Video Transcript
In this video, we will explore the
differences between physiological and structural support in plants. We will learn some examples of each
of these types of support and how they can be beneficial to the survival of a
plant.
Have you ever noticed that if a pea
rolls under the kitchen fridge and then you find it months later, it will have
become wrinkled, shrunken, and dry? If you try putting the same pea
into a glass of water, it would swell back up to its original size pretty quickly,
although eating it at this point would not be a good idea. This shrinking and swelling of the
pea is due to its cells losing and gaining water. And it displays one typical example
of a method of support in plant cells. While humans, like many animals,
have a skeleton to support them, plants do not. Therefore, plants need other
support mechanisms to maintain their cell shape and their overall shape and also to
protect them.
There are two methods that a plant
uses to maintain its shape and structure: physiological support and structural
support. Physiological support is temporary,
and it depends on maintaining a high volume of water in the plant cells to keep
their shape. Structural support, however, is
permanent. It involves the deposition of
strong polymers into certain cell walls in specific parts of the plant. This maintains the shape of those
cells and of the plant as a whole. The pea shrinking and swelling
displays an example of a physiological support mechanism, and we will look at this
support method first.
This diagram shows a typical plant
cell. Which structures can you recognize
in it? We can see the nucleus, some
chloroplasts, a rigid structural layer called a cell wall surrounding the outside of
the cell, and within it a thinner cell membrane. We can also see a large permanent
vacuole, which has been shown here in blue. As the nucleus and chloroplasts
aren’t particularly important to physiological support, let’s erase these from our
diagram. The vacuole in a plant cell is
usually a large structure filled with a liquid called cell sap.
Cell sap contains water molecules,
which have been shown in our diagram in blue, and dissolved solutes like sugars and
enzymes, which have been shown in our vacuole as pink. Lots of water can be stored in this
vacuole. And as the volume of water
increases, the vacuole exerts more turgor pressure on the cell membrane. Let’s have a look at how this
happens and why it is important. When plants have little water in
the environment surrounding their cells, there is a lower water potential outside
the cells than inside the cells. This causes water molecules to move
out of the plant cell’s vacuole and into the extracellular space through a process
called osmosis.
You may recall that osmosis is the
movement of water molecules from an area of high water potential to an area of low
water potential across the plasma membrane. This causes the plant cells to
eventually wrinkle and shrink. As so much water is lost from the
cell’s vacuole when it’s placed in a low-water environment, the low water
availability causes the cell membrane to pull away from the cell wall as there’s
less turgor pressure exerted upon it by the water in the vacuole. This lowers the cell’s turgidity,
eventually making the cell appear shrunk and wrinkled. When several plant cells shrink in
this way, it eventually leads to the plant, especially its leaves, visibly
wilting.
However, this process is
temporary. If the water availability outside
the cells increases again, the water can move into the plant cell’s vacuole by
osmosis from an area of high water potential to an area of low water potential,
returning the cell to its normal size. But what happens if the water
potential continues to increase in the extracellular environment? Well, even more water molecules
will move from the extracellular environment into the plant’s vacuole by osmosis
from an area of comparatively high water potential to an area of comparatively low
water potential.
As the vacuole now contains more
water molecules, it increases in size even further. And so it exerts far more turgor
pressure upon the cell cytoplasm, pushing the cell membrane against the cell wall
and increasing the cell’s turgidity. This makes the cell appear
swollen. You can see the effects of this
very high turgidity in many plant cells in the whole plant here. However, as we mentioned, this
process is temporary. So, if the water availability
decreases once more, the plant could still eventually wilt again. Now that we’ve learned some more
information about physiological support in plants, let’s learn about some structural
support mechanisms.
As we mentioned earlier, structural
support involves specific tough compounds being permanently incorporated into a
plant cell wall. Different compounds will be
deposited into the walls of different cells depending on their function. These compounds can help to
maintain the shape of the plant cells and therefore the plant itself, keeping it
upright and strong. Plant cell walls are made primarily
of a carbohydrate called cellulose. Cellulose fibers build up to form a
mesh within plant cell walls. This creates a physical barrier to
support the cell. If we were to magnify just one part
of one of these cellulose fibers, we would see that it’s made up of many cellulose
chains, which individually are made up of many monomers of glucose joined
together.
The fact that each strand of
cellulose is made up of thousands of glucose molecules joined together into a chain,
which are then joined together into cellulose fibers formed into a mesh, makes
cellulose very strong. Cellulose is also described as an
insoluble polymer, and it can help to maintain cell turgidity in the physiological
response that we just looked at. Cells without cell walls, like this
typical animal cell, will also absorb a lot of water molecules by osmosis if they’re
placed in an environment that has a higher water potential than the cell itself. However, without the cell wall,
when a typical animal cell gains too much water, the cell will burst. In the same situation, plant cells
will just become more swollen and firm as their rigid cell wall prevents them from
bursting. Let’s take a look at some specific
regions in the plant where cellulose cell walls might be useful for other
reasons.
This diagram is of a leaf. By cutting this leaf in half and
magnifying it, we can see certain tissues. And by magnifying it even further,
we might be able to see some cells. The cells in a plant epidermis,
which line the top and the bottom of its leaves, as well as other structures like
the plant stem, have cell walls particularly rich in cellulose. Plant leaves have both an upper and
a lower epidermis. Especially in the outer tissues of
a plant, such as in the epidermis, the cellulose cell walls form a barrier to
prevent disease-causing pathogens, like those shown here in red, from entering the
inner tissues. This protection is heightened even
more as the epidermis sometimes produces a waxy cuticle to coat it.
The cells that make up this cuticle
have a polymer called cutin deposited into their cell walls. Cutin is impermeable to water. This prevents the excess loss of
water from the surface of the leaves, and it helps to prevent waterborne pathogens
from settling on the leaf surface. Cutin increases the effectiveness
of this physical barrier by thickening the cell walls. Let’s take a look at some different
cells in this leaf.
The cells highlighted in blue in
our diagram make up a tissue called the xylem. We can see the magnified structure
of part of a xylem vessel here. The xylem is part of the plant’s
vascular transport system, responsible for transporting water and minerals from the
roots to the rest of the plant. The cell walls of xylem vessels
contain a polymer called lignin. Lignin is a really strong polymer,
and it provides additional support to the xylem by making its vessels more rigid and
helping them to remain upright to form a continuous column of water. Lignin is impermeable to water,
which is really helpful in the xylem as it reduces the chances of water leaking out
of the vessels, increasing the efficiency of water transport.
Lignin and cellulose can also be
incorporated into the cell walls in various other parts of the plant. Let’s take a quick look at the
typical simple tissue types that might include some of these polymers in their cell
walls. There are three main types of
simple tissues in plants: parenchyma tissue, collenchyma tissue, and sclerenchyma
tissue. You may recall that a simple tissue
is composed of cells that are structurally and functionally very similar to each
other.
Parenchyma cells typically have
thin cell walls made up of cellulose. They also tend to contain many
chloroplasts to carry out photosynthesis. Sclerenchyma cells, found in
sclerenchyma tissues, have thicker cell walls, and they’re reinforced with lignin
and extra cellulose. Sclerenchyma cells are generally
found in nongrowing parts of the plant to add structural support. For example, the walls of the xylem
vessels we looked at earlier will be full of sclerenchyma cells. Collenchyma cell walls are
reinforced with extra cellulose and sometimes additional substances like pectin to
provide extra support, typically in young stems, as collenchyma is far more flexible
than sclerenchyma. Collenchyma cells also usually
contain chloroplasts to carry out photosynthesis.
This celery stick provides us with
a nice relatable example of where we can find collenchyma and sclerenchyma
tissues. Though the majority of the stalk
will be made up of tough sclerenchyma tissues that make it crunchy and give it a
signature snap when we break it, collenchyma tissue makes up most of the veins that
run down the stalk. These stretchy strands are highly
flexible, which shows us a key characteristic of collenchyma tissue.
Let’s have a look at a final
structural support mechanism in plants. Some plants have a cork layer
surrounding organs like the stem, as we can see on the cork that’s making up part of
the bark of this mature tree. Cork is formed by the deposition of
a waxy impermeable substance called suberin into its cell walls. In some cells, like those of a
potato tuber shown here, an infection by a pathogen can cause cork to form. If the pathogen manages to make its
way past the outer cells of the potato tuber, a cork layer can form. A suberin is deposited into the
cell walls, protecting the innermost cells from infection.
When certain trees begin to lose
their leaves in preparation for winter, this forms a vulnerable region through which
pathogens might be able to access the plant and its tissues. But not to worry as a scar of cork
usually forms over this region, forming a protective layer that prevents pathogen
entry. As it is impermeable to water, and
therefore waterproof, cork provides another layer of support, both against losing
water and against pathogen entry.
Plants depend on these support
methods for their survival, so let’s explore some of the advantages they can provide
to plants. Plant support helps plants to
conserve water. Conserving water is advantageous to
plants as it’s a reactant in photosynthesis, which is the biological process by
which they make their own nutrition in the form of glucose. Therefore, water is vital to plants
for them to be able to make their own food. This glucose can then be used,
sometimes along with oxygen, which is also produced in photosynthesis, in a process
called cellular respiration to release energy. Water is also a useful transport
medium among its plentiful other functions. Plant support helps plants to
remain upright, which allows them to access more sunlight.
Light is another key requirement
for photosynthesis. So accessing more light means a
plant can increase its photosynthetic rate to make glucose faster. Plants compete for light to be able
to produce their food. Being tall and strong means that
one plant might have the edge when it comes to accessing light over its neighbors,
which means that they can be outcompeted for survival. Additionally, a strong stem allows
plants to withstand environmental pressures, for example, strong winds. This can be especially helpful if
the plant has heavy branches, fruits, or flowers that need to be supported, as if
these fruits or flowers are knocked off a plant prematurely and fall onto the
ground, they’re not so protected from some herbivorous insects, rot, decay, and
damage. And as we’ve mentioned, thick and
impenetrable cell walls can be very useful in restricting the entry and movement of
pathogens into the inner tissues of a plant.
Let’s review what we’ve learned
about support in plants by applying our knowledge to a practice question.
What will visibly happen to a plant
if cell turgidity is not maintained? (A) The stem will grow at a faster
rate. (B) The surface area of the leaves
will expand. (C) The leaves and the plant will
wilt. (D) The leaves will turn
yellow. Or (E) the flowers will drop
off.
This question is asking us about
the effects of low cell turgidity on the plant as a whole. So, first, let’s explore how
turgidity affects individual plant cells. Turgidity is the point at which the
plant cell membrane pushes against the cell wall, making the cell swollen and firm,
usually because it’s full of liquid. This diagram shows us a typical
turgid plant cell, which is what they are referred to when they’re full of water and
have a high turgidity. The vacuole in plant cells stores
water molecules and dissolved solutes.
When a plant cell is in an
environment that has a high water potential, which is represented by these many blue
dots showing water molecules outside the plant cell, water will move from outside
the cell, where there’s a high water potential, into the cell’s vacuole, where
there’s a comparatively lower water potential. The vacuole can then exert turgor
pressure onto the cell cytoplasm, which pushes against cell membrane and against
cell wall, making the cell turgid. This is an example of physiological
support in a plant. And when many cells become turgid,
the plant will stand upright and rigid.
However, this process is
temporary. And if the water potential in the
extracellular space decreases so much that it’s lower than the water potential in
the cell’s vacuole, the water molecules will move from an area of high water
potential in the cell’s vacuole to an area of comparatively lower water potential in
the extracellular space via osmosis. This means that the vacuole no
longer exerts so much turgor pressure upon the cell membrane. So the cell membrane pulls away
from the cell wall, resulting in a cell with low turgidity.
The plant cells in this condition
appear shrunk and wrinkled. And if this happens to many plant
cells, then the plant as a whole, especially its leaves, will visibly wilt. Therefore, we can deduce that the
effects of low cell turgidity on the plant as a whole is that the leaves and the
plant will wilt.
Now it’s time for us to recap some
of the key points that we’ve covered in this video about support in plants. Support in plants can be divided
into physiological support or structural support. While physiological support is
temporary, structural support is usually permanent. Physiological support refers to
plant cell’s ability to increase its turgidity via osmosis. Structural support is the
deposition of strong compounds like lignin into specific plant cell walls that need
extra support or waterproofing. Plant support mechanisms can be
advantageous as they help a plant to conserve water, access more sunlight, protect a
plant against damage, and even against disease-causing pathogens.
