This lesson describes how Fick’s law of diffusion is governed by the three main properties of gas exchange surfaces in living organisms. The PowerPoint and accompanying worksheets have been designed to cover points 2.1 (i & ii) of the Edexcel International A-level Biology specification and there is a particular focus on the relationship between the size of an organism or structure and its surface to volume ratio.
Adolf Fick is briefly introduced at the start of the lesson and the students will learn that his law of diffusion governs the diffusion of a gas across a membrane and is dependent on three properties. The students are likely to know that surface area is one of these properties but although they may have been introduced to the surface area to volume ratio at iGCSE, their understanding of its relevance tends to be mixed. Therefore, real life examples are included throughout the lesson that emphasise the importance of this ratio in order to increase the relevance. A lot of students worry about the maths calculations that are associated with this topic so a step by step guide is included at the start of the lesson to walk them through the calculation of the surface area, the volume and then the ratio. Through worked examples and understanding checks, SA/V ratios are calculated for cubes of increasing side length and living organisms of different size. These comparative values will enable the students to conclude that the larger the organism or structure, the lower the surface area to volume ratio. A differentiated task is then used to challenge the students to explain the relationship between the ratio and the metabolic demands of an organism and this leads into the next part of the lesson, where the adaptations of a human to increase the ratio at the gas exchange surface is covered. The students will calculate the SA/V ratio of a human alveolus (using the surface area and volume formulae for a sphere) and will see the significant increase that results from the folding of the membranes. The remainder of the lesson introduces concentration difference and thickness of membrane as the other two properties in Fick’s law of diffusion and students are reminded that the maintenance of a steep concentration gradient and a reduction in the diffusion distance are critical for this transport mechanism.
This lesson has been specifically planned to prepare students for the next lesson which describes how the structure of the mammalian lung is adapted for rapid gas exchange (specification point 2.1 [iii])
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