--- title: "Pressure Flow Hypothesis | General Biology I" description: "Pressure flow hypothesis explains how sugars move through phloem in General Biology I, using source loading, sink unloading, and pressure differences." canonical: "https://fiveable.me/college-bio/key-terms/pressure-flow-hypothesis" type: "key-term" subject: "General Biology I" unit: "Unit 30" --- # Pressure Flow Hypothesis | General Biology I ## Definition The pressure flow hypothesis is the plant model for moving sugars through the phloem from source to sink. It works by creating pressure differences through sugar loading and unloading, which pushes sap through the plant. ## What It Is The pressure flow hypothesis is the main explanation in General Biology I for how plants move sugars through the phloem. Instead of relying on a pump like the heart, the plant uses differences in pressure between a sugar source and a sugar sink to push sap through sieve elements. A source is any part of the plant that makes or releases sugar, usually mature leaves doing photosynthesis. In those cells, sucrose is loaded into the phloem, often with help from companion cells and active transport. When solute concentration rises in the phloem, water moves in from the nearby xylem by osmosis, which raises turgor pressure in that region. At the sink, sugar is removed from the phloem. Sinks include roots, growing shoots, fruits, seeds, and storage tissues. When sucrose leaves the phloem, solute concentration drops, water can move back out, and pressure falls. That pressure difference between source and sink is what drives bulk flow through the phloem. The important idea is that the sugar solution moves as a mass flow, not molecule by molecule. Once the pressure gradient is set up, sap moves from high pressure to low pressure through sieve elements. This makes transport efficient over long distances, especially in larger plants. This model also connects tightly to water movement in plants. The phloem does not work in isolation, because water constantly shifts between phloem and xylem to maintain the pressure difference. So if you are tracing plant transport, the pressure flow hypothesis sits right at the point where sugar transport and water potential meet. ## Why It Matters This term shows up any time General Biology I asks how plants distribute the products of photosynthesis. Leaves do not just make glucose for themselves, they export carbohydrates to the rest of the plant, and pressure flow explains how that export happens efficiently over distance. It also gives you a clean way to connect several plant transport ideas. If you know that sugar loading lowers water potential in the phloem, you can predict why water enters from xylem and why pressure rises at the source. If you know that unloading at the sink lowers solute concentration, you can predict why water leaves and pressure drops there. That cause-and-effect chain shows up in diagrams, short-answer questions, and lab-style reasoning about plant physiology. You may be asked to label source and sink tissues, explain why phloem transport is directional between those points, or compare phloem transport with xylem water movement. Pressure flow is the bridge concept that makes those answers hang together. It also helps you avoid a common mistake: thinking phloem transport is just passive diffusion. Diffusion alone would be too slow for long-distance movement. The plant spends energy on loading and unloading sugars so it can create the pressure gradient that drives bulk flow. ## Connections ### Phloem Pressure flow happens in the phloem, the plant tissue that carries sugars and other organic molecules. The sieve elements form the main transport tubes, while companion cells help with loading and maintaining those sieve tubes. If you are identifying plant vascular tissue, phloem is the structure that carries the pressure-driven flow. ### Source The source is where sucrose enters the phloem, usually a mature leaf making sugar by photosynthesis. Source cells load sugar into the transport system, which raises solute concentration and pulls in water. Without a source, there is no high-pressure starting point for the flow. ### Sink A sink is any tissue that uses or stores sugar, such as roots, fruits, seeds, or growing buds. At the sink, sugar is unloaded from the phloem, which lowers solute concentration and drops pressure. That unloading step is what keeps the movement going from source to sink. ### [Pressure Potential](/college-bio/key-terms/pressure-potential) Pressure potential is the physical pressure that builds up in the phloem sap during pressure flow. When water enters the phloem after sugar loading, turgor pressure rises and helps push sap along the tube. This is the pressure side of the mechanism, not just a background detail. ## On the AP Exam A quiz question may ask you to trace the path of sucrose from a leaf to a root or fruit and explain why the direction of flow makes sense. In a diagram, you might need to identify the source, sink, xylem, and phloem, then show where water enters and leaves. In a short-answer or lab question, the safest move is to describe the sequence: sugar loading at the source, water entry by osmosis, pressure buildup, bulk flow through phloem, and sugar unloading at the sink. If you see a data table or passage about plant transport, look for clues like high solute concentration, turgor pressure, or active transport. Those details usually point to pressure flow rather than simple diffusion. You may also be asked to compare phloem transport with xylem transport, so be ready to say that xylem moves water upward mainly through transpiration pull, while phloem moves sugars from source to sink through pressure-driven bulk flow. ## Key Takeaways - The pressure flow hypothesis explains how sugars move through phloem from a source to a sink. - Sugar loading at the source raises solute concentration, so water enters by osmosis and pressure builds up. - Sugar unloading at the sink lowers solute concentration, so water leaves and pressure drops. - The movement is bulk flow through sieve elements, not simple diffusion. - Phloem transport depends on water movement between phloem and xylem to maintain the pressure difference. ## FAQs ### What is pressure flow hypothesis in General Biology I? It is the model that explains how plants move sugars through the phloem from source tissues to sink tissues. Sugar loading creates high pressure at the source, sugar unloading creates lower pressure at the sink, and that difference drives flow. ### How does the pressure flow hypothesis work? Sugars are actively loaded into the phloem at the source, which lowers water potential and draws in water from the xylem. The resulting pressure pushes sap through the phloem, then sugars are unloaded at the sink so pressure can stay lower there. ### Is pressure flow the same as diffusion? No. Diffusion is too slow for long-distance transport in plants, and it does not create the directional bulk flow seen in phloem. Pressure flow uses energy to load and unload sugar, which creates the pressure gradient that drives movement. ### What are examples of sources and sinks? A mature leaf is a common source because it makes sucrose during photosynthesis. Roots, fruits, seeds, and growing shoots are common sinks because they use or store the sugar. The same organ can switch roles depending on the plant’s needs. ## Related Study Guides - [30.5 Transport of Water and Solutes in Plants](/college-bio/unit-30/5-transport-water-solutes-plants/study-guide/gAnMjvh5FBEkVOTJ) ## About This Document Canonical Fiveable pages are available as Markdown at the same path plus `.md`. - [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns - [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing - [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`) - [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/college-bio/key-terms/pressure-flow-hypothesis#resource","name":"Pressure Flow Hypothesis | General Biology I","url":"https://fiveable.me/college-bio/key-terms/pressure-flow-hypothesis","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/college-bio/key-terms/pressure-flow-hypothesis#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:21:10.328Z","isPartOf":{"@type":"Collection","name":"General Biology I Key Terms","url":"https://fiveable.me/college-bio/key-terms"},"author":{"@type":"Organization","@id":"https://fiveable.me/editorial#content-team","name":"Fiveable Content Team","url":"https://fiveable.me/editorial","parentOrganization":{"@id":"https://fiveable.me/#organization"},"member":[{"@type":"Person","name":"Amanda DoAmaral","sameAs":["https://www.linkedin.com/in/adoamaral"]},{"@type":"Person","name":"Becky Bahr","sameAs":["https://www.linkedin.com/in/becky-bahr-b52743132"]}]},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/college-bio/key-terms/pressure-flow-hypothesis#term","name":"pressure flow hypothesis","description":"The pressure flow hypothesis is the plant model for moving sugars through the phloem from source to sink. It works by creating pressure differences through sugar loading and unloading, which pushes sap through the plant.","url":"https://fiveable.me/college-bio/key-terms/pressure-flow-hypothesis","inDefinedTermSet":{"@type":"DefinedTermSet","name":"General Biology I Key Terms","url":"https://fiveable.me/college-bio/key-terms"}},{"@type":"FAQPage","mainEntity":[{"@type":"Question","name":"What is pressure flow hypothesis in General Biology I?","acceptedAnswer":{"@type":"Answer","text":"It is the model that explains how plants move sugars through the phloem from source tissues to sink tissues. Sugar loading creates high pressure at the source, sugar unloading creates lower pressure at the sink, and that difference drives flow."}},{"@type":"Question","name":"How does the pressure flow hypothesis work?","acceptedAnswer":{"@type":"Answer","text":"Sugars are actively loaded into the phloem at the source, which lowers water potential and draws in water from the xylem. The resulting pressure pushes sap through the phloem, then sugars are unloaded at the sink so pressure can stay lower there."}},{"@type":"Question","name":"Is pressure flow the same as diffusion?","acceptedAnswer":{"@type":"Answer","text":"No. Diffusion is too slow for long-distance transport in plants, and it does not create the directional bulk flow seen in phloem. Pressure flow uses energy to load and unload sugar, which creates the pressure gradient that drives movement."}},{"@type":"Question","name":"What are examples of sources and sinks?","acceptedAnswer":{"@type":"Answer","text":"A mature leaf is a common source because it makes sucrose during photosynthesis. Roots, fruits, seeds, and growing shoots are common sinks because they use or store the sugar. The same organ can switch roles depending on the plant’s needs."}}]},{"@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"General Biology I","item":"https://fiveable.me/college-bio"},{"@type":"ListItem","position":2,"name":"Key Terms","item":"https://fiveable.me/college-bio/key-terms"},{"@type":"ListItem","position":3,"name":"Unit 30","item":"https://fiveable.me/college-bio/unit-30"},{"@type":"ListItem","position":4,"name":"pressure flow hypothesis"}]}]} ```