The Movement of Water, Ions and Organic Solutes in Plants
Green plants can deceive. As we bustle past them in their hundreds, on
our way to school or work, we see them standing in formation or
swaying in the breeze in peopleÂ’s front gardens and they could be
mistaken for very idle organisms. However, we couldnÂ’t be more wrong Â–
as transport vessels, plants
are busy all the time; they transport the
products of photosynthesis (mainly sucrose) all over their internal
surface and amino acids, water, ions
and other solutes
travel from the
With so much to transport, and a lot of it up the plant, Nature is
posed a problem; a plant has no pumping organ, so how can vital
substances possibly reach the upper levels of plants without expending
a lot of energy? And when the products get there, how does the plant
know what to deposit, and where?
In this essay, I will seek to answer these questions while increasing
my understanding of the cells and mechanisms associated with the
movement of water, ions and organic solutes in plants.
To understand water movement through the plant, the key principles of
osmosis (and therefore diffusion) and water potential must be
v [IMAGE]The molecules that make up substances are constantly moving
about in a random way. This causes them to move from an area of higher
concentration to an area of lower concentration, along a concentration
gradient. Diffusion occurs in cells and across cell membranes when the
membrane is fully permeable to the solute present or the pores in the
membrane are big enough to allow the solute through. Diffusion is a
passive process Â– it requires no energy other than the kinetic energy
of the continuously moving molecules. A greater concentration
gradient, over a shorter distance, over a larger area and through a
thinner membrane (preferably with pores) will increase the rate of
v Osmosis is a particular kind of diffusion that occurs when water
diffuses across partially permeable membrane, from an area of higher
water potential (higher concentration of Â‘freeÂ’ water molecules) to an
area of lower water potential. The water molecules are smaller than
the solute molecules, so only they can fit through this type of
membrane. NB. A hypertonic solution has a higher concentration of
solute molecules and a hypotonic solution has a lower concentration of
the solute. When two solutions have the same solute concentration,
they are isotonic. This is the ultimate goal of both osmosis and
v The water potential of a solution (Ïˆ) is the potential of water
molecules to diffuse out of a solution. So, water is more likely to
diffuse out of a solution with a higher water molecule concentration,
a hypotonic solution, and it therefore has a high water potential.
From this, we can infer that pure water will have the highest water
potential, set at 0kPa (zero). Dissolving a solute into a solution
reduces the water potential, making it negative. Water potential in a
cell is affected by solute potential (the amount by which solute
molecules reduce water potential, Ïˆs) and pressure potential (the
pressure exerted on the water by the cell membrane, Ïˆp).
So, having cleared up the key areas, we can start on the journey of a
water molecule through a plant and why not begin at the journeyÂ’s very
commencement, in the rootsÂ…
The roots of plants are important organs in the process of water
uptake because they provide the link between the soil and the xylem,
thus allowing water into the plant in the first place. The images on
the next page show the structures of the root cells in typical
dicotyledonous roots. They show the main characteristics of the large
cortex made up of the parenchyma cells (used for storing starch and
some other substance), with small gaps in between them (important for
the aeration of the root tissue, which is non-photosynthetic). The
xylem and phloem are at the centre of the stele, surrounded by a ring
of cells called the pericycle (from which lateral roots arise), which
is itself surrounded by the endodermis (a single layer of cells
bearing a band of waterproof suberin, called the Casparian Strip).
The uptake of the water occurs by osmosis as there is a lower water
potential in the root hair cells than in the soil due to the
continuous loss of water through the stomata in the leaves. The root
hairs are all around the outside of young dicot roots, which is why
they donÂ’t feature in the above images (they are older dicot roots).
They increase the surface area for absorption and provide less of a
barrier for the water to cross because they have no waterproof layer.
Once the water has diffused into the epidermal cells of the root
through the hairs, there are three paths it can take to the xylem
v Around 90% of water takes the apoplast pathway. This goes through
the cell walls and water-filled dead cells in the cortex, which are
very absorbent and water can diffuse through and in between them.
However, when the pathway takes the water to the endodermis and it
hits the Casparian strip, it cannot penetrate the waterproof layer and
has to join one of the other paths.
v Some water also travels through the living parts of the root cells Â–
the cytoplasm. It crosses the gaps between cells through cytoplasmic
connections called plasmodesmata and diffuses straight into the xylem
after passing the endodermis (where it collects water from the
apoplast pathway). This route is called the symplast pathway.
v A small amount of water also passes into cell vacuoles by osmosis,
then from individual cell to individual cell to xylem vessel. This is
called the vacuolar pathway and is accountable for very little of the
water that reaches the xylem vessels.
The reason the water moves at all is due to a comparatively high Ïˆcell
near the epidermal cells and a lower Ïˆcell further inside the root,
around the vascular tissues. This may be due to less turgid cells,
lower wall pressure or more dissolved substances due to water being
removed by transpiration.
The water has reached the xylem, the Â‘business endÂ’ of water
transportation in plants. Like their solute-carrying counterparts,
phloem, xylem are found throughout the plant Â– the xylem tissue
transports water and mineral ions up through plants. The xylem are
also important in maintaining the structure of the plant body, despite
their thin appearance. This unexpected property is entirely
representative of the complex nature that the variety of cells present
in xylem tissue bring to this Â‘transport vesselÂ’.
The xylem is made up of five types of cell:
v Vessels, unique to angiosperms, are long tubes consisting of a lot
of dead vessel elements placed one on top of the other. They are
thickened with lignin secondary walls except on their end walls, which
are very thin and perforated to allow water to pass through them
easily in an uninterrupted flow. Vessel elements are generally wide in
diameter and can be looked at as a more advanced version of the
tracheid cell although, like tracheids, they still contain pits to
allow substances in and out of the vessel.
v The tracheid cells are long, lignified cells that do the same the
job as vessel elements. The difference is that tracheid cells overlap
each other so any water flow happens through the pits and bordered
pits at each end of it.
v Parenchyma cells are the only living cells in the xylem. Their walls
are not lignified so they perform little as far as structure
maintenance goes. Their role, however, is important; they store food
and allow radial transport (movement of substances across the stem,
from the outside to the middle, rather than up and down it).
v One of the cells that do perform a structural function are the fibre
cells. They are actually a type of sclerenchyma, along with sclereids.
They are elongated, thick, lignified cells, which give mechanical
support to tissues when they are bundled together. Their cell walls
are very thick, but they can afford a thin lumen because they do not
v The other sclerenchyma cell is the shorter scelreid cell. It differs
from fibre cells in its varying shapes and abundance of pitting. They
are found around the parenchyma cells.
[IMAGE]The vessel elements and tracheids are the cells that do the
transporting. They are surrounded by extra cells for exchange and
storage (parenchyma cells) and support (fibre and scelreid cells). The
diagram below shows the arrangement of the dead cells in vascular
The water travels some way up the xylem by the force of root pressure.
This is created by the very negative water potential of the root
cells, which are constantly expending energy to pump in ions. Water
therefore floods into the root cells, which become turgid enough for
them to exert a pressure on water in the xylem, forcing it up as far
as a meter. In very small plant, this can lead to water being forced
out through the leaves (guttation). After the effects of root
pressure, the transpiration stream takes over in transporting the
water to the leaves.
Plants lose water all the time through their stomata, despite
adaptations that reduce the loss (e.g. waxy cuticle). This rapid loss
of water from the leaves creates a pull on the water in the leaf
cells, since all water molecules cohere to each other due to their
special polar properties. So, water is drawn from the leaf cells,
creating a negative tension proportional to the diameter of the
curvatures in the cell walls. This is compensated for by drawing up
water from the xylem in the leaf, which is replaced by water from the
xylem in the stem, which is replaced with water from the roots etc.
etcÂ…. This process creates a continuous stream of water, from roots to
shoots, which is made possible by cohesion between water molecules,
adhesion between water and the dead xylem cells and the decreasing
concentration gradient as you go further up the plant or tree (from
around Ïˆ = -10kPa in the soil to as much as Ïˆ = -30,000kPa in the
The rate of transpiration is at its greatest in low humidity (the air
water concentration is low), high temperature (speeds up particle
movement and therefore, evaporation), heavy winds (disperse water
vapour well, maintaining the concentration gradient) and with a
considerable, consistent water supply (so the cells arenÂ’t flaccid and
the stomata donÂ’t close as a consequence).
The one negative aspect of this process is that any gases in solution
are drawn out as bubbles, or embolisms, which can block the flow of
water, creating cavitation. However, it is a sign of the
specialisation of vessel elements that, with their pits, they can
simply re-route the water around the embolisms until the night time,
when they dissipate as the negative pull on the xylem water deceases
when the stomata close.
Overall, it is the combined efforts of individual cells and the
conductive tissue of the vascular system that enable the plant to have
an important, constant supply of water to the paradoxical situation
that exists between photosynthetic gases and water in the stomata.
[IMAGE] To understand the uptake and use of ions in plants, the
principles of facilitated diffusion and active uptake (ion pumps) have
to be understoodÂ…
v Facilitated diffusion is, like diffusion, a passive process of
substance uptake. It works by selectively increasing the diffusion
rate for important larger molecules (e.g. glucose) and charged
molecules (ions) that cannot otherwise permeate the phospholipid
bilayer. They are allowed to pass through channel proteins
(pore-forming) or carrier proteins (shape changing). The latter are
specific to certain molecules, thus effectively increasing their rate
of diffusion. However, facilitated diffusion can only transport
molecules across the concentration gradient.
v [IMAGE]Active transport and, more specifically, ion pumps are a form
of transport that move substances against a concentration gradient
when the substances gained by facilitated diffusion do not suffice.
The process harnesses ATP, which releases a phosphate group onto a
carrier protein, giving it the energy to change shape and transport
the molecule of glucose, sodium etc. across the membrane and across
the concentration gradient.
Ions are needed by plants to produce a lot of the more important
substances in plant life Â– proteins, lipids, growth factors and
others. The path of ions through the plant is very simple, as part of
it takes place in the water content in the xylemÂ…
Plants need to take up only certain ions and need to get rid of
others. Many of these ions canÂ’t simply diffuse across the
concentration gradient because the plant either needs to hold on to
them even though there is a high concentration of them in the root
cells already, or there is a low concentration of them inside but the
plant doesnÂ’t even require that. So, after the few useful ions enter
and exit passively, through facilitated diffusion, active uptake needs
to occur. There are three main ways in which ions enter the roots
v The sodium-potassium pump is a specific carrier protein that uses
ATP to exchange sodium (Na+) for potassium (K+).
v ATP driven proton pumps use this energy to remove hydrogen ions (H+)
to the soil solution. This results in an imbalance of protons either
side of the membrane and the plasma membrane becomes negatively
The large concentration gradients created by the above two pumps can
be used to drive the transport of other molecules. For instance,
without the high concentration of H+ ions outside the root created by
the proton pump, the following transport wouldnÂ’t have enough energy
v In coupled transport, specific carrier proteins allow H+ ions back
in to the root, but they are now coupled with important nutrients such
as sucrose, which separate from each other inside the root and go
their separate ways.
After they are in the roots, the ions that need to be used in the stem
and leaves are transported as dissolved mineral ions in the water
travelling in the same direction, using exactly the same pathway as
described previously in the Â‘water transportÂ’ section. The ions are
deposited at the points they are needed, exiting the vessel elements
and tracheid cells.
The phloem deals with solute transportation in plants, mainly amino
acids and sugars, produced in the root tips and leaves, respectively.
There is some entry of sugars through the roots, and I have already
used it as an example of a secondary active pump. Sucrose enters roots
through the coupled transport method and then travels to the phloem
sieve tubes using both apoplastic and symplastic pathways. However,
because it is the translocation method that will be explained, I will
ignore this sucrose so as to avoid confusion.
To understand solute transport in the phloem, its structure has to be
understood first. The phloem tissue contains very different
specialised cell types to the xylem, the four of which are listed
below. The main difference, of course, is that the phloem cells are
living, as opposed to the xylem cells, which are (apart from the
parenchyma cells) dead:
v The sieve tube elements are the living cells that, joined end to
end, transport the solutes through the phloem. Their name comes from
the end walls of the cells, which have a lot of small perforations to
let the sap through (end walls are know as Â‘sieve platesÂ’). They have
a very thin layer of cytoplasm, but no nucleus.
v Text Box: Because of their lack of a nucleus, sieve tube cells
cannot control what goes in and out of them. This job falls to the
companion cells, which are adjacent to the sieve elements. These have
a dense and active cytoplasm, which tells part of the story because it
does the living for both itself and the sieve tube elements. For
instance, it controls the loading and unloading of solute into and out
of the sieve cells during translocation.
v Fibres are associated with phloem as they are with xylem, in
strictly supportive roles. Their characteristic very think cell wall
v Parenchyma cells also exist alongside the phloem and their roles
have remained as storage cells, although to a lesser importance than
in xylem vessels because some of the other live cells, like the
companion cells, can take care of this function, to an extent.
To move on to the function of the phloemÂ… Translocation is the method
by which the plant transports the organic products of photosynthesis
(sugars) from the site they are created, to the sites they are needed,
in the phloem. The theory to explain this transport is that of
Sugar is transferred from a source cell (where it is made by
photosynthesis) into sieve tube cells in the phloem, decreasing the Ïˆ
there. This Â‘loadingÂ’ is an active process, requiring a certain amount
of energy from respiration. This causes the sieve cells to take up
water from the xylem vessels, which travel across the vascular bundle
and into the phloem by osmosis. The water absorption creates turgor
pressure that gradually builds up as more solute is produced and more
water enters, until it is high enough to generate a bulk flow which
forces the phloem sap, laden with photosynthates, in the direction of
a sink cell (i.e. an area of higher Ïˆ). As the sap moves,
photosynthates are transported out of the phloem into the sink cells.
Their leaving the phloem causes the Ïˆ of the phloem sap to rise in the
positive direction until it becomes more positive than the xylem
(which is carrying ions as well as water), at which time the water
moves back into the xylem by osmosis, and continues once more on its
journey with the transpiration stream.
[IMAGE]The reason the phloem sap can deposit most of its solute load,
is because the sinks donÂ’t quickly attain a lower water potential than
the phloem sap as they receive the photosynthates, as might be
expected, because enzymes maintain the large concentration gradient by
modifying organic substances at the sink.
It has to be said that this explanation is very simple; indeed, only
now are scientists beginning to discover the subtle details of phloem
movement in plants. So much for thinking we know how translocation
really works - this essay will be invalid very soon! At least it can
be said that the issues of water and ion transportation have been
explained, along with the structures of the plant root, xylem and
phloem system, to give a deep knowledge of why, not just how, water,
ions and organic solutes are moved through plants.