Download File - Ms Curran`s Leaving Certificate Biology

Transport, food storage and
gas exchange in flowering
plants
Why do plants need a transport
system??
• Plants are Autotrophic they make their own food by
photosynthesis
• Plants store this food and use it for energy in respiration
• Other processes of metabolism in plants include cell
division, enzyme action and hormone action
• For all these processes to occur plants need to get and
transport water, carbon dioxide, oxygen and certain
minerals
Water transport in plants
Water uptake by roots
• The region of the root
with root hairs is
called the Piliferous
layer
• These are extensions
of epidermis cells but
don’t have a waxy
surface (cuticle) and
have thin walls
• How do you think this
makes them suitable
for absorption??
• The more root hairs
the more absorption
can take place….. Do
you think roots have
many or few root
hairs??
Osmosis
• The water around soil particles is relatively pure
and is called capillary water
• The cytoplasm in the root hairs is full of solutes
and is more concentrated than the water outside
in the soil
• Osmosis describes the way water will move from
low concentration to high concentration
How does water from root hair cells
get to the xylem?
• Where are root hairs
in relation to xylem?
• What type of tissue
does water need to
get across to reach
the xylem vessels?
• Water moves across
here by diffusion
Transverse section of root to show
movement of water
• The xylem form a hollow pipeline
throughout the plant going from the roots
up the stem into the petiole and from here
to the leaves
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?
Upward movement of water
•
2 mechanisms combine to cause upward
movement of water
1. Root pressure
2. Transpiration
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
• However root pressure isn’t strong enough to
push water through very tall plants
Transpiration
• The loss of water by evaporation through
the leaves + other aerial parts of a plant
• The water vapour escapes through tiny
holes on the undersides of leaves called
STOMATA
Trans section of a leaf
• Excess water in the ground tissue evaporates
into air spaces in the leaf and diffuses into the
atmosphere
• When the cells in the ground tissue lose water
they become less swollen and turgid
• For this reason the cytoplasm becomes more
concentrated and water from the xylem passes
into the ground tissue by osmosis
• 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
The control of Transpiration
• 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
• To prevent wilting plants need
to control how much water they
lose in transpiration
3 methods of controlling
transpiration
1. Leaves have a waxy cuticle through
which water cannot pass – this is 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
When are stomata open and
closed??
• Normally stomata open at day and close at
night
• 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
Conditions when stomata close at
day
2 reasons stomata may close at day
1.
2.
If the plant has lost too much water
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
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
Experiment to show the cohesion
of water molecules
• Capillary action is the process
which causes liquid in the soil
to rise up through roots and
stems
of plants or water to seep
through a sponge. Pick a
flower such as a carnation.
Carefully slice the stem in half
length ways. Place one half of
the stem in dark blue coloured
water and the other half of the
stem in dark red coloured
water. You will soon notice the
flower becoming half blue, half
red. The flower 'sucks up' the
water through narrow tubes in
its stem. The capillary action
overcomes the pull of gravity.
Outline of cohesion tension model
Summary points
• Water evaporates from the leaf,as each water molecule
evaporates another is pulled up through the thin xylem
column
• This pulling of water molecules puts all the water in the
xylem vessels under tension
• This tension is great enough to pull water to a height of
150m
• Stomata open in daylight and transpiration occurs
causing xylem vessels to become narrow, stems
therefore are narrower at day
• The strong lignin prevents xylem vessels collapsing
inward
• When transpiartion stops (at night) the tension is
released and xylem vessels return to their normal width
Water movement in the leaf by day
and night
Mineral uptake and transport
• Plants require minerals to function normally
• Calcium, magnesium, phosphates, potassium
etc. enter root hairs dissolved in water
• This entry requires energy so root hair cells
have lots of mitochondria
• Because they need energy to be transported the
process is called active transport
• Once inside the plant mineral get around
dissolved in water that moves in the xylem
Uptake and transport of
Carbon dioxide
• Photosynthesis takes
place mostly in the
mesophyll cells in the
leaf
• 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
The Rate of Photosynthesis
• A measure of how much CO2 is being absorbed
tells us the apparent rate of photosynthesis
• However CO2 is also produced by respiration in
the plant and this may be used in photosynthesis
• The true rate of photosynthesis is calculated by
adding the carbon dioxide taken in through the
stomata and the carbon dioxide formed in
respiration
What happens to the products of
photosynthesis?
• Oxygen produced in photosynthesis
diffuses into air spaces in the leaf and out
through stomata into the atmosphere
• However some of the oxygen produced is
needed for respiration
• 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,
monkeys and cattle
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
Food transport
• The precise mechanism of food transport
in phloem isn’t fully known
• 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
Food storage organs in plants
Modified Root
• In some plants with Tap roots (mainly
dicots) the root becomes swollen and
fleshy with stored food
• This food will be used by the plant to
produce flowers, seeds and fruits
• Humans harvest these before this
happens!
• Eg Carrots, turnips, sugar beet
Modified stem
• Potato plants develop an underground
stem system
• The tips become swollen with stored
starch
• These swollen tips are called stem tubers
• In nature a potato
tuber would remain
dormant in the soil
over winter and in
spring the buds on
the tuber (better
known to us as the
“eye”) would grow into
new plants
Modified Leaves
• Plants such as onions, daffodils and tulips
produce bulbs
• 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 see fig
25.11
• 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!
Modified Petioles
• Celery and Rhubarb are modified petioles
Gas Exchange in the leaf
Stomata
• Function of Stomata is Gas exchange
• Plants need carbon dioxide for
photosynthesis
• Carbon dioxide diffuses in through the
stomata from the atmosphere
Huge number of stomata on a leaf
• About 50,000 per cm2 !
• That’s the size of this
• This increases the rate of gas exchange
• Carbon dioxide diffuses through the
intercellular air spaces to the mesophyll
cells
Intercellular
air spaces
Mesophyll cells
How do air spaces affect gas
exchange??
• Air spaces increase the internal surface
area
• The inner surface are is about 20 times
more than the outer surface area of a leaf
• Think of a sponge
What gas exits the leaves??
• Photosynthesis produces oxygen which
diffuses out through the mesophyll cells
and air spaces then out through the
stomata
• When the earth was first formed there was
no oxygen plants are responsible for
releasing oxygen into the atmosphere (it
now makes up about 20% of air!)
What else exits the leaf through
stomata??
• Water vapour in transpiration
Recap – when are stomata open
and closed??
• Open at day for gas exchange
• Closed at night to reduce water loss
Gas exchange in stems
• Cells on the inside of stem and trunks
need oxygen for respiration
• When they respire they produce carbon
dioxide these gases need to get in + out
• The dermis and bark are thick and do not
allow substances in and out easily
Lenticles are openings in the barks
of trees and shrubs that allow gas
exchange
Stomatal opening and closing
• Each stomata has a pair of kidney shaped
guard cells
• These guard cells open and close stomata
by changing shape
• Remember this animation
• Wall of guard cells are thick on the insides
this causes them to curve when they
absorb water
• When water gets into guard cells by
osmosis they curve and open to reveal a
wide stomata
• When guard cells lose water they shrink
and the gap between them (the stomata
closes)
Control of Stomata opening and
closing
• The concentration of carbon dioxide in
the air spaces of the leaf plays a huge role
in controlling the opening and closing of
stomata
• High levels of CO2 cause the stomata to
close
• Low levels of CO2 cause the stomata to
open
Evening/Night time
• Photosynthesis stops when there is no
light
• The mesophyll cells are no longer
absorbing CO2 so it builds up in the air
spaces and the stomata close
Day time
• When it starts getting
light the mesophyll
cells absorb CO2
• This means the levels
of CO2 in the air
spaces drops so the
stomata open again
Proof!
• If a leaf is placed in a dark chamber with
no CO2 the stomata will open
• What does this tell us?
•
The exact mechanism of how CO2
controls stomatal opening is not fully
understood
• Other factors are also involved
1. The uptake and loss of patassium ions
by guard cells
2. An internal clock that tells guard cells to
open and close at regular intervals