14.1.1 Transport of Nutrients in Plants

Maintaining a Balance
Topic 14: Transport of Nutrients in Plants
Biology in Focus, HSC Course
Glenda Childrawi, Margaret Robson and Stephanie Hollis
 Describe current theories about processes responsible for
the movement of materials through plants in xylem and
phloem tissue
The role of transport in plants is
mainly to carry materials for
photosynthesis to the cells and
to move cell products away to
other parts of the plant. In small
plants, this may be achieved by
diffusion and active transport,
but in larger plants, specialised
vascular tissue has developed to
serve this transport function.
The vascular system in plants consists of vessels of xylem and
phloem and the movement of materials from one part of the
plant to another is known as translocation.
Chemical substances that are
needed for photosynthesis (such
as water mineral ions) are
carried by xylem tissue from
the roots (the site of
absorption) up to the leaves
where they will be used for the
manufacture of food
(photosynthesis). Xylem tissue
consists of xylem vessels,
tracheids, fibres and
parenchyma cells.
Phloem vessels are involved in
the transport of organic
nutrient products (particularly
sugars, amino acids and plant
hormones) to all parts of the
plant. Movement occurs in two
directions – up towards the
flowers and down to the roots.
Phloem tissue consists of
phloem fibres, phloem
parenchyma, sieve cells and
companion cells.
Theories about Xylem and Phloem
Experimental evidence has
shown the type of materials that
move through xylem and
phloem in plant stems and the
directions in which they move,
but the explanation of how this
movement occurs in each is
presented as a theory.
Remember a theory is a
scientist’s explanation of a
phenomenon, based on
Theories about Xylem and Phloem
The theories of how movement of substances occurs in plants
have been tested by examining whether their consequences
(predictions) are borne out by observation and experimentation.
They have modified over time, but the current most commonly
accepted theories are:
 The transpiration stream theory (cohesion-adhesiontension theory) of movement of water and mineral ions in
 The pressure flow theory (source-path-sink theory) of
translocation of organic nutrients
in phloem.
The Transpiration Stream Theory
The transpiration stream in
xylem occurs due to physical
forces that result from water
(and ions) being moved by
passive transport.
 A column of water is sucked
up the stem by the
evaporative pull of
transpiration and is known as
the transpiration stream.
The Transpiration Stream Theory
Once the water has been
absorbed into the roots of
plants (by osmosis) along with
mineral ions (by diffusion and
active transport), these
substances move across the
root into the xylem. A small
amount of root pressure results
from the continual influx of
more water and ions, forcing
the solution already present in
the xylem upwards.
The Transpiration Stream Theory
Most of the upward
movement in xylem
seems to be as a result of
the transpiration stream
– water is drawn up the
xylem tubes to replace
the loss of water from
the leaves by
transpiration. This is
based on evidence
gathered by biologists.
The Transpiration Stream Theory
 Xylem vessels are hollow and narrow,
offering very little resistance to the flow of
 The physical properties of water contribute
to the formation of a continuous stream.
Adhesive forces lead to capillarity (water
rises up the bore of xylem) and cohesive
forces (the attraction of water molecules to
each other) which ensure that a continuous
column of water that moves upwards is
maintained in the xylem vessels
The Transpiration Stream Theory
Evidence continued:
 A concentration gradient exists across the leaf:
 At the surface of the leaf, the osmotic pressure is high (water
concentration is low) because water is continually being lost by
evaporation through the stomata (transpiration)
 In the centre of the leaf the osmotic pressure is low (water
concentration is high)
The Transpiration Stream Theory
The flow of the
transpiration stream
can be explained by
this diagram:
The Pressure Flow Theory
Translocation in phloem tissue
moves products of
photosynthesis (such as glucose,
sucrose and amino acids) by
active transport. Up to 90% of
the dissolved substances in the
sap of phloem is sucrose
(common sugar). When sucrose
reaches the cells, it may be
converted back to glucose for
respiration or starch for
The Pressure Flow Theory
The flow of minerals in phloem
is an active process that requires
energy. The mechanism of flow
is driven by an osmotic pressure
gradient, generated by
differences in sugar and water
concentrations. It involves the
active loading of sugar into
phloem at one end and then the
active unloading from phloem
into the surrounding tissues at
the other end (the sink).
The Pressure Flow Theory
The loading of sugar into phloem at the source attracts water to
flow in and the offloading at the sink causes the water to flow out
of the phloem. This is why its called ‘pressure flow’.
The Pressure Flow Theory
Loading at the Source
Amino acids, sucrose and other mineral nutrients are loaded into
the phloem in the leaves. There are two theories as to how this
1. Symplastic loading – sugars and other nutrients move in the
cytoplasm from the mesophyll cells to the sieve elements
through the plasmodesmata (strands of cytoplasm that pass
through pits in the cell walls)
The Pressure Flow Theory
Loading at the Source
Apoplastic loading – sugars and nutrients move along a
pathway through the cell walls until they reach the sieve
element. They then cross the cell membrane to enter the
phloem tube. These sugars pass into the sieve cell by
active transport.
The Pressure Flow Theory
Loading at the Source
As sugars enter the phloem,
the phloem sap becomes more
concentrated and so the
osmotic pressure at the source
end is high. This draws water
into the phloem, from the
adjacent xylem tissue by
osmotic pressure. (blue
The Pressure Flow Theory
Offloading at the sink
Materials flow to the sink. At
the sink (for example roots,
flowers or any part of the plant
that needs nutrients), sugars
and materials are removed from
the phloem by active transport.
(red arrows)
The Pressure Flow Theory
Offloading at the sink
As sugars move out of the phloem, they draw water out with
them (by osmosis). This results in a lower osmotic pressure due
to the higher water concentration in the phloem at the sink
The Pressure Flow Theory
Pressure Flow (along the ‘path’)
This difference in osmotic pressure between the source and the
sink in the phloem drives the phloem sap to flow. The direction of
flow depends on where the sink areas (roots or flowers) of the
plant are, in relation to the source (leaves).
The Pressure Flow Theory
Pressure Flow (along the ‘path’)
Water can move into the phloem by
osmosis at any point along the
gradient. The flow is continuous,
because sucrose is continuous being
added at one end and removed at the
-Complete Transverse and Longitudinal Sections of Xylem and
Phloem PRAC

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