Excretion (OCR A-level biology)

This lesson describes the meaning of excretion, as well as the role of the liver, kidneys, lungs and the skin in the removal of carbon dioxide and urea. The engaging PowerPoint and accompanying resources have been designed to cover point 5.1.2 (a) of the OCR A-level Biology specification and also explains the importance of excretion for homeostasis.

The lesson begins by reminding students that excretion is one of the 7 characteristics of living organisms, as introduced within MRS GREN when they were younger. An A-level worthy definition of excretion is then introduced, and time is taken to ensure that students recognise that substances must be products of metabolism to be deemed to be excreted. In line with this, the students are challenged to spot that urea and carbon dioxide need to be excreted whilst faeces is egested.

Moving forwards, the role of the liver and then the kidneys in the excretion of urea are described. There is a focus on terminology, specifically prefixes and suffixes, to allow students to understand the meaning of deamination which occurs in the liver. The lesson doesn’t go into huge detail about this process and the subsequent ornithine cycle as these are both covered in an upcoming lesson about the functions of the liver. The transport of carbon dioxide is revisited and prior knowledge checks are used to allow the students to assess their recollection of hydrogen carbonate ions and carbaminohaemoglobin. All answers to these checks as well as any understanding checks are embedded into the PowerPoint.

The final part of the lesson explores how the skin is involved in excretion and a link is made to the maintenance of internal conditions within narrow limits by homeostasis.