Cellular Respiration: Overview

Cellular respiration is a multi-stage process by which cells extract energy from glucose to produce ATP, the cell's main energy currency. The process occurs in several stages, each with distinct locations and outcomes.

• Glycolysis: Occurs in the cytosol; breaks down glucose into pyruvate.

• Pyruvate Oxidation: Occurs in the mitochondria; converts pyruvate into acetyl-CoA.

• Citric Acid Cycle (Krebs Cycle): Further oxidizes acetyl-CoA, producing electron carriers.

• Oxidative Phosphorylation: Uses electron carriers to generate most of the cell's ATP.

Most ATP is produced during oxidative phosphorylation, but the earlier steps are essential for harvesting and transferring energy from glucose.

Glycolysis

Phases of Glycolysis

Glycolysis is the first stage of cellular respiration and takes place in the cytosol. It converts one molecule of glucose (6 carbons) into two molecules of pyruvate (3 carbons each), producing a small amount of ATP and NADH in the process. Glycolysis consists of two main phases:

• Energy Investment Phase

• Energy Payoff Phase

Energy Investment Phase

In this phase, the cell uses ATP to phosphorylate glucose and its intermediates, preparing them for subsequent breakdown.

• Glucose is the starting molecule.

• 2 ATP molecules are invested (used) to phosphorylate intermediates.

• The key product at the end of this phase is glyceraldehyde 3-phosphate (G3P).

• Each glucose yields two G3P molecules (due to splitting of a 6-carbon intermediate).

Energy Payoff Phase

This phase generates ATP and NADH by oxidizing G3P to pyruvate.

• Each G3P is converted through a series of steps to pyruvate.

• 2 NADH molecules are produced (one per G3P).

• 4 ATP molecules are produced by substrate-level phosphorylation (two per G3P).

• Since 2 ATP were used in the investment phase, the net gain is 2 ATP per glucose.

• Some water is released as a byproduct.

• Most of the energy from glucose remains in the pyruvate molecules.

Summary Table: Glycolysis

Key Terms and Concepts

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP to form ATP, catalyzed by an enzyme.

• NADH: An electron carrier that stores energy for use in the electron transport chain.

• Glyceraldehyde 3-phosphate (G3P): A 3-carbon intermediate that is central to the energy payoff phase.

Pyruvate Oxidation

Conversion of Pyruvate to Acetyl-CoA

After glycolysis, the two pyruvate molecules produced are transported from the cytosol into the mitochondria. Here, each pyruvate undergoes oxidation to form acetyl-CoA, which enters the citric acid cycle.

• Location: Mitochondrial matrix

• Process: Each pyruvate (3C) loses one carbon as CO2 (the first CO2 released in cellular respiration).

• The remaining 2-carbon fragment is oxidized, reducing NAD+ to NADH.

• The 2-carbon fragment is attached to coenzyme A, forming acetyl-CoA.

• This process occurs twice per glucose (once for each pyruvate).

Summary Table: Pyruvate Oxidation (per glucose)

Key Terms and Concepts

• Acetyl-CoA: A 2-carbon molecule that enters the citric acid cycle.

• NADH: Electron carrier produced during oxidation.

• CO2: Carbon dioxide, a waste product released during the process.

Equations

• Overall Glycolysis Reaction:

• Pyruvate Oxidation (per glucose):

Summary

• Glycolysis splits glucose into two pyruvates, producing a net gain of 2 ATP and 2 NADH.

• Pyruvate oxidation converts each pyruvate into acetyl-CoA, releasing CO2 and generating NADH.

• These steps prepare substrates and electron carriers for the citric acid cycle and oxidative phosphorylation, where most ATP is produced.