Overview
What you’ll learn
- Label every leaf tissue and match it to its job, from waxy cuticle to air spaces.
- Explain how root hair cells’ shape serves absorption.
- Compare xylem and phloem — structure, cargo and direction.
- Describe the transpiration stream from roots to leaf air spaces.
- Predict how temperature, humidity, wind and light change the transpiration rate.
- Turn potometer-style uptake data into rates and read transpiration graphs.
Specification checklist (AQA Trilogy 8464 · Edexcel 1SC0 · OCR Gateway J250)
Statements are paraphrased from the shared DfE combined science subject content and grouped by tier. Foundation content is studied by every student; Higher statements are examined on Higher-tier papers only. This chapter covers only what Combined Science (double award) examines — separate-science extras live on the GCSE Biology, Chemistry and Physics hubs. QuizQuip is not affiliated with AQA, Pearson Edexcel or OCR.
Foundation — every student
- 5.1Describe the leaf as a plant organ containing tissues: epidermis, palisade mesophyll, spongy mesophyll, xylem, phloem and guard cells around stomata, and relate each tissue to its function.
- 5.2Explain how root hair cells increase the absorbing surface area of roots, and describe the transport of water and mineral ions into the plant.
- 5.3Describe xylem tissue as hollow strengthened tubes that transport water and dissolved mineral ions from the roots to the stems and leaves, and explain the transpiration stream.
- 5.4Describe phloem tissue as living cells with pores in their end walls that transport dissolved sugars from the leaves to the rest of the plant (translocation), for immediate use or storage.
- 5.5Describe how stomata and guard cells open and close to control gas exchange and water loss, and explain the effect of temperature, humidity, air movement and light intensity on the rate of transpiration.
- 5.6Calculate rates of water uptake from practical data, and interpret graphs of transpiration rate against environmental conditions.
Key points
- 1Leaf layers: waxy cuticle → upper epidermis → palisade mesophyll (most chloroplasts) → spongy mesophyll (air spaces for gas exchange) → lower epidermis with stomata.
- 2The leaf is an organ: several tissues working together for photosynthesis and exchange.
- 3Xylem: dead, hollow, lignin-strengthened tubes carrying water + mineral ions upward only — the transpiration stream.
- 4Phloem: living cells with pores in the end walls, carrying dissolved sugars from leaves to the rest of the plant (translocation) — both directions.
- 5Root hair cells: long projections multiply the absorbing surface; minerals enter by active transport, water by osmosis.
- 6Guard cells open stomata in daylight for gas exchange; closing them conserves water in drought.
- 7Transpiration is faster when it is hotter, drier, windier and brighter; slower in cool, humid still air.
- 8Rate = volume ÷ time — a shoot taking 9 cm³ in 45 minutes runs at 0.2 cm³ per minute.
Notes · 1 of 4
Inside the leaf
A layered organ engineered for photosynthesis and gas exchange.
| Tissue | Job |
|---|---|
| Waxy cuticle | Waterproof layer limiting water loss |
| Upper epidermis | Thin, transparent — lets light through |
| Palisade mesophyll | Packed with chloroplasts — main photosynthesis site |
| Spongy mesophyll | Air spaces let carbon dioxide and oxygen diffuse to and from cells |
| Xylem | Carries water and mineral ions up to the leaf |
| Phloem | Carries dissolved sugars away from the leaf |
| Lower epidermis + guard cells | Stomata open and close to balance gas exchange against water loss |
Because the palisade layer sits directly under the transparent upper surface, it catches the strongest light first. Below it, the spongy layer’s air spaces put every mesophyll cell within a short diffusion distance of a stoma — the leaf’s version of the exchange-surface rules from chapter 2.
Notes · 2 of 4
Root hair cells and the xylem stream
Absorb at the bottom, haul to the top — a one-way water railway.
Root hair cells are extensions of root epidermal cells, pushing between soil particles. The hair shape multiplies surface area enormously, so water enters by osmosis and dissolved mineral ions enter by active transport — which is why the cells carry many mitochondria. The water then flows into the xylem, dead hollow tubes stiffened with lignin, and is drawn up the plant.
The upward flow is the transpiration stream. Water evaporates from the spongy mesophyll into the air spaces, diffuses out through open stomata, and the loss drags more water up through the xylem — a continuous chain from soil to sky. Along the way the stream also delivers the mineral ions the cells need for building proteins and chlorophyll.
Worked example — water uptake rate
A leafy shoot in a potometer-style setup takes up 9 cm³ of water in 45 minutes. Rate = volume ÷ time = 9 ÷ 45 = 0.2 cm³ per minute. Asked in cm³ per hour, that is 0.2 × 60 = 12 cm³ per hour — change the unit only after the division, and label the answer.
Notes · 3 of 4
Phloem and translocation
Living tubes shipping sugars to where they are spent or stored.
| Feature | Xylem | Phloem |
|---|---|---|
| Cells | Dead, hollow, walls stiffened with lignin | Living, with pores (sieve plates) in the end walls |
| Cargo | Water and dissolved mineral ions | Dissolved sugars (sucrose) |
| Direction | Upward only: roots → stem → leaves | Both ways: leaves → roots, growing shoots, storage organs |
| Process | Transpiration stream | Translocation |
Translocation moves the sugars made in photosynthesis to wherever the plant spends them — growing tips building new cells, roots that cannot photosynthesise, or storage organs such as potato tubers. That flexibility of direction is the big difference from the xylem’s one-way flow.
Notes · 4 of 4
Stomata and the rate of transpiration
Guard cells trade carbon dioxide for water — and the weather tips the balance.
Most stomata sit in the lower epidermis. Each pore is worked by a pair of guard cells: when they fill with water they bow apart, opening the stoma so carbon dioxide can diffuse in for photosynthesis — and water vapour can escape. In darkness or drought the guard cells lose water, straighten and close the pore, saving water at the cost of gas exchange.
| Condition | Effect | Why |
|---|---|---|
| Higher temperature | Faster | Water evaporates more readily and diffusion speeds up |
| Drier (less humid) air | Faster | A steeper water concentration gradient between leaf and air |
| More wind / air movement | Faster | Moving air sweeps vapour away, keeping the gradient steep |
| Higher light intensity | Faster | Stomata open wider for photosynthesis |
Worked example — reading a transpiration graph
A graph of water uptake against air movement shows 0.4 cm³/min in still air and 1.6 cm³/min beside a fan. The fan rate is 1.6 ÷ 0.4 = four times the still-air rate — an increase of 1.2 cm³/min — because moving air carries vapour away from the stomata, holding the concentration gradient steep. When explaining, name the gradient; that is the mark.
Revise
Key terms
The words examiners expect you to use precisely.
- Transpiration
- The loss of water vapour from a plant, mainly through stomata.
- Transpiration stream
- The upward flow of water and mineral ions from roots to leaves in the xylem.
- Translocation
- The transport of dissolved sugars around the plant in the phloem.
- Guard cells
- The paired cells that open and close a stoma by swelling or losing water.
- Stoma (plural stomata)
- A pore in the epidermis through which gases and water vapour pass.
- Lignin
- The strengthening material in xylem walls that keeps the tubes open.
Revise
Common mistakes
Where students most often lose marks in this chapter — and what to do instead.
Mistake: Saying the phloem carries water upwards
Water’s one-way lift belongs to the xylem.
Instead: Phloem carries dissolved sugars both directions (translocation); xylem carries water and ions upward.
Mistake: Describing xylem as living cells
Xylem functions because the cells have died, leaving hollow tubes.
Instead: Xylem is dead, hollow, lignin-strengthened tissue.
Show 10 more mistakesShow fewer mistakes
Mistake: Placing most stomata on the upper surface
The underside loses less water to direct sun.
Instead: Most stomata are in the lower epidermis.
Mistake: Saying guard cells photosynthesise to open stomata
Opening is a water-pressure response.
Instead: Guard cells swell with water to open and lose water to close.
Mistake: Claiming humidity speeds transpiration
Damp air shrinks the water concentration gradient.
Instead: Dry air speeds transpiration; humid air slows it.
Mistake: Forgetting mineral ions travel in the xylem
Students remember water but drop the ions.
Instead: The transpiration stream carries water AND dissolved mineral ions.
Mistake: Saying root hair cells take up minerals by osmosis
Osmosis moves water only.
Instead: Mineral ions enter by active transport; water enters by osmosis.
Mistake: Calling the leaf a tissue
A leaf contains many tissues.
Instead: The leaf is an organ — tissues such as mesophyll, xylem and epidermis working together.
Mistake: Mixing up palisade and spongy mesophyll jobs
Both photosynthesise but differ in packing.
Instead: Palisade: tightly packed, most chloroplasts, at the top; spongy: loose, with air spaces for gas exchange.
Mistake: Dividing time by volume for uptake rates
That gives minutes per cm³, not the rate.
Instead: Rate = volume ÷ time.
Mistake: Ignoring the unit when comparing rates
0.2 of anything is meaningless without cm³ per minute.
Instead: Always state cm³ per minute (or per hour) with the converted value.
Mistake: Saying wind closes stomata
Wind does not gate the pores — it clears the vapour.
Instead: Air movement blows water vapour away from the leaf, steepening the gradient and speeding transpiration.
Check yourself
Quick quiz
20 questions with instant explanations. About 5 minutes.
Question 1 of 3· free preview of 20
- Question 1: current question
- Question 2: not answered
- Question 3: not answered
Explain root hair cells and the xylem stream
Water and dissolved mineral ions travel up a plant inside the
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Practise
Topical questions
12 exam-style questions with model answers and mark schemes.
A set of structured and calculation questions matched to this chapter, in three tiers, each with a model answer, mark scheme and examiner tip. Higher-tier questions are tagged.
Practise topical questionsExam practice
Practice paper
A 30-mark paper in the style of a Combined Science paper, with a separate mark scheme. Give yourself 40 min.
Question paper
Chapter 5 Practice Paper
30 marks · 40 min · 5 pages
6 multiple-choice questions and structured questions with answer lines. Plant transport systems: leaf structure, root hair cells, xylem and the transpiration stream, phloem and translocation, stomata and the rate of transpiration.
Mark scheme
PROChapter 5 Practice Paper — mark scheme
Answers · Marking points · 3 pages
Model answers with a mark for each creditworthy point — for marking at home or in class.
How does this chapter fit the real exams?
Biology Paper 1
1 h 15 · 70 marks (Trilogy)
Cell biology, organisation, infection and response, bioenergetics (chapters 1–7)
Biology Paper 2
1 h 15 · 70 marks (Trilogy)
Homeostasis, inheritance variation and evolution, ecology (chapters 8–10)
Tiers
Foundation (grades 1-1 to 5-5) · Higher (4-3 to 9-9)
Two grades are awarded together; the 21 required practicals are examined on the written papers
Want a full paper? Try the Biology Paper 1-style (Foundation), Biology Paper 1-style (Higher), Biology Paper 2-style (Foundation), Biology Paper 2-style (Higher), Chemistry Paper 1-style (Foundation), Chemistry Paper 1-style (Higher), Chemistry Paper 2-style (Foundation), Chemistry Paper 2-style (Higher), Physics Paper 1-style (Foundation), Physics Paper 1-style (Higher), Physics Paper 2-style (Foundation), Physics Paper 2-style (Higher), Required practicals practice sets.
Questions
Frequently asked questions
Is plant transport in GCSE Combined Science?
Yes — leaf structure, root hair cells, xylem, transpiration, phloem and translocation are core Trilogy content in the DfE subject content shared by AQA Trilogy, Edexcel and OCR Gateway.
What is the easiest way to remember xylem vs phloem?
Xylem = up, water and ions, dead hollow tubes (think “x = water lift”). Phloem = food, sugars, both directions, living tubes with sieve pores.
What maths appears in this chapter?
Rate of water uptake = volume ÷ time, unit conversion (per minute to per hour), means from stomata counts, and ratio comparisons like “three times faster”.
How do examiners want the wind effect explained?
By the concentration gradient: moving air sweeps water vapour away from the stomata, keeping the gradient from leaf to air steep, so transpiration is faster.
Why do guard cells matter for the whole plant?
They set the balance between photosynthesis (carbon dioxide in) and survival (water retained) — opening by day, closing in drought.
Up next
Chapter 6: Infection and response