Auxins and cell elongation (CIE A-level biology)

This lesson describes the role of auxins in elongation growth, specifically in the plant responses of phototropism and gravitropism. The PowerPoint and accompanying resources have been designed to cover point 15.2 (2) of the CIE A-level biology specification.

The lesson begins with a prior knowledge check, where the students have to identify key terms encountered across topics 1 - 14, and use their 1st letters to form the term, tropism. Students are reminded of the meaning of a tropism, and how these directional growth responses are determined by the direction of the external stimuli. They should have met auxins at this previous level, but will now be introduced to IAA, and will complete several tasks which check that they understand the key features of these chemicals, such as their location of production and method by which they move through the shoots and roots. The students are guided through the movement of IAA to the shaded side in a shoot during phototropism, and will learn how this uneven distribution leads to uneven growth. An exam-style question presents them with two further scenarios, where the tip of the shoot has been cut off or is covered, and the students need to describe and explain what will happen to the appearance of the shoot after a week. Moving forwards, the students will learn how the pumping of hydrogen ions acidifies the cell wall and the subsequent activation of expansin proteins are involved in the cell elongation.

The remainder of the lesson discusses the response to gravity and explains how shoots and roots respond differently.

The lesson is full of understanding and prior knowledge checks and all answers are embedded into the PowerPoint.