Water in Plants

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