The shape of the lesson
By the end of the lesson, learners can
- Loading at the source: Choose zoom to source and watch the yellow protons, the pink sucrose and the blue water.
- Pressure drives the flow: Link each step to water potential and hydrostatic pressure.
- Xylem and phloem compared: A favourite exam task is to compare the two transport systems.
- Source makes or releases sucrose; sink uses or stores it.
- Proton pump uses ATP; cotransporter uses the H+ gradient.
- Sucrose in, water potential down, water in, pressure up.
- Mass flow from high to low hydrostatic pressure, source to sink.
One picture every idea returns to
Assimilates, mainly sucrose and amino acids, are carried dissolved in water in phloem sieve tubes from sources, where they are made or released (photosynthesising leaves, storage organs being emptied), to sinks, where they are used or stored (roots, growing tips, fruits, storage organs being filled). At the source, companion cells use ATP to drive proton pumps that move hydrogen ions out of the cell. The ions diffuse back in through cotransporter proteins, carrying sucrose with them against its concentration gradient, and the sucrose passes into the sieve tube through plasmodesmata. This lowers the water potential in the sieve tube, so water enters from the xylem by osmosis and the hydrostatic pressure rises. At the sink, sucrose is removed, water leaves and the pressure is lower, so the sap moves by mass flow down the pressure gradient from source to sink.
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, Sugar is pumped in, pressure does the rest.. Leaves make far more sucrose than they use, while roots, fruits and growing tips need it. 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: Loading at the source | Choose zoom to source and watch the yellow protons, the pink sucrose and the blue water. Project tab 7 of the lab (Transport in plants lab, phloem loading and mass flow) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Pressure drives the flow | Link each step to water potential and hydrostatic pressure. | Pairs |
| 21 to 30 min | Teach and check: Xylem and phloem compared | A favourite exam task is to compare the two transport systems. | Anticipation / Reaction Guide |
| 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, 22 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 Xylem and phloem compared, 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.
Anticipation / Reaction Guide
Before the lesson, students mark five statements true or false. After the teaching they mark them again and justify any change of mind in one sentence each. The statements come from the examiner traps below.
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 cotransporter does not use ATP directly. | ATP is used by the proton pump; the cotransporter uses the energy of H + ions diffusing back down their gradient. Saying "sucrose is actively pumped by the cotransporter using ATP" is a common error. |
| Mass flow is a passive movement down a pressure gradient. | The energy is spent at the source, on loading. Do not write that sieve tubes "pump" sap or that sucrose diffuses all the way to the sink. |
Translocation: loading the phloem and mass flow: worksheet
6 questions, 22 marks. Written in the style of the papers; no past paper question is reproduced.
Name two assimilates transported in the phloem and explain what is meant by a source and a sink.
Explain how companion cells load sucrose into phloem sieve tubes.
Explain how sucrose moves from a source to a sink by mass flow.
Compare the transport of water in the xylem with the transport of sucrose in the phloem.
A ring of bark, including the phloem but not the xylem, was removed from the stem of a tree. After several weeks the bark just above the ring was swollen and the roots had stopped growing. The leaves above the ring did not wilt. Explain these observations.
The sucrose concentration of sieve tube sap was 0.80 mol dm-3 near a leaf and 0.30 mol dm-3 near a root. Calculate the difference as a percentage of the concentration near the leaf, and explain how this difference relates to mass flow.
Answer key and marking notes
- Sucrose and amino acids (dissolved in water).
- Source: a part of the plant where assimilates are made or released into the phloem, for example a photosynthesising leaf.
- Sink: a part of the plant where assimilates are used or stored, for example a root, growing tip or developing fruit.
- Companion cells respire aerobically in their many mitochondria to make ATP.
- Proton pumps in the companion cell membrane use ATP to pump hydrogen ions out of the cell.
- This creates a high concentration of hydrogen ions outside the cell (a proton gradient).
- Hydrogen ions diffuse back in through cotransporter proteins, carrying sucrose with them against its concentration gradient.
- Sucrose then moves from the companion cell into the sieve tube element through plasmodesmata.
- Sucrose loaded at the source lowers the water potential of the sieve tube sap.
- Water enters from the xylem by osmosis, so the hydrostatic pressure at the source is high.
- At the sink sucrose is removed (used in respiration or stored as starch), water leaves by osmosis and the pressure is lower.
- Sap moves down the hydrostatic pressure gradient from source to sink, carrying the dissolved sucrose with it.
- Both involve movement of a solution by mass flow down a pressure gradient.
- Xylem transport is one way, from roots to leaves; phloem transport is from source to sink, up or down the plant.
- Xylem contents are under tension (pressure below atmospheric); phloem sap is under a high positive hydrostatic pressure.
- Xylem transport needs no energy from the plant's cells; phloem transport needs ATP for loading at the source (xylem cells are dead, sieve tube elements are living).
- Removing the phloem stopped translocation of sucrose from the leaves (source) down past the ring.
- Sucrose built up in the tissues just above the ring, so they swelled; the roots (sink) received no sucrose for respiration and growth.
- The xylem was intact, so water still reached the leaves and they did not wilt.
- Difference = 0.80 - 0.30 = 0.50 mol dm-3; 0.50 ÷ 0.80 × 100 = 62.5%.
- The high sucrose concentration at the leaf (source) gives a low water potential, so water enters and the hydrostatic pressure is high.
- At the root (sink) sucrose is removed, so the pressure is lower; the sap flows down this pressure gradient from leaf to root.
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.
In which cells does the mass flow of sap carrying sucrose take place?
- Aphloem sieve tube elements
- Bphloem companion cells
- Cxylem vessel elements
- Dmesophyll cells of the leaf
Which carbohydrate is the main form in which assimilates are translocated in the phloem?
- Asucrose
- Bglucose
- Cstarch
- Dcellulose
In spring a potato tuber sprouts new shoots, using its stored starch. At this time, how is the tuber acting?
- Aas a source, because its starch is converted to sucrose and loaded into the phloem
- Bas a sink, because it is a storage organ
- Cas a sink, because it is underground and receives no light
- Das neither, because tubers are not connected to the phloem
Which sequence shows the events in loading sucrose at a source in the correct order?
- AATP used to pump H+ out; H+ diffuses back in with sucrose via a cotransporter; sucrose enters the sieve tube; water follows by osmosis
- Bsucrose enters the sieve tube; water follows by osmosis; ATP used to pump H+ out; H+ diffuses back in
- Cwater enters the sieve tube by osmosis; sucrose is pulled in with the water; H+ pumped out; ATP made
- DH+ diffuses into the companion cell; ATP used to pump sucrose out; water leaves the sieve tube
An aphid feeding on a stem is cut off, leaving its mouthpart (stylet) in a sieve tube. Sap continues to flow out of the cut end of the stylet for hours. What does this show?
- Athe contents of sieve tubes are under a high hydrostatic pressure
- Bthe contents of sieve tubes are under tension, like xylem
- Cthe aphid's stylet actively pumps sap out of the phloem
- Dsucrose moves through the phloem by simple diffusion only
Companion cells were placed in a solution and left until the solution outside them became acidic. When sucrose was then added to the solution, the pH outside the cells rose and sucrose appeared inside the cells. Which explanation fits these results?
- AH+ ions re-entered the cells through cotransporters, carrying sucrose with them
- Bthe cells stopped respiring when sucrose was added, so ATP ran out
- Csucrose reacted with H+ ions outside the cells and neutralised them
- Dsucrose entered by osmosis and diluted the H+ ions outside
Mature leaf Q of the plant shown was supplied with radioactive carbon dioxide (14CO2) for one hour. Where would radioactive sucrose be found two hours later?
- Ain P, R and T, carried both up and down the stem in the phloem
- Bonly in P, because translocation in the phloem is upwards only
- Conly in T, because translocation in the phloem is downwards only
- Din S as much as in P, R and T, because sucrose spreads evenly to all leaves
Radioactive sucrose was detected 0.90 m down the stem from a labelled leaf 1.5 hours after labelling. What is the mean rate of translocation in mm min-1?
- A10 mm min-1
- B0.6 mm min-1
- C600 mm min-1
- D6 mm min-1
Quiz key
Sap moves by mass flow along sieve tubes, made of sieve tube elements joined by sieve plates. Companion cells load sucrose into them but are not the channel for the flow. Xylem vessels carry water and mineral ions upwards under tension. Mesophyll cells make sucrose but do not transport it over long distances.
Sucrose is the main transported sugar: it is soluble, so it travels dissolved in water, and is not used up by the cells it passes through as readily as glucose. Glucose is made in photosynthesis but is converted to sucrose for transport. Starch is insoluble and used for storage. Cellulose is a structural polysaccharide in cell walls.
An organ is a source when it releases assimilates into the phloem. The sprouting tuber breaks its starch down to sucrose and exports it to the growing shoots, so it is a source. It was a sink the previous summer while it was filling with starch; being a storage organ or being underground does not fix its role. Tubers are connected to the rest of the plant by phloem.
The proton pump uses ATP first, creating an H+ gradient; H+ diffusing back through the cotransporter carries sucrose into the companion cell; sucrose then passes through plasmodesmata into the sieve tube, lowering its water potential so water enters by osmosis. The second sequence puts loading before the energy step. Water does not pull sucrose in: water follows the sucrose. The last sequence reverses the direction of transport at the source.
Sap flows out on its own because the sieve tube sap is under positive hydrostatic pressure, as the mass flow hypothesis predicts. Under tension, air would be drawn in instead, as happens when xylem is cut. The aphid has been removed, so nothing is pumping. Diffusion is far too slow to push sap out of a stylet for hours.
Proton pumps had moved H+ out, making the outside acidic. When sucrose was available, H+ diffused back in through cotransporter proteins together with sucrose, so H+ outside fell (pH rose) as sucrose was taken up. There is no reason for respiration to stop, and a lack of ATP would not move sucrose in. Sucrose does not neutralise acid. Osmosis is the movement of water, not of solutes such as sucrose.
Leaf Q makes radioactive sucrose by photosynthesis and exports it to sinks: the young leaf P and the fruit R above and near it, and the roots T below. Phloem carries sap in both directions, in different sieve tubes, so it is found above and below Q. Translocation is not one way like transport in the xylem. An older mature leaf such as S is itself a source, so it receives little or none.
0.90 m = 900 mm and 1.5 h = 90 min, so 900 ÷ 90 = 10 mm min-1. 0.6 is the rate in metres per hour (0.90 ÷ 1.5) given with the wrong units. 600 mm min-1 converts metres to millimetres but leaves the time in hours. 6 mm min-1 treats 1.5 hours as 150 minutes instead of 90.
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.