- Cellular respiration introduction
- Introduction to cellular respiration and redox
- Steps of cellular respiration
- Overview of cellular respiration
- Oxidative phosphorylation and the electron transport chain
- Oxidative phosphorylation
- Fermentation and anaerobic respiration
- ATP synthase
- Cellular respiration
Steps of cellular respiration
Cellular respiration is a metabolic pathway that breaks down glucose and produces ATP. The stages of cellular respiration include glycolysis, pyruvate oxidation, the citric acid or Krebs cycle, and oxidative phosphorylation.
Cellular respiration is one of the most elegant, majestic, and fascinating metabolic pathways on earth. At the same time, it’s also one of the most complicated. When I learned about it for the first time, I felt like I had tripped and fallen into a can of organic-chemistry-flavored alphabet soup!
Luckily, cellular respiration is not so scary once you get to know it. Let's start by looking at cellular respiration at a high level, walking through the four major stages and tracing how they connect up to one another.
Steps of cellular respiration
Overview of the steps of cellular respiration.
- Glycolysis. Six-carbon glucose is converted into two pyruvates (three carbons each). ATP and NADH are made. These reactions take place in the cytosol.
- Pyruvate oxidation. Pyruvate travels into the mitochondrial matrix and is converted to a two-carbon molecule bound to coenzyme A, called acetyl CoA. Carbon dioxide is released and NADH is made.
- Citric acid cycle. The acetyl CoA combines with a four-carbon molecule and goes through a cycle of reactions, ultimately regenerating the four-carbon starting molecule. ATP (or, in some cases, GTP), NADH, and FADH_2 are made, and carbon dioxide is released. These reactions take place in the mitochondrial matrix.
- Oxidative phosphorylation. The NADH and FADH_2 produced in other steps deposit their electrons in the electron transport chain in the inner mitochondrial membrane. As electrons move down the chain, energy is released and used to pump protons out of the matrix and into the intermembrane space, forming a gradient. The protons flow back into the matrix through an enzyme called ATP synthase, making ATP. At the end of the electron transport chain, oxygen accepts electrons and takes up protons to form water.
During cellular respiration, a glucose molecule is gradually broken down into carbon dioxide and water. Along the way, some ATP is produced directly in the reactions that transform glucose. Much more ATP, however, is produced later in a process called oxidative phosphorylation. Oxidative phosphorylation is powered by the movement of electrons through the electron transport chain, a series of proteins embedded in the inner membrane of the mitochondrion.
These electrons come originally from glucose and are shuttled to the electron transport chain by electron carriers and , which become and when they gain electrons. To be clear, this is what's happening in the diagram above when it says or . The molecule isn't appearing from scratch, it's just being converted to its electron-carrying form:
To see how a glucose molecule is converted into carbon dioxide and how its energy is harvested as ATP and in one of your body's cells, let’s walk step by step through the four stages of cellular respiration.
- Glycolysis. In glycolysis, glucose—a six-carbon sugar—undergoes a series of chemical transformations. In the end, it gets converted into two molecules of pyruvate, a three-carbon organic molecule. In these reactions, ATP is made, and is converted to .
- Pyruvate oxidation. Each pyruvate from glycolysis goes into the mitochondrial matrix—the innermost compartment of mitochondria. There, it’s converted into a two-carbon molecule bound to Coenzyme A, known as acetyl CoA. Carbon dioxide is released and is generated.
- Citric acid cycle. The acetyl CoA made in the last step combines with a four-carbon molecule and goes through a cycle of reactions, ultimately regenerating the four-carbon starting molecule. ATP, , and are produced, and carbon dioxide is released.
- Oxidative phosphorylation. The and made in other steps deposit their electrons in the electron transport chain, turning back into their "empty" forms ( and ). As electrons move down the chain, energy is released and used to pump protons out of the matrix, forming a gradient. Protons flow back into the matrix through an enzyme called ATP synthase, making ATP. At the end of the electron transport chain, oxygen accepts electrons and takes up protons to form water.
Glycolysis can take place without oxygen in a process called fermentation. The other three stages of cellular respiration—pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation—require oxygen in order to occur. Only oxidative phosphorylation uses oxygen directly, but the other two stages can't run without oxidative phosphorylation.
Each stage of cellular respiration is covered in more detail in other articles and videos on the site. Try watching the overview video, or jump straight to an article on a particular stage by using the links above.
Want to join the conversation?
- if glycolysis requires ATP to start how did the first glycolysis in history happen?(55 votes)
- You must remeber that life on this planet has been evolving for billions of years, it is highly unlikely that the originating system resembles the current system. The development of celluar respiration began as a simple inefficient system progressing to it's current incarnation.(48 votes)
- What is the role of NAD+ in cellular respiration. Why is the role NAD+ plays so important in our ability to use the energy we take in?(15 votes)
- Hello Breanna!
NAD+ is an electron transport molecule inside the cristae of a cell's mitochondria. In glycolysis, the beginning process of all types of cellular respiration, two molecules of ATP are used to attach 2 phosphate groups to a glucose molecule, which is broken down into 2 separate 3-carbon PGAL molecules. PGAL releases electrons and hydrogen ions to the electron carrier molecule NADP+. Each PGAL molecule has a phosphate group added to it, forming a new 3-carbon compound. These phosphate groups and the phosphate groups from the first step are then added to adenosine diphosphate or ADP, forming 4 ATP molecules. This also produces 2 molecules of pyruvic acid.(36 votes)
- I have a question... Which part of the body will most likely use the cellular respiration? Is it lungs?(7 votes)
- Cellular Respiration happens in your cells and you entire body is made up of cells, it goes on all throughout your body including your lungs and brain.(27 votes)
- When it states in "4. Oxidative phosphorylation" that the NADH and the FADH2 return to their "empty" forms NAD+ FADH2, the author meant FAD when referring to the "empty" forms, right?(15 votes)
- the empty state of FADH2 is FADH, after oxidation it loses 1 h+ ion and elctron.(2 votes)
- When the electron carriers NAD+ and FAD gain electrons, why are 2 hydrogen ions also being added?(7 votes)
- The free energy from the electron transfer causes 4 protons to move into the mitochondrial matrix.
In other words, electrons provide energy for hydrogen atoms. Electrons fuel movement of protons. :)(7 votes)
- Does the glycolysis require energy to run the reaction?(4 votes)
- Yes glycolysis requires energy to run the reaction. 2 ATPs are used up by glycolysis this then begins the oxidative process of glycolysis.(6 votes)
- Aren't internal and cellular respiration the same thing?(4 votes)
- Cellular respiration is oxidative metabolism of glucose which takes place in mitochondria and in the cell.
Internal repsiration is gas exchange between blood and tissues.
- After oxidative phosphorylation, the ATP created is in the mitochondrial matrix, right? If so, how does it get out of the mitochondrion to go be used as energy?(2 votes)
- Just like the cell membrane, the mitochondrion membranes have transport proteins imbedded in them that bring in and push out materials.(8 votes)
- In the Citric Acid Cycle (Krebs Cycle), would the four-carbon molecule that combines with Acetyl CoA be Oxaloacetic acid?(4 votes)
- Yes. Acetyl CoA and Oxaloacetic Acid combine to form a six-carbon molecule called Citric Acid (Citrate).(4 votes)
- Is oxidative phosphorylation the same as the electron transport chain?(4 votes)
- Oxidative phosphorylation is a process involving a flow of electrons through the electron transport chain, a series of proteins and electron carriers within the mitochondrial membrane. This flow of electrons allows the electron transport chain to pump protons to one side of the mitochondrial membrane.
Meaning that it is not the same. Oxidative phosphorylation is the process while ETC is more like machinery made of components.(2 votes)