B3.1  Gas Exchange

Theme:  Form and Function

All biological gas exchange systems demonstrate form-function relationships where every structural feature directly serves the functional requirement of efficient molecular exchange across a biological boundary.

• Gas exchange surfaces share essential structural properties (permeability, thin tissue layers, moisture, and large surface area) that directly enable their function of facilitating molecular movement between organisms and their environment.
• Size constraints require structural solutions as cells increase in size, since decreasing surface area-to-volume ratios and increasing diffusion distances demand specialized adaptations to maintain efficient gas exchange.
• Mammalian lung structure maximizes efficiency through branched bronchioles creating extensive surface area, dense capillary networks maintaining concentration gradients, alveolar walls optimized for diffusion, and surfactant reducing surface tension.
• Ventilation mechanisms use structural arrangements of diaphragm, intercostal muscles, and ribs to create pressure changes that drive air movement, with measurable lung volumes reflecting structural capacity.
• Leaf structure balances competing needs: waxy cuticles minimize water loss while stomata enable gas exchange, spongy mesophyll creates internal air spaces for diffusion, and guard cells control stoma opening size.
• Tissue distribution in leaves creates functional zones with different roles.
• Hemoglobin structure enables efficient transport through cooperative binding that creates S-shaped dissociation curves, with different forms (foetal vs adult) having structural differences that match functional requirements.
• Bohr shift demonstrates how hemoglobin structure responds to carbon dioxide levels, increasing oxygen release where it's most needed through allosteric structural changes.

Guiding Questions:  
Guiding questions help students view the content of the syllabus through the conceptual lenses of both the themes and the levels of biological organization.

• How are multicellular organisms adapted to carry out gas exchange?
• What are the similarities and differences in gas exchange between a flowering plant and a mammal?

​​Linking Questions:  
Linking questions strengthen students’ understanding by making connections between topics.  The ideal outcome of the linking questions is networked knowledge.

• How do multicellular organisms solve the problem of access to materials for all their cells?
• What is the relationship between gas exchange and metabolic processes in cells?

Key Terms to Know: * higher level only

Abdominal Muscles
Affinity*
Allosteric Binding*
Alveoli
Bohr Shift*
Bronchiole
Capillary
Concentration Gradient
Cooperative Binding*
Cuticle (Leaf)
Diaphragm
Dissociation*
Epidermis (Leaf)
Expiratory Reserve

Foetal Haemoglobin*
Gas Exchange
Gills
Guard Cell
Haem Group*
Haemoglobin*
Inspiratory Reserve
Intercostal Muscles
Leaf
Leaf Cast
Lungs
Micrograph
Oxygen Dissociation Curve*
Permeability

Quantitative Data
Replicate Trials
Ribs
Spongy Mesophyll
Stomata / Stoma
Stomatal Density
Surface Area
Surface Area-To-Volume Ratio
Surfactant
Tidal Volume
Transpiration
Vein (Leaf)
Ventilation
Vital Capacity

B3.1.1— Gas exchange as a vital function in all organisms.

• Define gas exchange.
• State the role of diffusion in gas exchange. 
• Explain the need for structures of larger organisms to maintain a large enough surface area for gas exchange.​

B3.1.2— Properties of gas-exchange surfaces.

• Outline the function of the following properties of gas-exchange surfaces:   permeability, thin tissue layer, moisture and large surface area.​

B3.1.3— Maintenance of concentration gradients at exchange surfaces in animals.

• State the reason why concentration gradients must be maintained at exchange surfaces.
• Explain how cellular respiration, blood flow, and ventilation with air for lungs and with water for gills as mechanisms for maintaining concentration gradients at exchange surfaces in animals.

B3.1.4— Adaptations of mammalian lungs for gas exchange.

• State the locations of gas exchange in humans.
• Identify the structure of the airway that connects the alveoli to the outside of the body.
• Outline the structures of mammalian lungs that are adapted to maximizing gas exchange; include the presence of surfactant, a branched network of bronchioles, extensive capillary beds and a high surface area.
• Draw a diagram showing the structure of an alveolus and an adjacent capillary.​

B3.1.5— Ventilation of the lungs.

• ​Define ventilation, inspiration and expiration.
• State the relationship between gas pressure and volume.
• Outline the pressure and volume changes that occur during inspiration and expiration.​
• Outline the role of the diaphragm, intercostal muscles, abdominal muscles and ribs during inspiration and expiration.
  

B3.1.6- Measurement of lung volumes.

• Define ventilation rate, tidal volume, vital capacity, and inspiratory and expiratory reserve.
• List methods for measuring tidal volume, vital capacity, and inspiratory and expiratory reserve.

B3.1.7— Adaptations for gas exchange in leaves.

• State the direction of movement of gasses exchanged in leaves.
• Outline adaptations for gas exchange in leaves, including  epidermis, waxy cuticle, stoma, guard cells, air spaces, spongy mesophyll, and veins.

B3.1.8- Distribution of tissues in a leaf.

• State that a plan diagram shows the distribution of tissues, but not individual cells. 
• Draw and label a plan diagram to show the distribution of tissues in a transverse section of a leaf.  Include upper and lower epidermis, palisade and spongy mesophyll, xylem and phloem.

B3.1.9- Transpiration as a consequence of gas exchange in a leaf.

• Define transpiration.
• Outline the relationship between water evaporation and transpiration. 
• Discuss the effect of abiotic factors on the rate of transpiration; including temperature and humidity.
• Discuss the advantages of opening and closing stomata at different times of day.

B3.1.10- Stomatal density.

• Calculate stomatal density from a leaf cast or micrograph. 
• Interpret micrographs or cast of leaf surfaces to compare stomatal density on different leaf surfaces.

AHL ​B3.1.11- Adaptations of fetal and adult hemoglobin for the transport of oxygen.

• Describe the structure and function of hemoglobin.
• Define affinity.
• Outline the process of cooperative binding of oxygen to hemoglobin.
• Compare the oxygen affinity of adult and fetal hemoglobin.​
  

AHL B3.1.12- Bohr shift.

• Describe allosteric binding of CO2 to hemoglobin and the consequences for oxygen transport. 
• Define the Bohr shift. 
• Explain the mechanism and benefit of the Bohr shift.
• State the effect of the Bohr shift on the oxygen dissociation curve.

AHL B3.1.13- Oxygen dissociation curves as a means of representing the affinity of hemoglobin for oxygen at different oxygen concentrations.

• Define partial pressure.
• State the relative partial pressures of oxygen  in the atmosphere at sea level, in the alveoli, in alveoli blood capillaries, and in respiring tissue.
• Draw the oxygen dissociation curve to show affinity of hemoglobin for oxygen at different partial pressures of oxygen.
• Explain difference in the oxygen dissociation curves of adult and fetal hemoglobin.