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chapter 7 lecture Notes on Cellular Respiration and Fermentation

Cellular Respiration Overview

  • Cellular respiration is the metabolic process that occurs in living organisms to convert biochemical energy from nutrients into ATP (Adenosine triphosphate), and then release waste products. It primarily involves the breakdown of glucose in the presence of oxygen, taking place in the cytosol and mitochondria. This intricate process is fundamental for sustaining virtually all life on Earth by generating the energy required for cellular activities.

1. Importance of ATP

  • ATP (Adenosine Triphosphate):

    • Definition: The primary energy currency of the cell. It's composed of an adenine base, a ribose sugar, and three phosphate groups.

    • Role: Energy is released when the terminal phosphate bond is hydrolyzed, forming ADP (Adenosine Diphosphate) and inorganic phosphate (P_i). This hydrolysis reaction is highly exergonic and is coupled to endergonic cellular processes, powering various activities such as metabolism, active transport of molecules across cell membranes, nerve impulse propagation, and muscle contraction.

2. Types of Fat in Babies

  • Babies are born with brown fat:

    • Function: Specialized adipose tissue that generates heat (thermogenesis) without producing ATP, a process known as non-shivering thermogenesis.

    • Importance: Contains many mitochondria and a protein called uncoupling protein 1 (UCP1), which allows protons to bypass ATP synthase, converting the energy directly into heat. This is crucial for maintaining body temperature in small mammals and human infants, who have a high surface-area-to-volume ratio and limited shivering capacity.

3. Energy Sources and Breakdown

  • Carbohydrates:

    • Main source of energy, predominantly glucose, which is readily broken down through glycolysis to produce pyruvate, and subsequently ATP.

  • Fats:

    • Provide a high energy yield (approximately twice as much energy per gram compared to carbohydrates).

    • They are first hydrolyzed into glycerol and fatty acids. Glycerol can be converted to an intermediate of glycolysis, while fatty acids undergo a process called beta-oxidation in the mitochondrial matrix.

    • Beta-oxidation repeatedly cleaves two-carbon units from fatty acid chains, forming acetyl-CoA molecules, which then directly enter the citric acid cycle, generating significant amounts of NADH and FADH2.

  • Proteins:

    • Can also be used as an energy source, especially during prolonged starvation or when carbohydrate stores are low.

    • They are first broken down into individual amino acids. These amino acids then undergo deamination (removal of the amino group), with their remaining carbon skeletons entering cellular respiration at various points:

    • Some are converted to pyruvate.

    • Some are converted to acetyl-CoA.

    • Others enter directly as intermediates of the citric acid cycle (e.g., alpha-ketoglutarate, succinyl-CoA).

    • The specific entry point depends on the structure of the individual amino acid.

4. Aerobic vs. Anaerobic Respiration

  • Aerobic Respiration:

    • Definition: The complete breakdown of glucose in the presence of oxygen, yielding a large amount of ATP.

    • Occurs when oxygen is plentiful, primarily in the mitochondria of eukaryotic cells.

    • Produces ATP through a series of metabolic pathways:

    • Glycolysis (occurs in cytosol)

    • Pyruvate oxidation

    • Citric acid cycle (Krebs cycle)

    • Electron transport chain and oxidative phosphorylation (all in mitochondria)

  • Anaerobic Respiration:

    • Definition: The partial breakdown of glucose in the absence of oxygen.

    • Occurs when oxygen is scarce or absent, typically in the cytosol.

    • Includes processes like:

    • Lactic acid fermentation (in muscle cells during intense exercise)

    • Alcohol fermentation (in yeast and some bacteria).

4.1 Aerobic Respiration Steps

  1. Glycolysis:

    • Occurs in the cytosol.

    • The initial step in glucose metabolism. It splits one glucose molecule (C6H{12}O6) into two pyruvate molecules (C3H4O3).

    • This phase generates a net of 2 ATP (via substrate-level phosphorylation) and 2 NADH. It does not require oxygen and can occur under both aerobic and anaerobic conditions.

  2. Pyruvate Oxidation (or Link Reaction):

    • Converts each pyruvate molecule into acetyl-CoA in the mitochondrial matrix. This is a crucial transitional step.

    • For each pyruvate, 1 CO2 is released, and 1 NADH is generated, meaning a total of 2 CO2 and 2 NADH per glucose molecule.

  3. Citric Acid Cycle (Krebs cycle):

    • Acetyl-CoA enters the cycle and undergoes a series of redox reactions within the mitochondrial matrix.

    • For each acetyl-CoA (two per glucose), the cycle generates:

      • 3 NADH, 1 FADH2, 1 ATP (or GTP, equivalent to ATP). Therefore, per glucose, it produces a total of 6 NADH, 2 FADH2, and 2 ATP.

    • Releases two molecules of CO_2 per acetyl-CoA, completely oxidizing the original carbon atoms from glucose.

  4. Electron Transport Chain (ETC) and Oxidative Phosphorylation:

    • Occurs in the inner mitochondrial membrane.

    • NADH and FADH2 (electron carriers) donate their high-energy electrons to a series of four major protein complexes (Complex I, II, III, and IV) embedded in the inner mitochondrial membrane.

      • Complex I (NADH dehydrogenase) accepts electrons from NADH.

      • Complex II (Succinate dehydrogenase) accepts electrons from FADH2.

      • Complexes III and IV facilitate the sequential transfer of electrons. Ubiquinone (Q) acts as a mobile carrier between Complex I/II and III, while Cytochrome c mediates electron transfer between Complex III and IV.

    • As electrons pass down the chain, energy is released to pump protons (H^+) from the mitochondrial matrix into the intermembrane space, creating an electrochemical proton gradient. This gradient represents potential energy.

    • Oxygen serves as the final electron acceptor at the end of the chain, forming water (H_2O).

    • Protons flow back into the matrix through an enzyme called ATP synthase, driven by the proton motive force (PMF) through a process called chemiosmosis. This powers the synthesis of ATP.

    • This process accounts for the majority of ATP production (typically 28-30 ATP) from one glucose molecule, leading to a total theoretical yield of up to 30-32 ATP.

4.2 Anaerobic Respiration: Fermentation

  • Both types of fermentation aim to regenerate NAD^+ from NADH so that glycolysis can continue to produce ATP in the absence of oxygen.

  • Lactic Acid Fermentation:

    • Occurs in muscle cells under strenuous activity when oxygen supply is insufficient, and in some bacteria.

    • Converts pyruvate into lactate, consuming NADH and regenerating NAD^+ for glycolysis.

    • Total yield: 2 ATP per glucose molecule from glycolysis.

  • Alcohol Fermentation:

    • Occurs in yeast and some bacteria.

    • Converts pyruvate into acetaldehyde, releasing CO_2. Acetaldehyde is then converted into ethanol, consuming NADH and regenerating NAD^+.

    • Byproducts of this process: ethanol and CO_2 (which causes bubbles in dough and carbonation in alcoholic beverages).

4.3 Metabolic Pathway Regulation

  • Feedback Inhibition:

    • A crucial mechanism to regulate metabolic pathways, maintaining cellular homeostasis by preventing overproduction of end products.

    • Enzymes at an early step in a pathway are inhibited by the accumulation of the pathway's end product. This is often an example of allosteric regulation, where the end product binds to a site on the enzyme distant from the active site, altering its conformation and reducing its activity.

5. Connection to Photosynthesis

  • Photosynthesis and cellular respiration are complementary and interconnected processes, forming a global carbon cycle:

    • Photosynthesis converts carbon dioxide and water into glucose and oxygen using sunlight (energy storage).

    • Cellular respiration uses glucose and oxygen to produce carbon dioxide, water, and ATP (energy release).

    • The equations essentially swap reactants and products:

    • Photosynthesis:
      6CO2 + 6H2O + light \rightarrow C6H{12}O6 + 6O2

    • Cellular Respiration:
      C6H{12}O6 + 6O2 \rightarrow 6CO2 + 6H2O + ATP

6. Energetics of Metabolism

  • Understanding the energy yield from glucose breakdown is paramount, as our bodies rely entirely on ATP for all functions, from mechanical work to chemical synthesis and transport.

  • The stated annual caloric intake for an average adult (roughly 2,200 kilocalories/day) directly translates into the amount of energy required to sustain these ATP-dependent metabolic processes, split between basal metabolism (for basic life functions) and physical activity.

7. Conclusion

  • Metabolic pathways, including glycolysis, aerobic, and anaerobic respiration, ensure efficient energy conversion essential for survival.

  • The in-depth knowledge of these processes aids in understanding energy balancing, maintaining healthy body weight, and developing nutrition-focused health strategies, highlighting the intricate biological mechanisms that sustain life.