OxfordAQA International A Level Biology 3.1.5 Gas Exchange and the Transport of Oxygen in Living Organisms Exam Questions

About OxfordAQA International A Level Biology 3.1.5 Gas Exchange and the Transport of Oxygen in Living Organisms

This Unit 1 topic links organism size to the need for specialised exchange surfaces and mass transport. You need to explain how surface area to volume ratio changes with size, and how single-celled organisms, insects, dicotyledonous leaves and the human lungs are adapted for efficient gas exchange while limiting water loss. You also study ventilation, the mammalian circulation in outline, and how haemoglobin loads and unloads oxygen. It links to transport into cells, the heart and blood vessels in Unit 2, and respiration in Unit 3. Examiners like questions on the Bohr effect, interpretation of dissociation curves and adaptations of xerophytes and insects.

What you need to know

• Surface area to volume As an organism gets larger its surface area to volume ratio falls, so diffusion across the body surface is too slow. Larger organisms have flattened shapes or specialised exchange surfaces plus mass transport systems.
• Insects and leaves Insects have spiracles, tracheae and tracheoles that deliver oxygen directly to tissues; spiracles can close to reduce water loss. Dicotyledonous leaves have stomata, air spaces in the spongy mesophyll and a short diffusion distance. Xerophytes reduce water loss with features such as sunken stomata, hairs, rolled leaves and a thick waxy cuticle.
• Human gas exchange Air passes through the trachea, bronchi and bronchioles to the alveoli. The alveolar epithelium is one cell thick and next to capillary endothelium, giving a short diffusion pathway, with a large surface area and ventilation maintaining a steep gradient.
• Ventilation Inspiration: external intercostal muscles contract, ribs move up and out, diaphragm contracts and flattens, volume increases and pressure falls below atmospheric. Expiration at rest is mainly passive as muscles relax and lungs recoil.
• Haemoglobin Haemoglobin is a protein with four polypeptide chains, each with a haem group containing iron. It loads oxygen where the partial pressure is high and unloads it where it is low, giving an S-shaped dissociation curve.
• Bohr effect and variants Higher carbon dioxide concentration shifts the curve to the right, so oxygen is released more readily in respiring tissues. Haemoglobins with higher affinity (curve to the left), such as in animals living at high altitude, load oxygen at low partial pressures.

Common mistakes in 3.1.5 Gas Exchange and the Transport of Oxygen in Living Organisms

• Saying the alveolar wall is one cell membrane thick. It is one cell thick (squamous epithelium).
• Writing that air is sucked into the lungs. Volume increases, pressure falls below atmospheric pressure and air moves in down the pressure gradient.
• Mixing up the direction of the Bohr shift. More carbon dioxide lowers the affinity of haemoglobin for oxygen and shifts the curve to the right.
• Saying xerophytes have no stomata. They have fewer or sunken stomata and other features that trap moist air, but they still need stomata for gas exchange.

Try three 3.1.5 Gas Exchange and the Transport of Oxygen in Living Organisms exam-style questions