Back To CourseHigh School Biology: Tutoring Solution
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Tony has a BA in Biology and has taught secondary Life, Earth, and Physical Science, as well as Honors & AP Chemistry.
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.
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.
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.
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.
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.
Respiration is a cumulative function of three metabolic stages, which are diagrammed below:
Glycolysis, or the splitting of sugar, occurs outside of a mitochondrion and is the initial breakdown of glucose into two less complex molecules known as pyruvate, which can then enter the mitochondrion. Overall, the process produces the 2 pyruvate plus 2 molecules of water, 2 ATP, 2 molecules of NADH, and 2 hydrogen ions (H+). The NADH carries electrons to the oxidative phosphorylation step of cellular respiration, which occurs inside of the mitochondrion. As pyruvate enters the mitochondrion, it is converted into another molecule called Acetyl CoA, which then goes through the second stage of metabolism, known as the Krebs cycle. The Krebs cycle produces 6 molecules of carbon dioxide, 7 molecules of NADH, 2 molecules of FADH2 (another electron carrier), and 2 molecules of ATP. The hydrogen ions and the electrons being carried by NADH and FADH2 are then used in the last stage of metabolism, the electron transport chain, and oxidative phosphorylation.
Oxidative phosphorylation occurs within the inner mitochondrial membrane and the mitochondrial matrix. Embedded within the inner mitochondrial membrane are various proteins that pass the electrons being carried by NADH and FADH2 slowly downhill, releasing energy along the way in manageable amounts down to O2, molecular oxygen. If these electrons were to fall from NADH or FADH2 directly to O2, it would be two much energy released at one time, just like the flow of water through a dam also has to be slowed, or else the system would break.
The energy released from these falling electrons is used to pump hydrogen ions (H+ ) uphill. Then when these hydrogen ions fall back downhill, they do so through protein complexes called ATP synthases. This is the site of oxidative phosphorylation and the production of about 34 ATP molecules from one molecule of glucose. This diagram from earlier is a simplified model of these two interrelated processes needed to produce ATP through oxidative phosphorylation:
Let's review. Oxidative phosphorylation is using the energy released from electrons falling downhill to add phosphate to adenosine diphosphate to produce ATP. ATP, or adenosine triphosphate, is the higher energy molecule a cell uses to do work and survive. Oxidative phosphorylation is one stage of cellular respiration and occurs within the organelles, or tiny organs with specific jobs that help the cell survive, of the cell, specifically from the mitochondria, or the power plants of the cell. Along with glycolysis, or the splitting of sugar, it allows a living cell to metabolize glucose, converting it to a more useful form for the cell. Revisiting the dam analogy, it's like converting flowing water to a more useful form for us, electricity. The overall process of cellular respiration can be represented in the following chemical equation:
C6 H12 O6 + 6O2 ---> 6CO2 + 6H2 O + Energy (Heat + ATP), the last of which would be about 38 ATP and 686 Calories in the case of burning sugar in the presence of oxygen.
Translated into English, this equation says: 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.
Oxidative phosphorylation is the process through which ATP is produced in this equation we just looked at during cellular respiration.
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Back To CourseHigh School Biology: Tutoring Solution
36 chapters | 479 lessons