--- title: "Translocation and the Mass Flow Hypothesis" description: "Sugars are loaded into the phloem at a source, water follows by osmosis, and the pressure built there pushes sap towards a sink. The hypothesis is judged against ringing experiments and tracer evidenc" canonical: https://lightmysky.com/learn/science/translocation-and-the-mass-flow-hypothesis-mt_WZ8id0kou4 source: https://lightmysky.com/learn/science/translocation-and-the-mass-flow-hypothesis-mt_WZ8id0kou4.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Translocation and the Mass Flow Hypothesis Sugars are loaded into the phloem at a source, water follows by osmosis, and the pressure built there pushes sap towards a sink. The hypothesis is judged against ringing experiments and tracer evidence. Subject: Science · Area: Organisms & Life Processes · Ages 17 to 18 Page: https://lightmysky.com/learn/science/translocation-and-the-mass-flow-hypothesis-mt_WZ8id0kou4 ## Ready when they can - Separates the loading step, the pressure step and the unloading step. - Explains why phloem transport needs energy while xylem transport does not. - Weighs one piece of evidence for the hypothesis against one difficulty with it. ## Lesson: How sugary sap travels by pressure Translocation moves sugary sap from sources to sinks through the phloem. Sources are usually leaves making sugar, while sinks are roots, fruits or buds that consume or store it. Keep three steps separate in your head: loading at the source, pressure driven flow through the tubes, and unloading at the sink. The pressure gradient is built with water. Sucrose is actively loaded into the sieve tubes at the source, water follows by osmosis, and pressure rises there. At the sink, sucrose is unloaded for use or storage, water leaves too, and pressure falls. Sap therefore flows downhill in pressure from source to sink, and different tubes can run different ways as sources and sinks shift with the season. That loading step is why phloem transport needs energy while xylem transport does not. Pushing sucrose in against its gradient burns ATP, supplied by companion cells packed with mitochondria. The middle flow step is passive, and xylem water simply rides the transpiration pull with no loading cost at all. **Example.** Judge the mass flow hypothesis like an examiner: one strength against one weakness. Aphids tapped into phloem ooze sap under pressure, ringing a stem stops flow below the ring, and tracers fed to a leaf turn up in the roots. But sieve plates between tube elements look like they should choke the flow, and different substances can travel opposite ways at once. Weigh one piece of evidence against one difficulty and you answer evaluation questions the way examiners want. **Recap.** Loading builds pressure at the source, sap flows to the sink, and evidence and difficulties are weighed together. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Cohesion-Tension and Water Movement in the Xylem](https://lightmysky.com/learn/science/cohesion-tension-and-water-movement-in-the-xylem-mt_9_TORolYIl) - [Co-transport and Coupled Movement Across Membranes](https://lightmysky.com/learn/science/co-transport-and-coupled-movement-across-membranes-mt_pzQ8n3FWNV) - [Osmosis in Plant and Animal Cells](https://lightmysky.com/learn/science/osmosis-in-plant-and-animal-cells-mt_XciCLO69xG) ## Opens up - [Plant Growth Responses: Tropisms and the Hormones Behind Them](https://lightmysky.com/learn/science/plant-growth-responses-tropisms-and-the-hormones-behind-them-mt_q1N7OG2_6j)