The shape of the lesson
By the end of the lesson, learners can
- What each tissue carries, and where it is: Pick a section in the sim and rotate it.
- Built for the job: the xylem vessel: Extended.
- Xylem: water and mineral ions, plus support.
- Phloem: sucrose and amino acids.
- Positions: root = xylem star in the centre; stem = ring of bundles, xylem inside; leaf = veins, xylem on top.
- Extended: lignin (strength), no contents and no cross walls (continuous tube).
One picture every idea returns to
Xylem transports water and mineral ions and gives the plant support. Phloem transports sucrose and amino acids. In a root the xylem forms a star in the centre with phloem between its arms; in a stem the xylem and phloem are grouped in vascular bundles arranged in a ring, xylem towards the centre; in a leaf they run in the veins, xylem on the upper side. Extended: xylem vessels have thick walls with lignin, no cell contents and no cross walls, forming long continuous tubes.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Two tissues, two cargoes.. A flowering plant has two separate transport tissues. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: What each tissue carries, and where it is | Pick a section in the sim and rotate it. Project tab 1 of the lab (Xylem and phloem in a root, stem and leaf) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Built for the job: the xylem vessel | Extended. | Think-Pair-Share |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 16 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Built for the job: the xylem vessel, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Think-Pair-Share
Pose the question, give thirty seconds of silent thinking, then a minute to compare with a partner before a few pairs share with the class. The silent minute is the part that cannot be cut.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Do not mix up the cargoes. | Xylem never carries sucrose and phloem never carries the plant's water supply up from the roots. A quick memory aid: phloem carries food (sucrose and amino acids). |
| Lignin makes xylem strong; it does not make it "living" or help it carry sugar. | A xylem vessel is dead and empty: that is exactly what makes it a good pipe. |
Xylem and phloem: worksheet
6 questions, 16 marks. Written in the style of the papers; no past paper question is reproduced.
State the functions of xylem and of phloem.
A student is shown three transverse sections from a bean plant. Section P has a star-shaped central tissue. Section Q has separate bundles of tissue in a ring. Section R is thin and flat with a thick central region. Identify the organ each section came from and state where the xylem is in each.
Extended. Explain how three features of xylem vessels make them suited to their function.
The phloem in the stem of a plant is damaged by an insect. Suggest and explain one effect on the roots.
A ringed (bark-removed) branch still has green leaves a week later, but swelling appears just above the ring. Removing the bark removes the phloem but leaves the xylem. Explain why the leaves stay green.
Extended. Compare the structure of a xylem vessel with a phloem sieve tube. Give three differences.
Answer key and marking notes
- Xylem transports water and mineral ions (from the roots to the rest of the plant).
- Xylem also helps to support the plant.
- Phloem transports sucrose and amino acids.
- P is a root: the xylem is the star (cross) shape in the centre.
- Q is a stem: the xylem is in each vascular bundle, on the side towards the centre.
- R is a leaf: the xylem is in the veins / midrib, on the upper side, with the phloem below.
- Thick walls with lignin: strong, so the vessels do not collapse and they support the plant.
- No cell contents: no cytoplasm or organelles to block the flow of water.
- Cells joined end to end with no cross walls: form a long continuous tube, so water and mineral ions flow from root to leaf without interruption.
- Less sucrose (and fewer amino acids) reaches the roots through the phloem.
- So the root cells have less sucrose for respiration and growth (and less is stored), so root growth slows.
- The xylem is still intact, so water and mineral ions still reach the leaves.
- So the leaf cells stay supplied with water and remain alive (the swelling above the ring is where sucrose in the phloem builds up because it cannot pass).
- Xylem vessels are dead; sieve tubes are living cells.
- Xylem vessels have no cell contents; sieve tubes contain cytoplasm.
- Xylem has no cross walls; sieve tubes have sieve plates (perforated cross walls). Also accept: xylem walls are lignified, phloem walls are not.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which substance is transported in the phloem but not in the xylem?
- ASucrose
- BWater
- CMagnesium ions
- DNitrate ions
The diagram shows a transverse section of the root of a non-woody dicotyledonous plant. Which label points to the xylem?
- AA
- BB
- CC
- DD
In a vein in the midrib of a leaf, where is the phloem found?
- AOn the lower side of the vein, below the xylem
- BOn the upper side of the vein, above the xylem
- CIn the centre of the vein, surrounded by xylem
- DIn the palisade layer, away from the vein
A disease blocks the xylem in the stem of a plant. Which effect is seen first?
- AThe leaves wilt because they receive less water
- BThe roots shrink because they receive less sucrose
- CThe fruits stop growing because amino acids cannot reach them
- DThe leaves turn red because they make too much sucrose
As well as transporting water and mineral ions, the xylem also:
- Ahelps to support the plant
- Bcarries sucrose to the roots
- Cmakes glucose by photosynthesis
- Dabsorbs water from the soil
Which set of features describes a mature xylem vessel?
- Adead cells, lignified walls, no cross walls
- Bliving cells, lignified walls, sieve plates
- Cdead cells, thin walls, complete cross walls
- Dliving cells, cytoplasm present, no cross walls
Why is it an advantage that xylem vessels have no cell contents?
- ANothing obstructs the flow of water and mineral ions
- BThe vessel can carry out respiration to pump water
- CThere is room for chloroplasts to make sucrose
- DThe walls can be made of cellulose only
A mutant plant cannot make lignin. Which problem is it most likely to have?
- AIts xylem vessels collapse and its stems are weak
- BIts phloem cannot carry sucrose to the roots
- CIts root hair cells cannot take up mineral ions
- DIts leaves cannot open their stomata in the light
Quiz key
Phloem carries sucrose and amino acids. Water and mineral ions such as magnesium and nitrate come in from the soil and travel up in the xylem, so none of those three is the answer.
A is the star-shaped tissue in the centre of the root: the xylem. B is one of the small groups between the arms of the star: the phloem. C is the wide region of cortex cells. D is a root hair growing out from the outer layer.
In a leaf vein the xylem is on the upper side and the phloem on the lower side. Above the xylem is the wrong way round. The phloem is not surrounded by xylem in a vein, and it is part of the vein, not scattered in the palisade layer.
Blocked xylem stops water reaching the leaves, so the leaf cells lose water and the plant wilts. Sucrose and amino acids travel in the phloem, which is not blocked, so the roots and fruits are not the first to suffer. Making too much sucrose is not a result of blocked xylem.
Xylem has strong walls, so it helps to support the plant. Sucrose is carried by phloem, photosynthesis happens in mesophyll cells with chloroplasts, and water is absorbed from the soil by root hair cells, not by the xylem itself.
Xylem vessels are made of dead cells with no contents, thick walls with lignin, and no cross walls, forming a continuous tube. Living cells with sieve plates describe phloem sieve tubes. Thin walls and complete cross walls would make a weak pipe with barriers in it. Cytoplasm would block the flow.
With no cytoplasm, nucleus or vacuole, the vessel is an empty tube and water flows through without obstruction. Dead, empty cells cannot respire, so there is no pumping. Xylem vessels do not contain chloroplasts. The walls are thickened with lignin, not cellulose only.
Lignin thickens and strengthens xylem walls. Without it, the vessels can collapse inwards as water is pulled up, and the stem is weaker, so the plant is poorly supported. Phloem walls are not lignified, ion uptake by root hairs depends on active transport, and stomatal opening depends on guard cells, none of which needs lignin.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.