Metabolism and energyThis article is an open-access article distributed under the terms and conditions of the Creative Commons Attribution license http: Low testoserone key problem in understanding the origin of life is to explain the mechanism s that led to the spontaneous assembly of molecular building blocks anabolic reactions produce energy ultimately resulted in the appearance of macromolecular structures as they are known in modern biochemistry today. An indispensable thermodynamic prerequisite for such a primordial anabolism is the anabolic reactions produce energy coupling to processes that supplied the free energy required. Here I review different sources of free energy and discuss the potential of each form having been involved in the very first anabolic reactions that were fundamental to increase molecular complexity and thus were essential for life. How and where did life on Earth arise [ 1 ]? This is one who uses anabolic steroids the top 25 unanswered big questions facing science that were selected in the th anniversary issue of Anabolic reactions produce energy in [ 2 ]. The quest for a better understanding of the processes involved in the origin of life naturally depends on the ability to discriminate living from non-living things.
Anabolic and Catabolic Reactions
Autotrophs are organisms that utilize inorganic sources of carbon, such as carbon dioxide and calcium carbonate limestone.
Heterotrophs are organisms that utilize organic sources of carbon, such as glucose. Phototrophs are organisms that get their energy from sunlight or artificial light. Chemotrophs are organisms that get their energy from chemical reactions. Chemoheterotrophs get their carbon and their energy both from breaking down organic molecules. All animals, including humans, and all parasitic organisms, including those that cause infectious diseases, are chemoheterotrophs.
Chemoautotrophs and photoheterotrophs also exist, but are beyond the scope of this course. Metabolism is the sum total of all of the chemical reactions occurring within a cell. Dehydration synthesis removes a molecule of water from two simpler molecules, or monomers , as they are joined to form a dimer. Additional monomers may then be added by dehydration synthesis to form polymers , or complex molecules composed of many repeating units, or monomers.
Anabolic reactions are endergonic reactions, meaning that they require an input of energy. Catabolism is the process of breaking down complex molecules into simpler molecules. Catabolic reactions often employ hydrolysis in order to break down complex molecules by adding a molecule of water to the simple molecules that are the products. However, other catabolic reactions include decarboxylation , which involves the removal of a molecule of carbon dioxide, and dehydrogenation , the removal of two hydrogen ions and two electrons.
Catabolic reactions are exergonic , that is they release energy that can be captured and used to do cellular work or to carry out anabolic reactions. The unit of energy exchange that is used to capture energy released by catabolic reactions and to transfer that energy to perform cellular work or to carry out anabolic reactions is adenosine triphosphate ATP , a nucleotide, composed of the nitrogenous base adenine , the five-carbon sugar ribose , and three phosphate units coupled together.
ATP captures small quantities of energy by binding one phosphate unit to adenosine diphosphate ADP to synthesize, by dehydration synthesis, a high-energy molecule of ATP. The phosphate can either be captured in the form of inorganic phosphate a phosphate anion or as an organic phosphate, that is, it can be attached to a metabolite or organic intermediate in a metabolic reaction.
ATP can be produced in catabolic reactions in several ways. One of the more common ways that occurs in many of the catabolic reactions is called substrate phosphorylation , whereby the phosphate molecule is taken from one of the organic phosphate intermediates, or metabolites, and combined with ADP by dehydration synthesis.
Substrate phosphorylation takes place in glycolysis , the decarboxylation of metabolites such as pyruvic acid, and in the Krebs cycle. Another very efficient means of producing ATP is by oxidative phosphorylation , where the phosphate is in inorganic anionic form. This inorganic phosphate is coupled to ADP using energy released by the movement of hydrogen ions protons down a concentration gradient, across a membrane, by way of a channel protein that also serves as an enzyme that catalyzes the dehydration synthesis of ATP.
These hydrogen ions are then combined with electrons that have passed from molecule to molecule through a sequence of proteins in the membrane. This sequence of membrane proteins is called the electron transport system , and the movement of hydrogen ions through the channel protein-enzyme is called chemiosmosis. The final electron acceptor of the hydrogen ions and electrons is oxygen, thus the products of oxidative phosphorylation are H 2 O and ATP.
Oxidative phosphorylation is employed in aerobic respiration, which is carried out on the plasma membrane of prokaryotic cells, but on the inner membrane of the mitochondrion in eukaryotic cells. A third process for producing ATP, which occurs in photosynthetic autotrophs , is called photophosphorylation. Photophosphorylation takes place in plants, algae, cyanobacteria , and some eubacteria. However, since these organisms are autotrophs , they are not causative agents of infectious diseases.
Therefore, photophosphorylation is discussed in courses such as cellular and molecular biology, botany, phycology , general microbiology, and bacteriology. Chemoheterotrophic organisms may be divided into three groups, those that must always utilize aerobic respiration, and therefore, get most of their ATP by oxidative phosphorylation ; those that must always utilize fermentation and get all of their ATP by substrate phosphorylation ; and those that can do both.
Animals, including humans, belong to the first group. Causative agents of infectious diseases come from all three groups. Since the final electron acceptor in oxidative phosphorylation is oxygen, while the final electron acceptor in substrate phosphorylation is an organic compound, we can distinguish among these three groups by their need for oxygen.
Strict, or obligate, aerobes must have oxygen to grow, and cannot grow without it. They will grow only on the surface of a liquid growth medium broth. Strict, or obligate, anaerobes can only grow deep in the broth culture where oxygen cannot reach them because they are inhibited or even poisoned by oxygen. Facultative anaerobes can grow in the presence of oxygen using aerobic respiration, or in the absence of oxygen using fermentation.
Most causative agents of infectious disease are facultative anaerobes, but many are obligate anaerobes, and others are strict aerobes. The nutrient most frequently used by most organisms as a source of energy through catabolism of that nutrient is glucose , a six-carbon-atom sugar or hexose. The starting substance, or substrate, for glycolysis is usually glucose, but can be any six-carbon simple sugar or hexose that can be readily isomerized into glucose.
So, we say that the substrate for glycolysis is glucose. Two additional reactants that are required for glycolysis to proceed are ATP and a carrier molecule, called NAD, which carries hydrogen ions and electrons. Two ATP molecules are required to make the hexose 6-carbon-atom sugar molecule into a hexose biphosphate molecule, which is unstable, and immediately splits into two triose 3-carbon-atom sugar phosphate molecules.
These two triose biphosphate molecules give up one phosphate immediately to two ADP molecules to synthesize two ATP molecules. After a dehydration step in which the two triose phosphate molecules lose one molecule of water each, the phosphate is rearranged to make two high-energy, or unstable, triose phosphate molecules which give up their phosphates to two more ADP molecules to synthesize two ATP molecules.
The final organic product is called pyruvic acid. Organisms that carry out strictly aerobic respiration require oxygen and are called obligate aerobes. Coenzymes come from vitamins. B vitamins are mentioned. All organisms, including microbes, can be classified metabolically according to their nutritional pattern — their source of energy and their source of carbon. Photoautotrophs are organisms that get their carbon from CO 2 and their energy from sunlight, therefore, plants, algae, and cyanobacteria are photoautotrophs.
Anabolism is the process of building up more complex substances from simpler substances. Anabolic reactions in cells frequently employ dehydration synthesis as a mechanism for building complex macromolecules. Organisms that carry out strictly fermentation are obligate anaerobes. Organisms that can carry out both are called facultative anaerobes.
Sent to ETC if respiration Sent to fermentation if not.