Resource Acquisition and Transport in Vascular Plants

Green algae (ancestors of land plants) live in water.

Mosses (early plants) live in very moist environments, very low – non vascular

Land plants acquire resources: CO2 O2 Light Land plants acquire resources: Above ground – carbon dioxide and sunlight Below ground – water and minerals H2O Sugar H2O Minerals

Land plants compete for these resources.

CO2 O2 Natural selection favored the plants with efficient systems for long-distance transportation of Water Minerals Products of photosynthesis Light H2O Sugar H2O Minerals

Plants have to balance Acquisition of light and CO2 Evaporative loss of water

Various arrangement of leaves make it possible to maximize light and CO2 uptake and minimize water loss. Leaves that are self shaded undergo programmed cell death and fall off because their photosynthetic output is less than their metabolic needs.

Roots undergo modifications to increase intake of water and minerals. Roots associate with fungi to increase surface area (mycorrhizae) Early association with land plants made colonization of land plants possible.

Transport occurs: Short distance: passive and active transport Long distance: bulk flow

Cell membranes are fluid mosaic model made of phospholipid bilayer proteins

Most nutrients cannot diffuse across phospholipid bilayer Active transport: cell must expend ATP (energy) Transport proteins are involved in active transport of nutrients

Proton pump: energy from ATP pump protons, H+ out of the cells inside of the membrane had –ve charge, outside has +ve charge: membrane potential CYTOPLASM EXTRACELLULAR FLUID ATP Proton pump generates mem- brane potential and gradient.

Proton pump facilitates cation uptake CYTOPLASM EXTRACELLULAR FLUID Cations ( , for example) are driven into the cell by the membrane potential. Transport protein Membrane potential and cation uptake

Proton pump facilitates cotransport of anions with H+ Cell accumulates anions ( , for example) by coupling their transport to; the inward diffusion of through a cotransporter. Cotransport of anions

Proton pump facilitates transport of neutral solutes with H+ Plant cells can also accumulate a neutral solute, such as sucrose ( ), by cotransporting down the steep proton gradient. Cotransport of a neutral solute

Osmosis (diffusion of water) across semipermeable membrane. Concentration of water determines direction of water flow Water flow is affected by rigid cell walls which exerts pressure on plasma membrane.

Water potential (y): The physical property predicting the direction in which water will flow, governed by solute concentration and applied pressure; units megapascals (MPa). Water potential refers to water’s potential energy – water’s capacity to perform work when it moves region of high water potential to a region of low water potential.

y = ys + yp Where: y = water potential ys = solute potential/ osmotic potential yp = pressure potential

Free water has highest water potential Free water has highest water potential. When bound to solutes water potential goes down.

Pressure potential can be positive or negative. Usually cells are under positive water potential. Usually cell contents press against cell wall and cell wall presses against protoplast – turgor pressure.

Water potential and water movement in an artificial model. a) In absence of pressure ys determines net movement’ Addition of solutes 0.1 M solution Pure water H2O P = 0 S = –0.23  = 0 MPa P = –0.23 MPa

b) Positive pressure can raise y by increasing yp. Applying physical pressure b) Positive pressure can raise y by increasing yp. H2O P = 0 S = –0.23  = 0 MPa P = –0 MPa

c) Raising y on the right causes movement to the left. Applying physical pressure c) Raising y on the right causes movement to the left. H2O P = 0.30 S = –0.23  = 0 MPa P = –0.07 MPa

Negative pressure d) –ve pressure reduces yp, causes net movement to the left by reducing y . H2O P = –0.30 P = 0.30 S = –0.23 S = –0.23 P = –0.30 MPa P = –0.23 MPa

It becomes turgid when placed in pure water. Initial flaccid cell undergoes plasmolysis when placed in an environment with high solute concentration. It becomes turgid when placed in pure water. Initial flaccid cell:  P = 0  S = –0.7 0.4 M sucrose solution:  P = –0.7 MPa Distilled water:  P = 0  P = 0  S = –0.9  S = 0  P = –0.9 MPa  P = 0 MPa

What happens when you forget to water a plant What happens when you forget to water a plant? What happens when you water it?

Water transport is aided by transport proteins – aquaporins.

Three major pathways of transport: Apoplastic route Symplastic route Transmembrane route Key Symplast Apoplast Transmembrane route Apoplast Symplast Symplastic route Apoplastic route Transport routes between cells

Bulk flow requires more efficient transport than diffusion and active transport.

Transport of water and minerals into the xylem: pushing and pulling

Pushing up the xylem sap: root pressure Casparian strip Endodermal cell Pathway along apoplast Pathway through symplast Pushing up the xylem sap: root pressure Casparian strip Plasma membrane Apoplastic route Vessels (xylem) Symplastic route Root hair Epidermis Endodermis Vascular cylinder Cortex

When too much enters plants give out excess water through leaves – guttation

Transpiration: loss of water vapor through leaves and other aerial parts of plants. A single corn plant transpires 60L of water in the growing season.

Transpiration pull Y = –0.15 MPa Y = –10.00 MPa Cell wall Air-water LE 36-12 Transpiration pull Y = –0.15 MPa Y = –10.00 MPa Cell wall Air-water interface Airspace Low rate of transpiration High rate of transpiration Cuticle Upper epidermis Cytoplasm Evaporation Mesophyll Airspace Air space Cell wall Lower epidermis Evaporation Water film Vacuole Cuticle Stoma CO2 O2 CO2 O2 Xylem

Leaf (cell walls) = –1.0 MPa Cohesion and adhesion causes ascent of sap Xylem sap Outside air  = –100.0 MPa  Mesophyll cells Stoma Leaf (air spaces) = –7.0 MPa  Water molecule Transpiration Leaf (cell walls) = –1.0 MPa  Atmosphere Xylem cells Adhesion Cell wall Water potential gradient Trunk xylem = –0.8 Mpa  Cohesion, by hydrogen bonding Cohesion and adhesion in the xylem Water molecule Root hair Root xylem = –0.6 MPa  Soil particle Soil = –0.3 MPa  Water Water uptake from soil

Stomata regulates rate of transpiration: opening and closing of stomata are regulated by transport of K+. Cells turgid/Stoma open Cells flaccid/Stoma closed H2O H2O H2O H2O K+ H2O H2O H2O H2O H2O H2O Role of potassium in stomatal opening and closing

Cells turgid/Stoma open Cells flaccid/Stoma closed

Adpatations in desert plants Reduced life cycle Reduced period of leaf production Thicker cuticle Bristles to reflect the heat Reduced leaves, photosynthetic stems Growing underground Taking carbon dioxide at night

Cuticle Upper epidermal tissue Lower epidermal tissue Trichomes (“hairs”) Stomata 100 µm

Reduced leaves, photosynthetic stems

Movement of sugars from source to sink Translocation: transport of photosynthetic products for use and storage by phloem tissue. Sugar source: plant organ that is the net producer of sugar (leaves) Sugar sink: net consumer of depository sugar (growing tips, roots, buds, stems, fruits)

Positive pressure bulk flow in sieve tubes (phloem). Vessel (xylem) Sieve tube (phloem) Source cell (leaf) H2O Sucrose H2O stream flow Pressure Transpiration Sink cell (storage root) Sucrose H2O

Thinning helps prevent excess demand on sugar source (pruning in agriculture)