Cellular Respiration: Aerobic Harvesting of Energy

Overview of Cellular Respiration and Photosynthesis

Cellular respiration and photosynthesis are complementary processes that provide energy for life. While photosynthesis captures energy from sunlight to produce organic molecules, cellular respiration breaks down these molecules to release energy for cellular activities.

• Photosynthesis: Occurs in chloroplasts; uses sunlight to convert CO2 and H2O into glucose and O2.

• Cellular Respiration: Occurs mainly in mitochondria; consumes O2 and organic molecules, producing CO2, H2O, and ATP.

• Relationship: The products of photosynthesis serve as the reactants for cellular respiration and vice versa.

• Equation for Cellular Respiration:

• Breathing vs. Cellular Respiration: Breathing is the physical exchange of gases; cellular respiration is the metabolic process using O2 to produce ATP.

ATP: The Energy Currency of the Cell

ATP (adenosine triphosphate) is the main energy carrier in cells. The human body uses ATP for all activities, including maintenance and voluntary actions. Cellular respiration replenishes ATP continuously.

• Energy Transfer: Cellular respiration captures about 34% of the energy in glucose as ATP; the rest is lost as heat.

• Energy Balance: Maintaining a balance between energy intake and expenditure is essential for health.

Stages of Cellular Respiration

Overview of the Three Main Stages

Cellular respiration occurs in three main stages, each with distinct locations and functions:

• Stage 1: Glycolysis (cytosol)

• Stage 2: Pyruvate Oxidation and Citric Acid Cycle (mitochondrial matrix)

• Stage 3: Oxidative Phosphorylation (inner mitochondrial membrane)

Stage 1: Glycolysis

• Occurs in the cytosol.

• Breaks down one glucose molecule (6C) into two pyruvate molecules (3C each).

• Net yield: 2 ATP (via substrate-level phosphorylation) and 2 NADH.

• Consists of two phases: energy investment (uses 2 ATP) and energy payoff (produces 4 ATP and 2 NADH).

Stage 2: Pyruvate Oxidation and Citric Acid Cycle

• Pyruvate is transported into mitochondria and converted to acetyl CoA, releasing CO2 and producing NADH.

• The citric acid cycle (Krebs cycle) completes the breakdown of glucose:

  • Each acetyl CoA (2C) combines with oxaloacetate (4C) to form citrate (6C).

  • Each cycle turn releases 2 CO2, produces 3 NADH, 1 FADH2, and 1 ATP (or GTP).

Stage 3: Oxidative Phosphorylation

• Includes the electron transport chain (ETC) and chemiosmosis.

• NADH and FADH2 donate electrons to the ETC, which are passed to O2 (final electron acceptor), forming H2O.

• Energy from electron transfer pumps H+ ions into the intermembrane space, creating a gradient.

• ATP synthase uses the H+ gradient to synthesize ATP from ADP and Pi.

• Most ATP from glucose is produced in this stage (up to 28 ATP per glucose).

Summary Table: Stages of Cellular Respiration

Additional info: ATP yields are approximate and may vary depending on cell conditions.

Redox Reactions in Cellular Respiration

Electron Carriers and the Electron Transport Chain

• Redox Reactions: Involve the transfer of electrons from one molecule (oxidation) to another (reduction).

• NAD+ and FAD: Act as electron carriers, becoming NADH and FADH2 when reduced.

• Dehydrogenase: Enzyme that removes hydrogen atoms (electrons) from substrates and transfers them to NAD+ or FAD.

• Electron Transport Chain (ETC): Series of protein complexes in the inner mitochondrial membrane that transfer electrons to O2.

Fermentation: Anaerobic Harvesting of Energy

Fermentation Pathways

Fermentation allows cells to produce ATP without oxygen by recycling NAD+ for glycolysis.

• Lactic Acid Fermentation: Pyruvate is reduced to lactate; occurs in muscle cells and some bacteria.

• Alcohol Fermentation: Pyruvate is converted to ethanol and CO2; occurs in yeasts and some bacteria.

• ATP Yield: Only 2 ATP per glucose (from glycolysis).

Obligate vs. Facultative Anaerobes

• Obligate Anaerobes: Cannot survive in the presence of oxygen; rely solely on fermentation or anaerobic respiration.

• Facultative Anaerobes: Can use either fermentation or aerobic respiration depending on oxygen availability (e.g., yeast, some bacteria).

Special Topics

Brown Fat and Thermogenesis

• Brown Fat: Specialized tissue with mitochondria that can generate heat instead of ATP by allowing H+ to flow freely across the inner mitochondrial membrane (uncoupling oxidative phosphorylation).

• Function: Important for thermoregulation, especially in infants and lean adults exposed to cold.

Evolutionary Perspective: Glycolysis as an Ancient Pathway

• Occurs in nearly all organisms, in the cytosol, without requiring oxygen or organelles.

• Suggests glycolysis evolved early in the history of life, before the presence of atmospheric oxygen.

Connections Between Metabolic Pathways

Use of Other Organic Molecules as Fuel

• Carbohydrates: Broken down to glucose or intermediates for glycolysis.

• Fats: Glycerol enters glycolysis; fatty acids are converted to acetyl CoA for the citric acid cycle.

• Proteins: Amino acids are deaminated and enter glycolysis or the citric acid cycle.

Additional info: Humans can survive on diets low in carbohydrates by metabolizing fats and proteins for energy, but some tissues (e.g., brain) require glucose.

Biosynthesis and Regulation

• Intermediates from cellular respiration are used for biosynthesis of amino acids, nucleotides, and other macromolecules.

• Metabolic pathways are regulated by feedback inhibition to maintain cellular balance.