Metabolic Pathways and ATP Production

Definition of Metabolism: Metabolism encompasses the totality of all chemical reactions that occur within the body to provide the energy and substances necessary for growth.
Types of Metabolic Reactions: * Catabolic Reactions: These are energy-producing reactions that involve the breakdown of large, complex molecules into smaller molecules. * Anabolic Reactions: These are energy-requiring reactions that utilize $ATP$ energy to build larger molecules from smaller components.
The Role of ATP as an Energy Link: Adenosine triphosphate ( $ATP$ ) serves as the primary energy-storage molecule in the cell. It acts as a bridge, linking energy-producing catabolic reactions with energy-requiring anabolic processes. * Energy produced from catabolic reactions is stored when $ADP + P_i$ (inorganic phosphate) is converted to $ATP$ . * Energy is released for use by cellular processes when $ATP$ is hydrolyzed back into $ADP + P_i$ .

Catabolic reactions are organized into a tiered three-stage system to convert large food molecules into usable energy.
Stage 1: Digestion and Hydrolysis: * Large molecules are broken down into smaller ones that can enter the bloodstream. * Proteins are broken down into amino acids. * Polysaccharides are broken down into monosaccharides. * Lipids are broken down into fatty acids.
Stage 2: Degradation: * Molecules from Stage 1 undergo further breakdown and some oxidation. * These are converted into smaller molecules, specifically two- and three-carbon compounds.
Stage 3: Oxidation and Energy Release: * Small molecules are oxidized within the mitochondrion to provide $ATP$ energy. * Key components of Stage 3 include: * Acetyl CoA: A central molecule entering the next cycle. * Citric Acid Cycle: A series of reactions that oxidize the two-carbon units. * NADH and FADH2: Reduced coenzymes produced during oxidation. * Electron Transport and Oxidative Phosphorylation: The final process where energy is released to synthesize $ATP$ .

Structure of ATP: Adenosine triphosphate consists of three main components: * Adenine: A nitrogenous base. * Ribose: A five-carbon sugar. * Phosphate Groups: Three phosphate groups connected via high-energy phosphate bonds.
Nomenclature based on Phosphate Groups: * Adenosine monophosphate (AMP): Contains one phosphate group. * Adenosine diphosphate (ADP): Contains two phosphate groups. * Adenosine triphosphate (ATP): Contains three phosphate groups.
Hydrolysis of ATP: * Energy is required for physical and biological functions such as muscle contraction, substance transport across cellular membranes, nerve signal transmission, and enzyme synthesis. * The hydrolysis of $ATP$ to $ADP$ releases $7.3\,kcal$ per mole of $ATP$ . * Equation: $ATP \rightarrow ADP + P_i + 7.3\,kcal/mole$
Hydrolysis of ADP: * $ADP$ can also be hydrolyzed to form adenosine monophosphate ( $AMP$ ) and inorganic phosphate ( $P_i$ ). * Equation: $ADP \rightarrow AMP + P_i + 7.3\,kcal/mole$

Digestion Defined: The process that converts large food molecules into smaller ones capable of being absorbed by the body.
Major Sites of the Digestive System: * Mouth * Salivary glands * Stomach * Liver * Gallbladder * Pancreas * Small intestine

Digestion in the Mouth: * Salivary glands produce enzymes (salivary amylase). * These enzymes hydrolyze some of the $\alpha$ -glycosidic bonds in amylose and amylopectin. * Products generated: Maltose, glucose, and dextrins (which typically contain $3$ to $8$ glucose units).
Digestion in the Stomach: * Partially digested starches enter the acidic environment of the stomach. * The low pH of the stomach environment stops carbohydrate digestion by inactivating the salivary enzymes.
Digestion in the Small Intestine: * The small intestine maintains a pH of approximately $8$ . * The pancreas produces enzymes that hydrolyze the remaining dextrins into maltose and glucose. * Mucosal cells lining the small intestine produce enzymes that hydrolyze disaccharides: * Maltose is hydrolyzed into glucose. * Lactose and sucrose are hydrolyzed into monosaccharides.
Absorption and Metabolism: * Monosaccharides are absorbed through the intestinal wall into the bloodstream. * They are transported to the liver, where fructose and galactose are converted into glucose.

Mechanism: Milk contains the disaccharide lactose, which must be broken down by the enzyme lactase in the intestinal tract.
Condition: In many individuals, the production of lactase decreases as they age, leading to lactose intolerance.
Prevalence: This condition affects approximately $25\%$ of the population in the United States.
Management Strategies: * Avoiding products that contain lactose. * Ingesting commercial products that contain the lactase enzyme.

Main Energy Storage Molecule: The primary molecule for energy storage is $ATP$ (Adenosine Triphosphate).
Hydrolysis Products of ATP: The products resulting from the hydrolysis of $ATP$ are $ADP + P_i + 7.3\,kcal/mol$ .