Cambridge O Level Biology 5090: Transport in flowering plants (5090) – Revision Notes

For full explanations and worked examples, use the transport in flowering plants study guide. These notes are for the final weeks of revision.

They cover Topic 7, Transport in flowering plants, of the Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4): section 7.1 Uptake and transport of water and ions and section 7.2 Transpiration and translocation. The syllabus is not tiered, so every outcome here applies to all candidates.

Test yourself with the practice questions. The course hub is Cambridge O Level Biology, the printable checklist lists every outcome, and a free 10-minute diagnostic will show you which topics need work.

Key definitions

Term	Meaning
Transpiration	The loss of water vapour from leaves
Translocation	The movement of sucrose and amino acids in the phloem from sources to sinks
Source	A part of the plant that produces or releases sucrose and amino acids (e.g. a photosynthesising leaf)
Sink	A part of the plant that uses or stores sucrose and amino acids (e.g. a root, growing tip, fruit or seed)
Transpiration pull	The pull on the water column in the xylem caused by water loss from the leaves
Wilting	Drooping of leaves and soft stems when cells become flaccid because water loss exceeds water uptake

7.1 Uptake and transport of water and ions

Root hair cells (7.1.1)

• Long, thin extension → large surface area for uptake.
• Water enters by osmosis (soil water has a higher water potential than the cell sap).
• Mineral ions enter by active transport, using energy from respiration (many mitochondria).

The osmosis and active transport mechanisms are revised in the Movement into and out of cells revision notes.

Pathway of water (7.1.2)

Learn the four syllabus stops in order:

root hair cells → root cortex cells → xylem → mesophyll cells

The xylem runs from the root, up the stem, into the leaf veins. After the mesophyll cells, water evaporates into the air spaces and leaves through the stomata (that is transpiration, 7.2).

Stain practical (7.1.3): method in steps

1. Cut a leafy stem (e.g. celery) under water.
2. Stand it in a coloured stain (e.g. eosin) for a few hours in warm, bright conditions.
3. Cut thin transverse sections at different heights.
4. Observe with a hand lens or microscope.
5. Result: stain only in the xylem of the vascular bundles.

7.2 Transpiration and translocation

How transpiration happens (7.2.1–7.2.2)

1. Water evaporates from the surfaces of mesophyll cells into the air spaces.
2. Water vapour diffuses out through the stomata, down a concentration gradient.

Factors (7.2.3a)

Factor increased	Rate	Reason
Wind speed	↑	Removes water vapour near the leaf; gradient stays steep
Temperature	↑	More kinetic energy; faster evaporation and diffusion
Humidity	↓	More water vapour in the air; smaller gradient
Light intensity	↑	Stomata open wider

Wilting (7.2.3b)

Water lost by transpiration > water taken up → cells lose water → cells become flaccid, lose turgor pressure → no longer press against each other → leaves and soft stems droop.

If the plant is watered, or water loss falls (for example in the cool of the evening), the cells take in water by osmosis, become turgid again and the plant recovers.

Investigating transpiration (7.2.4): method in steps

Potometer

1. Cut a leafy shoot under water and fit it into the potometer under water, so no air gets in.
2. Make the joints airtight (petroleum jelly). Dry the leaves.
3. Introduce one air bubble into the capillary tube.
4. Measure the distance the bubble moves in a set time. Rate = distance ÷ time.
5. Change one factor: fan setting (wind), lamp distance with a heat filter (light), room temperature (temperature).
6. Keep other factors constant, allow time to settle, repeat and take a mean.

Weighing: seal the pot and soil in a plastic bag, weigh the plant at the start and after a set time; mass lost = water lost.

Calculations to know

Quantity	How to find it
Rate of bubble movement	distance ÷ time (e.g. mm per minute)
Volume taken up	distance × cross-sectional area of the capillary
Rate of mass loss	mass lost ÷ time
Percentage change	(new − original) ÷ original × 100

Worked reminder: a bubble moves 30 mm in 6 minutes along a capillary of cross-sectional area 0.4 mm². Rate = 30 ÷ 6 = 5 mm per minute. Volume = 30 × 0.4 = 12 mm³, so 2 mm³ per minute.

Transpiration pull (7.2.5)

1. Water vapour is lost from the leaves.
2. Water moves from the xylem into the mesophyll cells to replace it.
3. This creates a pull on the water in the xylem.
4. Forces of attraction between water molecules hold the column together.
5. The whole column of water is drawn up from the roots.

Translocation (7.2.6)

• In the phloem.
• Moves sucrose and amino acids.
• From sources (produce or release) to sinks (use or store).
• A storage organ can switch roles: a sink when it stores food, a source when it releases food to new growth.
• Phloem can carry substances up or down; xylem only carries water and ions upwards.

Worked reminder: on an apple tree in summer, the photosynthesising leaves are sources. The developing apples are sinks, because they store sugars, and so are the roots, because they use sucrose in respiration. Sucrose therefore moves up from leaves to fruits above them and down to the roots at the same time.

Transverse sections (7.2.7)

Tissue	Non-woody dicot root	Non-woody dicot stem
Xylem	Centre, star or cross shape	Inner side of each vascular bundle
Phloem	Between the arms of the xylem	Outer side of each vascular bundle
Cortex	Wide band around the central vascular tissue	Band between the surface and the ring of bundles

Xylem (7.2.8–7.2.9)

Functions: transport of water and mineral ions; support.

Feature	Function
Thick walls with lignin	Strength and support; vessels do not collapse
No cell contents	Unobstructed flow of water
End-to-end, no cross walls	Continuous tube for an unbroken water column

Must-know distinctions

• Transpiration vs translocation: water vapour lost from leaves vs sucrose and amino acids moved in phloem.
• Xylem vs phloem: water and mineral ions, upwards only, plus support vs sucrose and amino acids, up or down.
• Evaporation vs diffusion in the leaf: evaporation happens at mesophyll cell surfaces; diffusion carries the vapour out through stomata.
• Root vs stem section: central vascular tissue in a root; ring of bundles in a stem.
• Water uptake vs ion uptake: osmosis vs active transport.
• Potometer reading vs transpiration: a potometer measures water uptake, used as an estimate of water loss.

Quick self-test

1. Define transpiration.
2. Name, in order, the four parts of the water pathway listed in the syllabus.
3. Why does a root hair cell need many mitochondria?
4. State where water evaporates inside a leaf.
5. Explain why transpiration is slower on a humid day.
6. Explain why transpiration increases in bright light.
7. A potometer bubble moves 24 mm in 4 minutes. Calculate the rate.
8. A sealed potted plant falls from 250.0 g to 241.0 g in 6 hours. Calculate the rate of water loss and the percentage loss in mass.
9. What holds the water column together in the xylem?
10. A carrot root stores sugar in its first year. Is it a source or a sink then?
11. In a stem vascular bundle, which tissue is nearer the outside?
12. State two functions of xylem.

Answers

1. The loss of water vapour from leaves.
2. Root hair cells, root cortex cells, xylem, mesophyll cells.
3. To release energy by respiration for the active transport of mineral ions.
4. From the surfaces of mesophyll cells, into the air spaces.
5. The air outside holds more water vapour, so the concentration gradient between the air spaces and the air is smaller and water vapour diffuses out more slowly.
6. Stomata open wider in light, so more water vapour can diffuse out.
7. 24 ÷ 4 = 6 mm per minute.
8. Mass lost = 250.0 − 241.0 = 9.0 g. Rate = 9.0 ÷ 6 = 1.5 g per hour. Percentage loss = 9.0 ÷ 250.0 × 100 = 3.6%.
9. Forces of attraction between water molecules.
10. A sink: it stores food made elsewhere in the plant.
11. Phloem.
12. Transport of water and mineral ions; support.

Where marks are usually lost

• Writing “loss of water” for transpiration, with no mention of vapour or leaves.
• Describing water as evaporating from the stomata; it evaporates from mesophyll surfaces and the vapour diffuses out.
• Giving the direction of a factor’s effect without the reason (gradient or stomata).
• Linking light to temperature instead of to stomatal opening.
• Leaving out root cortex cells or mesophyll cells from the water pathway.
• Saying xylem transports sucrose or that phloem transports water and minerals.
• Swapping xylem and phloem positions in a stem bundle.
• Describing wilting without flaccid or turgor.
• Quoting a rate with no units, or dividing time by distance.

Official syllabus

Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028, version 4 (Cambridge University Press & Assessment), section 7 Transport in flowering plants.