Video Transcript
In this video, we will learn how
water moves from the roots through the plant to the leaves in a special type of
vascular tissue, the xylem. We will learn how various
scientists had various theories as to how exactly this transport happens and how one
theory after the other was rejected until finally one was found that can explain how
the plant does this, the cohesion-tension theory.
Water is essential for a plant’s
ability to thrive and grow. Why is this the case? Water is a key reactant in
photosynthesis, which is the process by which many plants are able to make their own
food in the form of glucose. Photosynthesis requires light
energy, which is usually supplied by the Sun. As a result, photosynthesis mostly
takes place in the leaves, which are found above ground trying to absorb as much
light as possible.
Plants require a lot of water, not
only as a reactant for photosynthesis, but also because as photosynthesis proceeds,
a large amount of water is lost directly to the atmosphere in the form of water
vapor, mostly through pores, the majority of which are found on the underside of
leaves, called stomata, or a singular stoma. This loss of water through
evaporation into the atmosphere is called transpiration. It’s estimated that around 90
percent of the water that’s absorbed daily by plants is lost through
transpiration. In this video, we’ll try to
understand how transpiration can actually be a driving force of water transport in
plants.
When watering a plant, we should
always try to water the soil in which the plant grows, as by watering the soil, we
are effectively watering the roots of the plant. The roots are the plant organs that
are usually responsible for absorbing water molecules from the soil. But how does the water that’s
absorbed in the roots make its way to the leaves where it’s needed for
photosynthesis? It flows upward from the roots to
the leaves through specialized vessels called xylem vessels. The main function of xylem vessels
is to transport water in one direction from the roots through the stem and to the
leaves.
Xylem vessels are very long
tubelike structures made of individual cells that are stacked end to end. There are no end walls between the
cells in xylem vessels. So the vessels form continuous open
tubes that water can flow through easily and without much force. Xylem tissue starts as living
cells, but as the cells mature, they die and form the hollow tubes that make up the
xylem vessels. For water transport through the
plant, the xylem vessel is a path of least resistance and probably the most direct
pathway to the leaves. The xylem sap that runs through the
xylem vessels mainly consists of water molecules. But it also contains some dissolved
mineral ions that have been absorbed into the roots from the soil. But how does the water flow upwards
from the roots to the leaves?
Gravity pulls water downward toward
the earth. So there must be some sort of
mechanisms that are stronger than the gravitational pull that are able to move the
water upwards through the xylem vessels. The first theories of how water is
transported in the plant focused on determining whether there were physical forces
that work in the xylem vessels that may help move water upward against gravity. The first scientist to attempt to
answer the question of how water is transported in plants focused on the
similarities between xylem vessels and blood vessels. Noting that both had a very small
diameter, it was suggested that water moved through the plant using capillary
action.
Capillary action is the ability of
water molecules to travel upward against the pull of gravity within a narrow
space. A simple experiment using a bowl of
water and a series of tubes with decreasing capillary sizes can demonstrate
this. This phenomenon was first described
by Leonardo da Vinci in the 17th century with this simple experiment. It showed that in plants, the xylem
vessels could raise the level of the water to its intended destination by becoming
smaller and smaller in diameter. This simple experiment with glass
tubes demonstrates that the statement “The smaller the capillary, the higher the
water rises” is true.
However, a series of calculations
by various other scientists showed that even the thinnest of capillaries would not
be able to lift water to heights above 150 centimeters. As most plants, especially tall
trees, are well over 150 centimeters in height, capillary action alone cannot
explain the movement of water up through the xylem vessel. Additionally, for capillary action
to work best, there must be direct contact between the xylem and the water in the
soil, which is not the case. Since the math and experiments both
failed to support this theory, it quickly fell out of favor.
In 1874, the great botanist doctor
Julius Sachs suggested a new view on water’s transport upward through plants based
on water absorption. Sachs supported the view that
imbibition, which is the absorption of water through the cell walls of the xylem
vessels, is the physical force behind water transport in the plant. The word “imbibition” derives from
the Latin word “imbibe,” meaning to drink. Given that plants absorb a lot of
water, imbibition theory seemed to make sense. As imbibition is the absorption of
water by the cell walls of plant cells, the cells found in dry seeds that are placed
into water like these ones become swollen in appearance, which makes the whole seed
become swollen. This process is typically observed
when dry dormant seeds absorb water molecules from soil through imbibition. This is one of the factors that
leads to seed germination.
Imbibition is important for seed
germination, as it helps the seeds to increase in size and sprout. The water from soil is absorbed
efficiently by the seed coat of a germinating seed. Upon closer inspection, the
absorption of water occurs in the roots that are embedded in the soil and not along
the entire length of the xylem vessel. The force of imbibition is also
very weak. Therefore, despite its importance
in seed germination, imbibition theory is not a reasonable explanation for the
movement of water up through the xylem vessels.
If you cut the stem of a plant at
its base, you will see the xylem sap ooze or flow up from the roots. This phenomenon is called
exudation. In 1920, the observation of
exudation in plants is what caused another plant biologist, doctor Priestley, to
suggest that root pressure might be the physical force behind water transport in
plants. You might recall that due to
pressure gradients, liquids like water tend to flow from areas of high pressure to
areas of comparatively lower pressure. It is this high pressure in the
roots that causes water to flow up through the stem. And it’s the phenomenon that doctor
Priestley used as the base of his theory.
Root’s pressure is a positive
pressure that develops in the xylem vessels in the root. This occurs due to the absorption
of water into the roots by osmosis. However, normally observed, root
pressure is pretty low, and it’s unable to raise water to the tops of very tall
trees. Furthermore, water transport does
not always require the roots. This can be best observed with a
fresh flower and two beakers of food coloring.
If a plant stem is split and each
part of the split stem is placed into a different beaker of colored water, the color
of the water in these beakers might start to show in the petals of the flowers from
a few hours to a day later. This is due to the transport of
water and the dissolved dyes through the xylem. In this simple experiment, we can
see that the transport of water has occurred in this cut flower, even in the absence
of roots. So, if water still moves up the
stem even in the absence of roots, root pressure is not enough to explain how water
is transported upward in plants.
Finally, an unexpected team of a
botanist and a physicist were able to figure out the best explanation for how water
transport in plants occurs. In 1894, a botanist named Henry
Dixon and a physicist named John Jolly developed the idea of cohesion tension. They suggested that the loss of
water in the leaves through transpiration might account for the movement of water
through the plant.
In order to understand their
theory, let’s first review what transpiration is and how it occurs. This diagram shows a side-view
cross section of some of the cells in a leaf. The leaves of many higher plants
have a waxy cuticle through which water cannot diffuse to reduce water loss. However, this would also prevent
the exchange of gases, like carbon dioxide and oxygen, from entering and leaving the
plant’s leaves. Plants must absorb carbon dioxide
as this is a key reactant in photosynthesis. So there are spaces in this waxy
cuticle that we looked at earlier, called stomata, through which carbon dioxide can
diffuse into the leaf. Once it’s in the leaf, carbon
dioxide can diffuse into the cells where it’s needed for photosynthesis.
Photosynthesis releases the
byproduct oxygen. Oxygen usually moves in the
opposite direction to carbon dioxide, first diffusing out of the leaves’
photosynthesizing cells, then through the stomata, and into the atmosphere. However, when the stomata are open
to allow this exchange of gases for photosynthesis, some water is also lost to the
atmosphere in the form of water vapor. This happens at an incredibly fast
rate through transpiration.
Transpiration might sound like a
bad thing as the plant is losing its precious water. However, transpiration also starts
a chain of events that helps to replace this lost water. Let’s simplify our diagram so that
we can see how this happens more clearly. When water evaporates from the
leaves and exits the stomata in the form of water vapor, a tension or pull on the
water in the xylem vessels arises. This draws water upwards from the
roots. It is the biochemical forces of the
water molecules that allow water to move against the downward pull of gravity up
through the plant.
The transpirational pull of water
evaporation from the leaves creates a chain reaction of pulls all down the column of
water molecules that line the xylem vessel. Let’s discuss the biochemical
forces of these water molecules in more detail. The water molecules in the xylem
vessel are strongly attracted to each other, which makes them stick together. This stickiness between water
molecules is called cohesion, and it’s been represented in this diagram by red
dashes between the water molecules. The cohesion between water
molecules is due to the strong hydrogen bonds that form between different water
molecules, causing them to stick together.
Cohesion between water molecules in
the xylem vessels is complemented by adhesion. In this diagram, adhesion has been
represented by orange dashes between the water molecules and the xylem vessel
walls. The stickiness of hydrogen bonds in
water also helps it to stick or adhere very tightly to the cell walls of the xylem
vessels. Both cohesion and adhesion are
related to sticking together, but the objects they’re sticking to are very
different. While cohesion occurs between
different water molecules, adhesion is the attraction between one water molecule and
the xylem vessel walls.
The cohesion between different
water molecules helps to form a continuous column of water, while adhesion helps
this column of water to move upward against the downward force of gravity. Cohesion and adhesion therefore
work together to pull water molecules from the roots, through the xylem vessels of
the stem, and out of the plant through the stomata on the leaves. This is often referred to as the
cohesion-adhesion or the cohesion-tension theory of water transport.
There are a few conditions that
need to be met for the transpiration pull to occur in the xylem. The xylem vessel must act as a
capillary tube. The tubes must be free of gas or
air bubbles, and there must be no breaks in the xylem vessels. You may recall that four main
environmental factors can affect the rate of transpiration. They are light, temperature,
humidity, and wind. These factors can significantly
impact the rate of transpiration. The more water that’s transpired,
the greater the force of the transpirational pull at the top of the xylem
vessels. For example, in the daytime, the
sunlight will stimulate the opening of the stomata in the leaves, which causes water
vapor to leave the stomata through diffusion, speeding up the transpiration
rate.
Increasing the temperature supplies
the water molecules with more thermal energy, which increases their kinetic
energy. This increases the speed of the
movement of the water molecules and therefore their rate of evaporation and
subsequent transpiration through the stomata. Furthermore, a low humidity or a
high wind speed can increase the rate of transpiration at the leaves by increasing
the concentration gradient for water vapor between the inside of the leaf and the
external environment. A steeper concentration gradient
between the leaf and the atmosphere means that water will transpire faster.
Let’s see how much we’ve learned
about transport in the xylem by applying our knowledge to a practice question.
The diagram provided shows a
simplified outline of the movement of water through a plant. Through what transport vessel does
the majority of water move? (A) Vein, (B) phloem, (C) cell
walls, or (D) xylem.
To answer this question, let’s go
through the different answer options and discuss what the purpose of each of the
different mentioned structures is.
A vein, when used in the context of
transport vessels, is a type of blood vessel that’s found in animals like
humans. Veins form part of the human
circulatory system that are responsible for transporting blood back toward the heart
from the body tissues. A different sort of vein can also
be found in leaves. While leaf veins do play a role in
transport, they are not transport vessels themselves. As the question is specifically
asking us the transport vessel through which water moves, we can eliminate option
(A).
While the phloem and the xylem are
both transport vessels that are found in plants, the cell wall is a rigid structural
layer that surrounds plant cells. Cell walls are responsible for
providing structural support and protection to plant cells, but cell walls are not a
transport vessel. So this option can also be
eliminated.
As we mentioned, the phloem and the
xylem are both transport vessels, but they differ in their structure and
function. The phloem transports substances
like sugars and amino acids both up and down the plant stem. While these sugars and amino acids
will be dissolved in water, so some water will be involved in phloem transport, it
is not the main function of the phloem to transport water.
The xylem, on the other hand, is
responsible for transporting the majority of water molecules in a plant. It also transports dissolved
mineral ions. The water molecules and ions are
generally absorbed from soil into the roots of a plant and then are transported from
the roots into the plant stem. Unlike in the phloem, where
substances can move both up and down the plant, the xylem only transports water and
mineral ions up the plant from the roots to the leaves and flowers and any other
organs that might require them.
If we take a look back at the
diagram provided by the question, we can see that water is only moving up the plant
from the roots to the leaves. This helps us to confirm that the
phloem is not the vessel through which the majority of water moves, as it transports
sugars and amino acids both up and down the plant stem. Instead, the role of the xylem is
to move the majority of water through the plant.
Let’s take a moment to wrap up the
video by reviewing the key points that we’ve learned. Water is first absorbed usually
from soil into a plant’s roots. And it then needs to be transported
against the pull of gravity up the plant to its photosynthetic parts. This transport of water occurs in
xylem vessels from the roots, through the stem, and to the leaves, where
photosynthesis can occur.
How exactly this happened was a
mystery for a long time, and several scientists have attempted to develop theories
to explain the mechanism of water transport through the xylem vessels. But ultimately, the theories of
capillary action, root pressure, and imbibition were all invalidated. The most validated theory was that
of transpiration, producing an upward pull of the water in the xylem vessels. Finally, the cohesion- tension or
cohesion-adhesion theory explained how the biochemical forces between individual
water molecules and between the water molecules and the xylem vessel walls allowed
their movement upward through the plant.
