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

This guide teaches Topic 7, Transport in flowering plants, for Cambridge O Level Biology 5090. It follows the Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4), and covers every learning outcome in sections 7.1 Uptake and transport of water and ions and 7.2 Transpiration and translocation. The syllabus is not tiered: every candidate studies all of this content.

Use it with the transport in flowering plants revision notes and the transport in flowering plants practice questions. The course hub is at Cambridge O Level Biology, and you can tick off outcomes on the printable checklist. To find your weak spots first, try a free 10-minute diagnostic.

Osmosis, turgor and active transport are taught in Movement into and out of cells; leaf structure and stomata in Plant nutrition.

What this topic covers

Section	What you must be able to do
7.1.1	Relate the structure of root hair cells to water and ion uptake
7.1.2	Outline the pathway of water: root hair cells, root cortex cells, xylem, mesophyll cells
7.1.3	Investigate the pathway of water in a cut stem using a stain
7.2.1–7.2.2	Describe transpiration; explain evaporation from mesophyll surfaces and diffusion out through stomata
7.2.3	Explain the effects of wind speed, temperature, humidity and light intensity, and how wilting occurs
7.2.4	Investigate the effects of wind speed, light intensity and temperature on transpiration rate
7.2.5	Explain how water moves up the xylem by transpiration pull
7.2.6	Describe translocation of sucrose and amino acids in the phloem from sources to sinks
7.2.7	Identify xylem, phloem and cortex in transverse sections of non-woody dicotyledonous roots and stems
7.2.8–7.2.9	State the functions of xylem and relate xylem vessel structure to function

7.1 Uptake and transport of water and ions

Root hair cells

A root hair cell is a cell on the outer surface of a young root with a long, thin extension that grows out between soil particles.

Feature	How it helps uptake
Long, thin hair extension	Gives a large surface area, so more water enters by osmosis and more ions by active transport
Thin cell wall	Short distance for water to cross
Cell sap with a lower water potential than soil water	Water moves into the cell by osmosis
Many mitochondria	Release energy by respiration for active transport of mineral ions

Water enters by osmosis. Mineral ions are usually less concentrated in the soil than in the cell, so they enter by active transport.

The pathway of water

The syllabus limits the pathway to four stops:

soil water → root hair cells → root cortex cells → xylem
           (root, then stem, then leaf vein) → mesophyll cells

Water crosses the root cortex cell to cell by osmosis to the xylem in the centre of the root. Xylem vessels carry it up the stem into the leaf veins, and it passes into the mesophyll cells. Mineral ions travel dissolved in the same water.

Practical: tracing water in a cut stem

Worked example: planning the stain investigation

Question: Describe how you would show the pathway of water through a stem.

Answer, step by step:

1. Cut the base of a leafy stem, such as a celery stalk, under water so no air enters the xylem.
2. Stand it in a beaker of coloured stain, such as eosin or a water-soluble dye.
3. Leave it in a warm, bright place for a few hours so water is pulled up quickly.
4. Remove it and cut thin transverse sections at several heights with a sharp blade. Cut away from your fingers on a tile.
5. Look at the sections with a hand lens or microscope.
6. Expected result: the stain appears only in the vascular bundles, in the xylem. Leaf veins also become coloured.
7. Conclusion: water travels up the stem in the xylem.

7.2 Transpiration and translocation

What transpiration is

Transpiration is the loss of water vapour from leaves.

It happens in two steps:

1. Water evaporates from the surfaces of the mesophyll cells into the air spaces inside the leaf.
2. The water vapour then diffuses out of the leaf through the stomata, from a higher concentration of water vapour inside the leaf to a lower concentration outside.

Most stomata are on the lower epidermis, so most water vapour leaves from the underside of the leaf.

Factors that affect transpiration rate

Factor	Change	Effect on rate	Explanation
Wind speed	Increases	Increases	Wind blows away water vapour from around the leaf, keeping the concentration gradient between inside and outside steep
Temperature	Increases	Increases	Water molecules have more kinetic energy, so they evaporate from mesophyll surfaces and diffuse faster
Humidity	Increases	Decreases	Air outside already holds more water vapour, so the concentration gradient is smaller and diffusion slower
Light intensity	Increases	Increases	Stomata open wider in light (for photosynthesis), so more water vapour can diffuse out

How wilting occurs

Wilting happens when a plant loses water by transpiration faster than it takes water up from the soil. The cells lose water, become flaccid and lose turgor pressure. Turgid cells normally press against each other and hold up the stem and leaves, so without turgor the leaves and soft stems droop.

Wilting is most likely on a hot, dry, windy day, or when the soil is dry.

Practical: investigating transpiration rate

Method 1: the potometer. A leafy shoot is fitted, under water, into a tube connected to a capillary tube. As the shoot takes up water, an air bubble moves along the capillary. The distance moved per minute gives the rate of water uptake. Almost all water taken up is lost by transpiration, so this is used as a measure of transpiration rate.

To change one factor at a time:

• Wind speed: a fan at a fixed distance, on different settings, or off.
• Light intensity: a lamp at different distances, with a heat filter (a beaker of water) between lamp and shoot.
• Temperature: the whole apparatus in rooms or cabinets at different temperatures.

Keep the same shoot, let it settle after each change, and repeat each reading.

Method 2: weighing. Seal the pot and soil of a potted plant in a plastic bag so only the shoot loses water, and weigh it before and after a set time.

Worked example: potometer calculations

A student ran a potometer for 5 minutes in each condition. The capillary has a cross-sectional area of 0.5 mm².

Condition	Bubble distance in 5 min / mm
Still air	15
Fan on	40

Rate in still air: 15 ÷ 5 = 3.0 mm per minute. Rate with fan: 40 ÷ 5 = 8.0 mm per minute. Volume taken up with fan: volume = distance × cross-sectional area = 40 × 0.5 = 20 mm³ in 5 minutes, which is 20 ÷ 5 = 4.0 mm³ per minute. Percentage increase from still air to fan: (8.0 − 3.0) ÷ 3.0 × 100 = 167% (to 3 significant figures).

Explanation: moving air removes water vapour from around the leaf, keeping the concentration gradient steep.

Worked example: weighing method

A sealed potted plant fell from 412.0 g to 398.5 g in 3 hours. Mass lost = 412.0 − 398.5 = 13.5 g. Rate = 13.5 ÷ 3 = 4.5 g per hour.

How water moves up the xylem

The syllabus explains this as a transpiration pull:

1. Water evaporates from mesophyll cells and diffuses out of the stomata.
2. Mesophyll cells replace this water from the xylem in the leaf veins.
3. This removal of water from the top of the xylem creates a pull on the water below.
4. Water molecules are held together by forces of attraction between water molecules (sometimes called cohesion).
5. So the whole column of water in the xylem is drawn upwards, from the roots, through the stem, to the leaves, in an unbroken column.

Worked example: explaining a result

Question: A student removes all the leaves from a shoot fitted to a potometer. The bubble almost stops. Explain why.

Answer: With no leaves, there are no mesophyll surfaces to evaporate from and no stomata for water vapour to leave by. Transpiration almost stops, so there is no transpiration pull to draw the column of water up the xylem, and little water is taken up.

Translocation

Translocation is the movement of sucrose and amino acids in the phloem, from parts of the plant that produce or release them (sources) to parts that use or store them (sinks).

Source (produces or releases)	Sink (uses or stores)
Photosynthesising leaves make sucrose	Growing shoot tips and young leaves use it for growth and respiration
A storage organ, such as a potato tuber, breaking down its starch when new shoots grow	Roots use it for respiration
	Developing fruits and seeds, and storage organs while they fill, store it

The same organ can be a sink at one time and a source at another. A potato tuber is a sink in summer, when it fills with stored starch, and a source in spring, when stored food is released to the new shoots. Because of this, phloem can carry substances up or down the plant. Xylem carries water only upwards.

Worked example: sources and sinks

Question: In early spring a tulip bulb sends up leaves before they can photosynthesise much. Identify the source and sink.

Answer: The bulb is the source: it releases stored food. The growing leaves and shoot are the sink: they use the sucrose for growth and respiration. Later the roles reverse.

Xylem, phloem and cortex in transverse sections

You need to identify three tissues in sections of non-woody dicotyledonous roots and stems.

	Root (transverse section)	Stem (transverse section)
Xylem	In the centre, often in a star or cross shape	In each vascular bundle, on the side towards the centre
Phloem	In patches between the arms of the xylem	In each vascular bundle, on the side towards the outside
Cortex	A wide band of cells between the outer surface and the central xylem and phloem	The band of cells between the outer surface and the ring of vascular bundles

In the stem, the vascular bundles form a ring near the outside: phloem faces out, xylem faces in.

Worked example: identifying a section

Question: A section shows a central star-shaped tissue with patches of a second tissue between its arms, surrounded by a wide band of cells. Is it a root or a stem? Name the three tissues.

Answer: It is a root, because the vascular tissue is in the centre rather than in a ring of bundles. The star-shaped tissue is xylem, the patches between its arms are phloem, and the wide band is cortex.

Xylem: functions and structure

Functions of xylem:

• transport of water and mineral ions, from the roots to the leaves
• support for the plant

Structure related to function:

Feature of xylem vessels	Link to function
Thick walls containing lignin	Strong and rigid, so they support the plant and do not collapse as water is pulled up
No cell contents	Nothing blocks the flow of water
Cells joined end-to-end with no cross walls, forming a long continuous tube	Water moves in an unbroken column from root to leaf

Details of how lignin is laid down are not required.

Common errors

• Writing that transpiration is “loss of water”. It is loss of water vapour, and from leaves.
• Saying water “evaporates out of the stomata”. It evaporates from mesophyll cell surfaces into air spaces, then the vapour diffuses out through stomata.
• Explaining humidity backwards. Higher humidity lowers the rate.
• Saying light increases transpiration “because it is hotter”. Link light to stomata opening.
• Saying xylem carries sucrose. Xylem carries water and mineral ions; phloem carries sucrose and amino acids.
• Calling the transpiration pull “suction from the roots”. The pull comes from water loss at the leaves.
• Putting phloem on the inside of a stem vascular bundle.

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.