Cellular Respiration and Fermentation: Mechanisms of Energy Harvest in Cells

Cellular Respiration and Fermentation

Catabolic Pathways and Energy Production

Catabolic pathways are metabolic routes that break down complex molecules into simpler ones, releasing stored energy. Cellular respiration and fermentation are two primary catabolic processes that enable cells to harvest energy from organic molecules.

• Photosynthesis uses CO2 and H2O to synthesize organic molecules and O2.

• Cellular respiration uses O2 and organic molecules to produce ATP, with CO2 and H2O as waste products.

• Fermentation is a partial degradation of sugars that occurs without oxygen.

• Aerobic respiration consumes organic molecules and oxygen, yielding ATP.

• Anaerobic respiration uses electron acceptors other than oxygen.

The overall equation for cellular respiration (using glucose) is:

$\mathrm{C_6H_{12}O_6 + 6\ O_2 \rightarrow 6\ CO_2 + 6\ H_2O + \text{Energy} \ (ATP + \text{heat})}$

The Principle of Redox Reactions

Redox (oxidation-reduction) reactions are chemical processes that transfer electrons between reactants. These reactions are central to energy transfer in biological systems.

• Oxidation: Loss of electrons from a substance.

• Reduction: Gain of electrons by a substance.

• Reducing agent: Electron donor.

• Oxidizing agent: Electron acceptor.

• Oxygen is highly electronegative and attracts electrons strongly.

Redox reactions that move electrons closer to oxygen release energy, which is harnessed for ATP synthesis.

Oxidation of Organic Fuel Molecules

During cellular respiration, organic fuel molecules (such as glucose) are oxidized, and oxygen is reduced. This process is a series of redox reactions that ultimately transfer electrons to oxygen, releasing energy.

NAD+ and the Electron Transport Chain (ETC)

NAD+ (nicotinamide adenine dinucleotide) is a coenzyme that acts as an electron carrier in cellular respiration. It is reduced to NADH, which stores energy used to generate ATP.

• Dehydrogenases are enzymes that remove hydrogen atoms from substrates, transferring electrons and protons to NAD+.

• Each NADH represents stored energy that can be used to synthesize ATP.

The electron transport chain (ETC) is a series of protein complexes embedded in the inner mitochondrial membrane. NADH and FADH2 donate electrons to the ETC, which are transferred through redox reactions, releasing energy used to pump protons and generate ATP.

Stages of Cellular Respiration

Cellular respiration consists of three main stages:

1. Glycolysis: Breaks down glucose into two molecules of pyruvate in the cytoplasm.

2. Pyruvate oxidation and the citric acid cycle: Completes the breakdown of glucose to CO2 in the mitochondrial matrix.

3. Oxidative phosphorylation: Includes the ETC and chemiosmosis, generating most of the cell’s ATP.

Glycolysis

Glycolysis is the first step of cellular respiration and occurs in the cytoplasm. It consists 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, 2 NAD+ are reduced to NADH, and 2 pyruvate and 2 H2O are formed.

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.

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle, also known as the Krebs cycle, completes the oxidation of organic fuel derived from pyruvate. For each turn, it generates:

• 1 ATP (or GTP)

• 3 NADH

• 1 FADH2

• 2 CO2 as waste

The cycle runs twice per glucose molecule. NADH and FADH2 produced carry electrons to the ETC.

Electron Transport Chain (ETC)

The ETC is a series of protein complexes in the inner mitochondrial membrane. NADH and FADH2 donate electrons, which are passed through the chain, releasing energy used to pump protons and create a proton gradient. Oxygen is the final electron acceptor, forming water.

Chemiosmosis and ATP Synthesis

As electrons move through the ETC, protons are pumped across the membrane, creating a proton motive force. Protons flow back through ATP synthase, driving the synthesis of ATP from ADP and inorganic phosphate (Pi). This process is called chemiosmosis.

ATP Production by Cellular Respiration

During cellular respiration, energy flows in the sequence: Glucose → NADH → ETC → proton motive force → ATP. Approximately 34% of the energy in glucose is transferred to ATP; the rest is lost as heat.

Fermentation

Fermentation allows ATP production in the absence of oxygen by regenerating NAD+ for glycolysis. There are two main types:

• Alcohol fermentation: Pyruvate is converted to ethanol, releasing CO2 and regenerating NAD+.

• Lactic acid fermentation: Pyruvate is reduced to lactate by NADH, regenerating NAD+ without releasing CO2.

Fermentation vs. Anaerobic and Aerobic Respiration

Feature	Fermentation	Anaerobic Respiration	Aerobic Respiration
Final Electron Acceptor	Organic molecule (e.g., pyruvate)	Inorganic molecule (not O2)	O2
ATP Yield	Low (2 ATP per glucose)	Moderate	High (up to 32 ATP per glucose)
ETC Used?	No	Yes	Yes

Similarities: All use glycolysis to oxidize glucose and harvest energy; NAD+ is the oxidizing agent.

Differences: Fermentation uses organic molecules as final electron acceptors, while respiration uses an ETC and inorganic acceptors.

Versatility of Catabolism

Catabolic pathways are versatile and can break down carbohydrates, proteins, and fats for energy:

• Carbohydrates enter glycolysis as glucose or other sugars.

• Proteins are digested to amino acids, which enter glycolysis or the citric acid cycle after deamination (removal of amino group).

• Fats are broken down into glycerol (enters glycolysis) and fatty acids (broken down by beta oxidation to acetyl CoA).

Additional info: Nitrogenous waste from amino acid catabolism is excreted as ammonia, urea, or other products depending on the organism.