7.2.6—Assimilate transport in phloem

Phloem translocation is the transport of assimilates from a source, where they are available, to a sink, where they are stored or used. Phloem sap is a solution containing mainly sucrose as well as water and other dissolved substances such as amino acids, hormones and minerals.

• Source: A green leaf or green stem can produce sugars by photosynthesis and export them as sucrose. A storage organ can also act as a source when stored material is mobilised for new growth.
• Sink: A developing seed, fruit, root or other storage/growing region receives assimilates for use or storage. The same organ can change from source to sink as development or demand changes.
• Pathway: Sieve-tube elements carry the phloem sap. Companion cells support the associated sieve-tube elements and the loading/unloading interface; the detailed proton-gradient loading mechanism belongs to the next card.
• Direction: Assimilates can move upwards or downwards in phloem because direction follows the current source-to-sink relationship. A plant therefore cannot be classified as having one fixed “up” or “down” phloem direction.
• Energy boundary: Loading at the source and unloading at the sink require metabolic energy. That local active work is distinct from the subsequent bulk movement through the sieve tube.

Reasoning cue: identify the current source and sink → identify the assimilate in phloem sap → trace the sieve-tube pathway → state why direction and loading demand are context-dependent.

Phloem is not simply a downward sugar tube and is not the same transport system as xylem. Source/sink status is functional and can change; pressure-driven mass flow is explained in the later card. Do not use ATP as if it directly pushes every assimilate along the whole plant. No image is needed for this card.