Transport in
Plants
AP Biology
2006-2007
Transport in plants
1. H2O & minerals


transport in xylem
transpiration
 Water potential, adhesion & cohesion
2. Sugars


transport in phloem
bulk flow
 Photosynthesis in leaves loads sucrose into
phloem
3. Gas exchange

photosynthesis
 CO2 in; O2 out
 stomata

respiration
 O2 in; CO2 out
 roots exchange gases within air spaces in soil
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Overview
 Sugars travel from
leaves to roots

through phloem
 Water and dissolved
minerals travel from root to
shoot

through xylem
It defies
gravity!
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http://www.youtube.com/watc
h?v=w6f2BiFiXiM
1. Transport of Water and Minerals
 Amount of water needed daily by plants
is small compared to the amount that is
lost through transpiration
 Transpiration:

evaporation of water from plant surface
 If water is not replaced, the plant will
wilt and may die.
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Water Potential
 Water movement is governed by differences in
water potential

The potential energy of water molecules
 Solute concentration and pressure
 Water moves from an area of higher water
potential to lower water potential


High solute concentration = low water potential
Low solute concentration – high water potential
PE
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PE
High Water
Potential
Low Water
Potential
Hydrostatic
pressure causes
water to travel
up tube
The Process - Roots
 Minerals from the soil



Actively transported into
the root hairs and start to
accumulate
High Water
Increase solute
Potential
concentration in root cells,
decrease water potential
Water moves in
through osmosis to
xylem cells
WATER
Low Water
Potential
root hair
H2O
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 As water enters the
xylem, it forces fluid up
the xylem due to
hydrostatic root pressure


positive pressure
This pressure can only
move fluid a short distance.
 The most significant force
moving the water and dissolved
minerals in the xylem
 The “pull” of water from
transpiration
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 cohesion & adhesion
Hydrostatic
pressure causes
water to travel
up xylem
Pull =
Negative
Pressure
Adhesion and Cohesion
 Water is a polar molecule


unequal sharing of
electrons in the covalent
bonds
oxygen atom has a stronger
attraction for electrons then
hydrogen
 O becomes slightly negatively
d–
charged
 H becomes slightly positively
charged
O
d+
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H
H
H2O
d+
Cohesion and Adhesion
 Water molecules are
attracted to one another
and other materials

Cohesion
 Due to: Hydrogen Bonds
 Force of attraction between
slightly “–” oxygen and
slightly “+” hydrogen of
adjacent water molecules

Adhesion
 Attraction between water
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molecules and the side of
xylem cells
Transpirational Pull Starts in Leaves
 Evaporation of water through stomata



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Lowers WP in the surrounding air spaces
Water moves from spongy cells (higher WP) to air spaces
(lower WP)
Water in spongy cells exerts a pull on column of water
molecules in the xylem all the way from the leaves to the
roots (adhesion, cohesion)
LOW
water
potential
HIGH
water
potential
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Mycorrhizae increase absorption
 Symbiotic relationship between fungi & plant



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symbiotic fungi greatly increases surface area for
absorption of water & minerals
increases volume of soil reached by plant
increases transport of minerals to host plant
Mycorrhizae
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2. Transport of Sugars
 Photosynthesis: CO2 + H2O  C6H12O6 + O2
 Storage form of sugar: Starch

Cannot be transported, must be broken down
into smaller components
 Transport form of sugar: Sucrose

Very sweet sap
 Usable form of sugar: Glucose
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Push and Pull
 Water and minerals are
mainly transported via
transpiration

negative pressure
or “pull”
 Sucrose is mainly
transported via:

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positive pressure
(hydrostatic pressure)
“push”
force (+)
pressure
due to
accumulati
on of water
“pull”
force (-)
pressure
due to
adhesion
& cohesion
Companion cells
 Cells that
surround phloem
 Contain a lot of
mitochondria
Why?
 A lot of active
transport!

ATP
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Transport of Sugars
 Mass flow hypothesis


“source to sink” flow
Source = leaf, Sink = root
 Phloem loading in leaf
 active transport of sucrose
into phloem
 increased sucrose concentration
decreases water potential

Water flows in from xylem
cells
 increase in pressure due to
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increase in water causes flow
 Hydrostatic pressure
ATP
can flow
1m/hr
 Phloem unloading
into root cells



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active transport of
sucrose
into root cells
Decreases pressure in
bottom of plant
Sucrose will travel
from high pressure
near leaves to low
pressure near roots
ATP
3. Gas Exchange
 What
environmental
conditions might
impact
transpiration of
water?
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Gas Exchange Regulation Epidermal cell
Guard cell
Chloroplasts
Nucleus
 In dry conditions



water leaves guard H2O
cells by osmosis
guard cells
become flaccid
H2O
stomata close to
prevent water loss
 In humid conditions



H2O
H2O
H2O
H2O
H2O
Thickened inner
cell wall (rigid)
water enters guard
cells by osmosis
H2O
H2O
guard cells
become turgid
Stoma open
stomata open to
water moves
facilitate water flow
into guard cells
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H2O
H2O
H2O
Stoma closed
water moves out
of guard cells
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
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Don’t get mad…
Get answers!!
Ask Questions!
AP Biology
2006-2007
Homework
 Section 9.5 – pg. 326 #1-9
 Read Transpiration Lab
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