Plant Tissues, Transport and Transpiration
Plants have two transport tissues: xylem, made of dead, hollow cells strengthened with lignin, carries water and dissolved mineral ions one way, from the roots to the leaves, in the transpiration stream, while phloem, made of living cells, carries dissolved sugars from the leaves to the rest of the plant in both directions, in a process called translocation. Transpiration is the loss of water vapour from the leaves, mostly through the stomata, and it is the pull created by evaporation that draws the transpiration stream up the plant.
Method
- Learn the two tissues as a contrast, not as two lists: xylem is dead, lignified, hollow, one-way, moves water and mineral ions upwards; phloem is living, has sieve plates, moves dissolved sugars in both directions, to growing regions and to storage organs.
- Learn the leaf structure in order from the top with a function each: waxy cuticle (reduces water loss), upper epidermis (transparent, lets light through), palisade mesophyll (packed with chloroplasts, main site of photosynthesis), spongy mesophyll (air spaces for gas diffusion), lower epidermis with stomata and guard cells.
- Learn the four factors that increase the rate of transpiration with the reason attached: higher temperature (water molecules evaporate faster), higher light intensity (stomata open wider for photosynthesis), more air movement (wind removes water vapour, keeping a steep concentration gradient), lower humidity (a steeper gradient between the leaf and the air).
- Explain guard cells as a trade-off: in light they take in water, become turgid and bow apart, opening the stoma so carbon dioxide can diffuse in; in the dark or when water is short they become flaccid and the stoma closes, which prevents wilting but also stops photosynthesis.
- For the potometer, be clear about what is measured: a potometer measures the rate of water uptake, which is used as an estimate of the rate of transpiration, because a small amount of the water taken up is used in photosynthesis and to keep cells turgid.
- For rate calculations, work out distance moved by the bubble divided by time, and convert to the unit asked for, for example millimetres per minute or cubic millimetres per hour if the tube's cross-sectional area is given.
Worked example
In a potometer investigation, an air bubble moves 45 mm along a capillary tube in 15 minutes. The tube has a cross-sectional area of 1.2 square millimetres. Calculate the rate of water uptake in cubic millimetres per hour.
- Find the volume of water taken up: volume = distance x cross-sectional area = 45 x 1.2 = 54 cubic millimetres.
- Note the time: 15 minutes, and the answer is wanted per hour.
- Convert the time to hours: 15 minutes = 0.25 hours.
- Divide the volume by the time: 54 / 0.25 = 216.
- Final answer: the rate of water uptake is 216 cubic millimetres per hour.
- State the assumption: water uptake is being used as an estimate of transpiration, so the true transpiration rate is slightly lower because some water is used by the plant.
Practice questions
Type your answer and press Check to be marked straight away, or reveal the answer and mark yourself.
Q1Which tissue transports water and mineral ions from the roots?Show answer
Answer: Xylem.
Q2Name the process by which dissolved sugars are transported in the phloem.Show answer
Answer: Translocation.
Q3Why are xylem cells strengthened with lignin?Show answer
Answer: Lignin gives the hollow tube strength and support, so it does not collapse under the pull of the transpiration stream.
Q4Where are most stomata found on a typical leaf, and why?Show answer
Answer: On the lower surface, which is shaded and cooler, reducing water loss by evaporation.
Q5State two conditions that increase the rate of transpiration.Show answer
Answer: Any two of: higher temperature, higher light intensity, more air movement, lower humidity.
Q6What happens to guard cells when a plant is short of water?Show answer
Answer: They lose water and become flaccid, so the stomata close, reducing further water loss but also stopping carbon dioxide entering.
Q7Why does a potometer measure water uptake rather than transpiration directly?Show answer
Answer: Some of the water taken up is used in photosynthesis and to keep cells turgid, so uptake is only an estimate of the water lost by transpiration.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
Describe two differences between xylem and phloem.
Show mark scheme
Tick each line you got. Your score builds from the marks on the scheme.
Nothing ticked yet - 4 available
A student measures the rate of transpiration of the same plant in still air and then in front of a fan. Predict the effect of the fan and explain it.
Show mark scheme
Tick each line you got. Your score builds from the marks on the scheme.
Nothing ticked yet - 3 available
Explain how the structure of a leaf is adapted both for efficient photosynthesis and for limiting water loss.
Show mark scheme
Tick each line you got. Your score builds from the marks on the scheme.
Nothing ticked yet - 6 available
See real GCSE Science past-paper questions, with official mark schemes →
Free printable worksheet
Want more practice on paper? Download the plant tissues, transport and transpiration worksheet pack - 17 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 14 of GCSE Biology Workbook, the whole course as one free printable PDF.
Next topics
Not quite what you needed?
Tell us what is missing on plant tissues, transport and transpiration, or which topic to write up next. Every request is read, and we reply to every one.
Build a full practice pack.
This topic is one of hundreds in the library - pick the ones a student needs and generate a printable PDF in minutes.