In this section, you will explore the following questions:
- How do electrons move through the electron transport chain and what happens to their energy levels?
- How is a proton (H+) gradient established and maintained by the electron transport chain and how many ATP molecules are produced by chemiosmosis?
Connection for AP® Courses
The electron transport chain (ETC) is the stage of aerobic respiration that uses free oxygen as the final electron acceptor of the electrons removed during glucose metabolism in glycolysis and the citric acid cycle. The ETC is located in membrane of the mitochondrial cristae, an area with many folds that increase the surface area available for chemical reactions. Electrons carried by NADH and FADH2 are delivered to electron acceptor proteins embedded in the membrane as they move toward the final electron acceptor, O2, forming water. The electrons pass through a series of redox reactions, using free energy at three points to transport hydrogen ions across the membrane. This process contributes to the formation of the H+ gradient used in chemiosmosis. As the protons are driven down their concentration gradient through ATP synthase, ATP is generated from ADP and inorganic phosphate. Under aerobic conditions, the stages of cellular respiration can generate 36-38 ATP.
Information presented and the examples highlighted in the section support concepts outlined in Big Idea 2 of the AP® Biology Curriculum Framework, as shown in the table. As shown in the table, concepts covered in this section also align to the Learning Objectives listed in the Curriculum Framework that provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A Learning Objective merges required content with one or more of the seven Science Practices.
|Big Idea 2||Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis.|
|Enduring Understanding 2.A||Growth, reproduction and maintenance of living systems require free energy and matter.|
|Essential Knowledge||2.A.1 All living systems require constant input of free energy.|
|Science Practice||1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively.|
|Science Practice||3.1 The student can pose scientific questions.|
|Learning Objective||2.4 The student is able to use representations to pose scientific questions about what mechanisms and structural features allow organisms to capture, store, and use free energy.|
|Essential Knowledge||2.A.1 All living systems require constant input of free energy.|
|Science Practice||6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices.|
|Learning Objective||2.5 The student is able to construct explanations of the mechanisms and structural features of cells that allow organisms to capture, store, or use free energy.|
Introduce oxidative phosphorylation using visuals such as this video.
Have students create a visual representation that shows an overview of glycolysis and the citric acid cycle and how the cycles relate to one another.
The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards:
[APLO 2.5][APLO 2.15][APLO 2.18][APLO 2.22]
You have just read about two pathways Introduce glucose catabolism—glycolysis and the citric acid cycle—that generate ATP. Most of the ATP generated during the aerobic catabolism of glucose, however, is not generated directly from these pathways. Rather, it is derived from a process that begins with moving electrons through a series of electron transporters that undergo redox reactions. This causes hydrogen ions to accumulate within the intermembranous space. Therefore, a concentration gradient forms in which hydrogen ions diffuse out of the intermembranous space into the mitochondrial matrix by passing through ATP synthase. The current of hydrogen ions powers the catalytic action of ATP synthase, which phosphorylates ADP, producing ATP.
Electron Transport Chain
The electron transport chain (Figure 7.11) is the last component of aerobic respiration and is the only part of glucose metabolism that uses atmospheric oxygen. Oxygen continuously diffuses into plants; in animals, it enters the body through the respiratory system. Electron transport is a series of redox reactions that resemble a relay race or bucket brigade in that electrons are passed rapidly from one component to the next, to the endpoint of the chain where the electrons reduce molecular oxygen, producing water. There are four complexes composed of proteins, labeled I through IV in Figure 7.11, and the aggregation of these four complexes, together with associated mobile, accessory electron carriers, is called the electron transport chain. The electron transport chain is present in multiple copies in the inner mitochondrial membrane of eukaryotes and the plasma membrane of prokaryotes.
To start, two electrons are carried to the first complex aboard NADH. This complex, labeled I, is composed of flavin mononucleotide (FMN) and an iron-sulfur (Fe-S)-containing protein. FMN, which is derived from vitamin B2, also called riboflavin, is one of several prosthetic groups or co-factors in the electron transport chain. A prosthetic group is a non-protein molecule required for the activity of a protein. Prosthetic groups are organic or inorganic, non-peptide molecules bound to a protein that facilitate its function; prosthetic groups include co-enzymes, which are the prosthetic groups of enzymes. The enzyme in complex I is NADH dehydrogenase and is composed of 44 separate polypeptide chains. Complex I can pump four hydrogen ions across the membrane from the matrix into the intermembrane space, and it is in this way that the hydrogen ion gradient is established and maintained between the two compartments separated by the inner mitochondrial membrane.
Q and Complex II
Complex II directly receives FADH2, which does not pass through complex I. The compound connecting the first and second complexes to the third is ubiquinone (Q). The Q molecule is lipid soluble and freely moves through the hydrophobic core of the membrane. Once it is reduced, (QH2), ubiquinone delivers its electrons to the next complex in the electron transport chain. Q receives the electrons derived from NADH from complex I, and the electrons derived from FADH2 from complex II. This enzyme and FADH2 form a small complex that delivers electrons directly to the electron transport chain, bypassing the first complex. Since these electrons bypass and thus do not energize the proton pump in the first complex, fewer ATP molecules are made from the FADH2 electrons. The number of ATP molecules ultimately obtained is directly proportional to the number of protons pumped across the inner mitochondrial membrane.
The third complex is composed of cytochrome b, another Fe-S protein, Rieske center (2Fe-2S center), and cytochrome c proteins; this complex is also called cytochrome oxidoreductase. Cytochrome proteins have a prosthetic group of heme. The heme molecule is similar to the heme in hemoglobin, but it carries electrons, not oxygen. As a result, the iron ion at its core is reduced and oxidized as it passes the electrons, fluctuating between different oxidation states: Fe++ (reduced) and Fe+++ (oxidized). The heme molecules in the cytochromes have slightly different characteristics due to the effects of the different proteins binding them, giving slightly different characteristics to each complex. Complex III pumps protons through the membrane and passes its electrons to cytochrome c for transport to the fourth complex of proteins and enzymes (cytochrome c is the acceptor of electrons from Q; however, whereas Q carries pairs of electrons, cytochrome c can accept only one at a time).
The fourth complex is composed of cytochrome proteins c, a, and a3. This complex contains two heme groups (one in each of the two cytochromes, a, and a3) and three copper ions (a pair of CuA and one CuB in cytochrome a3). The cytochromes hold an oxygen molecule very tightly between the iron and copper ions until the oxygen is completely reduced. The reduced oxygen then picks up two hydrogen ions from the surrounding medium to make water (H2O). The removal of the hydrogen ions from the system contributes to the ion gradient used in the process of chemiosmosis.
In chemiosmosis, the free energy from the series of redox reactions just described is used to pump hydrogen ions (protons) across the membrane. The uneven distribution of H+ ions across the membrane establishes both concentration and electrical gradients (thus, an electrochemical gradient), owing to the hydrogen ions’ positive charge and their aggregation on one side of the membrane.
If the membrane were open to diffusion by the hydrogen ions, the ions would tend to diffuse back across into the matrix, driven by their electrochemical gradient. Recall that many ions cannot diffuse through the nonpolar regions of phospholipid membranes without the aid of ion channels. Similarly, hydrogen ions in the matrix space can only pass through the inner mitochondrial membrane through an integral membrane protein called ATP synthase (Figure 7.12). This complex protein acts as a tiny generator, turned by the force of the hydrogen ions diffusing through it, down their electrochemical gradient. The turning of parts of this molecular machine facilitates the addition of a phosphate to ADP, forming ATP, using the potential energy of the hydrogen ion gradient.
Chemiosmosis (Figure 7.13) is used to generate 90 percent of the ATP made during aerobic glucose catabolism; it is also the method used in the light reactions of photosynthesis to harness the energy of sunlight in the process of photophosphorylation. Recall that the production of ATP using the process of chemiosmosis in mitochondria is called oxidative phosphorylation. The overall result of these reactions is the production of ATP from the energy of the electrons removed from hydrogen atoms. These atoms were originally part of a glucose molecule. At the end of the pathway, the electrons are used to reduce an oxygen molecule to oxygen ions. The extra electrons on the oxygen attract hydrogen ions (protons) from the surrounding medium, and water is formed.
The number of ATP molecules generated from the catabolism of glucose varies. For example, the number of hydrogen ions that the electron transport chain complexes can pump through the membrane varies between species. Another source of variance stems from the shuttle of electrons across the membranes of the mitochondria. (The NADH generated from glycolysis cannot easily enter mitochondria.) Thus, electrons are picked up on the inside of mitochondria by either NAD+ or FAD+. As you have learned earlier, these FAD+ molecules can transport fewer ions; consequently, fewer ATP molecules are generated when FAD+ acts as a carrier. NAD+ is used as the electron transporter in the liver and FAD+ acts in the brain.
Another factor that affects the yield of ATP molecules generated from glucose is the fact that intermediate compounds in these pathways are used for other purposes. Glucose catabolism connects with the pathways that build or break down all other biochemical compounds in cells, and the result is somewhat messier than the ideal situations described thus far. For example, sugars other than glucose are fed into the glycolytic pathway for energy extraction. Moreover, the five-carbon sugars that form nucleic acids are made from intermediates in glycolysis. Certain nonessential amino acids can be made from intermediates of both glycolysis and the citric acid cycle. Lipids, such as cholesterol and triglycerides, are also made from intermediates in these pathways, and both amino acids and triglycerides are broken down for energy through these pathways. Overall, in living systems, these pathways of glucose catabolism extract about 34 percent of the energy contained in glucose.
Use construction paper and other art materials to create your own diagram of the electron transport chain (ETC). Be sure to include all parts of the electron transport chain, as well as the electrons themselves, NAD+ and NADH, and oxygen. On your diagram, label all parts of the ETC that transfers the free energy from electrons to another form. Then, use your model to make predictions about each of the following. Then, share your answers with the class.
- What would happen to free energy release if a cytochrome failed to undergo one of the redox reactions involved in the electron transport chain?
- What ultimately happens to the free energy in the electrons that travel down the ETC?
- Did you remember to have a pair of electrons travel down the ETC? What would happen if only one electron reached oxygen?
- Dinitrophenol (DNP) is an uncoupler that makes the inner mitochondrial membrane leaky to protons. It was used until 1938 as a weight loss drug. What effect would DNP have on the change in pH across the inner mitochondrial membrane and the overall process of cellular respiration? Why do you think DNP might be an effective weight-loss drug? Why is DNP no longer used?
- Cyanide inhibits cytochrome c oxidase, a component of the electron transport chain. If cyanide poisoning occurs, would you expect the pH of the intermembrane space to increase or decrease? Explain the effect of cyanide on ATP synthesis.
This activity is an application of Learning Objective 2.4 and Science Practices 1.4 and 3.1 and Learning Objective 2.5 and Science Practice 6.2 because students will have the opportunity to create a model of the electron transport chain, allowing students to study and discuss the components of the electron transport chain that allow organisms to capture, store, and use free energy.
An extended lab investigation on cellular respiration is available from the College Board®. This activity involves respirometry of plant seeds. It is available from the College Board’s® AP Biology Investigative Labs: An Inquiry-Based Approach, Investigation 6.
The Think About It questions are applications of Learning Objective 2.4 and Science Practices 1.4 and 3.1 and Learning Objective 2.5 and Science Practice 6.2 because students are provided with situations that raise questions about cellular respiration and are then asked to explain the effects of factors that affect the process. Students are also connecting the structure of the mitochondrion to its role in cellular respiration.
- If a cytochrome failed to perform a redox reaction, the electrons could not travel to the next cytochrome, and possibly not reach the proton pumps. Even if they did reach the pumps, the ETC could not offload the electrons into oxygen, preventing any additional electrons from travelling down the ETC and also preventing any further ATP production.
- The free energy in the electrons is transferred to the proton pumps, allowing them to pump protons. Some is also lost as heat and the rest is transferred to oxygen at the end of the ETC.
- If only one electron reached oxygen, water would not form at the end of the electron transport chain until another electron travelled through the chain.
- In living cells, DNP acts as an agent that can directly shuttle protons across biological membranes. Therefore, it weakens the proton concentration gradient that drives protons to pass through ATP synthase. After DNP poisoning, the electron transport chain can no longer form a proton gradient, and ATP synthase can no longer make ATP. DNP is an effective diet drug because it uncouples ATP synthesis; in other words, after taking it, a person obtains less energy out of the food he or she eats. Interestingly, one of the worst side effects of this drug is hyperthermia, or overheating of the body. Since ATP cannot be formed, the energy from electron transport is lost as heat.
- After cyanide poisoning, the electron transport chain can no longer pump protons into the intermembrane space. The pH of the intermembrane space would increase, the pH gradient would decrease, and ATP synthesis would stop.