Cellular Metabolism

Overview – Cellular Metabolism

Cellular metabolism refers to the chemical processes that occur within cells to maintain life. These processes allow cells to extract energy from nutrients, construct new molecules, break down waste, and regulate internal functions. Understanding cellular metabolism is crucial in medicine, as disruptions underpin a wide range of diseases—from mitochondrial disorders to metabolic syndromes and cancer. This article provides a high-level summary of metabolic pathways, energy production, and regulatory mechanisms relevant to clinical study.

Definition

Metabolism is a cell’s capacity to acquire, convert, and use energy for growth, repair, survival, and function.

• Some metabolic reactions release energy (catabolism)
• Others consume energy (anabolism)
• All metabolic processes are enzyme-mediated and partly regulated by hormonal signals

Autotrophs vs Heterotrophs

Autotrophs

• “Self-feeders”
• Convert inorganic substrates (e.g. CO₂ and H₂O) into organic molecules (e.g. glucose)
• E.g. Photosynthesising plants and algae

Heterotrophs

• “Consumers”
• Must ingest complex molecules (sugars, fats, proteins) to extract energy
• E.g. Humans and other animals

ATP – The Energy Currency

• Adenosine Triphosphate (ATP) is the primary energy carrier in cells
• Generated via oxidation of carbohydrates, lipids, and amino acids
• Consumed in energy-requiring processes like active transport, synthesis, movement

ATP Formation

• ATP ↔ ADP + Pi + Energy
• ADP ↔ AMP + Pi + Energy
• Energy is released during phosphate bond hydrolysis

Aerobic vs Anaerobic Metabolism

Aerobic Metabolism

• Requires oxygen
• Occurs in most human tissues under normal conditions
• Involves oxidation of nutrients to produce ATP
• Highly efficient (high ATP yield)

Anaerobic Metabolism

• Occurs when oxygen demand exceeds supply
• Less efficient (lower ATP yield)
• Generates lactic acid as a by-product
• Used temporarily by skeletal muscle during intense activity or hypoxia

Major Metabolic Pathways

Carbohydrate Metabolism

• Glycolysis – cytoplasmic breakdown of glucose to pyruvate
• Citric Acid Cycle (TCA/Krebs Cycle) – mitochondrial processing of acetyl-CoA
• Electron Transport Chain (Oxidative Phosphorylation) – ATP production using oxygen
• Pentose Phosphate Pathway – alternative pathway generating NADPH
• Glycogenesis – glucose storage as glycogen
• Glycogenolysis – glucose retrieval from glycogen
• Gluconeogenesis – synthesis of glucose from non-carbohydrate sources

Lipid Metabolism

• Fatty Acid Oxidation (Beta-Oxidation) – breakdown of fatty acids for ATP
• Ketogenesis/Ketolysis – alternate fuel production/use (especially in starvation)
• Fatty Acid Synthesis – production of new fatty acids (anabolic)

Protein & Amino Acid Metabolism

• Amino Acid Metabolism – breakdown and reuse of amino acids
• Urea Cycle – detoxification of ammonia (by-product of amino acid catabolism)

Metabolic Specialisation by Tissue Type

• Muscle: prioritises glucose under normal conditions
• Liver: metabolically versatile; regulates blood glucose, detoxifies, synthesises cholesterol
• Heart: prefers fatty acids and ketone bodies
• Brain: uses glucose almost exclusively, can adapt to ketone bodies during starvation

Hormonal Regulation of Metabolism

• Metabolism is tightly regulated by hormones, including:
  • Insulin: promotes glucose uptake and storage
  • Glucagon: promotes glycogen breakdown and gluconeogenesis
  • Adrenaline: enhances glucose availability during stress
  • Cortisol: promotes catabolism during prolonged fasting
  • Thyroid Hormones: increase basal metabolic rate

Summary – Cellular Metabolism

Cellular metabolism includes all the energy-generating and energy-consuming processes within cells. These pathways enable cells to survive, grow, and respond to stress, and are driven by ATP and tightly regulated by hormones. A comprehensive understanding of metabolism underpins many areas of clinical medicine. For a broader context, see our Cell Biology & Biochemistry Overview page.