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Plant transport: Xylem and Phloem Chapter 4 pp. 47 -‐54
Lecture Outline: Evolu:on of vascular :ssue The xylem transports water and nutrients The phloem transports sugars and other organic compounds Xylem and phloem structure and func:on How do roots take up water and nutrients from the soil? Cohesion-‐tension mechanism and transpira:on relate to the physical proper:es of water The func:on of stomata in transpira:on Sugar transport in the phloem The pressure-‐flow hypothesis describes transport of sugar in plants
Bloodroot Sanguinaria canadensis
Plant classifica:on and vascular :ssue
nonvascular plants
seedless vascular plants
gymnosperms (cone-‐bearing seed plants)
angiosperms (flowering seed plants)
H2O and minerals
O2
CO2
CO2 O2
H2O
Light
Sugar
Tracheids
100 µm
Xylem
Vessel element
Pits
Vessel Tracheids
Tracheid
Long-‐distance transport in the phloem and xylem
Phloem
Sieve-tube element (left) and companion cell: cross section
Sieve-tube elements: longitudinal view Sieve plate with pores
Transpira/on is the evapora:ve loss of water
from the leaves of a plant.
H2O and minerals
H2O
But first, water and minerals must travel from the soil through the dermal and ground :ssue into the vascular cylinder (xylem/phloem) of a root…
eudicot root cross sec:on
100 µm
Epidermis
Cortex
Endodermis
Vascular cylinder
Pericycle
Xylem
Phloem
Dermal Ground Vascular
Key to labels
Cross sec:on of a eudicot root
Pathway along apoplast
Casparian strip
Endodermal cell
Pathway through symplast
Casparian strip
Plasma membrane
Apoplastic route
Symplastic route
Root hair
Epidermis
Cortex
Endodermis
Xylem
Vascular cylinder
Transport of water and mineral nutrients from root hairs to the xylem
Transpira:on is the movement of water against gravity, from the soil to the leaves, without using any energy… HOW do plants do this?
Transpira:on depends on: 1. the evapora:on of H2O
from the leaves pulls water upwards from the roots (tension)
2. the physical proper:es of water (cohesion) hTp://photo.accuweather.com
Xylem sap
Mesophyll cells Stoma Stoma
Water molecule
Transpiration Atmosphere
Adhesion by hydrogen bonding
Cell wall
Xylem cells
Cohesion and adhesion in the xylem
Cohesion by hydrogen bonding
Water molecule
Root hair Soil particle
Water Water uptake from soil
The ascent of xylem sap (water and minerals)
Water has cohesive behavior
– H
H
O —
—
+ +
+
–
–
H2O = water
The ascent of xylem sap depends
on hydrogen bonds (-‐-‐-‐)
between water molecules
Xylem sap
Stomata function in transpiration
Water molecule
Transpiration Atmosphere
Adhesion by hydrogen bonding Cell
wall Xylem cells
Cohesion and adhesion in the xylem
Cohesion by hydrogen bonding
Water molecule
Root hair Soil particle
Water Water uptake from soil
The ascent of xylem sap (water and minerals)
K+
H2O
H2O
H2O
H2O H2O
H2O
H2O
H2O
H2O
H2O
Guard cells turgid
Guard cells flaccid
Osmosis of water causes stomata (leaf pores) to open and close
Water flows into the cells, stoma opens
Water flows out of the cells, stoma closes
Xylem sap
Water molecule
Transpiration Atmosphere
Adhesion by hydrogen bonding Cell
wall Xylem cells
Cohesion and adhesion in the xylem
Cohesion by hydrogen bonding
Water molecule
Root hair Soil particle
Water Water uptake from soil
The cohesion-‐tension theory
explains the ascent of xylem sap (water and minerals)
The Pressure-‐flow hypothesis describes
how sugars are transported from sources to sinks via
the phloem (this does require
energy!) H2O and minerals
O2
CO2
CO2 O2
H2O
Light
Sugar
Source to sink
Daucus carota roots are sources in the springHme, and shoot is the sink
Daucus carota leaves are sources in the summer Hme, and flowers/fruits and developing organs are sinks
Starch -‐ how plants store sugar
Chloroplast
Starch
starch molecules are simply glucose molecules linked together (similar to cellulose)
Sieve-tube element
Sieve plate
Nucleus of companion cells
Sieve-tube elements: longitudinal view Sieve plate with pores (SEM)
10 µm
Structure of the phloem
4
Fig. 36-‐20
3
2
1
1
2
3
Vessel (xylem)
Sieve tube (phloem)
Source cell (leaf)
Load sugar into the phloem at the source (requires ENERGY!)
Water from the xylem flows into the phloem (OSMOSIS) , crea:ng pressure
Unload sugar at the sink (requires ENERGY!) Water from the phloem flows back into the xylem (OSMOSIS) – water is recycled, crea:ng a constant circuit!
Sink cell (storage root)
Sugar
H2O
H2O
Bulk flo
w
H2O
Sugar
Bu
lk flow
Pressure-‐flow hypothesis
Lecture Review, Chap 4 • Do any plants lack vascular :ssue? If so, give an example of a
kind of plant that lacks this par:cular :ssue type. • Relate structure to func:on in sieve-‐tube cells, vessel elements,
and tracheid cells. • How are water and mineral nutrients from the soil transferred
into the vascular cylinder of a root? • Define transpira:on. • Describe the cohesion-‐tension mechanism and relate it to the
func:on of the stomata and the proper:es of water. • Why do water molecules s:ck together? How does this relate to
why plants can move water against the force of gravity without using any energy?
• Trace the path of sugar in the phloem from source to sink. What is the name of the hypothesis to describe this flow?