Gas exchange in insects, fish and mammals (Edexcel A-level Biology B)

This lesson describes how the surfaces in insects, fish and mammals are adapted for gas exchange. The PowerPoint and accompanying worksheets have been designed to cover the detail of point 4.3 (i) of the Edexcel A-level Biology B specification.

The lesson has been intricately planned to challenge the students on their understanding of the surface area to volume ratio (as covered in topic 4.1) and to make direct links to upcoming lessons on the transport systems in humans. The lesson begins by explaining that single-celled organisms are able to diffuse oxygen and carbon dioxide across their body surface but that as organisms increase in size and their SA/V ratio decreases, they need adaptations at their gas exchange surfaces to be able to obtain the oxygen to meet their metabolic demands. This leads into the next part of the lesson which describes the roles of the following structures in insects and bony fish:

• spiracles, tracheae, tracheoles and tracheole fluid
• operculum, gill arch, gill filaments and lamellae

The next task challenges the students to use their knowledge of topics 1, 2 and 3 to come up with the letters that form the key term, countercurrent flow. This is a key element of the lesson and tends to be a principle that is poorly understood, so extra time is taken to explain the importance of this mechanism. Students are shown two diagrams, where one contains a countercurrent system and the other has the two fluids flowing in the same direction, and this is designed to support them in recognising that this type of system ensures that the concentration of oxygen is always higher in the oxygenated water than in the blood in the lamellae.

As the alveoli as a structure of gas exchange was introduced at GCSE, this final part of the lesson has been written to challenge the recall of that knowledge and to build on it.
The main focus is the type of epithelium found lining the alveoli and students will discover that a single layer of flattened cells known as simple, squamous epithelium acts to reduce the diffusion distance. Again, students will have met this in a lesson in topic 2 on specialised cells (and tissues) so a number of prior knowledge checks are used alongside current understanding checks.

The following features of the alveolar epithelium are also covered:

• Surface area
• Moist lining
• Production of surfactant
• The maintenance of a steep concentration gradient

As a constant ventilation supply is critical for the maintenance of the steep concentration gradient, the final task considers the mechanism of ventilation