chapter 30-movement of water and solutes in plants

• What is the role of transpiration in the plant?

Transpiration creates a "pull" that helps draw water and dissolved minerals from the roots up through the xylem to the leaves and other parts of the plant.

• When CO2 comes into contact with a moist cell surface, causes water to be exposed to unsaturated air causing evaporation to occur (ensures CO2 can enter the leaves for photosynthesis)

• How is stomatal opening, and thus transpiration rate, regulated?

• open in the morning as light levels increase on leaf surface and close as light levels decrease

• Build up of solutes causes osmotic movement of water into guard cells and build up of turgor pressure

• Decline in guard cell solutes → stomatal closing, water moves out of guard cells and turgor pressure decreases

most ideal conditions:

• light present

• CO2 not too high

• moderate temperature (not too high)

• not too humid

• How does the cohesion-tension theory account for the movement of water to the top of tall trees?

Water evaporates from the leaves, creating a negative pressure (tension) in the xylem vessels.

• Adhesion of water to wall of tracheids and vessels of xylem

gets water throughout tree

• Threats to the cohesion of xylem sap

• What happens when salt builds up?

• Insufficiently lignified tracheid or vessel could collapse if there is a lack of water to pull upward

• Cavitation: the formation of air bubbles

• May be due to herbivore damage or wounding

• In a vessel element or tracheids, cavitation can lead to embolism, or break in cohesion of the fluid

• Reduced water uptake

• How is the impact of embolisms, that would break cohesion, minimized?

• Embolism: filling of vessels and/or tracheids with air or water vapor

• Embolized xylem conduits cannot conduct water

• The xylem vessels are narrow and interconnected, which helps limit the spread of embolisms.

• In some plants, root pressure can refill embolized vessels by pushing water upward, dissolving trapped air bubbles.

• Living cells surrounding the xylem can actively pump water into embolized vessels, restoring their functionality

• What is the role of the roots in water and ion transport?

• From root hairs, the water moves through the cortex

• Once in the conduction element of the xylem, water moves upward through root and stem into leaves

• Ions actively transported in two places

  • Into root hairs and into xylem

• How does the osmotically-generated pressure-flow mechanism account for the movement of sugars from source to sink?

• Sugars produced during photosynthesis are actively transported into phloem sieve tubes at source

• Draw water from xylem via osmosis

• Water from surrounding xylem flows into phloem by osmosis due to lower water potential

• At sink, sugars are actively or passively removed from phloem for storage or use

Root pressure

• Turgor pressure from solute loading in the roots can force water into and up the xylem

• One consequence is guttation, formation of water droplets through hydathodes on the stems and leaves of herbaceous plants

• But root-pressure is relatively weak compared to cohesion-adhesion-tension and can raise water no more than a meter

• How is the flow of material through the phloem and xylem connected?

• Water from the xylem is needed to load sugars into the phloem at the source (like leaves)

• As sugars accumulate in the phloem, they create a high osmotic pressure that pulls in water from nearby xylem vessels. This increases pressure inside the phloem, pushing the sugary solution (called phloem sap) toward the sink tissues (like roots or fruits).

• At the sink, sugars are unloaded from the phloem, water potential rises, and water may re-enter the xylem.