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Cellular Respiration and Energy Production

Cellular Respiration Overview

Cellular respiration is a metabolic pathway that breaks down glucose and produces ATP, the primary energy currency of the cell. It involves a series of catabolic reactions that occur in the cytoplasm and mitochondria.

  • Concepts Covered:

    • Glycolysis

    • Kreb's Cycle (also known as Citric Acid Cycle or TCA cycle)

    • Oxidative Phosphorylation (including Electron Transport Chain and Chemiosmosis)

    • Fermentation (anaerobic alternative when oxygen is absent)

Learning Objectives

  • Diagram the different pathways that can lead to cellular energy production from a single molecule of glucose.

  • List the inputs, outputs, and important intermediates in:

    • Glycolysis

    • Kreb’s Cycle

    • Oxidative Phosphorylation

    • Fermentation

  • Describe how the electron transport chain produces an H^+ electrochemical gradient.

  • Explain how ATP synthase utilizes the H^+ electrochemical gradient to synthesize ATP.

  • Describe the role of oxygen in cellular respiration.

Pathways to Energy Production

  • Key Molecules:

    • Glucose: The primary six-carbon monosaccharide used as the starting molecule for energy production.

    • Pyruvate: A three-carbon molecule, the product of glycolysis. It represents a branching point: under aerobic conditions, it enters the Krebs cycle; under anaerobic conditions, it undergoes fermentation.

    • Lactate: A three-carbon molecule, the result of lactic acid fermentation under anaerobic conditions, particularly in muscle cells during intense exercise and in some microorganisms.

    • Ethanol: A two-carbon molecule, produced during alcoholic fermentation by yeast and some bacteria.

  • Methods:

    • Fermentation (anaerobic): An incomplete breakdown of glucose that occurs in the absence of oxygen to regenerate NAD^+ for glycolysis to continue.

      • Lactic Acid Fermentation: Pyruvate is converted to lactate.

      • Alcoholic Fermentation: Pyruvate is converted to acetaldehyde, then to ethanol, regenerating NAD^+ and releasing CO_2.

    • Aerobic Respiration: The complete breakdown of glucose in the presence of oxygen, yielding a large amount of ATP. It consists of multiple stages:

      • Glycolysis: Initial breakdown of glucose occurring in the cytoplasm.

      • Krebs Cycle: Further oxidation of carbon fuels occurring in the mitochondrial matrix.

      • Oxidative Phosphorylation: ATP synthesis powered by electron transport, occurring on the inner mitochondrial membrane.

Overview of Aerobic Respiration

Aerobic respiration efficiently extracts energy from glucose by completely oxidizing it to carbon dioxide and water, capturing the released energy in the form of ATP.

  • Stages of Aerobic Respiration:

    1. Glycolysis: Occurs in the cytoplasm.

    2. Pyruvate Oxidation (Acetyl-CoA formation) and Krebs cycle: Occurs in the mitochondrial matrix.

    3. Electron transfer phosphorylation (ATP formation via the Electron Transport Chain and Chemiosmosis): Occurs on the inner mitochondrial membrane.

  • Overall Reaction of Aerobic Respiration:
    C6H{12}O6 \text{ (glucose)} + 6O2 \text{ (oxygen)} \rightarrow 6CO2 \text{ (carbon dioxide)} + 6H2O \text{ (water)} + \text{Energy (ATP + heat)}

  • Key Coenzymes:

    • NADH: (Nicotinamide Adenine Dinucleotide, reduced form) A high-energy electron and hydrogen ion carrier. It carries electrons harvested during glycolysis and the Krebs cycle to the electron transport chain.

    • FADH2: (Flavin Adenine Dinucleotide, reduced form) Another high-energy electron and hydrogen ion carrier, similar in function to NADH but typically entering the electron transport chain at a different point.

Second Stage of Aerobic Respiration (Mitochondrial Stages)

  • Completes Glucose Breakdown: These stages complete the oxidation of glucose, initiated by glycolysis, by fully breaking down pyruvate into carbon dioxide.

  • Location: Occurs within the mitochondria. Pyruvate enters the mitochondrial matrix.

  • Reactions Involved:

    • Acetyl-CoA Formation (Pyruvate Oxidation):

      • Two pyruvate molecules (from one glucose) are transported into the mitochondrial matrix.

      • Each pyruvate is converted to acetyl-CoA (a two-carbon molecule) by the Pyruvate Dehydrogenase Complex.

      • During this conversion, NAD^+ is reduced to NADH, and a molecule of CO_2 is released per pyruvate.

      • Net output per glucose (2 pyruvates): 2 Acetyl-CoA, 2 NADH, 2 CO_2.

    • Krebs Cycle (Citric Acid Cycle or TCA Cycle):

      • This cycle begins with the entry of acetyl-CoA. The acetyl group from acetyl-CoA combines with a four-carbon molecule, oxaloacetate, to form a six-carbon molecule, citrate.

      • Through a series of reactions, two molecules of CO_2 are released, and oxaloacetate is regenerated, making the cycle truly cyclic.

      • Per Cycle (per acetyl-CoA, thus per pyruvate):

        • Produces:

          • 3 NADH (carrying high-energy electrons)

          • 1 FADH2 (carrying high-energy electrons)

          • 1 ATP (or 1 GTP, which is readily converted to ATP) via substrate-level phosphorylation.

          • 2 CO_2

      • Total from 2 Cycles per Glucose (from 2 acetyl-CoA molecules):

        • 6 NADH

        • 2 FADH2

        • 2 ATP (or GTP)

        • 4 CO_2

Electron Transfer (Oxidative) Phosphorylation

This is the final and most productive stage of aerobic respiration, where the majority of ATP is generated. It consists of the Electron Transport Chain (ETC) and Chemiosmosis.

  • Function of Coenzymes: NADH and FADH2, carrying high-energy electrons and H^+ ions, deliver their electrons to the protein complexes embedded in the inner mitochondrial membrane, initiating the electron transport chain.

  • Process (Electron Transport Chain):

    • Electrons from NADH and FADH2 are passed through a series of four large protein complexes (Complex I, II, III, IV) and mobile carriers (ubiquinone and cytochrome C) on the inner mitochondrial membrane.

    • As electrons move down the chain, they release energy in a stepwise manner.

    • This released energy is used to actively pump H^+ ions (protons) from the mitochondrial matrix into the intermembrane space, creating a steep H^+ electrochemical gradient (proton-motive force) across the inner mitochondrial membrane.

  • ATP Synthesis (Chemiosmosis):

    • The H^+ ions, driven by their electrochemical gradient, flow back down their concentration gradient from the intermembrane space into the mitochondrial matrix.

    • This flow occurs through a specialized enzyme complex called ATP synthase, which is also embedded in the inner mitochondrial membrane.

    • The kinetic energy of the flowing H^+ ions causes a rotational change in ATP synthase, which drives the phosphorylation of ADP to ATP (ADP + P_i \rightarrow ATP).

  • Role of Oxygen: Oxygen acts as the final electron acceptor at the end of the electron transport chain (at Complex IV). It combines with electrons and H^+ ions to form water (H_2O). This removal of electrons is crucial; without oxygen, the electron transport chain would become backed up, preventing further electron flow and halting ATP production from NADH and FADH2.

Summary: The Energy Harvest

  • Energy Yield from Glucose:

    • The theoretical maximum yield from the complete breakdown of one glucose molecule is typically estimated to be 36 to 38 molecules of ATP. Actual yields can vary due to factors like the cost of transporting NADH from glycolysis into the mitochondria and proton leakage.

    • ATP Production Breakdown (Theoretical Maximum):

      • Glycolysis: 2 ATP (net via substrate-level phosphorylation) + 2 NADH (yielding 4-6 ATP in ETC)

      • Acetyl-CoA Formation and Krebs Cycle: 2 ATP (or GTP via substrate-level phosphorylation) + 8 NADH + 2 FADH2 (yielding approx. 28-30 ATP in ETC)

      • Electron Transfer Phosphorylation: Approximately 32-34 ATP (from the oxidation of NADH and FADH2 generated in all stages).

    • Overall, the majority of ATP (around 90%) is produced during oxidative phosphorylation.

Conclusion

Understanding cellular respiration is fundamental for comprehending how cells produce and utilize energy efficiently. The integration of glycolysis, pyruvate oxidation, the Krebs cycle, and oxidative phosphorylation demonstrates the complexity of metabolic pathways and their importance to cellular function. This intricate process ensures a continuous supply of ATP, vital for maintaining life.