Transport In Plants Flashcards
(19 cards)
Describe the structure of xylem vessels.
Xylem vessels are long, tube-like structures formed from dead cells called vessel elements. Their walls are thickened with lignin and have pits that allow lateral movement of water.
What is the role of lignin in xylem vessels?
Lignin strengthens the cell walls, prevents collapse under tension, and provides waterproofing.
What are bordered pits in xylem vessels?
Bordered pits are areas in the lignified walls where lignin is absent, allowing water to move between adjacent vessels.
Describe the structure of phloem tissue.
Phloem is made of sieve tube elements and companion cells. Sieve tube elements have no nucleus and have sieve plates, while companion cells have nuclei and many mitochondria to support the sieve tubes.
How do companion cells support sieve tube elements?
They provide ATP and carry out metabolic processes required for active transport of substances in and out of sieve tubes.
What is the role of root hair cells in water uptake?
Root hair cells increase surface area for water and mineral ion absorption from the soil via osmosis and active transport.
Why is water potential lower inside root hair cells than in the soil?
Due to the presence of solutes like ions and sugars, which draws water in by osmosis.
Compare the apoplast and symplast pathways.
Apoplast pathway moves water through cell walls and is faster. Symplast pathway moves water through cytoplasm via plasmodesmata and allows selective movement.
What happens at the endodermis during water uptake?
The Casparian strip blocks apoplast pathway, forcing water into the symplast so the plant can control entry of solutes into the xylem.
What is the process of transpiration?
Evaporation of water from mesophyll cells into air spaces and out through stomata, creating a negative pressure that pulls water up from the roots.
How does temperature affect transpiration rate?
Higher temperature increases kinetic energy of water molecules, increasing evaporation and diffusion rate.
How does wind affect transpiration rate?
Wind removes the humid layer around the leaf surface, increasing the water potential gradient and thus transpiration rate.
Explain the cohesion-tension theory.
Water molecules stick to each other (cohesion) and to xylem walls (adhesion). Water is pulled in a continuous column due to transpiration, creating tension.
Why is water transport in the xylem a passive process?
Because it relies on physical forces like cohesion, adhesion, and tension, not metabolic energy.
What is the source-sink relationship in plants?
Sources produce sugars (e.g., leaves) and sinks use/store them (e.g., roots, fruits). Sugars move from sources to sinks via phloem.
Describe the mass flow hypothesis.
Sucrose is actively loaded into phloem at sources, lowering water potential. Water enters by osmosis, increasing pressure. Sap flows to sinks, where sucrose is unloaded.
What evidence supports the mass flow hypothesis?
Companion cells have many mitochondria; translocation can be stopped by metabolic poisons; phloem pressure changes along the length of sieve tubes.
List four xerophytic adaptations and their functions.
Thick cuticle (reduces water loss), sunken stomata (trap moist air), leaf hairs (reduce air movement), reduced leaf area (less surface for evaporation).
List three hydrophytic adaptations and their purposes.
Air spaces in tissues (buoyancy), stomata on upper leaf surface (exposure to air), reduced vascular tissue (less need for support and transport).
