Metabolic reactions are the foundation of life, transforming nutrients into energy and building blocks. These processes involve breaking down complex molecules and building new ones, all powered by ATP, the cellular energy currency.

Cellular metabolism is a delicate balance of oxidation-reduction reactions and hormonal regulation. These processes work together to maintain energy levels, build essential molecules, and keep the body functioning smoothly in various conditions.

Metabolic Reactions

Breakdown of polymers to monomers

• Catabolic reactions break down complex molecules into simpler ones through a process called hydrolysis
  • Polymers such as polysaccharides (starch, glycogen), proteins, and triglycerides are broken down into their respective monomers (glucose, amino acids, fatty acids, and glycerol)
  • Hydrolysis reactions use water molecules to cleave the chemical bonds between monomers, a process catalyzed by specific enzymes to increase reaction rates
• Monomers released from catabolic reactions serve as energy sources or building blocks for anabolic reactions
  • Glucose from polysaccharide breakdown enters glycolysis or the citric acid cycle for ATP production
  • Amino acids from protein breakdown can be used to synthesize new proteins or converted to glucose or fatty acids
  • Fatty acids from triglyceride breakdown undergo beta-oxidation to generate acetyl-CoA for the citric acid cycle

Formation of polymers from monomers

• Anabolic reactions construct complex molecules from simpler ones through dehydration synthesis
  • Monomers such as monosaccharides (glucose), amino acids, fatty acids, and glycerol are combined to form their respective polymers (polysaccharides, proteins, triglycerides)
  • Dehydration synthesis removes water molecules to create covalent bonds between monomers, a process catalyzed by specific enzymes to increase reaction rates
• Anabolic reactions require an input of energy, typically in the form of ATP, to drive the formation of new chemical bonds
  • Glucose monomers are combined to form storage polysaccharides like starch in plants and glycogen in animals
  • Amino acids are linked together to form polypeptide chains, which fold into functional proteins
  • Fatty acids and glycerol are combined to form triglycerides for energy storage in adipose tissue

ATP as metabolic energy currency

• ATP (adenosine triphosphate) serves as the primary energy currency in living organisms
  • ATP consists of an adenosine molecule bonded to three phosphate groups
• Energy is released when the terminal phosphate group is hydrolyzed, converting ATP to ADP (adenosine diphosphate) and inorganic phosphate (Pi)
  • $ATP + H_2O \rightarrow ADP + P_i + Energy$
• The energy released from ATP hydrolysis drives endergonic reactions that require an energy input
  • Anabolic reactions like protein synthesis and glycogen synthesis utilize ATP to form new chemical bonds
  • Active transport mechanisms use ATP to move molecules against their concentration gradients across membranes
• ATP is regenerated from ADP and Pi through substrate-level phosphorylation or oxidative phosphorylation
  • $ADP + P_i + Energy \rightarrow ATP$
  • Substrate-level phosphorylation occurs during glycolysis and the citric acid cycle
  • Oxidative phosphorylation takes place in the electron transport chain of mitochondria

Cellular Metabolism

Oxidation-reduction in cellular metabolism

• Oxidation-reduction (redox) reactions involve the transfer of electrons between molecules
  • Oxidation is the loss of electrons, while reduction is the gain of electrons
• Electron carriers like NAD+ (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide) play crucial roles in cellular metabolism
  • NAD+ is reduced to NADH, while FAD is reduced to FADH2 upon accepting electrons
• Redox reactions are essential in catabolic pathways that break down nutrients to generate ATP
  • Glycolysis, the citric acid cycle, and the electron transport chain all involve redox reactions
  • Electrons from NADH and FADH2 are used to create a proton gradient that drives ATP synthesis through oxidative phosphorylation
• Redox reactions also participate in anabolic pathways that synthesize complex molecules
  • Fatty acid synthesis and amino acid synthesis require reducing power in the form of NADPH (reduced nicotinamide adenine dinucleotide phosphate)

Hormones in metabolic regulation

• Insulin, secreted by pancreatic beta cells, is a key anabolic hormone
  • Stimulates glucose uptake by muscle and adipose tissue, promoting glycogen synthesis and lipogenesis
  • Enhances protein synthesis and inhibits protein breakdown in muscle tissue
• Glucagon, secreted by pancreatic alpha cells, is a major catabolic hormone
  • Stimulates glycogenolysis (glycogen breakdown) and gluconeogenesis (glucose synthesis from non-carbohydrate precursors) in the liver, increasing blood glucose levels
  • Promotes lipolysis (triglyceride breakdown) in adipose tissue, releasing fatty acids into the bloodstream
• Cortisol, secreted by the adrenal cortex, is a catabolic hormone involved in the stress response
  • Stimulates proteolysis (protein breakdown) in muscle tissue, providing amino acids for gluconeogenesis
  • Enhances lipolysis in adipose tissue and gluconeogenesis in the liver, ensuring adequate glucose supply during stress
• Growth hormone (GH), secreted by the anterior pituitary gland, has anabolic effects
  • Stimulates protein synthesis and muscle growth, promoting positive nitrogen balance
  • Induces lipolysis in adipose tissue, mobilizing fatty acids for energy production
  • Antagonizes insulin action, reducing glucose uptake and utilization by tissues

Metabolic Regulation and Control

• Metabolic pathways are interconnected series of chemical reactions that occur in cells to maintain homeostasis
• Enzymes are protein catalysts that significantly increase the rate of metabolic reactions
  • Coenzymes are non-protein organic molecules that assist enzymes in catalyzing reactions
• Feedback inhibition is a regulatory mechanism where the end product of a metabolic pathway inhibits an earlier step in the pathway
• Allosteric regulation involves the binding of molecules to sites on enzymes other than the active site, altering enzyme activity