B3.1 Gas exchangeIB Biology SL: Revision notes

All organisms must exchange oxygen and carbon dioxide with their environment. As an organism gets larger, its surface area-to-volume ratio falls (a cube of side x has SA:V = 6/x), and the distance from its centre to the surface increases. Diffusion alone becomes too slow and the surface too small, so large organisms need specialised gas-exchange surfaces and transport systems.

Gas-exchange surfaces are permeable to O₂ and CO₂, made of a thin tissue layer (short diffusion distance), moist (gases dissolve before diffusing) and have a large surface area.

In animals, steep concentration gradients are maintained by a dense network of blood vessels, continuous blood flow that removes O₂ and brings CO₂, and ventilation — with air in lungs, and with water pumped over the gills in fish.

• Branched network of bronchioles carries air to millions of alveoli.
• Many alveoli give a high surface area.
• Alveolus walls are one layer of thin cells.
• Extensive capillary beds surround each alveolus.
• Surfactant reduces surface tension in the moist lining, stopping the alveoli sticking together and collapsing when air is exhaled.

Inhalation: external intercostal muscles contract, moving the ribs up and out; the diaphragm contracts and flattens. Thorax volume rises, pressure falls below atmospheric, air flows in.

Exhalation: at rest, these muscles relax and elastic recoil reduces volume. In forced exhalation, the internal intercostals pull the ribs down and in and the abdominal muscles push the diaphragm up.

Tidal volume = volume in or out per resting breath. Inspiratory reserve = extra volume that can be inhaled after a normal breath in. Expiratory reserve = extra that can be forced out after a normal breath out. Vital capacity = tidal volume + inspiratory reserve + expiratory reserve.

In a transverse section of a dicotyledonous leaf, from top to bottom: waxy cuticle, upper epidermis, palisade mesophyll, spongy mesophyll with large air spaces, lower epidermis with stomata controlled by guard cells, and a vein (xylem above phloem) in the midrib. The cuticle reduces water loss; stomata let CO₂ in and O₂ out; air spaces give a large, moist internal surface; veins supply water and remove sugars.

Transpiration is the loss of water vapour from leaves. It is a consequence of gas exchange: stomata must open for CO₂ uptake, so water vapour diffuses out. Rate increases with higher temperature, higher light intensity (stomata open), lower humidity and more air movement.

Stomatal density = number of stomata ÷ area of field of view (πr²). Count several fields, because biological material is variable: repeating measurements increases reliability.