Two transport tissues. Two different journeys.
Core + labelled SupplementB8.1–B8.4Two 45-minute sessions + optional consolidationResponses save in this browser when storage is available. Download a copy before changing device. Nothing is submitted to MSH or a teacher.
Suggested pacing: Session 1: starter 5 min, learn/watch 25 min, activities 15 min. Session 2: retrieval 5 min, quiz 10 min, exam practice 25 min, exit ticket 5 min. Pause or split further to suit your class.
Phloem transports sucrose and amino acids to parts that use or store them. Xylem carries water and mineral ions from roots and helps support the plant.
| Feature | Xylem | Phloem |
|---|---|---|
| Main substances | Water and mineral ions | Sucrose and amino acids |
| Role | Transport and support | Transport of organic nutrients |
| Typical transport direction | From roots towards leaves | From source to sink; can be up or down in different tubes |
| Useful structural link | Thick, lignified vessel walls help support the plant | Living tissue transports assimilates |
Use position, not a memorised colour: real microscope images are not necessarily blue and orange. In a non-woody dicot root, xylem commonly forms a central star with phloem between its arms. In a stem, vascular bundles form a ring: xylem is towards the centre, phloem towards the outside. In a leaf vein, xylem is nearer the upper surface and phloem nearer the lower surface.
A root hair cell is an epidermal cell with a long, narrow extension. This provides a large surface area in contact with soil water, increasing uptake of water and mineral ions. From B3: water enters by osmosis when soil water has a higher water potential than the cell sap; mineral-ion uptake can involve active transport.
Transpiration is loss of water vapour from leaves. Liquid water evaporates from moist mesophyll-cell surfaces into the leaf’s air spaces. Water vapour then diffuses through open stomata into the outside air, down its concentration gradient.
As leaves lose water, water supplied by xylem replaces it. The resulting transpiration stream links the root supply to the leaf. The plant does not pump water using a heart.
Write the two stages separately: evaporation from mesophyll surfaces; diffusion of water vapour through stomata. Water does not “evaporate from the xylem straight out of the leaf”.
| Change, other factors constant | Usual effect | Explanation [Supplement] |
|---|---|---|
| Higher temperature | Rate increases over a suitable range | Faster evaporation and faster diffusion of water vapour. |
| Greater wind speed | Rate increases | Moving air removes humid air near the leaf, maintaining a steeper water-vapour concentration gradient. |
| Higher humidity [Supplement] | Rate decreases | Outside air contains more water vapour, so the gradient from leaf to air is smaller. |
These comparisons assume water is available and stomata remain open. Under severe water stress, stomatal closure can change the pattern.
If water loss exceeds uptake for long enough, cells lose water and turgor pressure falls. Soft tissues become less rigid; leaves and stems droop. Wilting is not the same as plasmolysis in every situation.
Translocation moves sucrose and amino acids in phloem from sources to sinks. A source releases these substances into transport; a sink uses them or stores them.
| Situation | Source | Sink |
|---|---|---|
| Mature leaf supplying a growing root | Photosynthesising leaf | Growing root |
| Leaf supplying a developing fruit | Photosynthesising leaf | Fruit storing nutrients |
| Stored reserves mobilised during new growth | Storage organ | Growing shoot |
A root is not always a sink: a storage root can become a source when its reserves are mobilised. “Source” and “sink” describe current function, not a permanent organ label.
Look for a single elongated cell with a narrow projection. It is not a whole root. The cell membrane lies just inside the wall; its partially permeable nature matters for osmosis.
Cognito · Open on YouTube. Internet access is required. If your school blocks the player, use the written lesson below.
Predict before moving a control. This is an illustrative model, not measured plant data. Water is available and stomata are assumed open.
Index = 100 × (temperature ÷ 20) × [(wind + 1) ÷ 2] × [(100 − humidity) ÷ 50]. It is a teaching rule, not a physiological law. Baseline 20 °C, wind 1 and 50% humidity = 100.
A bubble moves along a capillary as a shoot takes up water. For a fixed capillary, bubble distance ÷ time is a useful uptake-rate proxy. It is not a direct measurement of all transpiration: some absorbed water is used or retained.
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Original Cambridge-style practice, not copied past-paper questions. Write first, then reveal mark points and self-assess. Written answers are not automatically graded; self-scores are not official grades.
Suggested marking: one mark per valid point, up to 3.
Suggested marking: one mark per valid point, up to 2.
Suggested marking: one mark per valid point, up to 3.
Suggested marking: one mark per valid point, up to 2.
Illustrative results using the same shoot and capillary; temperature, humidity and light are kept constant.
| Fan setting | Trial 1 / mm min⁻¹ | Trial 2 / mm min⁻¹ | Trial 3 / mm min⁻¹ |
|---|---|---|---|
| Off | 2.0 | 2.2 | 1.8 |
| Low | 3.9 | 4.1 | 4.0 |
| High | 6.1 | 5.9 | 6.0 |
Suggested marking: one mark per valid point, up to 3.
Suggested marking: one mark per valid point, up to 2.
Suggested marking: one mark per valid point, up to 2.
Suggested marking: one mark per valid point, up to 3.
Teacher-led setup: cut the shoot under water to reduce air entry; seal joints; handle glass and blades safely. This page uses supplied data and requires no practical equipment.
This lesson follows Cambridge 0654 syllabus for exams in 2025–2027, B8.1–B8.4. The verified sequence after B6 is B7 Human nutrition, B8 Transport in plants, B9 Transport in animals. Check the examination-year syllabus for other cohorts.
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Continue the series: B7 Human nutrition · B8 Transport in plants · B9 Transport in animals