Cellular Respiration and Fermentation: Catabolic Pathways and ATP Production

Cellular Respiration and Fermentation

Overview of Cellular Respiration

Cellular respiration is a series of metabolic pathways that extract energy from organic molecules, primarily glucose, to produce ATP, the main energy currency of the cell. This process involves both aerobic and anaerobic mechanisms and is central to energy flow in biological systems.

• ATP (Adenosine Triphosphate): The molecule that drives most cellular work by providing energy for cellular processes.

• Catabolic Pathways: Pathways that break down complex molecules into simpler ones, releasing stored energy.

• Photosynthesis vs. Cellular Respiration: Photosynthesis stores energy in organic molecules, while cellular respiration releases that energy for cellular use.

Catabolic Pathways Yield Energy by Oxidizing Organic Fuels

Catabolic pathways, such as cellular respiration and fermentation, release energy by breaking down organic molecules. Electron transfer plays a key role in these processes, especially through redox reactions.

• Fermentation: Partial degradation of sugars without oxygen.

• Aerobic Respiration: Consumes organic molecules and oxygen, yielding ATP.

• Anaerobic Respiration: Similar to aerobic but uses electron acceptors other than oxygen.

• Glucose: The primary molecule traced in cellular respiration, though fats and proteins can also be used as fuel.

Redox Reactions: Oxidation and Reduction

Redox (oxidation-reduction) reactions involve the transfer of electrons between molecules, releasing energy stored in organic compounds. This energy is harnessed to synthesize ATP.

• Oxidation: Loss of electrons from a substance.

• Reduction: Gain of electrons by a substance (reduces positive charge).

• Reducing Agent: Electron donor.

• Oxidizing Agent: Electron acceptor.

Example: In the combustion of methane, methane is oxidized and oxygen is reduced, releasing energy.

Stepwise Energy Harvest via NAD+ and the Electron Transport Chain

Nicotinamide adenine dinucleotide (NAD+) is a coenzyme that acts as an electron carrier. Enzymes called dehydrogenases remove hydrogen atoms from substrates, transferring electrons to NAD+ to form NADH. The electron transport chain (ETC) then transfers these electrons through a series of redox reactions, ultimately producing ATP.

• NAD+: Oxidizing agent; accepts electrons and becomes NADH.

• NADH: Stores energy used to synthesize ATP.

• Electron Transport Chain: Series of molecules in the mitochondrial inner membrane (or plasma membrane in prokaryotes) that transfer electrons in a controlled manner, releasing energy in steps.

The Stages of Cellular Respiration: A Preview

Cellular respiration consists of three main stages:

1. Glycolysis: Occurs in the cytoplasm; breaks down glucose into two pyruvate molecules.

2. Pyruvate Oxidation and Citric Acid Cycle (Krebs Cycle): Completes the breakdown of glucose to CO2 in the mitochondrial matrix.

3. Oxidative Phosphorylation: Electron transport chain and chemiosmosis produce most of the cell’s ATP.

Substrate-Level Phosphorylation: ATP formed directly by transferring a phosphate group from a substrate to ADP during glycolysis and the citric acid cycle.

Glycolysis: Oxidizing Glucose to Pyruvate

Glycolysis is the first step in cellular respiration, occurring in the cytoplasm and consisting of two phases:

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

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

Glycolysis does not require oxygen and all carbon from glucose ends up in pyruvate.

Oxidation of Pyruvate to Acetyl CoA

Before entering the citric acid cycle, pyruvate is converted to acetyl coenzyme A (acetyl CoA) by the enzyme pyruvate dehydrogenase. This process involves:

• Oxidation of pyruvate’s carboxyl group, releasing CO2.

• Reduction of NAD+ to NADH.

• Combination of the remaining two-carbon fragment with coenzyme A to form acetyl CoA.

The Citric Acid Cycle (Krebs Cycle)

The citric acid cycle completes the breakdown of glucose by oxidizing acetyl CoA. Each turn of the cycle generates:

• 1 ATP (by substrate-level phosphorylation)

• 3 NADH

• 1 FADH2

• 2 CO2 (waste product)

The cycle runs twice per glucose molecule, as two pyruvate are produced from each glucose.

Oxidative Phosphorylation: Electron Transport and Chemiosmosis

Oxidative phosphorylation is the process by which most ATP is generated during cellular respiration. It involves two main components:

• Electron Transport Chain (ETC): Embedded in the inner mitochondrial membrane, the ETC transfers electrons from NADH and FADH2 to oxygen, releasing energy in steps.

• Chemiosmosis: The energy released by the ETC is used to pump protons (H+) across the membrane, creating a proton gradient. Protons flow back through ATP synthase, driving the synthesis of ATP.

Proton-Motive Force: The H+ gradient across the membrane, which powers ATP synthesis.

ATP Yield from Cellular Respiration

During cellular respiration, about 34% of the energy in glucose is transferred to ATP, producing approximately 32 ATP molecules per glucose. The rest is lost as heat. The exact ATP yield can vary due to several factors, including the coupling of redox reactions and the use of the proton-motive force for other cellular work.

Fermentation and Anaerobic Respiration

When oxygen is not available, cells can produce ATP through fermentation or anaerobic respiration.

• Anaerobic Respiration: Uses an electron transport chain with a final electron acceptor other than oxygen (e.g., sulfate).

• Fermentation: Extension of glycolysis that regenerates NAD+ by transferring electrons to pyruvate or its derivatives.

Types of Fermentation:

• Alcohol Fermentation: Pyruvate is converted to ethanol and CO2; used by yeast in brewing and baking.

• Lactic Acid Fermentation: Pyruvate is reduced to lactate; used by fungi, bacteria, and muscle cells.

The Versatility of Catabolism

Cells can use other organic molecules besides glucose for energy:

• Proteins: Digested to amino acids, which are deaminated and enter cellular respiration at various points.

• Fats: Broken down into glycerol (used in glycolysis) and fatty acids (converted to acetyl CoA via beta oxidation). Fats yield more than twice as much ATP per gram as carbohydrates.