Why do large organisms need specialised exchange surfaces, and how are mammalian, fish and insect gas-exchange systems adapted?
3.1.1 Exchange surfaces: the need for specialised exchange surfaces as size and metabolic rate increase and surface-area-to-volume ratio falls; the features of an efficient exchange surface; the structure and function of the mammalian gas-exchange system, the counter-current system in fish gills, and the tracheal system of insects.
A focused answer to the OCR H420 3.1.1 dot point on exchange surfaces and gas exchange. Covers why surface-area-to-volume ratio drives the need for exchange surfaces, the features of an efficient surface, the mammalian lung, the fish counter-current system and the insect tracheal system.
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What this dot point is asking
OCR wants you to explain why surface-area-to-volume ratio forces large organisms to evolve specialised exchange surfaces, list the features that make a surface efficient, and describe and compare gas exchange in mammals, bony fish and insects.
The answer
Why exchange surfaces are needed
Substances enter and leave cells across the surface by diffusion. As an organism gets larger, its volume increases faster than its surface area, so the surface-area-to-volume (SA:V) ratio falls and the distance to the innermost cells rises. A small single-celled organism has a high SA:V ratio and a short diffusion distance, so simple diffusion across its surface is enough. Large, active organisms cannot rely on this: diffusion would be too slow, so they evolve specialised exchange surfaces served by a mass transport (circulatory) system. A high metabolic rate (in active, often endothermic animals) raises demand further.
Features of an efficient exchange surface
- Large surface area (more space for exchange).
- Thin barrier (a short diffusion distance, often one cell thick).
- A steep concentration gradient maintained, by a good blood supply and (in lungs) ventilation that constantly replaces the air.
- Permeable to the substances exchanged.
These features all increase the rate of diffusion, in line with Fick's law: rate is proportional to (surface area times concentration difference) divided by diffusion distance.
The mammalian gas-exchange system
Air passes down the trachea (held open by C-shaped cartilage rings), into two bronchi, then many bronchioles, ending in clusters of alveoli. The alveoli are the exchange surface:
- millions of alveoli give an enormous total surface area;
- the alveolar wall and the capillary wall are each a single flattened (squamous) epithelial cell, so the diffusion distance is tiny;
- a dense capillary network and continuous ventilation keep a steep oxygen and carbon dioxide gradient.
Ventilation moves air in and out. In inspiration the diaphragm contracts and flattens and the external intercostal muscles contract, raising the ribs; thoracic volume increases, pressure falls below atmospheric, and air flows in. Expiration is largely passive at rest: the muscles relax, volume decreases, pressure rises and air flows out.
Gas exchange in bony fish
Fish use gills: stacks of gill filaments, each bearing many thin lamellae with a rich blood supply. The crucial adaptation is the counter-current system: water flows over the lamellae in the opposite direction to blood flow within them. Because water is always more oxygenated than the blood it meets, a diffusion gradient is maintained along the whole length of the lamella, so oxygen diffuses into the blood across the entire surface. In a parallel system the two would reach equilibrium partway and exchange would stop.
Gas exchange in insects
Insects have a tracheal system: air enters through spiracles (valved pores) into a network of tracheae, which branch into fine tracheoles that deliver oxygen directly to respiring tissues. Gas exchange is mainly by diffusion down the tracheoles; some larger or active insects also ventilate by abdominal pumping movements, and at high activity fluid is withdrawn from the tracheole ends to bring air closer to the cells. Insects do not use their blood (haemolymph) to transport oxygen.
Examples in context
Example 1. Why elephants need lungs but amoebae do not. An amoeba has a high SA:V ratio and exchanges gases across its whole surface; an elephant's low ratio and long diffusion distances make specialised lungs and a circulatory system essential, a clear illustration of the size argument.
Example 2. Emphysema. In emphysema, alveolar walls break down and alveoli merge, reducing the total surface area for gas exchange; less oxygen is absorbed, causing breathlessness, a common applied context that tests the efficient-surface features.
Try this
Q1. State three features of an efficient gas-exchange surface. [3 marks]
- Cue. Large surface area, thin (short diffusion distance), and a maintained concentration gradient (good blood supply and ventilation); permeable is also acceptable.
Q2. Explain why ventilation helps maintain a high rate of gas exchange in the alveoli. [2 marks]
- Cue. It continually replaces the air, keeping a high oxygen and low carbon dioxide concentration in the alveolus, so a steep concentration gradient is maintained for diffusion.
Q3. Name the valved pores through which air enters an insect's tracheal system. [1 mark]
- Cue. Spiracles.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H420/01 20194 marksExplain how the counter-current system in the gills of a bony fish increases the efficiency of oxygen uptake compared with a parallel-flow system.Show worked answer →
The key is maintaining a diffusion gradient along the whole length of the lamella.
In a counter-current system, blood and water flow in opposite directions across the gill lamella. This means that water, which is always more oxygenated, meets blood that is progressively more oxygenated, so a concentration gradient is maintained along the entire length of the lamella and oxygen diffuses into the blood all the way across.
In a parallel-flow system the blood and water would reach equilibrium about halfway along, after which no further diffusion occurs, so less oxygen is absorbed.
Markers reward "opposite directions", "gradient maintained along the whole length", and the contrast with equilibrium in parallel flow.
OCR H420/01 20213 marksA cube-shaped organism has sides of 2 cm. Calculate its surface-area-to-volume ratio, and explain why a larger organism cannot rely on diffusion across its body surface alone for gas exchange.Show worked answer →
Work out both quantities, then link to diffusion distance.
Surface area . Volume . Ratio (2 marks: working and ratio).
For the third mark: as an organism gets larger, its volume increases faster than its surface area, so the surface-area-to-volume ratio falls and the diffusion distance to inner cells increases. Diffusion alone would then be too slow to supply oxygen to all cells, so a specialised exchange surface and a transport system are needed.
Markers reward the correct ratio and the link between falling ratio, greater diffusion distance and the need for an exchange surface.
Related dot points
- 3.1.2 Transport in animals: the structure of the mammalian heart and the events of the cardiac cycle (atrial systole, ventricular systole and diastole), the pressure and volume changes that open and close the valves, and the myogenic control of heart rate by the SAN, AVN, bundle of His and Purkyne tissue, including the interpretation of electrocardiograms (ECGs).
A focused answer to the OCR H420 3.1.2 dot point on the mammalian heart. Covers the heart's structure, the three stages of the cardiac cycle, how pressure changes open and close the valves, myogenic control by the SAN, AVN, bundle of His and Purkyne tissue, and how to read an ECG.
- 3.1.2 Transport in animals: the structure and functions of arteries, arterioles, capillaries, venules and veins; the formation of tissue fluid from plasma at the arterial end of a capillary bed and its return at the venous end and via the lymphatic system, explained in terms of hydrostatic and oncotic (osmotic) pressure.
A focused answer to the OCR H420 3.1.2 dot point on blood vessels and tissue fluid. Covers the structure and function of arteries, arterioles, capillaries, venules and veins, and how hydrostatic and oncotic pressure form tissue fluid at the arterial end and return it at the venous end and via the lymph.
- 3.1.2 Transport in animals: the role of haemoglobin in transporting oxygen, the oxygen dissociation curve and cooperative binding, the Bohr effect, the higher oxygen affinity of fetal haemoglobin, and the transport of carbon dioxide including the formation of hydrogencarbonate ions and the chloride shift.
A focused answer to the OCR H420 3.1.2 dot point on oxygen transport. Covers haemoglobin and cooperative binding, the sigmoidal oxygen dissociation curve, loading and unloading, the Bohr effect, fetal haemoglobin, and carbon dioxide transport as hydrogencarbonate with the chloride shift.
- 3.1.3 Transport in plants: the structure and function of xylem and phloem; the cohesion-tension theory of water transport in the xylem and the factors affecting transpiration; the mass flow hypothesis of translocation in the phloem from source to sink; and the adaptations of xerophytes for reducing water loss.
A focused answer to the OCR H420 3.1.3 dot point on transport in plants. Covers xylem and phloem structure, the cohesion-tension theory of transpiration, the factors affecting transpiration rate, the mass flow hypothesis of translocation, and xerophyte adaptations.
- 2.1.1 Cell structure: the ultrastructure of eukaryotic and prokaryotic cells, the function of organelles including the role of the rough endoplasmic reticulum and Golgi apparatus in producing and secreting proteins; the use, calibration and resolution of light and electron microscopes.
A focused answer to the OCR H420 2.1.1 dot point on cell structure and microscopy. Covers every required eukaryotic and prokaryotic organelle, the protein secretory pathway, the three microscopes, eyepiece-graticule calibration and the magnification equation.
Sources & how we know this
- OCR A Level Biology A (H420) Specification — OCR (2023)