plant transport Flashcards
WHY DO PLANTS NEED A TRANSPORT SYSTEM?
- Gaseous exchange do not need a transport system – they can diffuse
between cells - But transport systems are needed for distribution of water, inorganic and organic nutrients, as well as other substances such as plant hormones.
Transport systems are needed for the following reasons:
- To move substances from where they are absorbed to where they are needed
- To move substances from where they are produced to where they are needed for metabolism
- To move substances to a different part of the plant for storage
main organs involved in transport within plants.
Stems, roots and leaves
Organs are composed of more than one
tissue
Tissues
collections of cells specialised for a particular function.
The cells may be of the same type[simple tissues
such as parenchyma, or of different types[complex tissues], as in xylem and phloem.
plant tissues
dermal tissue
ground tissue
vascular tissue
meristematic tissue
dermal tissue types
epidermis
endodermis
epiodermis
- A continuous layer on the outside of the plant, one cell thick, that provides protection.
- covered with a waxy cuticle which is waterproof
and helps to protect the organ from drying out and from infection.
epidermis in leaves
it also has pores called stomata which
allow entry of carbon dioxide for photosynthesi
epidermis in roots
it may have extensions called root hairs
to increase the surface area for absorption of
water and mineral salts.
endodermis
It surrounds the vascular tissue in stems and roots;
one cell thick;
High-power detail of a transverse section
of leaf epidermis
4 types of ground tissue
1) parenchyma cells
2) collenchyma cells
3) sclerenchyma
4) cambium
Parenchyma cells:
Living cells; thin-walled; used as packing tissue;
support plant by being turgid; helps in photosynthesis; forms cortex in
root and stem, and pith in stem; storage; e.g. spongy and palisade
mesophyll cells
Collenchyma cells:
modified form of parenchyma with extra cellulose
deposited at the corners of the cells; provides extra strength; midrib of
leaves contains collenchym
. Sclerenchyma
a type of plant tissue that provides mechanical
strength to various parts of the plant. Cells with thick, lignified cell walls.
These cells are dead at maturity and contribute to the rigidity and
protection of plant
. Cambium:
a cylindrical layer of meristematic cells located between the
xylem and phloem; responsible for producing new xylem and phloem cells
The mesophyll is made up of
specialised parenchyma cells found between the
lower and upper epidermis of the leaf
mesophyll is specialised for
photosynthesis and therefore contain chloroplasts
2 types of mesophyll
palisade mesophyll and spongy mesophyll
Spongy mesophyll is so-called because
in three dimensions it is spongy in
appearance, because it has many large air
spaces between the cells.
Palisade mesophyll cells are near the
upper surface of the leaf where they receive more
sunlight.
* They therefore contain more chloroplasts
than spongy mesophyll cells
Pericycle:
This is a layer of cells, one to several cells thick, just inside the
endodermis and next to the vascular tissue
pericycle in roots
it is one cell thick and new roots can grow from this layer.
pericycle in stems
it is formed from a tissue called sclerenchyma
pericycle has
dead, lignified cells for extra strength.
MERISTEMATIC TISSUES
A meristem is a collection of undifferentiated cells that can
divide and become other specialized types of cells in the plant.
Meristem tissue is important because
it allows for plants to
grow and repair damaged tissue.
2 transport systems
xylem and phloem
xylem
carries mainly water and inorganic ions (mineral salts) from roots to
the parts above ground. The xylem sap contained in the xylem can move in only one direction, from roots to the rest of the plant.
phloem
The second system is phloem. This carries substances made by
photosynthesis from the leaves to other areas of the plant. At any one time, phloem sap can be moving in different directions in different parts of the phloem.
DISTRIBUTION OF XYLEM AND PHLOEM IN DICOT STEM
xylem and phloem distributed in a specific pattern;
scattered as vascular bundles in a circular arrangement; vascular cambium seen in between xylem and phloem; xylem inside and phloem outside the ring
in dicot stem
DISTRIBUTION OF XYLEm and pHLOEM IN DICOT root
vascular tissue surrounded by endodermis; xylem at
the centre; phloem seen surrounding the xylem; inner layer of
endodermis is the pericycle;
in dicot root
DISTRIBUTION OF XYLEm and pHLOEM IN DICOT leaves
Xylem and phloem distributed within the leaf
veins, xylem on the upper side of the veins, while phloem located on
the lower side of the leaf veins;
dicot leaves
Water always move from a region of
higher water
potential to a region of lower water potential
driving force of water
– evaporation from leaves – transpiration
evaporation starts in
mesophyll cells and through diffusion water vapor moves out
This reduces the wp in leaves, thus creating a
water potential gradient throughout the plant
* Water moves down the water potential gradient moving from soil into the xylem tissue in the centre of root, then to stem and leaves through xylem
Movement of water from leaf to atmosphere –
TRANSPIRATION
Mesophyll cells have many air spaces around them – mesophyll
cell walls are wet [EVAPORATION] – air inside the leaf is
usually saturated with water vapour – air inside leaf is in
direct contact with air outside the leaf through small pores
called stomata – there is usually a water potential gradient
between the air inside the leaf [higher wp] and the air outside
the leaf [lower wp] – water vapour diffuses down the potential
gradient called transpiration
Transpiration
loss of water vapour from a plant / leaves /
aerial plant parts, to its environment, by diffusion down a water
potential gradient; most transpiration takes place through the
stomata in the leaves.
Factors affecting transpiration rate:
humidity
light intensity
wind speed
how does humidity affect transpiration rate
Humidity: as humidity in the atmosphere increases, rate of transpiration decreases
how does wind speed affect transpiration rate
Transpiration may also be increased by an increase in wind speed or rise in
temperature
how does light intensity affect transpiration rate
As light intensity increases, rate of transpiration increases [photosynthesis increases –
need more CO2 – opens stomata , for intake of CO2 – water vapour escapes]
In hot conditions, transpiration plays an important
role in cooling the leaves.
As water evaporates
from the cell walls inside the leaf, it absorbs heat
energy from these cells, thus reducing their
temperature.
If the rate at which water vapour is lost by transpiration
exceeds the rate at which a plant can take up water from the
soil, then the
amount of water in its cells decreases. The cells
become less turgid and the plant wilts as the soft parts such
as leaves lose the support provided by turgid cells. In this
situation the plant will also close its stomata
Measuring the rate of transpiration
- rate at which
transpiration is
happening directly
affects the rate of
water uptake - The apparatus used
to measure the rate
at which water is
taken up by a plant
is called a
potometer
XEROPHYTES
plants that live in places where water is in
short supply.
xerophytes are adapted to
minimize water loss
xerophyte examples
marram grass, opuntia, phlomis, euphorbia
marram grass xerophytic adaptations
– roll up, due to
shrinkage of hinge cells, exposing water
proof layer called cuticle to the air outside.
SUNKEN STOMATA - Stomata are found only
in the upper epidermis and therefore open
into the enclosed humid space in the
middle of the ‘roll’ ; HAIRS / TRICHOMES -
trap humid air close to the leaf, thus reducing
the steepness of the diffusion gradient for
water vapour
marram grass
Phlomis
a small shrub that lives in dry habitats in the Mediterranean
regions of Europe and North Africa.
Scanning electron micrograph of a TS through a Phlomis italica leaf
showing its trichomes (×20). These are tiny hair-like structures that
act as a physical barrier to the loss of water, like the marram grass
hairs.
Opuntia
a cactus with flattened, photosynthetic stems that
store water. The leaves are reduced to spines, which lessens the
surface area from which transpiration can take place and
protects the plant from being eaten by animals.Its leaves
are in the form of needles, greatly reducing the surface area
available for water loss. In addition, they are covered in a layer of
waterproof wax and have sunken stomata, as shown here
opuntia
phlomis
The cardon Euphorbia
a canariensis grows in dry areas of Tenerife. It
has swollen, succulent stems that store water and
photosynthesise. The stems are coated with wax, which cuts
down water loss. The leaves are extremely small.
cardon euphorbia
Xylem tissue has two functions,
structural support and
transport.
xylem contains several different types of cells so its a
complex tissue
In flowering plants, xylem tissue
contains
(i) Xylem vessel with vessel elements
(ii) Xylem tracheids
(iii) fibres
(iv) parenchyma cells
Vessel elements and tracheids
cells that are involved with the
transport of water
Sclerenchyma fibres
elongated cells with lignified walls that help
to support the plant. They are dead cells; they have no living
contents.
Parenchyma cells:
living cells; thin walled; used as packing tissue;
metabolically active; storage [starch and fat]; helps in short distance
transportation of water; supports, when fully turgid; air spaces in
between these cells allow gas exchange; water and mineral salts are
transported through the walls and through the living contents of the
cells; forms the cortex in roots and stems, and the pith in stems; in
leaves – modified to palisade and mesophyll cells
Vessels are made up of many
elongated cells called vessel
elements, arranged end to end. The end walls of neighbouring
vessel elements break down completely, to form a continuous
tube
Each vessel element begins life as a
normal plant cell in whose wall
lignin is laid down.
Lignin is
a very hard, strong substance, which is impermeable to
water
As lignin builds up around the cell,
the contents of the cell die, leaving
a completely empty space, or lumen, inside.
Cellulose lining is
hydrophilic to maintain a column of water
xylem vessel structure
non living
thick cell wall wall made up of cellulose
cell wall with lignin
no end walls
large lumen
pits
how does xylem being non living ?
no cytoplasm, no organelles, hollow lumen,
therefore, greater volume of water can flow without resistance
how does xylem being thick cell made up of cellulose ?
structural support, allows
adhesion of water
how does the cell wall with lignin affect xylem
: prevents inward collapse as xylem
vessel is under tension; lignin is a strong, hard, water proof
substance
how does the no end walls affect xylem
less resistance to flow of water; forms a
continuous tube joined end to end
how does the large lumen affect xylem
large volume of water can be transported
how does the pits affect xylem
formed from plasmodesmata; because no lignin deposits
in plasmodesmata – leading to formation of pits; allows lateral
movement of water to connect to all parts of the plant; if there
is an air bubble blocking vessel, pits allow water to move into
another xylem vessel and bypass airlock
pits in cells
in those parts of the original cell walls where groups of
plasmodesmata are found, no lignin is laid down. These non-lignified
areas can be seen as ‘gaps’ in the thick walls of the xylem vessel, and
are called pit
pits are not
pores; they are crossed by
permeable, unthickened cellulose cell wall. The pits in one cell link
with those in the neighbouring cells, so water can pass freely from
one cell to the next.
xylem
What keep the water in a xylem
vessel moving as a continuous
column?
- Cohesion and adhesion help to keep
the water in a xylem vessel moving as
a continuous column
cohesion
– refers to the property of
water molecules attracted to each
other by hydrogen bonding
adhesion
refers to the property of
water molecules attracted to the
cellulose and lignin in the walls of the
xylem vessels.
FROM XYLEM ACROSS THE LEAF
As water evaporates from the cell walls of mesophyll cells, more water is drawn into the walls to replace it.
* This water comes from the xylem vessels in the leaf.
* Water constantly moves out of these vessels through the unlignified parts of the xylem vessel walls.
* The water then moves down a water potential gradient from cell to cell in the leaf along two possible pathways
symplastic pathway
water moves from cell to cell via the plasmodesmata
apoplastic pathway
water moves through the cell walls.
apoplastic pathway
symplastic pathway
Movement of water through xylem from root
to leaf
Removal of water from the xylem vessels in leaf, leads to a
tension[negative pressure] in the water left in the xylem
vessels or it reduces the hydrostatic pressure in the xylem
vessels
* Water potential at the top of the xylem vessel becomes lower
compared to the wp of the xylem vessel at the bottom
* This tension or this pressure difference causes water to
move up the xylem vessels
* This movement is by mass flow [means that all the water
molecules and any dissolved solutes move together, as a
body of liquid]
* Air lock – the condition when an air bubble disrupts the flow
of water in xylem. Water stops moving upwards. The pits in
the vessel walls allow water to move out into neighbouring
vessels and so bypass such an air lock
Cohesion-tension theory
says that the movement of water
in the upwards direction against gravity is guided by the
attractive forces between the particles of water which is
known as cohesion
Transpiration pull
causes a suction effect [leading to a
negative pressure] on the water column and water rises up,
aided by its capillary action.
Water is taken up by root hairs because the
water potential inside the xylem vessels is lower than the
water potential in the root hairs. Therefore, the water moves down this
water potential gradient across the root
The water takes two
routes through the cortex
– symplastic and
apoplastic pathway
Root hair increases the
surface area for water and mineral ion uptake.
Once the water reaches the endodermis,
the apoplastic pathway is blocked by a thick, waterproof, waxy band of suberin in their cell walls called the Casparian strip that forms an impenetrable barrier to water in the walls of the endodermis cells
The only way for water to cross the endodermis is
through the cytoplasm of the endodermal cells.
As the endodermal cells get older,
the suberin deposits become more extensive,
except in certain cells called passage cells, through which water can continue to pass freely.this arrangement gives a plant control over what mineral ions pass into its xylem vessels, as everything has to cross cell surface membranes.
* It may also help with the generation of root pressure.
- Once across the endodermis,
water continues to move down the water potential gradient across the pericycle and towards the xylem vessels [through the pits in their walls] It then moves up the vessels towards the leaves.
Capillary action or capillarity
is the tendency of a liquid to move up against gravity when confined within a narrow tube (capillary)
Capillarity occurs due to three properties of water:
1) surface tension
2) adhesion
3)cohesion
how does surface tension affect cappilarity
which occurs because hydrogen bonding between
water molecules is stronger at the air-water interface than among
molecules within the water
how does adhesion affect cappilarity
which is the molecular attraction between “unlike”
molecules. In the case of xylem, adhesion occurs between water
molecules and the molecules of the xylem cell walls
how does cohesion affect cappilarity
which is the molecular attraction between “like”
molecules. In water, cohesion occurs due to hydrogen bonding between
water molecules.
ROOT CAP
The tip of the roots are covered by a tough,
protective root cap and is not permeable to water known as root
cap
ROOT HAIRS
long, thin extensions in the epidermis, are called root hairs.
These reach into spaces between the soil particles, from
where they absorb water. Water moves into the root hairs by
osmosis down a water potential gradient.
soil water has a
relatively high water potential and the cytoplasm and cell sap inside the root hairs have a relatively low water potential. Water, therefore, diffuses down this water potential gradient, through the partially permeable cell surface membrane and into the cytoplasm and vacuole of the root hair cell
root hair cells provide a
large surface area, thus increasing the rate at which water can be absorbed.
Root hairs are also important for the
absorption of mineral ions such as nitrate and magnesium.
Root pressure
a force or the hydrostatic pressure generated in the roots that
help in driving the fluids and other ions from the soil into the xylem.
Root pressure is raised by the
active secretion of solutes, for example mineral
ions, into the water in the xylem vessels in the root.
Cells surrounding the xylem
vessels use energy to
pump solutes across their membranes and into the xylem by
active transport. The presence of the solutes lowers the water potential of thesolution in the xylem, thus drawing in water from the surrounding root cells.This influx of water increases the water pressure at the base of the xylem vessel
transpiration reduces the water (hydrostatic) pressure at the top of a xylem vessel compared with the pressure at the base, so
causing the water to flow up the vessels.Plants may also increase the
pressure difference between the top and bottom by raising the water pressure at the base of the vessels. Although root pressure may help in moving water up xylem vessels, it is not essential and is probably not significant in causing water to move up xylem in most plants.
Water can continue to move up through xylem even if
he plant is dead. Water transport in plants is largely a passive process, driven by transpiration from the leaves. The water simply moves down a continuous water potential gradient from the soil to the air.
Mineral ions in solution are absorbed along with
water by the roots hairs.
route for mineral ions
apoplastic and symplastic pathways ,from the xylem they enter the apoplastic and symplastic pathways
again – after reaching the target regions
- mineral ions can also move by
diffusion and active transport.
- mineral ions can also move by
diffusion, facilitated diffusion and active
transport
Facilitated diffusion and active transport allow cells to
control what ions enter or leave cells.
One important control point is the root endodermis
where the Casparian strip forces ions to pass through living cells before they can enter the xylem
Osmotic pressure is responsible for the
movement of water from the root hairs to
the cortical cells and then finally to the xylem vessels
Osmotic pressure
It is the force required to resist the movement of water through a semi-permeable membrane down the concentration gradient.
There are water molecules present in the spaces between
the soil particles.
This water is available for the plants
the imbibition of water
the imbibition of water
This is the process of absorption of water. The water then enters the cells of the root hair by osmosis. The movement of water is due to the difference in the osmotic pressure
The osmotic pressure of water in roots is high and so
the water enters the root hair cells
Water uptake in a root hair cell is by
osmosis.
Role of the Casparian strip
- It stops materials that have been moving through the apoplast and forces them to move into the cytosol of the
endodermis. - It regulates the flow of water between outer tissues and the vascular cylinder at the center of the root.
- It prevents the backflow of water and nutrients into the soil.
Mineral ions are taken up, mostly by
active transport against the
concentration gradient via carrier
proteins, but also by diffusion
PHLOEM TISSUE
complex tissue – many different cells performing one function –
transport of assimilates – H A S
phloem tissue is composed of
i. Sieve tube made of sieve elements
ii. Companion cells
iii. Parenchyma
iv. fibres
TRANSLOCATION
used more commonly to describe the transport of soluble
organic substances within a plant. e.g. assimilates
[hormones, amino acids, sucrose]
assimilates are transported in
sieve elements.
sieve elements are found in
phloem tissue, along with several other types of cells including companion cells, parenchyma and fibres
SIEVE TUBES
tube-like structures made of living cells. A sieve tube
is made up of many elongated sieve elements (also known as sieve
tube elements), joined end to end vertically to form a continuous tube
SIEVE ELEMENT
❑ is a living cell
❑ has a cellulose cell wall, a cell surface membrane and cytoplasm
containing endoplasmic reticulum and mitochondria
❑ thin layer of cytoplasm
❑ no nucleus, no ribosomes
❑ end walls present - where the end walls of two sieve elements
meet, a sieve plate is formed. This is made up of the walls of both
elements, perforated by large pores
COMPANION CELLS
❑Each sieve element has at least one companion cell lying close beside it.
❑Companion cells - cellulose cell wall, a cell surface membrane,
cytoplasm, small vacuole , nucleus, high number of mitochondria and
ribosomes
❑the cells are metabolically very active
❑Companion cells are very closely associated with their neighboring sieve elements - regarded as a single functional unit.
❑Numerous plasmodesmata pass through their cell walls, making direct contact between the cytoplasm of the companion cell and that of the sieve element.
Phloem sap:
The liquid inside phloem sieve tubes
Callose:
When a sieve tube is cut, the release of pressure inside the
tube causes a surge of its contents towards the cut. When the contents
come up against a sieve plate, they may block it. This helps to prevent
escape of the contents of the sieve tube. Then, within minutes, the
sieve plate is properly sealed with a carbohydrate called callose, a
process sometimes called ‘clotting’.
The contents of phloem sieve tubes
– sucrose, potassium ions, amino
acids, plant growth substances [like auxin, cytokinin, gibberellin]
Chloride ions, phosphate ions, magnesium ions, sodium ions, ATP,
nitrate ions,
SOURCE
Any area of a plant in which sucrose is loaded into the phloem
is called a source. This is usually a photosynthesising leaf or a storage
organ.
SINK
Any area where sucrose is taken out of the phloem is called a sink
– for example, young leaf, bud, flower or root, or a storage point, e.g. seed,
fruit or tuber. Sinks can be anywhere in the plant, both above and below
the photosynthesizing leaves . Thus, sap flows both upwards and
downwards in phloem (in contrast to xylem, in which flow is always
upwards
In some plants, aphids may be used to
sample sap. Aphids such as greenfly
feed using tubular mouthparts called stylets. Phloem sap flows through
the stylet into the aphid. If the stylet is cut near the aphid’s head, the sap
continues to flow and can be collected.
Transport of assimilates in phloem is an
energy requiring process
Mass flow occurs because of the pressure difference created by
active loading of sucrose into the sieve elements at the source
Loading a high concentration of sucrose into a sieve element from the
source greatly decreases the
water potential in the sap inside it.
Therefore, water enters the sieve element, moving down a water
potential gradient by osmosis. This causes a correspondingly high build
up in pressure (equivalent to about six times atmospheric pressure).The
pressure is referred to as hydrostatic pressure, turgor pressure or
pressure potential [Ψp]. A pressure difference is therefore created
between the source and the sink. This pressure difference causes a mass
flow of water and dissolved solutes through the sieve tubes, from the high
pressure area to the low pressure area
At the sink, sucrose may be
removed and used,
causing the water to follow by osmosis, and thus
maintaining the pressure gradient
Loading sucrose into phloemLoading sucrose into phloem
- In leaf mesophyll cells, photosynthesis in chloroplasts produces triose sugars, some of
which are converted into sucrose. - The sucrose, in solution, then moves from the mesophyll cell, across the leaf to the
phloem tissue - by the symplastic pathway, moving from cell to cell via
plasmodesmata or by the apoplastic pathway - The companion cells and sieve elements work together.
- Sucrose is loaded into a companion cell or directly into a sieve element by active transport.
- Hydrogen ions (H+) are pumped out of the companion cell [active transport] into its
cell wall, using ATP as an energy source through PROTON PUMPS. This creates a large excess of hydrogen ions in the apoplastic pathway outside the companion cell.
The hydrogen ions can move back into the cell down their concentration gradient[facilitated diffusion], through a protein that acts as a carrier for both hydrogen ions and sucrose simultaneously. The sucrose molecules are carried through this co-transporter molecule into the companion cell, against the concentration
gradient for sucrose. - The sucrose molecules can then move from the companion cell into the sieve tube, through the plasmodesmata which connect them (the symplastic pathway).
Unloading sucrose from phloem
- Unloading occurs into any tissue which requires sucrose.
- using both symplastic and apoplastic routes
- Phloem unloading requires energy, and similar methods to those used
for loading are probably used. - Once in the tissue, the sucrose is converted into something else by
enzymes, so decreasing its concentration and maintaining a
concentration gradient. - One such enzyme is invertase, which hydrolyses sucrose to glucose and
fructose
XYLEM
passive process – pressure difference created by the water potential gradient in between soil and air causes mass flow
Mass flow is unidirectional [from soil to root to leaf to air
Movement by mass flow down a water potential gradient
Xylem vessel formed by vessel elements stacked end to endHappens in dead cells
Contents can leak out as they are non living tubes
Have lignified cell walls
End plates are destroyed so no resistance in mass flow
phloem
active process –pressure difference is created by active loading of sucrose into the sieve elements at the source
Mass flow is bidirectional [from source to sink]
Movement by mass flow down a pressure gradient
Sieve tube formed by sieve elements stacked end to end
Occurs in living cells
Entry and exit of contents are well controlled by living membranes
no lignified cell walls
Sieve plates present[helps in pressure build up]; but absence of many organelles including nucleus reduces resistance
Xylem on the other hand has to withstand
high negative pressure (tension) inside its
tubes and buckling is prevented by its
lignified walls.
.Callose formation:
Phloem sap has a high turgor pressure
because of its high solute content, and
would leak out rapidly if the holes in the
sieve plate were not quickly sealed.
* help to prevent the entry of microorganisms
which might feed on the nutritious sap or
cause disease
there are two factors that determine the water potential of a solution
the concentration of the solution, and the pressure applied to it.
solute potential :
The contribution of the concentration of the solution to water potential is called solute potential.
We can think of solute potential as being the extent to which the solute
molecules decrease the water potential of the solution
The more solute there is, the lower the tendency for water to move out of the solution. Just like water potential, solute potential is 0 for pure water, and has a negative value for a solution. Adding more solute to a solution decreases its water potential. So the greater the concentration of the solute, the more negative the value of the solute potential. The psi symbol can be used to show the solute potential, but this time with the subscript s – ψs
pressure potential :
The contribution of pressure to the water potential of a solution is called pressure potential ; it increases its water potential. Pressure potential can be shown using the symbol ψp
water potential is a combination of
solute potential and pressure potential. This can be
expressed in the following equation:
ψ = ψs+ ψp
Adaptations of xerophytes:
- small leaves / needles / needle-like leaves; R ‘spines’ / thorns / narrow / fewer leaves
- reduce / small surface area;
- temporary / shed leaves;
- leaves dry out and then rehydrate;
- fleshy leaves / succulent leaves / leaves with hypodermis;
- curled / rolled, leaves; R curved /
folded / coiled - (very) thick / waxy / impermeable, cuticle;
- stomata surrounded by hairs / hairy leaves / hairs trap moisture;
- sunken stomata / stomata in pits / crypts / grooves;
R inverted / few stomata - stomata closed during the day / stomata open at night;
max 2 for features given above - (so) reduces / slows down (rate of) transpiration / water loss /
Pathway of water from root hair cells to xylem vessels:
- through cortex / via cortical cells ;
apoplast pathway - (by) via cell walls (of adjacent cells) ; R if named as symplast pathway ;
symplast pathway - via cytoplasm and plasmodesmata ; R if named as apoplast pathway
- ref. vacuolar pathway ;
- ref. apoplast to symplast / pathway described, at endodermis ;
- (via) passage cells ;
- ref to, suberised / Casparian, strip ; in correct context
Explain how the structure of sieve tube elements helps the translocation of substances in the phloem.
- little/watery/peripheral, cytoplasm/no tonoplast/no vacuole/ few organelles/few ribosomes/so little resistance/AW e.g. easy transport/move more easily/minimum obstruction;
- pores in sieve plate provide little resistance/permit continuous flow/allows movement/AW e.g. as above;
- sieve plate braces/prevents cell bulging under pressure/collapsing;
- plasmodesmata only between sieve tube element and companion cell allows pressure to build up;
- plasmodesmata allows loading/AW e.g. sucrose to be transported in from companion/transfer cell;
- (strong) cellulose walls prevent, excessive/too much, bulging/expansion;
- mitochondria (and starchy plastids) for ATP, for repair/maintenance;
R reference to mitochondria in companion cells
Describe the role of companion cells in translocation in the phloem.
- sucrose/sugars/assimilates, are pumped/loaded (by companion cells);
- reference to pumping H+;
- reference to co-transport/AW e.g. H+ carry sucrose with them;
- mitochondria provide, ATP for active transport;
Explain how the sucrose is transported in phloem along the stem from the leaf to the fruit.
- (sucrose) loaded at, source / leaf;
- role of companion cells;
- further detail, e.g. H+ pumped out, sucrose moves in through co-transporter;
- absorption of water / water enters by osmosis;
- hydrostatic pressure builds up;
- mass flow;
- (sucrose) unloaded at, sink / fruit / root / AW;
- gives a difference in pressure (between source and sink);
Describe how the assimilate is moved from source to sink.
- H+ / protons, (move) out of companion cells by, active transport / AW ; R diffuse by active transport
- H+ / protons, diffuse (back) in with / cotransport sucrose, into companion cells ; A description of (facilitated) diffusion R active transport (ref. to companion cell required only once for mps 1 and 2)
- via, cotransporter / cotransporter described ;
- sucrose, diffuses / AW, into (phloem) sieve, tube / element, via plasmodesmata ;
- (entry of sucrose into sieve tube so) water potential lowers ;
- water enters by osmosis ;
- (hydrostatic) pressure builds up ; A pressure difference created
- unloading at, sink / named sink, gives a difference in pressure (between source and sink) ; AW
- (so) mass flow ; term to be used in context
Function of water stored in the vacuoles of plant cells
- (raw material) for photosynthesis; A for photolysis
- maintains turgidity / provides support;
- pushes chloroplasts to edge of cell;
- used in hydrolysis reactions;
- solvent for, ions / named ion / pigment / named pigment;
Describe and explain how xylem is suitable for its function: [5] [each numbered point counts as 1 mark]
Transport of mineral ions and water;
Elongated cells, end to end forms tube for transport
No endwalls allows water column to flow unimpeded/uninterrupted/ minimal resistance AW
Hollow, no cytoplasm, minimal organelles; more space for greater volume of flow; allows wat
er acolumn to flow unimpeded/uninterrupted/ minimal resistance
Cellulose lining (A cellulose walls) so hydrophilic adhesion of water molecules to maintain water column
Lignified walls to prevent collapse/to withstand negative pressure (R prevents bursting)
Lignified walls so waterproof/prevents loss of water/ prevents leakage
Additional ref. To lingin eg. for support of plant
Pits in walls allows sideways movement of water/ connects all parts of the plants
Ref. to diameter of lumen eg. thin for adhesion, not breaking water column
Explain how structure of sieve tube elements helps in translocation of substances in phloem:
Little cytoplasm, very few organelles so easy movement/transport
Sieve plates with pores to provide little resistance to flow
Sieve plate braces prevent collapse of under pressure
Plasmodesmata allows loading or sucrose to be transported from companion cell
Strong cellulose wall prevents extra bulging
Many mitochondria for ATP for repair R reference to mitochondria in companion cells
Describe role of companion cells in translocation in phloem:
sucrose/assimilates are loaded/pumped
Reference to pumping of H+
Reference to co-transport
Mitochondria to provide ATP for active transport
With reference to structure of a cell, explain the difference between evaporation and transpiration:
Evaporation is the conversion of water into water vapour (from liquid to gas form)
At the surface of cell wall on mesophyll layer
Using heat energy
Transpiration is the loss of water vapour from the leaf
Through diffusion from area of high water potential to area of low water potential down the gradient
Through the stoma
Why is there a low rate of transpiration at night?
At night, the stoma closes
It is closed to prevent water loss
There is no sunlight for photosynthesis
Explain why rate of water uptake in a potometer will NOT be the same as the rate of transpiration
Water uptake may not all be lost by transpiration
It could be used for other things such as photosynthesis, hydrolysis reactions, maintaining turgidity
Explain the statement “transpiration is an inevitable consequence of gas exchange in leaves”
Stoma is open for gas exchange
For CO2 to diffuse into leaves for photosynthesis
Water vapour diffuses out of leaf through stoma down water potential gradient
Explain how hydrogen bonding is involved in the movement of water in the xylem
adhesion of water to, cellulose / lining / walls (of xylem vessels) ;
A adhesive force
ref to, hydrophilic / polar, property of cellulose (fibres) ;
A hydrophilic / polar, parts of lignin
cohesion between water molecules ; cohesive force
maintains column of water / prevents water column breaking / AW ;
ref. to transpiration pull / AW ; I transpiration unqualified
Outline the properties of water that contribute to the apoplastic movement of water to the spongy mesophyll cells and to the movement of water into the intercellular air spaces.
hydrogen bonding (between water molecules) ;
water molecules are polar ;
movement to spongy mesophyll cells
adhesion / attraction, to, cellulose / cellulose fibres / cell walls ;this is in context of leaf cells but also allow for xylem R cell walls of lignin A hydrophilic parts of lignin
cohesion between water molecules / (water molecules are) cohesive ;
idea that movement of water (molecules) towards, spongy / mesophyll, cells, pulls / AW, other water molecules ; A transpiration pull / continuous column / unbroken column I continuous stream
movement to intercellular air spaces
water molecules absorb heat (energy) ;
bonds break between water molecules ;
evaporation / water to water vapour ; I latent heat of vaporisation
from spongy cell, walls / surfaces ;
Cell wall of xylem is made up of
LIGNIN AND CELLULOSE
xylem vs phloem
Is translocation an active process?
YES
-At source, the specific proton pump carrier proteins on the cell surface membrane
f companion cells pumppumps protons (H+ ions) into the cell wall of the source cells, creating a proton gradient that will cause the protons to come back to the companion cells but associated with sucrose from the source through specific cotransporter proteins in the cell surface membrane of the companion cell by facilitated diffusion down the concentration gradient; then, the sucrose moves intoin to the phloem sieve tube elements by simple diffusion in a symplast pathway through plasmodesmata while the proton pumps are pumping more protons to intake more sucrose, and this is the process of Active loading
The movement of sucrose into the phloem sap in the phloem sieve tube element
increases the solute potential in the phloem sieve tube element, causing the water potential to drop, so water from nearby cells move by osmosis down water potential gradient into the phloem sieve tube elements nt. This will cause the hydrostatic pressure at source to be very high.
the movement of sucrose from the phloem sieve tube elements (near the sink) to the sink
causes the solute potential in the phloem sap to decrease increasing the water potential causing water to move by osmosis down water potential gradient to OTHER surrounding cells from the phloem sieve tube elements (near the sink). This is why there is low hydrostatic pressure near the sink.
The high hydrostatic pressure at source,
and the low hydrostatic pressure at sink, generates pressure difference at the two causing the phloem sap to move by mass flow theory.
at sink, the proton pumps on the cell surface membrane of the companion cells pumps protons into the cell walls of the
phloem sieve tube elements to also create a proton gradient and protons come back to the =companion cells associated with sucrose through cotransporter proteins, then sucrose goes to sink cells through plasmodesmata by simple diffusion in a symplast pathway to be then used in aerobic respiration or to be stored in form of starch (amylose and amylopectin)
H+ ions pumped out of companion cells into the cell wall of source cells
his creates a proton gradient so H+ moves back into the companion cell and co-transports sucrose with it. Sucrose moves into companion cells and then can move to phloem sieve tubes by plasmodesmata. This decreases the water potential of phloem sieve tubes near source cells. So water moves in from the xylem hence hydrostatic pressure increases at source. At the sink, there is a high water potential so it moves out of the sink by osmosis and has low hydrostatic pressure. The phloem sap moves down the hydrostatic pressure gradient by mass flow transporting sucrose with it to the sink. The sucrose unloads at the sink.
Outline the importance of water as a solvent in plants.
Dissolves mineral ions/salts/named polar compounds
For transport in xylem and phloem
Many metabolic reactions occur in the water
Dissolved oxygen and carbon dioxide for photosynthesis and respiration
Storage of solutes in water
Symplast is known to be the
cytoplasmic connection and does not include
intercellular spaces, cell walls etc.
some soil borne fungi cause wilting in crop plants by growing within
xylem vessels. which process will be directly affected by these fungi?
they re basically stopping the flow of water so conduction in the apoplast
is affected
processes involved in transpiration
- diffusion of water vapour thru stomata
- the mass flow of water thru the xylem
- evaporation of water from spongy mesophyll cells
- evaporation of water vapour from exposed leaves
- evaporation of water vapour from exposed some extra stuff based on graphs (idt its important)
- xylem sucrose conc remains constant thru out 24 hours
- phloem and leaves sucrose conc is highest during the night and
lowest during the day
why is stem cut + connected to potometer under water?
to prevent air entering the xylem vessels
which feature of xylem vessels helps the cohesion of water?
the vessel elements form narrow tubes
what determines the rate of water movement from root → leaves?
evaporation of water from the mesophyll cell walls
advantage of transpiration to the plant
stomata are open for gas exchange
how will the root pressure in a plant be affected by waterlogged soil?
it will decrease due to a lack of oxygen in the soil
in order for root pressure to form, cells surrounding the xylem actively
secrete mineral ions etc. into the xylem. Active secretion requires ATP
from mitochondria which in turn require oxygen to function efficiently.
more active secretion → more root pressure
less oxygen → less atp cuz mitochondria needs o2 → less active
secretion → less rp
xylem is made up of
dead cells
metabolic poison affects transport in phloem not
xylem (dead cell thing)
The amount of hormones detected is dependent upon the cell surface
area, because
this will determine the number of glycoproteins and
glycolipids which are detectors for hormones. [co2 produced is
dependent on cell volume]
solute potential of phloem sieve tube is low cuz
mass flow and
everything being transported is dissolved only
diameter of tree trunk decreases during the day because
the cohesive
tension forces increase during the day (why? cuz of more transpiration)
transpiration more →
cohesive tension forces more → diameter of tree
trunk less
swollen leaves means
out of proportion (they do help reduce water loss)
pressure potential is never
negative
water potential will approach zero in the conditions described
[plasmolysed plant cell is placed in pure water]
water travelling through the apoplastic pathway will only be subjected to
cohesive forces (symplastic is due to differences in water potential)
as in plain diffusion,
both directions (even if its
in an isotonic solution, there is movemen tin both directions ig there is
just no net movement)
casparian strip is present in
the root/endodermis (so root hair cell →
xylem is by symplast) - so in xylem, its only symplast
xylem vessels could collapse due to
increased tension (but they have
lignin so they good)
companion cells have peripheral cytoplasm w
no nucleus to provide as
little resistance to flow as possible
water arriving at the spongy mesophyll cells via the
symplast pathway
must move by osmosis through the cells surface membrane before
evaporation from the surface of the cells
being moved into sink wp
increases and volume of liquid decreases (cuz
we need less hydrostatic pressure)
being taken out of sink wp
decreases and volume of liquid increases
xylem has thick
lignified walls
water can pass thru
lignifief cell walls (via pits)
transpiration transfers
heat energy
large numbers of mitochondria is not an adaptation for
water uptakev from soil bevause water uptake is a passive process
stomata r more likely to close when
soil wp is low (less water enters plant, so plant wants to minimise water loss)
pathway of water in root
epidermis →
cortex —> endodermis [casparian strip] —> pericycle —>
xylem
transpiration factors
