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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
SL

Gas Exchange Must Match Cellular Respiration

Respiration consumes O₂ and produces CO₂. A gas-exchange system is needed when diffusion across the outer body surface cannot move these gases fast enough for all cells.

small/thin organism → high SA:V + short diffusion path → direct exchange may suffice
large or active organism → lower SA:V + long internal distances + high demand → specialized surface, ventilation and transport

The system does not create diffusion. It maintains a large area, a short path and steep partial-pressure gradients so diffusion can meet metabolic demand.

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.

Mobile-friendly annotated alveolus-capillary diagram showing a thin moist permeable exchange surface, large combined area, short diffusion path, and opposite directions of oxygen and carbon dioxide diffusion.

For every claimed adaptation, name the changed variable: area, gradient, distance or permeability. Then connect it to faster O₂ or CO₂ diffusion.

Ventilation and Blood Flow Preserve Opposite Gradients

At an alveolus, ventilation replaces air while perfusion replaces blood. Together they keep alveolar O₂ above venous-blood O₂ and venous-blood CO₂ above alveolar CO₂.

alveolar air high O₂ → O₂ diffuses into blood → haemoglobin binding removes dissolved O₂
venous blood high CO₂ → CO₂ diffuses into alveolus → expiration removes it

Reduced ventilation lowers alveolar O₂; reduced perfusion leaves no blood to load. Either mismatch lowers whole-system exchange even if the membrane is intact.

Alveoli Combine Area, Thinness and Gradient Maintenance

Millions of alveoli create a very large moist surface. Type I pneumocytes and capillary endothelium form a short diffusion path; elastic fibres support recoil and surfactant limits collapse.

airway branching → alveolar air → type I pneumocyte → basement membrane region → capillary endothelium → plasma → erythrocyte haemoglobin

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.

Show thorax-ventilation-sequence so the learner can trace Inspiration and Expiration.

inspiration: muscles contract → thorax expands → pressure falls → air enters
quiet expiration: muscles relax + elastic recoil → volume falls → pressure rises → air leaves

Read a Spirometer Trace as Volume over Time

Tidal volume is the change during one normal breath. Breathing rate is cycles per minute. Ventilation rate = tidal volume × breathing rate; vital capacity spans maximum inspiration to maximum expiration.

1 identify axes and calibration
2 measure peak-to-trough height for volume
3 count complete cycles across a known time
4 multiply using consistent units
5 separate amplitude change from frequency change

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.

Leaf gas-exchange tradeoff diagram showing waxy cuticle limiting water loss, lower epidermal stomata with guard cells controlling pores, spongy mesophyll air spaces for diffusion, and veins supplying water and removing assimilates.

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.

Clean labelled dicot leaf cross-section with cuticle, epidermis, palisade mesophyll, spongy mesophyll air spaces, stomata, xylem, and phloem, optimized for mobile reading.

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

Transpiration Responds to the Vapour Gradient and Stomata

Transpiration is water loss by evaporation from mesophyll surfaces followed by diffusion of water vapour out through stomata. Its rate depends on stomatal aperture and the leaf-to-air vapour gradient.

  • light often opens stomata → rate rises
  • higher temperature → faster evaporation and often steeper gradient
  • higher humidity → smaller gradient → rate falls
  • wind removes moist boundary air → rate rises until stomata respond

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.

A hand applies clear nail varnish with a brush to a small area of a green leaf beside an open bottle labeled clear nail varnish.

clear nail-varnish impression → peel and mount → calibrate field area → count stomata in random fields → calculate each density → repeat and report variation

SL Summary: One Diffusion Model

Feature Lung Leaf
area many alveoli broad blade + mesophyll walls
path thin barrier thin leaf + air spaces
gradient air + blood flow CO₂ use + stomata

Spirometry measures ventilation; a potometer estimates water uptake; a leaf cast measures stomatal density. Match each method to its direct measurement.

Gas exchange as vital function

[ 3 ]

Outline the process of gas exchange necessary for aerobic respiration in a unicellular eukaryotic organism.

Properties of gas-exchange surfaces

[ 7 ]

Explain the process of gas exchange taking place in the alveoli.

Maintaining concentration gradients

[ 4 ]

Describe how a concentration gradient of oxygen is maintained between the lungs and blood capillaries.

Mammalian lung adaptations

[ 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.

Lung ventilation

[ 9 ]

Explain the mechanism of ventilation in the lungs in order to promote gas exchange for cell respiration.

Lung volume measurements

[ 2 ]

Outline how ventilation rate could have been monitored in this study.

Leaf gas exchange adaptations

[ 2 ]

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

Leaf tissue distribution

[ 8 ]

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

Drawing canvas

Transpiration exam focus

[ 8 ]

Explain how abiotic factors affect the rate of transpiration in terrestrial plants.

Stomatal density

[ 1 ]

Outline how stomatal density in busy Lizzie leaves can be estimated within a known field of view.