Unit 5: Metabolism, Enzymes, and Cellular Respiration
Metabolism refers to the sum total of every chemical reaction in every organism. Cells use enzymes and metabolic pathways to conduct these chemical reactions. It is essential to understand the reactions that comprise metabolism to learn how organisms acquire and use energy to survive. Since this process is quite complicated, we will explore it from several different angles in this unit.
Completing this unit should take you approximately 9 hours.
5.1: Energy and Thermodynamics
Energy flows through all living systems. The chemical reactions that build and break down energy-containing compounds are vital to all living organisms. Biochemical pathways involve the building and breaking down through anabolism and catabolism, respectively. Notice that all of these pathways involve chemical reactions that require enzymes, which are the biological catalysts we will discuss in the next section.
Remember that the laws of thermodynamics state that energy is neither created nor destroyed, but transformed from one form to another. Biochemical reactions follow these same laws of thermodynamics which predict whether reactions will occur spontaneously, or without any energy required.
5.2: Metabolic Pathways, Enzymes, and Regulation
Metabolism refers to all chemical reactions in the cell. Some reactions break down large molecules and release energy. Other reactions require energy to build up large molecules. These reactions are connected via metabolic pathways that must be regulated to conserve resources and energy.
5.3: The Role of Cellular Respiration, Energy, and ATP
In this section, we explore cellular respiration, the metabolic pathway that breaks down our food in the form of glucose to produce oxygen, water, and ATP, which is the most usable energy source for the cell. This process takes place in the mitochondria within the cell. Aerobic respiration occurs in the presence of oxygen; anaerobic respiration occurs without oxygen and builds lactic acid.
Living organisms perform an important catabolic pathway that breaks down organic compounds to yield energy in the form of ATP. The molecules are completely broken down aerobically through cell respiration. Scientists have defined three primary stages in cell respiration, which we will review in detail: glycolysis, the citric acid cycle, and oxidative phosphorylation.
Glycolysis is the first step in the process of cellular respiration. It involves the partial breakdown of glucose into two pyruvate molecules. Glycolysis is the first step in harvesting potential energy from the bond of glucose. Notice that glycolysis, a multi-step biochemical pathway, produces a small amount of energetic resources.
5.5: The Krebs Cycle (Citric Acid Cycle)
The Krebs cycle (also called the citric acid cycle and TCA cycle) is a cyclical biochemical pathway that begins the completion of the oxidation of glucose. While it forms a small amount of ATP via substrate-level phosphorylation, its main role is the formation of energetic electron carriers that are needed for the electron transport chain.
5.6: Oxidative Phosphorylation: Electron Transport Chain and Chemiosmosis
Oxidative phosphorylation is the final stage of cellular respiration and is made up of two closely connected components: the electron transport chain and chemiosmosis. In the electron transport chain, electrons are passed from one molecule to another: the energy released during these electron transfers is used to form an electrochemical gradient. In chemiosmosis, the energy stored in the gradient is used to produce ATP.
The electron transport chain is a collection of proteins located in the inner membrane of the mitochondria known for its role in creating a proton gradient that is necessary for oxidative phosphorylation. These proteins are part of the final step of cellular respiration where glucose is completely oxidized by oxygen. The electrons, which are temporarily carried by previous redox electron carriers, move through the chain and reduce oxygen gas.
5.7: Anaerobic Cellular Respiration: Fermentation
Organic molecules are partially broken down when they lack oxygen. Some organisms use the process of fermentation to break glucose down when oxygen is not available following the electron transport chain.
Unit 5 Assessment
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