Exchange of Water and Ions in Plants
Some
of the sections in these notes may seem rather repetitive and, to an extent,
they are! This is intentional. The problem is that candidates tend not to answer
questions as they are set on this section of the syllabus.
When asked to relate the structure of a root to its function, they will talk
about the apoplast and symplast, or the transpiration stream. If asked about
how external factors affect transpiration, they will talk about the cohesion
tension theory. Asked about the structural modifications of xerophytes, they
talk about external factors affecting transpiration! The notes have been
constructed to enable you to relate questions to specific part of the syllabus.
Extracts from the syllabus are quoted in italics and
are then followed by an explanation of what it means - what you need to know or
do.
You
should know that plants take in water and mineral salts from the soil.
· Like all land-living
organisms, plants need a supply of water to replace water lost to the
environment; especially the air.
· They use their roots to get
water from the soil.
· Plants also need a supply of
mineral ions, such as nitrate, magnesium and phosphate.
· These mineral ions are found
as mineral salts in solution, in soil water.
· The concentration of these
ions in the soil water is usually lower than in the cells of the plant.
· With a supply of mineral
ions, plants can make all the biological molecules they need from the products
of photosynthesis.
You
should know that much of the water taken in by the plants is lost via
transpiration.
· All land-living/terrestrial
organisms lose water to the atmosphere and they all have adaptations to reduce
and control this water loss.
· Plants have a waterproof, waxy cuticle covering the leaves
and green stems.
· This allows very little
water out and the thicker the cuticle, the less water gets through.
· The surfaces of leaves can not be covered by an unbroken
layer of cuticle, because the plant has to carry out gaseous exchange, to get carbon dioxide and oxygen in and out.
Note. Gaseous exchange in plants
is covered in sections 3.2 and 3.3 of By03.
· There are openings in the leaf for gaseous exchange
which are called stomata (singular,
stoma).
· These are small pores,
guarded on each side by special guard
cells.
· The guard cells can alter the size of the stomatal opening.
· Most of the water taken up by the roots is eventually lost through the
stomata.
· This loss is the most
important part of water loss by transpiration.
· The stomata open onto air spaces in the leaf which are lined by leaf
cells.
· The cell walls of these cells are wet.
· If the leaf is warmed up by absorbing energy from sunlight, the
water in the cell walls begins to evaporate into the air spaces as water
vapour.
· It is this evaporation of water that provides the
energy for transpiration.
· The air spaces in the leaf become saturated
with water vapour.
· Under normal environmental
conditions, the external atmosphere will contain less water vapour.
· This means that there is a higher concentration of water in the air
spaces than in the atmosphere.
· The air in the air spaces
has a higher/less negative water potential than the atmosphere.
· As a result, water vapour diffuses out of the stomata
along a water potential/concentration gradient.
· The opening and partial
closing of the stomata by the guard cells is important in controlling water
loss by transpiration.
· Stomata are usually found on
the lower surfaces of leaves, to reduce transpiration.
· Since warm, moist air would
rise by convection through stomata on the upper surface of a leaf.
· Only a very, very small amount of the water taken up by a plant is used
for photosynthesis.
You
should know that Xerophytic plants may have structural modifications to reduce
the rate of transpiration.
· Xerophytic plants have modifications
which allow them to live in environments where there is a shortage of water.
· This may be due to a low
annual rainfall, as found in dry deserts.
· It might be because the
water in the soil is frozen for much of the year.
· It might be because the soil
water contains a lot of dissolved mineral ions and has a water potential
lower/more negative than the roots of normal plants, making it difficult to
take up water. This is found in places like salt marshes.
· Xerophytes have any of the
following structural modifications to
reduce water loss by transpiration.
· Thicker cuticle. Many xerophytes have a very thick waxy
cuticle over their leaves and stems. As a result, they often appear to have
very glossy leaves.
· Smaller leaves. This reduces the surface area for losses by
evaporation (through the cuticle).
· It also reduces the surface
area for stomata to be present.
· Leaves are often reduced to spines, with photosynthesis
being carried out by a modified stem.
· This stem is likely to be
rounded, to give a smaller surface area for evaporation.
· Curling of leaves. Some plants have leaves which curl, so that
the lower surface is not exposed directly to the atmosphere.
· The air inside collects some
of the water vapour lost from the stomata.
· This reduces the water
potential/concentration gradient for the diffusion of water from the stomata
and reduces transpiration.
· Air moving across stomata increases the rate of transpiration.
· Curling the leaf reduces air movements past the stomata.
· Hairs. Hairs on the lower surface of the leaf/stem reduce
air movements over the stomata.
· Stomata in pits. The stomata are found in pits in the lower
surface of the leaf. The pits often contain hairs as well.
· This has the same sort of
effect as curling the leaves. Hairs reduce the air movements in the pits even
more.
You
should be able to relate the structure of a primary dicotyledonous root to its
functions.
· The functions of the root
are;
· to take
up water from the soil,
· to
take up mineral ions from the soil water,
· to
anchor the plant in the soil and prevent it falling over.
· The structure is shown in
the diagram.
· Near the root tip, the root
is covered with a layer of epidermal
cells.
· Water and mineral ions enter
the root through his layer.
· Some epidermal cells have
long outgrowths called root hairs.
· These increase the surface
area for the absorption of water and mineral ions.
· The cells of the cortex are packing /parenchyma cells.
· They give some stiffness to
the root, through being turgid (full of water).
· The endodermis is a layer of cells that surrounds the vascular tissue
(xylem and phloem) in the centre of the root.
· The endodermal cells have
waterproof strips of suberin in their cell walls called Casparian strips.
· They control the entry and
exit of water and mineral ions from the xylem.
· The xylem carries water and
mineral ions in solution up to the shoot.
· The xylem cells have highly
thickened cell walls.
· The thick cell walls also
make the vascular tissue in the centre of the root very strong.
· This makes the root less
likely to snap when it is pulled on by the stem - for example, during a storm.
· The phloem carries dissolved
sugar/sucrose and amino acids up and down the plant - depending upon where they
are needed.
You
should understand the role of the apoplast, symplast and vacuolar pathways in
the movement of water from the soil to the xylem across the root; and from the
xylem out through the stomata of the leaf.
· There are three
pathways/routes by which water can move from one plant cell to another.
· Apoplast pathway. (between the cell walls)
· The cell walls of most plant cells are freely permeable to water and substances in solution.
· This is because they are
made of a meshwork of cellulose fibres, with microscopic spaces between them.
· It means that most plant
cells can not regulate the movement
of solutions along this pathway.
· Movement will be by diffusion, along concentration
gradients/water potential gradients.
· The cell walls of
neighbouring cells are in close contact with each other.
· This means that water and
mineral ions can move along the apoplast
pathway from one cell wall to the next.
· Symplast pathway. (through the cytoplasm of the cells)
· To move through the cytoplasm of living plant cells, water and mineral
ions have to enter the cell across a
cell surface membrane.
· This membrane is selectively permeable.
· Osmosis and active transport take place across the
membrane.
· The cells can regulate the entry of water and different
types of mineral ion.
· The cells can take in
essential mineral ions against the
concentration gradient, using ATP from respiration.
· Active transport of mineral
ions can generate water potential
gradients.
· This allows water to enter
the cells/uptake of water by osmosis.
Note. This is synoptic with
sections 1.3, 1.4 and 1.5 of By01.
· Once in the plant cell,
water and mineral ions can move directly to neighbouring plant cells through plasmodesmata.
· These are strands of
cytoplasm that connect the plant cells.
· There is no membrane barrier across these
strands.
· Vacuolar pathway. (through the cell vacuoles)
· Once inside the plant cell,
water can travel through the cell vacuole.
· It has to cross the membrane
surrounding the vacuole to get in and out.
· This means that water enters and leaves the vacuole by
osmosis.
· This can be considered an
extension of the symplast pathway.
· The diagram represents the
three pathways in the root;
A is the apoplast,
B is the symplast,
C is the vacuolar.
Note, only the symplast and vacuolar
pathways are possible through the endodermis - see below.
· Water and dissolved mineral
ions enter the root through the apoplast and symplast (and vacuolar) pathways.
· Soil solution can move freely into the cell walls of the epidermal cells and
cells of the cortex.
· Epidermal cells can selectively take up certain mineral ions by
active transport.
· This creates a far higher
concentration inside these cells than that of the soil solution.
· The water potential of the
epidermal cells is lower/more negative than soil solution.
· As a result, water will enter the cells by osmosis.
· This brings the water (and mineral ions) into the symplast pathway.
· The problem in moving water further than the cortex is that the xylem is made up of dead cells, with no
cell membrane.
· Xylem cells can not take up
mineral ions by active transport, or water by osmosis.
· Xylem cells can not regulate what enters and leaves them.
· The endodermal cells control the entry of water and mineral ions into the
xylem.
· The endodermis is a layer of
cells that surrounds the vascular tissue.
· The cell walls of these
cells contains a strip of suberin, the
Casparian strip.
· This is a waxy, waterproof,
material that blocks the apoplast
pathway.
· This means that water and mineral ions entering (or leaving) the xylem
have to pass through the living endodermal cell.
· To do this they have to
cross a cell membrane and become part of the symplast pathway.
· Endodermal cells selectively
pump mineral ions into the xylem by active
transport
· and water passes through
them by osmosis into the xylem.
· Water moves up the xylem in the
root and stem in the transpiration
stream. · The
water passes into the xylem in the veins
of the leaf.
· It then enters the apoplast pathway in the cell walls of
the leaf cells.
· Some of the water will cross
the cell surface membranes of the leaf cells by osmosis and enter the symplast
pathway.
· Eventually, the water
reaches the cell walls of leaf cells that line one of the air spaces in the leaf.
· The water will evaporate and form water vapour in the
air in the air space.
· The energy to evaporate the water comes from sunlight.
· Water molecules then diffuse out of the stomata and are lost
to the atmosphere.
· This happens because of the water potential gradient between the
air space and the atmosphere.
You
should understand how external (light, temperature, humidity, air movement) and
internal factors affect the rate of transpiration.
· One of the main factors
affecting the rate of transpiration is how wide open the stomata are.
· In most plants, the stomata
open wider as light intensity increases (up to an optimum value).
· This allows more rapid
exchange of gases (especially carbon dioxide) and a higher rate of
photosynthesis.
· At night (in the dark), the
stomatal openings become much smaller.
· This reduces water losses
but still allows enough oxygen to enter for respiration.
Remember,
all cells respire all the time!
· Temperature affects the rate
at which water evaporates into the air spaces in the leaf.
· It also affects the rate of
diffusion of water molecules.
· A rise in temperature
increases both the rate of evaporation and diffusion.
· Temperature will tend to
rise towards the middle of the day, fall towards evening and reach a low point
overnight.
· Transpiration rate will tend
to rise and fall with temperature and, of course, the opening of the stomata in
response to light.
· The rate at which water
diffuses out of the stomata depends to a large extent on the size of the water potential gradient
between the air spaces in the leaf and the atmosphere.
· The more humid the atmosphere is, the smaller the water potential gradient
and the slower the rate of diffusion and transpiration.
· Obviously, a very dry
atmosphere will lead to a rapid rate of diffusion and transpiration.
· The steepness of the water potential gradient also affects the rate of
diffusion.
· In still air, the
concentration of water molecules decrease gradually with distance from the
stomata.
· If air is moving past the surface of the leaf, the steepness of the water
potential gradient increases.
· This is because the moving
air takes water molecules away from the opening of the stomata.
· The concentration of water
molecules decreases very rapidly with distance from the stomata.
· The steeper gradient leads
to a faster rate of diffusion and transpiration.
· The faster the air movement over the surface of the leaf, the faster the rate
of transpiration.
· One of the main internal
factors affecting the rate of transpiration is the amount of water in the leaf.
· On days which are hot, or
very dry, or very windy (or a combination of the three), the rate of water loss
by transpiration may be greater than the rate at which the roots can take up
water.
· If this happens, the cells
in the leaves will start to lose some of the water in their vacuoles which
keeps them turgid/hard.
· This makes the leaf tissues
softer and the leaf will start to droop/wilt.
· The guard cells will also lose turgor/become less turgid.
· This will lead to closing of the stomata and a reduction
in transpiration.
· This will reduce the rate of
photosynthesis but save the plant from death by desiccation/drying out.
· It is not unusual to find at
least a partial closure of stomata during the hottest part of the day.
· If the roots are short of
water, they may produce a plant hormone ABA.
· This travels up in the
transpiration stream and has its effect on the guard cells.
· It causes them to close the
stomata and thus reduce water loss by transpiration.
You
should be able to interpret evidence for the ‘starch-sugar’ and potassium movement’
hypotheses of the mechanism of stomatal movement.
· There are two main
hypotheses about how stomata open and close.
· It seems that some plant
species may use one of these and some species the other, so both may be thought
of as true!
· Both involve a lowering/making more negative of the water potential of
the guard cells.
· This leads to water entering
the guard cells from neighbouring epidermal cells by osmosis.
· The guard cells expand slightly as they fill with water/become more
turgid.
· The outer cell wall of the
guard cell is thinner than the inner one (the one next to the stoma).
· Because of the uneven thickening of their cell walls,
the guard cells bend as they expand.
· This bending opens the stoma wider.
· Starch-sugar hypothesis.
· The guard cells are the only
cells of the leaf epidermis that contain chloroplasts.
· During the day/in light
their rate of photosynthesis is greater than their rate of respiration.
· So, their chloroplasts use
up carbon dioxide in photosynthesis faster than it is produced in respiration.
· This leads to a slight
increase in the pH of the guard cells, because there is less carbonic acid
formed from carbon dioxide.
· This increases the activity
of an amylase enzyme which coverts insoluble starch into soluble sugars.
· The increase in
concentration of these sugars lowers/makes more negative the water potential of
the guard cells and leads to stomatal opening.
· At the end of the day, the
rate of photosynthesis falls below the rate of respiration in the guard cells
and carbon dioxide increases in the cells.
· This lowers the pH in the
cells and increases the activity of a phosphorylase enzyme, which converts
soluble sugars into insoluble starch.
· This raises/makes less
negative the water potential of the guard cells.
· Water leaves the cells by
osmosis, the cells become less turgid and the stoma closes.
· Potassium movement hypothesis.
· This involves the pumping /active
transport of potassium ions into the guard cells from neighbouring epidermal
cells.
· This is linked to the light
dependant stage of photosynthesis in the chloroplasts of the guard cells.
· In the light, K+
ions are actively transported into the guard cells and lower/make more negative
their water potential.
· Water enters by osmosis, the
guard cells become turgid and the stoma opens.
· You may be asked to
interpret evidence for one of these mechanisms for stomatal opening.
· You should look for
disappearance of starch, appearance of simple/reducing sugars, the role of ATP
and changes in water potential in water cells.
· You might be data concerning
radioactively labelled potassium ions, relating to their movements.
· You must know that a lower/more negative water potential will lead to
the uptake of water by guard cells by osmosis; which then leads to stomatal
opening.
You
should be able to explain how xerophytic modifications reduce the rate of water
loss.
· The thick, waxy, cuticle of xerophytes reduces water losses by evaporation through the surface of the
leaf/stem.
· Most xerophytes have a reduced surface area to volume ratio.
· This also reduces the
surface area for water loss by evaporation.
· The reduction in surface
area may involve leaves with a reduced surface area, as in needle-like leaves.
· It might involve leaves
reduced to spines/or absent (with
the stem taking over photosynthesis).
· In many xerophytes the
stomata are protected, to reduce the rate of diffusion/transpiration of water
molecules out of the plant.
· The stomata may be in pits in the lower surface of the leaf.
· The surface of the leaf may
be covered in hairs.
· Both of these produce still air over the openings of the
stomata.
· This leads to a build-up of water vapour/higher humidity
next to the stomata.
· This reduces the water potential gradient from the stomata to the atmosphere
and reduces the rate of diffusion of water molecules/transpiration.
· The still air also reduces the steepness of the diffusion gradient from
the stomata to the atmosphere and reduces the rate of transpiration.
· The leaves of the xerophyte
may roll-up, to protect the stomata.
· They now face out into the
still air inside the rolled-up leaf.
· The advantages of still air
then apply.
· Some xerophytes only open
their stomata at night, when the temperature is much lower and water loss is
much less.
· These plants have mechanisms
for storing carbon dioxide at night.
· This carbon dioxide is then
used during the day, when the stomata are closed to save water.
Common
Mistakes
Not understanding/mentioning the importance of the
evaporation of water to form water vapour, and the sun providing the necessary
heat. This is what drives transpiration.
Not using correct terms and concepts, such as
diffusion, osmosis, water potential, active transport, movement along gradients.
Not appreciating the links between active uptake of
mineral ions from the soil and respiration and ATP.
Confusing the apoplast and symplast.
Not understanding the role of the endodermis in
controlling the entry of ions to the xylem via the symplast.
Not being able to explain how external factors affect
the rate of transpiration in terms of evaporation rate, diffusion gradients and
water potential gradients.
Over-complex explanations (often very confused) of
stomatal opening and closing mechanisms.
