles contract during normal inhalation? A. internal intercostals and diaphragm B. external intercostals and diaphragm C. abdominal muscles D. internal intercostals only

Solution

BA1

WORKED EXAMPLE 17 Paper 2 question (exam level)

Explain the roles of type I and type II pneumocytes.

Solution

Type I: thin, gas exchangeR1

Type II: secrete surfactant to reduce surface tensionR1

23 Oxygen dissociation curves (HL)

Haemoglobin and cooperative binding.

Haemoglobin binds oxygen cooperatively: binding of one O2 makes the next bind more easily, giving an S-shaped (sigmoid) dissociation curve. The Bohr shift: higher CO2 (lower pH) shifts the curve to the right, so haemoglobin releases more oxygen in active tissues. Fetal haemoglobin has a higher affinity than adult haemoglobin.

WORKED EXAMPLE 18 Bohr shift (exam level)

Explain the advantage of the Bohr shift during exercise.

Solution

Active muscles produce CO2, lowering pHR1

Haemoglobin releases more oxygen to respiring tissuesR1

24 Ventilation and exercise

Responding to demand.

During exercise, both tidal volume and ventilation rate increase, controlled by the respiratory centre in the medulla, which responds to rising CO2 (falling blood pH) detected by chemoreceptors.

Quick check

1. Which change in blood is the main stimulus for increased breathing?

Answers: 1. rising CO2 (falling pH)

25 High altitude adaptation

Less oxygen available.

At high altitude the partial pressure of oxygen is lower. Acclimatization includes increased ventilation, more red blood cells (EPO secretion) and more capillaries. Populations such as Tibetans have genetic adaptations for efficient oxygen use.

In the real world

Endurance athletes often train at altitude to stimulate red blood cell production before competing at sea level.

Quick check

1. Why does the body produce more red blood cells at altitude?

Answers: 1. to carry more oxygen when less is available per breath

26 Common misconceptions and links

Students often think…	Actually…
the lungs suck air in	volume change lowers pressure; air flows in
transpiration is deliberate water loss	it is an unavoidable consequence of gas exchange
a potometer measures transpiration directly	it measures water uptake
the Bohr shift moves the curve left	high CO₂ shifts it right

How B3.1 connects to the rest of the course

Topic	Connection
B2.3 Cell specialization	pneumocytes; SA:V ratio
B3.2 Transport	blood and xylem
C1.2 Respiration	why cells need oxygen
C1.3 Photosynthesis	CO₂ uptake
D3.3 Homeostasis	control of ventilation

27 Exam-style question with markscheme

Try this under exam conditions (about 15 minutes) before reading the markscheme.

Figure 4. A simplified spirometer trace.

(a) Using the trace, estimate the tidal volume. [1]

(b) Explain the mechanism of inspiration. [3]

(d) Explain why windy conditions increase transpiration. [2]

(e) Explain the advantage of the Bohr shift to an exercising muscle. [3]

Markscheme

Part	Answer	Marks
(a)	about 0.5 dm³	A1
(b)	external intercostals and diaphragm contract	R1
	thorax volume increases, pressure falls	R1
	air flows in down pressure gradient	R1
(c)	thin walls: short diffusion distance	R1 R1
	capillary network: maintains gradient / large surface area	R1 R1
(d)	removes humid air near leaf	R1
	steeper water vapour gradient; faster diffusion	R1
(e)	high CO₂ in respiring muscle	R1
	lowers haemoglobin affinity for O₂	R1
	more O₂ released to muscle	R1

Examiner's comments. In (b), answers must link muscle contraction to volume, pressure and air flow in that order.

28 Check your understanding

Section A: multiple choice

1. During inspiration the diaphragm

A. contracts and flattens B. relaxes and domes C. stays still D. moves up

2. Vital capacity is

A. a normal breath B. air left after exhalation C. the maximum volume exhaled after maximum inhalation D. breaths per minute

3. Surfactant

A. carries oxygen B. prevents alveoli from collapsing C. digests bacteria D. increases surface tension

4. Stomata are opened by

A. guard cells losing water B. the cuticle C. xylem D. guard cells becoming turgid

5. A potometer measures

A. rate of photosynthesis B. rate of water uptake C. rate of respiration D. oxygen production

6. Transpiration decreases when

A. humidity increases B. temperature increases C. wind increases D. light increases

7. Fetal haemoglobin has

A. a lower affinity B. no haem groups C. the same curve D. a higher affinity for oxygen than adult haemoglobin

8. Most CO₂ is transported as

A. dissolved CO₂ B. carbaminohaemoglobin C. hydrogen carbonate ions D. carbon monoxide

Section B: structured

9. Explain the properties of gas exchange surfaces using the alveolus as an example. [4]

10. Describe the mechanism of ventilation. [5]

11. Explain how a leaf is adapted for gas exchange. [4]

12. Explain the shape of the oxygen dissociation curve. [3]

Answers

1. A 2. C 3. B 4. D 5. B 6. A 7. D 8. C

9. Large area (millions of alveoli), thin (one cell), moist, capillaries and ventilation maintain gradient.

10. Inspiration: intercostals and diaphragm contract, volume up, pressure down, air in; expiration reverse.

11. Thin, flat, air spaces, stomata, moist mesophyll, cuticle.

12. Cooperative binding: first O₂ increases affinity for next; steep middle section releases O₂ in tissues.

29 Summary and topic map

Idea	Remember
Exchange surfaces	large area, thin, moist, gradient
Ventilation	muscles change volume, pressure, air flow
Alveoli	pneumocytes, surfactant, capillaries
Lung volumes	tidal volume, vital capacity, reserves
Leaves	air spaces, stomata, guard cells
Transpiration	potometer; temperature, humidity, wind, light
HL	cooperative binding; S-curve; fetal Hb; Bohr shift; CO₂ transport

Before the exam, make sure you…

•link each adaptation to a property of exchange surfaces
•explain ventilation through volume and pressure
•state that potometers measure uptake
•read lung volumes from spirometer traces

Gas Exchange on one page

Cover this page with a sheet of paper and try to rebuild it from memory. Whatever you can't recall is what to revise next.

Figure 5. Topic map for Gas Exchange.

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