Video Transcript
In this video, we’ll discover how
respiration works in a plant and examine the close relationship that exists between
respiration and photosynthesis. We’ll also look at some simple
experiments we can do to investigate respiration as well as finding out how plants
are adapted for this vital process.
If you thought it was just animals
that got to do all the cool stuff, then think again. Although plants can’t run around
like we do, they still carry out a lot of complex processes which require energy,
and they get this energy through cellular respiration. Respiration is the cellular process
whereby glucose is broken down to release energy in the form of adenosine
triphosphate, or ATP for short. When we’re talking about
respiration, we need to be careful to say that energy is released rather than made
because according to the first law of thermodynamics, energy cannot be created or
destroyed.
There are two types of
respiration. The first is aerobic, which
requires oxygen, and the second is anaerobic, which doesn’t require oxygen. Although some organisms, such as
bacteria, get their energy exclusively from anaerobic respiration, plants are
capable of respiring both ways. Aerobic respiration is most common
in plants, however, so that’s the one we’ll be concentrating on in this video. So, where does respiration take
place in a plant?
Just like animal cells, plant cells
contain mitochondria, with each one being known as a mitochondrion. And it is here that glucose
undergoes the oxidation reaction known as aerobic respiration to release energy. Before we explore this reaction in
more detail, let’s remind ourselves of some important definitions.
In a chemical reaction, a reactant
is a substance which is present at the start and undergoes a change during the
reaction. On the other hand, a product is a
substance made during the reaction, which is therefore present at the end of it. Now let’s find out about the
reactants and products of respiration by having a look at its word and chemical
equations. These equations show that
respiration is the process whereby glucose reacts with oxygen to produce carbon
dioxide and water and also to release energy in the form of ATP. In reaction equations such as
these, the reactants are always shown to the left of the arrow, and the products are
always shown to the right of the arrow. So for respiration, the reactants
are glucose and oxygen, and the products are carbon dioxide and water, as well as
the energy that’s released.
While animals like us humans get
glucose from our food when we digest it, you may recall that plants use light from
the Sun to make their own glucose through a process called photosynthesis. Let’s have a look at the equations
for photosynthesis. We can tell from these that
photosynthesis is the process whereby carbon dioxide reacts with water in the
presence of light energy to produce glucose and oxygen. What do you notice about the
photosynthesis reaction compared to the respiration reaction?
Hopefully, you spotted that the
products of respiration are the reactants for photosynthesis, and vice versa, the
products of photosynthesis are the reactants for respiration. This means that these two reactions
have a very close relationship. While respiration is happening in
the mitochondria, photosynthesis is taking place in the nearby chloroplasts. It’s also important to note that
respiration can occur night and day, whereas photosynthesis can only happen in the
day because it relies on light energy from the Sun.
We can investigate respiration and
its relationship with photosynthesis by carrying out some simple experiments in a
laboratory. This diagram shows the setup for
the first experiment. We have a potted plant and a beaker
containing lime water. Lime water is a clear colorless
solution of calcium hydroxide, which is often used in scientific experiments. In the presence of carbon dioxide,
a calcium carbonate precipitate is formed which causes the lime water to turn
cloudy. We can therefore use this in an
investigation to see whether carbon dioxide is being made or not.
In our experimental setup, the
plant and the lime water are placed underneath a bell jar. This prevents gases such as oxygen
and carbon dioxide from entering or leaving the area surrounding the plant. Finally, the bell jar is covered
with a thick dark sheet to prevent any light from reaching the plant. We then leave the setup undisturbed
for 24 hours. Pause the video for a moment and
see if you can predict what we’re going to see when we uncover the bell jar after 24
hours. Let’s see if you’re correct.
We’ve already said that plants are
respiring constantly, breaking down glucose and oxygen into carbon dioxide and
water. The carbon dioxide produced in
respiration is usually used up as a reactant for photosynthesis. But because the plant can’t get any
light in our experiment, it can’t carry out photosynthesis. So, the carbon dioxide is released
from the plant into the air inside the bell jar. This carbon dioxide causes the lime
water to turn cloudy, which demonstrates that respiration is occurring inside the
plant. Now, let’s see how a similar
experimental setup can be used to investigate the relationship between respiration
and photosynthesis.
As you can see in the diagram, the
second experiment is set up in almost exactly the same way as the first. Except this time, instead of
covering the bell jar with a dark sheet, the setup is exposed to a light source, in
this case a lamp. So what will happen if we leave
this setup for 24 hours? This time, as well as respiring,
the plant is also able to carry out photosynthesis as it’s receiving light energy
from the lamp. This means the carbon dioxide
produced during respiration is immediately used as a reactant for photosynthesis, so
no carbon dioxide is released into the atmosphere by the plant and the lime water
remains clear. This demonstrates that
photosynthesis occurs hand in hand with respiration in plants and that the two
reactions interact through their products and reactants.
Although the products of
respiration provide the reactants for photosynthesis and vice versa, this is not the
full story. You may be familiar with the idea
that a plant is a net carbon sink. This means it takes in more carbon
dioxide for photosynthesis than it gives out following respiration despite the fact
that it can only photosynthesize during the daytime. It is also a net oxygen source,
which means it produces more oxygen through photosynthesis than it consumes during
respiration.
So, how is a plant adapted for this
exchange of gases with its environment? If you zoom in on a section of a
leaf, you can see that it has lots of tiny openings on its surface. Each of these openings is called a
stoma, and together they’re known as stomata. Stomata can open and close to
control the diffusion of carbon dioxide and oxygen in and out of the plant. When they’re open, they also allow
water, a product of respiration to diffuse out of the plant and into the atmosphere
as water vapor. The stem of a plant also contains
important gas exchange structures.
The pores you can see on this
diagram are called lenticels. And unlike stomata, they’re always
open. Lenticels facilitate the exchange
of carbon dioxide, oxygen, and water vapor with the atmosphere. The final adaptation we’re going to
look at is found in the roots. Roots contain specialized cell
types called root hair cells, like the one you can see in this diagram. Root hair cells have a large
surface area in order to maximize the amount of water they can absorb from the
soil. This water can also be a source of
dissolved oxygen, which can then be used by the plant for respiration.
Now, we’ve learnt all about
respiration in plants. Let’s have a go at some practice
questions to test our knowledge of this topic.
Which of the following gases is
produced by aerobic respiration? (A) Sulfur dioxide, (B) carbon
monoxide, (C) carbon dioxide, (D) oxygen, or (E) hydrogen.
Respiration is the cellular process
whereby glucose is broken down to release energy in the form of adenosine
triphosphate, or ATP for short. Just like animal cells, plant cells
are capable of two types of respiration: aerobic, which requires oxygen, and
anaerobic, which doesn’t require oxygen. This question is asking us about
aerobic respiration, which is the more common type. So let’s have a look at it in more
detail.
You may recall that aerobic
respiration takes place in organelles called mitochondria. Let’s see the equation for the
chemical reaction that happens here. From this equation, we can see that
glucose reacts with oxygen to produce carbon dioxide, water, and energy in the form
of ATP. The substances shown to the left of
the arrow are known as the reactants because they are present at the start of the
reaction. And the substances shown to the
right of the arrow are known as the products because they are produced during the
reaction.
Now, let’s return to our
question. The only two gases involved in
aerobic respiration are carbon dioxide and oxygen. And the question is asking us to
decide which one is produced by the reaction. We know that oxygen is a
reactant. Therefore, the gas which is
produced by aerobic respiration is (C) carbon dioxide.
Let’s try another question.
Assuming the plant is alive and
healthy, which of the following conclusions about the experimental setup shown below
is correct? (A) The plant will be respiring but
not photosynthesizing. (B) The plant will be
photosynthesizing but not respiring. (C) The plant will not be respiring
or photosynthesizing. Or (D) the plant will be
photosynthesizing and respiring.
Let’s remove the multiple-choice
options for now and remind ourselves about respiration and photosynthesis. You may recall that plant cells
carry out two very important chemical reactions. The first is respiration, and the
second is photosynthesis. Aerobic respiration is the process
whereby glucose is broken down in the presence of oxygen to produce carbon dioxide
and water and to release energy in the form of ATP. This is how plants get all the
energy they need to survive. Photosynthesis is the process
whereby plants convert carbon dioxide and water into glucose and oxygen using light
energy.
Because photosynthesis relies on
light energy, it can only happen when the Sun is out or in the presence of some
other light source. Respiration, on the other hand,
happens constantly with or without a light source. Now, let’s consider the
experimental setup shown in the diagram.
We’re told that the potted plant is
alive and healthy; therefore, we know that it will be respiring. We can also see that the plant is
receiving light energy from this lamp, which means it will also be
photosynthesizing. We have therefore concluded that
with this experimental setup, the correct answer is (D); the plant will be
photosynthesizing and respiring.
Let’s summarize what we’ve learned
in this video by reviewing the key points. We found out that respiration is
the process whereby glucose is broken down to release energy in the form of ATP. Aerobic respiration takes place in
the mitochondria. Glucose and oxygen are the
reactants for respiration; carbon dioxide, water, and energy are the products.
Respiration is closely related to
photosynthesis; the products of one reaction provide the reactants for the
other. Respiration in plants happens all
the time, whereas photosynthesis can only happen in the light. And finally, plants have stomata,
lenticels, and root hair cells, which allow them to exchange gases and water with
their environment.
