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Transport in Plants Revision

Transport systems in dicotyledonous plants
The need for plant transport systems:
Metabolic demands:
• The cells of the green parts of the plant make their own glucose and oxygen by
photosynthesis
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They need oxygen and glucose transported to them and the waste products of
cell metabolism removed
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• Mineral ions absorbed by the roots need to be transported to all cells to make
the proteins required for enzymes and the structure of the cell
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Perennial plants (that live a long time and
reproduce year after year) are large
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The
mountain ash in Australia is up to 114m tall
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Surface area: volume ratio
• Leaves are adapted to have a relatively large SA:V ratio for the exchange of gases
with the air
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This means they cannot rely on diffusion alone to supply their cells
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There are herbaceous dicots with soft tissues and a
relatively short life cycle
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Dicotyledonous plants have a series of transport vessels
running through the stems, roots and leaves – the vascular
system
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Transport systems in dicotyledonous plants
Observing xylem vessels in a living plant stem:
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Xylem vessels can be stained and seen in
transverse and longitudinal sections of plant
stems/roots on prepared slides
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Plant material, like celery stalks with lots of leaves,
or the roots of germinating seeds, are put in water
containing a strongly coloured dye for at least 24
hours
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1) In one specimen, make clean transverse cuts along
the stem with a sharp blade on a white tile
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3) In another specimen, make a longitudinal cut
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The structure and functions of the xylem:
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The xylem is a largely non-living tissue
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The flow of materials in the xylem is up from the roots to the shoots
and the leaves
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The xylem vessels are the main structures
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Thick-walled xylem parenchyma packs around the xylem vessels,
storing food and containing tannin deposits
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Xylem fibres are long cells with lignified secondary walls that
provide extra mechanical strength but do not transport water
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It can form rings, spirals or relatively solid tubes with lots of small
unlignified areas called bordered pits
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Transport systems in dicotyledonous plants
The structure and functions of the phloem:
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Phloem is a living tissue that transports food in the form of organic solutes around the plant
from the leaves where they are made for photosynthesis
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The flow of materials in the phloem can go up and down the plant
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Like the xylem, the phloem sieve tubes are made up of many cells joined end to end to form a
long, hollow structure
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In the areas between the cells, the walls are perforated to form sieve plates, which let the
phloem contents through
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The phloem becomes a tube filled with phloem sap and the mature phloem cells have no
nucleus
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)
Companion cells maintain their nucleus and organelles
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Phloem tissue also contains supporting tissues including fibres and sclereids (cells with
extremely thick cell walls
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Turgor pressure (or hydrostatic pressure)
as a result of osmosis in plant cells provides
a hydrostatic skeleton to support the stems
and leaves
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The loss of water by evaporation helps to
keep plants cool
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Water is a raw material for photosynthesis
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A root hair is a long, think extension from a root hair cell, a specialised
epidermal cell found near the growing root tip
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• Each microscopic hair has a large SA:V ratio and there are thousands on each
growing root tip
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• The concentration of solutes in the cytoplasm of root hair cells maintains a
water potential gradient between the soil water and the cell
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Cytoplasm and vacuolar sap of the root hair cell contain many different
solvents (sugars, mineral ions, amino acids) so the water potential in the cell is
lower
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Water transport in multicellular plants
Movement of water across the root:
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Once the water has moved into the root hair cell, it
continues to move across the root to the xylem in one of two
different pathways
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The root hair cell has a higher water potential than the next
cell along – the result of water diffusing in from the soil to
make the cytoplasm more dilute
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This process continues from cell to cell across the root until
the xylem is reached
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This is the movement of water through the apoplast (cell
walls and intercellular spaces
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As water molecules move into the xylem, more water
molecules are pulled through the apoplast behind them due
to the cohesive forces between the water molecules
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Water transport in multicellular plants
Movement of water into the xylem:

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Water moves across the root in the apoplast and symplast pathways until it reaches the endodermis – layer of cells surrounding the
xylem and phloem of the roots
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The Casparian strip is a band of waxy material called suberin that runs around each of the endodermal cells forming a waterproof layer
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This diversion to the cytoplasm is significant, as to get there water must pass through the selectively permeable cell surface membranes
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Once forced into the cytoplasm, the water joins the symplast pathway
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The endodermal
cells also move mineral ions into the xylem by active transport
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This increases the rate of water moving into the xylem by osmosis down a water potential gradient from the endodermis through the
symplast pathway
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The active pumping of mineral ions into the xylem to produce movement of water by osmosis results in root pressure, and it is
independent of the effects of transpiration
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Water transport in multicellular plants
Evidence for the role of active transport in root pressure:
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Some poisons, like cyanide affect the mitochondria and prevent the production of
ATP
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Root pressure increases with a rise in temperature and falls with a decrease in
temperature, which suggests that chemical reactions are involved
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Xylem sap may exude from the cut ends of stems at certain times
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This is guttation
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Carbon dioxide and water are both
needed so, for successful
photosynthesis to take place in a
leaf, water must be transported
there from the roots and carbon
dioxide must be taken into the cells
of the leaf from the air
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In a process of gaseous exchange,
oxygen also moves out of the leaf
cells into the air spaces by diffusion
down a concentration gradient
(oxygen is a waste product of
photosynthesis
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The process of transpiration:
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Leaves have a very large surface area for capturing sunlight/carrying out photosynthesis
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This is an
important adaptation that prevents the leaf cells losing water rapidly and constantly by
evaporation from their surfaces
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They are
usually on the underside of the leaf
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When the stomata are open, water vapour also moves out by diffusion and is lost
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Transpiration is an inevitable consequence of gaseous exchange
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But during the day, a
plant needs to take in carbon dioxide for photosynthesis
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So at least some stomata need to be open all the time

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