Chapter 18
Transport in the Flowering Plant
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Plants are Autotrophic, they make their own
food during Photosynthesis in the leaves.
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Why do plants need a transport
system?
To provide the materials needed for various
plant metabolic processes including
Photosynthesis, Respiration , Growth and
Reproduction
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What materials are transported in plants
1. Water
2. Carbon dioxide
3. Minerals
4. Carbohydrates produced in photosynthesis
5. Plant growth regulators
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Water uptake
Root epidermal cells absorb water by osmosis.
Adaptations for absorption:
1.Root hairs give a large surface area
2.Have thin walls
3.Don’t have a waxy cuticle
4.The cell sap has a lower water concentration
than soil water
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Water uptake
Root Hair
Ground tissue
Xylem
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Water
Xylem form
continuous hollow pipes
from roots to leaf
Water movement through the root
Water moves from
1. the epidermal cells across
2. the ground tissue of the cortex and into
3. the xylem vessels.
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Giant Redwoods
The largest and oldest trees in the world
A single mature giant redwood can draw
650,000 litres of water up through it in one
season!
How is this possible?
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Upward movement of water
Two mechanisms combine to cause upward
movement of water through the stem in the
xylem.
They form a continuous hollow pipeline from
the roots to the leaves.
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1. Root pressure – When water is drawn into
roots by osmosis the extra volume causes
pressure to build up. This pressure pushes
water up through the xylem
Root Pressure
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2. Transpiration - the loss of water vapour
from the plant. As water evaporates from
the leaf, more water is pulled upwards
through the xylem into the leaf.
Transpiration
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Role of Transpiration
Water evaporates from the leaf cells during
transpiration – 99%.
The cells become less turgid.
The evaporation of water from the leaf cells
causes more water to be pulled upwards as a
contionus column to replace it
through the xylem vessels.
Animation of Transpiration stream
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 Water_movement
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Learning check 1
1. Define Autotroph.
2. Why do plants need a transport system?
3. What materials are transported in plants?
4. How are root hairs are adapted to the process of
absorbing water?
5. Outline the pathway taken by water into the plant.
6. Name the two mechanisms combined to cause the
upward movement of water through the stem in
the xylem
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The control of Transpiration
Leaves need to replace the water they lose in
transpiration or they may wilt and die.
To prevent wilting plants need to control how
much water they lose in transpiration.
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Plasmolysis of plant cell
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1. Presence or absence of a cuticle
Leaves have a waxy cuticle through which
water cannot pass – this is mainly on the
upper side of a leaf as this side is more
exposed and more water can evaporate
here.
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2. Opening and closing of the stomata
Openings on the lower epidermis of leaves
for gas exchange.
Guard cells control the opening - increases
water loss and the closing - reduces water
loss of the Stomata.
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Structure and working of Stomata
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 Summary of Transpiration
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When are stomata open and closed?
• Stomata open during the day when
photosynthesis is taking place to allow water
vapour out and CO2 in.
• Stomata close at night reducing water loss
and CO2 intake as photosynthesis is not
occurring.
Animations showing movement of water during transpiration
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Conditions when stomata close at day
Two reasons stomata may close during the
day:
1. If the plant has lost too much water
2. If temperatures are too high
By closing stomata the plant reduces water
loss.
In dry conditions stomata remain closed for
long periods, photosynthesis cannot occur
and food crops are reduced.
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Learning Check 2
1. What is Transpiration?
2. Explain the role of Transpiration in water
transport.
3. What controls Transpiration?
4. What controls the loss of water from leaf?
5. 3 Methods of controlling transpiration are?
6. When are stomata open and closed?
7. Explain the role of Root pressure in water
transport.
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Cohesion-Tension Model
of Xylem Transport
Need to Know
1. How plants move water up to great heights
against the force of gravity
2. Know the contribution of Irish scientists
Dixon and Joly to plant biology
3. Understand the terms transpiration,
cohesion, adhesion, tension, osmosis and
use them to explain water movement up
through xylem
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Cohesion-Tension Model of Xylem Transport:
explains how water is transported in plants to
extreme heights against the force of gravity.
Cohesion-Tension Model of Xylem Transport
Theory proposed by two Irish scientists
Henry Dixon
John Joly
Working in Trinity College 1894
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The cohesion-tension model of
water transport in xylem
• Two Irish Scientists working in Trinity College
Henry Dixon and John Joly put forward this
model in 1894
• Cohesion – the sticking of similar molecules to
each other, water molecules stick to each other
• Adhesion – when different molecules stick
together, water adheres to the walls of xylem but
this force is not as great as the cohesive forces of
water
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Cohesion
Similar molecules sticking together e.g. water
sticking to water
H
H
H
O
H
H
O
H
H
O
H
O
Attraction between molecules
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Cohesion
 Animation showing Cohesion by hydrogen bonding in
water
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Adhesion
Different molecules sticking together e.g. water
sticking to xylem walls
H
H
O
Attraction
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The Cohesion – Tension model
1. Cohesion between water
molecules in the narrow xylem
tubes causes the water to form
into a continuous column or
stream in the xylem
H2O
H2O
H2O
H2O
H2O
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2. Water molecules evaporate due
to transpiration at the leaf.
• Cohesion between the water
molecules replaces the water by
pulling the next water molecule
up the xylem.
• As the column of water is hard to
break this pull is felt down the
entire column of water to the
root.
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H2O
H2O
H2O
H2O
H2O
 As each water molecule is “pulled” from the
xylem another water molecule is “pulled” up
from the root.
 This pulling force is passed from water
molecule to water molecule all the way down
the plant.
 This is how water is pulled up through the
plant by transpiration.
 Animations of Cohesion-tension
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3. Transpiration from the leaf
• Puts the column of water under tension
• Tension can pull a column of water to great
heights in plants
• Tension causes the column of water to be
stretched
• But the cohesive forces between the water
molecules are strong enough to prevent the
column breaking
• The strong lignin prevents xylem vessels
collapsing inward
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View An Animation of sugar and water moving through
xylem and phloem
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Outline of cohesion tension model
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Summary points
1. Water evaporates from the leaf, as each
water molecule evaporates another is
pulled up through the thin xylem column
2. This pulling of water molecules puts all the
water in the xylem vessels under tension
3. This tension is great enough to pull water to
a height of 150m
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Summary points
4. Stomata open in daylight and transpiration
occurs causing xylem vessels to become
narrow, stems therefore are narrower at day
5. The strong lignin prevents xylem vessels
collapsing inward
6. When transpiration stops (at night) the
tension is released and xylem vessels return
to their normal width
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Learning Check 3
1. Explain the terms Cohesion, Adhesion,
Transpiration, Tension, Osmosis
2. Name the Irish scientists who proposed the
tension cohesion model of water movement.
3. Explain how plants move water to great
heights against the force of gravity.
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Mineral uptake and transport
Examples
Calcium ……
Helps make cell walls
Magnesium …..
Part of chlorophyll
Potassium
Are absorbed by active
transport which requires energy
Nitrates
Phosphates
Are transported from the
roots in the xylem, dissolved
in water
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Uptake & transport of Carbon dioxide
• Most of the Carbon dioxide comes in through
the stomata from the atmosphere
• It diffuses into the air spaces and then into
the photosynthesising cells in the ground
tissue
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Carbon Dioxide Sources
1. Produced in the leaf during respiration
2. Diffuses from the air in though the stomata
CO2
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Photosynthesis Products
Glucose
• Glucose is converted to starch
• Starch and glucose are transported in the
Phloem
Oxygen
• Some diffuses into air spaces in the leaf and
out through stomata into the atmosphere
• some of the oxygen produced is needed for
respiration
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• Glucose is the carbohydrate made in
photosynthesis
• This may be used immediately for energy for
the plant in respiration or it may be stored
as starch
• Some of this starch is stored in the spongy
mesophyll cells in leaves
• Starch stored in leaves is important in the
diet of leaf eating animals such as horses
and cattle
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Trans-section of a leaf
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Glucose converted to sucrose
• When glucose get converted to sucrose it
enters phloem sieve tubes and is transported
through the plant
• This sugary water is called phloem sap
An insect extracting
sugary phloem sap
from a plant
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Food transport
• Phloem carries food to all parts of the plant,
some food is sent to growth areas such as
buds, flowers and roots.
• Food is used to form new plant structures, for
respiration or it is stored as starch.
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Learning Check 4
1. Name 4 Minerals taken up by the plant.
2. How are they transported?
3. Describe the uptake and transport of Carbon
dioxide.
4. List the Products of Photosynthesis & state
their use
5. What is the function of the phloem?
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Food storage organs in plants
• Some parts of plants may become swollen and
fleshy with stored food
• This food will be used by the plant to produce
flowers, seeds and fruits
Modified Root
E.g. Tap roots of Carrots, turnips, sugar beet
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Modified stem
• Potato plants develop an underground stem
system
• The tips become swollen with stored starch
• These swollen tips are called stem tubers
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Modified Leaves
E.g. onion bulbs , daffodils bulbs and tulips bulbs
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• A bulb contains a tiny underground stem
which have swollen fleshy leaves attached
• This stem has an apical bud and lateral buds
can be seen where the leaves meet the
reduced stem
• The entire bulb is protected by dry scaly
leaves
• Some bulbs are edible (onion, garlic) while
others are poisonous (daffodil, tulip) this
prevents organisms in the soil from eating
them!
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Modified Petioles
• Celery and Rhubarb are modified petioles
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Learning Check 5
1. Name 4 food storage organs in plants.
2. What is the function of the stored food?
3. What are Potato plants?
4. Describe the structure of a bulb.
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Websites on storage organs in
plants & Transpiration
 http://theseedsite.co.uk/bulbs.html
 http://www.cactus-art.biz/notebook/Dictionary/Dictionary_S/dictionary_storage_organ.htm
 http://youtu.be/fNtFwQ4NpDg
 Told in storybook fashion, this short film narrates the struggle of
one water droplet, Vladimir, who faces the bourgeoisie-promoted
process of transpiration.
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