Plants need an internal transport system because roots, stems, and leaves do different jobs in different places. Xylem carries water and dissolved minerals from the roots toward the leaves, where water is used in photosynthesis and lost by evaporation. Phloem carries sugars made in the leaves to growing tissues, roots, fruits, and storage organs.
Together, these vascular tissues let large plants move materials over long distances without a heart or pump.
Key Facts
- Xylem mainly transports water and mineral ions upward from roots to stems and leaves.
- Phloem transports sucrose and other organic nutrients from sources to sinks, often in both directions within different tubes.
- Transpiration pull occurs when water evaporates from leaf stomata and creates tension that pulls xylem sap upward.
- Cohesion and adhesion help xylem water columns stay continuous as water moves through narrow vessels.
- Pressure-flow model: sugar loading into phloem lowers water potential, water enters by osmosis, and pressure pushes sap toward sinks.
- Water potential guides water movement: water moves from higher water potential to lower water potential.
Vocabulary
- Xylem
- Xylem is vascular tissue that transports water and dissolved minerals, usually upward, through dead hollow cells.
- Phloem
- Phloem is vascular tissue that transports sugars and other organic molecules through living sieve tube cells.
- Transpiration
- Transpiration is the loss of water vapor from plant leaves, mainly through stomata.
- Source
- A source is a plant region, such as a mature leaf, that produces or releases sugar into the phloem.
- Sink
- A sink is a plant region, such as a root, fruit, or growing shoot, that uses or stores sugar from the phloem.
Common Mistakes to Avoid
- Saying xylem carries sugar is wrong because xylem mainly carries water and mineral ions, while phloem carries sugars.
- Thinking phloem only moves downward is wrong because phloem sap can move upward or downward depending on where sources and sinks are located.
- Assuming plants pump xylem sap with active muscles is wrong because most upward xylem movement is driven by transpiration pull, cohesion, and adhesion.
- Ignoring water potential is wrong because osmosis into and out of phloem depends on differences in water potential created by sugar concentration.
Practice Questions
- 1 A plant loses 18 mL of water by transpiration in 3 hours. What is its average transpiration rate in mL per hour?
- 2 A leaf loads sucrose into phloem at a rate of 4 mg per minute. How much sucrose is loaded in 25 minutes?
- 3 A plant has mature leaves, growing root tips, and developing fruits. Identify likely sources and sinks, and explain the expected direction of sugar movement in the phloem.