AQA GCSE Combined Science (8464), Higher tier · Biology › Organisation › Plant tissues, organs and systems
Practise Plant organ system. 14 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.
Grade by grade
What you need to be able to do, from the first marks up to the top 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.
4
Define transpiration and translocationTranspiration is the loss of water vapour from the leaves; translocation is the movement of dissolved sugars through the phloem.
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.
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.
6
Explain factors affecting the rate of transpirationHigher temperature, lower humidity, more air movement and higher light intensity all increase the rate.
6
Calculate the rate of transpirationRate = volume (or mass) of water lost or taken up ÷ time, e.g. from a potometer.
7
Explain how guard cells control water lossGuard cells become turgid to open the stomata and flaccid to close them, balancing gas exchange against water loss.
7
Explain how water moves through the plantWater evaporates from leaf cells and diffuses out through the stomata, and more water is drawn up the xylem to replace it.
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
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.
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.
Stomata and guard cells
Stomata and guard cells control gas exchange and water loss.
When the plant has plenty of water, the guard cells take in water, become turgid and bend apart, opening the stoma. When the plant is short of water, they lose water, become flaccid and the stoma closes, reducing water loss. grade 7+
Stomata usually close in the dark, when no carbon dioxide is needed for photosynthesis, which saves water.
Cheatsheet
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
Volume of water in a potometer tube = \(\pi r^2 \times\) distance moved by the bubble grade 7+
How to answer each type of question
Explain the effect of a factor on transpiration
2 to 3 marks6
Say whether the rate increases or decreases.
Explain with evaporation, diffusion and the concentration gradient of water vapour.
For light, link to more stomata opening for photosynthesis.
Example. Explain why a plant loses water more quickly on a windy day than on a still day.
Show the model answer
The wind blows water vapour away from the surface of the leaf (1). This keeps a steep concentration gradient of water vapour between the air spaces in the leaf and the air outside (1), so water vapour diffuses out through the stomata faster (1).
Calculate the rate of transpiration
2 to 4 marks7
Distance rate = distance moved by the bubble ÷ time.
Volume = \(\pi r^2 \times\) distance (use the radius, not the diameter, and square it).
Give the unit, e.g. mm per minute or mm3 per minute.
Example. A student used a potometer. The air bubble moved 45 mm in 5 minutes. The capillary tube has a radius of 0.5 mm. (a) Calculate the rate at which the bubble moved, in mm per minute. (b) Calculate the volume of water taken up by the shoot in 5 minutes. Use: volume = \(\pi r^2 \times\) distance. Give your answer to 3 significant figures.
Explain how root hair cells, xylem or phloem are adapted
2 to 4 marks5
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 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).
Explain how guard cells control water loss
3 marks7
Say what happens to the guard cells (lose water, become flaccid).
Say what happens to the stomata (they close).
Say the effect: less water vapour is lost (but less carbon dioxide can get in, so photosynthesis slows).
Example. On a hot, dry day, a plant runs short of water and its stomata close. Explain how the stomata close and how this helps the plant.
Show the model answer
The guard cells lose water and become flaccid (1), so the stomata close (1). Less water vapour diffuses out of the leaf, so less water is lost by transpiration (1).
Compare xylem and phloem
2 marks5
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 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).
Shortcuts and 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.
Where marks are lost
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.
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.
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)
so more water vapour can diffuse out of the leaf
Question 3Hard7 marks
A student wanted to investigate the effect of light intensity on the rate of transpiration of a leafy shoot using a potometer.
(a) Plan an investigation the student could carry out. Your plan should produce valid results.[6]
(b) Give the reason why the student should use the same leafy shoot throughout the investigation.[1]
Show the answer and mark scheme
(a)
cut the shoot under water and fit it into the potometer under water; make the joints airtight (e.g. with petroleum jelly); dry the leaves
introduce an air bubble into the capillary tube
vary light intensity by placing a lamp at different distances from the shoot, e.g. 10, 20, 30, 40 cm (or use a light meter)
use a heat shield / tank of water between the lamp and the shoot to keep the temperature constant
allow the shoot to adjust for a few minutes at each light intensity
measure the distance the bubble moves in a set time, e.g. 5 minutes, and reset the bubble with the reservoir
keep other variables the same: same shoot, same temperature, humidity and air movement (no draughts)
repeat at each light intensity and calculate the mean rate (distance or volume per minute)
Marked with levels of response: the full level descriptors are in the app.
(b)
different shoots have different numbers / areas of leaves / numbers of stomata