Leaves make far more sucrose than they use, while roots, fruits and growing tips need it. Companion cells spend ATP to load sucrose into sieve tubes, water follows by osmosis, and the pressure it builds pushes the sap to wherever sugar is being used.
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.
Choose zoom to source and watch the yellow protons, the pink sucrose and the blue water. Then zoom to the sink, return to the whole tube and untick ATP supply. Watch the pressure at the source and the flow to the sink.
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.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the phloem tab, what does the companion cell pump out of the cell using ATP?
In the lab, how does sucrose enter the companion cell?
You untick ATP supply. What happens to the readings over the next few seconds?
In the lab, where does the water that enters the sieve tube at the source come from?
Link each step to water potential and hydrostatic pressure. This chain is the core of every mass flow answer.
| Place | What happens | Result |
|---|---|---|
| source | sucrose is loaded into the sieve tube, lowering its water potential | water enters from the xylem by osmosis; hydrostatic pressure is high |
| along the tube | sap moves through the open lumens and sieve plate pores | mass flow from high pressure to low pressure, carrying all the solutes together |
| sink | sucrose is unloaded and used in respiration or converted to starch, raising the water potential in the sieve tube | water leaves by osmosis; hydrostatic pressure is lower |
Whether a part of the plant is a source or a sink can change: a potato tuber is a sink while it fills with starch in summer and a source when it sprouts the following spring. Different sieve tubes can therefore carry sap in different directions at the same time, up to young leaves and down to roots.
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.
A favourite exam task is to compare the two transport systems. Learn the contrasts as pairs.
| Feature | Xylem | Phloem |
|---|---|---|
| what is carried | water and mineral ions | assimilates such as sucrose and amino acids, in water |
| direction | one way, from roots to leaves | from source to sink, so up or down |
| driving force | tension (pressure below atmospheric) from transpiration | high hydrostatic pressure at the source |
| energy from the plant | none: passive, powered by evaporation | ATP used at the source to load sucrose |
| cells | dead, empty, lignified vessel elements | living sieve tube elements with companion cells |
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Which of these is acting as a sink for sucrose?
Why is the hydrostatic pressure in a sieve tube high at the source?
A respiratory inhibitor is applied to the companion cells of a leaf. What is the most likely effect?
What provides the energy for a cotransporter protein to move sucrose into a companion cell?
This skill is now recorded on your profile. Keep the chain going.