AQA GCSE Biology Foundation (8461), Foundation tier · Organisation › Plant tissues, organs and systems
Practise Plant organ system. 8 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How the roots, stem and leaves work together as an organ system to transport water, mineral ions and sugars: root hair cells, xylem, phloem, transpiration, translocation, and the role of stomata and guard cells. Expect 'explain' questions on the factors that affect transpiration and calculations of transpiration rate from potometer data.
Key facts
Roots + stem + leaves = the plant transport organ system
Root hair cells: water by osmosis; mineral ions by active transport
Xylem: water and mineral ions, roots → leaves; hollow tubes strengthened by lignin
Phloem: dissolved sugars, leaves → rest of plant (translocation); pores in the end walls
Transpiration = loss of water vapour from the leaves
Faster transpiration: higher temperature, lower humidity, more air movement, higher light intensity
Rate of transpiration = volume of water lost ÷ time
Notes
Roots, xylem and phloem
The roots, stem and leaves form a plant organ system for transporting substances around the plant.
Root hair cells have a long, thin extension that gives a large surface area. They absorb water by osmosis and mineral ions by active transport, which needs energy from respiration, so they have many mitochondria.
Xylem transports water and mineral ions from the roots to the stems and leaves. It is made of hollow tubes strengthened by lignin, suited to carrying water in the transpiration stream.
Phloem transports dissolved sugars from the leaves to the rest of the plant, for immediate use or for storage. This is translocation. Phloem is made of tubes of elongated cells, and cell sap moves from one cell to the next through pores in the end walls.
Transpiration
diagram
Transpiration is the loss of water vapour from the leaves.
Water evaporates from the surfaces of cells inside the leaf, and the water vapour diffuses out through the stomata. Water is drawn up the xylem to replace it. This flow is the transpiration stream.
The transpiration stream. The arrows show the movement of water.
Temperature up → faster: water molecules have more energy, so they evaporate and diffuse faster.
Humidity up → slower: there is more water vapour in the air, so the concentration gradient is smaller.
Air movement up → faster: water vapour is blown away from the leaf, keeping the concentration gradient steep.
Light intensity up → faster: more stomata open to let in carbon dioxide for photosynthesis, so more water vapour escapes.
Rate of transpiration = volume of water lost ÷ time. A potometer measures water uptake, which is close to, but not exactly, the water lost.
A potometer. The distance the bubble moves in a set time shows the rate of water uptake.
Stomata and guard cells
diagram
Stomata and guard cells control gas exchange and water loss.
Guard cells control the size of the stoma.
Stomata usually close in the dark, when no carbon dioxide is needed for photosynthesis, which saves water.
How to answer each type of question
Explain how root hair cells, xylem or phloem are adapted
2 to 4 marksGrade 5
Give a feature of the cell or tissue.
Link it to its job: absorbing, transporting water or transporting sugars.
Example. Explain how root hair cells are adapted to absorb water and mineral ions from the soil.
Show the model answerHide the model answer
Root hair cells have a long, thin extension that gives a large surface area (1), so more water can be absorbed by osmosis (1). They have many mitochondria (1), which release energy from respiration for the active transport of mineral ions (1).
Compare xylem and phloem
2 marksGrade 5
Compare what each carries, and then how each is built.
Use 'whereas' so each point covers both tissues.
Example. Give two differences between xylem and phloem.
Show the model answerHide the model answer
Xylem transports water and mineral ions, whereas phloem transports dissolved sugars (1). Xylem is made of hollow tubes strengthened by lignin, whereas phloem is made of elongated cells with pores in the end walls (1).
Don’t lose marks
Saying water enters root hair cells by diffusion or active transport. Water moves in by osmosis; mineral ions move in by active transport.
Saying phloem carries water, or xylem carries sugars.
Saying transpiration is the loss of 'water' from the plant in general. It is the loss of water vapour from the leaves.
Saying higher humidity increases transpiration. It decreases it, because the concentration gradient is smaller.
Saying a potometer measures water loss. It measures water uptake, and some water taken up is used by the plant.
Using the diameter instead of the radius, or forgetting to square it, in potometer volume calculations.
More tips
Memory tricks
Transpiration is fastest in the conditions that dry washing on a line fastest: warm, dry, windy and sunny.
Humidity is the odd one out: more humidity means slower transpiration.
Turgid guard cells swell and bow apart, like two bent sausages, opening the stoma.
Root hair cells: Water by Osmosis, Minerals by Active transport.
Potometer check: as the shoot takes up water, the bubble moves towards the plant.
Exam technique
For factor questions, state the effect (increase or decrease), then explain it using the concentration gradient or the stomata.
In calculations, check the units (mm, mm3, minutes, hours) and give the unit with your answer.
Keep transpiration (water vapour, xylem) and translocation (sugars, phloem) separate: examiners do not accept them swapped.
When describing a transpiration graph, quote values from it and say where the rate changes or levels off.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Name the parts of the plant transport systemThe roots, stem and leaves form an organ system; xylem carries water and phloem carries sugars.
Grade 4
Define transpiration and translocationTranspiration is the loss of water vapour from the leaves; translocation is the movement of dissolved sugars through the phloem.
Grade 5
Describe how root hair cells absorb substancesThey take up water by osmosis and mineral ions by active transport, helped by their large surface area.
Grade 5
Compare the structure of xylem and phloemXylem is hollow tubes strengthened by lignin; phloem is tubes of elongated cells with pores in the end walls.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Name the tissue that transports water and mineral ions from the roots to the leaves.
xylem
Water evaporates from the leaves of plants and diffuses out through the stomata. Name the cells that open and close the stomata.
guard cells
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
The roots, stem and leaves of a plant form an organ system for transporting substances.
(a) Name the tissue that transports water and mineral ions from the roots to the leaves.[1]
(b) Name the tissue that transports dissolved sugars around the plant.[1]
(c) What is the movement of dissolved food molecules through the phloem called? Tick (✓) one box.[1]
Diffusion
Osmosis
Translocation
Transpiration
(d) What is meant by transpiration?[1]
Show the answer and mark scheme
(a)Answer: xylem
xylem
(b)Answer: phloem
phloem
(c)Answer: Translocation
(d)
the loss of water (vapour) from the leaves (by evaporation through the stomata)
Question 2Medium6 marks
The rate of transpiration is affected by environmental conditions.
(a) Explain why the rate of transpiration increases on a windy day.[2]
(b) Explain why the rate of transpiration decreases when the air is humid.[2]
(c) Explain why the rate of transpiration is higher in bright light than in the dark.[2]
Show the answer and mark scheme
(a)
moving air removes water vapour from around the leaf
so the concentration gradient is kept steep and water vapour diffuses out faster
(b)
there is more water vapour in the air around the leaf
so the concentration gradient is smaller and water vapour diffuses out more slowly
(c)
more stomata are open / stomata open wider in the light (for photosynthesis)