7.2.67 · Transport in Plants

The Sucrose Loading Mechanism — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 10 parts Lesson 13 of 15 Unit quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

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Lesson summary

7.2.6 The Sucrose Loading Mechanism — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain how sucrose is actively loaded into sieve tubes by companion cells using the proton pump and co-transporter, and how unloading occurs at sinks.

Against the gradient
Preferred sugar
Symplast route
Apoplast route
Final step
Secondary active transport

Part 1 of 8Why loading is needed

1Source

Source sucrose is made in mesophyll cells of the leaf.

2Mass flow

Mass flow sucrose must be loaded into sieve tubes at a high concentration, which lowers sieve tube water potential so water enters.

3Against the gradient

Against the gradient phloem sap is far more concentrated than the mesophyll, so loading needs active transport.
- (about 20 per cent sucrose)

4Energy

ATP comes from the many mitochondria of the companion cells.

5Preferred sugar

Preferred sugar sucrose is transported rather than glucose because it is a less reactive non-reducing sugar and carries more energy.

Part 2 of 8Routes from mesophyll to phloem

1Symplast route

Symplast route sucrose moves through cytoplasm via plasmodesmata by diffusion, which is passive.

2Apoplast route

Apoplast route sucrose moves through the cell walls, and this route needs active steps.
It leaves mesophyll cells by facilitated diffusion into the cell wall space.
It then diffuses in the walls to the companion cell.

3Final step

Final step in many plants the apoplast route is used.

Part 3 of 8The loading mechanism

Proton pumps (H⁺-ATPase) in the companion cell membrane use ATP to pump H⁺ out into the cell wall space.

This gives a high H⁺ concentration outside and a low one inside, forming a proton gradient across the membrane.

H⁺ moves back into the companion cell down its gradient through a co-transporter protein that carries sucrose with it.

Sucrose accumulates in the companion cell above its concentration in the wall.

Sucrose passes through plasmodesmata into the sieve tube element.

The loading mechanism

Secondary active transport

Secondary active transport co-transport gets its energy from the proton gradient, not directly from ATP.

Transfer cells

Transfer cells they have folded walls and membranes that give a large surface area for many such proteins, along with many mitochondria.

Concentration achieved

Concentration achieved phloem sucrose can reach up to three times the mesophyll concentration.

Part 4 of 8Types of transport in loading

Type of transport	Energy source	Example in loading
Active transport	ATP directly	H⁺ pumped out of the companion cell
Co-transport	Proton gradient	H⁺ and sucrose enter together
Facilitated diffusion	Concentration gradient	Sucrose leaving mesophyll cells (apoplast route)

Part 5 of 8Consequence for water potential

1Sieve tube

Sieve tube as sucrose accumulates, its water potential falls.

2Water entry

Water entry water enters from the neighbouring xylem by osmosis.

3Pressure

Pressure this raises the hydrostatic pressure at the source end and starts mass flow towards the sink.

Part 6 of 8Unloading at the sink

Removal

Removal at a root or fruit, sucrose is removed from the sieve tube by diffusion or active transport,
- then moves out of the phloem by apoplast or symplast.

Conversion

Conversion sucrose is respired or converted to other substances, for example invertase hydrolyses it to glucose and fructose,
- or it becomes starch for storage.

Gradient

Gradient this keeps sink sucrose concentration low so unloading continues.

Water

Water removal raises the sieve tube water potential,
so water leaves by osmosis and hydrostatic pressure falls at the sink end, maintaining the pressure gradient.

Part 7 of 8Evidence that loading is active

1Metabolic poisons

Metabolic poisons they reduce loading.

2Mitochondria and ingrowths

Mitochondria and ingrowths companion cells are packed with mitochondria and have wall ingrowths, matching a heavy demand for ATP and membrane area.

3Concentration

Concentration sucrose is more concentrated in the phloem than in surrounding cells.

4pH experiments

pH experiments lowering the pH around leaf discs (more H⁺ outside) increases sucrose uptake,
- and a chemical making the membrane leaky to H⁺ reduces uptake as the gradient is lost.

5Proteins involved

Proteins involved the proton pump (H⁺ out), the co-transporter (H⁺ and sucrose in) and, in some plants, a facilitated diffusion channel letting sucrose into the wall space.

6Link

Link co-transport of sodium ions and glucose in the ileum uses the same principle.

Part 8 of 8More from the specification

The Transpiration Stream in Plants

Transpiration is the evaporation of water vapour from plant leaves through stomata, creating negative hydrostatic pressure that draws water continuously upward through xylem vessels from roots.

Apoplast and Symplast Transport Pathways

In root cortex tissue, water moves via the apoplast pathway through freely permeable cell walls and intercellular spaces,
or via the symplast pathway through the living cytoplasm and plasmodesmata.

Casparian Strip and Endodermal Control

The suberin Casparian strip in root endodermal cell walls blocks the apoplast pathway, forcing all water and dissolved mineral ions into the symplast for selective cellular transport.

Active Phloem Sucrose Loading

In source tissues, companion cells actively pump protons out into cell walls;
- proton-sucrose symport proteins then co-transport sucrose molecules against their concentration gradient into the phloem.

Apoplast pathway

Movement of water through cell walls and spaces without crossing a membrane.

Symplast pathway

Movement of water through the cytoplasm and plasmodesmata.

Transpiration stream

Continuous movement of water up the xylem from roots to leaves.

Transpiration

Loss of water vapour from leaves by evaporation and diffusion through stomata.

Movement

An action by an organism or part of it causing a change of position or place.

Stomata

Pores in the leaf through which water vapour leaves.

Tissue

A group of cells of similar structure that perform the same function.

Assimilates

Organic substances such as sucrose and amino acids transported in the phloem.

Mineral

A nutrient such as calcium or iron needed in small amounts.

Vessel

A long tube formed from dead xylem cells.

Solute

A substance dissolved in a solvent.

Key terms

Term	Meaning
Loading	Movement of sucrose into the sieve tubes at a source.
Proton pump	A membrane protein that uses ATP to move H⁺ across a membrane.
Co-transport	Movement of one substance coupled to the movement of another through the same protein.
Active transport	Movement of molecules against a concentration gradient using energy.
Unloading	Removal of sucrose from the sieve tube at a sink.
Hydrostatic pressure	Pressure exerted by a liquid, here the sap in the sieve tube.
Facilitated diffusion	Passive movement through a channel or carrier protein.

Avoid these mistakes

Saying sucrose is actively transported directly by the proton pump — the pump moves H⁺ and sucrose is moved by the co-transporter.

Saying co-transport uses ATP directly.

Saying water is loaded by active transport — water follows by osmosis.

Forgetting that H⁺ moves back into the cell down its gradient.

Saying sucrose moves into the sieve tube by diffusion at the source.

Last updated: 15 October 2026