Rucete ✏ Campbell Biology In a Nutshell
Unit 6 PLANT FORM AND FUNCTION — Concept 36.5 Sugars Are Translocated from Sources to Sinks via the Phloem
Plants move sugars from where they are made (sources) to where they are needed or stored (sinks) using the phloem. This long-distance transport system relies on pressure-driven bulk flow, allowing sugar-rich sap to move efficiently through the plant.
Phloem Structure and Sap Composition
- Phloem sap flows through sieve-tube elements connected by sieve plates.
- Sap is rich in sucrose, along with amino acids, hormones, and minerals.
- Phloem uses living cells, unlike xylem, which relies on dead cells.
Sugar Sources and Sinks
- Sugar source: net producers like mature leaves or storage organs in dormancy.
- Sugar sink: net consumers like roots, fruits, and buds.
- Sinks receive sugars from the nearest source.
- Example: upper leaves feed shoot tips; lower leaves feed roots.
- Tubers and bulbs can act as sources or sinks seasonally.
Sugar Loading and Unloading
- Sucrose loading often requires active transport using:
- Proton pumps and H⁺/sucrose cotransporters
- Moves across apoplastic and symplastic pathways
- Companion cells assist in sugar loading and maintaining pressure gradients.
- At the sink, sucrose is used or stored, lowering its concentration and promoting diffusion.
Pressure Flow Mechanism (Bulk Flow)
- Transport is driven by positive pressure:
- Sugar loading lowers water potential → water enters by osmosis.
- Increased turgor pressure pushes sap toward the sink.
- Unloading at sink reduces pressure, maintaining flow.
- Xylem recycles water from sink to source.
- Flow rates can reach up to 1 meter/hour.
- Sieve tubes connect specific sources and sinks; adjacent tubes may flow in opposite directions.
Regulation and Adaptation
- Sink strength varies with organ type and development stage.
- Self-thinning: plants may abort some fruits or seeds if resources are limited.
- Horticultural practices (e.g., fruit thinning) can enhance size and sweetness.
Experimental Support: Aphid Studies
- Aphids use stylets to feed directly from phloem sap.
- Higher sugar concentrations are found near sources than sinks.
- This evidence supports the pressure-flow hypothesis.
In a Nutshell
Sugars travel from source to sink through living phloem cells via pressure-driven bulk flow. Loading and unloading processes maintain a flow gradient, with companion cells and transport proteins enabling movement. This dynamic system allows plants to allocate energy efficiently across tissues for growth, storage, and reproduction.