The shape of the lesson
By the end of the lesson, learners can
- Two routes across the root: Choose apoplast, then symplast, then both.
- Holding the column together: Press Transpiration on and watch the water molecules rise.
- Evidence for tension: Exam questions often give you data and ask you to link it to cohesion-tension.
- Apoplast: cellulose walls and spaces, no membranes, fast.
- Symplast: cytoplasm and plasmodesmata.
- Casparian strip (suberin) blocks the apoplast at the endodermis: control of ions.
- Cohesion water to water, adhesion water to cellulose, both hydrogen bonds; tension from transpiration.
One picture every idea returns to
Mineral ions and some organic compounds are carried around a plant dissolved in water. Water enters root hair cells by osmosis and crosses the cortex by two routes: the apoplast pathway through the permeable cellulose cell walls and spaces between cells, and the symplast pathway through the cytoplasm, passing from cell to cell through plasmodesmata. At the endodermis a band of waterproof suberin, the Casparian strip, blocks the apoplast, so all water must pass through the endodermal cells, letting the plant control which ions reach the xylem. In the xylem, water molecules form hydrogen bonds with each other (cohesion) and with the cellulose of the walls (adhesion). As water is lost from the leaves, the continuous column is pulled up under tension: the transpiration pull. Lignin keeps the vessels from collapsing and makes their walls waterproof.
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, A column of water, pulled from the top.. Water crosses the root through cell walls or through cytoplasm, is forced through living cells at the endodermis, then rises in the xylem as an unbroken column held together by hydrogen bonds and pulled up by the leaves. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Two routes across the root | Choose apoplast, then symplast, then both. Project tab 3 of the lab (Transport in plants lab, root pathways) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Holding the column together | Press Transpiration on and watch the water molecules rise. Project tab 5 of the lab (Transport in plants lab, cohesion-tension) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Evidence for tension | Exam questions often give you data and ask you to link it to cohesion-tension. | Frayer Model |
| 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, 20 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 Evidence for tension, 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.
Frayer Model
Pairs fill a four-box grid for the key term: definition, characteristics, examples and non-examples. The non-examples box is where the misconceptions surface, so give it the most time.
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 |
|---|---|
| The Casparian strip blocks the apoplast only. | It does not stop water reaching the xylem: water simply has to go through the cytoplasm of the endodermal cells (the symplast) for that step. Never write that the Casparian strip "stops water entering the xylem". |
| Water moves up the xylem passively: the energy comes from the Sun evaporating water in the leaf, not from ATP in the xylem. | Xylem vessel elements are dead and cannot pump anything. |
From soil to xylem, and up the stem: worksheet
6 questions, 20 marks. Written in the style of the papers; no past paper question is reproduced.
State two substances, other than water, that are transported in a plant dissolved in water, and name the tissue that carries each.
Describe the apoplast pathway taken by water from the soil to the xylem of a root.
Explain the importance of the Casparian strip to a plant.
Explain how hydrogen bonding allows water to move up the xylem from the roots to the leaves.
The diameter of a tree trunk was measured every two hours. It was largest at 04:00 and smallest at 14:00. Explain these results.
A student sets up a potometer by cutting a leafy shoot in air and then fitting it into the apparatus. Very little water uptake is recorded. Suggest and explain why.
Answer key and marking notes
- Mineral ions (for example nitrate or magnesium ions) are carried in the xylem.
- Organic compounds such as sucrose or amino acids are carried in the phloem.
- Water moves through the cellulose cell walls and the spaces between cells of the cortex.
- It does not cross any cell membranes; the cellulose walls are freely permeable.
- At the endodermis the Casparian strip (waterproof suberin) blocks the walls, so water must enter the cytoplasm of the endodermal cells (join the symplast).
- It then passes into the xylem through pits in the lignified walls.
- It is a band of suberin, which is waterproof, in the walls of endodermal cells, so it blocks the apoplast pathway.
- All water and dissolved ions must therefore pass through the partially permeable cell surface membranes of endodermal cells.
- This lets the plant control (select) which mineral ions enter the xylem and keep out toxic substances.
- Water molecules are polar and form hydrogen bonds with each other.
- This gives cohesion, so the water in a xylem vessel forms a continuous column.
- Water molecules also form hydrogen bonds with the cellulose in the vessel walls: adhesion.
- Water evaporating from the leaves (transpiration) is replaced from the xylem, so the column is pulled upward under tension (transpiration pull).
- Lignified walls stop the vessels collapsing under this tension.
- Transpiration rate is highest in the early afternoon (warm, bright, stomata open) and lowest at night.
- A high transpiration rate pulls the water column up with greater tension.
- The tension pulls the xylem walls inward, so the trunk is narrowest at 14:00; at night tension is low and the trunk returns to its widest.
- Cutting the shoot in air lets air bubbles into the xylem vessels.
- An air bubble breaks the continuous water column, because there are no hydrogen bonds across the gap.
- So the transpiration pull cannot draw water up from the potometer; the shoot should be cut and fitted under water.
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.
Through which part of root cortex cells does water travel in the apoplast pathway?
- Athe cellulose cell walls and the spaces between cells
- Bthe cytoplasm, passing through plasmodesmata
- Cthe vacuoles, crossing the tonoplast of each cell
- Dthe lignified walls of xylem vessels in the cortex
What are plasmodesmata?
- Astrands of cytoplasm passing through pores in the cell walls of neighbouring cells
- Bwaterproof bands of suberin in the walls of endodermal cells
- Cgaps in the lignin of xylem walls where only cellulose remains
- Dpores in the sieve plates between sieve tube elements
A root is treated with an enzyme that breaks down the suberin in its endodermis, without harming the cells. What is the most likely effect?
- Awater and ions could reach the xylem through the apoplast without being selected by cell membranes
- Bno water could reach the xylem, because the symplast would be blocked
- Cthe endodermal cells would become waterproof and stop taking up ions
- Dwater would move only through the symplast across the whole root
Which statement explains cohesion between water molecules in a xylem vessel?
- Ahydrogen bonds form between the slightly negative oxygen of one molecule and a slightly positive hydrogen of another
- Bcovalent bonds form between the hydrogen atoms of neighbouring water molecules
- Chydrogen bonds form between water molecules and the cellulose of the vessel walls
- Dionic bonds form between water molecules and mineral ions dissolved in the sap
A student says that ATP is used to move water up the xylem of a tall tree. Why is the student wrong?
- Axylem vessels are dead, and the energy comes from evaporation of water in the leaves
- Bxylem vessels use energy from the phloem instead of their own ATP
- Cwater rises only because of root pressure, which needs no energy
- Dthe hydrogen bonds between water molecules release the energy needed
The graph shows the rate of transpiration from the leaves of a tree and the rate of water uptake by its roots over 24 hours. Which conclusion is best supported by the data?
- Achanges in transpiration come first and drive water uptake by the roots
- Bwater uptake by the roots pushes water up and causes transpiration
- Ctranspiration and water uptake are always equal at every time of day
- Dthe tree loses no water at all between 00:00 and 06:00
Which set of water potentials would allow water to move by osmosis from the soil through a root hair cell and a cortex cell and into the xylem?
- Asoil −10 kPa, root hair −200 kPa, cortex −350 kPa, xylem −500 kPa
- Bsoil −500 kPa, root hair −350 kPa, cortex −200 kPa, xylem −10 kPa
- Csoil −10 kPa, root hair −350 kPa, cortex −200 kPa, xylem −500 kPa
- Dsoil −200 kPa, root hair −10 kPa, cortex −350 kPa, xylem −500 kPa
During a dry, hot day, air bubbles form in several xylem vessels of a stem, yet water still reaches the leaves above them. Which feature of xylem explains this?
- Apits allow water to move sideways into neighbouring vessels and bypass the blocked ones
- Bsieve plates trap the air bubbles and let the water pass through
- Ccompanion cells pump the air bubbles out of the vessels using ATP
- Dlignin dissolves the air bubbles so the column joins up again
Quiz key
The apoplast is the continuous system of cellulose cell walls and intercellular spaces; water moves through it without crossing any membrane. Cytoplasm and plasmodesmata make up the symplast. Moving through vacuoles also means crossing membranes, so it is not the apoplast. The cortex is made of parenchyma, not xylem, and its walls are not lignified.
Plasmodesmata are fine strands of cytoplasm, lined by cell surface membrane, that run through the walls and join the cytoplasm of adjacent cells. A waterproof band of suberin is the Casparian strip. Gaps in the lignin of xylem are pits. Sieve plate pores are larger openings in phloem end walls.
The suberin of the Casparian strip blocks the apoplast at the endodermis, forcing water and ions through membranes, where the plant controls which ions pass. Without it, the apoplast would run straight into the xylem and that control would be lost. The symplast is not blocked by removing suberin, so water still reaches the xylem. Suberin is what waterproofs walls, so removing it cannot make cells more waterproof. With the apoplast open, much of the water would travel in the walls, not only the symplast.
Cohesion is the attraction of water molecules to each other through hydrogen bonds between the partially negative oxygen and the partially positive hydrogen of a neighbour. Covalent bonds hold the atoms within one molecule, not between molecules. Bonding to cellulose is adhesion, not cohesion. Interactions with dissolved ions are not what holds the column together.
Mature xylem vessel elements are dead, so they cannot use ATP. Water is pulled up passively by the tension created when water evaporates from the leaves, using heat energy from the Sun. The phloem does not power the xylem. Root pressure exists but is small and does use energy (active uptake of ions), so it is neither the main mechanism nor energy free. Hydrogen bonds hold the column together; breaking or making them does not drive the flow.
Transpiration rises and peaks about two hours before water uptake, so the loss of water from the leaves comes first and the pull is transmitted down the xylem to the roots, as cohesion-tension predicts. If roots pushed water up, uptake would rise first. The two curves differ for most of the day (the tree draws on water stored in its tissues while transpiration exceeds uptake). Both rates stay a little above zero at night, so some water is still lost.
Water only moves by osmosis down a water potential gradient, so the values must become more negative at every step from soil to xylem: −10, −200, −350, −500 kPa. The second set runs the wrong way (water would leave the root). In the third set water could not move from the root hair (−350) into the cortex (−200). In the fourth set water could not enter the root hair (−10) from the soil (−200).
An air bubble breaks the column in that vessel, but water can move laterally through pits into adjacent vessels whose columns are still continuous, and carry on upwards. Sieve plates are found in phloem, not xylem. Companion cells belong to phloem and cannot pump air out of xylem. Lignin strengthens and waterproofs walls; it does not dissolve gases.
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.