Cellular Respiration

Introduction to Respiration

Cellular respiration is a fundamental metabolic process by which cells extract energy from organic molecules, primarily glucose, through a series of stepwise chemical reactions. This process involves redox reactions, where electrons are transferred from one molecule to another, ultimately leading to the production of ATP, the cell's main energy currency.

• Redox reactions are based on electron transfer; electrons move towards more electronegative atoms, releasing energy.

• This energy is harnessed to replenish cellular ATP levels.

• Electron transfer is facilitated by electron carriers such as NAD+ and FAD.

• The main stages of cellular respiration are glycolysis, pyruvate oxidation, citric acid cycle, and oxidative phosphorylation.

Glycolysis

The Process of Glycolysis

Glycolysis is the first stage of cellular respiration, occurring in the cytosol. It breaks down one molecule of glucose (6 carbons) into two molecules of pyruvate (3 carbons each), generating ATP and NADH in the process.

• Energy Investment Phase: 2 ATP molecules are consumed to split glucose into two three-carbon sugars.

• Energy Payoff Phase: 4 ATP molecules are produced (net gain of 2 ATP), and 2 NAD+ are reduced to 2 NADH.

• Pyruvate is formed as the end product.

• ATP is produced by substrate-level phosphorylation.

Net Inputs and Outputs of Glycolysis:

The Logistics of Glycolysis

All six carbons from glucose are accounted for in the two molecules of pyruvate produced. Glycolysis is an anaerobic process, meaning it does not require oxygen and does not release carbon dioxide.

• Occurs in the cytosol.

• Can proceed in both aerobic and anaerobic conditions.

Visual Summary of Glycolysis

The following summarizes the main products of glycolysis:

• Net Gain: 2 ATP, 2 NADH + H+, and 2 pyruvate molecules per glucose.

Pyruvate Oxidation

The Process of Pyruvate Oxidation

Pyruvate oxidation links glycolysis to the citric acid cycle. It occurs in the mitochondrial matrix (in eukaryotes) or cytosol (in prokaryotes). Each pyruvate molecule is converted into acetyl-CoA, releasing carbon dioxide and generating NADH.

• Pyruvate dehydrogenase catalyzes the reaction.

• Each pyruvate loses a carboxyl group as CO2 (decarboxylation).

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

• The two-carbon acetyl group is attached to Coenzyme A, forming acetyl-CoA.

Summary Equation:

Citric Acid Cycle (Krebs Cycle)

The Process of the Citric Acid Cycle

The citric acid cycle is an eight-step process that completes the oxidation of glucose by converting acetyl-CoA into carbon dioxide. It generates ATP, NADH, and FADH2 for use in oxidative phosphorylation.

• Each acetyl-CoA combines with oxaloacetate to form citrate.

• Citrate is progressively oxidized, regenerating oxaloacetate.

• Produces 3 NADH, 1 FADH2, 1 ATP (or GTP), and 2 CO2 per cycle.

• Since each glucose yields 2 acetyl-CoA, the cycle runs twice per glucose.

Oxidative Phosphorylation

Electron Transport Chain (ETC)

Oxidative phosphorylation is the final stage of cellular respiration, occurring in the inner mitochondrial membrane. NADH and FADH2 donate electrons to the electron transport chain, a series of protein complexes that transfer electrons to oxygen, forming water.

• Electrons are passed through a series of cytochrome carriers (proteins with iron groups).

• As electrons move down the chain, they lose energy, which is used to pump protons (H+) into the intermembrane space.

• Oxygen is the final electron acceptor, forming H2O.

Chemiosmosis and ATP Synthesis

The proton gradient created by the ETC drives protons back into the mitochondrial matrix through ATP synthase, a molecular machine that synthesizes ATP from ADP and inorganic phosphate.

• Proton flow through ATP synthase causes it to spin, catalyzing ATP formation.

• This process is called chemiosmosis.

Summary Equation for ATP Synthesis:

Overall ATP Yield from Cellular Respiration

Cellular respiration produces about 34% of the total energy stored in a glucose molecule as ATP. The theoretical maximum yield per glucose is approximately 30-32 ATP molecules.

Summary of Cellular Respiration

Cellular respiration is a stepwise process of redox reactions that converts chemical energy stored in glucose into ATP. Glycolysis breaks glucose into pyruvate, which is converted to acetyl-CoA. Acetyl-CoA enters the citric acid cycle, generating electron carriers that transport electrons to the ETC. As electrons move through the ETC, chemiosmosis drives the flow of protons through ATP synthase, producing ATP.

• Glycolysis: Glucose → Pyruvate

• Pyruvate Oxidation: Pyruvate → Acetyl-CoA

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

• Oxidative Phosphorylation: NADH/FADH2 → ETC → ATP

Example: Muscle cells use cellular respiration to generate ATP for contraction during exercise.