B3.1 Gas exchange
Gas exchange links animal ventilation, alveolar diffusion, leaf stomata, transpiration and haemoglobin affinity to oxygen supply and carbon dioxide removal.
- Syllabus
- First assessment 2025
- Topic
- B3.1
- Level
- HL
Learning objectives
B3.1.1Gas exchange as vital function• Gas exchange supplies respiratory gases between organisms and environment• Animals take in oxygen and release carbon dioxide for aerobic respiration• Larger or active organisms need specialized exchange surfaces because SA:V decreasesB3.1.2Properties of gas-exchange surfaces• Efficient gas-exchange surfaces are permeable, thin, moist, and large• Large surface area and steep concentration gradients increase diffusion• Short diffusion paths speed movement of oxygen and carbon dioxideB3.1.3Maintaining concentration gradients• Dense capillary networks provide large exchange area and rapid transport• Continuous blood flow carries oxygen away and carbon dioxide toward the surface• Ventilation refreshes air or water to maintain steep gas gradientsB3.1.4Mammalian lung adaptations• Many small alveoli provide large surface area and thin squamous walls• Capillary beds maintain gradients and bring red blood cells close to alveolar air• Surfactant reduces surface tension, while elastic fibres support ventilation and recoilB3.1.5Lung ventilation• Diaphragm and intercostal muscles change thoracic volume and pressure• Inspiration uses diaphragm and external intercostal contraction to draw air in• Expiration reduces thoracic volume; internal intercostals and abdominal muscles aid forced exhalationB3.1.6Lung volume measurements• Spirometry records breathing patterns, ventilation rate, and lung volumes• Tidal volume is air moved during normal relaxed breathing• Vital capacity equals tidal volume plus inspiratory and expiratory reserve volumesB3.1.7Leaf gas exchange adaptations• Stomata allow carbon dioxide, oxygen, and water vapour to diffuse• Guard cells open and close stomata by changes in turgor• Waxy cuticle, lower epidermal stomata, air spaces, mesophyll, and veins balance gas exchange with water lossB3.1.8Leaf tissue distribution• Dicot leaves have waxy cuticle, upper and lower epidermis, palisade and spongy mesophyll• Vascular bundles contain xylem and phloem for water and assimilate transport• Spongy mesophyll air spaces and stomata create internal diffusion routesB3.1.9Transpiration• Transpiration is evaporation from mesophyll walls followed by diffusion of water vapour through stomata• Temperature, humidity, wind, and light alter concentration gradients and stomatal opening• Potometers estimate transpiration indirectly by measuring water uptakeB3.1.10Stomatal density• Stomatal density is the number of stomata per unit leaf area• Higher density can increase carbon dioxide uptake and water loss• Leaf casts or micrographs allow stomatal counts and comparisons between speciesB3.1.11(HL)—Haemoglobin adaptations• Haemoglobin has four subunits with haem groups that bind oxygen reversibly• Cooperative binding increases oxygen loading in high pO2 and unloading in low pO2• Foetal haemoglobin has higher oxygen affinity than adult haemoglobin for placental transferB3.1.12(HL)—Bohr shift• Increased carbon dioxide lowers pH and reduces haemoglobin affinity for oxygen• The oxygen dissociation curve shifts right at higher carbon dioxide concentration• The Bohr effect promotes oxygen release in actively respiring tissuesB3.1.13(HL)—Oxygen dissociation curves• Oxygen dissociation curves plot haemoglobin saturation against partial pressure of oxygen• A sigmoid curve shows cooperative binding between haemoglobin subunits• Curve position indicates oxygen affinity, loading in lungs, and unloading in tissues
Four Properties Increase Diffusion Rate
Fick’s law predicts faster gas transfer with greater surface area and partial-pressure difference, and with a thinner diffusion barrier. A moist, permeable surface lets gases dissolve before crossing.
For every claimed adaptation, name the changed variable: area, gradient, distance or permeability. Then connect it to faster O₂ or CO₂ diffusion.
Muscle Contraction Creates the Pressure Difference for Airflow
Air moves because thoracic volume changes alter lung pressure. Inspiration is active: diaphragm and external intercostals contract, increasing volume and lowering pressure below atmospheric.
inspiration: muscles contract → thorax expands → pressure falls → air enters
quiet expiration: muscles relax + elastic recoil → volume falls → pressure rises → air leaves
A Stoma Trades Carbon Entry for Water Loss
CO₂ enters and water vapour exits mainly through stomata. Guard cells change pore width, balancing photosynthetic demand against dehydration risk.
CO₂: atmosphere → stoma → substomatal air space → moist mesophyll wall → photosynthetic cell
H₂O: xylem → mesophyll wall → evaporation → air spaces → diffusion through stoma
Leaf Layers Coordinate Light, Gas and Transport
Leaf tissue distribution places palisade cells near light, spongy mesophyll beside interconnected air spaces, stomata in epidermis and vascular bundles within diffusion range of cells.
upper epidermis/cuticle → light entry + water conservation
palisade → chloroplast-rich light capture
spongy mesophyll → moist exchange area + air spaces
xylem/phloem → water supply + assimilate export
Stomatal Density Measures Pores per Leaf Area
Stomatal density = number of stomata counted ÷ sampled epidermal area. It estimates potential pore abundance, while aperture determines how open those pores are at a given moment.
clear nail-varnish impression → peel and mount → calibrate field area → count stomata in random fields → calculate each density → repeat and report variation
Haemoglobin Loads Oxygen Cooperatively
Haemoglobin has four haem-containing subunits and binds O₂ reversibly. Binding one O₂ promotes a higher-affinity conformation, producing the sigmoid oxygen-dissociation curve.
low pO₂: initial binding is difficult
middle range: small pO₂ change causes large saturation change
high pO₂: near plateau, loading remains high despite modest fluctuation
Read Dissociation Curves at a Fixed Partial Pressure
Curve height gives percentage saturation at a chosen pO₂. A left-shifted curve has higher affinity; a right-shifted curve has lower affinity and releases more oxygen at the same tissue pO₂.
choose pO₂ on x-axis → draw vertically to each curve → read saturation horizontally → subtract saturations → explain the difference using affinity and biological context