Lesson Video: Respiration in Plants Biology

In this video, we will learn how to describe the process of respiration in a plant and discuss the relationship between respiration and photosynthesis.

12:17

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.

Nagwa uses cookies to ensure you get the best experience on our website. Learn more about our Privacy Policy.