Exam Focus

Exam Focus

Transpiration is one of the most reliably examined topics in GCSE Biology. Edexcel 1BI0 Paper 2 (Topic 6: Plant Structures and Their Functions) tests this with stimulus-based questions — you will commonly be given a graph of transpiration rate against an environmental variable and asked to describe and explain the trend. The potometer required practical is frequently assessed with questions asking you to evaluate the experimental method or calculate rate. These question types appear repeatedly:

• Graph interpretation (2-4 marks): You will be given a graph of transpiration rate against one environmental factor (temperature, humidity, light intensity, wind speed) and asked to describe the trend and/or explain it. Always: describe what the graph shows in numbers, then explain the mechanism.
• Potometer required practical (3-5 marks): Describe the method, state the limitation (measures uptake, not transpiration directly), explain why the shoot must be cut underwater (to prevent air entering xylem and blocking water flow — an "air lock" or "air embolism").
• Guard cell mechanism (3-4 marks Higher): "Explain how stomata open in the light." Walk through the ion-osmosis-turgor-shape change sequence. Potassium ions move in → water follows by osmosis → guard cells become turgid → thick inner walls cause cells to bow → pore opens.
• Xerophyte adaptations (3-6 marks): For each adaptation, state the feature AND explain how it reduces transpiration. Never just list features — examiners want mechanism.
• 6-mark extended response: "Explain how the structure of guard cells allows them to control the rate of transpiration." Plan: structure (thick inner wall, thin outer wall, kidney shape) → mechanism (K⁺ movement → osmosis → turgor change → shape change → pore opens/closes) → impact on transpiration.

Most common command words in this topic: explain, describe, suggest, evaluate, calculate (rate from potometer data).