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What is Aerobic Respiration?

Elizabeth Schap, Meredith Mikell, Amanda Robb
  • Author
    Elizabeth Schap

    Elizabeth Schap has taught high school biology, environmental science, chemistry and research at various ability levels for over 16 years. She has a Master's in Secondary Science Education from Towson University and a BA with a double concentration in Biology and Communication Arts from Notre Dame of Maryland University. She also has written lesson plans for Scholastic Inc and curricula for National Aquarium in Baltimore.

  • Instructor
    Meredith Mikell

    Meredith holds a B.S. in marine science with a minor in philosophy, as well as a master's of aeronautical science with a space science emphasis. She has taught subjects including marine science, biology, astronomy, math, and reading to students from kindergarten through high school.

  • Expert Contributor
    Amanda Robb

    Amanda has taught high school science for over 10 years. She has a Master's Degree in Cellular and Molecular Physiology from Tufts Medical School and a Master's of Teaching from Simmons College. She is also certified in secondary special education, biology, and physics in Massachusetts.

Learn the definition, steps, final products, and formula of aerobic respiration. Explore how much usable energy is extracted from one glucose molecule. Updated: 08/15/2021

What Is Aerobic Respiration?

Aerobic respiration is the process by which glucose molecules are broken down into usable cellular energy called adenosine triphosphate (ATP) while in the presence of oxygen. This cellular process is completed by single-celled and multi-celled organisms (including plants) that use glucose as energy, since glucose holds a lot of energy that must be processed into an easier to use form. This process takes place both in the cytoplasm of cells and in the mitochondria. Oxygen is needed to complete a series of steps to release the most usable energy from each glucose molecule.

Aerobic respiration is one of two types of cellular respiration used by organisms to break down organic compounds into usable cellular energy. When oxygen is not present, cells can undergo a process called anaerobic respiration, but this process releases less usable energy for cells.

Plants have mitochondria that complete the process of aerobic respiration.

tropical plant on brown background, top view

Aerobic Respiration Formula

The balanced aerobic respiration formula is:

{eq}\rm C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + ATP {/eq}

As shown in the equation, six molecules of glucose enter the process with six molecules of diatomic oxygen. These reactants are then combined with two molecules of ATP, which is a catalyst, to cycle through the steps of aerobic respiration. As glucose is broken down and rearranged into ATP, waste products of six water molecules and six carbon dioxide molecules are released by the cells.

Aerobic Respiration

Take a deep breath...now exhale. Do you feel it? Every time you breathe in oxygen and exhale carbon dioxide, you are exchanging gases that are a crucial part of your energy metabolism. We don't normally equate oxygen and breathing with metabolizing food, yet the latter cannot occur without the former.

Most living organisms undergo this process, from single-celled bacteria to the multi-celled blue whale. While many microorganisms can accomplish this task within their single cell, we larger macroorganisms have evolved body organs dedicated to efficiently obtaining enough oxygen, and eliminating enough carbon dioxide, to undergo enough aerobic respiration to fuel our large, complex selves.

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Oxygen is used as a reactant in aerobic cell respiration.

C6H12O6 + O2 yields CO2 + H2O + Energy

Aerobic Respiration Steps

Aerobic respiration follows a catabolic pathway because it breaks down a larger molecule into smaller products. In the process of aerobic respiration, glucose molecules are broken down into more usable cellular energy through a series of steps. There are four large steps to the process:

  • Glycolysis
  • Pyruvate Oxidation (formation of acetyl coenzyme A)
  • Citric Acid Cycle (also known as the Krebs cycle)
  • Oxidative Phosphorylation (process of creating ATP from the transfer of electrons from their carriers to oxygen)

Glycolysis

In glycolysis, the reactants glucose and 2 molecules of ATP enter the process. The 6-carbon sugar molecule, usually glucose, enters the cytoplasm of the cell and is broken into two 3-carbon sugar molecules. These products, known as pyruvate, are produced with multiple other products.

The total products produced from one complete cycle of glycolysis are:

  • 4 ATP molecules - 2 are sent back to the start of glycolysis to begin the next cycling.
  • 2 NADH molecules - converted from the electron acceptor NAD+
  • 2 free hydrogen ions - sent to the electron transport chain to complete oxidative phosphorylation
  • 2 molecules of water - released as waste
  • 2 pyruvate molecules - sent to the other three steps of aerobic respiration for further breakdown

As outlined above, each product of this first step of aerobic respiration either continues through the process to release more ATP or exits the cell as waste.

Glycolysis is the first step of aerobic respiration

Glycolysis in aerobic respiration

Formation of Acetyl Coenzyme A

The two pyruvate molecules enter the mitochondria to be able to further release the chemical energy stored. This occurs in the mitochondria due to the needed reactant - oxygen. Without oxygen present, the process could not continue.

Pyruvate is broken down by a specialized enzyme that oxidizes the pyruvate molecules to create an end product called acetyl coenzyme A or acetyl-CoA. During this step, a CO2 molecule is released as a waste product while the three-carbon pyruvate molecules are turned into two-carbon molecules with an attached sulfur group. This is the acetyl-CoA molecule needed to enter the next step of aerobic respiration.

In addition to acetyl-CoA and CO2, 2 molecules of NADH and 2 H+ ions are released as products. These will be used as reactants in the steps that follow.

Krebs Cycle

The Krebs cycle, or the citric acid cycle, continues the process in the mitochondria, releasing energy as acetyl-CoA, NADH, and oxygen molecules enter the pathway. Here acetyl-CoA is rearranged into carbon chains, allowing more electrons to be oxidized and sent to the final step in the process. The electrons, in the form of NADH, H+ ions, and FADH2, will be needed to create the largest amount of ATP.

As electrons are taken from the acetyl-CoA, they are attached to electron carriers NAD+ and FADH. These help to transport the electrons needed to finish the process. Carbon atoms are also attached to diatomic oxygen molecules to be released as waste products.

In addition to CO2, the products of the Krebs Cycle are:

  • 6 NADH
  • 2 FADH2
  • 2 ATP

The citric acid cycle in aerobic respiration

citric acid cycle in aerobic respiration

Definition of Aerobic Respiration

Respiration is the metabolic process of most living things in which food molecules or glucose are turned into usable energy for the cell, called ATP. Respiration is the anti-process to photosynthesis, the process in which plants use sunlight and carbon dioxide to build food molecules releasing oxygen as a waste product.

During aerobic respiration, oxygen is present and helps the process to crank out energy very efficiently. Some organisms can also undergo anaerobic respiration, in which oxygen is absent, and a somewhat less efficient method of metabolism takes place. While photosynthesis takes place in the chloroplasts of plant and algae cells, aerobic respiration takes place in the cytoplasm, or the gooey inner cell space and mitochondria of all eukaryotic cells.

The Steps of Aerobic Respiration

It all starts with a sugar! An organism takes in carbohydrates for energy, and the digestion process breaks the carbs down into their smallest units, glucose, a type of sugar molecule. Cells then make energy by breaking the glucose molecule down and releasing its electrons, which are later used to help crank out ATP. There are three main steps in this process.

There are three main steps in this process. It begins with glycolysis. During glycolysis, the 6-carbon glucose molecule undergoes a series of reactions that break it down into two 3-carbon pyruvate molecules. The purpose of this process is to release electrons from the bonds in the glucose, which are scooped up by an acceptor molecule called NAD+, turning it into NADH when it accepts the electrons. In this process, two molecules of ATP are made. This step occurs in the cytoplasm, and the pyruvate and NADH molecules then enter the mitochondria for the next step.

The next phase of aerobic respiration is the citric acid cycle, also known as the Kreb's cycle, named for the biochemist who discovered it. To prepare for this stage, the pyruvate molecules from glycolysis are converted to a 2-carbon compound called Acetyl CoA. What happened to the third carbon? You just exhaled it in the form of carbon dioxide!

With each turn of the cycle, the Acetyl CoA is broken down and rebuilt into carbon chains. The purpose is to extract electrons from them and generate more ATP, similar to the more simple process of glycolysis. NAD+ is used again to pick up the electrons released, as is another acceptor molecule, FADH, which becomes FADH2 when reduced. These acceptor molecules get loaded up with electrons, like cargo trucks, and carbon dioxide is released as the carbon chains are broken down and new Acetyl CoA comes in. Exhale!

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Video Transcript

Aerobic Respiration

Take a deep breath...now exhale. Do you feel it? Every time you breathe in oxygen and exhale carbon dioxide, you are exchanging gases that are a crucial part of your energy metabolism. We don't normally equate oxygen and breathing with metabolizing food, yet the latter cannot occur without the former.

Most living organisms undergo this process, from single-celled bacteria to the multi-celled blue whale. While many microorganisms can accomplish this task within their single cell, we larger macroorganisms have evolved body organs dedicated to efficiently obtaining enough oxygen, and eliminating enough carbon dioxide, to undergo enough aerobic respiration to fuel our large, complex selves.

Definition of Aerobic Respiration

Respiration is the metabolic process of most living things in which food molecules or glucose are turned into usable energy for the cell, called ATP. Respiration is the anti-process to photosynthesis, the process in which plants use sunlight and carbon dioxide to build food molecules releasing oxygen as a waste product.

During aerobic respiration, oxygen is present and helps the process to crank out energy very efficiently. Some organisms can also undergo anaerobic respiration, in which oxygen is absent, and a somewhat less efficient method of metabolism takes place. While photosynthesis takes place in the chloroplasts of plant and algae cells, aerobic respiration takes place in the cytoplasm, or the gooey inner cell space and mitochondria of all eukaryotic cells.

The Steps of Aerobic Respiration

It all starts with a sugar! An organism takes in carbohydrates for energy, and the digestion process breaks the carbs down into their smallest units, glucose, a type of sugar molecule. Cells then make energy by breaking the glucose molecule down and releasing its electrons, which are later used to help crank out ATP. There are three main steps in this process.

There are three main steps in this process. It begins with glycolysis. During glycolysis, the 6-carbon glucose molecule undergoes a series of reactions that break it down into two 3-carbon pyruvate molecules. The purpose of this process is to release electrons from the bonds in the glucose, which are scooped up by an acceptor molecule called NAD+, turning it into NADH when it accepts the electrons. In this process, two molecules of ATP are made. This step occurs in the cytoplasm, and the pyruvate and NADH molecules then enter the mitochondria for the next step.

The next phase of aerobic respiration is the citric acid cycle, also known as the Kreb's cycle, named for the biochemist who discovered it. To prepare for this stage, the pyruvate molecules from glycolysis are converted to a 2-carbon compound called Acetyl CoA. What happened to the third carbon? You just exhaled it in the form of carbon dioxide!

With each turn of the cycle, the Acetyl CoA is broken down and rebuilt into carbon chains. The purpose is to extract electrons from them and generate more ATP, similar to the more simple process of glycolysis. NAD+ is used again to pick up the electrons released, as is another acceptor molecule, FADH, which becomes FADH2 when reduced. These acceptor molecules get loaded up with electrons, like cargo trucks, and carbon dioxide is released as the carbon chains are broken down and new Acetyl CoA comes in. Exhale!

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  • Activities
  • FAQs

Aerobic Respiration Analogy

In this activity, students will be writing an analogy for aerobic respiration. Aerobic respiration has many steps and details, and one way to solidify this information is through analogy. For example, students might choose to compare the process of aerobic respiration to manufacturing their favorite shoes or creating their favorite meal. Students should consider what the "reactants" are in these examples and what steps are needed to create the products.

Directions

In this activity, you're going to be applying your knowledge of aerobic respiration to create an analogy for the process. Your analogy should include a description of the reactants and products and at least three key steps that align with glycolysis, the citric acid cycle and oxidative phosphorylation. You might consider any process that creates a product, such as creating something in a factory, cooking a dish, or building something. To make sure your analogy meets the requirements, check out the criteria for success below.

Criteria for Success

  • Analogy compares another process to aerobic respiration
  • Analogy discusses the reactants and products of the process
  • Analogy uses at least three steps to compare to glycolysis, the citric acid cycle and oxidative phosphorylation
  • Analogy clearly explains the relationship between the chosen process and aerobic respiration
  • Analogy is at least 500 words

What are the 3 products of aerobic respiration?

The three products of aerobic respiration are carbon dioxide, water and energy in the form of adenosine triphosphate (ATP). The carbon dioxide and water are considered waste products, while the ATP is the desired product.

What is aerobic respiration?

Aerobic respiration is the breaking down of glucose into usable cellular energy known as adenosine triphosphate, or ATP, while in the presence of oxygen.

What are the 3 steps of aerobic respiration?

The three steps of aerobic respiration are glycolysis, the Krebs cycle, and the electron transport chain. Sometimes, these steps are also called glycolysis, the citric acid cycle, and oxidative phosphorylation. Glycolysis takes place outside the mitochondria, while the other two steps occur within it.

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