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Biology: Unit F211: Cells, Exchange and Transport
Module 1: Cells
Transport in Plants
Plants have two tissues, both of which are adapted to transport functions; the xylem carries
water and minerals (inorganic ions like Na and Mg) upwards from roots to leaf; the phloem
carries sucrose from the source (where it is made) to the sink (where it is used). A transport
system like the one found in mammals is not required however. This is because;
1. Plants have a low metabolic rate – they are not very active
2. Leaves provide a large surface area : volume ratio for photosynthesis.
The branching shape gives lots of surface area, allowing gases to move
quickly between atmosphere and cells by diffusion.
The xylem and phloem are found together in vascular bundles.
In the root…
The vascular bundle in the root is found at
the centre. The xylem is found in the shape
of an X with the phloem found between the
arms of the X shaped xylem.
Xylem
Epidermis
Cortex
Phloem
Endodermis
In the stem…
The vascular bundles are on the outside of the
stem, with the phloem closest to the
epidermis. Each year a plant will grow a new
layer of vascular bundle – these are the rings
of a tree. You are seeing the waterproof
chemical on the walls of the xylem - the lignin.
Xylem
Phloem
Cambium
Cortex
Upper Epidermis
In the leaf…
The vascular bundle forms the midrib
and the veins of the leaf. In each
vein, the xylem are on top and the
phloem are beneath.
Stomata
Xylem
Phloem
Spongy Mesophyll
Palisade Mesophyll
Xylem…
Xylem is a tissue because they are made from three different types of cells; xylem vessels, tracheids (refilling) and
fibres (support). They work together to perform the specific function of carrying water and minerals.
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The end walls of the dead cells have fallen away to form
one continuous unbroken hollow tube from root to leaf.
Walls are hard, rigid, waterproof and strong containing
cellulose and lignin; this provides support and means that
they are waterproof, and so are unaffected by osmosis.
Cells are dead, and have no organelles to get in the way
of the passive process of transpiration.
The lumen is large so there is little resistance to the
flow of water.
There are small holes in the xylem called pits – where
lignification was not complete. These pits allow water to
move in and out of adjacent vessels to allow ‘refilling’.
This means that the water column can remain continuous.
1. Water moves into the root
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Root hair cells take up minerals by active transport using ATP
This reduces the water potential of the cytoplasm, so water moves in by osmosis from the soil to the root hair cell.
Water travels through the apoplast and symplast pathways from the root hair cells, through the cortex, the
endodermis and the pericycle.
The cells of the endodermis actively transport minerals into the xylem, lowering the water potential of the xylem
vessel, so water enters the xylem vessel by osmosis. This creates root pressure – water moves from an area of high
hydrostatic pressure to low hydrostatic pressure by mass flow.
Simplast Pathway –
water crosses the
plasma membrane of the
first cell and then travels
through the cytoplasm
to the xylem, going
through the gaps which
join each cell to the next,
called plasmodesmata.
Highest Water
Potential
Apoplast Pathway –
water moves along the
non selective cell wall.
The Casparian Strip – a line of cells in the endodermis which
are lined with suberin – a hard waterproof substance. This
blocks the apoplast pathway, forcing all of the fluid through
the plasma membrane in the symplast pathway. This allows
the plant to select what enters the xylem (no fertilisers etc.)
Lowest Water
Potential
2. Water moves up the stem
 Water moves in one unbroken column up the xylem from the root to
the leaf
 This is possible because of water’s ability to make hydrogen bonds
 The water molecules are polar, and the opposite dipoles of the
oxygen (δ-) and hydrogen (δ+) attract each other causing water
molecules to stick together – this is cohesion.
 The water molecules are also able to form hydrogen bonds with the
cellulose cell wall – this is adhesion.
3. Water moves out of the leaf
 The water moves from the xylem through the pits and into the
spongy mesophyll via apoplast and symplast pathways.
 The spongy mesophyll have a higher water potential than the air
spaces and so water evaporates into the air spaces, down the
concentration gradient.
 Water vapour diffuses out of the air spaces through the stomata
into the outside air, down the concentration gradient. Hydrogen
bonds mean that a continuous stream of water leaves the leaf.
Transpiration is the loss of water vapour by evaporation from the stomata on
the underside of the leaf.
Number of leaves – a plant with more
leaves has a larger surface area over
which water vapour can be lost.
Number/size/position of stomata – large stomata on the top side
of the leaf means that water vapour is lost more easily. Small
stoma on the underside means that less water vapour is lost.
Temperature – higher temperatures mean increased rates of
transpiration. The water molecules have greater kinetic energy and so
evaporate at an increased rate and diffuse out more quickly.
Light – in light, stomata open
in order to allow carbon
dioxide to enter for
photosynthesis. This means
that water is lost more easily.
Presence of Cuticle – a waxy cuticle
is waterproof and so reduces
evaporation from the leaf surface.
Humidity – higher humidity levels mean less transpiration. Greater
humidity means more moisture in the air – a higher concentration of water.
There is therefore a lower concentration gradient between the leaf and air,
so there is a slower rate of diffusion of water vapour out of the leaf.
Wind speed – a higher wind speed means a greater rate of transpiration. Wind removes the water vapour from
around the leaf, maintaining the steep concentration gradient between the leaf and the air.
The sugar sucrose is moved in the phloem by an active pumping mechanism. Sucrose
is always moved from where it is made (source) to where it is used (sink). The
translocation of sugars occurs by mass flow; from a high pressure at the source to a
low pressure at the sink.
Phloem is a tissue because it consists of 2 types of cell; the sieve element and the companion cells.
5. The water entering creates a
high pressure, pushing the sucrose
along the sieve element by mass
flow, from the area of high
pressure at the source to an area
of low pressure at the sink. This
low pressure is created because
sucrose is removed; increasing the
water potential of the sieve
element, so water follows out by
osmosis, reducing the hydrostatic
pressure.
1. H+ ions are actively
transported out of the
companion cells, against the
concentration gradient using
energy.
4. The high concentration of
sucrose in the sieve
element at the source
lowers the water potential,
so water moves in by
osmosis.
3. Now the sucrose has been loaded onto the companion cell,
it is at high concentration. It therefore diffuses through the
plasmodesmata into the sieve element.
2. H+ ions can only move
back into the companion
cell, down the
concentration gradient,
using a co-transporter
protein – this is passive. The
co-transporter only allows
H+ to move back if it is
accompanied by sucrose
(the co-transporter has two
binding sites)
How do we know that the phloem is used?
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If a plant is put into an atmosphere containing radioactive carbon dioxide,
radioactive sucrose appears in the phloem. The carbon dioxide had been used in
photosynthesis to make sugars, and these are then transported via the phloem.
If a tree is ringed and the phloem are removed, a swelling forms either above or
below the ring cut. This is because the sucrose being carried in the phloem
cannot move any further beyond the point where the cut has been made. The
swelling is above the ring in summer, because sucrose is moving down, from the
source of the leaves to the sink of the root. In spring, the swelling is below the
ring, because the sucrose is moving up, from the source in the roots to the sink at
the leaf.
How do we know that it requires metabolic energy?
 The companion cells have many mitochondria which produce ATP from
aerobic respiration
 A poison which stops respiration will stop translocation – respiration is
required to produce the ATP needed for actively transporting H+ ions out of
the companion cells.
Measuring the rate of transpiration – A Potometer
1. Work out the volume of water lost; πr2h
2. Work out water loss per unit of time; volume
lost / time
Method:
Cut the shoot at an angle under water
Allow time for the plant to adjust to the
surroundings, and keep environmental conditions constant
Time how long it takes the meniscus to move a set distance
Reset the meniscus with water from the reservoir, and take
more readings so that you can calculate the mean rate of
uptake.
The stem is cut at an angle so
that air bubbles in the xylem are
removed and the surface area of
the xylem is maximised
Adjustment time is required so
that the plant acclimatises to
the environment and transpires
at a consistent rate.
The seal must be airtight to stop the loss of water vapour.
Taking multiple results removes anomalous results
The potometer does not measure water loss accurately; it works on the
assumption that all of the water which enters the plant at the roots
leaves via transpiration at the leaf. This however is not the case; water
is used in photosynthesis, or used to make cells turgid etc.
Xerophytes – a plant which has structural adaptations to reduce water loss by
transpiration due to living in water poor environments.
There are three main principles:
1. Reduce the surface area of the
stomata or leaf
2. Reduce the concentration gradient
between leaf and air
3. Waterproof to prevent evaporation
Less water lost by evaporation
Smaller leaves, needles, spines, using the stem
for photosynthesis, densely packed spongy
mesophyll so that there is a smaller surface area
form which water can diffuse out of the cells,
leaves rolling so that a tough waterproof cuticle
is exposed instead of the stomata.
Hair on the surface of leaves trap water vapour,
reducing the water potential gradient, which reduces
the rate of water lost by diffusion from the leaf. The
cells having a lower water potential also has the same
effect.