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…
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…
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…
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…
Skeletal muscles continuously produce ATP to provide the energy that enables muscle contractions. Skeletal muscle fibers can be categorized as type I, type IIA, or type IIB based on differences in their contraction speed and how they produce ATP, as well as physical differences related to these factors. Most human muscles contain all three muscle fiber types, albeit in varying proportions. Type I, or slow oxidative, muscle fibers appear red due to large numbers of capillaries and high levels of myoglobin, an oxygen-storing protein. Type I muscle fibers contain more mitochondria, which produce ATP through oxidative phosphorylation, than type II fibers. Slow oxidative muscle fibers use aerobic respiration, involving oxygen and glucose, to produce ATP. In addition to contracting more slowly than type II fibers, type I fibers receive nerve signals more slowly, contract for longer periods, and are more resistant to fatigue. Type I fibers primarily store energy as fatty substances called triglycerides. Type II, or fast, muscle fibers often appear white. Relative to type I fibers, type II fibers receive nerve signals and contract more quickly, but contract for shorter periods and fatigue more quickly. Type II muscle fibers primarily store energy as ATP and creatine phosphate. Type IIA, or fast oxidative, muscle fibers primarily u…
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…
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…
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…
Most DNA resides in the nucleus of a cell. However, some organelles in the cell cytoplasm—such as chloroplasts and mitochondria—also have their own DNA. These organelles replicate their DNA independently of the nuclear DNA of the cell in which they reside. Non-nuclear inheritance describes the inheritance of genes from structures other than the nucleus.
Mitochondria are present in both plants and animal cells. They are regarded as the “powerhouses” of eukaryotic cells because they break down glucose to form energy that fuels cellular activity. Mitochondrial DNA consists of about 37 genes, and many of them contribute to this process, called oxidative phosphorylation. Chloroplasts are found in plants and algae and are the sites of photosynthesis. Photosynthesis allows these organisms to produce glucose from sunlight. Chloroplast DNA consists of about 100 genes, many of which are involved in photosynthesis. Unlike chromosomal DNA in the nucleus, chloroplast and mitochondrial DNA do not abide by the Mendelian assumption that half an organism’s genetic material comes from each parent. This is because sperm cells do not generally contribute mitochondrial or chloroplast DNA to zygotes during fertilization. While a sperm cell primarily contributes one haploid set of nuclear chromosomes to the zygote, an egg cell contrib…
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…
Glucose is the source of nearly all energy used by organisms. The first step of converting glucose into usable energy is called glycolysis. Glycolysis occurs in the cytosol of the cell over two phases: an energy-requiring phase and an energy-releasing phase. Over the first three steps, glucose is converted into different forms and attaches to two phosphate groups donated by two ATP molecules, resulting in an unstable sugar. In the next two stages, the unstable sugar splits into two sugar isomers which are either converted or used directly in the next phase of glycolysis. First, glucose receives a phosphate group from ATP converting it into a more reactive form (glucose 6-phosphate). Because glucose attached to the negatively-charged phosphate cannot cross the hydrophobic cell membrane, the addition of a phosphate group also traps glucose inside the cell. Next, the more reactive form of glucose is converted into one of its isomers, fructose 6-phosphate, which is required for subsequent energy-requiring steps of glycolysis. Fructose 6-phosphate then receives a phosphate group from a second ATP molecule. This converts fructose 6-phosphate into fructose 1,6-bisphosphate, an unstable sugar. This unstable sugar splits into two distinct three-carbon sugar isomers, glyceraldehyde 3-phosphate and DHAP. Glyceraldehyde 3-phosphate can be directly use…
1Lady Davis Institute for Medical Research, Jewish General Hospital, 2Integrated Program in Neuroscience, McGill University, 3Department of Cellular and Molecular Medicine, University of Ottawa, 4Department of Oncology, Faculty of Medicine, McGill University
1Laboratory of Mitochondrial Biology and Metabolism, National Heart, Lung, and Blood Institute, National Institutes of Health, 2Laboratory of Immunogenetics, National Institute of Allergy and Infectious Diseases, National Institutes of Health, 3Laboratory of Immunology, National Institute of Allergy and Infectious Diseases, National Institutes of Health
Creating a Structurally Realistic Finite Element Geometric Model of a Cardiomyocyte to Study the Role of Cellular Architecture in Cardiomyocyte Systems Biology
1Cell Structure and Mechanobiology Group, University of Melbourne, 2Systems Biology Laboratory, Melbourne School of Engineering, University of Melbourne, 3Department of Biomedical Engineering, University of Melbourne, 4School of Mathematics and Statistics, Faculty of Science, University of Melbourne, 5Department of Engineering Science, University of Auckland, 6Advanced Microscopy Facility, Bio21 Molecular Science and Biotechnology Institute, University of Melbourne, 7ARC Centre of Excellence in Convergent Bio-Nano Science and Technology, University of Melbourne, 8School of Medicine, Faculty of Medicine, Dentistry and Health Sciences, University of Melbourne, 9Living Systems Institute, University of Exeter
1Chemistry Department, Elon University, 2Department of Neurobiology and Anatomy, Wake Forest School of Medicine, 3Neuroscience Graduate Program, Wake Forest School of Medicine, 4ALS Center Translational Science Unit, Wake Forest School of Medicine
1Department of Nuclear Medicine, Klinikum rechts der Isar, Technische Universität München, 2Department of Chemistry, Technische Universität München, 3GE Global Research, 4Zentralinstitut für Medizintechnik der Technischen Universität München (IMETUM), Technische Universität München, 5Institute for Biological and Medical Imaging (IBMI), Helmholtz Zentrum München, 6IDG Institute of Developmental Genetics, Helmholtz Zentrum München
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
Intra-cardiac Side-Firing Light Catheter for Monitoring Cellular Metabolism using Transmural Absorbance Spectroscopy of Perfused Mammalian Hearts
1Department of Microbiology, Biochemistry and Molecular Genetics, New Jersey Medical School, Rutgers, The State University of New Jersey, 2Public Health Research Institute, Rutgers, The State University of New Jersey
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
Measuring Liver Mitochondrial Oxygen Consumption and Proton Leak Kinetics to Estimate Mitochondrial Respiration in Holstein Dairy Cattle
1Animal Sperm Research Center, Department of Animal Sciences, Robert H. Smith Faculty of Agriculture, Food and Environment, Hebrew University of Jerusalem, 2Department of Animal Sciences, Robert H. Smith Faculty of Agriculture, Food and Environment, Hebrew University of Jerusalem
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
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
Saccharomyces cerevisiae Exponential Growth Kinetics in Batch Culture to Analyze Respiratory and Fermentative Metabolism
1Department of Chemistry, Universidad Autónoma de Querétaro, 2Department of Biochemical Engineering, Instituto Tecnológico de Morelia, 3Department of Biochemical Engineering, Instituto Tecnológico Superior de Ciudad Hidalgo
Sensitive Measurement of Mitophagy by Flow Cytometry Using the pH-dependent Fluorescent Reporter mt-Keima
1Peripheral Neuropathy Research Center, College of Medicine, Dong-A University, 2Department of Biochemistry, College of Medicine, Dong-A University, 3Aging Institute, Department of Medicine, University of Pittsburgh School of Medicine