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Electron Transport: The process by which Electrons are transported from a reduced substrate to molecular Oxygen. (From Bennington, Saunders Dictionary and Encyclopedia of Laboratory Medicine and Technology, 1984, p270)

Electron Transport Chains

JoVE 10742

The final stage of cellular respiration is oxidative phosphorylation, which consists of (1) an electron transport chain and (2) chemiosmosis.

The electron transport chain is a set of proteins and other organic molecules found in the inner membrane of mitochondria in eukaryotic cells and the plasma membrane of prokaryotic cells. The electron transport chain has two primary functions: it produces a proton gradient—storing energy that can be used to create ATP during chemiosmosis—and generates electron carriers, such as NAD+ and FAD, that are used in glycolysis and the citric acid cycle. Generally, molecules of the electron transport chain are organized into four complexes (I-IV). The molecules pass electrons to one another through multiple redox reactions, moving electrons from higher to lower energy levels through the transport chain. These reactions release energy that the complexes use to pump H+ across the inner membrane (from the matrix into the intermembrane space). This forms a proton gradient across the inner membrane. NADH and FADH2 are reduced electron carriers produced during earlier cellular respiration phases. NADH can directly input electrons into complex I, which uses the released energy to pump protons into the intermembrane space. FADH2 inputs electrons into complex II, the only co

 Core: Cellular Respiration

Electron Carriers

JoVE 10744

Electron carriers can be thought of as electron shuttles. These compounds can easily accept electrons (i.e., be reduced) or lose them (i.e., be oxidized). They, therefore, play an essential role in energy production because cellular respiration is contingent on the flow of electrons.

Over the many stages of cellular respiration, glucose breaks down into carbon dioxide and water. Electron carriers pick up electrons lost by glucose in these reactions, temporarily store the electrons and input them into the electron transport chain. Two such electron carriers are NAD+ and FAD, which are both derived from B vitamins. The reduced forms of NAD+ and FAD, NADH and FADH2, respectively, are produced during earlier stages of cellular respiration (glycolysis, pyruvate oxidation, and the citric acid cycle). The reduced electron carriers NADH and FADH2 pass electrons into complexes I and II of the electron transport chain, respectively. In the process, they are oxidized to form NAD+ and FAD. Additional electron carriers in the electron transport chain are flavoproteins, iron-sulfur clusters, quinones, and cytochromes. With the assistance of enzymes, these electron carriers eventually transfer the electrons to oxygen molecules. The electron carriers become oxidized as they donate electrons and reduc

 Core: Cellular Respiration

Characterizing Electron Transport through Living Biofilms

1Center for Bio/Molecular Science and Engineering, Naval Research Laboratory, 2George Mason University, 3Chemistry Division, Naval Research Laboratory, 4Departments of Physics, Biological Sciences, and Chemistry, University of Southern California, 5Department of Chemical Engineering and Materials Science, Michigan State University

JoVE 54671



JoVE 10743

Oxidative phosphorylation is a highly efficient process that generates large amounts of adenosine triphosphate (ATP), the basic unit of energy that drives many processes in living cells. Oxidative phosphorylation involves two processes—electron transport and chemiosmosis. During electron transport, electrons are shuttled between large complexes on the inner mitochondrial membrane and protons (H+) are pumped across the membrane into the intermembrane space, creating an electrochemical gradient. In the next step, protons flow back down their gradient into the mitochondrial matrix via ATP synthase, a protein complex embedded within the inner membrane. This process, called chemiosmosis, uses the energy of the proton gradient to drive the synthesis of ATP from adenosine diphosphate (ADP). The electron transport chain is a series of complexes that transfer electrons from electron donors to electron acceptors via simultaneous reduction and oxidation reactions, otherwise known as redox reactions. At the end of the chain, electrons reduce molecular oxygen to produce water. The shuttling of electrons between complexes is coupled with proton transfer, whereby protons (H+ ions) travel from the mitochondrial matrix to the intermembrane space against their concentration gradient. Eventually, the high concentration of protons in the interm

 Core: Cellular Respiration

Photosystem I

JoVE 10752

Like Photosystem II (PS II), Photosystem I (PS I) captures photons and transports them through chlorophyll molecules into a reaction center. In PS I, the photons reenergize the electrons that have entered PS I from PS II. From the reaction center, the high energy electron is sent through an electron transport chain and ultimately joins with an additional electron and a proton to reduce NADP+ into NADPH. Thus, similar to PS II that captures energy to generate ATP, PS I captures energy to create NADPH. The pigments of the light-harvesting complex in Photosystem I absorb photons and relay the energy to the reaction center (P700). Following oxidation, a high-energy electron is passed from the specialized pair of chlorophyll a to the primary electron acceptor. This time, however, the missing electrons from the chlorophyll a pair are replaced by the electrons traveling from Photosystem II (instead of splitting of water as in PS II). On their way from PS II to PS I, the electrons pass through the electron transport chain, comprising the carrier molecule plastoquinone, the dual-protein cytochrome complex, and plastocyanin. Once the electron was excited in the reaction center of PS I, it enters a second electron transport chain—the protein complex ferredoxin. The single electron then joins with another electron and a proton (H+)

 Core: Photosynthesis

Photosynthesis- Concept

JoVE 10565


Almost all living organisms on Earth depend on photosynthesis, which is the process that converts sunlight energy into a simple sugar called glucose. This molecule can be used as a short-term energy source or to build more complex carbohydrates like starches for long-term energy storage. Autotrophs are organisms that capture light energy using photosynthesis. Also known …

 Lab Bio

What is Cellular Respiration?

JoVE 10976

Organisms harvest energy from food, but this energy cannot be directly used by cells. Cells convert the energy stored in nutrients into a more usable form: adenosine triphosphate (ATP).

ATP stores energy in chemical bonds that can be quickly released when needed. Cells produce energy in the form of ATP through the process of cellular respiration. Although much of the energy from cellular respiration is released as heat, some of it is used to make ATP. During cellular respiration, several oxidation-reduction (redox) reactions transfer electrons from organic molecules to other molecules. Here, oxidation refers to electron loss and reduction to electron gain. The electron carriers NAD+ and FAD—and their reduced forms, NADH and FADH2, respectively—are essential for several steps of cellular respiration. Some prokaryotes use anaerobic respiration, which does not require oxygen. Most organisms use aerobic (oxygen-requiring) respiration, which produces much more ATP. Aerobic respiration generates ATP by breaking down glucose and oxygen into carbon dioxide and water. Both aerobic and anaerobic respiration begin with glycolysis, which does not require oxygen. Glycolysis breaks down glucose into pyruvate, yielding ATP. In the absence of oxygen, pyruvate ferments, producing NAD+ for continued glycoly

 Core: Cellular Respiration

Photosystem II

JoVE 10751

Photosystem II is a multi-protein complex embedded within the thylakoid membrane where it harvests light energy. Chlorophyll molecules transfer energy to a specific pair of chlorophyll a molecules in the reaction center of Photosystem II. Here, the chlorophyll a molecules lose an electron (oxidation), transferring it to a primary electron acceptor. The donated electrons pass through the electron transport chain into Photosystem I. Splitting a water molecule releases one oxygen atom, two protons (H+) and two electrons. The electrons replace the donated electrons of the two chlorophyll a molecules in the reaction center. The oxygen atom immediately reacts with another oxygen atom, producing O2 that is released into the atmosphere. The protons accumulate and create a concentration gradient across the thylakoid membrane that drives ATP synthesis in a process called chemiosmosis. The multi-protein complex Photosystem II harvests photons and transfers energy through its bound pigments chlorophyll a and b, and carotenoids. Carotenoids have a protective function as they help dissipate the vast amount of energy taken in that could otherwise damage the plant tissue. Energy travels from chlorophyll molecule to chlorophyll molecule until it reaches a pair of specialized chlorophyll a molecules in a region called the re

 Core: Photosynthesis

ATP Yield

JoVE 11008

Cellular respiration produces 30-32 ATP molecules per glucose molecule. Although most of the ATP results from oxidative phosphorylation and the electron transport chain (ETC), 4 ATP are gained beforehand (2 from glycolysis and 2 from the citric acid cycle).

The ETC is embedded in the inner mitochondrial membrane and comprises four main protein complexes and an ATP synthase. NADH and FADH2 pass electrons to these complexes, which in turn pump protons into the intermembrane space. This distribution of protons generates a concentration gradient across the membrane. The gradient drives the production of ATP when protons flow back into the mitochondrial matrix via the ATP synthase. For every 2 input electrons that NADH passes into complex I, complexes I and III each pump 4 protons and complex IV pumps 2 protons, totaling 10 protons. Complex II is not involved in the electron chain initiated by NADH. FADH2, however, passes 2 electrons to complex II, so a total of 6 protons are pumped per FADH2; 4 protons via complex III and 2 via complex IV. Four protons are needed to synthesize 1 ATP. Since 10 protons are pumped for every NADH, 1 NADH yields 2.5 (10/4) ATP. Six protons are pumped for every FADH2, so 1 FADH2 yields 1.5 (6/4) ATP. Cellular respiration produces a maximum of 10 NADH and 2 FADH2 pe

 Core: Cellular Respiration

What is Glycolysis?

JoVE 10737

Cells make energy by breaking down macromolecules. Cellular respiration is the biochemical process that converts “food energy” (from the chemical bonds of macromolecules) into chemical energy in the form of adenosine triphosphate (ATP). The first step of this tightly regulated and intricate process is glycolysis. The word glycolysis originates from Latin glyco (sugar) and lysis (breakdown). Glycolysis serves two main intracellular functions: generate ATP and intermediate metabolites to feed into other pathways. The glycolytic pathway converts one hexose (six-carbon carbohydrate such as glucose), into two triose molecules (three-carbon carbohydrate) such as pyruvate, and a net of two molecules of ATP (four produced, two consumed) and two molecules of nicotinamide adenine dinucleotide (NADH). Did you know that glycolysis was the first biochemical pathway discovered? In the mid-1800s, Louis Pasteur determined that microorganisms cause the breakdown of glucose in the absence of oxygen (fermentation). In 1897, Eduard Buchner found that fermentation reactions can still be carried out in cell-free yeast extracts, achieved by breaking open the cell and collecting the cytoplasm which contains the soluble molecules and organelles. Shortly thereafter in 1905, Arthur Harden and William Young discovered that the rate of fermentation decreases wit

 Core: Cellular Respiration

Products of the Citric Acid Cycle

JoVE 10977

The cells of most organisms—including plants and animals—obtain usable energy through aerobic respiration, the oxygen-requiring version of cellular respiration. Aerobic respiration consists of four major stages: glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation. The third major stage, the citric acid cycle, is also known as the Krebs cycle or tricarboxylic acid (TCA) cycle. For every glucose molecule that undergoes cellular respiration, the citric acid cycle is carried out twice; this is because glycolysis (the first stage of aerobic respiration) produces two pyruvate molecules per glucose molecule. During pyruvate oxidation (the second stage of aerobic respiration), each pyruvate molecule is converted into one molecule of acetyl-CoA—the input into the citric acid cycle. Therefore, for every glucose molecule, two acetyl-CoA molecules are produced. Each of the two acetyl-CoA molecules goes once through the citric acid cycle. The citric acid cycle begins with the fusion of acetyl-CoA and oxaloacetate to form citric acid. For each acetyl-CoA molecule, the products of the citric acid cycle are two carbon dioxide molecules, three NADH molecules, one FADH2 molecule, and one GTP/ATP molecule. Therefore, for every glucose molecule (which generates two acetyl-CoA molecules), the citric acid cycle yiel

 Core: Cellular Respiration

The Citric Acid Cycle

JoVE 10741

The citric acid cycle, also known as the Krebs cycle or TCA cycle, consists of several energy-generating reactions that yield one ATP molecule, three NADH molecules, one FADH2 molecule, and two CO2 molecules.

Acetyl CoA is the point-of-entry into the citric acid cycle, which occurs in the inner membrane (i.e., matrix) of mitochondria in eukaryotic cells or the cytoplasm of prokaryotic cells. Prior to the citric acid cycle, pyruvate oxidation produced two acetyl CoA molecules per glucose molecule. Hence, the citric acid cycle runs twice per glucose molecule. The citric acid cycle can be partitioned into eight steps, each yielding different molecules (italicized below). With the help of catalyzing enzymes, one acetyl CoA (2-carbon) reacts with oxaloacetic acid (4-carbon), forming the 6-carbon molecule citrate. Next, citrate is converted into one of its isomers, isocitrate, through a two-part process in which water is removed and added. The third step yields α-ketoglutarate (5-carbon) from oxidized isocitrate. This process releases CO2 and reduces NAD+ to NADH. The fourth step forms the unstable compound succinyl CoA from α-ketoglutarate, a process that also releases CO2 and reduces NAD+ to NADH. The fifth

 Core: Cellular Respiration


JoVE 10694

Mitochondria and peroxisomes are organelles that are the primary sites of oxygen usage in eukaryotic cells. Mitochondria carry out cellular respiration—the process that converts energy from food into ATP—the primary form of energy used by cells. Peroxisomes carry out a variety of functions, primarily breaking down different substances such as fatty acids.

Peroxisomes contain up to 50 enzymes and are surrounded by a single membrane. They carry out oxidative reactions that break down molecules and produce hydrogen peroxide (H2O2) as a by-product. H2O2 is toxic to cells, but the peroxisome contains an enzyme—catalase—that converts H2O2 into harmless water and oxygen. In addition, catalase uses H2O2 to break down alcohol in the liver into aldehyde and water. However, since H2O2 is produced in very low quantities in the body, other enzymes primarily degrade alcohol. A critical function of the peroxisome is to break down fatty acids in a process called β oxidation. The resulting product—acetyl-CoA—is released into the cytosol and can travel to the mitochondria, where it is used to produce ATP. In mammalian cells, the mitochondria also carry out β oxidation, as well as using products from the catabolism o

 Core: Cell Structure and Function

Outcomes of Glycolysis

JoVE 11006

Nearly all the energy used by cells comes from the bonds that make up complex, organic compounds. These organic compounds are broken down into simpler molecules, such as glucose. Subsequently, cells extract energy from glucose over many chemical reactions—a process called cellular respiration.

Cellular respiration can take place in the presence or absence of oxygen, referred to as aerobic and anaerobic respiration, respectively. In the presence of oxygen, cellular respiration starts with glycolysis and continues with pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation. Both aerobic and anaerobic cellular respiration start with glycolysis. Glycolysis yields a net gain of two pyruvate molecules, two NADH molecules, and two ATP molecules (four produced minus two used during energy-requiring glycolysis). In addition to these major products, glycolysis generates two water molecules and two hydrogen ions. In cells that carry out anaerobic respiration, glycolysis is the primary source of ATP. These cells use fermentation to convert NADH from glycolysis back into NAD+, which is required to continue glycolysis. Glycolysis is also the primary source of ATP for mature mammalian red blood cells, which lack mitochondria. Cancer cells and stem cells rely on aerobic glycolysis for ATP. Cells that use aerobic respiration cont

 Core: Cellular Respiration

Cellular Respiration- Concept

JoVE 10567

Autotrophs and Heterotrophs

Living organisms require a continuous input of energy to maintain cellular and organismal functions such as growth, repair, movement, defense, and reproduction. Cells can only use chemical energy to fuel their functions, therefore they need to harvest energy from chemical bonds of biomolecules, such as sugars and lipids. Autotrophic organisms, namely…

 Lab Bio

Energy-releasing Steps of Glycolysis

JoVE 10739

While the first phase of glycolysis consumes energy to convert glucose to glyceraldehyde 3-phosphate (G3P), the second phase produces energy. The energy is released over a sequence of reactions that turns G3P into pyruvate. The energy-releasing phase—steps 6-10 of glycolysis—occurs twice, once for each of the two 3-carbon sugars produced during steps 1-5.

The first energy-releasing step—considered the 6th step of glycolysis overall—consists of two concurrent events: oxidation and phosphorylation of G3P. The electron carrier NAD+ removes one hydrogen from G3P, oxidizing the 3-carbon sugar and converting (reducing) NAD+ to form NADH and H+. The released energy is used to phosphorylate G3P, turning it into 1,3-bisphosphoglycerate. In the next step, 1,3-bisphosphoglycerate converts ADP to ATP by donating a phosphate group, thereby becoming 3-phosphoglycerate. The 3-phosphoglycerate is then converted into an isomer, 2-phosphoglycerate. Subsequently, 2-phosphoglycerate loses a water molecule, becoming the unstable molecule 2-phosphoenolpyruvate, or PEP. PEP easily loses its phosphate group to ADP, converting it into a second ATP molecule and becoming pyruvate in the process. The energy-releasing phase releases two molecules of ATP and one molecule of NADH per converted sugar. Because

 Core: Cellular Respiration

Mössbauer Spectroscopy

JoVE 10448

Source: Joshua Wofford, Tamara M. Powers, Department of Chemistry, Texas A&M University 

Mössbauer spectroscopy is a bulk characterization technique that examines the nuclear excitation of an atom by gamma rays in the solid state. The resulting Mössbauer spectrum provides information about the oxidation state, spin…

 Inorganic Chemistry

An Introduction to Cell Metabolism

JoVE 5652

In cells, critical molecules are either built by joining together individual units like amino acids or nucleotides, or broken down into smaller components. Respectively, the reactions responsible for this are referred to as anabolic and catabolic. These reactions require or produce energy typically in the form of a “high-energy” molecule called ATP. Together,…

 Cell Biology

The ATP Bioluminescence Assay

JoVE 5653

In fireflies, the luciferase enzyme converts a compound called luciferin into oxyluciferin, and produces light or “luminescence” as a result. This reaction requires energy derived from ATP in order to proceed, so researchers have exploited the luciferase-luciferin interaction to gauge ATP levels in cells. Given ATP’s role as the cell’s currency of…

 Cell Biology

Comprehensive Autopsy Program for Individuals with Multiple Sclerosis

1Department of Neurosciences, Lerner Research Institute, Cleveland Clinic, 2Department of Biomedical Engineering, Lerner Research Institute, Cleveland Clinic, 3Mellen Center for Treatment and Research in Multiple Sclerosis, Neurological Institute, Cleveland Clinic

JoVE 59511


Analysis of Non-Human Primate Pancreatic Islet Oxygen Consumption

1Department of Molecular Physiology and Biophysics, Vanderbilt University, 2Agilent Technologies, 3Department of Biological Sciences, Vanderbilt University, 4Department of Veterans Affairs Tennessee Valley, 5Department of Medicine, Vanderbilt University Medical Center, 6Department of Cell and Developmental Biology, Vanderbilt University

Video Coming Soon

JoVE 60696

 JoVE In-Press

Measurement of Mitochondrial Mass and Membrane Potential in Hematopoietic Stem Cells and T-Cells by Flow Cytometry

1Department of Oncology UNIL CHUV, Ludwig Institute for Cancer Research Lausanne, University of Lausanne, 2Swiss Institute for Experimental Cancer Research (ISREC), School of Life Sciences, Swiss Federal Institute of Technology Lausanne (EPFL), 3Center of Experimental Therapeutics, Department of Oncology, Centre Hospitalier Universitaire Vaudois, 4Hematology Service, Department of Oncology, Centre Hospitalier Universitaire Vaudois (CHUV)

Video Coming Soon

JoVE 60475

 JoVE In-Press

Analyzing Oxygen Consumption Rate in Primary Cultured Mouse Neonatal Cardiomyocytes Using an Extracellular Flux Analyzer

1Division of Cardiology, Department of Medicine, University of California San Diego, 2Department of Pharmacology, University of California San Diego, 3Cardiology Section, Department of Medicine, Veterans Administration Healthcare, San Diego

JoVE 59052


Measurement of Energy Metabolism in Explanted Retinal Tissue Using Extracellular Flux Analysis

1Division of Metabolism, Endocrinology and Lipid Research, Department of Medicine, Washington University School of Medicine, 2Department of Biomedical Engineering, Washington University in Saint Louis, 3Department of Ophthalmology and Visual Science, Washington University School of Medicine

JoVE 58626


Isolation of Primary Mouse Hepatocytes for Nascent Protein Synthesis Analysis by Non-radioactive L-azidohomoalanine Labeling Method

1Department of Pharmacology and Systems Physiology, College of Medicine, University of Cincinnati, 2Division of Endocrinology, Cincinnati Children's Hospital Medical Center, 3Division of Developmental Biology, Cincinnati Children's Hospital Medical Center, 4Department of Pediatrics, College of Medicine, University of Cincinnati

JoVE 58323


A Flow Cytometry-based Assay for Measuring Mitochondrial Membrane Potential in Cardiac Myocytes After Hypoxia/Reoxygenation

1State Key Laboratory of Cardiovascular Disease, Department of Cardiology, Fuwai Hospital, National Center for Cardiovascular Diseases, Chinese Academy of Medical Sciences and Peking Union Medical College, 2State Key Laboratory of Cardiovascular Disease, Fuwai Hospital, National Center for Cardiovascular Diseases, Chinese Academy of Medical Sciences and Peking Union Medical College

JoVE 57725

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