Cellular Respiration

Overview of Cellular Respiration

Cellular respiration is a fundamental metabolic process by which cells extract energy from organic molecules, primarily glucose, to produce ATP, the energy currency of the cell. This process occurs in both prokaryotic and eukaryotic organisms and involves a series of redox reactions.

• Definition: Cellular respiration is a set of metabolic pathways that break down organic molecules to release energy for cellular work.

• Purpose: To convert the chemical energy stored in glucose and other organic molecules into ATP.

• Location: In eukaryotes, most steps occur in the mitochondria; in prokaryotes, they occur in the cytoplasm and plasma membrane.

• Organism Classification:

  • Autotrophs: Organisms that produce their own organic molecules through photosynthesis (e.g., plants, algae).

  • Heterotrophs: Organisms that obtain organic molecules by consuming other organisms (e.g., animals, fungi).

Key Concept: High-energy electrons from food molecules are transferred through a series of redox reactions, ultimately reducing a final electron acceptor and generating ATP.

Major Steps of Cellular Respiration

• Glycolysis: Splitting of glucose into two molecules of pyruvate.

• Pyruvate Oxidation: Conversion of pyruvate into acetyl-CoA.

• Citric Acid Cycle (Krebs Cycle): Oxidation of acetyl-CoA to CO2 and reduction of electron carriers.

• Electron Transport Chain & Chemiosmosis: Transfer of electrons to oxygen (in aerobic respiration) and synthesis of ATP via ATP synthase.

Types of Cellular Respiration

• Aerobic Respiration: Final electron acceptor is oxygen (O2).

• Anaerobic Respiration: Final electron acceptor is an inorganic molecule other than oxygen (e.g., nitrate, sulfate).

• Fermentation: Final electron acceptor is an organic molecule (e.g., lactic acid, ethanol).

Glycolysis: Splitting Glucose

Overview of Glycolysis

Glycolysis is the first stage of cellular respiration, occurring in the cytoplasm. It involves the breakdown of one glucose molecule (6 carbons) into two molecules of pyruvate (3 carbons each), producing a small amount of ATP and NADH.

• Location: Cytoplasm of the cell.

• ATP Production: Net gain of 2 ATP molecules per glucose by substrate-level phosphorylation.

• Electron Carrier: 2 NADH molecules are produced per glucose.

Phases of Glycolysis

• Energy Investment Phase (Steps 1-3): 2 ATP molecules are used to phosphorylate glucose and its intermediates.

• Cleavage Phase (Steps 4-5): 6-carbon molecule is split into two 3-carbon molecules.

• Energy Payoff Phase (Steps 6-10): 4 ATP and 2 NADH are produced; pyruvate is formed.

Net Reaction of Glycolysis:

Key Points

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP to form ATP.

• Initial substrate: Glucose.

• Net products: 2 pyruvate, 2 ATP, 2 NADH, and 2 H2O per glucose.

Pyruvate Oxidation: Producing Acetyl-CoA

Overview of Pyruvate Oxidation

After glycolysis, pyruvate is transported into the mitochondria (in eukaryotes) and oxidized to form acetyl-CoA, which enters the citric acid cycle. This step links glycolysis to the citric acid cycle and generates NADH and CO2.

• Location: Mitochondrial matrix in eukaryotes; plasma membrane in prokaryotes.

• Enzyme Complex: Pyruvate dehydrogenase catalyzes the reaction.

• Products (per pyruvate):

  • 1 CO2

  • 1 NADH (from NAD+)

  • 1 Acetyl-CoA (2-carbon molecule attached to coenzyme A)

Reaction:

Fate of Pyruvate

• If oxygen is present: Pyruvate is oxidized to acetyl-CoA and enters the citric acid cycle.

• If oxygen is absent: Pyruvate is reduced during fermentation to regenerate NAD+.

ATP Synthesis Mechanisms

Substrate-Level Phosphorylation

ATP is formed by the direct transfer of a phosphate group from a substrate to ADP. This occurs during glycolysis and the citric acid cycle.

Oxidative Phosphorylation

ATP synthase uses the energy from a proton gradient, established by the electron transport chain, to synthesize ATP from ADP and inorganic phosphate.

Comparison Table: Types of Respiration

Key Terms and Definitions

• Redox Reaction: A chemical reaction involving the transfer of electrons between two species.

• Electron Carrier: Molecules such as NAD+ and FAD that transport electrons during cellular respiration.

• ATP (Adenosine Triphosphate): The main energy currency of the cell.

• NADH/FADH2: Reduced forms of electron carriers that donate electrons to the electron transport chain.

• Substrate-Level Phosphorylation: Direct formation of ATP by transfer of a phosphate group to ADP.

• Oxidative Phosphorylation: ATP synthesis powered by the transfer of electrons through the electron transport chain and chemiosmosis.

Summary Diagram (Described)

• Glycolysis: Glucose → Pyruvate (in cytoplasm)

• Pyruvate Oxidation: Pyruvate → Acetyl-CoA (in mitochondria)

• Citric Acid Cycle: Acetyl-CoA → CO2 + NADH + FADH2

• Electron Transport Chain & Chemiosmosis: NADH/FADH2 → ATP (via ATP synthase)

Additional info: The provided images illustrate the flow of carbon and electrons through the main stages of cellular respiration, highlighting the role of mitochondria and the interconnection with photosynthesis in autotrophs.