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Home , Guard cell, Osmosis, Stoma, Transpiration

Exchange & Transport in

Exchange & Transport Exchange & Transport in Vascular Plants

I. and Solute Uptake by Cells II. Local Transport III. Long Distance Transport IV.

Passive Transport () Water and Solutes - Uptake by Cells • Net movement of molecules from a region of high concentration to a region of low concentration  Caused by random (Brownian) movements of molecules  (Increase entropy)  Each type of molecule follows its own concentration gradient  At equilibrium, movement is equal in both directions

Osmosis: simple diffusion of the (water) Water and Solute Uptake by Cells Water moves across bilayer and through Osmotic ( gate ) • Water diffuses Move the water by moving the solutes! according to its . Hypertonic : higher concentration of concentration gradient solutes • ↑Osm → ↓[water] ↓ Osm →↑[water] . Hypotonic solution: lower concentration of • can solutes generate force () . Isotonic solution: equal solute concentrations

Heyer 1 Exchange & Transport in Plants

Water and Solute Uptake by Cells Water and Solute Uptake by Cells Hypotonic Isotonic Hypertonic solution solution solution Plant cells flaccid

Animal

Plant cell Plant cells turgid  : pressure exerted on wall of turgid cell = 75–100psi

Water Potential (Ψ) (Ψ)

• Osmotic pressure pulls water to the • Physical pressure • Pure water, ΨS = 0 MPa. • pressure potential [ΨP] could right. (= pressure potential [ΨP]): . 1 MPa = 10 atm = ~160 psi be negative solution on right has potential • Osmotic potential • solutes lower energy to push water to the left ΨS (=solute potential [ΨS]): solution on left has potential . 0.1 Osm = –0.23 MPa • If ΨS & ΨP are equal but energy to push water to the right opposite→ no net flow • Water moves from

• ↓Osm→↑ ΨS high Ψ to low Ψ. ⇒ Ψ = ΨS + ΨP = 0

Water and Solute Uptake by Cells Selective permeability Water moves across and • Except for water and small nonpolar solutes, permeability of cell through aquaporins is selective and regulated. • Permeability determined by transporter proteins. Ions and small molecules use – Channels and carriers are solute specific channels and pumps – If no transporter, than that solute cannot cross membrane • (Artificial membranes are only semipermeable —i.e., only discriminate based upon molecular size.)

Heyer 2 Exchange & Transport in Plants

Types of cellular transport • : driven by Brownian motion Water and Solute Uptake by Cells – Simple diffusion & osmosis – (carrier mediated passive transport) • : requires chemical energy (ATP) – Carrier mediated – Can transport against concentration gradient

Water and Solute Uptake by Cells Water and Solute Uptake by Cells

Water and Solute Uptake by Cells Water and Solutes — Local Transport

compartments & membranes

Heyer 3 Exchange & Transport in Plants

Water and Solutes — Local Transport Water and Solutes — Local Transport  Transmembrane Transport: from cell-to-cell across plasma  membranes [SLOW!]  Symplastic Transport: from cell-to-cell through plasmadesmata  Apoplastic Transport: around cells through porous cell walls

Water and Solutes Water and Solutes — Transport — Long Distance Transport Apoplastic movement of xylem —pushing Via xylem and Pressure: Bulk Flow: active transport in roots movement of fluids → minerals accumulate in xylem through vessels → water follows Must generate big → ↑ pressure pressure differences Limited to 1–2 m, if at all Where’s the pump? : pushes water out

Water and Solutes — Xylem Transport Water — a polar molecule δ– Apoplastic movement of xylem sap —pulling δ+

Transpiration- • Polar: one end slightly positive (δ+), Cohesion-Tension the other end slightly negative (δ–) Mechanism • Cohesion: – water molecules stick to each other

Heyer 4 Exchange & Transport in Plants

Water — a polar molecule Water and Solutes — Xylem Transport

• Adhesion (wetting): Pop quiz! – water molecules are attracted to and stick to What are the big other polar molecules pipes called? – Like the of What are the xylem walls smaller pipes • called? – “lead” water molecules attracted to “dry” cellulose for adhesion – Pull rest of water along by cohesion

Water and Solutes — Xylem Transport Regulating

Guard cells turgid: Guard cells flaccid: open stoma closed

K+ pumped into central Gates allow ; K+ diffusion water out of cell; Transpiration: the loss of follows water follows from the stomata of leaves

Regulating Transpiration Negative ψp → negative ψ in leaves

Transpiration pull moves xylem sap upward

Guard cells turgid: Guard cells flaccid: stoma open stoma closed

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Water and Solutes Water and Solutes — Phloem Transport — Phloem Transport Symplastic Flow Solute [osmotic] potential (Ψs) creates pressure gradient Movement of phloem sap  Source tissue — pushing only • or breakdown → ↑ [solute] in phloem sap solution → ↓Ψs Translocation: → Absorb water from xylem in phloem moving → ↑ΨP from sources  Sink organ •Starch synthesis → to sinks ↓sucrose [solute] in phloem sap solution → ↑Ψs phloem loses water to xylem Fig. 36.19 local → in phloem transport: sucrose → ↓ΨP loading into phloem  Bulk flow from ↑ΨP to ↓ΨP in phloem, from source to sink Secondary effect: With leaf-to-root translocation, xylem flow is increased without transpiration

Gas Exchange Gas Exchange Why must plants do gas exchange?

Photosynthesis (chlorenchyma): Photosynthesis (chlorenchyma):

CO2 + H20 + energy  CH2O + O2 CO2 + H20 + energy  CH2O + O2

Respiration (all tissues): Respiration (all tissues):

CH2O + O2  CO2 + H20 + energy CH2O + O2  CO2 + H20 + energy

Photosynthetic mesophyll Photosynthetic mesophyll (chlorenchyma): (chlorenchyma): cells are inside the leaf cells are inside the leaf

• Need to deliver adequate CO2 from air to epidermis chlorenchyma • But exposure to dry air causes water loss mesophyll

mesophyll epidermis

epidermis

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Air: composition & Gas Exchange & Water Loss

partial  High demand for CO2 in leaves in daylight; but water loss is a big problem.  N : 78%; P = 0.78 atm  limits water loss through epidermis. 2 N2 (↑H O/↓CO ): Stomata open to let air circulate.  O : 21%; P = 0.21 atm 2 2 2 O2 (↓H2O/↑CO2): Stomata close to limit water loss.  CO : 0.03%; P = 0.0003 atm 2 CO2 Other gases bring total up to 1 atmosphere.

Gas Exchange & Water Loss Gas Exchange & Water Loss

 Plants have tricks to balance gas exchange & water loss.  : plants adapted for low-moisture habitats • epidermis of the epiphytic Rhipsalis , windy, seashore – Surface view: The crater-shaped depressions with a each at their base can be recognized.  Oleander: stratified epidermis & stomata in hairy pits. – Cross-section: The guard cells are deeply countersunk, the cuticle is extremely thick.

Surface view Cross-section

Gas Exchange & Water Loss Gas Exchange & Water Loss . Layer of dead, air-filled cells in epidermis . Air-pockets are silvery and insulating → keep leaves cool .No leaves! . Living tissues displaced from surface → reduce moisture loss . make hairy surface . Dense trap humid air

Cactus Ocotillo Old man cactus  “leaf” primordia → spines  Leafless most of year   Photosynthetic stem Small, short-lived leaves Maui silversword in rainy season

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Gas Exchange & Water Loss What about ?

 Some plants store CO2 at so they can keep stomata closed all day.

• : elongated creating air gaps in the peridermal – permit gas-exchange between the atmosphere and the metabolically active cells below the – Often develop under site of stoma in primary epidermis This is covered in 6B!

What about Oxygen? What about Oxygen? Special issues for submerged plants

Mangroves: Pneumatophores carry O2 to roots in mud 5°C 35°C

% O2 in air 21% 21%

% O2 in water 0.9%* 0.5%* O in 2 1 1 water/air /25 x /40 x

* At equillibrium with air. Stagnation may decrease to 0.

What about Oxygen?

Some aquatic plants need special tricks for oxygen.

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