Video Transcript
In this video, we will learn how
plants can produce metabolic waste products such as carbon dioxide, water, and
nitrogenous waste. We will then explore how some
plants are adapted to excrete some of these waste products through various
mechanisms and structures.
All living organisms, including
plants, carry out several metabolic processes for their growth and survival. This diagram shows a highly
magnified view of a typical plant cell. In the chloroplasts that are found
in many plant cells, food in the form of glucose is synthesized through
photosynthesis. Many plant cells also contain
mitochondria. These mitochondria are the site of
cellular respiration, where the glucose that was synthesized in photosynthesis is
broken down to release energy. Plants also need to absorb mineral
ions into their roots from the soil to synthesize proteins, pigments, and other
essential compounds. These compounds are involved in
various different reactions, the sum of which make up the plants’ metabolism.
Metabolism describes all of the
chemical reactions that occur within living organisms to maintain life, so plants
must carry out these metabolic reactions in order to survive. These chemical reactions all
generate products, which the plant does not always need and may even potentially be
toxic. These byproducts are called
metabolic waste and must often be eliminated via excretion if they cannot be reused
or recycled by the organism in another metabolic process. Excretion is the removal of
metabolic waste products from an organism’s body. Unlike animals, plants do not have
a specialized organ system for excretion. Instead, they have several
different mechanisms by which potentially dangerous waste products can either be
recycled or excreted.
Let’s learn about the different
types of metabolic waste products that plants generate and how these waste products
are eliminated. Plants are autotrophs, which means
that they synthesize their own food or nutrition. In this case, through
photosynthesis, they synthesize glucose. We’ve also learned that cellular
respiration releases the energy from carbon-containing compounds like glucose. Let’s look at this in more
detail. The primary form of cellular
respiration involves breaking down glucose by reacting it with oxygen to produce
carbon dioxide and water. This process also releases energy
that the cell can use. The reactants of cellular
respiration are shown in orange, while the products are shown in pink.
Photosynthesis, on the other hand,
reacts carbon dioxide with water in the presence of light energy to produce glucose
and oxygen. In this case, we’re shown the
reactants in pink and the products in orange. This is because these two reactions
are almost the exact opposite of each other. Both of these reactions liberate
gaseous waste products. While photosynthesis liberates
oxygen, cellular respiration liberates carbon dioxide and water vapor. In plants, photosynthesis and
cellular respiration go hand in hand.
While the products of
photosynthesis form the reactants of cellular respiration, the products of cellular
respiration form the reactants of photosynthesis. This shows an example of how some
products of metabolic reactions can be reused in plants. However, in some cases, these
gaseous waste products need to be excreted. Plants can eliminate excess
quantities of these gaseous waste products by releasing them into the atmosphere by
a process known as gas exchange. This is also the way in which the
plants will absorb gases like carbon dioxide. However, water, like mineral ions,
will need to enter the plant through its roots. The stems and leaves of a plant
have specialized openings on their surfaces through which gaseous molecules like
oxygen, carbon dioxide, and water vapor may diffuse into the atmosphere.
Let’s take a look at each of these
structures to understand how excretion of gases in plants can occur. Leaves contain openings called
stomata or a singular stoma. These are tiny pores found in
amongst the epidermis cells mostly on the underside of leaves. The stomata are the site at which
gas exchange between the leaves and the external atmosphere takes place. Through these pores, gaseous
metabolic waste products, like the oxygen that’s produced in photosynthesis and not
used in cellular respiration, can diffuse out of the leaf and into the
atmosphere. Other gases can also diffuse
between the leaf and the external environment through the stoma, and we’ll come back
to these in more detail in just a little while.
The stem of some plants can also
play a key role in gas exchange. For example, the stem that makes up
the woody trunk of this tree contains many pores on its surface called
lenticels. Through these lenticels, oxygen,
carbon dioxide, and water vapor can be exchanged with the atmosphere. Lenticels are raised, oval,
circular, or in this case elongated openings on woody stems in trunks and even on
some roots. Plants primarily release excess
water into the atmosphere as the gas water vapor through a process called
transpiration. Transpiration is the loss of water
through evaporation from the aerial, or upper, parts of a plant into the
atmosphere. So water won’t only be lost in the
form of water vapor through transpiration from the lenticels, but also from the
stomata, which you’ll recall are pores on the underside of many leaves.
If we take a look at a cross
section of some of the main cells in a leaf, we can see how transpiration occurs
more clearly. We can still see the stoma on the
underside of the leaf. We can also still see the
surrounding lower epidermis cells at the bottom of the leaf and the upper epidermis
cells that are found at the top of the leaf. Sometimes the cells of the
epidermis produce a waxy cuticle to coat them. There are three main types of
transpiration: stomatal transpiration, lenticular transpiration, and cuticular
transpiration. Let’s look at stomatal
transpiration in a little more detail first.
We already know that this includes
the evaporation of water molecules from the stomata. But let’s have a look at this in a
side view of the leaf to see how it happens more clearly. During the daytime when light
intensity is high, many plant cells will be carrying out photosynthesis. As we know, oxygen is produced in
photosynthesis. And the oxygen molecules that can’t
be used in respiration will diffuse through the air spaces in between the cells and
the leaf and out of the stomata. And carbon dioxide, which is needed
for photosynthesis, will diffuse from the atmosphere through the stomata between the
air spaces and into the cells that require it for photosynthesis.
At the same time, however, stomatal
transpiration will take place. Liquid water that’s produced in
cellular respiration in cells will accumulate in the intercellular spaces. There, the water molecules can
evaporate into water vapor and the water vapor exits the leaf through the stomata
into the atmosphere. Overall, stomatal transpiration
accounts for about 90 percent of the water lost from a plant through
transpiration.
But you might remember that water
is a key reactant in photosynthesis. So in order to make its own food
and survive, the plant can’t afford to lose large volumes of water. Therefore, at night, when the light
intensity is too low for photosynthesis to occur, the stomata close as there’s no
reason to absorb carbon dioxide into the leaf if photosynthesis cannot occur. This prevents the excess loss of
water through stomatal transpiration when light intensity is low.
Lenticular transpiration is the
loss of water through the lenticels as water vapor. Only a minimal volume of water,
about 0.1 percent of the total water lost through transpiration, is lost through the
lenticels. Cuticular transpiration is the
evaporation of water from the cuticle. The cuticle is a waxy layer, and
this waxiness makes the surface of the plant slightly less prone to water loss. But it’s still possible and
cuticular transpiration can occur even when the stomata are closed. Overall, cuticular transpiration
accounts for less than 10 percent of the total water lost through transpiration.
The rate of cuticular transpiration
depends on the thickness of the waxy cuticle. Plants growing under extremely hot
and dry conditions can develop extra thick cuticles to prevent extra water loss
through transpiration. Lenticular and cuticular
transpiration can occur throughout the day or night, while stomatal transpiration
can only occur in the daytime.
Let’s summarize the information
we’ve learned about stomatal, lenticular, and cuticular transpiration in a
table. Stomatal transpiration occurs
through pores on the leaf surface called stomata, and it accounts for about 90
percent of the water that’s lost through transpiration. But it can only occur during the
daytime as stomata only open when light is present for photosynthesis to occur. Lenticular transpiration occurs
through lenticels, which are raised openings of various shapes and sizes on the
stems and sometimes the roots of woody plants. It only accounts for about 0.1
percent of the total water lost through transpiration. But lenticels don’t close, so
lenticular transpiration can occur throughout the day and the night.
Cuticular transpiration occurs from
the cuticle, which are waxy layers that coat the epidermis generally of the
leaves. Cuticle transpiration accounts for
less than 10 percent of the total water lost through transpiration, and it can occur
throughout the day and the night.
Aside from transpiration, water can
also be eliminated from the bodies of some plants in liquid form through a process
called guttation. Water is first absorbed in plants
from the soil into their root hair cells. Mineral ions, which have been shown
here in green, are also absorbed from the soil in a similar way. Water is then carried along with
dissolved mineral ions up through the plant to its aerial organs like the leaves
through long tubelike structures called xylem vessels. When in the xylem vessels, the
water and dissolved minerals are known as xylem sap. The absorption of water molecules
from the soil and into the root cells creates an upward pressure through the xylem
vessels. This is aptly named the root
pressure.
Excess xylem sap is exuded in the
form of water droplets through structures called hydathodes, which are found in the
margins of leaves. This process is called guttation,
or sometimes droplet exudation, where the xylem sap rich in dissolved minerals is
exuded through the hydathodes. And it’s a result of the root
pressure that’s exerted by water moving into root hair cells. The water droplets produced by
guttation should not be confused with the dew drops that are often found on grass in
the early morning. Instead, dew drops are formed
through the condensation of atmospheric water onto the surface of plants. While transpiration occurs mainly
in the daytime, guttation is more likely to occur at night or in the early morning
when the stomata are closed but the plant needs to eliminate excess volumes of water
and mineral ions in large quantities.
Another type of plant waste is
nitrogenous waste. Just like some animals, plants can
generate nitrogenous waste like urea, nitrates, and ammonium. These are formed as a result of
protein metabolism in which proteins are broken down into smaller peptides, which
can subsequently be broken down into smaller amino acids. Amino acids can then be converted
into other substances or used in various metabolic reactions. These metabolic reactions will
produce metabolic waste that needs to be excreted. Alternatively, these amino acids
might be recycled in protein synthesis to make new proteins for growth and
repair.
Nitrogen in the form of ammonium
and nitrates can actually be used to synthesize amino acids again too. As amino acids are the building
blocks of proteins, these waste products can theoretically be recycled through
protein synthesis to form proteins that are needed for growth and development.
Let’s take a look at another type
of plant waste. Sometimes these waste products are
in the form of mineral salts or acids. These compounds might have a toxic
effect on a plant if they’re allowed to accumulate. Instead, these compounds can be
converted into crystals. In this crystal form, which is
represented in this diagram as pink dots, they can be stored in the vacuole or
cytoplasm of certain cells. This can prevent these potentially
toxic compounds from spreading to different parts of the plant and causing harmful
effects. These crystals can accumulate in
structures that are fairly disposable like leaves, bark, and fruits. These structures can eventually be
shed, leaving the plant free of toxic substances, so it can regrow these new
structures from scratch.
For example, plants like this
potato plant grown in soil with excess calcium tend to accumulate insoluble crystals
of calcium oxalate inside their leaves, roots, and tubers. These are sometimes called
raphides. Sometimes organic acids can be
helpful to the plant to make nutrients in the soil more soluble so they can be
reabsorbed by the roots.
The final method of plant excretion
that we’ll look at in this video is how they can accumulate certain substances that
can then be removed via secretions. For example, some plants can store
certain waste products in resins and gums, which accumulate in old xylem
vessels. These substances can then be
excreted out of a plant, for example, in response to damage. In fact, some plants can even
produce these substances in response to injury to block up a site of damage to
prevent the entry of pathogens. Secretions like latex and oils can
also contain metabolic waste products that accumulate in bark, stems, and
leaves. Let’s see how much we can remember
about excretion in plants by applying our knowledge to a practice question.
Water can also be lost from a woody
plant through small pores in the stem. What are these pores called? (A) Glands, (B) hydathodes, (C)
lenticels, or (D) stomata.
The stem of a plant plays an
important role in gas exchange, as well as in the absorption and diffusion of
water. The surfaces of the stems of some
woody plants like those mentioned in the question contain raised openings called
lenticels. Lenticels are a site of gas
exchange between the stem and the atmosphere surrounding the stem. More precisely, they are the site
of lenticular transpiration. This describes how water vapor,
which is shown in our diagram as blue dots, can move from the stem through the
lenticels and into the external environment. Though only a minimal volume of
water is lost from the plant through lenticular transpiration, it does describe one
of the ways that water can be lost from a woody plant stem.
Water can also be lost from a plant
through stomatal transpiration. But as stomata are only found on
leaves and the question is asking us about pores found on the stem through which
water can be lost, stomata cannot be our correct answer. Water can also be lost from plants
through a process called guttation through structures called hydathodes. But as hydathodes, like stomata,
are found on plant leaves and not on the stem, this cannot be our correct
answer.
Glands are structures typically
found in animal bodies, which produce hormones. Although plants do produce
hormones, they don’t have glands. Also, glands are not generally
associated with water loss, so this is not our correct answer. So we’ve worked out that the pores
in a woody plant stem through which water can be lost are called lenticels.
Let’s review some of the key points
that we’ve addressed in this video about excretion in plants. Plants generate metabolic waste
products that need to be either excreted or reused. The products generated in
photosynthesis are used as reactants in respiration, and the products of respiration
can be used as reactants in photosynthesis if they’re not released into the
atmosphere. Water can be eliminated through
evaporation via stomatal, lenticular, and cuticular transpiration. Water can be eliminated in the form
of xylem sap by guttation through hydathodes. Nitrogenous waste products can
often be reused in protein synthesis.