Surface area to volume ratio (Edexcel A-level Biology B)

This lesson explains why large organisms with a low surface area to volume ratio need specialised gas exchange surfaces and a mass transport system. The PowerPoint and accompanying worksheets have been designed to cover points 4.1 (i & ii) of the Edexcel A-level Biology B specification and have been specifically planned to prepare students for the upcoming lessons on gas exchange (4.3) and circulation (4.4)

The students are likely to have been introduced to the surface area to volume ratio at GCSE, but 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 this 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 that walks 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 a single-celled and multicellular organisms and this leads into the next part of the lesson, where the adaptations of large organisms to increase this ratio at the exchange surfaces are 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. In addition to the ratio, time is taken to discuss and describe how the maintenance of a steep concentration gradient and a thin membrane are important for the rate of diffusion and again biological examples are used in humans and other organisms to increase the understanding. Fick’s law of diffusion is also introduced as a mechanism to help the students to recall that surface area, concentration difference and thickness of membrane govern the rate of simple diffusion.

The final part of the lesson considers how a mass transport system is needed alongside the specialised gas exchange surface to allow the oxygen to be delivered to the respiring cells to enable them to continue to carry out aerobic respiration to generate ATP.