Chpt. 25: Transport,
Food Storage and Gas
Exchange in Flowering
Plants
You need to….
Know the definition of Autotroph
Understand the role of osmosis, diffusion, root pressure,
transpiration and stomata in plant transport
Learn how water is taken up by plant roots and the path
taken by the water through the root, stem and leaf
Why do plants need a Transport System??
•Plants are Autotrophic they make their own food by
photosynthesis
•Some processes of metabolism in plants include
Photosynthesis, Respiration, Growth and Reproduction.
•For all these processes to occur plants need to get and
transport water, carbon dioxide, oxygen, carbohydrates
(produced in photosynthesis) and certain minerals.
Water transport in plants
Water uptake by roots:
The region of the root with root hairs is called the
Piliferous layer
Water enters the root hair cells by osmosis
Root hairs are adapted to this
process by having:
1. Thin walls
2. A large surface area
Osmosis:
The water around soil particles is relatively pure and is
called capillary water (high water conc.)
The cytoplasm in the root hairs is full of solutes and is
more concentrated than the water outside in the soil (low
water conc.)
Osmosis describes the way water will move from an area
of high water concentration to an area of low water
concentration
Water movement through the root :
Water moves by diffusion from the root hairs into the
ground tissue and eventually reaches the xylem
Root Hair
Ground tissue
Xylem
Xylem form
continuous hollow pipes
from roots to leaf
Water
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!!
Upward Movement of Water
Two mechanisms combine to cause upward movement of
water through the stem in the xylem:
1. Root Pressure
2. Transpiration (this is the loss of water by evaporation
through the leaves and other aerial parts of a plant)
1. Root pressure
As water moves
into the root by
osmosis it builds
up a pressure
that pushes
water up the
xylem.
2. Transpiration
As water
evaporates from
the leaf by
transpiration,
more water is
pulled upwards
through the xylem
into the leaf
More detail on role of
Transpiration
*Water evaporates from the
leaf cells in transpiration.*
The ground tissue cells in
the leaf become less turgid
Causing water to move out of
the Xylem to the ground
tissue cells in the leaf.
This creates an Osmotic
Gradient
As each water molecule is “pulled” from the xylem
another water molecule is “pulled” up from the root
Water molecules
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
Transverse section of a leaf
The control of Transpiration:
Why????
Leaves need to replace the
water they lose in
transpiration or they may
wilt and die
At certain times particularly
dry weather and drought it
is difficult for plants to
absorb water from the soil
3 methods of controlling transpiration:
1. Leaves have a waxy cuticle through which water
cannot pass – this is thicker on the upper side of a leaf
as this side is more exposed and more water can
evaporate here
2.
Stomata are normally found on the lower surface of a
leaf as less evaporation occurs here
3. Each stoma has two guard cells that can open or close
the stoma by changing shape.
Stomata opening increases water loss
Stomata closing reduces water loss
More Detail on Stomata:
Normally stomata open by day this allows water vapour
out and CO2 in when photosynthesis is taking place
Stomata close at night reducing water loss and CO2 intake
as photosynthesis is not occurring
However, 2 reasons stomata may close by 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.
Cohesion-Tension Model of Water Transport
in Xylem
Need to ........
(Higher Level Only)
•
Know how plants move water up to great heights
against the force of gravity
•
Know the contribution of Irish scientists Dixon and
Joly to plant biology
•
Understand the terms transpiration, cohesion,
adhesion, tension, osmosis and use them to explain
water movement up through xylem
Cohesion-Tension Model
of Xylem Transport explains how water is
transported in plants to extreme heights
against the force of gravity
Theory proposed by two Irish scientists:
Working in Trinity College 1894:
Henry Dixon
John Joly
Important Terms:
Cohesion: similar molecules sticking together e.g. water
molecules sticking together = high cohesion
H
H
H
O
H
H
O
H
O
Attraction between molecules
H
H
O
Adhesion: different molecules sticking together e.g.
water sticking to xylem walls
Xylem Wall
H
H
O
Attraction
* Note: the force of attraction between water molecules and xylem wall is not
as great as the cohesive forces of water
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
2.
As water molecules evaporate in transpiration
at the leaf, cohesion between the water
molecules replaces the water by pulling the
next water molecule up the xylem.
H2O
H2O
H2O
As the column of water is hard to break this
pull is felt down the entire column of water to
the root
H2O
H2O
3.
Transpiration from the leaf puts the column of water
under tension
This tension causes the column of water to be
stretched but the cohesive forces between the water
molecules are strong enough to prevent the column
of water breaking and lignin in the walls prevents the
xylem from collapsing in.
4. The tension in the xylem due to transpiration is great
enough to pull the column of water to great heights in
plants
5. Stomata open in daylight and transpiration occurs
The tension produced in the water column causes
the xylem to become narrower and as a result the
stem becomes slightly narrower by day.
Stomata close at night and transpiration stops. The
lack of tension allows the xylem to return to its
original wider shape and subsequently the stem
becomes wider.
Learning Check
Explain the terms Cohesion, Adhesion, Transpiration,
Tension, Osmosis
Name the Irish scientists who proposed the tension
cohesion model of water movement.
Explain how plants move water to great heights against
the force of gravity
Mineral Uptake and Transport
Need to ........
Learn how minerals are absorbed and transported
Learn how CO2 is obtained by the leaf
Learn how carbohydrates and oxygen produced by
photosynthesis are transported away from the leaf
Mineral uptake and transport
• Plants require numerous minerals e.g. calcium,
magnesium.
• Minerals exist in soil dissolved in water.
• Minerals along with other dissolved substances
(nitrates, phosphates, potassium ions) enter the root
hairs from the soil dissolved in water.
• Minerals enter root hairs by a process called active
transport which requires energy.
• Once inside the root minerals etc. are transported
throughout the plant via xylem, dissolved in water
Uptake and Transport of Carbon Dioxide
Sources of Carbon Dioxide:
1. Diffuses from the air in through the stomata.
- the rate of absorption of carbon dioxide is a
measure of the apparent rate of photosynthesis
2. Produced in the leaf cells during respiration.
High
=
increased
=
increased
temperature
respiration
production of CO2
True Rate of
Photosynthesis
=
Rate of CO2 +
absorbed by
stomata
Rate of CO2
produced in
respiration
The Fate of the Products of Photosynthesis
• Oxygen:
- is transported through the air spaces and out
through the stomata by diffusion.
- some of the oxygen produced can be used in leaf
cells for respiration.
• Glucose:
- used for respiration
- converted to starch for storage
Note:
- starch stored in leafs important in diet of leaf
eating animals.
- Glucose
sucrose
phloem sap
Food Storage Organs in Plants
Need to know:
One example of a root, stem and leaf modified as a
food storage organ
Food Storage Organs in Plants
Plants can alter their roots stems and leaves to act as
food storage organs
Root modified to store food:
• some dicots produce a large V-shaped root, tap root,
that penetrates deep into the soil
• Generally tap root – anchors plant
- absorbs water
• In plants such as carrots, turnips and sugar beet the
tap root becomes stored food later used for the
production of flowers, seeds and fruit.
• Tap root crops are harvested before they can produce
flowers.
Root modified to store food:
Example Carrot
Dicot plants, e.g. carrots
produce one main tap root
This root becomes fleshy
and swollen with stored starch
Stem modified to store food:
Example Potatoe
Potatoes have an underground
stem system
The tips of these stems become
swollen with stored starch
The swollen tips are called Tubers
Swollen stem tip is a potato tuber
Apical Bud
Lenticel
Lateral Bud
Leaf modified to store food:
Example
Onion bulb
Is a reduced underground stem
With fleshy leaves swollen with
stored starch surrounding a central
apical bud
Dry Scaly leaves
Fleshy leaves
Apical Bud
Reduced stem
Adventitious roots
Learning Check
Give an example of a stem modified to store food.
Give an example of a root modified to store food. In
what form is the food stored ?
What is a bulb ?
Gas Exchange in the Leaf and Stem
Need to Know:
Explain the role of the leaf (stomata) & stem (lenticel)
in gas exchange.
Gas Exchange in the Leaf:
• Function of stomata – gas exchange in leaves
• Photosynthesis requires carbon dioxide and it
is via the stomata that CO2 diffuses from the
atmosphere into leaves.
• Large number of stomata increases the rate of
gas exchange:
CO2 enters
O2 + H2O leave
• Inside the leaf CO2 diffuses to the mesophyll cells
through the air spaces between these cells
• Similarly oxygen, produced during photosynthesis,
diffuses from the mesophyll cells, into the
intercellular air spaces and out of the leaf through
the stomata.
• Water vapour also diffuses out of the leaf via the
stomata.
Gas Exchange in Stems:
Cells on the inside of stems or trunks of trees and
shrubs require oxygen for respiration and subsequently
produce carbon dioxide, so:
 How do gases [oxygen and carbon
dioxide] get through the waterproof,
corky surface of bark?
 They do so through lenticels, which are
areas where the packing of bark cells is
loosened up a bit.
 Lenticels often look like tiny raised
blisters on a branch or twig.
• Normally in lenticels :
O2 enters
CO2 + H2O leave
Stomatal Opening and Closing
• Each stoma is a gap between two specialised cells,
called guard cells.
• The guard cells have a thicker wall on the sides that
face each other.
• When water enters the guard cells, via osmosis, they
expand into a curved shape and a gap (the stoma)
opens up between the two cells.
• When guard cells lose water they shrink in size causing
the gap (stoma) between them to close.
Higher Level Only
Control of Stomatal Opening and Closing
Whether a stoma opens or closes depends on how much
CO2 is in the air spaces of the leaf.
Low levels of CO2 cause stomata to open:
Photosynthesis starts when the sun rises in the
morning:
• CO2 starts to get used up in photosynthesis and so
there is a drop in CO2 levels in the air spaces.
• This causes stomata to open.
High levels of CO2 in the air spaces of the leaf cause
stomata to close:
At night, the sun goes down and photosynthesis stops:
• The combination of less CO2 being absorbed by
mesophyll cells and continuing respiration raises the
CO2 levels in the air spaces.
• As a result the stomata close.
Other factors affecting stomatal opening and closing:
- Uptake and loss of potassium ions in guard cells
- Internal Clock