Grade by Grade
F. Plants: Anatomy, Growth, and Function · Plants: Anatomy, Growth, and Function

Moving Sugar: Translocation and the Pressure-Flow Model

F3.1 F2.1 F1.1 A1.10 A1.13About 65 minutes
Learning goals
  • I can define translocation and identify sources and sinks in a plant at different times of year.
  • I can explain the pressure-flow model of phloem transport, step by step.
  • I can describe evidence for phloem transport, including aphid and girdling experiments.
  • I can compare transport in xylem and phloem.
  • I can apply what I know to maple syrup production.

Words to know

translocation
The movement of sugars and other organic molecules through the phloem, from sources to sinks.
source
A part of a plant that releases sugar into the phloem, such as a mature leaf, or a storage organ in spring.
sink
A part of a plant that uses or stores sugar, such as roots, growing shoots, flowers, fruits and seeds.
phloem loading / unloading
Moving sucrose into sieve tubes at a source (often by active transport) / out of sieve tubes at a sink.
pressure-flow model
The explanation that loading sugar at a source draws in water by osmosis, building pressure that pushes phloem sap toward sinks, where sugar is removed.
bulk flow
The movement of a whole fluid together, driven by a difference in pressure, like water in a hose.
turgor pressure
The pressure of water inside a plant cell or sieve tube pushing against its walls.
honeydew
The sugary liquid that aphids excrete after feeding on phloem sap.

Sugar on the move

A potato plant makes sugar in its leaves, in sunshine, above ground. Yet the starch ends up in the potatoes, underground, in the dark. Apples get sweet on the branch. A sugar maple builds new leaves in spring before it has any leaves to make sugar with. In each case, sugar travels long distances inside the plant, in the phloem. This movement is called translocation.

Remember from Grade 10 and Lesson 3

In Grade 10 you learned that a source releases sugar and a sink uses or stores it, so phloem can carry sugar up or down. In Lesson 3 you saw that phloem is made of living sieve tube elements joined by sieve plates, each with a companion cell. Now you'll see what actually pushes the sap along.

What's in phloem sap?

Phloem sap is mostly water with a lot of dissolved sucrose, often 10 to 30% by mass, much sweeter than a soft drink. It also carries amino acids, some minerals and hormones (signals between plant parts).

Why sucrose rather than glucose? Sucrose is less chemically reactive, so it can travel long distances without being used up or reacting along the way, and it carries more energy per molecule.

Sources and sinks change with the season

  • Summer: mature leaves are sources. Sinks include growing shoot tips, young leaves, roots, flowers, developing fruits and seeds, and storage organs such as potatoes and carrot roots.
  • Spring: before new leaves open, storage organs (roots, tubers, the trunk) become sources, sending stored sugar to the growing buds, which are sinks.
  • A young leaf starts as a sink and becomes a source once it is big enough to make more sugar than it uses.

One sieve tube carries sap in only one direction at a time, but different sieve tubes in the same stem can carry sap in opposite directions.

source leaf→ growing budsource leaf→ fruitsource leaf→ storage rootsink: budsink: fruitsink: storage root
In summer, mature leaves are sources; growing buds, fruits and storage roots are sinks. Sugar flows through the phloem from each source to the nearby sinks, both upward and downward.

The pressure-flow model

The best-supported explanation for translocation is the pressure-flow (mass-flow) model, first proposed by the German botanist Ernst Münch around 1930.

  1. Loading at the source. Companion cells use ATP to actively transport sucrose into the sieve tubes, so the sucrose concentration there becomes very high.
  2. Water follows. The high solute concentration draws water in from the nearby xylem by osmosis. This raises the turgor pressure inside the sieve tube at the source end.
  3. Bulk flow. The sap flows along the sieve tubes from the high-pressure source end toward the low-pressure sink end, like water in a garden hose. Sieve plates let it pass from cell to cell.
  4. Unloading at the sink. Sink cells take sucrose out of the sieve tubes and use it for respiration or growth, or convert it to starch for storage. This keeps the sucrose concentration (and pressure) low at the sink end.
  5. Water returns. With less solute, water leaves the sieve tube by osmosis and goes back into the xylem.

So translocation is powered by active loading at the source. The flow itself is passive, driven by the pressure difference.

xylemphloem (sieve tube)low pressureSOURCE (mature leaf)sugar made by photosynthesisSINK (root, fruit, bud)sugar used or stored① sucroseloaded (ATP)② water in③ bulkflow④ sucroseunloaded⑤ water outwater risesin xylemhigh pressure
The pressure-flow model. ① Sucrose is actively loaded at the source; ② water follows by osmosis, raising pressure; ③ sap flows to the sink; ④ sucrose is unloaded; ⑤ water leaves and returns to the xylem.

Evidence for the model

  • Aphid experiments. Aphids are small insects that push a needle-like mouthpart (a stylet) into a single sieve tube to feed. If a feeding aphid is removed and its stylet is left in place, sap keeps oozing out of the stylet for hours. This shows the sap is under pressure, and lets scientists collect and analyse pure phloem sap.
  • Girdling (ringing) experiments. If a ring of bark (including the phloem) is removed from a branch, the bark just above the ring swells and sugars build up there, because sugar coming down from the leaves can't get past.
  • Radioactive tracers. If a leaf is given carbon dioxide containing radioactive carbon-14, the radioactive sugar it makes soon shows up in nearby sinks (fruits, roots, growing tips), and the pattern matches the source-to-sink idea.

Comparing transport in xylem and phloem

FeatureXylemPhloem
CarriesWater and mineralsSugars (mainly sucrose), amino acids, hormones
DirectionUp only (roots → leaves)Up or down (source → sink)
Conducting cellsDead: tracheids, vessel elementsLiving: sieve tube elements with companion cells
Driving forceTension from transpiration (pull)Pressure from active loading (push)
Energy used by the plantNone directly (sun-driven evaporation)ATP for loading sucrose
Pressure in the tubesUsually negative (tension)Positive (high pressure)
Xylem pulls; phloem pushes.

Maple syrup: transport you can taste

In late winter and early spring, sugar maples convert starch stored in the living cells of the wood and roots into sucrose. Unusually, much of this sugar travels in the xylem sap. When nights are below freezing and days are above freezing, the freeze-thaw cycle builds pressure in the trunk, and sap drips out of a taphole. The sap is only about 2 to 3% sugar, so it must be boiled down a lot: syrup must be at least 66% sugar.

Maple sugaring is a living tradition of many First Nations, including Anishinaabe and Haudenosaunee communities, who were making maple sugar long before European settlers arrived and continue today. Producers use a handy rule called the Rule of 86:

litres of sap needed for 1 L of syrup ≈ 86 ÷ (sugar % of the sap)

Worked example

Let's use the Rule of 86

A maple producer measures sap at 2.0% sugar early in the season and 2.5% later. (a) How many litres of sap are needed for 1 L of syrup each time? (b) How many litres of sap are needed for 50 L of syrup at 2.0%?

  1. (a) At 2.0%: 86 ÷ 2.0 = 43 L of sap per litre of syrup.
  2. At 2.5%: 86 ÷ 2.5 = 34.4 L ≈ 34 L of sap per litre of syrup.
  3. Sweeter sap means less water to boil away, which saves fuel and time.
  4. (b) 50 L × 43 L of sap per L of syrup = 2150 L of sap (about 2.2 × 10³ L).

(a) About 43 L at 2.0% and about 34 L at 2.5%. (b) About 2150 L of sap for 50 L of syrup.

Worked example

Let's find the sources and sinks

A potato plant in (a) July and (b) a seed potato planted in May, before any leaves have come up. For each, name the main source and sink and the direction of phloem flow in the underground stem.

  1. (a) July: source = mature leaves; sinks = developing tubers (and roots, shoot tips). Flow in the underground stem is downward, toward the tubers, where sucrose is unloaded and turned into starch.
  2. (b) May: source = the seed potato (its stored starch is broken down into sucrose and loaded into the phloem); sink = the growing shoot and new roots. Flow is upward toward the shoot.
  3. The same tissue carries sap in opposite directions at different times because sources and sinks change.

(a) Leaves → tubers (downward). (b) Seed potato → shoot (upward).

Just for fun

Phloem sap is under so much pressure that an aphid barely has to suck: the sap flows in by itself, often faster than the aphid can use it. The extra sugar comes out the other end as sticky honeydew, which some ants collect so eagerly that they guard aphids like tiny livestock.

Hands-on activity

Model osmosis-driven pressure with a potato

You need: a large potato, a teaspoon, sugar, water, a shallow dish, a pin or toothpick for a marker, and a knife used by a grown-up.

  1. A grown-up cuts a flat base on the potato and peels the sides near the bottom, then hollows out a cup in the top about 2 cm deep (keep the walls about 1 cm thick).
  2. Stand the potato in a dish with about 2 cm of water.
  3. Put 2 teaspoons of sugar in the hollow with a few drops of water. Mark the starting liquid level with a pin.
  4. Check after 1 hour, 4 hours and the next morning.
  5. Control: set up a second potato the same way, but with no sugar in the hollow.

What to look for: in the sugar potato, the liquid level in the hollow rises, sometimes overflowing. Water moved by osmosis from the dish, through the potato's living cells, toward the high sugar concentration, building up volume and pressure. The control shows little change. This is like step 2 of the pressure-flow model: loading sugar draws in water. Compost the potatoes afterwards.

Bonus: look on garden plants or houseplants for aphids (tiny, soft, often green insects on new shoots). Shiny, sticky leaves below them are coated in honeydew.

Your turn

1

Define translocation.

2

Give two examples of sources and two examples of sinks in a summer apple tree.

3

Why does water enter the sieve tube at the source?

4

What keeps the pressure low at the sink?

5

Where does the plant spend energy in phloem transport?

6

How do aphids help scientists study phloem?

7

How many litres of sap at 3.0% sugar are needed for 1 L of maple syrup?

More practice: warm-up

1

Which sugar is mainly transported in phloem?

2

Is phloem sap under pressure or under tension?

3

Which way can phloem sap move?

4

What do sink cells often turn sucrose into for storage?

5

Who first proposed the pressure-flow model?

More practice: on your own

1

Explain why a branch swells just above a ring where the bark was removed.

2

Explain why a young, growing leaf is a sink but a mature leaf is a source.

3

Compare the driving forces of xylem and phloem transport.

4

A chemical stops companion cells from making ATP. Predict the effect on translocation.

5

Why might maple sap become less sweet and the sap season end when warm nights arrive?

Challenge

1

Scientists gave one leaf on a bean plant ¹⁴CO₂. After a few hours, radioactivity was found in the roots and the shoot tip above, but very little in the other mature leaves. Explain this pattern.

2

A producer collects 1200 L of sap at 2.4% sugar. How much syrup can she make? Show your work and state the answer with appropriate precision.

3

Use the pressure-flow model to explain why removing developing fruits from a tomato plant makes the remaining fruits grow larger.

Check your understanding

Quiz

1.

In the pressure-flow model, what directly causes water to enter the sieve tubes at the source?

2.

Which part of a plant is a sink in summer?

3.

Phloem transport differs from xylem transport because phloem:

4.

The sap oozing from a cut aphid stylet is evidence that:

5.

Using the Rule of 86, sap that is 2.0% sugar needs about how many litres per litre of syrup?

6.

In early spring before leaves open, the main source in a sugar maple is:

For parents and teachers

Big idea: F3.1 includes translocation. The key idea is the pressure-flow model: active loading of sugar at a source draws in water by osmosis, creating a pressure difference that pushes sap to sinks. Compare it with the xylem's pull from Lesson 6.

How to help: The potato osmometer is easy and dramatic; start it in the evening. In March or early April, a visit to a sugar bush makes this lesson memorable, and many offer tours. Ask your teen to explain where the energy for each transport process comes from (sun-driven evaporation for xylem; ATP for phloem loading).

Common difficulties:

  • Thinking phloem only flows downward.
  • Mixing up which end has high pressure (the source).
  • Forgetting that water in the phloem comes from (and returns to) the xylem.
  • The maple-sap twist: spring maple sap is collected from the xylem, which is unusual; it carries sugar because the tree loads stored sugar into it.