IB Biology HL B3.1 Gas Exchange Questions

Practise IB Biology HL B3.1 by linking respiratory surfaces and ventilation to oxygen transport, dissociation curves and transpiration evidence.

Exam points

Explain how lower oxygen partial pressure at high altitude reduces the diffusion gradient for gas exchange.
Relate alveolar area, thin walls, capillaries and surfactant to efficient gas exchange.
Trace ventilation muscles, thoracic pressure and airflow, interpreting pressure or spirometry data.
Calculate spirometric volumes and compare ventilation responses as exercise intensity increases.
Explain stomatal control, guard-cell responses and leaf adaptations that limit water loss.
Interpret leaf tissue distribution and internal air spaces as routes for light capture and gas diffusion.
Define transpiration, predict environmental water loss and evaluate potometer measurements.
Compare stomatal density and indicator patterns between leaf surfaces.
Explain haemoglobin subunits, haem binding, cooperative loading and fetal oxygen affinity.
Explain the Bohr shift from carbon dioxide and pH, predicting oxygen unloading from curve shifts.
Read oxygen dissociation curves for saturation, pO2, plateaus, cooperativity and relative affinity.

Covered syllabus points

B3.1.1—Gas exchange as vital function B3.1.2—Properties of gas-exchange surfaces B3.1.3—Maintaining concentration gradients B3.1.4—Mammalian lung adaptations B3.1.5—Lung ventilation B3.1.6—Lung volume measurements B3.1.7—Leaf gas exchange adaptations B3.1.8—Leaf tissue distribution B3.1.9—Transpiration B3.1.10—Stomatal density B3.1.11 (HL)—Haemoglobin adaptations B3.1.12 (HL)—Bohr shift B3.1.13 (HL)—Oxygen dissociation curves

[Maximum number: 7]

Adult humans may absorb more than five hundred litres of oxygen per day. Explain how gas exchange is maintained in the human respiratory system.

a. ventilation/inhaling brings fresh air/air with high oxygen concentration to the lungs OR ventilation/exhaling gets rid of stale air/air with high concentration of carbon dioxide

b. ventilation due to muscle contractions causing pressure/volume changes in the thorax

c. contraction of external intercostal muscles AND diaphragm occurs during inspiration OR contraction of internal intercostal muscles/abdomen wall muscles during «forced» expiration

d. alveoli surrounded by «many» capillaries

e. blood flow/pumping of heart «brings blood to/takes blood away from alveoli/lungs»

f. concentration gradients «of oxygen/ $\mathrm{CO}_{2}$ » maintained «by ventilation/blood flow»

g. $\mathrm{O}_{2}$ AND $\mathrm{CO}_{2}$ diffuse

h. $\quad \mathrm{CO}_{2}$ from capillaries/blood/vessel to alveolus/air AND $\mathrm{O}_{2}$ from alveoli into capillaries/blood/vessel

i. large numbers of alveoli increase surface area

j. short distance so rapid diffusion/gas exchange

k. type I pneumocytes/alveolus wall/capillary walls are one cell thick/very thin

l. alveoli «lining» moist for dissolving of gases/rapid diffusion OR

type II pneumocytes keep the «lining of» the alveolus moist

m. type II pneumocytes secrete surfactant to reduce surface tension/prevents alveoli from collapsing

[Maximum number: 3]

Carbon dioxide and oxygen are essential gases in many biological processes.

Describe conditions necessary in the lungs for efficient gas exchange in humans.

a. alveoli provide a large surface area (for easy diffusion of gases);

b. thin walls/ 1 -cell thick walls /type 1 pneumocytes/cells lining the alveoli are (extremely) thin (to facilitate gas exchange);

c. a moist surface on the cells (to dissolve the gases/oxygen and carbon dioxide);

d. type II pneumocytes secrete surfactant to reduce surface tension/prevent sides of alveolus sticking together;

e. blood capillaries have thin walls/1-cell thick walls (for short path/easy diffusion of gases);

f. good supply of blood / blood flow through capillaries around alveoli to maintain the concentration gradients of gases/ higher oxygen concentration in alveoli than in blood capillaries/ higher $\mathrm{CO}_{2}$ concentration in blood capillaries than in alveoli;

g. ventilation/breathing maintains concentration gradients of gases (between air in alveoli and blood capillaries) (to facilitate diffusion);

Mpb. Refers to thin walls, not membranes.

Mpd. Requires function of surfactant.

3 marks

[Maximum number: 3]

The graph shows the relationship between partial pressure of oxygen in air and altitude.

Altitude in metres plotted against partial pressure of oxygen in air in kPa. Labelled points mark the summit of Everest, highest human habitation, Pikes Peak, Denver and Ben Nevis.

Explain the consequence of high altitude for gas exchange in humans.

a. lower oxygen partial pressure reduces oxygen availability (for the blood);

b. less oxygen diffuses/is exchanged with the blood OR less oxygen diffuses across the alveolar membrane/alveoli;

c. less saturation of hemoglobin with oxygen;

d. more $\mathrm{CO}_{2}$ remains in the blood OR less $\mathrm{CO}_{2}$ is exhaled;

e. breathing rate/depth is increased (in the short term);

f. (to compensate for the lack of oxygen) the body will produce more erythrocytes/RBCs/hemoglobin;

Mp a: Do not accept lower oxygen concentration at high altitude. Partial pressure must be mentioned.

Mp c: Do not accept increased hemoglobin affinity for oxygen instead of saturation.

Mp d: Allow higher $\mathrm{CO}_{2}$ decreasing hemoglobin affinity for oxygen.

Marking guidance:

3 max

[Maximum number: 6]

A supply of oxygen is needed for aerobic respiration in mitochondria. Describe the features of alveoli in human lungs that adapt them for efficient absorption of oxygen.

[Maximum number: 8]

Explain the mechanism of ventilation of human lungs.

[Maximum number: 1]

Measurements of the lung capacity of a student were recorded using a spirometer and displayed with a data logger. Initially the student was at rest, then changed to carrying out strenuous exercise. The results are displayed in the graph.

Calculate the ventilation rate at rest, giving the units.

[Maximum number: 2]

The micrograph shows part of a lilac (Syringa vulgaris) leaf in vertical section.

Explain the roles of two leaf structures that help with the process of gas exchange in the leaf.

a. spongy mesophyll provides a large/moist/permeable surface area (for gas exchange);
b. guard cells form pores/stomata (through the epidermis) / open and close stomata OR stomata are pores (through the epidermis that) allow $\mathrm{CO}_{2} / \mathrm{O}_{2} /$ gases to enter/exit the leaf/to pass through the epidermis;
c. (air) spaces for diffusion/movement/transfer of gases (to/from spongy mesophyll cells and stomata);

[Maximum number: 8]

Explain how the distribution of tissues in the leaf of a dicotyledonous plant is adapted to production and distribution of products of photosynthesis.

a. leaf has large surface area for absorption of light;
b. upper epidermis (thin) allowing light to pass;
c. (waxy translucent) cuticle to (allow light in and) prevent water loss;
d. palisade mesophyll contains many (cells with) chloroplasts;
e. palisade mesophyll close to upper layer to receive more light;
f. spongy mesophyll contains chloroplasts which allow photosynthesis;
g. spongy mesophyll (cells loosely packed) allows gaseous exchange;
h. stoma allow $\mathrm{CO}_{2}$ for photosynthesis to diffuse in;
i. stoma allow $\mathrm{O}_{2}$ produced in photosynthesis to diffuse out;
j. xylem brings water (for reactions);
k. phloem carries away products of photosynthesis/sucrose;
l. guard cells open and close stoma (for gas exchange);

[Maximum number: 2]

The rate of transpiration can be measured using a data-logging pressure sensor connected to a plant cutting via a plastic tube. In an experiment, a control set-up was connected to the tube and placed in a well-lit room with normal humidity levels.

Outline how this control set-up could be modified to test the effect of either humidity or temperature on the rate of transpiration.

[Maximum number: 1]

The stomatal density is the number of stomata per area of leaf epidermis. A study was done on the stomatal density of the lower epidermis of leaves from Protium decandrum, a tree found in the Amazon forest.

The image shows the epidermis stripped from the lower surface of a leaf.

Micrograph of lower leaf epidermis showing irregular interlocking cells and several oval stomata. A scale bar at the lower right represents 15 micrometres.

The mean stomatal density for the lower epidermis of P. decandrum was around 600 per $\mathrm{mm}^{2}$ . Predict how the stomatal density for the upper epidermis would compare.

[Maximum number: 2]

Oxygen dissociation curves show the percentage of oxygen saturation of adult haemoglobin (Hb) at different oxygen concentrations, as measured in partial pressure of oxygen ( $\mathrm{pO}_{2}$ ).

(a)

On the graph, sketch an oxygen dissociation curve for foetal haemoglobin.

[ 1 ]

curve should be the sigmoidal and drawn to the left of the oxygen dissociation curve;

Labels not necessary.

Not necessary that curves meet at high $\mathrm{pO}_{2}$ .

(b)

Describe an adaptation of haemoglobin for the transport of oxygen.

[ 1 ]

(c)

Identify the percentage of oxygen saturation of haemoglobin at a partial pressure of 40 mmHg .

[ 1 ]

(d)

Describe the relationship between partial pressure of oxygen ( $\mathrm{pO}_{2}$ ) and percent saturation of haemoglobin.

[ 2 ]

a. as $\mathrm{pO}_{2}$ increases, \% saturation increases

OR

positive correlation/relationship;

b. \% saturation levels off/plateaus/OWTTE (at higher partial pressure);

c. sigmoidal relationship/S curve;

a., b. Accept quoting values for increase and levelling off.

c. Do not accept other terms than sigmoid or S shaped curve.

Marking guidance:

Do not accept 100\% saturation.

[Maximum number: 3]

(a)

The graph shows the oxygen dissociation curve for adult hemoglobin.

[ 3 ]

(i)

Using the graph, draw a line to show how the oxygen dissociation curve changes with the Bohr shift.

[ 1 ]

(ii)

Explain the role of the Bohr shift during vigorous exercise.

[ 2 ]

a. more $\mathrm{CO}_{2}$ is produced which lowers the pH of the blood;
b. hemoglobin releases more oxygen (at lower pH ) for same partial pressure of oxygen;
c. more oxygen is available to respiring tissues;

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