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Oxidative Phosphorylation: Definition, Steps & Products

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Instructor: Anthony Grattini

Tony has a BA in Biology and has taught secondary Life, Earth, and Physical Science, as well as Honors & AP Chemistry.

Oxidative phosphorylation is the process that enables the body to produce adenosine triphosphate (ATP), which is needed for the transfer and storage of energy in cells. Explore the definition, steps, and products of oxidative phosphorylation, review an analogy, and learn how the process works, including the three stages of respiration and making ATP. Updated: 10/09/2021

Analogy for Oxidative Phosphorylation

In order to understand a chemical process like oxidative phosphorylation at the molecular level, I'd like to start off by using an analogy everyone can visualize: comparing the steps within oxidative phosphorylation to a hydroelectric dam's use of falling water to do other work.

Hydroelectric dams use falling water to convert kinetic energy into all of the electrical energy we use in our homes. Falling water is thus indirectly powering my computer, washing machine, dishwasher, desk lamp, printer, and ceiling fan. This process can be easily understood by an observer like you and me because it is on a large enough scale. A huge amount of water is held up on top of a cliff, with the potential to release a lot of energy when gravity is allowed to naturally pull it down through the dam. The dam collects this released energy and then converts it into electrical energy by using the flow of falling water to spin magnets. These spinning magnets create electricity, which is collected in a conducting material, like copper wires, and sent to our homes to do other work.

Matter and energy at the molecular level also flow in a predictable direction. Energy travels from where there is a lot of energy to where there is less energy. Matter travels from where there is a lot of that specific type of matter to where there is less of that matter. To help visualize this property, keep the hydroelectric dam analogy in mind throughout the following discussion of oxidative phosphorylation, which uses falling electrons and hydrogen ions to do other work, like making ATP.

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  • 0:00 Analogy for Oxidative…
  • 1:35 What Is ATP and…
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What Is ATP and Phosphorylation?

ATP

The molecule shown above is known as ATP, short for adenosine triphosphate. It is the central character in bioenergetics, or the transformation of energy to do work in a living system. It is composed of a nucleic acid (adenine), a sugar (ribose), and a triphosphate tail (3PO4 3-).

The triphosphate tail of ATP is the chemical equivalent of a loaded spring. Losing a phosphate from its tail is like a spring returning to its most useless state, or relaxing. The cell taps this energy source by using enzymes to transfer phosphate groups from ATP to other compounds, which are then said to be phosphorylated. Phosphorylation primes a molecule to undergo some kind of change that performs work, and the molecule loses its phosphate group in the process. If the cell were like a clock, ATP would be like the coiled spring that provides the energy necessary to move all of the gears within the clock. But unlike a clock, which needs to be recoiled, a living cell can recycle used ATP by adding more phosphates to its used forms, ADP or AMP (adenosine diphosphate or adenosine monophosphate) through the process of oxidative phosphorylation.

What Is Oxidative Phosphorylation?

Oxidative phosphorylation is the use of electrons falling from the hydrogen in glucose to the oxygen in a living cell. These falling electrons provide the energy necessary to pump H+ ions up a hill. When these H+ ions fall back down the hill, this energy is used to phosphorylate, or attach a phosphate group (-PO4 3-), to ADP to make the high-energy molecule ATP, which the cell can now use again to do vital work.

How the Process Works

Just like our body has various organs with specific jobs to perform, every cell within our body has organelles, which are tiny organs with specific jobs that help the cell survive. The organelle where oxidative phosphorylation occurs is known as the mitochondrion, and it is shown in the model below, along with the electron-microscopic image.

Mitochondrion

Mitochondria are like the power plants of the cell. The number of mitochondria within a cell is directly related to how active the cell is, or how much power the cell requires. One of the major energy-carrying molecules within a cell is ATP, and it is made by mitochondria through a three-step process that we'll learn about in the next section. The overall process the mitochondria perform is known as cellular respiration, and the following chemical equation summarizes the process of burning sugar in the presence of oxygen, which produces carbon dioxide, water, and energy (heat & ATP):

C6 H12 O6 + 6O2 ---> 6CO2 + 6H2 O + Energy (about 38 ATP & 686 Calories)

Here is the translation of this equation into a more understandable sentence: One molecule of glucose in the presence of six molecules of oxygen will produce six molecules of carbon dioxide, six molecules of water, about thirty-eight molecules of ATP, and give off 686 Calories.

This process is similar to the combustion of gasoline in the presence of oxygen to drive the pistons of a car. Both have carbon dioxide, water, and energy as their end products. The car and the living cell then use the energy from the breakdown of the organic molecule (gasoline or glucose) to perform other tasks and give off a lot of heat.

The 3 Stages of Respiration

Respiration is a cumulative function of three metabolic stages, which are diagrammed below:

Cellular Respiration

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