Revision Notes

8.3 Transpiration

What is Transpiration?

  • Water travels up xylem from the roots into the leaves of the plant to replace the water that has been lost due to transpiration
  • Transpiration is defined as the loss of water vapour from plant leaves by evaporation of water at the surfaces of the mesophyll cells followed by diffusion of water vapour through the stomata
  • Xylem is adapted in many ways:
    • A substance called lignin is deposited in the cell walls which causes the xylem cells to die
    • These cells then become hollow (as they lose all their organelles and cytoplasm) and join end-to-end to form a continuous tube for water and mineral ions to travel through from the roots
    • Lignin strengthens the plant to help it withstand the pressure of the water movement
  • Movement in xylem only takes place in one direction – from roots to leaves (unlike phloem where movement takes place in different directions)

Water uptake, transport and transpiration, IGCSE & GCSE Biology revision notesWater uptake, transport and transpiration


Transpiration in plants, IGCSE & GCSE Biology revision notesTranspiration in plants

  • Transpiration has several functions in plants:
    • transporting mineral ions
    • providing water to keep cells turgid in order to support the structure of the plant
    • providing water to leaf cells for photosynthesis
    • keeping the leaves cool (the conversion of water (liquid) into water vapour (gas) as it leaves the cells and enters the airspace requires heat energy. The using up of heat to convert water into water vapour helps to cool the plant down)
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How does Transpiration Occur?

  • Evaporation takes place from the surfaces of spongy mesophyll cells
  • The many interconnecting air spaces between these cells and the stomata creates a large surface area
  • This means evaporation can happen rapidly when stomata are open
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How is the Transpiration Stream Created?

  • Water molecules are attracted to each other by cohesion – creating a continuous column of water up the plant
  • Water moves through the xylem vessels in a continuous transpiration stream from roots to leaves via the stem
  • Transpiration produces a tension or ‘pull’ on the water in the xylem vessels by the leaves
  • As water molecules are held together by cohesive forces (each individual molecule ‘pulls’ on the one below it), so water is pulled up through the plant
  • If the rate of transpiration from the leaves increases, water molecules are pulled up the xylem vessels quicker
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  • If more water evaporates from the leaves of a plant than is available in the soil to move into the root by osmosis, then wilting will occur
  • This is when all the cells of the plant are not full of water, so the strength of the cell walls cannot support the plant and it starts to collapse


A wilted plant cannot support itself and starts to collapse, IGCSE & GCSE Biology revision notesA wilted plant cannot support itself and starts to collapse

Investigating Factors that Affect Transpiration

Comparision between xylem and phloem tissue table, IGCSE & GCSE Biology revision notes


Investigating the role of environmental factors in determining the rate of transpiration from a leafy shoot

  • Cut a shoot underwater to prevent air entering the xylem and place in tube
  • Set up the apparatus as shown in the diagram and make sure it is airtight, using vaseline to seal any gaps
  • Dry the leaves of the shoot (wet leaves will affect the results)
  • Remove the capillary tube from the beaker of water to allow a single air bubble to form and place the tube back into the water
  • Set up the environmental factor you are investigating
  • Allow the plant to adapt to the new environment for 5 minutes
  • Record the starting location of the air bubble
  • Leave for a set period of time
  • Record the end location of air bubble
  • Change the light intensity or wind speed or level of humidity or temperature (only one – whichever factor is being investigated)
  • Reset the bubble by opening the tap below the reservoir
  • Repeat the experiment
  • The further the bubble travels in the same time period, the faster transpiration is occurring and vice versa


Investigating transpiration rates using a potometer, IGCSE & GCSE Biology revision notesInvestigating transpiration rates using a potometer

  • Environmental factors can be investigated in the following ways:
    • Temperature : Temperature of room (cold room and warm room)
    • Humidity : Spray water in plastic bag and wrap around plant

Exam Tip

Remember when designing an investigation to ensure a fair test you must keep all factors the same other than the one you are investigating.

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Temperature & Humidity on Transpiration Rate

Factors affecting rate of transpiration:

Temperature & Humidity on Transpiration Rate table, IGCSE & GCSE Biology revision notes

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.

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