Chapter 36:
Transport in Plants
Plants

Leaves  roots
may be 100m apart.
Question ?

How do plants move
materials from one
organ to the other ?
Levels of Plant Transport
1. Cellular
2. Short Distance
3. Long Distance
3 Levels of Plant Transport

A) Cellular Transport


The transport of solutes and
water across cell
membranes.

Problem: very slow

Mechanisms


Types of transport:
1.



Passive Transport
Diffusion and Osmosis.
Requires no cellular energy.
Materials diffuse down
concentration gradients.

Transport Proteins
Ex: Carrier Proteins
Selective Channels
Potassium Channel



Found in most plant cell
membranes.
Allow K+ but not Na+ to pass.
Often “gated” to respond to
environmental stimuli.
B. Active Transport
C. Water Transport
2. Active Transport


Requires cell energy.
Moves solutes against a concentration gradient.

Ex: Proton Pump (another example of Chemiosmosis)




Membrane Potentials



Uses ATP to move H+ out of cells.
H+ creates a membrane potential.
H+ allows cotransport.
Allow cations to moved into the cell.
Ex: Ca+2, Mg+2
Cotransport


Couples H+ with anions to move both into cell.
Ex: NO3-
Summary
3. Water Transport



Osmosis - water moves from high concentration to low
concentration.
Water Potential

The potential energy of water to move from one location to another.

Abbreviated as
y
Has two components:
yr
yp

Pressure potential:

Solute potential:
y = yr + yp
Problem



Cell wall creates a pressure in the cells.
Water potential must account for this pressure.
Pressure counteracts the tendency for water to
move into plant cells.
Bulk Flow




The movement of water between two
locations due to pressure.
Much faster than osmosis.
Tension (negative pressure).
May cause bulk flow against the diffusion
gradient.
Tension


Is a very important force to "pull" water from
one location to another.
Plant Vacuoles


Create Turgor Pressure against the cell wall.
Affect water potential by controlling water
concentrations inside cells.
Tonoplast



Name for the vacuole membrane.
Has proton pumps.
Comment – genetic modification of these
pumps gives plants salt tolerance.
Proton Pumps



Drives solutes inside the vacuole.
Lowers water potential (yp ) inside the
vacuole.
Result



Water moves into the vacuole.
Vacuole swells.
Turgor pressure increases.
Turgor Pressure


Important for non-woody plant support.
Wilting:


Loss of turgor pressure.
Loss of water from cells.
Turgid
Flaccid

Aquaporins



Water specific facilitated diffusion transport
channels.
Help water move more rapidly through lipid
bilayers.
Short Distance Transport
1. Transmembrane route
2. Symplast route
3. Apoplast route
1. Transmembrane

Materials cross from cell
to cell by crossing each
cell's membranes and cell
walls.
2. Symplast

The continuum of
cytoplasm by
plasmodesmata bridges
between cells.
3. Apoplast

Extracellular pathway
around and between
cell walls.
Point

Movement of materials can take place by all 3
routes.
Long Distance Transport


Problem: diffusion is too slow for long
distances.
Answer: tension and bulk flow methods.
Start - Roots


Absorb water.
Take up minerals.
Root Hairs


Main site of absorption.
Comment - older roots
have cork and are not very
permeable to water.
Root Cortex



Very spongy.
Apoplast route very
common.
Problem

Can't control uptake
of materials if the
apoplast route is
used.
Solution

Endodermis with its
Casparian Strip.
Casparian Strip




Waxy layer of suberin.
Creates a barrier between the cortex and the
stele.
Forces materials from apoplast into
endodermis symplast.
Result

Plant can now control movement of materials into
the stele.
Casparian Strip
Endodermis
Mycorrhizae



Symbiotic association of fungi with roots of
plants.
Help with water and mineral absorption
(replaces root hairs in some plants).
May also prevent toxins from entering the
plant.
Mycorrhizae
Xylem Sap


Solution of water and minerals loaded into the
xylem by the endodermis.
Endodermis - also prevents back flow of
water and minerals out of the stele.
Xylem Sap Transport Methods
1. Root Pressure
2. Transpiration (Ts)
 Root Pressure




Root cells load minerals into xylem.
Water potential (yp) is lowered.
Water flows into xylem.
Result


Volume of water in xylem increases
Xylem sap is pushed up the xylem tissues creating root
pressure.
Comments



Root Pressure: limited way
to move xylem sap.
Most apparent at night.
Excess water may leave
plant through Guttation.
Transpiration (Ts)


Evaporation of water from aerial plant parts.
Major force to pull xylem sap up tall trees.
TCTM Theory




Transpiration
Cohesion
Tension
Mechanism
How does TCTM work?


Water evaporates from leaves, especially from
the cell walls of the spongy mesophyll.
Reason: water potential of the air is usually
much less than that of the cells.
As water evaporates:



Cohesion: water molecules sticking together
by H bonds.
Adhesion: water molecules sticking to other
materials (cell walls etc.).
Result

The loss of water from the leaves creates
“tension” or negative pressure between the air
and the water in the plant.
Tension causes:

Xylem sap to move to replace the water lost
from the mesophyll cells.
Xylem Sap

Is “pulled” by the resulting tension all the way
down the plant to the roots and soil.
Summary

Xylem sap moves along a continual chain of
water potential from:
air leaf stem roots soil
Comments


Tension is a negative pressure which causes a
decreased in the size of xylem cells.
Xylem cells would collapse without
secondary cell walls.
Factors that Affect Transpiration Rate
1. Environmental
2. Plant Structures
Stomatal Crypt
Multiple Layer Epidermis
Environmental Factors
1. Humidity
2. Temperature
3. Light
4. Soil Water Content
5. Wind
Plant Structure Factors
1. Cuticle
2. Stomate Number
3. Hairs
Stomates



Openings in the epidermis that allow water
and gas exchange.
Controlled by Guard Cells.
Control rate of Ts and Ps.
Guard Cells



Turgid: Swell - open stomata.
Flaccid: Shrink - close stomata.
Size of the cells is a result of turgor pressure
changes.
Turgid - Open
Flaccid - Closed
Turgor Pressure of Guard cells

Controlled by K+ concentrations.
To Open Stomata:
1. K+ enters the guard cells.
2. Water potential lowered.
3. Water enters guard cells.
4. Turgor pressure increases.
5. Guard cells swell and Stomata opens.
To Close Stomata:
1. K+ leaves guard cells.
2. Water leaves guard cells.
3. Turgor pressure decreases.
4. Guard cells shrink and Stomata close.
K+ Movement


Regulated by proton pumps and K+ channels.
Controlled by:



Light (Blue)
CO2 concentrations
Abscisic Acid (water stress)
Comment

Plant must balance loss of water by
transpiration with CO2 uptake for Ps.
Adaptations for Balance


C4 Ps
CAM Ps
Phloem Transport


Moves sugars (food).
Transported in live cells.

Ex: Sieve & Companion Cells
Source - Sink Transport

Model for movement of phloem sap from a
Source to a Sink.
Source


Sugar production site
Ex: Ps
Starch breakdown in a
storage area.
Sink


Sugar uptake site.
Ex: Growing areas
Storage areas
Fruits and seeds
Comment

The same organ can serve as a source or a
sink depending on the season.
Result

Phloem transport can go in two directions
even in the same vascular bundle.
Xylem Transport: In Contrast to Phloem



Usually unidirectional.
Endodermis prevents back flow.
Dead cells.
Phloem Loading at the Source:
1. Diffusion
2. Transfer Cells
3. Active Transport
Phloem Loading
Transfer Cells

Modified cell with ingrowths of cell wall to
provide more surface area for sugar diffusion.
Result



Sugar loaded into phloem.
Water potential (yp) decreases.
Bulk flow is created.
Bulk Flow


Movement of water into phloem.
Pressure forces phloem sap to move toward
the sink.
At the Sink:



Sugar is removed.
Water potential is raised.
Water moves out of phloem over to xylem.
Phloem: summary



Source - builds pressure.
Sink - reduces pressure.
Pressure caused by:


Sugar content changes
Water potential changes
Comment

Plants move materials without "moving"
parts, unlike animals.
Summary



Know various ways plants use to move
materials.
Know how Ts works and the factors that
affect Ts.
Know how phloem transport works.