Mass Transport in Plants: Xylem and Phloem
The mass transport mechanisms in xylem and phloem are essential for plant survival and growth. This page provides an overview of these transport systems and their functions in plants.
Xylem Structure and Function
Xylem is responsible for transporting water and mineral ions from the roots to the leaves. Its structure is adapted for efficient water transport:
Highlight: Xylem vessels are long with no end walls, allowing water to move freely upwards through the plant.
The movement of water through the xylem is driven by transpiration, which creates tension in the water column:
Definition: Transpiration is the evaporation of water from plant surfaces, primarily through the leaves.
Water evaporates from the leaf surfaces, creating a suction effect that pulls more water up through the xylem due to the cohesive properties of water molecules.
Phloem Structure and Function
Phloem transports dissolved substances, primarily sucrose, throughout the plant. Its structure includes:
- Sieve tube elements
- Companion cells
Vocabulary: Companion cells are specialized cells that support the function of sieve tube elements in the phloem.
Companion cells provide energy for active transport of solutes into the sieve tubes.
Mass Flow Hypothesis
The mass flow hypothesis explains the mechanism of translocation in phloem:
- Solutes are actively loaded into the phloem at source tissues (e.g., leaves).
- This creates a concentration gradient and lowers water potential in the phloem.
- Water enters the phloem from the xylem via osmosis, increasing pressure.
- The pressure gradient pushes solutes along the phloem towards sink tissues.
- At sink tissues, solutes are removed and used or stored.
Example: The higher the concentration of sucrose in the phloem, the faster the rate of translocation.
Factors Affecting Transpiration
Several environmental factors influence the rate of transpiration:
- Light intensity: Higher light intensity increases transpiration as stomata open wider for CO₂ uptake.
- Wind: Increased wind speed accelerates transpiration by removing water vapor from leaf surfaces.
- Temperature: Higher temperatures increase evaporation rates, leading to faster transpiration.
- Humidity: Low humidity increases the water potential gradient, promoting faster transpiration.
Highlight: These factors affect the water potential gradient between the leaf and the surrounding air, which drives transpiration.
Understanding these mass transport mechanisms in plants is crucial for A Level Biology students studying topics such as water movement, nutrient transport, and plant physiology.
