Cellular Metabolism: Energy, Reactions, and Pathways

Posted on Dec 15, 2024 in Biology

Metabolism: Cellular Reactions and Energy

Metabolism is the set of reactions that occur within a cell, through which it gains and uses energy. These reactions maintain ionic concentrations and continuously regenerate molecules and structures. The spontaneity of a chemical reaction is measured using a thermodynamic quantity called free energy.

Exergonic Reactions (Catabolism)

Exergonic reactions occur spontaneously, releasing energy. These degradation reactions result in products with less energy than the reactants. The released energy is used to synthesize ATP, which is then degraded into ADP + Pi, driving endergonic reactions.

Endergonic Reactions (Anabolism)

Endergonic reactions do not occur spontaneously and require an external energy supply. These synthesis reactions result in products with more energy than the reactants. The energy released from exergonic reactions is used to synthesize ATP, which then powers endergonic reactions.

Enzymes: Biological Catalysts

Enzymes are biocatalysts that increase the rate of specific reactions, allowing cellular chemical reactions to occur fast enough to sustain life.

Metabolic Pathways

Cellular chemical reactions are arranged in sequences called metabolic pathways. The product of one reaction becomes the substrate for the next. These pathways can be linear, cyclic, or branched and are interconnected, forming a complex network.

Redox Reactions: Energy Transfer

Energy transfer in cells involves the transfer of electrons from one molecule to another. These are redox reactions.

Oxidation

Oxidation occurs when a molecule loses electrons. The substance is oxidized and is called the electron donor or reducing agent.

Reduction

Reduction occurs when a molecule gains electrons. The molecule is reduced and is called the electron acceptor or oxidizing agent. These two processes are coupled.

Energy Transfer in Redox Processes

Each redox pair has a standard oxidation-reduction potential, which measures the tendency to give up electrons. Electrons travel spontaneously from pairs with a negative potential to those with a positive potential, releasing free energy.

Catabolism vs. Anabolism

Catabolism

  • Degradative phase where complex organic molecules are broken down into simpler compounds.
  • Exergonic processes that release free energy, used to synthesize ATP.
  • Involves oxidation of organic molecules, with electrons transferred to molecules like NADP+, which is reduced to NADPH.
  • Convergent pathways where many initial compounds form a few final products.

Anabolism

  • Construction phase where simple molecules are used to build complex molecules.
  • Endergonic processes that require energy from ATP hydrolysis.
  • Involves the reduction of molecules using electrons from reduced transporters like NADPH.
  • Divergent pathways where a variety of end products are formed from a few precursors.

Electron Transporters

Coenzymes

The main coenzymes are nicotinamide nucleotides (NAD+ and NADP+) and flavins (FAD and FMN).

  • NAD+ and NADH: NAD+ acts as a coenzyme in dehydrogenases, accepting electrons. NADH is used for ATP synthesis.
  • NADP+ and NADPH: NADPH participates in anabolic processes like synthesis and reduction.
  • FMN, FMNH2, FAD, FADH2: These coenzymes act in flavoproteins and are part of the electron transport chain.

Other Groups

Coenzyme A (CoA) acts as a carrier of acyl groups.

Energy (ATP)

Besides ATP, other nucleotide triphosphates like GTP, UTP, and CTP are involved in energy exchange. The hydrolysis and synthesis of ATP form the ATP-ADP cycle.

Use of Energy from ATP Hydrolysis

The energy released from ATP hydrolysis is used for:

  • Mechanical work (e.g., muscle contraction)
  • Active transport of substances
  • Building membrane potential
  • Heat production
  • Radiant energy forms like bioluminescence

Catabolic Processes and Electron Acceptors

Cells are classified based on the final electron acceptor used:

  • Aerobic: Use molecular oxygen as the final electron acceptor.
  • Anaerobic: Use a molecule other than oxygen as the final electron acceptor.
  • Optional: Can use either aerobic or anaerobic processes.

Carbohydrates: Main Fuel

Glucose is the main fuel used by living beings. Sources of glucose include:

  • Heterotrophic organisms obtaining it from food.
  • Autotrophs obtaining it through photosynthesis.
  • Transformation of other organic molecules (gluconeogenesis).
  • Hydrolysis of starch or glycogen (glycogenolysis).

Glycogenolysis

  1. Glycogen phosphorylase cleaves glycosidic bonds using inorganic phosphate.
  2. Glucose-1-phosphate molecules are obtained.
  3. (1->6) glucosidase transfers glucose to a non-reducing end.
  4. Glucose 1-phosphate molecules are derived.

Glucose Oxidation

Glucose oxidation occurs in a series of reactions, gradually releasing energy stored in ATP molecules. The process has two phases:

Phase 1: Glycolysis

Glucose is oxidized in glycolysis, yielding ATP, pyruvate, and NADH + H+.

Phase 2: Pyruvate Oxidation

Pyruvate is oxidized in two ways:

  • Under anaerobic conditions, through fermentation.
  • Under aerobic conditions, through cellular respiration, where pyruvate is completely oxidized to CO2 and H2O.

Glycolysis

Glycolysis degrades glucose into two pyruvate molecules, releasing energy stored in ATP and NADH + H+.

Features of Glycolysis

  • Catabolic and anaerobic process in the cytoplasm.
  • Universal pathway, believed to be one of the oldest.

Balance of Glycolysis

For each glucose molecule, glycolysis yields: 2 pyruvate molecules, 2 ATP molecules (net), and 2 NADH molecules. If glucose is derived from glycogenolysis, more ATP is produced.