Example 1: potometer rate
A potometer bubble moves 36 mm in 4 minutes. Find the rate.
- Rate = distance ÷ time = 36 ÷ 4.
- Rate = 9.0 mm per minute.
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IB DemystifiedMYP Sciences
Transport in plants
A tall tree can lift hundreds of litres of water from its roots to its highest leaves every day, with no heart and no pump. Understanding how plants move water and sugar explains wilting, irrigation and why trees cool a hot city.
Recommended for MYP 4 · eAssessment priority · About 3 lessons · Criteria A, B, C and D
On this page
- Learning objectives
- Before you start
- Key vocabulary
- Understanding the ideas
- Factors affecting transpiration
- Transport in plants in the real world
- Worked examples
- In the eAssessment
- Check your understanding
- Practice questions
- Investigation
- Criterion-linked questions
- Challenge questions
- Topic check
- Review your mistakes
- Your progress
Learning objectives
By the end of this topic you should be able to:
- describe the structure and function of xylem and phloem
- explain how root hair cells absorb water and mineral ions
- explain transpiration and the role of stomata and guard cells
- explain how light, temperature, wind and humidity affect transpiration
- analyse potometer and stomata data
- discuss water use in farming and cities
Before you start
You will use these skills. If any feel shaky, review them first.
- cells, diffusion and osmosis (see Cell organisation)
- photosynthesis and respiration
- surface area and concentration gradients
Key vocabulary
- Xylem
- Dead, hollow tubes strengthened with lignin that carry water and mineral ions upwards.
- Phloem
- Living tissue that carries dissolved sugars up and down the plant (translocation).
- Transpiration
- The loss of water vapour from leaves, mainly through stomata.
- Stoma (plural stomata)
- A pore in the leaf surface, opened and closed by two guard cells.
- Root hair cell
- A root cell with a long extension that increases surface area for absorbing water and ions.
- Potometer
- Apparatus that measures the rate of water uptake by a leafy shoot.
Understanding the ideas
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What is it?
Plants need water for photosynthesis, to keep cells firm and to carry mineral ions. Water enters root hair cells by osmosis, and mineral ions such as nitrate are absorbed by active transport, which needs energy from respiration.
-
Why does it happen?
Water moves up the xylem in a continuous column. As water evaporates from the leaves and diffuses out through the stomata, more water is pulled up to replace it: the transpiration stream. Sugars made in the leaves are carried in the phloem to wherever they are needed, including roots, fruits and growing tips.
-
How do we know?
Scientists measure transpiration with potometers and by weighing plants, and count stomata by taking nail-varnish impressions of leaf surfaces. Dye experiments with celery show the path of water through the xylem.
-
Why does it matter?
Agriculture uses most of the world's fresh water, and understanding how plants take up and lose water helps farmers irrigate efficiently. Transpiration from trees also cools cities during heatwaves.
-
What does it connect to?
Transport in plants links to photosynthesis, osmosis and ecosystems in biology, to the water cycle and climate in Earth science, and to evaporation and energy transfer in physics.
Factors affecting transpiration
- Light: brighter light opens more stomata, increasing transpiration.
- Temperature: warmer conditions make water evaporate and diffuse faster.
- Wind: moving air removes water vapour, keeping a steep concentration gradient.
- Humidity: moist air reduces the gradient, so transpiration slows.
- Adaptations to reduce water loss: waxy cuticle, fewer or sunken stomata, small leaves or spines, stomata mostly on the lower surface.
Transport in plants in the real world
Farmers in Punjab and Sindh are trying drip irrigation to save water. Cities including Karachi and Lahore have planted dense urban forests for cooling and shade. Florists cut flower stems underwater to stop air entering the xylem, so flowers last longer.
Worked examples
Example 2: explaining a factor
Explain why transpiration is slower on a humid day.
- Humid air already contains a lot of water vapour.
- The concentration gradient between the inside of the leaf and the air is smaller.
- So water vapour diffuses out of the stomata more slowly.
In the eAssessment
Plant transport questions often use potometer data, leaf diagrams and stomata counts. Expect:
- Explain water movement from roots to leaves in clear steps.
- Process potometer data into rates and volumes.
- Explain the effect of each environmental factor on transpiration.
- Discuss irrigation, drought and water use.
Common ways to lose marks: mixing up xylem and phloem; saying mineral ions enter by osmosis; forgetting that a potometer measures uptake, not loss; and saying transpiration only happens in hot weather.
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Investigation: comparing stomata across species
Partially guided investigation · about 45 minutes · pairs
- Research question
- How does the number of stomata per mm² on the lower leaf surface differ between plants that grow in full sun and plants that grow in shade?
- Scientific background
- Stomata let in carbon dioxide for photosynthesis but also let water vapour out. Plants in different habitats may have different numbers of stomata to balance these needs.
- Hypothesis
- Write your own prediction, with a scientific justification.
- Variables
- Identify your independent, dependent and control variables, and explain how you will control them.
- Apparatus
- Leaves from at least three sun plants and three shade plants, clear nail varnish, clear sticky tape, microscope slides, microscope with a calibrated field of view.
- Method
- Paint a small patch of nail varnish on the lower surface of each leaf and let it dry.
- Lift it off with clear tape and stick it on a slide.
- Count the stomata in the field of view at high power in three different places, and convert to numbers per mm².
- Calculate a mean for each species and compare sun and shade plants.
Safety. Use nail varnish in a well-ventilated room, away from flames. Wash hands after handling plants; some plant sap irritates skin. Take only a few leaves from each plant.
Then evaluate: how representative were your leaves, and how could you make the comparison fairer?
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
| Skill | Correct | Status |
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