Exchange surfaces (3.1.1)
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- Content: A clear outline of the content covered by the delivery guide
- Thinking Conceptually: Expert guidance on the key concepts involved, common difficulties students may have, approaches to teaching that can help students understand these concepts and how this topic links conceptually to other areas of the subject
- Thinking Contextually: A range of suggested teaching activities using a variety of themes so that different activities can be selected which best suit particular classes, learning styles or teaching approaches.
Content (from AS and A-level)
The content from the specification that is covered by this delivery guide is:
|3.1.1 Exchange Surfaces|
|(a)||the need for specialised exchange surfaces||To include surface area to volume ratio (SA:V), metabolic activity, single-celled and multicellular organisms.
M0.1, M0.3, M0.4, M1.1, M2.1, M4.1,HSW1, HSW3, HSW5, HSW8
|(b)||the features of an efficient exchange surface||To include,
• increased surface area - root hair cells
• thin layer – alveoli
• good blood supply/ventilation to maintain gradient – gills/alveolus.
|(c)||the structures and functions of the components of the mammalian gaseous exchange system||To include the distribution and functions of cartilage, ciliated epithelium, goblet cells, smooth muscle and elastic fibres in the trachea, bronchi, bronchioles and alveoli.
|(d)||the mechanism of ventilation in mammals||To include the function of the rib cage, intercostal muscles (internal and external) and diaphragm.
|(e)||the relationship between vital capacity, tidal volume, breathing rate and oxygen uptake||To include analysis and interpretation of primary and secondary data e.g. from a data logger or spirometer. M0.1, M0.2, M0.4, M1.3|
HSW2, HSW3, HSW4, HSW5, HSW6
|(f)||the mechanisms of ventilation and gas exchange in bony fish and insects||To include,
• bony fish – changes in volume of the buccal cavity and the functions of the operculum, gill filaments and gill lamellae (gill plates); countercurrent flow
• insects – spiracles, trachea, thoracic and abdominal movement to change body volume, exchange with tracheal fluid.
|(g)||the dissection, examination and drawing of the gaseous exchange system of a bony fish and/or insect trachea||PAG2|
|(h)||the examination of microscope slides to show the histology of exchange surfaces.||PAG1|
The structural design of systems of gas exchange at an organ dissection, tissue histology and cytology level form the foundations of understanding of topic 3.1.1 Exchange surfaces. Students need to recognise and name new cells and build visual awareness of how cells and tissues relate to one another in the mammalian lungs, the gills of a bony fish and the tracheal system of insects.
Practical work could involve examining whole lungs from the butcher as well as dissecting beneath the operculum of the head of a large fish, such as a salmon from the fishmonger. Light microscope examination of slides of mammalian trachea, bronchi and alveoli, fish gills and insect tracheae is also required.
The resources listed include guides for carrying out these practical investigations, online histology atlas resources and resources detailing ventilation mechanisms in mammals and fish.
Step 1 provides photographs and an outline of the dissection of the head to reveal the gills and can be used alone without dissecting a whole fish as described in further steps. Salmon heads are recommended because they are large, so easy to work on, and either free or very cheap as normally discarded at the fresh fish counter. To see gills better, float the severed gill bars in a beaker of water. This practical relates to PAG2.
Students could also try to make their own temporary mounts of tiny sections of gill filament (linking to Module 1, 2.1.1(b) and PAG1).
Stage 1 of this activity involves observation of breathing movements and spiracles of live locusts. This activity could be extended to measure the rate of dorso-ventral flattening and anterio-posterior telescoping of the locust’s abdomen at different temperatures, related to the increase in metabolic activity in the ectothermic locust as ambient temperature rises. Calculation of the mean ventilation rate with appropriate units would involve maths skill M1.2 and M0.1 and plotting the results against temperature could cover M3.2.
Stage 2 instructs on locust tracheal system dissection. A downloadable worksheet is provided. The dissection of the dead locust relates to PAG2 and gives an opportunity to practise biological drawing.
An informative five minute video on counter-current exchange in the gills of a fish. Students are asked to choose between two different bio-engineering options to maximise oxygen uptake. Figures are used to explain diffusion gradients. This would be a useful follow-up to fish dissection and revision of diffusion gradients generally.
The video can be accessed directly from the YouTube link or selected from the web page via the link 'Biodiversity IV - Animals'.
Approaches to teaching the content
Teaching could begin with basic principles and then consider the three different examples of exchange systems in animals and one in plants. Most of 3.1.1 Exchange surfaces is concerned with exchange of respiratory gases in mammals, fish and insects, but there is one reference to root hair cells (3.1.1b), where the exchange concerns water and mineral ions instead.
An alternative route would be to start by relating structure to function in each organism in turn, and to derive from the anatomical studies the common features of gas exchange surfaces (3.1.1b) and the idea of increasing specialisation and efficiency of gas exchange surfaces as body size increases (3.1.1a). An approach that climbs the phylogenetic hierarchy would deal with the animal examples in the order: insect, fish, mammal. Another approach would be to stress the difference between acquiring gaseous oxygen from air in a terrestrial environment by considering insects and mammals together, and contrasting this with obtaining dissolved oxygen from water in fish (synoptic link to 4.2.2g adaptations).
After studying the gross anatomy of the exchange systems, either ventilation mechanisms or the cell level detail for each organism could come next. Some drawing together of common themes at the end of the topic is a good idea even if the principles 3.1.1(a) (the need for) and (b) (the features of) were taught at the beginning.
Common misconceptions or difficulties students may have
Areas that may pose particular conceptual difficulties are:
- the mathematical skills required to calculate surface area to volume ratio of different sizes and shapes of bodies 3.1.1(a)
- grasping the dynamic nature of diffusion gradients at exchange surfaces 3.1.1(b)
- analysing spirometer data 3.1.1(e)
Conceptual links to other areas of the specification – useful ways to approach this topic to set students up for topics later in the course
The grouping of exchange surfaces with transport in animals (3.1.2) and transport in plants (3.1.3) clearly indicates that acquiring respiratory gases (animals) or water and minerals (plants) should be linked functionally to the systems for transporting these and other substances.
There is also the conceptual link that both types of system, exchange systems and transport systems, are necessitated by increasing size and complexity of organisms, and by increased rate of metabolism (3.1.1(a), 3.2.1(a), 3.1.3(a) and 5.1.5(j)).
An opportunity to measure physiological function (such as ventilation rates) is mentioned in learning outcome 5.1.1(d), ectotherms and endotherms.
Animal exchange and transport link synoptically to the topic of respiration (5.2.2) while plant exchange at the roots and water transport link to photosynthesis (5.2.1).
The overlap between the epithelia (2.1.6(h) and (i)) and the histology knowledge of the lungs required for 3.1.1(c) and (h) provides an opportunity for reinforcement in teaching.
Understanding of diffusion (2.1.5d) and osmosis (2.1.5e) is an important prerequisite for teaching exchange surfaces. Practical activities investigating diffusion in model cells such as agar cubes (2.1.5d(ii)) can be used to illustrate both the importance of the diffusion distance with regard to a need for a transport system and the importance of surface area to volume ratio regarding the need for additional exchange surfaces in larger organisms.
Students are given several pieces of modelling clay, each of the same weight or volume. Their brief is to design animals for different habitats and levels of activity, so that some maximise oxygen uptake and some are less efficient according to their needs. The students could name their creations and have the chance to explain each one’s adaptations in pairs or to the class. Fast and slow organisms, terrestrial and aquatic habitats and low oxygen environments (high altitude, polluted water, tidal mud) could be considered.
The surface area to volume ratio of each creature should be estimated (M0.4) before being calculated (M4.1) and the creatures could be ranked in order of surface area to volume ratio.
If the fiction were established that every mm of clay actually represents a µm, the calculations of surface area and volume would be more relevant to diffusion of gases. Some micrograph images of tiny multicellular animals like tardigrades, rotifers and water fleas could be shown on the whiteboard to compliment the students’ ideas.
Suggested contexts for developing a broader and deeper understanding of the topics in 3.1.1 Exchange surfaces include:
- a project for students to build their own spirometer
and resources for discussion or individual student research into:
- gas exchange in giant insects
- humans breathing at very high altitude
- humans breathing underwater
- the use of oxygen in a medical context.
Meganeura Giant Dragonfly (Naked Science)
Giant Bugs a Thing of the Past (National Geographic 2007)
Why Giant Bugs Once Roamed the Earth (National Geographic 2011)
Students could be asked ‘How big do insects grow?’ in the context of the comparative inefficiency of the insect tracheal system. A Google image search for ‘largest insect’ should allow students to estimate the maximum size of modern insects like the Goliath Beetle, Titanic Longhorn Beetle and Atlas Moth. Dimensions and images of fossilised giant dragonflies could then be presented (e.g. with the video clip link) with the question as to why they grew so big then but not now. The two article links provided give different answers, showing HSW7, HSW11.
SkyDance (SkyDance SkyDiving)
Space jump (Pro-stunts Ltd)
Search terms: HALO, HAHO
The physiological zone for humans is from sea level to 10,000 feet. Above this supportive equipment is needed to avoid hypoxia. The conundrum is to assume that a person jumps with a parachute but no breathing apparatus. Should they open the parachute immediately and delay their fall though the altitudes where oxygen partial pressure is too low? Or delay opening the parachute to fall faster through this danger zone, and risk losing consciousness before opening the parachute?
This problem is presented as something to stimulate student discussion or research. In fact two types of jump occur in the military, HALO (high altitude low opening, also used in skydiving) and HAHO (high altitude, high opening).
Discussion of this topic should be based on a secure understanding of diffusion gradients at the alveoli so that the link is made between 3.1.1(b), (c) and (d) and the new context.
Why can’t we breathe underwater? (How Stuff Works)
Human fish – artificial gills (Daily Mail)
Aquaman Oxygen Crystal (International Business Times)
Harry Potter & the Goblet of Fire book pp. 428-429 (J.K.Rowling, Bloomsbury)
An optional context for adding an element of fun to this topic is to use the example of Harry Potter needing to breathe underwater for an hour. Most students will be familiar with the idea of scuba diving apparatus which could be referenced here. The explanation of why we can’t extract oxygen from water with lungs could be used as the starting point if the Harry Potter context is left out. Discussion or research work on this topic could relate to learning outcomes 3.1.1(c), (d), (f) and (g) and HSW1 and HSW9.
Medical Oxygen (BOC)
Helium-Oxygen mixtures for patients in respiratory distress (BOC)
Oxygen therapy uses (News Medical)
These resources could be used to explore situations when lung function can no longer meet a person’s oxygen needs unaided and the science behind medical intervention to prevent hypoxia, related to 3.1.1(c), (d) and (e) and HSW9.
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