Alberts - Essential Cell Biology (4th ed.)

Overview of Cellular Energy Acquisition

• Cells utilize various molecules generated from the breakdown of sugars and fats to create organic molecules.

• Discussion of ATP production mechanisms is deferred until Chapter 14.

Breakdown of Sugars and Fats

• Oxidizing glucose directly to CO2 and H2O would release excessive energy at once, making it unrecoverable. Instead, cells use enzymes for controlled oxidation.

• Enzymatic breakdown occurs in steps, yielding energy in small amounts captured by activated carriers (e.g., ATP, NADH).

• ATP can be synthesized through:

  • Direct coupling of exergonic reactions to ATP formation.

  • Oxidative phosphorylation, detailed in Chapter 14.

Stages of Food Breakdown

• Food molecule breakdown occurs in three stages:

  1. Stage 1: Breakdown of macromolecules to simple subunits.

  2. Stage 2: Conversion of simple subunits to acetyl CoA, producing limited ATP and NADH.

  3. Stage 3: Complete oxidation of acetyl CoA, generating significant ATP.

Catabolism Process

• Stage 1: Conversion of polymers (proteins, polysaccharides, fats) into monomeric subunits (amino acids, sugars, fatty acids).

• Stage 2: Glycolysis splits glucose into pyruvate, producing ATP and NADH in the cytosol. Pyruvate enters the mitochondria for further processing.

• Stage 3: Acetyl CoA enters the citric acid cycle within the mitochondrial matrix, driving further oxidation and ATP production via oxidative phosphorylation.

Glycolysis Mechanics

• Glycolysis is a ten-step process crucial for breaking down glucose:

  • Investment: Initial steps cost two ATP to prepare glucose for breakdown.

  • Payoff: Subsequent steps yield four ATP and produce NADH, leading to a net gain of two ATP per glucose.

  • Each reaction in glycolysis is catalyzed by a specific enzyme.

Substrate-Level Phosphorylation

• ATP formation occurs through substrate-level phosphorylation in glycolysis, where phosphate groups are directly transferred from substrate to ADP.

• NADH generated in glycolysis contributes to ATP production when electrons are transferred to the electron-transport chain during oxidative phosphorylation.

Fermentation

• Under anaerobic conditions, glycolysis can occur without oxygen, leading to fermentation, which generates limited ATP while converting pyruvate to lactate or ethanol, thus regenerating NAD+.

Anaerobic Respiration

• Anaerobic microorganisms use a different final electron acceptor in anaerobic respiration, contrasting with fermentation by utilizing the electron-transport chain.

Energetics of ATP Production

• ATP and NADH generated in catabolic processes are critical for driving biosynthetic reactions and maintaining cellular functions.

Citric Acid Cycle

• Pyruvate produced from glycolysis is converted to acetyl CoA and subsequently processed in the citric acid cycle generating NADH, FADH2, and GTP, primarily within the mitochondrial matrix.

Fatty Acid Catabolism

• Fatty acids are also converted to acetyl CoA, enhancing energy extraction during catabolism within mitochondria.

Oxidative Phosphorylation

• NADH and FADH2 donate high-energy electrons to the electron-transport chain, triggering ATP synthesis through chemiosmosis.

• Oxygen is vital as it serves as the terminal electron acceptor in mitochondrial respiration, producing water.

Regulation of Metabolism

• Cellular metabolism is tightly regulated through feedback mechanisms, allowing cells to adapt and maintain energy balance.

• Gluconeogenesis is regulated to synthesize glucose in response to energy needs, utilizing specific bypass enzymes for irreversible steps in glycolysis.

Storage of Energy Reserves

• Cells store glucose as glycogen and fats as triacylglycerols for future energy needs, mobilizing energy in response to hormonal signals during fasting or strenuous activity.

• Glycogen serves as a readily available glucose source for brief energy demands, while fats provide a denser energy reserve over prolonged periods.

Key Concepts in Energy Acquisition

• Food breakdown is organized in distinct cellular compartments.

• Processes of glycolysis, citric acid cycle, and oxidative phosphorylation underlie cellular energy metabolism.

• Regulated feedback controls allocate resources between energy production and storage, shaping cellular responses to fluctuating energy supply.