Cell Energy Processes and Pathways

How Cells Obtain Energy from Food

• Cells need to harvest energy from food to function properly, similar to powering a device but essential for survival.

Cellular Respiration

• Definition: Process by which cells extract energy from the chemical bonds in sugars.

  • Sugars are broken down and oxidized, resulting in carbon dioxide and water as byproducts.

• Energy Capture: Energy released during these reactions is stored in high-energy carriers like ATP (Adenosine Triphosphate) and NADH (Nicotinamide adenine dinucleotide).

ATP and ADP

• ATP Production Methods:

  1. Direct Coupling: Energetically favorable enzyme-catalyzed reactions linked to ATP synthesis from ADP and phosphate (Pi).

  • Oxidation of food molecules provides energy for ATP creation.

  1. Oxidative Phosphorylation: Most ATP synthesis involves using energy from activated carriers (NADH, FADH2) to generate ATP.

  • Occurs in the inner mitochondrial membrane of cells.

Electron Transport Chain (ETC) and ATP Synthase

• Process Overview:

  • ETC consists of a series of electron carriers in the inner mitochondrial membrane.

  • Electrons from NADH and FADH2 pass through carriers, releasing energy to pump protons (H+) across the membrane.

  • This creates a proton gradient used by ATP synthase to convert ADP and Pi into ATP.

Breakdown of Food Molecules

• Stages of Breakdown:

  • Catabolism: Enzymatic degradation of complex molecules into simpler ones.

  1. Stage 1 - Digestion: Breakdown of food into simpler molecules (e.g., proteins → amino acids, polysaccharides → sugars, fats → fatty acids and glycerol).

  2. Stage 2 - Acetyl CoA Formation: Simple subunits are converted into acetyl CoA, producing some ATP and NADH.

  3. Stage 3 - Complete Oxidation: Oxidation of acetyl CoA yields large amounts of ATP within mitochondria.

Fermentation

• Anaerobic Process:

  • When oxygen is scarce, pyruvate and NADH stay in the cytosol instead of going to ETC.

  • Pyruvate is converted to lactate (in muscle cells) or ethanol and carbon dioxide (in yeast).

  • NADH donates electrons to regenerate NAD+, allowing glycolysis to continue without oxygen.

  • Definition: Fermentations are energy-producing pathways that operate without oxygen.

Anabolic Pathways

• Relationship with Catabolism:

  • Catabolic processes (like glycolysis and the citric acid cycle) not only generate ATP but also provide building blocks for synthesis.

  • Intermediates from these pathways can be redirected to synthesize amino acids, nucleotides, lipids, and more through anabolic processes.

Electron Transport Chain and Oxidative Phosphorylation

• Mechanism:

  • ETC uses high-energy electrons from food oxidation to produce ATP.

  • Electrons are passed through a series of acceptors, causing protons to be pumped across the inner mitochondrial membrane, generating a gradient.

  • This proton gradient drives ATP synthesis as protons flow back into the matrix through ATP synthase.

Glycogen and Gluconeogenesis

• Importance of Glucose:

  • During fasting or intense exercise, glucose reserves deplete faster than they can be replenished.

  • Gluconeogenesis: Converts pyruvate back into glucose, an energy-intensive process requiring ATP.

  • Glycogen: The body stores glucose in the form of glycogen, primarily in liver and muscle cells, which can be mobilized quickly to meet energy needs.

Summary

• Cellular respiration allows cells to create energy from food via catabolic processes, ATP production, and electron transport while also providing the necessary components for anabolic pathways. Fermentation serves as an alternative energy source in anaerobic conditions, and glycogen storage helps maintain glucose levels during demanding periods.