Cellular Respiration & Glycolysis

• Exam 3 covers chapters 7-8 and takes place March 13th at 4pm.

Cellular Respiration

• Series of catabolic reactions converting energy in fuel molecules into ATP.
• Sugar molecules like glucose are broken down into carbon dioxide and water in the presence of oxygen.
• Releases energy because the potential energy of the reactants exceeds that of the products.
• ATP is generated via substrate-level phosphorylation and oxidative phosphorylation.
• A redox reaction involving the transfer of electrons between molecules.
• Oxidation is the loss of electrons, and reduction is the gain of electrons.
• Electron carriers transfer electrons to an electron transport chain to generate ATP.
• It is a four-stage process: glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation.

Glycolysis

• It involves the partial oxidation of glucose, producing pyruvate, ATP, and reduced electron carriers.
• Glycolysis occurs in the cytoplasm through 10 reactions, oxidizing glucose to pyruvate.
• It has preparatory, cleavage, and payoff phases.
• Net gain per glucose molecule: two ATP and two NADH molecules.

Pyruvate Oxidation

• Pyruvate is oxidized to acetyl-CoA, linking to the citric acid cycle.
• Occurs in the mitochondrial matrix.
• Conversion of pyruvate to acetyl-CoA produces one NADH and one carbon dioxide molecule.

The Citric Acid Cycle

• Results in thorough oxidation of fuel molecules, generating ATP and reduced electron carriers.
• It takes place in the mitochondrial matrix.
• The acetyl group of acetyl-CoA is completely oxidized.
• Regenerates oxaloacetate, the cycle begins with acetyl-CoA combining with oxaloacetate.
• A complete turn produces one GTP (converted to ATP), three NADH, and one FADH2 molecule.
• Intermediates are starting points for synthesizing organic molecules.

Oxidative Phosphorylation

• Involves the transfer of electrons to oxygen along an electron transport chain to pump protons and synthesize ATP.
• NADH and FADH2 donate electrons to the electron transport chain in the inner mitochondrial membrane.
• Electrons move through redox couples in the electron transport chain.
• Electron transfer is paired with the movement of protons across the inner mitochondrial membrane into the intermembrane space.
• Proton buildup creates a proton electrochemical gradient, storing potential energy.
• Movement of protons back into the mitochondrial matrix through the Fo subunit of ATP synthase is coupled with ATP formation, catalyzed by the F₁ subunit.
• Complete oxidation of one glucose nets 32 ATP molecules.

Anaerobic Metabolism

• Glucose is broken down by fermentation, producing a modest amount of ATP without oxygen.
• Pyruvate, the end product of glycolysis, is processed differently with or without oxygen.
• Pyruvate enters fermentation pathways without oxygen, and is reduced to lactic acid in lactic acid fermentation.
• In ethanol fermentation, pyruvate is converted to acetaldehyde, then reduced to ethanol.

Metabolic integration

• Pathways are integrated, controlling the energy level of cells.
• Excess glucose stored as glycogen in animals and starch in plants.
• Dietary carbohydrates are converted into glycolysis intermediates.
• Fatty acids in triacylglycerols provide energy storage, broken down through β-oxidation.
• Phosphofructokinase-1 controls a key step in glycolysis, regulated by allosteric activators (ADP and AMP) and inhibitors (ATP and citrate).
• ATP for muscle exercise comes from lactic acid fermentation, aerobic respiration, and β-oxidation.

Photosynthesis

• The main pathway for incorporating energy and carbon into carbohydrates.
• Carried out by eukaryotic organisms and photosynthetic bacteria.
• Two stages: light capture (energy from sunlight produces NADPH and ATP) and carbon fixation (NADPH and ATP synthesize carbohydrates using CO₂).
• Water is the electron donor, releasing oxygen, and carbon dioxide is reduced to form carbohydrates.
• In eukaryotes, photosynthesis occurs in chloroplasts: the photosynthetic electron transport chain occurs along the thylakoid membrane, and the Calvin cycle takes place in the stroma.
• Energy from sunlight is used to produce NADPH and ATP.
• Chlorophyll absorbs visible light and is associated with proteins in photosystems.
• Photosystems absorb light and transfer energy and electrons.
• Antenna chlorophylls transfer absorbed light energy to the reaction center.
• Reaction center chlorophylls transfer electrons to an electron-acceptor molecule, initiating the photosynthetic electron transport chain.
• Electron transfer occurs within protein complexes and compounds.
• Water is the electron donor, and NADP+ is the final electron acceptor.
• Photosystem II pulls electrons from water to produce oxygen and protons.
• Photosystem I passes electrons to ferredoxin, then reducing NADP+ to produce NADPH.
• Movement of electrons is coupled with the transfer of protons from the stroma to the lumen.
• Buildup of protons in the lumen drives ATP production by photophosphorylation.
• Cyclic electron transport increases ATP production by redirecting electrons from ferredoxin back into the electron transport chain.

The Calvin Cycle

• A three-phase process that uses carbon dioxide to synthesize carbohydrates.
• Phases: carboxylation, reduction, and regeneration.
• Carboxylation: CO₂ is added to RuBP, catalyzed by rubisco
• Reduction: energy is captured by sunlight with donation of a phosphate group by ATP, and the transfer of electrons from NADPH to produce triose phosphate molecules, and are the product of the second phase.
• Regeneration: RuBP is regenerated from triose phosphates.
• Starch formation stores carbohydrates.

Photosynthetic Challenges

• Challenges: excess light energy and oxygenase activity of rubisco.
• Imbalance between NADPH production and use can lead to reactive oxygen species.
• Protection from excess light energy includes antioxidant molecules and xanthophyll pigments.
• Rubisco can catalyze a reaction with oxygen, resulting in energy loss and CO2 release.
• Rubisco has evolved to favor CO2 over oxygen, but with reduced speed.
• Carbohydrate synthesis through the Calvin cycle results in energy losses via photorespiration.
• Maximum theoretical efficiency is approximately 4% of total incoming solar energy.

The Evolution of Photosynthesis

• Photosynthesis had a profound impact on life on Earth.
• Cyanobacteria evolved the ability to use water as an electron donor.
• Cyanobacteria evolved two photosystems by genetic transfer or gene duplication and divergence.
• Photosynthesis in eukaryotes likely evolved by endosymbiosis.
• Organisms with two photosystems produce all of Earth's atmospheric oxygen.