Also Known As Chapter 36!! Transpiration + Vascularity

Transport Overview

3 Types of Transport in Vascular Plants 1. Transport of water & solutes by individual cells 2. Short-Distance transport of substances between cells at the tissue level 3. Long-distance transport within the xylem & phloem among the entire plant

From B4  Passive Transport – movement down a gradient  Does NOT require energy  Active Transport – Movement against a electrochemical gradient  Requires energy  Most solutes must use transport proteins  Aquaporin – channel (transport) protein for water

Proton Pumps  Uses energy from ATP to pump out hydrogen ions from the cell  Establishes a proton gradient with higher [H+] outside the cell  The electrical difference is called a membrane potential  Potential energy is therefore created  As the H+ ions diffuse back in, they can do work

 Name two processes that used proton pumps.  What is cotransport?  How does it relate in the processes named above?

Water Potential ( Ψ )  Water moves from High concentration to Low concentration via osmosis  Equivalently, water moves from high water potential to low water potential  Water potential is the combined effect of  Solute Concentration  Physical Pressure  Ψ = Ψ s + Ψ p

Water Potential (Page 2)  Solute potential ( Ψ s) is proportional to the number of dissolved solute particles  Also called Osmotic Potential  Ψ s of water = 0  Addition of solute  Decrease in potential  Ψ s ≤ 0

Water Potential (Page 3)  Pressure Potential ( Ψ p)  Physical pressure on a solution  Can be (+) or (-)  Water is usually under a positive pressure potential Turgor pressure – when cell contents press the plasma membrane against the cell wall  Drying out = Negative pressure potential

Water Potential Examples

Roots & Water Absorption  Root hairs = absorption of water  Root epidermis  cortex  vascular cylinder  To rest of plant via xylem  Mycorrhizae are important for absorption as well

Tonoplast (Vacuolar Membrane)  Regulates molecular traffic between the cytosol & vacuolar contents Symplast  Cytoplasmic continuum consisting of the cytosol of cells and the plasmodesmata connecting the cytosols.  Crosses membrane early in the process Apoplast  Continuum of cell walls + extracellular spaces  Only crosses a membrane at endodermis

Casparian Strip  In the endodermis  Ensures that any water or solutes must pass through a plasma membrane before entering xylem  Critical control point  Again, plasma membrane controls what can enter the xylem

Bulk Flow  Water movement from regions of high pressure to regions of low pressure  In xylem, water & minerals travel by bulk flow  Called xylem sap  Xylem travels from Roots  Stem (xylem)  Leaves (exit thru stomata)

1. Root pressure or “push” 2. Transpiration or “pull” What powers the bulk flow?

VASCULAR TRANSPORT -- Starts with stomatal opening -- Transpirational Pull

Root Pressure or “push”  Water diffusing into the root cortex = positive pressure  This pressure forces fluid UP the xylem  Weak force – can only propel fluids up a couple of feet

Transpirational Pull  Your book calls this: transpiration-cohesion-tension mechanism  In leaves, water is lost through stomata  Why? Lower water pressure in air than in leaves  Water is drawn up in to this area of negative pressure  Water molecules pull up other water molecules  Cohesion – water on water action  Adhesion – water to cell wall action  Via Hydrogen bonds

Transpiration/Cohesion  1 molecule of H2O evaporates due to transpiration, another molecule is drawn from the roots to replace it. High humidity = DECREASE transpiration Wind = INCREASE transpiration Increasing light intensity = INCREASE transpiration Close stomata = NO transpiration

Stomata  90% of water lost by plants is through stomata  Stomata account for 1% of leaf surface area  Guard cells control opening & closing of stomata

What causes stomatal opening? 1. Depletion of CO 2 within air spaces Photosynthesis consumes CO 2 = stomata open IF placed in chamber w/o CO 2, stomata open 2. Light 3. An increase in K+ ions  Lowers water potential of guard cells  Water flows into guard cells  stomata open

What closes stomata? 1. Lack of water – guard cells lose volume & close 2. High Temp. – stimulates cell respiration & CO 2 accumulates inside leaf 3. Abscisic Acid – produced in mesophyll cells in response to dehydration

Phloem  Translocation  Photosynthetic products leave the leaves and travel throughout the plant  Mechanism is called pressure flow  Sieve tube elements carry sugar from source to sink  Source – leaves (net producer of sugar)  Sink – roots (net consumer of sugar)

Sucrose is added at the sugar source (leaves)  Electrochemical gradient is created to move sucrose into phloem by cotransport  Decreases water potential in phloem, so creates positive pressure