Life Processes · Lesson 11 of 13
Transportation in Plants
“Plants operate two transport networks without a single pump.”
• Explain why large plants need transport tissues. • Compare xylem and phloem. • Describe water and mineral entry into roots. • Explain root pressure and transpiration pull. • Describe translocation through phloem. • Explain why phloem transport requires energy.
A tall tree must move water from soil to leaves many metres above the ground and send sugars made in leaves to roots, fruits and growing buds. Diffusion alone cannot cover these distances fast enough, so plants use two organised conducting systems.
Need For Transportation In Plants
Roots contact soil and absorb water and minerals, while leaves capture light and manufacture carbohydrates. These materials are needed in regions far from where they enter or form. Plants have lower energy needs than actively moving animals and many supporting tissues are dead, so transport may be slower, but the distances can be enormous.
| Feature | Xylem | Phloem |
|---|---|---|
| Main materials | Water and dissolved minerals | Sucrose, amino acids and other organic substances |
| Typical source | Roots | Leaves or storage tissues |
| Direction | Mainly upward | Upward or downward according to need |
| Main conducting structures | Vessels and tracheids | Sieve tubes with companion cells |
| Energy | Largely driven by physical forces | Loading and translocation use ATP |
Transport Of Water
Xylem vessels and tracheids connect roots, stems and leaves to form continuous channels. Root cells actively take up mineral ions, creating a concentration difference. Water enters the root and then the xylem, producing root pressure that can push a water column upward. Root pressure is particularly important when transpiration is low, such as at night.
Transpiration is the loss of water as vapour from the aerial parts of a plant, mainly through stomata.
When water evaporates from leaf cells, it creates suction that pulls water from leaf xylem. Because the water column is continuous, this pull is transmitted downward and draws water upward from roots. During the day, when stomata are open, transpiration pull is the major force lifting water in tall plants. Transpiration also transports minerals and helps cool the plant.
Observing Transpiration
Cover the soil of two similar pots so evaporation from soil is prevented. Place a plant in one pot and a stick of similar height in the other, then cover each with a transparent bag and place them in bright light. Water droplets collect mainly inside the bag surrounding the plant. Because soil evaporation was blocked and the stick does not release water, the droplets provide evidence of water-vapour loss from the plant.
Transport Of Food And Other Substances
Translocation is the transport of soluble products of photosynthesis and other organic substances through phloem from sources to regions that use or store them.
Companion cells use ATP to load sucrose into sieve tubes. The high solute concentration draws water in osmotically, raising pressure. Material then moves toward tissues with lower pressure, such as roots, fruits, seeds or growing buds. Because sources and sinks change, phloem flow can be upward or downward. In spring, stored sugar may move from roots or stems toward developing buds.
Problem
What is the main force moving water to the top of a well-watered tree on a sunny day?
- 1.Sunlight promotes stomatal opening and evaporation from leaves.
- 2.Evaporation creates suction in leaf xylem.
- 3.The continuous water column transmits this pull down the xylem.
- 4.Therefore, transpiration pull is the main daytime driving force.
Problem
Why can some upward water movement continue at night when transpiration is low?
- 1.Root cells continue active ion uptake.
- 2.The concentration difference draws water into roots and root xylem.
- 3.Incoming water creates positive root pressure.
- 4.Root pressure can push water upward over a limited distance.
Problem
A leafless plant begins growing buds using sugar stored in its roots. Identify the transport tissue and direction.
- 1.The transported material is stored organic food, not water and minerals.
- 2.Organic food moves through phloem.
- 3.Roots act as the source and buds as growing sinks.
- 4.The sugar therefore moves upward through phloem using energy-dependent loading and a pressure gradient.
Xylem is mainly upward, but phloem is not permanently downward. Phloem moves from the current source to the current sink, so direction depends on the plant’s needs.
Quiz
Which tissue carries water and minerals?
What is the major daytime force for upward water movement?
Why is ATP used in phloem transport?
Which phloem direction is possible?
What additional role does transpiration have?
Practice Problems
- Compare xylem and phloem by materials, direction, structures and energy use.
- Explain how active ion uptake can produce root pressure.
- Trace a water molecule from soil to loss through a leaf stoma.
- Explain why removing a ring of phloem can cause sugars to accumulate above the cut.
- During fruit development, identify likely sources and sinks and predict the direction of phloem flow.
Key Takeaways
• Large plants need conducting tissues because diffusion is insufficient over long distances. • Xylem transports water and minerals through continuous channels. • Root pressure can push water upward, especially when transpiration is low. • Transpiration pull is the major daytime force in tall plants. • Transpiration also transports minerals and cools leaves. • Phloem carries sugars, amino acids and other substances. • Energy-dependent loading creates the pressure gradient for translocation. • Phloem direction depends on the locations of sources and sinks.