Transcript Transport in plants

a. state the functions of xylem and
phloem.
b. identify the positions of xylem and
phloem tissues as seen in transverse
section of roots, stems and leaves.
c. investigate, using a suitable stain,
the pathway of water in a cut stem.
How is water transported
against gravity from the
roots, up the xylem and
to the leaves?
Think Like a Scientist
Scientists use ‘thought experiments’ to
help them solve problems.
3
I wonder where
trees get water
from?
Well,
obviously
from the
ground.
What are the
processes involved?
How does water move
through the transport
system of a plant IF
it does not have a heart
to act as a pump?
PAUSE to
PONDER
•How is water lifted against gravity
from the ground to the leaves
through this transport system?
• Are the products of photosynthesis
also carried in a set of vessels from
the leaves to the roots?
5
How do nitrate ions
get into plants?
Are they directly
absorbed from the air?
No. Even though
the air has 79% of
nitrogen, it is
highly unreactive.
PAUSE to
PONDER
•How are ions transported around in plants?
Transport in Plants
• Plants need a transport
system so that cells deep
within the plants tissues can
receive the nutrients they
need for cell processes
• The problem in plants is
that roots can obtain water,
but not sugar, and leaves
can produce sugar, but can’t
get water from the air
What substances need to be moved?
• The transport system in
plants is called vascular
tissue
• Xylem tissue transports
water and soluble
minerals
• Phloem tissue
transports sugars
Structure of Xylem
• Used to transport water
and minerals from roots
to leaves
• Consists of tubes for
water, fibres for support
and living parenchyma
cells
Xylem vessels
• Obvious in dicotyledonous plants
• Long cells with thick walls containing lignin
• Lignin waterproofs walls of cells and strengthens
them
• Cells die and ends decay forming a long tube
• Lignin forms spiral, annular rings or broken rings
(reticulate)
• Some lignification is not complete and pores are left
called pits or bordered pits, allowing water to move
between vessels or into living parts
Adaptations of Xylem to Function
• Xylem can carry water and minerals from roots to
shoot tips because:
• Made of dead cells forming continuous column
• Tubes are narrow so capillary action is effective
• Pits allow water to move sideways
• Lignin is strong and allows for stretching
• Flow of water is not impeded as: there are no end
walls, no cell contents, no nucleus, lignin prevents
tubes collapsing
Structure of Phloem
• Function to transport
sugars from one part to
another
• Made of sieve tube
elements and
companion cells
Sieve Tubes
• Sieve tube elements not true cells as they
have little cytoplasm
• Lined up end to end to form a tube
• Sucrose is dissolved in water to form a sap
• Tubes (known as sieve tubes) have a few walls
across the lumen of the tube with pores (sieve
plates)
Companion cells
• In between sieve tubes
• Large nucleus, dense
cytoplasm
• Many mitochondria to load
sucrose into sieve tubes
• Many gaps in cell walls
between companion cells
and sieve tubes for flow of
minerals
The Vascular Tissues
• Xylem and phloem are
found together in
vascular bundles, that
sometimes contain
other tissues that
support and strengthen
them
Vascular bundles: xylem & phloem
Xylem transports
water & dissolved
minerals from roots
to leaves.
Phloem transports
food (sugar) made in
leaves to all other
parts of the plant.
Stem
• The vascular bundles are found near the outer edge of the stem
• The xylem is found towards the inside of each vascular bundle, the phloem is
found towards the outside
• In between the xylem and phloem is a layer of cambium
• Cambium is a layer of meristem cells that divide to make new xylem and
phloem
Xylem & phloem tissues in stems.
Phloem
Xylem
Stem
Root
• The vascular bundle is found in
the centre
• There is a large central core of
xylem- often in an x-shape
• This arrangement provides
strength to withstand the pulling
forces to which roots are exposed
• Around the vascular bundle are
cells called the endodermis which
help to get water into the xylem
vessels
• Just inside the endodermis is the
periycle which contains meristem
cells that can divide (for growth)
xylem
phloem
Distribution of
vascular bundles in
roots
xylem
phloem
Xylem vessel
wall
Xylem vessel
lumen
Phloem
Endodermis
Starch
grains
Root
Leaf
• The vascular bundles (xylem
and phloem) form the
midrib and veins of the leaf
• A dicotyledon leaf has a
branching network of veins
that get smaller as they
branch away from the
midrib
• Within each vein, the xylem
can be seen on top of the
phloem
xylem
phloem
Distribution of vascular bundles in leaves.
Diagram showing a
section through a leaf.
xylem
phloem
A = Xylem
B = Phloem
C/D = Upper/Lower
epidermis
Leaf
Root vs. stem vs. leaf
The vascular bundle
differs depending on if
it is a root or stem
Transport Structures
Leaf
Xylem
Stem
Root
Phloem
Xylem
Phloem
Xylem
Phloem
d. describe the structure and functions
of root hair cells in relation to their
surface area, and to water and ion
uptake.
e. define transpiration.
f. describe how factors (e.g humidity,
temperature, light intensity) affect
the rate of transpiration.
g. describe how wilting occurs.
Transport of Water
Leaves
Stem
Root
Water uptake in ROOT
Animation
Root Pressure
Osmotic pressure that build up within the root cells which
forces water up the root xylem
Recap on Osmosis
How does the root
maintain a
concentration
gradient?
Root Pressure
 Uptake of water by the root hair by Osmosis
 Uptake of dissolved minerals through active
transport
 Cell sap within the root hairs becomes more
concentrated than the water in the soil
Root Pressure
• Occurs when the soil moisture level is high
either at night or when transpiration is low
during the day
• Can only raise the water in some plants up to
20cm
• Not the main force
Water transport in STEM
You have learnt that water
is transported from the
root to the stem and the
leaf.
How does the plant
transport water
upwards against
gravity??
Transpiration
Evaporation of water from the plant
 Loss of water vapor through the stomata on the
underside of the leaves
1. The mesophyll cells in the leaves are covered with
a thin layer of moisture
 Essential for efficient gas exchange to occur
2. Some of this moisture evaporates into the
intercellular spaces which diffuses through the
stomata into the ‘drier’ air
 Water is pulled upwards through osmosis
(Transpiration Pull)
3. The waterway in which the water moves from a
higher water potential to a lower water potential

Transpiration Stream
4 Environmental Factors that affect
Animation
Transpiration
1.
2.
3.
4.
5.
Wind speed
Humidity
Light intensity
Temperature
Water supply
Light intensity
During the day, stomata of
the leaves open. Why?
Photosynthesis!!
Gases exchange (CO2 & O2)
Water vapor also evaporates
(Transpiration)
Temperature
The higher the temperature, the higher the air
water capacity to hold moisture
At 30ºC, a leaf
may transpire 3
times as fast as it
does at 20ºC
5 Factors that affect Transpiration
1. Wind speed
◦
Increases  Increases transpiration rate
2. Humidity
◦
Increases  Decreases transpiration rate
3. Light intensity
◦
Increases  Increases transpiration rate
4. Temperature
◦
Increases  Increases transpiration rate
5. Water supply
◦
Decreases  Decreases transpiration rate
Potometer
The rate at which plants take up water depends on the rate of
transpiration- the faster a plant transpires, the faster it takes up
water.
Capillary Action
 The attractive force between the molecules of a
particular liquid is known as Cohesion
Water coheres to each other via chemical bonds called
hydrogen bonds (holds the droplets of water together)
• The attractive force between two unlike materials is
known as Adhesion
Adhesion causes water to stick to the inside of the glass
When water passes up the thin xylem vessels, it
adheres to the surface of the vessels, while the
force of osmosis gently ‘pushes’ the water
molecules, which cohere to each other, upwards
Capillary Action
Forces that promotes uptake of water
1. Root Pressure
 Promotes uptake of water in the root
2. Transpiration pull
 Main force to ‘suck’ up the water
3. Capillary Action
 Pushes the water upwards due to the adhesion
between the walls of the xylem vessels and water
molecules as well as the cohesion between the
water molecules
Transport of Food
Transport of Food in plant
 Food substances (sugar & amino acids) are
manufactured in the green leaves through the
process called photosynthesis
 Translocation is the process of transporting food
substances downwards from the leaves to all other
parts of the plant, through the phloem
Animation
Animation
3 experiments to show phloem
transport food substances
1. Using Aphids
Aphid penetrates the stem into
the phloem using its mouthpart
called stylet and sucks the plant
sap
A feeding aphid can be
anaesthetized and the stylet cut
off
The phloem sap flows out
through the stylet and can be
analysed. It is found to contain
sugars and other organic
substances
3 experiments to show phloem
transport food substances
2. The Ringing Experiment
1. Swelling above the ring
2. Reduced growth below the
ring
3. Leaves are unaffected
3 experiments to show phloem
transport food substances
3. Using radio-isotopes
Wilting of plant
Wilt
Temporary or permanent loss of turgor pressure due
to excessive transpiration of the leaves & shoot
Result in flaccid appearances of leaves & shoot
Characterized by drooping & shriveling of soft tissue
a.
Is wilting necessary a disadvantage to the plants?
Why?
b.
Wilting reduces loss of water in leaves. How does
it affect intake of water for photosynthesis?
c.
How is it the prolong process of wilting cause
harm to plants?
d.
What other conditions may cause plants to wilt?
Advantage of wilting
 Rate of transpiration reduces as the leaves folds up, reducing surface
that is exposed to sunlight.
 This will cause guard cells to become flaccid and the stomata will
close to conserve water in the plant.
Disadvantage of wilting
 Rate of photosynthesis will be reduced due to the following reasons:
 Water becomes a limiting factor now.
 The amount of sunlight that can be absorbed becomes a limiting
factor as leaves shriveled up.
 The amount of carbon dioxide becomes a limiting factor as the
stomata are closed.
Things to ponder
• What time of the day should you water your
plant?
• Too much water, is it good for your plant?
People often give potted houseplants more
fertilizer than they need. As a result, the plants
begin to wilt and eventually die instead of
getting larger and healthier. What could be the
reason for this result?
“Girdling” or ring-barking is a term referring to
the bark of the tree in a complete ring around
the trunk or a branch. Predict the effect that
girdling will have on a tree. Explain. Beneficial
or Destructive to plants?