Cellular respiration is the process responsible for converting chemical energy, and the reactants/products involved in cellular respiration are oxygen, glucose (sugar), carbon dioxide, and water. While the exact steps involved in cellular respiration may vary from species to species, all living organisms perform some type of cellular respiration.
Without cellular respiration, living organisms wouldn’t be able to produce the chemical energy they need, and their cells would not be able to carry out the tasks needed to sustain themselves.
The Reactants Involved In Cellular Respiration
Let’s take a closer look at the reactants of cellular respiration.
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Glucose, or sugar, has the chemical formula C6H12O6. While this formula can potentially be applied to a variety of different molecules, depending on how the atoms within the molecule are arranged, most molecules with this chemical formula are sugars of one form or another. The most notable formation of C6H12O6 is glucose, which is sometimes referred to as blood sugar or dextrose. The cells of animals convert glucose into a substance known as pyruvate through a process called glycolysis. The glycolysis process takes glucose and generates two molecules of ATP, or energy, with it.
Dioxygen, frequently just called oxygen, it made up of two oxygen atoms and it is what vertebrates used to breathe. Oxygen makes up about 21% of our atmosphere and vertebrates bring oxygen into their lungs where it is absorbed by red blood cells that transport the oxygen to other parts of the body. While ATP can be generated without the use of oxygen, the utilization of oxygen lets the cells of the body more efficiently convert glucose into ATP.
Vertebrates release carbon dioxide and water as the byproducts of cellular respiration. Carbon dioxide is released by many different microorganisms during not only the process of cellular respiration but also the process of fermentation. Plants use carbon dioxide to create their own energy, much as heterotrophic organisms use glucose and oxygen to create energy. The carbon dioxide will enter the cells of the plant through small holes in the leaves referred to as stomata. After the carbon dioxide has entered the cells of the plant, the chloroplasts within the cell will begin the process of photosynthesis and create carbohydrates as a result.
Water, also referred to as dihydrogen monoxide, has the chemical formula H2O. This molecule can be found everywhere on earth, and also within the cells of most organisms. In addition to carbon dioxide and sunlight, plants also need water to produce energy through photosynthesis. Water is held within the cells of a plant in structures referred to as vacuoles.
The Balanced Chemical Equation For Cellular Respiration
Now that we know what the reactants of cellular respiration are, let’s take a look at how they interact with one another.
What follows cellular respiration’s balanced equation/formula:
C6H12O6 + 6O2 –> 6CO2 + 6H2O + 38 ATP
In plain English, this can be read as:
Glucose + oxygen –> carbon dioxide + water + energy
This is the basic cellular respiration process,
During the course of cellular respiration, oxygen and glucose are utilized to create carbon dioxide, water, and energy. The oxygen that an organism breathes in is used to break down the sugars found in food. This produces heat energy, similar to how burning a piece of wood releases heat. With cellular respiration, after oxygen breaks down the sugar and its energy is released, carbon dioxide is released as a byproduct. The energy released by the breakdown of the sugar molecules is stored within the cells of the organism for later use.
Some of the ATP that the cells use originates as a result of the reactions that transform glucose. Yet much of the ATP is made as a result of a process called oxidative phosphorylation, a phase of cellular respiration. Cellular respiration, in this case, aerobic respiration (respiration that uses oxygen), can be divided into four different steps and oxidative phosphorylation is the final step in the cellular respiration process.
The Stages of Cellular Respiration
The four stages of cellular respiration are:
- Link Reaction (pyruvate oxidation)
- Krebs cycle (Citric acid cycle)
- Electron transport chain
The first stage of cellular respiration is referred to as glycolysis, and during this phase, glucose is hit with a number of different chemical transformations and converted into different molecules. Glycolysis happens within the cytosol/cytoplasm of cells, and it doesn’t actually need oxygen to occur. Aerobic respiration involves the conversion of glucose into two pyruvate molecules. When the two molecules of pyruvate become oxidized, two NADH are produced as a result. These two NADH molecules assist in carrying electrons to the other reactions within the cell. Two molecules of ATP are also produced during this step.
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Pyruvate oxidation is the next phase of cellular respiration and it occurs when the pyruvate made in glycolysis enters the innermost part of the mitochondria, the mitochondrial matrix. In this matrix the pyruvate will be linked together with a substance dubbed coenzyme A. This creates acetyl CoA, a new molecule with two carbons. More NADH is generated here, and carbon dioxide is released as a result.
The Krebs cycle, sometimes referred to as the tricarboxylic acid cycle or just the citric acid cycle, is where oxaloacetic acid is combined with the acetyl CoA produced in the last step. This creates citric acid, which will then go through various reactions in a cycle. The final step of the citric acid cycle is to create more oxaloacetic acid, which sets up the cycle to begin again. Carbon dioxide is released during the citric acid cycle, and ATP, FADH2, and NADH are produced here. The electrons within FADH2 and NADH are then sent to the next portion of the cellular respiration process, the electron transport chain.
The molecules of FADH2 and NADH that were created during the previous cellular respiration steps will now transfer their electrons into the electron transport chain. This process of transferal is called oxidative phosphorylation. Since these molecules are now no longer weighted down with electrons, they become their simplest forms – FAD and NAD+. The movement of the electrons across the electron transport chain releases energy. Protons are pushed out of the mitochondrial matrix by the process, creating a gradient. An enzyme called ATP synthase is used to create ATP, and it returns the protons to the matrix. The electron transport chain comes to an end when molecules of oxygen bond with protons and accept electrons, creating water.
As for how much ATP is generated by this process, around 30 units of ATP are likely to be created. The process of oxidative phosphorylation will generate between 26 to 28 units of ATP, and substrate phosphorylation will typically generate between 4 to 6 more ATP units, for a total of between 30 to 34. However, setting up for glycolysis uses a bit of ATP so the actual yield is a few units lower.
The previously mentioned processes occur when there is enough oxygen for aerobic respiration to take place. If there is not an adequate supply of oxygen, anaerobic respiration will take place instead. Anaerobic respiration can produce ATP without an oxygen supply, but it is much less efficient than aerobic respiration, producing around 1/18th the amount of energy that aerobic respiration does.
Fermentation is one form of anaerobic respiration. Fermentation differs from other forms of energy production because in fermentation the glycolysis pathway is solely responsible for extracting ATP. Though glycolysis creates pyruvate, the pyruvate won’t proceed through the rest of the pathway. This means that the oxidization process, the Krebs/citric acid cycle, and the electron transport chain are all skipped. Because the electron transport chain isn’t operating during fermentation, NADH will not drop its electrons.
“Fermentation is the exhalation of a substance through which the admixture of a ferment which, by virtue of its spirit, penetrates the mass and transforms it into its own nature.” — Andreas Libavius
To compensate for the lack of oxidation, citric acid cycle, and electron transport chain, fermentation has a few extra reactions that will create NAD+ from NADH. This is done by allowing NADH to take an organic molecule such as pyruvate and remove the electrons that it carries, ensuring that NAD+ is created and that the glycolysis process can keep going.
How Cellular Respiration Relates To Photosynthesis
6CO2 + 6H2O → C6H12O6+ 6O2
You may have noticed that this equation is basically the opposite of cellular respiration. The cells of animals combine hydrogen and oxygen to create water and carbon dioxide as a byproduct. Meanwhile, plants use carbon dioxide and water to power the photosynthetic process, releasing glucose and oxygen as the end products of this system.
This intertwined and complex relationship is referred to as the carbon cycle. This is what allows molecules of carbon to be recycled and work their way through the whole biosphere, moving from plants to animals, to the atmosphere, and then back into plants.
Photosynthesis is the process plants used to create the energy they need. Photosynthetic organisms have organelles within their cells called plastids, which have pigments in them capable of trapping certain wavelengths of light. The sunlight they trap is converted into carbohydrates by the plant cells.