Topic 3 Transpiration Coursebook section 8.3 Teaching ideas ◆
Transpiration is the driving force for the movement of water up a plant. This is a crucial point for students to understand. Transpiration reduces hydrostatic pressure at the top of xylem vessels, so that water flows up them from higher pressure regions at their base. Use Figure 8.11 to introduce the idea of the transpiration stream. ◆ Students could do Activity 8.3 To see which surface of a leaf loses most water. They will need to think back to what they know about the structure of leaves, and remember where the stomata are situated. ◆ Students could set up Activity 8.4 To measure the rate of transpiration of a potted plant. This will need to be revisited at regular intervals over the next few days. ◆ Activity 8.5 involves a potometer, but if you do not have one, it is easy to do this experiment using a long glass tube, as described in the Notes. Common misunderstandings and misconceptions ◆
It is extremely common for students to fail to understand what makes water move up a plant. They may have no idea at all, or they may think that it moves up by osmosis. ◆ Students often think that water is lost from leaves as a liquid, and fail to understand that it evaporates from wet cell walls and then diffuses out through the stomata. ◆ Students may think that a large proportion of the water taken up is used in photosynthesis. In fact, only a tiny proportion is used for this; almost all of it is lost by transpiration through the leaves. Homework ideas ◆ ◆ ◆ ◆ ◆
Coursebook questions 8.8 to 8.11 End-of-chapter questions 1 and 3 Workbook exercise 8.1 A transpiration experiment Workbook exercise 8.2 Tissues in a root Workheet 8.2 The mechanism of water movement through a plant
Topic 4 Transport of manufactured food Coursebook section 8.4 Teaching ideas ◆
Remind students of the positions of xylem and phloem in roots, stems and leaves, and ask them to tell you what is transported in phloem. ◆ Show students plants in various stages of growth – for example, a recently germinated seed, an adult growing plant with green leaves, a potato tuber beginning to sprout. Ask them to suggest where the main source of ‘food’ for the plant is made or stored. Introduce the word ‘source’. Then ask which parts of the plant need most ‘food’ – students should be able to suggest the regions that are growing. Introduce the word ‘sink’. ◆ Discuss the fact that materials can be transported both up and down the plant in phloem (and in different directions at different stages of growth), in contrast to xylem in which transport is always from roots to stem. Original material © Cambridge University Press 2014 © Cambridge University Press 2014
IGCSE Biology
Chapter 8: Teaching ideas
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