Cellular Energetics

Cellular Energetics: Cellular Respiration

Overview

• Importance of Energy in Living Cells

  • Living cells require a constant input of energy.

  • Energy input must exceed energy loss to maintain order and power cellular processes.

  • Loss of energy flow results in the death of living organisms.

Catabolic Pathways

• Catabolic pathways occur when molecules are broken down, resulting in the transfer of electrons and energy release.

  • Types of Catabolic Pathways:

  • a. Fermentation:

    • Involves the partial degradation of sugars without the use of oxygen.

  • b. Cellular Respiration (Aerobic Respiration):

    • The most prevalent and efficient catabolic pathway.

    • Requires oxygen and organic fuel.

Redox Reactions

• Definition: Redox reactions involve the transfer of electrons from one reactant to another through oxidation and reduction.

  • Oxidation: A process in which a substance loses electrons, thus becoming oxidized.

  • Reduction: A process in which a substance gains electrons, thus becoming reduced.

Cellular Respiration Process

• During cellular respiration, glucose is oxidized and oxygen is reduced.

• Chemical Equation for Cellular Respiration: $ext{Glucose (C}<em>6 ext{H}</em>{12} ext{O}<em>6) + ext{Oxygen (O}</em>2) ightarrow ext{Water (H}<em>2 ext{O}) + ext{Carbon Dioxide (CO}</em>2) + ext{Energy (ATP)}$

  • Reactants: Glucose and oxygen.

  • Products: Water, carbon dioxide, and energy in the form of ATP (Adenosine Triphosphate).

Mitochondria

• Role: Mitochondria serve as the powerhouse of the cell.

  • Functions similar to a digestive system: ingests nutrients, breaks them down, and produces energy-rich molecules (ATP).

• Structure:

  • Mitochondria possess many folds, increasing surface area to facilitate more chemical reactions, which enhances energy production.

Main Chemical Processes

• 1. Photosynthesis: Building of sugar molecules.

• 2. Glycolysis: Breaking down of sugar molecules (first step in cellular respiration).

Power Molecules

• ATP (Adenosine Triphosphate):

  • The primary power molecule used by all cells to perform work.

• NADH, NADPH, FADH:

  • These molecules are equally important for cellular energy but are utilized less frequently than ATP.

Recycling of ATP

• ATP is a renewable energy source.

  • Energy necessary for ATP synthesis comes either from the food consumed in animals or produced in plants.

  • Enzymes facilitate the process of breaking down ATP to release energy and re-synthesize ATP from ADP (Adenosine Diphosphate) by adding a phosphate group.

Cellular Respiration Steps

• Step 1: Glycolysis

  • Glucose enters the cell and is broken down into two pyruvate molecules.

  • Produces 2 ATP through substrate-level phosphorylation.

  • Electrons are harvested and stored in NADH for later use.

• Step 1½: Conversion of Pyruvate

  • Pyruvate is rearranged, H+ is added, CO2 is released, leading to the production of Acetyl CoA.

• Step 2: Krebs Cycle (Citric Acid Cycle)

  • Occurs in the mitochondrial matrix.

  • Acetyl CoA is further broken down, producing:

  • CO2,

  • 2 ATP through substrate-level phosphorylation,

  • Electrons captured in NADH and FADH2.

• Step 3: Oxidative Phosphorylation (Electron Transport Chain and Chemiosmosis)

  • Takes place in the inner mitochondrial membrane and the intermembrane space.

  • Electrons from NADH and FADH2 are transferred through a series of electron acceptors.

  • This process captures free energy from electrons and produces 32 ATP.

  • Oxygen acts as the final electron acceptor, forming water.

Protein Complex in Electron Transport Chain

• Electron transport involves multiple protein complexes that transport electrons and H+ ions across the mitochondrial membrane.

• Electrons from NADH and FADH2 pass through four protein complexes (I to IV) generating a proton gradient, driving ATP production.

Chemiosmosis

• Definition: The mechanism through which ATP is produced using the energy stored in the H+ ion gradient.

  • ATP Synthase: The enzyme that synthesizes ATP, utilizing the flow of H+ ions down their gradient to produce ATP from ADP and inorganic phosphate.

ATP Production Accounting

• The energy flow during cellular respiration generally follows this pathway: glucose → NADH → electron transport chain → proton-motive force → ATP.

• Total ATP produced from one glucose molecule:

  • 30 to 38 ATP, depending on the efficiency of the electron transport mechanism and the shuttle used.

Fermentation

• Fermentation allows anaerobic organisms to produce ATP without oxygen.

• Glycolysis: Can occur in both aerobic and anaerobic conditions, generating ATP through fermentation when oxygen is absent.

• Comparison of Respiration and Fermentation:

  • Fermentation: Does not require oxygen; results in 2 ATP per glucose molecule.

  • Cellular Respiration: Requires oxygen; yields up to 36 ATP per glucose molecule.

  • Types of Fermentation:

  • Plants produce ethanol.

  • Animals produce lactate.

Evolutionary Significance of Glycolysis

• Glycolysis is a highly conserved process, occurring in nearly all organisms, and likely predates atmospheric oxygen development, indicating its ancient origin in prokaryotes.

Metabolism of Various Molecules

• Catabolism also includes the breakdown of various food molecules, including:

  • Amino Acids: Converted through various biochemical pathways.

  • Sugars: Broken down into glucose via glycolysis.

  • Glycerol and Fatty Acids: Enter pathways for energy production during cellular respiration.