The organisation of plant cells into tissues and organs - their structure and
leaf structure-adaptations, the transport and gas exchange in plants & absorption of nutrients
- importance and deficiency problems,
potometer - measuring rates of transpiration experiments
Diffusion, osmosis, transport and active transport
school biology revision notes: GCSE biology, IGCSE biology, O level
biology, ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old
students of biology
This page will help you answer questions
such as ... How does gas exchange take place in
plants? How does a plant absorb mineral
nutrients? How does a plant transport minerals
around in itself?
Sub-index for this page
Types of plant cells and their organisation into tissues
Flowering plant transport systems structure and
function of plant xylem and phloem cells
Structure and function of roots, absorption &
necessity of nutrients, osmosis
The importance of minerals to
- need for absorption of
nutrients through the roots
Investigating minerals and plants, effect of
mineral deficiencies, relevance of fertilisers
More on leaf adaptations to aid photosynthesis
Factors affecting the rate of transpiration and the
function of the stomata and guard cells
More on the environmental factors affecting the rate of
water loss - rate of transpiration
An experiment to investigate the rate of transpiration
- using a potometer
Types of plant cells and their organisation into tissues
Plant cells are organised into
tissues and these form plant organs such as leaves, roots and stems.
These organs must function together
in such way to work as an organ system - to ensure the plant gets
all its needs to survive and grow into a mature plant.
Transportation of e.g.
nutrients or waste products is one of the most important function of a
plant organ system.
Plant organs are made of
summary of those
you must know are briefly described below and in more detail later where
Epidermal tissue covers the
whole surface of the plant - its the equivalent of our 'skin'!
Meristem cells are found in
the growing tips of shoots and roots.
They can differentiate into
all the different types of plant cell needed for growth and
Palisade mesophyll tissue
Most photosynthesis occurs in
the palisade mesophyll tissue, part of the leaves.
Spongy mesophyll tissue
Spongy mesophyll tissue forms
part of the leaf and contains lots of air spaces to let gases
diffuse in and out of the leaf structure.
Xylem and phloem
Xylem and phloem are tubular
cell networks that allow the transportation of mineral ions,
food e.g. sugars and water around the plant - the leaves, roots
and stems must be all connected together.
The cuticle is
a water repellent protective layer covering the epidermal cells
of leaves and other parts and limits water loss.
and this is how some of the above fit
together in the structure of a
More on the
tissue structures of a leaf and their functions - adaptations of leaves
So, starting from the top layers, and
all marked on the above diagram ...
The epidermal tissue on the
upper side of the leaf are covered with a waxy cuticle layer
which is water repellent - this helps water loss by evaporation.
The upper epidermal layer is
transparent to visible light, so light can penetrate to the
palisade cell layer where it is needed for photosynthesis.
The palisade mesophyll layer
is made of the palisade cells which are packed with lots of chloroplasts - the
sites of photosynthesis - note that the palisade cells are near the
upper surface to receive the most light.
The xylem and phloem are
networks of vascular sheathed bundles of cells that are the backbone
veins of the
plant's transport system.
The details of these are described in detail in
the next section.
The leaf tissues are adapted for
efficient gas exchange.
The broad green leaves of plants
exposed to light provide a large surface area for the light
absorbing sites of photosynthesis - more than the thinner stem.
The leaves are thin so the
absorbed carbon dioxide has only a short distance to diffuse to the
Leaves have veins (vascular
bundles) that support the leaf and transport water and minerals to
the leaves and glucose away from the leaves.
The lower epidermal tissue
is full of tiny holes (stomata, pores) which allow carbon dioxide to
diffuse into the leaf for photosynthesis.
dioxide + water == light/chlorophyll ==> glucose + oxygen
The opening and closing of
stomata is controlled by guard cells which respond to changes
in environmental (ambient) conditions including the movement of
water in and out of leaves.
The spongy mesophyll tissues
contain air spaces which increase the rate of diffusion gases in
(carbon dioxide) and out (oxygen).
PAGE for SUB-INDEX
Flowering plants have
two separate transport systems which must reach all parts of the plant
The structure and function of plant
xylem and phloem cells - the transport tissues
Plants have two networks of
'fine tubes' to transport molecules and ions (xylem tubes and phloem tubes).
xylem vessel tissue
transports water and minerals from the root hairs to all the rest of plant
i.e. through the roots and stem to the very tips of all the leaves.
This xylem vessel process is
phloem vessel tissue
transports dissolved sugars from the leaves (where they are made from photosynthesis)
to all parts of the plant e.g. for growth of new cells or to storage tissue
where they are converted to starch.
This function of the phloem to
move sugar molecules (mainly dissolve sucrose), amino acid molecules
and mineral ions around the plant is called
These systems are essential for a
plant to be healthy.
In some trees the transport
systems run through the bark.
Unfortunately, some animals like
to chew this bark.
The transport systems are
disrupted and the tree sadly dies!
Consider the xylem first
The xylem tissue transports water and mineral ions
from the roots to the stem and leaves.
Xylem tubes are made of
cells joined together 'end to end' in such a way they form a complete fine tube through
which water and mineral ions are freely transported from the roots, up through
the stem to the leaves.
The xylem cells have no end wall but a hole down the
middle (lumen) allows the free movement of fluid - but in only one
The strong xylem cell walls are made from cellulose
strengthened-stiffened by a material called lignin - these give the plant
The movement of water from the roots
through the xylem and out of the leaves is called the transpiration stream
and is caused by the diffusion of water and its subsequent evaporation. The
transpiration stream only flows in one direction - up through the
Water evaporating through
the stomata in the leaves, causes it to be replaced by water absorbed by the roots and this
water moves up via the
xylem tube system through the stem and to all the leaves - from the roots to
ALL of the plant and carrying essential mineral ions too.
If the stomata pores are open, evaporation of water will
always take place because the concentration of water in the air is less than
the concentration off water in outer layers of a leaf.
The diffusion and evaporation of
water from the leaves produces a water deficiency in the plant, so water is
automatically (if available) drawn up through the xylem tube system, so the
transpiration stream is driven by this evaporation of moisture from the
Water is essential to the plant
for both transportation and photosynthesis.
Evolution adaptation notes:
Transpiration is a necessary
adaptation to work in conjunction with photosynthesising leaves -
the stomata allow the gas exchange - carbon dioxide in, and water
vapour and oxygen out.
Even the narrow roots of plants
are further covered in tiny root hairs that greatly increase the
surface area even more, and hence increase the efficiency of water
This adaptation means the water
has only got to move a short distance to the xylem to be transported
up through the whole of the plant.
Now consider the phloem cells and
compare their structure and function with xylem cells
Phloem cells are elongated living cells
and the phloem tube tissues carry dissolved sugars
(food - glucose, sucrose) from the leaves to the rest of the
plant, including the tissue growing regions and the storage organs.
The phloem cells also transport other
important materials like amino acids for protein synthesis and mineral
ions for the function of chlorophyll and enzymes.
This process is called
and can operate fluid flow in both directions - another useful
The sugars from photosynthesis enter
the phloem system by active transport and transported around by
water which enter the phloem cells by osmosis.
Phloem cells are elongated with end
walls that have pores in them to allow fluids to flow through and the
phloem system allows transportation in both directions.
Phloem tubes are sometimes called sieve tube elements and the perforated end-plates allow fluids to
The phloem cells (sieve tube
elements) have no nucleus and can't survive on their own, so each one
has a companion cell (not shown, but has nucleus) that controls the living functions for
The companion cell, with a
nucleus, has lots of mitochondria to provide energy for the
phloem's transportation function.
They allow the transport of water and dissolved substances
to all part of the plants where nutrients are needed for immediate use in
growth or converted to starch for storage
The phloem tubes mainly
carry the sugars made in the leaves from photosynthesis up and down
the stem to all parts of the plant for immediate use in respiration, new
growth or to the food storage organs to form starch.
Phloem cells contain a little
cytoplasm, but no nucleus and have very few
sub-cellular structures, this adaptations increase the efficiency of
transportation - also aided by the sieve plates of adjoining
Throughout the plant the chains of xylem and
phloem cells are held together in vascular bundles.
You find them in the roots, stem and in the leaf -
where you see them as the veins in leaves.
PAGE for SUB-INDEX
The structure and function of
roots, absorption and necessity of nutrients
and function of roots - absorption
of minerals - diffusion and active transport
In plants, most of the water and mineral ions
(e.g. magnesium ions or nitrate ions) are absorbed
by roots, these in turn will be translocated up through the stem to the rest
of the plant using the xylem tube system.
I'm concentrating on minerals
here, but remember plants need lots of water for their
internal transport systems (xylem and phloem cells),
photosynthesis and to keep cells turgid and support the
physical structure of the plant as a whole.
Whereas most of the plant consists of
green photosynthetic tissue, the roots are non-photosynthetic and pale in
Therefore sugars would be translocated
from photosynthetic tissues like leaves to non-photosynthetic tissues like
roots using the phloem tubes.
The root hair cells are connected to root cortex
cells, which in turn, are closely connected to xylem vessels which
transfer minerals and nutrients around the plant efficiently.
Root hair cells are well adapted for their
large surface area for absorbing water
- which will also contain minerals,
they have no cuticle, but a thin membrane for
efficient absorption - short diffusion distance,
they have a thin cell wall, again reduces the
distance for water transfer in osmosis,
a large permanent vacuole to absorb and store
(temporarily) as much water as possible,
surface area of the
roots is increased by root hairs and the surface area of leaves is increased
by the flattened shape and internal air spaces.
Each branch of a root will have
millions of root hair cells creating a massive surface area to
absorb water and mineral ions.
The cells on the surface of a plant's
roots grow into long cells that shape into 'hairs' that stick out
into the soil.
The fine root hairs considerably
increase the surface area of the roots for absorbing water and minerals
and each branch of the root is covered millions of these microscopic
Root hair cells are very
elongated combining into fine hair-like structures, which greatly increases
the surface area of contact with the soil through which most of the plant's water and mineral
intake are absorbed.
Osmosis and active transport
As long as the water
concentration is higher in the soil, the root hairs will naturally absorb
water by osmosis.
The water potential of the soil
is usually greater than that of the fluid in the root, so water will
be naturally absorbed by osmosis.
However, the concentration of minerals in
the root hair cells is higher than in the moisture surrounding the roots,
and therefore an absorption problem.
This is because the plants cells would naturally lose essential mineral
ions by diffusion back into the soil moisture.
Not good, it means the root hair
cells can't use diffusion on its own to absorb minerals from the
soil, in fact, without active transport, mineral ions would move out
of the root hairs.
Therefore, active transport
systems must be used by the plant to counteract the natural direction of
diffusion from a high mineral concentration in the plant cells to a low
mineral concentration in the soil moisture.
The process requires energy from
respiration which powers the process by which the plant's root cells
can absorb minerals from the soil, even if they are only present in
a very dilute solution of the soil's moisture.
So, energy from respiration is
usually required to absorb minerals into the roots from the soil moisture by
working against the concentration gradient - active transport mechanism.
It has been shown
experimentally that when there is an increase in the uptake of
minerals by plants, there is an increase in active transport,
accompanied by an increase in respiration.
Examples of moving substances by
active transport from a low concentration to a higher concentration:
The concentration of nitrate
is usually greater in the root hairs cells than the surrounding
soil water. Therefore the natural diffusion gradient for the
movement of nitrate ions is in the direction of root to soil,
but pant need the nitrogen in nitrate ions to synthesise
proteins. Therefore active transport is used to move nitrate
ions from the soil water into the root fluids against their
natural diffusion gradient.
The same argument applies to
magnesium ions which chlorophyll molecules need in
Active transport is much more
complicated than 'simple' osmotic water exchange in plant cells.
comparison of diffusion, osmosis
and active transport
PAGE for SUB-INDEX
The importance of
- need for absorption of
nutrients through the roots
Mineral ions can only be absorbed
by plants through the hairs of the root cells.
The concentration of minerals
is quite low in soil compared to the concentration in the
fluids of root cell hairs - hence the need for active
transport for the plant to absorb them - and why there is a
high concentration of mitochondria in root hair cells to
provide the energy needed.
Several ions found in soil are
The health of any plant is
harmed if it is deficient in access to these mineral ions.
Just look at their
life-supporting roles in the biochemistry of plant cells listed
Nitrogen for amino acids
and protein synthesis (including enzymes) is obtained from the
nitrate ion (NO3-).
Lack of nitrogen gives
The magnesium ion (Mg2+)
is required by the chlorophyll molecule to function in
photosynthesis and is also essential for the metabolism of
If a plant is deficient
in magnesium not enough chlorophyll can be made and the
plant suffers from chlorosis - a yellowing of the leaves,
and photosynthesis is much reduced - as is the supply of food
and energy for the plant.
The potassium ion (K+)
is involved in the mechanism for opening and closing stomata, the
activation of some enzymes, involved in photosynthesis, and the production of ATP
phosphate ions. A phosphate structure is part of the ATP
molecule from respiration - energy source, and part of the structure of
cellular DNA and RNA.
Iron ions (e.g. Fe2+
or Fe3+) are important in the synthesis of
chlorophyll and some enzyme functions.
You can also get chlorosis in
plants from iron deficiency. (see magnesium above)
Borate ions supply the element boron
which plays a key role in many plant functions including cell wall
formation and stability, maintenance of structural and functional
integrity of biological membranes, the movement of sugar for energy
into growing parts of plants.
Soil that is deficient in any of
these minerals can be improved by using synthetic artificial
fertilisers (NPK types for nitrogen, phosphorus and potassium) often
supplemented with lots of additives to supply other trace elements
like iron or magnesium that plants need. You must compost or manure
if you are a true organic gardener!
For more see
Plant diseases and defences against pathogens
Investigating mineral requirements of plants and the effect of
All plants need a variety of
minerals to grow and maintain themselves in a healthy state.
In the previous section, I've
described some of the uses plants make of some nutrients.
If plants do not receive any of
their essential mineral nutrients ('mineral deficiency'), symptoms will be observed as the
plant declines in health e.g.
lack of nitrates - poor
growth and yellowing of leaves,
lack of potassium - poor
fruits and flowers and discolouring of leaves,
lack of phosphorus - poor
root growth and discolouring of leaves
lack of magnesium - yellowing
lack of iron - loss of green
colour in stems and leaves - known as chlorosis,
lack of boron - the growing tips of the
root or shoot show stunting and distortion of the growing tip
that can lead to tip death, brittle foliage, and yellowing of
lower leaf tips.
How do mineral deficiencies
(i) Naturally poor soil
Some soils are naturally
very low in, or devoid of, some essential minerals.
Lime-rich soils are
deficient in iron.
This land is not
productive unless it is treated with a synthetic inorganic
fertiliser or an organic fertiliser to increase the mineral
content of the soil.
(ii) Farming methods
From the prehistoric
times, throughout the world, woodland has been cut down and
burnt to provide land to grow crops - known as 'slash and
Repeated planting of the
same annual crops on the cleared land uses up the minerals
in the soil and so the soil becomes deficient in essential
More woodland is then cut
down, a process that continues to this day in many forested
areas e.g. the rainforests of Brazil.
Again, use of
fertilisers, enables repeated growing of crops.
inorganic fertilisers have become more complex in their
formulation and may contain a dozen different elements that
plants need for healthy growth and development.
How can we investigate which
minerals are essential for healthy plant growth
You can grow plants without soil
if you suspend them in a solution of the minerals they need - the
culture solution of a
hydroponic technique - no soil used - the experiment is illustrated in the diagram above
(I guess I'm no artist, but you should get the 'picture').
The plant is suspended in a culture solution of nutrients,
so that the roots can absorb what they need for healthy
growth and development - water for transpiration, mineral
ions (for reasons discussed above) and oxygen (for
The culture vessel is covered to
simulate the darkness that roots would experience in soil.
A tube supplies air, containing
oxygen, to keep the culture solution aerated.
You grow control plants
with a culture solution containing all the nutrient minerals they
You can then grow the same plants
in culture solutions that do NOT contain any of a specific mineral -
to artificially create a deficiency of a plant nutrient.
The growth of the plant can be
monitored to see the effect of a particularly deficiency -
observed symptoms resulting from the specific deficiency
You must only omit one mineral
at a time in the investigation, otherwise you could not be sure
which one caused the symptoms you are observing.
Using this relatively simple
technique, you can investigate the effects
of deficiencies in nitrate ions (for nitrogen), phosphate ions (for
phosphorus), magnesium ions (Mg2+), potassium ions (K+),
iron ions (Fe2+/Fe3+) and borate ions (for
of nutrient knowledge to food production
If you know what minerals are
soil is deficient in, you can add a fertiliser to provide the
Farming, agriculture in general,
uses two types of fertiliser.
Organic fertilisers come from
animal ('smelly muck') or plant ('compost') waste material and
are considered environmentally superior to inorganic
They produce better quality
soil that is more resistant to erosion and better at retaining
Organic fertilisers only
break down slowly in the soil to release their nutrient minerals
- 'intensive farming' farmers would see this as an economic
Good quality organic
fertiliser should supply all the essential minerals a plant
needs to grow and develop.
These are concentrated
formulations of particular essential elements plants need for
They can be made to any
composition to suit particular soil deficiencies in any soil.
The inorganic fertiliser
formulation can also be made to promote the growth of a
particular crop to increase the yield.
They do not smell, easily
stored (chemically stable), easily and evenly spread on the soil
as small pellets and the chemicals are quickly absorbed by plant
The solid pellets can be
designed to break down slowly to minimise the polluting
effects from run-off into water courses.
These fertilisers do not
improve the quality of the soil, but must be regularly
applied-replaced, especially if fields are used to grow several
yields of crops each year (same or different).
PAGE for SUB-INDEX
on leaf adaptations to aid
See also detailed notes on
explained, limiting factors affecting rate
gcse biology revision notes
Gas exchange in plants
In plants carbon dioxide
enters leaves by diffusion and then diffuses into cells where photosynthesis
Oxygen will diffuse out from the leaf surface.
Reminder: Photosynthesis takes place inside the
subcellular structures called chloroplasts in the palisade cells.
carbon dioxide + water ==
==> glucose + oxygen
+ 6CO2(g) == sunlight/chlorophyll ==> C6H12O6(aq)
In daylight more carbon dioxide will be taken
in for photosynthesis in than
given out from respiration and more oxygen given out than taken in - the effect of more
photosynthesis than respiration - the surplus glucose is converted into
At night-time the opposite will happen, more carbon
dioxide from respiration will be given out than taken in, and more oxygen taken in than given
out - respiration increases and food stored as starch becomes the source of
energy in the dark.
Beneath the apparently flat
surface of a leaf is quite a porous layer of air spaces between the outer
layers of cells - particularly on the underside of leaves - quite often the
lower surface of a leaves feel rougher and 'roughness' means a more
disrupted surface of a larger gas exchange surface area.
Photosynthesis and diffusion
(with reference to the above diagram of leaf structure)
Plants have stomata
(tiny pores or holes), mainly on the underside of leaves in the spongy
mesophyll, to obtain
carbon dioxide gas from the atmosphere for photosynthesis and to give out the
'waste' oxygen gas produced
as a by-product in
Carbon dioxide is absorbed from
air and water from the roots for
Carbon dioxide diffuses into the
leaves through the stomata and is depleted through photosynthesis.
Therefore as photosynthesis
proceeds, the internal carbon dioxide concentration in the leaf is
much lower than in the surrounding air, so carbon dioxide will
diffuse into the leaf down this concentration gradient.
The rate of diffusion of the
carbon dioxide (and any other gas) is increased by:
Increasing the surface
area of the leaf - always the broadest part of any plant.
The smaller the distance
the molecules have to travel as they diffuse - thin leaves
with an even thinner mesophyll layer.
An increase in the carbon
dioxide concentration gradient - always be there while
photosynthesis is taking place.
As the CO2 is
absorbed, wind blows by fresh supplies of carbon dioxide to
maintain a high inward concentration gradient.
Oxygen from photosynthesis diffuses out through the
stomata, and most water is lost in the same way (transpiration).
The air spaces in the leaf
structure create a larger surface area to allow this diffusion to
take place efficiently.
Leaves are also thin, so distance
and diffusion times are short, further increasing the efficiency of
Carbon dioxide can diffuse in
through the stomata and oxygen can diffuse out and stomata also allow water
vapour to escape as part of the process of transpiration (details in
Since carbon dioxide is being
used up in photosynthesis, the concentration gradient enables more carbon
dioxide to diffuse in through the stomata.
The size of the stomata are
controlled by guard cells (more on this in the next two sections on
The flattened shape of leaves
increases the surface area over which efficient gas exchange can
take place - greater chance of carbon dioxide to diffuse into the
leaves (see photographs below).
Inside the leaf the cell walls
form another exchange surface and the air spaces between these cells
further increase the surface are for gas exchange - carbon dioxide
in, oxygen and water vapour out.
Water vapour evaporates from the
surfaces of the leaf cells.
The higher concentration of water vapour
in the leaves means there is a natural diffusion gradient to the
outside air so the it can exit the leaves in the process of
A high humidity reduces the
concentration gradient of water vapour between the interior and
exterior of the leaf, slowing down the diffusion of water vapour
- slowing down transpiration.
Conversely, very dry air (low
humidity) will increase the concentration gradient and increase
the rate of water loss from leaves.
leaf surface area
If a surface has a regular shape like a
square or rectangle it is easy to measure and calculate the surface area
(length x breadth).
However, in the case of a leaf, you have
an irregular shaped surface, even if there is a line of symmetry down the
If the leaf is laid out on a marked
out grid, like the ones illustrated above, you can count the squares to
arrive at an estimate of the leaf's surface area. Make sure the leaf is
fully flattened out.
If a square is mainly filled with
leaf (over ½ filled) you count it towards the total.
If a square is not well filled (less
than ½ filled), it should not be counted in the total area.
Using this method I estimate that the
has an area of 52 cm2.
(±1, do you agree?)
In this case I found it easier to
count the blank squares.
On the right, I estimate the
smaller leaf has
an area of 32 cm2.
(± do you agree?)
In this case I found it easier to
count the green squares.
side of a leaf is smoother and greener - richer in chloroplasts to
capture the sunlight
side of a leaf is rougher - more 'porous' for efficient gas exchange
and the veins more prominent
A summary of adaptations
for the effective functions of leaves - some very important for
transparent waxy cuticle
light through and protect leaf from excess water loss
shaped cells arranged in end-on, near the upper surface,
maximises chloroplasts at the top of the cells
maximum amount of light to be absorbed
shaped cells creating air spaces
the surface are for gas exchange - CO2 in, O2
out - increases efficiency
stomata (pores) for gas exchange surrounded by pairs of guard
cells open and close each stoma (pore) to control the diffusion
of the gas exchange
xylem and phloem tubes in the veins
substances around the plant including to, and from, the leaves
Plant adaptations and controlling water
PAGE for SUB-INDEX
Factors affecting the rate of
transpiration and function of the stomata and guard cells
Plants are constantly losing water,
but cannot be healthy without a balancing water intake. The water is
needed for transportation and photosynthesis - in fact most of the water
is used in the transport of materials through the plant, only a few% is
used in photosynthesis.
Transpiration is result of the way
leaves have become adapted to facilitate photosynthesis - the
stomata aiding the transport system by allowing gas exchange - carbon
dioxide, oxygen and water vapour.
The process of water movement from the roots
through the xylem and out of the leaves is called transpiration and
is essential for a plant's transport system.
Water is absorbed through the root
hairs, passes up the root, continues up the stem and spreads into all
Transpiration is caused by the
evaporation and diffusion of water from a plant's surface - mostly from
Most of the loss of water vapour takes place
through the stomata on the surface of leaves.
Water on the spongy surface of the
mesophyll evaporates and diffuses out of the leaves.
Plants continually lose
water because the concentration of water in the plant fluids is greater than
the concentration of water in the air outside - the concentration gradient
is in the 'outward' direction.
Since plants need water all the
time, water is continually transported through the xylem in
The loss of water from leaves by
a small shortage of water in the leaves and so more water is drawn up
from the rest of the plant through the xylem tubes to replace the water
Therefore, this causes in turn,
more water to be absorbed and
drawn up from the roots.
So, there is a constant flow of water
up through the plant - the transpiration stream - which carries the
mineral ions dissolved from the soil up into the whole of the plant.
So, important functions of
Water is needed for
Water carries dissolved
substances around the plant.
Evaporation from leaves cools
Cells filled with water give
the plant physical support.
Evaporation is more rapid in hot, dry and windy
If plants lose water faster than it is replaced by the roots,
the stomata can close to prevent wilting.
If too much water is lost through
the stomata, plants will wilt ('flop') and die.
As we have said, plants are
constantly losing water by evaporation, but cannot be healthy
without a balancing water intake. If plants lose water too fast they
will wilt - the leaves droop and hang down. This reduces the surface
area available for evaporation through the stomata. The stomata
close and photosynthesis stops to prevent water loss. There is a
danger the plant will overheat. Plants will stay wilted until they
can absorb water and the temperature falls and no longer in
The size of stomata is
controlled by pairs guard cells, which surround them.
Therefore stomata and guard cells control the
rate of evaporation from leaves.
(Note: stoma is singular, stomata is
Two guard cells surround each
The size of the opening of the
stomata (diagram on left) must be controlled by the guard cells or a plant might lose too much
water and wilt.
It is the guard cells that
regulate the rate of transpiration.
It is the guard cells that control
the rates of water loss and gain AND the rate of gas exchanges.
The 'kidney shaped' guard cells can change shape to control the size
of the pore.
Water diffuses out of the spongy
mesophyll producing a film of water on the surface of the cells. Water
evaporates into air spaces between the cells and the water vapour
diffuses down the concentration gradient to the stomata and escapes from
the leaf into the surrounding air.
Water will diffuse out and
evaporate away much faster in less humid-drier, hotter or windier weather
Stomata close automatically if
the water supply begins to 'dry up' to reduce water loss.
The guard cells will respond to
the ambient conditions ie close up the stomata if the rate of water loss is
to great for water to be replenished from the roots.
When the plant has lots of
water, the guard cells become swollen with water (turgid) and the
stomata are open to increase the rate of water loss, but also
increase the intake of carbon dioxide for photosynthesis (and oxygen
When the plant is short of water,
guard cells lose water (flaccid, 'limp') and
the stomata are closed to decrease the rate of water loss.
If the plant is very short of
water the cytoplasm inside the cells shrinks and the cell
membrane comes away from the rigid cell wall. This process is called
plasmolysis and the cell is said to plasmolysed.
Three more points
(i) Adaptations of guard
Apart from their shape, guard
cells have other adaptations which help them in their function
to aid in controlling gas exchange and water loss.
They have thin outer walls
and thickened inner walls which allow the opening and closing
mechanism to work efficiently.
The guard cells also
respond to light levels - they close at night to save water -
conserved for photosynthesis and open up again when daylight returns
to allow the exchange of gases.
(ii) You usually find more
stomata on the underside of leaves compared to the top.
The lower leaf surface is
more shaded and cooler, this reduces water loss, compared to the
water loss that would happen on the upper surface.
Plants growing hot climates
need to conserve water and so they have fewer and smaller
stomata on the underside of the leaves and no stomata on the
upper epidermal surface.
plant adaptations - examples in
(iii) It is changes in the concentration of ions inside the guard
cells that facilitate the opening and closing of stomata.
When guard cells lose
water, it causes the cells to become flaccid and the stomata
openings to close - reducing water loss. This occurs when plants
has lost an excessive amount of water OR if light levels drop
and the use of carbon dioxide in photosynthesis decreases.
cells respond to light, if light levels increase, potassium
ion (K+) are pumped-absorbed into them.
This increases the
concentration of particles in the guard cells fluid.
This decreases the
concentration of water molecules (decreases the cell's water
Therefore water diffuses into
the guard cells by osmosis making the guard cells turgid and the
stoma opens - carbon dioxide can enter for photosynthesis.
The reverse happens light
levels or water levels are low.
Then, potassium ions exit
the guard cells, the concentration of water molecules
increases (increasing the cell's water potential).
Water will then move out
of the guard cells by osmosis, they become flaccid and the
stoma closes reducing the loss of water, and not as much
carbon dioxide is available for photosynthesis - not needed
at all at night.
Extra note on plant cells and
(i) When you water a plant it
increases the water potential of the soil around it.
Therefore the plant cells
will draw water in by osmosis until they become turgid -
fatter and swollen.
The cell fluids (contents of
the cell) will push against the cell wall, known as turgor
pressure, and this helps support the plant tissues
(therefore the plant as a whole).
(ii) If the soil is very dry,
lacking in water, the plant starts to wilt and the water potential
of the plant is greater than the surrounding soil.
The result is the plant cells
become flaccid and begin to lose water.
The plant doesn't droop
(flop) completely and retains much of its shape because the
strong cellulose cell wall is relatively inelastic and helps the
plant retain its shape.
PAGE for SUB-INDEX
More on the environmental
factors (ambient conditions) affecting the rate of water loss -
the rate of
1. Air flow
The more air that flows over
the leaves, e.g. stronger wind, the greater the rate of
transpiration. Conversely, the lower the wind speed, the slower
the rate of transpiration.
The more quickly the water
vapour is removed and swept away by a greater air flow, the steeper the
water diffusion gradient out of the leaf is - the concentration of water vapour is
much greater in the stomata than in the air in and surrounding the leaf
- which is much lower because the water vapour is being
constantly carried away in the air current across the surface of
If the air is quite still,
the water vapour accumulates around the leaf, considerably
reducing the diffusion gradient because the water vapour
concentrations become similar. The concentration of water vapour
is high in the stomata and BUT only a bit less in the
A particle model of diffusion in gases
Reminder that the net flow of a substance in
diffusion is from a higher concentration to a lower
concentration e.g. the movement of the 'green' water
particles in the diagram sequence below.
The less humid (more drier)
the air surrounding the leaves, the greater the rate of
The lower the water
concentration on the outside of leaves, the steeper the water
diffusion gradient from the leaves to the external air, the faster the rate of water
loss by transpiration.
When the air is very humid
with a high water concentration, there is a smaller difference
in the higher (in leaf) and lower (outside leaf) concentrations
- so a smaller water diffusion gradient resulting in smaller
rate of water loss by transpiration.
3. Light intensity
The greater the intensity of
light (e.g. the brighter the sunlight) the greater the rate of
transpiration because the rate of photosynthesis increases. This
stimulates the guard cells to open up the
stomata more to let more carbon dioxide in and water vapour
and oxygen out.
As it gets darker,
photosynthesis rate decreases and the stomata begin to close up
- they don't need to be open to allow carbon dioxide to diffuse
in. When the stomata are closed, little water can escape - I
presume enough oxygen can get in for the plant's respiration at
The warmer the surrounding
air the greater the rate of transpiration.
This is because the water
molecules have more kinetic energy to escape the intermolecular
forces at the surface of the liquid water in the stomata. So,
the water molecules can evaporate more quickly and diffuse out
of the stomata.
Also, the rate of
photosynthesis increases with increase in temperature - just
like any other chemical reaction - therefore more water is
required to be drawn up through the roots.
The daily cycle of the rate of
The graphs above show how the
rate of transpiration is likely to vary through the day for two
The rates of both photosynthesis
and transpiration increase and decrease with change in light
intensity over a 24 hour daily cycle.
On average, at midday (noon) the
sun is at its maximum height, the sunlight intensity is at a
maximum, so photosynthesis can be at a maximum, but only if the
transpiration rate maximises too, to supply the water for
The peak heights will vary
depending on the effect of the factors that control
transpiration (discussed above) and
The light intensity has a greater
effect on the rate of transpiration of plant B compared to plant A, even though it starts from a lower
base at midnight.
In the night time, when
photosynthesis is at a minimum, the water uptake through the roots
is greater than the rate of transpiration.
Through the day, and peaking at
midday, the transpiration rate exceeds the rate of water uptake.
In daylight the rate of
transpiration cannot be the same as the rate of water uptake because
some of the water is used in photosynthesis and the rest of the
plant's metabolic processes and the rate of evaporation increases
That rate of transpiration
exceeds the rate of water uptake as the rate of photosynthesis
As the light intensity
increases, the stomata open to allow in more carbon dioxide for
BUT, this also allows more
water vapour to evaporate.
Plant adaptations and controlling water
PAGE for SUB-INDEX
An experiment to investigate the
rate of transpiration - using a potometer
A much more sophisticated experiment to
look at the rate of transpiration.
The potometer - a way of
measuring the uptake of water by a plant - it measures the rate of
A potometer consists of a vertical tube with a plant
shoot sealed in it.
The plant shoot should be cut
under water to prevent air entering the xylem and at an angle to
increase the surface area for water absorption.
The tube is connected to a reservoir of water,
controlled by a tap, used to replace water lost by transpiration.
The horizontal capillary tubing, connected to the plant
tube and reservoir, has a scale set up beside it.
Inside the capillary tube, adjacent to the scale is an
air bubble which will move along to the left as water evaporates from
The whole apparatus must be
airtight and watertight or the readings will be inaccurate.
You need to let the plant
acclimatise ('settle down') to the laboratory conditions before
starting the experiment.
You then let one air bubble into
the capillary tube which is then back under the water in the beaker.
At the start and after a measurement of transpiration has been made, the
reservoir tap is opened to allow water to flow in and move the air
bubble to the right near the start of the scale.
Excess water runs out into the beaker which also acts as
a reservoir of water itself during an experimental run, rather than
sucking in air.
Making a measurement
You note the starting position of the
air bubble on the right.
You then measure the time it takes for the air bubble to move
from right to left and note the total distance moved.
To repeat the experiment you let
water in from the reservoir to bring the bubble back to near the start
of the scale.
Estimated transpiration rate =
distance air bubble travels / time taken
So your rate of transpiration units might be
mm or cm/min (an arbitrary scale based on the experimental setup).
BUT, note that the experiment assumes the water uptake by the plant through its roots is directly
related to the water loss by evaporation from the leaves.
The sorts of investigations you can do
with a potometer
Wherever possible keep everything constant
except the one factor you are investigating e.g. constant temperature,
constant air humidity in the laboratory.
You can now investigate various
environmental conditions by comparing the relative uptake of water.
1. Varying light intensity: You can use a reasonably powerful light to increase
light intensity, placing it quite close to the plant, with the bulb at
the same height as the centre of the plant shoot.
You measure the distance of the centre of the lamp
bulb to the centre of the plant shoot (d).
Run the experiment and measure the rate of
You can then move the same lamp back,
measuring the new longer distance and re-measure the transpiration
rate. You should repeat all measurements for varying lamp distances.
The intensity of light on
the plant is proportional to1/d2 or you can
measure the intensity with a light meter.
You can then plot a graph of
transpiration rate against light intensity.
You must make sure the
temperature and humidity are constant and there is no air flow over the plant
It would be easy to do
comparative experiments in a brightly lit room and a darkened room.
2. Varying air flow: You could
use a hair dryer to blow cold air (room temperature) at different speeds
over the plant to see if increased air flow increases transpiration
rate. Difficult to get data to produce a meaningful graph, but no air
blowing, low blowing setting and a high blowing setting should give you
You must make sure the
temperature, light intensity and humidity are constant and the light intensity stays the same.
3. Varying temperature: This
is also tricky to get good quantitative data.
You can easily compare blowing
cold air and warm air over the plant, and that should give a
difference in transpiration rate.
BUT, not that easy to keep
You can measure the temperature of
the air near the plant during the experiment.
It might be convenient to do
the experiment in a cold room and then in a warm room.
You must make sure the air flow
is constant (completely still air is the best condition), and the
humidity and light intensity stay the same.
PAGE for SUB-INDEX
Practical investigations you might have encountered
potato slices in different concentrations of liquid in terms of mass gain
and mass loss - this is to illustrate the process of osmosis.
designing an investigation to measure the mass change of potato
when placed in a series of molarities of sucrose solution
the relationship between concentrations of sugar solution and change in
length of potato strips
placing shelled eggs in different concentrations
of liquid to observe the effect
placing slices of fresh beetroot in
different concentrations of liquid to observe the effect, and then taking
thin slices to observe the cells
observing guard cells and stomata using
observing water loss from plants by placing in a plastic bag
with cobalt chloride paper.
investigating flow rate in xylem
using celery, which can include calculation of flow rate
content of artificial phloem and xylem given knowledge of the appropriate
planning an investigation using a potometer to measure the effect of temperature
or wind speed on the
Some learning objectives for this page
You should know that plant organs include stems,
roots and leaves.
Details of the internal
structure are only needed for the leaf.
Know the structure and function
of palisade cells and guard cells in plants.
Palisade cells contain
chlorophyll and are adapted for photosynthesis.
Guard cells are adapted to open
and close the pores which allow gas exchange and water evaporation.
You should know examples of plant tissues
epidermal tissues, the outer
layers which cover
the whole plant,
mesophyll, between two epidermis
layers, where most photosynthesis happens,
xylem and phloem, which transport substances
around the plant eg sugars like sucrose and glucose, minerals and water.
PAGE for SUB-INDEX
General PLANT BIOLOGY revision notes
explained, limiting factors affecting rate, leaf adaptations
Plant cells, transport, gas exchange in plants,
transpiration, absorption of nutrients, leaf & root structure
Diffusion, osmosis, active transport, exchange of
substances - examples fully explained
Respiration - aerobic and anaerobic in plants (and
biology revision notes
Hormone control of plant growth and uses of plant hormones
gcse biology revision notes
Plant diseases and defences against pathogens and pests
gcse biology revision notes
Adaptations, lots explained including
gcse biology revision notes
and a section on
Stem cells and uses - meristems in plants (at the end of the
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