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.
Understanding Biology: Xylem and Phloem
Xylem is built for a hard physical job. Most xylem vessels are made from cells that die as they mature. Their empty interiors join to form long tubes with strong walls.
A tough substance called lignin strengthens these walls, stopping them from collapsing when the water inside is under tension. This is one reason wood is strong. In a tree trunk, much of the wood is old xylem.
The narrow width of xylem vessels matters too. Water molecules attract one another, so they can form an unbroken column.
They are attracted to vessel walls as well. These forces help the column resist gravity, although the pull from evaporating leaf surfaces provides most of the upward movement.
Water leaves mainly through tiny pores called stomata. Guard cells around each stoma control how wide it opens. When stomata open, carbon dioxide can enter for photosynthesis, but water vapour escapes.
This creates a trade off for the plant. On a hot, dry day, water loss can become dangerous. Many plants partly close their stomata to conserve water, but this reduces carbon dioxide intake and can slow photosynthesis.
If the water column breaks because of drought, freezing, or damage, an air bubble may form in xylem. This is called cavitation. Some plants can bypass blocked vessels, while others replace damaged xylem during new growth.
Phloem has a different design because its contents are living materials. Its main conducting cells are called sieve tube elements. They remain alive but have very little internal equipment, leaving space for sap to move.
Nearby companion cells provide energy and control the loading of sucrose into the sieve tubes. A leaf making more sugar than it needs acts as a source. A developing root, flower, seed, fruit, or bud can act as a sink.
A potato tuber is a sink while it stores sugar as starch. Later, when shoots grow from the tuber, stored material can be released and the tuber becomes a source. This shows that source and sink describe a role, not a fixed part of a plant.
Students often see transport evidence in simple practical work. Celery placed in coloured water can show the path of xylem in a stem, especially in a cross section. Removing a ring of bark from a woody stem disrupts phloem near the outside of the stem.
Sugars then build up above the ring, while tissues below may slowly starve. This demonstrates that xylem and phloem occupy different positions. When drawing a stem or root cross section, pay attention to vessel arrangement as well as labels.
In roots, xylem is often central. In many stems, xylem lies closer to the centre and phloem lies nearer the outside. Linking structure to function makes these patterns easier to remember.
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.