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. to very tips of 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 allows the free movement of fluid. The xylem cell walls are
strengthened by a material lignin.
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 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
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..
This allows 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 to all parts of the plant e.g. for new
growth or to the food storage organs.
The function of roots
In plants most of the water and mineral ions are absorbed
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.
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.
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 because the plants cells would naturally lose essential mineral
ions by osmotic diffusion back into the soil moisture, not good!
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.
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 sugar solution.
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
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 surface area.
Plants have stomata
(tiny pores or holes), mainly on the underside of leaves, to obtain
carbon dioxide gas from the atmosphere and to give out the oxygen gas produced in
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.
Since carbon dioxide is being
used up in photosynthesis, the concentration gradient enables more carbon
dioxide to diffuse in through the stomata.
Water, and the oxygen produced
in photosynthesis diffuse out.
More on transpiration
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 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 too
Factors affecting the rate of
transpiration and the function of the stomata and guard cells
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.
The size of the opening of the
stomata must be controlled by the guard cells or a plant might lose too much
water and wilt.
The 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
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 guard cells are swollen with water (turgid) the
stomata are open to increase the rate of water loss.
When the guard cells are low on water (flaccid, 'limp')
the stomata are closed to decrease the rate of water loss.
More on the environmental
factors affecting the rate of water loss
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 surrounding the leaf
- which is much lower because the water vapour is being
constantly carried away in the air current.
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 greater the intensity of
light (e.g. the brighter the sunlight) the greater the rate of
transpiration because the rate of photosynthesis increases. The
stomata must be open to let 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.
An experiment to investigate the
rate of transpiration - using a potometer
The potometer - a means of
measuring the uptake of water by a plant
A potometer consists of a vertical tube with a plant
shoot sealed in it.
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
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.
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
Remember - wherever possible keep everything constant
except the one factor you are investigating.
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 is constant and that there is no air flow over the plant
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 is 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. You can measure the temperature of
the air near the plant during the experiment.
You must make sure the air flow
is constant (still air best) and the light intensity stays 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
See also cell biology section
explained, limiting factors affecting rate, leaf adaptations
gcse biology revision notes
Transport and gas exchange in plants,
transpiration, absorption of nutrients, leaf and root structure gcse biology revision notes
Diffusion, osmosis, active transport, exchange of
substances - examples fully explained
Respiration - aerobic and anaerobic in plants gcse
biology revision notes
Hormone control in plants 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