7.2.7—Companion cells transfer assimilates

Companion cells load assimilates such as sucrose into phloem sieve tubes. The route may be symplastic and passive, or—when sucrose takes the apoplastic route—may use ATP-dependent proton pumping and H⁺–sucrose co-transport.

1. Create the gradient: In a modified companion cell (transfer cell), a proton pump uses ATP to move H⁺ from the cytoplasm into the cell wall. This creates a high H⁺ concentration outside the cell.
2. Use the gradient: H⁺ moves back down its concentration gradient through a co-transporter protein. The same transport step carries sucrose into the companion cell against sucrose’s concentration gradient.
3. Enter the sieve tube: Sucrose moves from the companion cell into the associated sieve-tube element through plasmodesmata. Companion-cell membrane infoldings increase transport surface area, while many mitochondria supply ATP for the proton pump.
4. Recognise the alternative: If sucrose uses the symplastic route, it can move through cytoplasm and plasmodesmata by diffusion; the exact route is not presented as universal.
5. Pressure boundary: Loading raises the solute concentration in phloem and can lower its water potential, setting up water-entry and turgor consequences that the next mass-flow card connects to bulk movement. This card stops at loading/unloading rather than explaining the whole pressure-flow model.

Chain cue: ATP → proton pump → H⁺ gradient → H⁺–sucrose co-transport → companion cell → plasmodesmata → sieve tube.

Do not invent a named transporter or say ATP pushes every sucrose molecule along the entire plant. ATP powers the local proton pump; co-transport loads sucrose, while later mass flow is the long-distance process. Unloading occurs at sinks and may use apoplastic or symplastic routes, but its full pressure-flow consequence belongs to 4595. No image is generated for this card.