Cellular Respiration Overview

Cellular Respiration Overview

• Cellular respiration is the process by which cells convert glucose into ATP (adenosine triphosphate), the primary energy currency of the cell.
• The process involves several stages, including glycolysis, the transition reaction, the Krebs cycle, and oxidative phosphorylation.

Glycolysis

• Glycolysis occurs in the cytoplasm and is the breakdown of glucose (a 6-carbon molecule) into two molecules of pyruvate (a 3-carbon molecule).
• Two ATP molecules are used as initial investment, but four ATP molecules are produced, resulting in a net gain of 2 ATP molecules.
• Two molecules of NAD+ are also reduced to NADH; carrying electrons to the electron transport chain.

Transition Reaction

• The transition reaction links glycolysis to the Krebs cycle.
• Pyruvate is transported into the mitochondrial matrix and converted to acetyl CoA (a 2-carbon molecule).
• A carbon dioxide molecule is released, and one NAD+ is reduced to NADH.

Krebs Cycle (Citric Acid Cycle)

• The Krebs cycle takes place in the mitochondrial matrix.
• Acetyl CoA combines with oxaloacetate to form citrate, starting the cycle.
• The cycle releases two molecules of carbon dioxide.
• One ATP, three NADH, and one FADH2 are produced per cycle, per acetyl CoA entering.
• The cycle regenerates oxaloacetate, allowing the cycle to continue.

Oxidative Phosphorylation

• Oxidative phosphorylation occurs across the inner mitochondrial membrane.
• NADH and FADH2 deliver electrons to the electron transport chain.
• Electrons are passed through a series of protein complexes, releasing energy that is used to pump protons (H+) across the membrane, creating a proton gradient.
• The flow of H+ back into the matrix through ATP synthase drives the synthesis of ATP, a process called chemiosmosis.
• Oxygen is the final acceptor of electrons in the electron transport chain, forming water.

Summary of ATP Production

• Glycolysis produces 2 ATP.
• Transition reaction produces no ATP directly.
• Krebs cycle produces 2 ATP.
• Oxidative phosphorylation produces 32-34 ATP (depending on cell).

Other cellular processes

• Pyruvate enters the mitochondria only when O2 is present.
• The citric acid cycle has eight steps, each catalyzed by a specific enzyme.
• The acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate.
• The reactions of the citric acid cycle follow a chemical logic.
• The TCA cycle extracts the max potential energy from the pyruvate.
• The cycle oxidizes organic fuel derived from pyruvate.