Cellular Respiration

Overview of Cellular Respiration

Cellular respiration is a fundamental metabolic process by which cells convert the energy stored in food molecules, such as glucose, into ATP, the energy currency of life. This process is essential for powering cellular activities, including movement, growth, and maintenance.

• Life requires energy: All living organisms need energy to sustain life processes.

• ATP: Adenosine triphosphate (ATP) is the primary molecule used by cells to store and transfer energy.

• Glucose: A common source of chemical energy, derived from carbohydrates.

• Main steps of cellular respiration: Glycolysis, pyruvate processing, citric acid cycle, electron transport chain (ETC), and ATP synthase.

• Aerobic vs Anaerobic: Cellular respiration can occur with (aerobic) or without (anaerobic) oxygen.

• Fermentation: An alternative pathway when oxygen is absent.

Characteristics of Life and Energy

Energy and metabolism are central to life. Organisms obtain energy from their environment, either from sunlight (plants) or food (animals).

• Cells: Basic unit of life.

• Replication: Ability to reproduce.

• Evolution: Adaptation and change over time.

• Information: Genetic and environmental information guides cellular function.

• Energy: Required for all cellular processes.

ATP: Life’s Energy Currency

ATP is a molecule with high potential energy, making it unstable and unable to be stored. Cells continuously synthesize ATP to meet their energy demands.

• ATP allows cells to overcome energy barriers.

• ATP is produced and used rapidly.

Glucose: Common Source of Chemical Energy

Cells harvest energy from glucose through a series of controlled chemical reactions. The conversion of storage carbohydrates to glucose monomers is followed by ATP production.

• Hydrolysis: Breakdown of carbohydrates to glucose.

• Cellular respiration vs fermentation: Degree of glucose oxidation differs.

Glucose Oxidation

Complete oxidation of glucose releases energy, carbon dioxide, and water. In cellular respiration, this process is controlled and stepwise, preventing the release of all energy as heat.

• Overall reaction:

• Fermentation: Incomplete oxidation, less energy released, produces reduced organic molecules as waste.

Main Steps of Cellular Respiration

Definition and Electron Transport Chain (ETC)

Cellular respiration encompasses reactions that use electrons harvested from high-energy molecules to produce ATP via an electron transport chain.

• Electrons from NADH and FADH2 move through the ETC via redox reactions.

• ETC: Series of molecules that oxidize NADH and FADH2, transferring electrons to oxygen.

Glycolysis

Glycolysis is the first step in cellular respiration, occurring in the cytosol of both eukaryotic and prokaryotic cells. It consists of two phases: energy investment and energy payoff.

• Energy investment phase (reactions 1-5): ATP is used to raise the free energy of glucose, making it more reactive.

• Energy payoff phase (reactions 6-10): ATP and NADH are produced.

• Substrate-level phosphorylation: ATP is generated directly from a substrate.

• Regulation: Glycolysis is regulated by phosphofructokinase, which has two ATP binding sites (active and regulatory). Feedback inhibition slows the reaction when ATP is abundant.

Pyruvate Processing

Pyruvate, produced from glycolysis, is processed differently in eukaryotes and prokaryotes.

• Eukaryotes: Pyruvate moves from cytosol to mitochondrial matrix via carrier proteins.

• Prokaryotes: Pyruvate processing occurs in the cytosol.

• Enzyme: Pyruvate dehydrogenase oxidizes pyruvate, releasing CO2 and transferring the acetyl group to CoA.

• NAD+ reduction: Energy released reduces NAD+ to NADH.

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle completes the oxidation of glucose.

• Two cycles per glucose molecule.

• Produces 3NADH, 1FADH2, and 1ATP/GTP via substrate-level phosphorylation.

• Glucose is fully oxidized to CO2.

Electron Transport Chain (ETC)

The ETC is molecules in the inner mitochondrial membrane (eukaryotes) or plasma membrane (prokaryotes) that transfer electrons from NADH and FADH2 to oxygen.

• Redox reactions: Electrons move through molecules with increasing redox potential, releasing small amounts of energy at each step.

• Q and Cytochrome c: Shuttle electrons between complexes.

• Oxygen: Final electron acceptor, forming water.

• Energy released: Used to actively transport H+ from matrix to intermembrane space, creating a proton gradient.

ATP Synthase and Chemiosmosis

ATP synthase is an enzyme that uses the proton gradient generated by the ETC to synthesize ATP from ADP and inorganic phosphate (Pi).

• Chemiosmosis: The movement of H+ down its gradient drives ATP synthesis.

• Proton motive force: The energy stored in the proton gradient.

• Mechanism: Flow of H+ through ATP synthase causes rotation and conformational changes, catalyzing ATP formation.

Net Yield of Cellular Respiration

Cellular respiration yields significantly more ATP under aerobic conditions than anaerobic or fermentation pathways.

• Aerobic respiration: Oxygen is the final electron acceptor, producing water and a large proton gradient.

• Anaerobic respiration: Other molecules (NO3-, SO42-) serve as terminal electron acceptors in some prokaryotes.

• Fermentation: Occurs when oxygen is absent; less ATP is produced.

Fermentation

Fermentation Pathways

Fermentation is an emergency backup pathway for ATP production when oxygen or other electron acceptors are unavailable. It allows glycolysis to continue by oxidizing NADH to NAD+.

• Lactic acid fermentation: Pyruvate is reduced to lactate; some lactate is converted back to glucose in the liver (gluconeogenesis).

• Alcohol fermentation: Pyruvate is converted to ethanol and CO2 (e.g., yeast).

• Other types: Yogurt, cheese, vinegar production.

• Inefficiency: Fermentation produces only 2 ATP per glucose, compared to 29 ATP in aerobic respiration.

Comparison: Fermentation vs Anaerobic Respiration

Practice and Application

Key Questions

• Where does glycolysis occur in eukaryotic and prokaryotic cells? Answer: Cytoplasm.

• Which high-energy molecules are produced during glucose oxidation? Answer: NADH, FADH2, ATP.

• Which molecule possesses the highest chemical energy? Answer: NADH (pattern: most energy is transferred to electron carriers).

• Most electrons within glucose are extracted during which step? Answer: Citric acid cycle.

• Impact of blocking cytochrome c: Answer: ETC would halt, ATP production would cease, leading to cell death.

Summary Table: Cellular Respiration Steps

Additional info:

• Feedback inhibition is a common regulatory mechanism in metabolism.

• ATP synthase is a rotary enzyme, unique in its mechanism.

• Fermentation is less efficient but crucial for survival in anaerobic conditions.