Introduction to Metabolism (PDF)

An Introduction to Metabolism An organisms’s metabolism transforms matter and energy, subject to the laws of thermodynamics Metabolism is the totality of an organism’s chemical reactions A metabolic pathway begins with a specific molecule and ends with a product Each step is catalyzed by a s...

An Introduction to Metabolism An organisms’s metabolism transforms matter and energy, subject to the laws of thermodynamics Metabolism is the totality of an organism’s chemical reactions A metabolic pathway begins with a specific molecule and ends with a product Each step is catalyzed by a specific enzyme Common Enzymes in Metabolic Processes Isomerases: Catalyze conversions of one isomer to another Hydrolases: Catalyze hydrolysis of a substrate by the addition of water (e.g: maltase, lactase) Phosphotransferases: Transfers a phosphate group from one molecule to another Regulation of enzyme activity helps control metabolism Chemical chaos would result if a cell’s metabolic pathways were not tightly regulated A cell does this by switching on or off the genes that encode specific enzymes or by regulating the activity of the genes Feedback Inhibition Enzyme function can be inhibited through allosteric regulation This happens when a regulatory molecule binds to the allosteric site of an enzyme, changing its shape so it can no longer function in the pathway This prevents the cell from wasting chemical resources by making more product than is needed 🡪 if the concentration is high, this molecule binds to an enzyme often high in the metabolic pathway, preventing it from working. When this molecule is used up by the cell and the concentration decreases, the enzyme begins to function again Metabolic Pathways Catabolic pathways release energy by breaking down complex molecules into simpler compounds Cellular respiration, the breakdown of glucose in the presence of oxygen, is an example of a pathway of catabolism  Anabolic pathways consume energy to build complex molecules from simpler ones The synthesis of protein from amino acids is an example of anabolism Bioenergetics is the study of how organisms manage their energy resources Forms of Energy Energy is the capacity to cause change Energy exists in various forms, some of which can perform work Kinetic energy: associated with motion Thermal energy (heat): is kinetic energy associated with random movement of molecules or atoms Potential energy: is energy that matter possesses because of its location or structure Chemical energy: is potential energy available for release in a chemical reaction Energy can be converted from one form to another  What Can Cells do with Energy? Cells can use energy for: Chemical work Mechanical work Transport work The Laws of Energy Transformation Thermodynamics is the study of energy transformations A closed system, like that modelled by liquid in a thermos, is isolated from its surroundings In an open system, energy and matter can be transferred between the system and its surroundings Organisms are open systems The First Law of Thermodynamics According to the first law of thermodynamics, the energy of the universe is constant Energy can be transferred and transformed, but it cannot be created or destroyed The Second Law of Thermodynamics According to the second law of thermodynamics: Every energy transfer or transformation increases the entropy (disorder) of the universe During every energy transfer or transformation, some energy is unusable and is often lost as heat  Living cells unavoidably convert organized forms of energy to heat Spontaneous processes occur without energy input; they can happen quickly or slowly When a process occurs without energy input, it increases the entropy of the universe.  Exergonic and Endergonic Reactions in Metabolism An exergonic reaction proceeds with a net release of free energy and is spontaneous (reactants have more energy than the products, so energy is released when larger molecules are broken down) An endergonic reaction absorbs free energy from its surroundings and is nonspontaneous (synthesizing large polymers from monomers) Another way to view the reactions The Importance of ATP in metabolism ATP (Adenosine triphosphate) is composed of ribose sugar, adenine, and three phosphate groups To do work, cells use the process of energy coupling, which is mediated by ATP  The three types of cellular work (mechanical, transport, and chemical) are powered by the hydrolysis of ATP The bond between the last two phosphates can be broken by hydrolysis Energy is released when this happens This is an exergonic reaction which can be coupled with an endergonic reaction 🡪 the coupled reactions are exergonic overall (release energy)  ATP drives endergonic reactions by phosphorylation 🡪 transferring a phosphate group to some other molecule, such as a reactant This molecule is called a phosphorylated intermediate  The Regeneration of ATP ATP is renewable, and can be reformed by the addition of a phosphate group to ADP (Adenosine diphosphate) The energy for this reaction comes from catabolic reactions in the cell  REDOX Reactions Most of the important energy changes occurring in the body involve types of chemical reactions known as oxidations and reductions in which one or more electrons are transferred from one reactant to another The process of losing electrons is called OXIDATION and the process of gaining electrons is called REDUCTION Enzymes involved in these processes are: Dehydrogenase (removal of hydrogen) Oxidase (add oxygen to hydrogen, forming water)  In the cells of living organisms, oxidation usually involves the removal of electrons belonging to hydrogen atoms (in many cases the proton goes along with the electrons so the entire hydrogen atom is removed) These electrons (transferred with a hydrogen atom) are accepted by an oxidizing agent, and it becomes reduced Electron Transport Chains In some cases, a series of redox reactions occur in which the product of one redox reaction is the reactant of the next in the series In this case, a substance that was reduced in the first reaction becomes oxidized in the next… and this can continue happening down a chain of increasingly stronger electron acceptors (this releases free energy along every step of the way) This is the basis for several key reactions in photosynthesis and cellular respiration