Transcript Xylem and Phloem - Mrs Miller's Blog | Science Revision

Transport in Plants
• Explain the need for transport systems in
multicellular plants in terms of size and surface
area:volume ratio;
• Describe, with the aid of diagrams and
photographs, the distribution of xylem and
phloem tissue in roots, stems and leaves of
dicotyledonous plants;
• Describe, with the aid of diagrams and
photographs, the structure and function of xylem
vessels, sieve tube elements and companion
cells;
Transport in Plants
• Plants need a transport
system so that cells deep
within the plants tissues
can receive the nutrients
they need for cell
processes
• The problem in plants is
that roots can obtain
water, but not sugar, and
leaves can produce
sugar, but can’t get water
from the air
What substances need to be
moved?
• The transport system
in plants is called
vascular tissue
• Xylem tissue
transports water and
soluble minerals
• Phloem tissue
transports sugars
The Vascular Tissues
• Xylem and phloem
are found together in
vascular bundles, that
sometimes contain
other tissues that
support and
strengthen them
Root vs. stem vs. leaf
The vascular bundle
differs depending on if
it is a root or stem
Root
• The vascular bundle is found
in the centre
• There is a large central core of
xylem- often in an x-shape
• This arrangement provides
strength to withstand the
pulling forces to which roots
are exposed
• Around the vascular bundle
are cells called the endodermis
which help to get water into the
xylem vessels
• Just inside the endodermis is
the periycle which contains
meristem cells that can divide
(for growth)
Stem
• The vascular bundles are found near the outer edge of the stem
• The xylem is found towards the inside of each vascular bundle, the
phloem is found towards the outside
• In between the xylem and phloem is a layer of cambium
• Cambium is a layer of meristem cells that divide to make new xylem
and phloem
Leaf
• The vascular bundles
(xylem and phloem) form
the midrib and veins of
the leaf
• A dicotyledon leaf has a
branching network of
veins that get smaller as
they branch away from
the midrib
• Within each vein, the
xylem can be seen on top
of the phloem
Phloem
Xylem
Stem
A = Xylem
B = Phloem
C/D = Upper/Lower epidermis
Leaf
Xylem vessel wall
Xylem vessel
lumen
Phloem
Endodermis
Starch
grains
Root
Structure of Xylem
• Used to transport
water and minerals
from roots to leaves
• Consists of tubes for
water, fibres for
support and living
parenchyma cells
Xylem vessels
• Obvious in dicotyledonous plants
• Long cells with thick walls containing lignin
• Lignin waterproofs walls of cells and strengthens
them
• Cells die and ends decay forming a long tube
• Lignin forms spiral, annular rings or broken rings
(reticulate)
• Some lignification is not complete and pores are
left called pits or bordered pits, allowing water to
move between vessels or into living parts
Adaptations of Xylem to Function
• Xylem can carry water and minerals from roots
to shoot tips because:
• Made of dead cells forming continuous column
• Tubes are narrow so capillary action is effective
• Pits allow water to move sideways
• Lignin is strong and allows for stretching
• Flow of water is not impeded as: there are no
end walls, no cell contents, no nucleus, lignin
prevents tubes collapsing
Structure of Phloem
• Function to transport
sugars from one part
to another
• Made of sieve tube
elements and
companion cells
Sieve Tubes
• Sieve tube elements not true cells as they
have little cytoplasm
• Lined up end to end to form a tube
• Sucrose is dissolved in water to form a
sap
• Tubes (known as sieve tubes) have a few
walls across the lumen of the tube with
pores (sieve plates)
Companion cells
• In between sieve tubes
• Large nucleus, dense
cytoplasm
• Many mitochondria to
load sucrose into sieve
tubes
• Many plasmodesmata
(gaps in cell walls
between companion cells
and sieve tubes) for flow
of minerals
Water route between cells
• Apoplast: between cell
walls of neighbouring
cells
• Symplast: through
plasma membrane and
plasmodesmata to
cytoplasms from cell to
cell
• Vacuolar: same as
symplast, but also
through vacuoles
Water uptake from the soil
• Epidermis of roots contain root hair cells
• Minerals absorbed by active transport
using ATP
• Minerals reduce the water potential in the
cell cytoplasm (more negative) so water is
taken up by osmosis
Movement across the root
•
•
•
•
Active process occurring at the endodermis (layer of cells surrounding the
xylem, some containing waterproof strip called casparian strip)
Casparian strip blocks the apoplast pathway (between cells) forcing water
into the symplast pathway (through the cytoplasm)
The endodermis cells move minerals by active transport from the cortex into
the xylem, decreasing the water potential (more negative), thus water moves
from the cortex through the endodermal cells to the xylem by osmosis
A water potential gradient exists across the whole cortex, so water is moved
along the symplast pathway (through cytoplasm) from the root hair cells
across the cortex and into the xylem
Casparian Strip
• Blocks the apoplast pathway (cell walls)
• Water and dissolved nitrate ions have to pass
into the cell cytoplasm through cell membranes
• There are transporter proteins in the cell
membranes that actively transport nitrate ions
into the xylem lowering the water potential (more
negative)
• Water enters xylem down concentration gradient
and cannot pass back
Water movement up stem
• Root pressure: minerals move into xylem by
active transport, forcing water into xylem and
pushes it up the stem
• Transpiration Pull: loss of water at leaves
replaced by water moving up xylem. Cohesiontension theory- cohesion between water
molecules and tension in the column of water
(which is why xylem is strengthened with lignin)
means the whole column of water is pulled up in
one chain
• Capillary action: adhesion of water to xylem
vessels as they are narrow
How water leaves the leaf
• Through stomata
• Tiny amount through the waxy
cuticle
• Water evaporates from the
cells lining the cavity between
the guard cells, lowering water
potential and meaning that
water enters them by osmosis
from neighbouring cells, which
is replaced by further
neighbouring cells and so on
Transpiration
• Loss of water vapour from upper parts of the
plant
• Water enters leaf from xylem and passes to
mesophyll cells by osmosis
• Water evaporates from surface of mesophyll
cells to form water vapour (air spaces allow
water vapour to diffuse through leaf tissue)
• Water vapour potential rises in air spaces, so
water molecules diffuse out of the leaf through
open stomata
Transpiration: three processes
• Osmosis from xylem to mesophyll cells
• Evaporation from surface of mesophyll
cells into intercellular spaces
• Diffusion of water vapour from intercellular
spaces out through stomata
Water use in plant
•
•
•
•
•
Photosynthesis
Cell growth and elongation
Turgidity
Carriage of minerals
Cools the plant
Measuring transpiration
• Potometer is used to
estimate water loss
Factors affecting transpiration
• Leaf number: more leaves, more transpiration
• Number, size, position of stomata: more and large, more
transpiration, under leaf, less transpiration
• Cuticle: waxy cuticle, less evaporation from leaf surface
• Light: more gas exchange as stomata are open
• Temperature: high temperature, more evaporation, more
diffusion as more kinetic energy, decrease humidity so
more diffusion out of leaf
• Humidity: high humidity, less transpiration
• Wind: more wind, more transpiration
• Water availability: less water in soil, less transpiration
(e.g. in winter, plants lose leaves)
Too much water loss
• Less turgidity
• Non-woody plants wilt and die
• Leaves of woody plants die first then it will
die if water loss continues
Xerophytes
• Smaller leaves reducing surface area e.g. pine tree
• Densely packed spongy mesophyll to reduce surface area, so less
water evaporating into air spaces
• Thick waxy cuticle e.g. holly leaves to reduce evaporation
• Closing stomata when water availability is low
• Hairs on surface of leaf to trap layer of air close to surface which can
become saturated with water, reducing diffusion
• Pits containing stomata become saturated with water vapour
reducing diffusion
• Rolling the leaves so lower epidermis not exposed to atmosphere
also traps air which becomes saturated
• Maintain high salt concentration to keep water potential low and
prevent water leaving
Marram Grass
Leaf rolled up to trap
air inside
Thick waxy cuticle to
reduce water
evaporation from the
surface
Trapped air in the
centre with a high
water potential (less
negative)
Stomata in pits to
trap air with moisture
close to the stomata
Hairs on lower
surface reduce
movement of air
Movement of Sugars
• Translocation: movement of assimilates (sugars
and other chemicals) through the plant
• Source: a part of the plant that releases sucrose
to the phloem e.g. leaf
• Sink: a part of the plant
that removes sucrose from
the phloem e.g. root
Sucrose Entering the Phloem
• Active process (requires energy)
• Companion cells use ATP to transport hydrogen
ions out of their cytoplasm
• As hydrogen ions are now at a high
concentration outside the companion cells, they
are brought back in by diffusion through special
co-transporter proteins, which also bring the
sucrose in at the same time
• As the concentration of sucrose builds up inside
the companion cells, they diffuse into the sieve
tubes through the plasmodesmata (gaps
between sieve tubes and companion cell walls)
Sucrose movement through
phloem
• Sucrose entering sieve tube lowers the water
potential (more negative) so water moves in by
osmosis, increasing the hydrostatic pressure
(fluid pushing against the walls) at the source
• Sucrose used by cells surrounding phloem and
are moved by active transport or diffusion from
the sieve tube to the cells. This increases water
potential in the sieve tube (makes it less
negative) so water moves out by osmosis which
lowers the hydrostatic pressure at the sink
Movement along the phloem
• Water entering the phloem at the source,
moving down the hydrostatic pressure
gradient and leaving at the sink produces
a flow of water along the phloem that
carries sucrose and other assimilates. This
is called mass flow. It can occur either up
or down the plant at the same time in
different phloem tubes
Evidence for translocation
• Radioactively labelled carbon from carbon dioxide can appear in the
phloem
• Ringing a tree (removing a ring of bark) results in sugars collecting
above the ring
• An aphid feeding on the plant stem contains many sugars when
dissected
• Companion cells have many mitochondria
• Translocation is stopped when a metabolic poison is added that
inhibits ATP
• pH of companion cells is higher than
that of surrounding cells
• Concentration of sucrose is higher at
the source than the sink
Evidence against translocation
• Not all solutes move at the same rate
• Sucrose is moved to parts of the plant at
the same rate, rather than going more
quickly to places with low concentrations
• The role of sieve plates is unclear
Useful Revision Sites
• http://scienceaid.co.uk/
• http://www.s-cool.co.uk/
• http://www.sparknotes.com/biology/