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
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 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 bundles of cells that are the backbone of the
plant's transport system.
These are described in detail in
the next section.
The leaf tissues are adapted for
efficient gas exchange.
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).
Flowering plants have
two separate transport systems which must reach all parts of the plant
Plants have two networks of
'fine tubes' to transport molecules and ions (xylem tubes and phloem tubes).
The xylem vessels
transport water and minerals from the root hairs to all the rest of plant
i.e. through the stem to the very tips of all the leaves.
The phloem vessels
transport 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.
The xylem tissue transports water and mineral ions
from the roots to the stem and leaves.
Xylem tubes are made of dead
cells joined together 'end to end' in such a way they form a complete fine tube through
which water and minerals 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
direction. 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 that evaporating through
the stomata causes it to be replaced by water absorbed by the roots and this
water moves up via the
xylem tube system. 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.
Phloem cells are elongated living cells
and the phloem tube tissues carry dissolved sugars
(food) from the leaves to the rest of the
plant, including the growing regions and the storage organs.
This process is called
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) that control the living functions for
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 have very few
The function of roots
In plants most of the water and mineral ions are absorbed
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.
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.
The 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.
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 osmotic 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.
Simple demonstration of osmosis.
Water diffuses out of the potato cells
into the solution at higher concentrations of sugar.
If you wanted to reverse the process, and
make water go back into the potato cells, you would have to use an energy
'input' system to do so. In living organisms this is known as 'active
For more details on this experiment see
Diffusion, osmosis and active transport
Leaf adaptations to aid
photosynthesis and more on transpiration
See 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.
In daylight more carbon dioxide will be taken in than
given out 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 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
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.
Plants have stomata
(tiny pores or holes), mainly on the underside of leaves, to obtain
carbon dioxide gas from the atmosphere for photosynthesis and to give out the oxygen gas produced
as a by-product in
Carbon dioxide is absorbed from
air and water from the roots fuel
Oxygen diffuses out through the
stomata and most water is lost in the same way.
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
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
Factors affecting the rate of
transpiration and the function of the stomata and guard cells
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.
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. 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 diffusion gradient
is in the 'outward' direction.
The loss of water from leaves creates
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
loss. 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.
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.
The size of stomata is
controlled by guard cells, which surround them.
(Note: stoma is singular, stomata is
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 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) 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.
(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.
More on the environmental
factors (ambient conditions) affecting the rate of water loss - rate of
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 by a greater air flow, the steeper the
diffusion gradient 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, 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.
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 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.
An experiment to investigate the
rate of transpiration - using a potometer
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.
TOP OF PAGE
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
General PLANT BIOLOGY revision notes
explained, limiting factors affecting rate, leaf adaptations
gcse biology revision notes
Plant cells, transport and gas exchange in plants,
transpiration, absorption of nutrients, leaf and 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