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How water is used in plants
Photosynthesis, enzyme actions & chemical activity, moist mesophyll, maintain cell turgor pressure, most lost in transpiration ~ 400 H2O lost for each CO2 gained
Diffusion
Move from high to low concentration
Osmosis
Diffusion of water
Pressure
Required to prevent osmosis
Potential
Balanced by cell wall resistance
Pressure Potential (Turgor Pressure)
Pressure that develops against cell wall b/c of water entering the cell
Turgid Cell
Firm cell due to water gained
Plasmolysis
Loss of water through osmosis, accompanied by shrinkage if cytoplasm away from cell wall, cell is flaccid
Cycling of Water Summary
Water → Roots → Pathway → Xylem → C-T → Stomata → Atmosphere
How Water Enters the Root
Absorbed with osmosis by root hair or epidermis
Pathways to go through the cortex
Apoplastic, Symplastic, both use the plasmodesmata
Apoplastic
Water moves through the cell walls
Symplastic
Water moves through the inside of cells
What does the endodermis do
Forces water to use the symplastic route
How water moves through the xylem
Moves up via cohesion-tension mechanism
Cohesion
Water molecules stick together with hydrogen bonds
Tension
Created by transpiration in leaves, pulls water from adjacent cell and creates a water column
Transpiration
Loss of water vapor through stomata, regulated by stomatal apparatus, effected by humidity, light, and CO2 conc.
Stomatal Apparatus
Stoma and 2 guard cells
Open Stomata
When photosynthesis occurs, guard cells expend energy to acquire K+ and H2O
Stomata Close
Photosynthesis not occurring, K+ leaves guard cells and water follows
Modifications for Stomata and Water Conservation
Stomata open at night instead of day, modified CAM photosynthetic pathway, stomata recessed
Guttation
loss of liquid water, when transpiration losses are low, minerals absorbed at night are pumped into intercellular space around xylem, water follows and pressure builds, so water is forced out of hydathodes at tips
Pressure-Flow-Hypothesis
Organic solutes flow from source where water enters by osmosis, to sinks where food is utilized and water exits
Phloem Loading
Sugar enters by active transport into sieve cells
Effect of Phloem Loading
Water potential in sieve tubes decrease so water enters
Effect of Decreased Water Potential
Turgor pressure develops an drives fluid through sieve tubes toward sinks
What Occurs at Sinks
Food substances are actively removed with water, lowering pressure in sieve tubes, then water diffused back into xylem
Transport of Food Summary
Phloem Loading → Decrease water potential → Increase turgor pressure → Move toward sinks → Remove food substances→ Water back into xylem