Cellular Respiration and Fermentation Summary

I) Overview

• Energy Forms

  • Cells require energy for survival and function, as every cellular activity, from metabolism to movement, depends on it.

  • The primary energy currency in cells is ATP (Adenosine Triphosphate), which is essential for energy transfer.

  • ATP can be synthesized from ADP (Adenosine Diphosphate) and inorganic phosphate (P) when energy is added through processes like substrate-level phosphorylation or oxidative phosphorylation.

  • Both autotrophs (organisms that produce their own food through processes like photosynthesis) and heterotrophs (organisms that consume other organisms for energy) undergo cellular respiration to convert biochemical energy from nutrients into ATP.

• Redox Reactions

  • Refers to Reduction-Oxidation Reactions, crucial for energy transfer in cells. In these redox reactions, when one molecule is oxidized (loses electrons), another is reduced (gains electrons).

  • Examples:

    • NAD (oxidized form) + 2H → NADH (reduced form)

    • FAD (oxidized) + 2H → FADH₂ (reduced)

  • Electrons are transported within cells by electron carriers (shuttles) like NADH and FADH₂, which play key roles in the electron transport chain, enabling efficient ATP production.

• Overview of Energy Harvest from Glucose

  • Glucose is the primary fuel source for ATP synthesis, serving as a central metabolite in cellular respiration.

  • The process can be subdivided into four main stages:

    • Glycolysis: Conversion of glucose to pyruvate.

    • Pyruvate Oxidation: Conversion of pyruvate to Acetyl CoA.

    • Krebs Cycle (Citric Acid Cycle): Comprehensive breakdown of Acetyl CoA.

    • Electron Transport Chain (ETC): Utilizes high-energy electrons to synthesize ATP.

II) Glycolysis

• Known as "sugar splitting," glycolysis involves a series of 10 enzymatic reactions that convert one glucose molecule into two pyruvate molecules.

• Major points:

  • Occurs in the cytosol (the fluid part of the cytoplasm), making it an anaerobic process (does not require oxygen).

  • Produces:

    • A net gain of 2 ATP through substrate-level phosphorylation.

    • 2 NADH for later use in oxidative phosphorylation.

    • 2 pyruvate molecules (3 carbons each), which serve as substrates for further energy extraction in subsequent stages.

III) Krebs Cycle / Citric Acid Cycle

A) Pyruvate Oxidation

• This vital step occurs in the mitochondrial matrix, where each pyruvate is converted to Acetyl CoA, releasing CO₂, and generating NADH.

• Acetyl CoA then fuels the Krebs Cycle.

B) Citric Acid Cycle

• The Krebs Cycle consists of 8 distinct metabolic steps and occurs in the mitochondrial matrix.

• It completes the oxidative breakdown of glucose. For each glucose molecule, the outputs are:

  • 2 ATP (directly produced),

  • 2 GTP (equivalent to ATP),

  • 6 NADH + H⁺ (carrying high-energy electrons),

  • 2 FADH₂ (another electron carrier).

• The high-energy electrons from NADH and FADH₂ will proceed to the electron transport chain, where a significant amount of ATP is produced.

IV) Electron Transport Chain (ETC)

• Located on the inner mitochondrial membrane, the ETC is critical for ATP production.

• NADH and FADH₂ donate high-energy electrons to the chain, initiating a flow of electrons that contributes to the formation of an electrochemical gradient.

  • 1 FADH₂ contributes to the synthesis of approximately 1.5 ATP.

  • 1 NADH contributes to the synthesis of approximately 2.5 ATP.

• ATP synthase utilizes the established electrochemical gradient to generate ATP from ADP and inorganic phosphate, highlighting the process of chemiosmosis.

V) Review of Oxidation of Glucose

• The overall reaction for cellular respiration can be summarized as:

  • Glucose + O₂ → CO₂ + H₂O + Energy (ATP).

• The process is approximately 38% efficient in terms of ATP production from glucose. Breakdown of ATP production includes:

  • Glycolysis: 2 ATP produced,

  • Citric Acid Cycle: 2 ATP produced,

  • Electron Transport Chain: 30-32 ATP produced, showcasing the efficiency of aerobic respiration.

VI) Interconversions

• ATP can be synthesized from other macromolecules such as carbohydrates, fats, and proteins, allowing the body to adapt to varying energy demands.

• Reversal of reactions allows for the synthesis of these macromolecules from ATP, except for essential amino acids and fatty acids, which must be obtained from the diet.

VII) Toxins/Poisons

• Substances that inhibit cellular respiration include:

  • Cyanide: Binds to cytochrome c oxidase, preventing electron transport.

  • Carbon monoxide (CO): Competes with oxygen for binding to hemoglobin, affecting oxygen transport essential for aerobic respiration.

• The presence of such toxins underscores the importance of efficient respiratory pathways in cellular metabolism.