Explanation:
Steps of cellular respiration
Overview of the steps of cellular respiration.
1. Glycolysis. Six-carbon glucose is converted into two pyruvates (three carbons each). ATP and NADH are made. These reactions take place in the cytosol.
2. 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.
3. 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.
4. 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.
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
NAD
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NAD
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start text, N, A, D, end text, start superscript, plus, end superscript and
FAD
FADstart text, F, A, D, end text, which become
NADH
NADHstart text, N, A, D, H, end text and
FADH
2
FADH
2
start text, F, A, D, H, end text, start subscript, 2, end subscript when they gain electrons. To be clear, this is what's happening in the diagram above when it says
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NADH
NADHstart text, N, A, D, H, end text or
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FADH
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FADH
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start text, F, A, D, H, end text, start subscript, 2, end subscript. The molecule isn't appearing from scratch, it's just being converted to its electron-carrying form:
NAD
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NAD
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start text, N, A, D, end text, start superscript, plus, end superscript
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2
e
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2e
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H
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2H
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2, start text, H, end text, start superscript, plus, end superscript
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NADH
NADHstart text, N, A, D, H, end text
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H
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H
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start text, H, end text, start superscript, plus, end superscript
FAD
FADstart text, F, A, D, end text
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2e
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H
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2H
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2, start text, H, end text, start superscript, plus, end superscript
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→right arrow
FADH
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FADH
2
start text, F, A, D, H, end text, start subscript, 2, end subscript
To see how a glucose molecule is converted into carbon dioxide and how its energy is harvested as ATP and
NADH
NADHstart text, N, A, D, H, end text
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FADH
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FADH
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start text, F, A, D, H, end text, start subscript, 2, end subscript 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
