1

AP Biology – Chapter 36

AP Biology 2006-2007

Chapter 36 - Transport in

Plants

AP Biology

Transport in plants - Overview   H2O & minerals

  transport in xylem   transpiration

  evaporation, adhesion & cohesion   negative pressure

  Sugars   transport in phloem   bulk flow

  Calvin cycle in leaves loads sucrose into phloem   positive pressure

  Gas exchange   photosynthesis

  CO2 in; O2 out   stomates

  respiration   O2 in; CO2 out   roots exchange gases within air spaces in soil

AP Biology

Transport in Plants   Occurs on 3 levels

1.  Uptake and loss of water and solutes by individual cells (cellular level)

2.  Short distance transport of substances from cell to cell at the level of tissues or organs

3.  Long distance transport of sap within xylem and phloem at the level of the whole plant

2

AP Biology – Chapter 36

AP Biology

Cellular Transport   Passive transport

  Movement of molecules down their concentration gradient

  Occurs without direct energy expenditure   Active transport

  Pumping of solutes across membranes against their concentration gradient

  Requires energy expenditure – ATP to transport solutes uphill

  Aided by transport proteins   Proteins embedded in the cellular membrane   Speed movement across the membrane   Some bind selectively to a solute on one side – release

on opposite side   Selective channels – ion channels – Na+ K+ pump

AP Biology

Water & mineral absorption  Water absorption from soil

 osmosis  aquaporins

 Mineral absorption  active transport  proton pumps

  active transport of H+

H2O

root hair

aquaporin

proton pumps

AP Biology

Mineral absorption  Proton pumps

 active transport of H+ ions out of cell   chemiosmosis  H+ gradient

 creates membrane potential   difference in charge   drives cation uptake

 creates gradient   cotransport of other

solutes against their gradient

3

AP Biology – Chapter 36

AP Biology

Proton Pumps  Chemiosmosis

  Transmembrane proton gradient links energy releasing processes to energy consuming processes

  Produces H+ gradient; Membrane potential

  Energy from gradient can be used for active transport

  Drives cations into cell   Cotransport - Moves anions and

sugars into cell with H+ returns

AP Biology

Na+ K+ Pump   Membrane potential created from proton pumps

drive the active transport of different solutes   Actively pumps K+ ions into the cell   Actively pumps Na+ ions out of the cell

AP Biology

Water Absorption – osmosis   Osmosis – passive transport of water (down conc.

gradient) across a membrane   Direction of water movement depends on solute

concentration and pressure

  Water potential – combined effects of solute concentration and physical pressure   Determines the direction of water movement   Water ALWAYS moves from an area of high water

potential to low water potential   Ex: water moves into a cell with a high solute

concentration   Water potential is measured in Ψ (psi), in units called

megapascals MPa – one MPa is equal to 10atm of pressure

4

AP Biology – Chapter 36

AP Biology

Water Potential (Ψ)  Both solutes and pressure affect water

potential  Solute potential (ΨS) – also called

osmotic potential – proportional to the number of dissolved solute molecules  ΨS of pure water is 0

 Pressure potential (ΨP) – physical pressure on a solution. Can be positive or negative  Water in living cells is usually under

positive pressure

Ψ = ΨS +ΨP

AP Biology

Solute Potential (ΨS)   When solutes are added to water

  Solutes bind water molecules reducing the number of free water molecules

  Adding solute lowers the water potential   Water potential affects the absorption and loss of

water by a plant cell   Aquaporins – transport proteins change the rate

of water movement   Do NOT affect the water potential gradient or

direction of water flow   Increase the RATE at which water diffuses

down its gradient

AP Biology

Water Absorption   Plants in a hypotonic environment

  Solute concentration inside the cell is HIGHER   Water enters the cell the plant cell becomes

turgid

5

AP Biology – Chapter 36

AP Biology

Water Absorption   Plants in a hypertonic environment

  Solute concentration inside the cell is LOWER   Water leaves the cell the plant cell becomes

plasmolyzed (flaccid/limp as it loses water)

AP Biology

Major Pathways of Transport – short distance transport   Apoplast – continuum formed by cell walls,

extracellular spaces and dead interiors of tracheids and vessels

  Ions can be diffused across a tissue entirely though the apoplast

  Symplast – continuum of cytosol, neighboring cells connected by plasmodesmata

  Transmembrane – cell walls and cytosol pathway

AP Biology

Bulk Flow – long distance transport   Movement of water from high pressure to low pressure -

Movement of water/solutes through xylem and phloem   Water taken up by epidermis   Roots hairs/mycorrhizae increase surface area   Phloem – hydrostatic pressure forces sap to the opposite

end of the tube   Xylem – tension/negative pressure drives transport

  Transpiration – evaporation of water from a leaf reduces pressure in the leaf xylem

  Creates a tension that pulls xylem sap upward from the roots

6

AP Biology – Chapter 36

AP Biology

Ascent of xylem fluid Transpiration pull generated by leaf

AP Biology

Transpiration-Cohesion-Tension Method   Xylem moves water/minerals one way

  Transpiration – evaporation of water from leaves, due to lower water potential of air

  Flow of water transported up from the xylem replaces water lost in transpiration

  Also carries minerals to the shoot system   Creates NEGATIVE pressure to pull water up from

roots and shoots   Water forms column and moves by capillary action

  Cohesion – water molecules stick to each other due to hydrogen bonding

  Adhesion – water molecules sick to surface of xylem cells

AP Biology

Root Pressure   At night – transpiration is very low

  Root cells continue to expend energy while pumping minerals into xylem

  Accumulation of minerals lowers the water potential – causing a positive pressure = root pressure forces fluid up the xylem   Root pressure causes guttation – forcing water droplets

out of the leave blade

7

AP Biology – Chapter 36

AP Biology

Transpirational Pull   Root pressure is not the major mechanism driving

ascent of xylem sap   Primarily – xylem sap is pulled upward by the

leaves   Transpiration provides the pull   Cohesion and adhesion of water due to hydrogen

bonding transmits the upward pull of the xylem   Depends on the generative of negative pressure

(tension) in the leaf   Tension created by adhesion and surface tension

lowers the water potential drawing water from an area of high water potential to an area of lower water potential

AP Biology

Stomata regulate rate of transpiration   Large surface area of leaves

  Enhance light absorption for photosynthesis   Increase water loss through stomata

  CO2 diffuses into and O2 diffuses out of the leaf via stomata

  A leaf may transpire more than its weight in water each day

  Amount of water lost depends on number of stomata

AP Biology

Stomata   Each stomata is flanked by a pair of guard cells

  Control the diameter of the stoma by changing shape (widen or narrow the gap between the two cells)

  Guard cells take in water turgid, increases the gap between cells

  Guard cells lose water flaccid, space between the cells close

8

AP Biology – Chapter 36

AP Biology

Chloroplasts Epidermal cell

Nucleus Guard cell

Thickened inner cell wall (rigid)

Stoma open Stoma closed

H2O

water moves into guard cells

H2O H2O H2O

H2O H2O

H2O

H2O

H2O H2O H2O H2O

Control of Stomates

K+

K+

K+

K+

K+ K+

K+ K+

K+ K+ K+ K+

water moves out of guard cells

  Uptake of K+ ions by guard cells   proton pumps   water enters by

osmosis   guard cells

become turgid   Loss of K+ ions

by guard cells   water leaves by

osmosis   guard cells

become flaccid

AP Biology

Stomatal opening cues   Blue light receptors stimulate activity of proton

pumps uptake of K+

  Depletion of CO2 as photosynthesis begins   Stomatal opening is an internal “clock” located in

guard cells   Circadian rhythm – opening and closing cycle

of the stomata (cycles with an interval of 24 hours)

  Environment stresses can cause stomata to close during the date   Water deficiency   Hormone responses

AP Biology

Control of transpiration  Balancing stomate function

 always a compromise between photosynthesis & transpiration   leaf may transpire more than its weight in

water in a day…this loss must be balanced with plant’s need for CO2 for photosynthesis

9

AP Biology – Chapter 36

AP Biology

Transport of Nutrients in Phloem   Translocation – the transport of nutrients through the

phloem   Sieve tube members – specialized cells that function in

translocation (arranged to form long sieve tubes)   Sieve tubes always carry food from a sugar source to a

sugar sink   Sugar source – plant organ in which sugar is being

produced by photosynthesis (mature leaves)   Sugar sink – organ that is the consumer of sugar

(growing roots, buds, stems, fruits)   Phloem sap moves from the source to sink – driven by

positive pressure   Companion cells help load sugars into sieve tube elements   High sugar concentration reduces water potential (water

moves INTO tubes)   Removal of sugar at sink increases water potential (water

moves OUT of tubes into xylem)

AP Biology On a plant…

What’s a source…What’s a sink?

can flow 1m/hr

Pressure flow in phloem   Mass flow hypothesis

  “source to sink” flow   direction of transport in phloem is

dependent on plant’s needs   phloem loading

  active transport of sucrose into phloem

  increased sucrose concentration decreases H2O potential

  water flows in from xylem cells   increase in pressure due to

increase in H2O causes flow

AP Biology

Transport of sugars in phloem   Loading of sucrose into phloem

 flow through cells via plasmodesmata  proton pumps

  cotransport of sucrose into cells down proton gradient

10

AP Biology – Chapter 36

AP Biology

Summary of Phloem Transport   Pressure flow in a sieve tube drives the flow of

phloem sap   Lading of sugar into the sieve tube at the

source reduces water potential inside the sieve tube uptake of water

  The absorption of water generates hydrostatic pressure forces sap to flow along the tube

  Pressure relieved by unloading sugar and loss of water from the tube at the sink

  Xylem recycles water form sink to source   Sap always flows from source to sink