1

Plant Transport Today�s topics:

•  Absorption by Roots •  Root hairs •  Soil nutrients

•  Water potential •  Osmosis again •  Ψ = Ψs + Ψp

•  Xylem pulls water and nutrients •  cohesion tension model •  Stomata

•  Phloem pushes water and sugar •  sources to sinks

8 November 2021

Plant growth occurs in meristems

Root meristem Shoot meristem

3-year woody stem Annual Rings

Root structure

Monocot Dicot

Root vascular cylinder is in the center

See section 37.1

2

Energy is required to get nutrients into

the cell.

clay

Proton pumps are used to get ions into root cells

Low [Solute]

H2O

High [Solute}

Selectively permeable membrane

Osmosis

Osmosis

See also Chapter 7

Fig. 36-8c

ψP = ψS = −0.23

(c) Increased positive pressure

H2O

ψ = 0.07 MPa

ψP = 0 ψS = 0 ψ = 0 MPa

0.30

Water Potential

ψ = ψS+ ψP

Water potential = solute potential + pressure potential

Water always moves from areas of higher water potential to areas of lower water potential

Fig. 36-8d

(d) Negative pressure (tension)

H2O

ψP = −0.30 ψS =

ψP = ψS = −0.23

ψ = −0.30 MPa

ψ = −0.23 MPa

0 0

Water Potential

ψ = ψS+ ψP

Water potential = solute potential + pressure potential

Water always moves from areas of higher water potential to areas of lower water potential

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Transpiration pulls water up through the xylem

Vessel element

Vessel elements, with perforated end walls Tracheids

Pits

Vessel Tracheids 100 µm

Vascular Tissue

Outside air Ψ= -58.5 Mpa Inside leaf Ψ= -2.0 MPa Xylem of trunk Ψ= -1.2 MPa Xylem of root Ψ= -0.6 MPa Soil water Ψ= -0.3 MPa

Example: Transpiration pulls water up through the xylem

Leaf Structure

4

Stomata

K+

H2O

H2O

H2O

H2O H2O

H2O

H2O

H2O

H2O

H2O

Guard cells turgid/ Stoma open

Guard cells flaccid/ Stoma closed

K+ ions are used to move water in and out of guard cells

What controls when stomata are open or closed?

Movement in the Phloem

3

2

1

1

2

3 4

Vessel (xylem)

Sieve tube (phloem)

Uptake of water

Unloading of sugar

Sink

Sucrose

H2O

H2O

Bul

k flo

w b

y ne

gativ

e pr

essu

re

H2O Sucrose

Bul

k flo

w b

y po

sitiv

e pr

essu

re

Osmotic pressure drives

bulk flow of water and

sugars

Source (leaf)

Active transport of sucrose