In this explainer, we will learn how to describe the process of respiration in a plant and discuss the relationship between respiration and photosynthesis.
We know that plants are capable of synthesizing their own food. Through the process of photosynthesis, green plants convert carbon dioxide and water into glucose and oxygen in the presence of sunlight. Plants utilize this glucose for the energy they need to survive, but how is this energy obtained from glucose molecules? In both plants and animals, the liberation of energy from the macromolecules in food is accomplished through the process of respiration.
Photosynthesis is the process by which green plants convert carbon dioxide and water into sugars, such as glucose, and oxygen in the presence of sunlight.
Cellular respiration can be described as the process by which carbon-containing compounds, such as glucose, are broken down within the cells to liberate energy. This energy is stored as chemical energy, in the form of a molecule called ATP. The compounds that are broken down are called the substrates or reactants, and the resulting compounds are called the products of the reaction.
Definition: Cellular Respiration
Cellular respiration is a process in living organisms in which carbon-containing compounds, such as glucose, are broken down to release energy in the form of ATP.
Some organisms, such as archaea and bacteria, live in environments with low levels of oxygen. These cells respire by breaking down molecules in the absence of oxygen, a process called anaerobic respiration. In most plants and other higher organisms, however, such breakdown of molecules is achieved in the presence of oxygen, through a complete oxidation reaction. This form of respiration is called aerobic respiration.
Definition: Aerobic Respiration
Aerobic respiration is the process by which energy is released in the cells in the presence of oxygen.
The most common substrate for cellular respiration is glucose, although in some cases protein and fat molecules may also be used. Glucose is a simple sugar molecule that is made up of a ring of six carbon atoms. In aerobic respiration, the chemical bonds between the carbon atoms are broken down through a reaction with oxygen, and this releases energy along with carbon dioxide and water.
Key Term: Glucose
Glucose is a simple monosaccharide sugar molecule made up of six carbon atoms and is considered the most common substrate in cellular respiration.
The energy released is stored as ATP, or adenosine triphosphate. When the cells need energy for other processes, ATP molecules can be broken down to release the energy stored in them. However, ATP molecules differ from other storage molecules, such as carbohydrates and fats, in that they function as a quick and easy energy source for body cells, which does not require as much energy to be broken down as those macromolecules would require. ATP is, therefore, described as the “energy currency” of living organisms.
Key Term: ATP
ATP, or adenosine triphosphate, is the molecule that stores chemical energy in living organisms.
Example 1: The Products of Aerobic Respiration
Which of the following gases is produced by aerobic respiration?
- Carbon monoxide
- Sulfur dioxide
- Carbon dioxide
Living organisms obtain energy from food through the process of respiration. In plants and most higher organisms, this occurs in the presence of oxygen, in a process called aerobic respiration.
The complex compounds that are broken down during respiration are called the reactants, whereas the resulting compounds are called the products. In cellular respiration, glucose is the most common reactant. Glucose is a simple sugar that is made of six carbon atoms.
During the process of aerobic respiration, glucose is reacted with oxygen to release energy, carbon dioxide, and water.
The gas produced by aerobic respiration is, therefore, carbon dioxide.
Cells contain organelles that perform specific functions. In plant cells, photosynthesis takes place in organelles called chloroplasts, which are found in the green parts of plants. Chloroplasts contain a pigment called chlorophyll, which gives plants their green color. Chlorophyll is one of the pigments in plant cells that allows them to absorb energy from sunlight, enabling them to convert carbon dioxide and water into glucose and oxygen.
Key Term: Chloroplasts
Chloroplasts are the chlorophyll-containing organelles in plant cells where photosynthesis takes place.
Respiration, on the other hand, mainly takes place in organelles called mitochondria. Mitochondria are commonly called the “powerhouses of the cell,” since they break down glucose into energy that can be carried on energy-carrying molecules, such as ATP, and used by the cells when needed.
Key Term: Mitochondria
Mitochondria (singular: mitochondrion) are the organelles in cells where respiration takes place and energy is released.
These two key organelles can be seen in Figure 1, which shows a simplified diagram of a plant cell.
In plants, photosynthesis and respiration go hand in hand. The products of each of these reactions form the reactants for the other. To understand this clearly, let’s take a look at the equations of the two processes.
You may notice that these two reactions are almost the exact opposite of each other. The products of the photosynthesis reaction are the same molecules as the reactants of the respiration reaction, and vice versa.
During respiration, the glucose generated by photosynthesis is broken down through a reaction with oxygen, another product of photosynthesis. This is how energy, in the form of ATP, is released from the food synthesized by photosynthesis. On the other hand, the carbon dioxide and water obtained from respiration are used as the reactants for photosynthesis. This is represented in Figure 2.
Example 2: Obtaining Glucose as a Reactant for Cellular Respiration
Plants require glucose to carry out respiration. By what method do they obtain the majority of this glucose?
- Glucose is produced during transpiration.
- Glucose is actively transported from the soil and into the roots.
- Glucose is produced during photosynthesis.
- Glucose diffuses into the leaf via the open stomata.
Green plants synthesize their own food in the presence of sunlight via a process called photosynthesis. In plants, photosynthesis and respiration go hand in hand. The products of one reaction form the reactants of the other. This can be understood by taking a look at the equations of each of these reactions.
As we can see, the glucose and oxygen that are formed as a result of photosynthesis are used as the reactants for respiration, to be broken down into carbon dioxide, water, and energy.
Plants, therefore, obtain the majority of glucose through photosynthesis.
Let’s discuss the concept of respiration in plants and address some questions about how this process differs from respiration in animals.
Generally, the question of whether plants breathe does not really have a straightforward answer. Plants require oxygen for aerobic respiration, as we have learned, to break down glucose into energy. During this process, carbon dioxide is liberated. However, unlike animals, plants have no specialized respiratory organ system for gas exchange.
As we have seen, several gaseous molecules are involved in both respiration and photosynthesis. Plants can obtain the gaseous molecules they need from the atmosphere and release the products of their reactions through gas exchange.
Key Term: Gas Exchange
Gas exchange is the process by which plants obtain the gaseous molecules they need from the atmosphere and release the gaseous products of their reactions.
The rate of gas exchange in plants is quite low compared to that in animals, as the products of respiration are usually used up as the reactants for photosynthesis. This means that they do not need to diffuse into the atmosphere in large quantities. Most of the time, plants do not need to take up a great amount of oxygen from the atmosphere for respiration, since it is already supplied by photosynthesis.
Each part of the plant takes care of its own needs for gas exchange, and there is not much transfer of gases between plant parts. The roots, stems, and leaves all have structures and adaptations that allow them to obtain the reactants they need from the atmosphere and to release the products of respiration and photosynthesis.
Let’s have a look at each of these structures and understand how they help plants exchange gases with the atmosphere.
Leaves have openings called stomata, which are tiny pores found on their surfaces. Stomata open and close depending on the need for gas exchange, as represented in Figure 3. These openings also play a role in allowing the diffusion of water vapor from the plants into the atmosphere.
Stomata (singular: stoma) are pores on the epidermis of leaves through which gas exchange takes place with the atmosphere.
The stem and roots of a plant also play an important role in gas exchange, as well as in the absorption and diffusion of water. They have pores on their surfaces, called lenticels, through which oxygen, carbon dioxide, and water can be exchanged with the atmosphere. Roots also possess specialized structures called root hair cells, which extend into the air spaces in the soil, as you can see in Figure 4 below.
Lenticels are openings on the surfaces of stems and roots through which gas exchange takes place with the atmosphere.
Definition: Root Hair Cells
Root hair cells are fine, hair-like structures that extend into the soil and maximize the surface area of the roots for the absorption of water.
Root hair cells, like the ones that you can see in Figure 4, have a large surface area to maximize the efficiency of water absorption from the soil, which may contain dissolved oxygen and nutrients.
Example 3: Obtaining Oxygen as a Reactant for Cellular Respiration
Plants require oxygen to carry out aerobic respiration. Which of the following is not a method by which plants obtain this oxygen?
- Oxygen produced by the plant during photosynthesis is used in respiration.
- Oxygen is dissolved into water and enters the roots of the plant.
- Oxygen diffuses into the leaf through stomata.
- Oxygen is breathed in by the plant through open airways in the leaf.
Aerobic respiration is the process by which glucose is broken down in the presence of oxygen to yield energy. Plants obtain oxygen for this process through a variety of methods.
Plants synthesize their own food through photosynthesis. Photosynthesis involves the conversion of carbon dioxide and water into glucose and oxygen. The processes of photosynthesis and respiration go hand in hand, and the products of each of these reactions form the reactants of the other. The glucose and oxygen produced during photosynthesis are used in aerobic respiration to release energy.
Each individual part of a plant takes care of its own respiratory needs. The roots, stems, and leaves of a plant all respire and have specialized structures for this purpose. Leaves have tiny pores called stomata on their surfaces, which open and close depending on the plant’s need for gas exchange with the atmosphere. If a plant requires oxygen from the atmosphere, it can diffuse into the leaf through these pores.
Roots have special structures called root hairs, which extend into the air spaces in the soil. These root hairs can take up water from the soil into the plant, which may contain dissolved oxygen and nutrients.
Of the statements mentioned, the only way in which plants cannot obtain oxygen is through open airways in the leaf. As we have learned, leaves do not have open airways; they have tiny pores called stomata, which open and close depending on the need for gas exchange.
Interestingly, in plants, all living cells have a surface that is in contact with the surrounding air. This means that gases can easily diffuse into the atmosphere from the surfaces of the plant, without needing an organ system in place. Even in trees with thick, woody stems and roots, the living cells are arranged in thin layers, just under the surface of the bark. This arrangement allows the presence of air spaces so that all the living cells are exposed to the air. The cells found deeper within the stems of such plants are dead cells, which provide mechanical support and, therefore, do not need to respire.
It is important to remember that while plants only perform photosynthesis in the daytime when sunlight is available, cellular respiration takes place constantly, day and night. Atmospheric gas exchange is, therefore, higher at night when the plant is respiring, but photosynthesis does not take place due to the absence of sunlight. Oxygen, rather than being supplied by photosynthesis, is taken up from the atmosphere at night. Similarly, the carbon dioxide produced by respiration at night is released into the atmosphere, instead of being used as a reactant for photosynthesis.
Let’s now take a look at a simple experiment that can be conducted to understand cellular respiration.
Figure 5 shows a small potted plant and an open vial filled with limewater, placed under a large overturned jar. This setup is covered with a thick, dark sheet and left undisturbed for 24 hours.
Limewater is another name for the solution of a compound called calcium hydroxide. In the presence of carbon dioxide, calcium hydroxide is converted into calcium carbonate, a white compound that precipitates out of the solution. This gives limewater a cloudy appearance.
When the sheet is removed, the limewater in the vial turns from a clear liquid to a cloudy suspension. Let’s apply our knowledge of respiration in plants to understand this observation.
Plants, as we have learned, respire constantly, breaking down glucose and oxygen into carbon dioxide and water. Usually, the carbon dioxide released through respiration is used up as a reactant in photosynthesis. In the dark, however, when photosynthesis cannot take place, the carbon dioxide is instead released into the atmosphere through pores on the surfaces of the leaves, stems, and roots. In the experimental setup shown above, this carbon dioxide reacts with the limewater, or calcium hydroxide, and converts it into calcium carbonate. The word equation for this reaction is given below.
Equation: Limewater and Carbon Dioxide
Another interesting way in which this experimental setup can be used is to demonstrate the relationship between photosynthesis and respiration.
Using a fresh vial of limewater, instead of covering the overturned jar with a dark sheet, the setup is exposed to a light source, as shown in Figure 6.
After being left undisturbed for 24 hours, the vial of limewater does not turn cloudy, in contrast to what happened in the previous experiment. This is because, in the presence of light energy, the plant both performs photosynthesis and constantly respires. The carbon dioxide released through respiration is, therefore, utilized as a substrate for photosynthesis, instead of being released into the atmosphere. Therefore, the limewater, or calcium hydroxide, is not converted into calcium carbonate.
Let’s go over the key points of what we have learned about respiration in plants.
- Plants obtain energy through cellular respiration, which breaks down glucose into carbon dioxide and water to liberate energy.
- Photosynthesis and respiration go hand in hand, as the products of one reaction form the reactants of the other.
- Respiration occurs day and night, whereas photosynthesis can only take place in the daytime, in the presence of sunlight.
- Aerobic cellular respiration takes place in the mitochondria.
- The energy released through cellular respiration is stored in ATP molecules, which can be broken down and used when needed by the cell.
- Plants have stomata, lenticels, and root hairs that enable gas exchange with the atmosphere.