Plant transport explains how water, minerals, and sugars move through a plant so cells can photosynthesize, grow, and stay firm. This cheat sheet focuses on the two main vascular tissues, xylem and phloem. Students need these ideas to connect plant structure with processes such as photosynthesis, transpiration, and nutrient distribution.
It is especially useful for comparing what moves, where it moves, and what forces drive movement.
Key Facts
- Xylem transports water and dissolved minerals mainly upward from roots to stems and leaves.
- Phloem transports sugars, mainly sucrose, from sources to sinks and can move materials up or down the plant.
- Transpiration is the evaporation of water from leaf surfaces, mostly through stomata, and it helps pull water through xylem.
- The cohesion-tension mechanism depends on water molecules sticking to each other and being pulled upward by transpiration.
- Adhesion helps water molecules stick to xylem vessel walls, supporting the upward movement of the water column.
- A source is a plant part that produces or releases sugar, such as a mature leaf, and a sink is a plant part that uses or stores sugar, such as a root or fruit.
- Pressure flow in phloem moves sugar from high-pressure source regions to lower-pressure sink regions.
- Guard cells control stomata opening and closing, which affects gas exchange, water loss, and the rate of transpiration.
Vocabulary
- Xylem
- Vascular tissue that carries water and dissolved minerals from roots toward the rest of the plant.
- Phloem
- Vascular tissue that carries dissolved sugars and other organic nutrients between sources and sinks.
- Transpiration
- The loss of water vapor from a plant, mainly through stomata in the leaves.
- Cohesion-tension
- The process in which water molecules stick together and are pulled upward through xylem by transpiration.
- Stomata
- Small pores in leaves that allow gas exchange and release water vapor.
- Source-sink relationship
- The movement pattern in which sugars travel from producing or storage areas to areas that use or store them.
Common Mistakes to Avoid
- Saying xylem moves sugar is wrong because xylem mainly transports water and minerals, while phloem transports sugars.
- Assuming phloem only moves upward is wrong because phloem transport can move up or down depending on where sources and sinks are located.
- Thinking transpiration is the same as photosynthesis is wrong because transpiration is water loss, while photosynthesis makes glucose using light, carbon dioxide, and water.
- Forgetting the role of stomata is wrong because stomata control both carbon dioxide entry and much of the water loss that drives transpiration.
- Saying roots push all water to the top of tall trees is wrong because root pressure is limited, and cohesion-tension from transpiration is the main force in tall plants.
Practice Questions
- 1 A plant loses 18 mL of water by transpiration over 6 hours. What is the average transpiration rate in mL per hour?
- 2 A student measures sugar concentration in phloem sap as 0.8 g/mL. How many grams of sugar are in 25 mL of sap?
- 3 A leaf is acting as a source and a growing fruit is acting as a sink. In which tissue will most sugar move between them, and what is the likely direction of movement?
- 4 If a hot, dry, windy day causes stomata to close, explain how this would affect transpiration, xylem water movement, and carbon dioxide intake.
Understanding Plant Transport Xylem and Phloem
The design of each tissue matches its job. Most xylem conducting cells are dead when mature. Their empty interiors form long tubes with strong, waterproof walls containing lignin.
Lignin prevents the tubes from collapsing when water is under tension. In woody plants, xylem makes up much of the wood. New xylem is produced each growing season, which can create annual rings.
Phloem cells are living. Sieve tube elements line up end to end, with perforated end walls called sieve plates.
Each sieve tube element depends on a nearby companion cell. Companion cells provide energy and control the loading of sugars into the transport pathway.
Water movement is closely linked to water potential, which describes the tendency of water to move from a wetter region to a drier region. Roots take in water because soil water can enter root cells by osmosis. Mineral ions may be actively moved into root tissues, lowering water potential there and encouraging more water to enter.
A band of cells called the endodermis acts as a checkpoint before water reaches the xylem. Its waxy Casparian strip blocks movement through cell walls.
Water must cross cell membranes instead, allowing the plant to control which ions enter its transport system. This matters because some soil minerals are useful while others can be harmful.
The pull through xylem is powerful but vulnerable. A continuous column of water can break if drought, freezing, or physical damage introduces an air bubble. This break is called cavitation.
It stops flow in that vessel because gases compress much more easily than water. Plants reduce this risk with narrow vessels, reinforced walls, and connections that allow water to move around blocked areas. On hot, dry days, guard cells may close stomata to slow water loss.
Closing stomata protects the water supply, but it also limits carbon dioxide entering the leaf. The plant must balance conserving water with maintaining photosynthesis.
Phloem transport requires energy at the source. Companion cells use energy from respiration to load sucrose into sieve tubes. Water then enters from nearby xylem by osmosis, raising pressure in the sieve tube.
At a sink, sucrose is removed for respiration, growth, or conversion into starch. Water follows out, so pressure becomes lower there. A single plant can have many sources and sinks at once.
Young leaves often act as sinks until they can photosynthesize enough to export sugar. Developing fruits, seeds, roots, buds, and storage organs may compete for the same supply.
When studying diagrams, trace the route separately for water and sugar. Pay attention to cell type, energy use, pressure changes, and the reason a particular organ is acting as a source or sink.