Chapter 10, Transport in Multicellular Plants

Particular requirements of plants: Carbon dioxide Oxygen Organic nutrients Inorganic ions and water Energy / Sunlight Required for photosynthesis Required for cell respiration Must be transported to cells that don’t do photosynthesis K+, phosphate, nitrate, also Fe, Mg, others Leaves have high surface area to catch

Plants have two transport systems  One for water & inorganic ions  One for carrying products of photosynthesis (sugars)  Oxygen and carbon dioxide ARE NOT TRANSPORTED VIA VESSLES, but are exchanged by diffusion alone xylem phloem

The transport of water Root hairs – roots – xylem – up to stems & leaves *Review water potential -Ψ = Ψs + Ψp  Where Ψs = solute potential (Ψs of pure water = 0, adding solute makes Ψs negative)  And where Ψp = pressure potential (positive value) Water moves DOWN water potential gradient (from high Ψ to low Ψ) Therefore Ψ root hairs > Ψ roots > Ψ xylem > Ψ leaves

From soil to root hair  Root cap – impermeable to water  Epidermal cells – have extensions called root hairs (high surface area)  Mycorhizas – symbiotic soil fungi o further increase surface area o increase absorption of phosphate

From root hair to xylem Root cross section: Epidermis Cortex Stele – Pericycle – Xylem – Phloem Skin Food storage Vascular tissue Waterproof barrier

Two possible routes through cortex of root:  Apoplast pathway – through cell walls, ends at the stele o Endodermis / Casparian strip  blocks apoplast pathway  forces water through cytoplasm (filtered by cell membrane) of endodermis cells  Symplast pathway Doesn’t go through cell membrane Cell membrane excludes undesirable solutes Goes through cell membrane into cytoplasm, cell to cell via plasmodesmata Waxy, waterproof

Lateral transport of minerals and water in roots Casparian strip Pathway along apoplast Pathway through symplast Plasma membrane Apoplastic route Symplastic route Root hair Epidermis Cortex Endodermis Vascular cylinder Vessels (xylem) Casparian strip Endodermal cell

Xylem tissue Support and transport Vessel elements and tracheids Fibers Parenchyma cells Two types of tubes Strengthened by lignified schlerenchyma fibers Lignin monomers Small numbers of live, supportive cells between tubes.

Xylem vessels Vessel elements Lignin Lumen Plasmodesmata – microscopic channels through plant cell walls Tracheids Not alive at maturity, end plates disappear, making long, unobstructed tubes Hard, strong, impermeable to water (the opening), again, unobstructed Pits covered by permeable cellulose remain in cell wall where plasmodesmata were, water can infiltrate Tapered ends with pits, found in all plants. Main vessels in ferns & conifers.

From leaf to atmosphere – transpiration Mesophyll layers (“middle leaf”) – Photosynthetic cells – Air spaces – Wet surface Stomata (in lower epidermis) – Control gas exchange & water loss Transpiration – Ψ gradient between leaf & air Temperature, wind will increase transpiration

Cells flaccid/Stoma closed Cells turgid/Stoma open Radially oriented cellulose microfibrils Cell wall Vacuole Guard cell

From xylem to leaf Mass flow Cohesion Adhesion Water molecules move together because of cohesion & adhesion (to lignin) Attraction of water molecules to each other Attraction of water to other molecules

Root pressure Active transport of ions – Into root cortex from soil – Into xylem from surrounding cells Solute causes decreased Ψ – Water enters xylem by osmosis – Causes increased pressure (Ψp), which pushes water up Effects of root pressure are relatively small

Comparing rates of transpiration Potometer Xerophytes Device for measuring water loss Plants adapted to dry environments. Waterproof cuticles Reduced surface area Sunken stomata Trichomes (leaf “hairs”)

Translocation Assimilates Phloem tissue – Sieve elements – Companion cells – Parenchyma – Fibers Products of photosynthesis (mostly glucose) Lack nucleus, but have ER & other organelles have nucleus, connect to sieve elements via plasmodesmata Small number of living cells, add support lignified sclerenchyma cells, for support

Phloem Tissue (cont.) Sieve elements – cell wall – minimal cytoplasm – ER – Mitochondria – NO NUCLEUS or ribosomes Thin, cellulose, sieve (end) plate is perforated by large pores

Phloem tissue (cont.) – Companion cells all plant cell organelles large numbers of ribosomes & mitochondria many plasmodesmata with neighboring sieve elements

– Parenchyma – Fibers Phloem tissue (cont.) ?

Sieve elements Sieve plate – pores (plasmodesmata) Open, allowing free flow of fluid between cells

Contents of phloem sieve tubes phloem sap – water – sucrose – potassium ions – amino acids – other ions (Cl-, phosphate, Mg++, Na+) and ATP

“clotting” if phloem is cut – phloem protein – blocks plasmodesmata – clotting carbohydrate – callose similar to cellulose, β-glucose polymer bonded by 1-3 glycosidic bonds (not 1-4 like cellulose)

sucking insects (aphids) – mouth parts (stylet) is too narrow, phloem loss is slow, doesn’t start clotting

Vessel (xylem) H2OH2O H2OH2O Sieve tube (phloem) Source cell (leaf) Sucrose H2OH2O Sink cell (storage root) Sucrose 3 1 Transpiration stream Pressure flow

How translocation occurs Mass flow – active loading of sucrose into sieve elements at source causes decreased water potential in sieve element water follows by osmosis pressure increases at source

How translocation occurs – unloading of sucrose from sieve elements at sink (root, fruit, growing shoot) increases water potential at sieve elements water leaves by osmosis pressure decreases at sink – phloem sap moves down pressure gradient from source to sink

Loading of sucrose into phloem Sucrose & water move from mesophyll (photosynthetic cells) – via symplast and apoplast pathway – actively transported into companion cells co-transport of sucrose and H+ ion – diffuse through plasmodesmata to sieve elements

Uploading of sucrose from phloem  exact mechanism not known, but probably diffusion  converted to glucose & fructose (invertase enzyme), maintaining concentration gradient

Evidence for the mechanism of phloem transport materials in phloem move 1,000 times faster than they would by diffusion rates match those predicted by measuring pressure at source and sink evidence for H+ co-transport mechanism – high pH of phloem sap (around pH 8) – membrane potential of -150 mV – large amounts of ATP are on hand in sieve elements, to fuel active transport

Differences between sieve elements and xylem vessels Xylem Dead lignified cell walls end walls disappear low pressure – transport by cohesion- tension carries water, some ions can’t clot – doesn’t need to Phloem alive non-lignified cell walls end walls form sieve plates high pressure, transport by mass flow carries sugar, amino acids, etc. clotting mechanism to prevent sap loss