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—elongated cells connected by sieve plates.
Sap is rich in sucrose (up to 30% by weight), along with amino acids, hormones, and minerals.
Unlike xylem, which uses dead cells, phloem transport requires living cells to maintain pressure gradients and load/unload sugars.
Sugar Sources and Sinks
Sugar source: a net producer of sugar (e.g., mature leaves, storage organs during dormancy).
Sugar sink: a net consumer or storer of sugar (e.g., growing roots, fruits, buds).
Sinks are supplied by their nearest sources.
Example: upper leaves may feed growing tips; lower leaves may feed roots.
Organs like tubers or bulbs can act as either sources or sinks depending on the season.
Sugar Loading and Unloading
Sucrose loading into phloem often requires active transport using:
Proton pumps and H⁺/sucrose cotransporters.
Occurs across apoplastic and symplastic pathways.
Companion cells assist in loading and maintaining the pressure gradient.
At the sink, sucrose is used or stored (e.g., converted to starch), keeping concentration low and promoting passive diffusion from phloem.
Pressure Flow Mechanism (Bulk Flow)
Transport occurs via positive pressure—a process called pressure flow:
Loading sugar at the source lowers water potential in the sieve tube → water enters by osmosis.
This increases turgor pressure, pushing sap toward the sink.
Unloading at the sink reduces pressure, maintaining flow.
Xylem recycles water from sink to source.
Flow rates can reach up to 1 meter/hour, far faster than diffusion or cytoplasmic streaming.
Each sieve tube connects specific sources and sinks, so neighboring tubes may carry sap in opposite directions.
Regulation and Adaptation
Sink strength (capacity to accept sugar) varies by organ and development stage.
Self-thinning: when sources can’t support all sinks, plants may abort fruits or seeds.
Horticultural practice: removing some flowers/fruits (e.g., apples) improves size and sweetness.
Experimental Support: Aphid Studies
Aphids use stylets to tap phloem sap.
Researchers observed that sap near sources had higher sugar concentrations than sap near sinks.
These results support 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.