Systems for Long-Range Transport in Plants (Lecture 3) Flashcards
What is the direction of water flow in plants?
Water flows upwards, from the soil –> to the plant –> to the atmosphere
Why is water flow important?
It brings mineral nutrients from the roots to the shoots.
As well, evaporation from the leaves cools the leaves that are exposed to the sun.
How can water rise tremendous distances, without a pump?
Thanks to the COHESION-TENSION MODEL. Water forms a continuous column under tension in the xylem, which allows it to be drawn to the leaves, from the roots.
What makes water molecules and adhesive to the cell walls, allowing this cohesion-tension model?
Water’s polarity!
What force drives or pulls the water column upwards, against gravitational forces?
A gradient of increasingly negative water potential, from the soil to the atmosphere.
What causes the negative pressure/tension that pulls water upwards?
EVAPORATION = More negative water potential = Gradient pulling the water column up.
Leaves have stomata (pores) and air spaces within the leaf mesophyll - this facilitates evaporation to the atmosphere.
As water evaporates out of the leaf, the remaining water is held under increasing tension. Water recedes into the pores in the cell wall and is held tightly by the carbohydrates in the cell wall.
Water movement in xylem is __________.
Unidirectional - up only.
Why is the xylem important in this cohesion-tension mechanism?
Xylem is an airtight system of tubes that prevents leakage and can withstand strong negative pressures.
What helps xylem cells resist the negative pressure from the tension in the water column, and keeps cells from collapsing?
The cell walls, which are reinforced with lignin.
What type of movement is that of water through the xylem?
Bulk flow! Everything (water, nutrients, etc.) is moving, not just individual molecules.
The rate can reach 15-45m/hr in trees with large vessels.
How do the stomata regulate transpiration (evaporation) from the leaf?
The stomata open and close, using solute (osmotic) potential to regulate cell turgor.
When the guard cells contain K+ (solute), the stomata open.
When K+ is released via the gated channels, the stomata close.
What is a minor force for water transport?
Root pressure! Here, ions are pumped into xylem cells into the root. The higher solute inside the xylem draws in water, which in turn generates a positive pressure, causing upward flow.
How are sucrose and organic nutrients transported in the phloem?
Via the up or down BULK FLOW movement of sieve tube contents.
How do we determine the direction of nutrient flow?
The flow is from cells with high sucrose concentration (photosynthetic leaves = SOURCE TISSUES) to low sucrose regions (roots, growing shoot tips = SINK TISSUES).
What is the main output of photosynthesis?
Sucrose.
