Chapter 14: Cellular Metabolism and Bioenergetics PDF

# Chapter 14: Basic Concepts of Cellular Metabolism and Bioenergetics ## Introduction to Metabolism Metabolism is broadly defined as the "sum total of all chemical reactions in an organism." However, this definition is oversimplified. Instead, metabolism encompasses the study of thousands of reac...

# Chapter 14: Basic Concepts of Cellular Metabolism and Bioenergetics ## Introduction to Metabolism Metabolism is broadly defined as the "sum total of all chemical reactions in an organism." However, this definition is oversimplified. Instead, metabolism encompasses the study of thousands of reactions occurring within a cell. This includes investigating their coordination, regulation, and energy requirements. ## Autotrophs and Heterotrophs All living organisms fall into two major categories: 1. **Autotrophs** ("self-feeding" organisms) are capable of using CO2 as their sole source of carbon, building complex carbon-containing biomolecules in the process. These organisms primarily rely on the sun's energy for biosynthesis and are generally self-sufficient. Examples include photosynthetic bacteria and higher plants. 2. **Heterotrophs** ("feeding on others") obtain energy by ingesting complex carbon-containing compounds such as carbohydrates and fats. Heterotrophs, such as higher animals and numerous other organisms, depend on autotrophs for sustenance. Heterotrophs can be further categorized into two subclasses based on their need for molecular oxygen: - **Aerobes** utilize molecular oxygen for metabolic reactions and thrive in oxygen-rich environments. - **Anaerobes** do not require oxygen for survival and some are even poisoned by it. ## 14.1 Intermediary Metabolism - In _E. coli_, there are at least a thousand biochemical reactions. A human cell contains as many as 3000 enzymes, each catalyzing a specific reaction. - Metabolism in all living organisms is comprised of sequential enzyme-catalyzed reactions. ## Metabolons - Functional Units of Metabolism - The interior of a cell is a gel-like matrix that hinders the movement of small molecules between enzymes in a metabolic pathway. - Enzymes involved in specialized metabolic sequences are organized into functional units called **Metabolons**, facilitating the efficient transfer of metabolites (reactants) from one enzyme's active site to the next. This minimizes loss due to diffusion, making the process more efficient. There are several types of Metabolons: - **Loosely held:** Enzymes connected by weak, noncovalent bonds (e.g., glycolysis) - **Tightly associated:** Enzymes forming a multienzyme complex within the cytoplasm (e.g., pyruvate dehydrogenase complex). - **Membrane-bound:** Enzymes forming a multienzyme complex within a cell membrane. ## Two Directions of Metabolism Metabolic pathways are categorized into two distinct paths based on their biochemical purpose: - **Catabolism** (degradative path): - Complex organic molecules such as fats, carbohydrates, and proteins are broken down into simpler molecules like lactate, pyruvate, ethanol, CO2, H2O, and NH3. - Characterized by: - Oxidation reactions - Release of free energy captured in ATP - Convergence - **Anabolism** (biosynthetic path): - Construction of large, complex biomolecules from smaller precursor molecules. For example, glucose synthesis from pyruvate or DNA synthesis from nucleotides. - Characterized by: - Construction of large, complex biomolecules - Reduction reactions - Input of energy supplied by ATP, NADH, and NADPH (derived from catabolism) - Divergence **Important note about metabolic reactions**: - Not every reaction is exclusively catabolic or anabolic; some may be neither. - Some reactions may neither release nor require energy. - Oxidation/reduction reactions are not the only type. - Anabolism and catabolism are interconnected, linked by the ATP energy cycle. ## Stages of Metabolism ### Catabolism **Stage I:** - The breakdown of macromolecules (proteins, fats [triacylglycerols], and polysaccharides) into their building blocks. - Major breakdown reactions: - **Proteins** are broken down into amino acids. - **Triacylglycerols** are broken down into fatty acids and glycerol. - **Polysaccharides** are broken down into disaccharides and, primarily, monosaccharides. - This stage involves no release of useful energy in the form of NADH or ATP. **Stage II:** - Amino acids, fatty acids, and monosaccharides are oxidized to a common metabolite, Acetyl CoA. - Some released energy is captured in the form of NADH or ATP. **Stage III:** - Acetyl CoA enters the citric acid cycle, where it is oxidized to CO2, the end product of aerobic carbon metabolism. - NADH and FADH2 are formed, releasing electrons to the electron transport system. - Oxygen is used in oxidative phosphorylation, producing water and energy. - Energy release is directly coupled to ATP synthesis from ADP and Pi. ### Anabolism (Biosynthesis) - Anabolism is divided into three stages: - **Monosaccharide and polysaccharide synthesis** may begin with CO2, oxaloacetate, and pyruvate. - **Amino acid synthesis** is accomplished using acetyl CoA or by the amination of pyruvate and alpha-keto acids derived from the citric acid cycle. - **Triacylglycerol synthesis** is accomplished using fatty acids synthesized from acetyl CoA. - Many anabolic pathways utilize energy from ATP and NADPH. **Important points about anabolism:** - While anabolism and catabolism share intermediates and some enzymes, they are not entirely reversible processes. - The pathways differ in terms of cellular location and regulation. - Glucose degradation primarily occurs in active muscle cells. - Glucose synthesis primarily occurs in liver cells. - Fatty acid synthesis primarily occurs within the cytoplasm of adipose cells. - Fatty acid degradation primarily occurs in the mitochondria of resting muscle cells. ## 14.2 The Chemistry of Metabolism Metabolic reactions are catalyzed by enzymes. The simplest single-celled organisms may contain hundreds of enzyme-catalyzed reactions, while a human cell can perform as many as 3000 reactions. Six main categories of biochemical reactions are utilized in metabolism: 1. **Oxidation-reduction reactions (Redox):** - Transfer of electrons between molecules. - Identified by the movement of hydrogen atoms. 2. **Group-transfer reactions:** - Transfer of functional groups between molecules. - Important examples include phosphate groups from ATP and acyl groups from coenzyme A. 3. **Hydrolysis reactions:** - Breakdown of molecules using water. - Important examples include the hydrolysis of: - Esters (e.g., triacylglycerols to fatty acids) - Amides (e.g., proteins to amino acids) - Glycosides (e.g., oligosaccharides to monosaccharides) 4. **Nonhydrolytic cleavage reactions:** - Breakdown of molecules without water. - Often involves breaking carbon-carbon bonds. 5. **Isomerization and rearrangement reactions:** - Rearrangement of atoms within a molecule. - Examples include aldose-ketose isomerization, which is particularly important in carbohydrate metabolism. 6. **Bond formation reactions:** - Synthesis of new bonds, requiring energy often supplied by ATP. ## Biochemical Redox Agents - Many redox reactions rely on coenzyme redox pairs such as NAD+/NADH, NADP+/NADPH, and FAD/FADH2. ## Combining Reaction Types - Organisms can integrate different reaction types within a single step. - One example is the decarboxylation of isocitrate combining a redox reaction with a nonhydrolytic cleavage reaction. **Summary of Chemical reactions within Metabolism**: These six reaction types correlate with the six classes of enzymes described in Chapter 5.