Understanding Metabolism: A Comprehensive Guide to Cellular Energy Conversion

Posted on May 8, 2024 in Biology

WHAT IS METABOLISM?

Metabolism refers to the set of life-sustaining chemical reactions occurring within organisms. These reactions serve three primary purposes:

  1. Converting food energy into usable energy for cellular processes.
  2. Transforming food into building blocks for proteins, lipids, nucleic acids, and certain carbohydrates.
  3. Eliminating metabolic waste products.

Metabolic reactions are organized into pathways, where one chemical undergoes a series of enzyme-facilitated steps to become another chemical.

METABOLIC PATHWAYS

There are three main types of metabolic pathways:

  • Catabolic reactions: Break down complex substances into simpler ones.
  • Anabolic reactions: Build complex substances from simpler ones.
  • Amphibolic pathways: Involve both catabolism and anabolism.

MAIN CHARACTERISTICS OF METABOLISM

  • Role of enzymes and coenzymes: Enzymes act as catalysts, regulating the rate of metabolic reactions. Coenzymes like NAD and FAD participate in redox reactions.
  • Metabolism as a Series of Reduction-Oxidation Reactions: Redox reactions involve changes in the oxidation state of atoms.
  • Anaerobic and aerobic pathways: Anaerobic pathways occur without oxygen, while aerobic pathways require oxygen.

THE ROLE OF ATP IN METABOLISM

Adenosine Triphosphate (ATP) is the primary energy currency of cells. It stores and releases energy through the breaking and forming of phosphate bonds.

Phosphorylation

Phosphorylation is the process of adding a phosphate group to a molecule, often ADP to form ATP. There are three main types of phosphorylation:

  • Substrate-level phosphorylation: Direct transfer of a phosphate group from a substrate to ADP.
  • Photophosphorylation: ATP synthesis using sunlight energy during photosynthesis.
  • Oxidative phosphorylation: ATP generation through the electron transport chain and ATP synthase.

GLYCOLYSIS

Glycolysis is an anaerobic pathway that breaks down glucose into two pyruvate molecules, producing ATP and NADH.

Glycolysis consists of two phases:

  1. Preparatory Phase: Glucose is converted into two three-carbon sugar phosphates, consuming ATP.
  2. Pay-off Phase: The sugar phosphates are converted into pyruvate, generating ATP and NADH.

GLUCONEOGENESIS (GNG)

Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors, such as pyruvate and lactate.

Gluconeogenesis Pathways

  • Opposite Pathways: Gluconeogenesis and glycolysis share enzymes but use different reactions.
  • Energy Requirements: Gluconeogenesis is energy-consuming, requiring ATP or GTP.
  • From pyruvate to PEP: Pyruvate is converted to phosphoenolpyruvate (PEP) through a series of steps.
  • From Lactate to Glucose: Lactate can be converted into pyruvate and then glucose.

Gluconeogenesis and Glycolysis Regulation

Glycolysis and gluconeogenesis are reciprocally regulated to prevent simultaneous energy waste.

GLYCOGENOLYSIS AND GLYCOGENESIS

  • Glycogenesis: The process of storing excess glucose as glycogen.
  • Glycogenolysis: The breakdown of glycogen into glucose for energy.

FERMENTATIONS

Fermentation is an anaerobic pathway that breaks down glucose to regenerate NAD+ for glycolysis.

Lactic Acid Fermentation

Pyruvate is converted to lactate, regenerating NAD+.

Alcoholic Fermentation

Pyruvate is converted to ethanol and carbon dioxide, regenerating NAD+.

Production of Acetic Acid in Vinegar

Acetic acid bacteria ferment alcohol into acetic acid in the presence of oxygen.

OXIDATIVE DECARBOXYLATION OF PYRUVATE

Pyruvate is converted to acetyl-CoA, releasing carbon dioxide and linking glycolysis to the Krebs cycle.

KREBS CYCLE OR CITRIC ACID CYCLE

The Krebs cycle oxidizes acetyl-CoA, producing ATP, NADH, and FADH2.

ATP SYNTHASE

ATP synthase uses an electrochemical gradient to drive ATP synthesis from ADP and Pi.

ANAEROBIC CELLULAR RESPIRATION

Similar to aerobic respiration, but uses alternative electron acceptors in low-oxygen environments.

BETA (β) OXIDATION (LYNEN HELIX)

Fatty acids are broken down into acetyl-CoA, NADH, and FADH2.

PHOTOSYNTHESIS

Plants and other organisms convert light energy into chemical energy stored in carbohydrates.

Chemosynthesis

CO2 fixation using energy from the oxidation of inorganic compounds.

Light-Dependent Reactions

Light energy is used to split water, release oxygen, and generate ATP and NADPH.

Calvin Cycle

CO2 is incorporated into organic molecules using ATP and NADPH to form carbohydrates.