This lesson describes the mass flow hypothesis for the mechanism of translocation in plants and includes details of active loading at the source. Both the detailed PowerPoint and accompanying resources have been designed to cover the 4th part of point 3.4.2 of the AQA A-level Biology specification.
The lesson begins by challenging the students to recognise the key term translocation when it is partially revealed and then the rest of the lesson focuses on getting them to understand how this mechanism involves the mass flow of assimilates down the hydrostatic pressure gradient from the source to the sink. It has been written to tie in with an earlier lesson in topic 3.4.2 where the structure of the phloem tissue was initially introduced and the students are continually challenged on this prior knowledge. A step-by-step guide is used to describe how sucrose is loaded into the phloem at the source by the companion cells. Time is taken to discuss key details such as the proton pumping to create the proton gradient and the subsequent movement back into the cells by facilitated diffusion using co-transporter proteins. Students will learn that the hydrostatic pressure at the source is high, due to the presence of the water and sucrose as cell sap, and that this difference when compared to the lower pressure at the sink leads to the movement along the phloem.
A number of quick quiz rounds are included in the lesson to maintain engagement and to introduce key terms and the lesson concludes with a game of SOURCE or SINK as students have to identify whether a particular plant structure is one or the other (or both)
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