transpiration. slide 2 of 32 transport overview plants need co 2, sunlight and h 2 o in the leaves ...

Transpiration

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Transport Overview Plants need CO2, Sunlight and H2O in the leaves

ONLY H2O needs to be transported to the leaves

CO2 gets in via stomata

Water is most of the mass of a plant

Carbon accounts for most of the mass of a dried plant

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Fundamental Forces

Physical forces drive transport of materials in plants

Movement by concentration gradient-- Movement due to random molecular motion-- Diffusion or facilitated diffusion for things other than water-- Osmosis is for water-- Solutes move independently of water concentration

Movement by pressure gradient-- Bulk Flow – movement of water and solvents due to

pressure gradient

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3 Types of Transport in Vascular Plants

1. Transport of water & solutes by individual cells-- ALWAYS accomplished by diffusion-- Example: from soil to root hair cell-- Example 2: from one tracheid to another tracheid

2. Short-Distance transport of substances between cells at the tissue level-- ALWAYS accomplished by diffusion

3. Long-distance transport within the xylem & phloem among the entire plant -- ALWAYS accomplished by bulk flow (pressure gradient)

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Individual Cell Movement

Passive Transport – movement down a gradient Does NOT require energy Simple diffusion, osmosis or facilitated diffusion

Active Transport – Movement against a electrochemical gradient Requires energy

Most solutes must use transport proteins Aquaporin – channel (transport) protein for

water

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Water Potential (Ψ)

Water moves from High concentration (of water, not solute concnetration) to Low concentration via osmosis

Water mover from high pressure to low pressure via bulk flow

Water potential is the combined effect of Solute Concentration Physical Pressure

Ψ = Ψs + Ψp

Conclusion: water moves from high water potential to low water potential

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Solute Potential (Ψs)

Solute potential (Ψs) is proportional to the number of dissolved solute particles Also called Osmotic Potential Ψs = -iCRT

Ψs of water = 0 Addition of solute Decrease in water potential

More solute = less water (realtively) = lower water potential Ψs ≤ 0

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Pressure Potential (Ψp)

Pressure Potential (Ψp) Physical pressure on a solution Created by placing physical pressure (+) or by vacuum/sucking

(-) 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

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Water Potential Examples

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Short-Distance Transport

SymplastCytoplasmic continuum (called Symplast) consists 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

Transmembrane Self-evident & highly inefficient

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Long Distance Transport

Accomplished by Bulk Flow Water movement from regions of high pressure to regions of

low pressure

Movement in both xylem and phloem is driven by pressure differences between opposite ends of vessels or sieve tubes.

Diffusion is a poor driver over long distances (roots to leaves)

In xylem, water & minerals travel by negative pressure Transpiration and root push

In phloem, hydrostatic pressure forces materials down

Follow a molecule of water or mineral…

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Roots & Water Absorption

Root hairs = absorption of water Root hairs increase surface area for absorption Hydrophilic cell walls absorbs soil solution (water and minerals)

Mycorrhizae are important for absorption as well

Root epidermis cortex vascular cylinder (xylem) Called Lateral Transport (Short Distance Transport) To rest of plant via xylem

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Casparian Strip

In the endodermis

Waxy material encircling the cells of the endodermis

Ensures that any water or solutes must pass through a plasma membrane before entering xylem

Impedes apoplastic transfer

Critical control point

Again, plasma membrane controls what can enter the xylem

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Xylem moves vertically, how?

After water or minerals gets past the endodermis, most will find its way to the xylem

BULK FLOW, not concentration differences drives this transport

2 PRESSURE differences drive this Root Pressure or root push Transpiration (much more important)

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Root Pressure

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

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Transpiration

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

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Transpiration (Page 2)

Transmitted all the way from Leaves to the soil solution

Again, due to PRESSURE differential, not concentration

Small diameter of vessel elements and tracheids increases adhesion

Transpiration is ultimately due to stomata Necessary water loss for CO2 uptake and O2 removal If stomata closed, then less photosynthesis and plant may

overheat

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Transpiration (Page 3)

1 molecule of H2O evaporates due to transpiration, another molecule is drawn from the roots to replace it.

Factors that influence transpiration High humidity = DECREASE transpiration Wind = INCREASE transpiration Increasing light intensity = INCREASE transpiration Close stomata = NO transpiration

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90% of water lost by plants is through stomata

Stomata account for 1% of leaf surface area

Guard cells control opening & closing of stomata

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Phloem Translocation

Photosynthetic products (Phloem Sap) are translocated through the phloem Translocation literally means “movement from place to

place” 30% of phloem sap is sucrose, but it can be any

assimilate form of sugars (G3P)

Translocation is NOT a one-way transport mechanism

Sieve tube elements carry sugar from source to sink Source – leaves (net producer of sugar) Sink – roots (net consumer of sugar)

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Sucrose is added at the sugar source (leaves)

Sucrose first moves in by diffusionH2O follows

Once sucrose concentration is too high, an electrochemical gradient is created to move sucrose into phloem by cotransport

Decreases water potential in phloem, so creates positive pressure

Phloem sap is propelled away from the source

Where sugar is used, negative pressure is found

Used in respirationConverted to starch or cellulose

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Sugar loading into the sieve-tubes is necessary prior to any bulk flowMovement through the sugar source cells can be either apoplastic or symplasticSymplastic movement occurs via plasmodesmata

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Where sugar is used = sink

Concentration in sink is lower than in phloem

So sugar concentration gradient = diffusion of sugar and then water out of the phloem

So lower pressure at the sink

Sugar may be -- Used in respiration-- Converted to starch

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Pressure Flow Hypothesis

Also called mass flow (bulk flow) hypothesis

Phloem sap moves from source to sink at 1 m/hr, which is far faster than diffusion or cytoplasmic streaming

So it is the PRESSURE differential that moves phloem sap Pressure builds at source Pressure falls at sink

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Sucrose Loading

From cell to cell through the plasmodesmata (Symplast) OR Along cell walls (apoplast)

Surface membranes of companion cells actively pump sucrose into the sieve tube’s cytoplasm.

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The accumulation of sucrose and other solutes, such as amino acids, in sieve elements lowers the water potential so that water diffuses in by osmosis from adjacent cells and from the xylem.

This creates pressure in the sieve elements causing the liquid (phloem sap) to flow out of the leaf.

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Sucrose is unloaded at sinks.

This is taken up by the cells and is respired or stored as starch.

This reduces the concentration of phloem sap and lowers the pressure, so helping to maintain a pressure gradient form source to sink so the sap keeps flowing in the phloem.

transpiration. slide 2 of 32 transport overview plants need co 2, sunlight and h 2 o in the leaves ...

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