Chpt 36: Transport In Plants

Transport Overview

1- uptake and loss of water and solutes by individual cells (root cells)

2- short-distance transport from cell to cell (sugar loading from leaves to phloem)

3- long-distance transport of sap within xylem and phloem in whole plant

Whole Plant Transport 1- Roots absorb water and

dissolved minerals from soil 2- Water and minerals are

transported upward from roots to shoots as xylem sap

3- Transpiration, the loss of water from leaves, creates a force that pulls xylem sap upwards

4- Leaves exchange CO2 and O2 through stomata 5- Sugar is produced by photosynthesis in leaves 6- Sugar is transported as phloem sap to roots and

other parts of plant 7- Roots exchange gases with air spaces of soil

(supports cellular respiration in roots)

Proton pumps (chpt 8)

Passive transport--Solutes diffuse down their gradients – requires no E (proteins in plants that passively transport water = aquaporins) Cross slowly unless they can pass through transport

proteins

Active transport – pumping solids across membrane against gradient – requires E— Carried out by special proteins

Proton pump -most impt active transporter in plants p.150

Membrane proteins store E by generating voltage across (separating charges) membranes.

Uses ATP for power Moves pos charges (H+) so creating a

voltage difference & a chemical difference that can be used to drive other processes

You expend a little E & get more in return

Water potential (ψ) differences drive water transport

Plant survival depends on balancing water uptake with water loss!

Hypotonic –lower solute concentration Hypertonic – higher solute concentration Water will move from hypo to hyper Since plant cells have a cell wall, pressure must also be

taken into consideration Water potential includes concentration & pressure

Water potential & concentration

Water moves from higher water potential to lower water potential

This moving water can do work so it has the potential to do work!

Measured in megapascals (Mpa) 1MPa = 10 atm Adding solutes lowers water potential so, if [↑] then

ψ↓

Water potential & pressure

If pressure goes ↑ then ψ↑ So, ψ = ψpressure + ψ[solute]

Look at p.751

Cellular Transport Water transport

√ Osmosis; hyper-more solute; hypo-less solute; iso- same solute

Cell wall creates physical pressure to go along with [solute]

Water moves from high to low water potential Flaccid (limp, isotonic –plants need to be in

hypotonic envr.); Plasmolysis (cell loses water in a hypertonic

environment; plasma membrane pulls away); Turgor pressure (influx of water due to osmosis;

hypotonic environment)

flaccid

Cells with vacuoles (vacuolated) have 3 compartments

Cell wall Cytosol Vacuole –membrane around vac. called

tonoplast

Transport within tissues/organs Tonoplast - vacuole membrane– controls traffic

between vacuole & cytosol Plasmodesmata – connections between dif. Cells -

cytosolic connection Symplast route (lateral) – all of the plant cells are

connected & this is called a cytoplasmic continuum or symplast route –materials move from inside 1 cell to inside the next cell

Apoplast route (lateral) – walls of adjacent cells are also in contact = continuum of cell walls –water etc… travel across tissue via cell walls & extracellular spaces

Diffusion process is fine for cell to cell, but to slow for long distances

Bulk flow (long distance) -movement of a fluid drive by pressure (in xylem)

Transport of xylem

Capillary action (rise of liquids in a narrow tube) –due to adhesion (water sticks to walls) is minimal

Osmosis creates Root pressure: at night (low transpiration), roots cells continue to pump minerals into xylem; this generates pressure, pushing sap upwards; usually doesn’t create a great amt of pressure

Root pressure causes guttation – exuding water droplets on tips of grass blades in morn.

Transport of Xylem Sap – cohesion-tension theory

Transpiration: loss of water vapor from leaves pulls water from roots (transpirational pull or tension); cohesion (water sticks to water) produces a single polymer-

like column of water from roots to leaves

Bulk Flow occurs when water evaps from leaves– as molecule evaps it pulls up a column behind it (sun causes transpiration so sun provides E for the process!)

Remember – water moves from greater water potential to lesser!

Transpiration control – balance between opening for CO2 & closing to maintain water

Stomata close when Temp is high- reduces H2O loss, but stops photosyn

Stomata open when CO2 levels low – CO2 required for photosyn.

Stomata close at night ([CO2] high due to respiration so no need to open….)

Guard cells control the size of the stomata

Scanning electron micrograph of Equisetum (horsetail or scouring rush) epidermis. Note the oval stomatal apparatuses in the center of the stem.

Pea Leaf Stoma, Vicea sp.

Transpirational Control Stomata opening- ↑K+ ions create a gradient that’s used to move

H2O into guard cells causing g-cells to enlarge & open stomata (K+ charge gradient in dif plant species balanced by H+ out or Cl- ions in)

Photosynthesis-Transpiration compromise… Xerophytes (plants adapted to arid environments)~ thick cuticle;

small spines for leaves

Translocation of Phloem Sap Translocation: movement of

food through phloem to a sink

Sugar source: sugar production organ (mature leaves)

Sugar sink: sugar storage organ (growing roots, tips, stems, fruit)

1-sugar enters sieve-tube members via active transport – this creates a concentration gradient between source (higher) & sink (lower)

2-water enters sieve-tube members by diffusion – passive transport

3- pressure gradient in tube moves water & sugars to sieve-tube members at sink (bulk flow due to pressure)

Pressure-flow hypothesis

4 – pressure at sink reduced as sugars are removed to by used –removing sugars decreases conc of solutes which lowers water pressure at sink & helps keep process going

5- in plants the sugar can also be “removed” by changing it to starch – it doesn’t have to be immediately used

6- xylem then recycles water from sink to source

Driving force for water movement in plants is transpiration & cohesion!

Driving force for sugar movement in plants is a concentration gradient!