Cellular Respiration and Fermentation

Introduction to Cellular Energy Production

Cells require energy to perform essential functions, and this energy is primarily supplied in the form of ATP (adenosine triphosphate). Cellular respiration and fermentation are metabolic pathways that harvest energy from high-energy molecules, such as glucose, to produce ATP. The energy released during these processes is used to add a phosphate group to ADP, forming ATP.

Overview of Cellular Respiration

Cellular respiration is a series of metabolic reactions that oxidize glucose to produce ATP. Most cells continuously synthesize ATP, as it is unstable and only lasts for a short period. Glucose is obtained either through photosynthesis (in plants) or from food (in other organisms), and is stored as glycogen or starch.

• Cellular respiration consists of four main processes: Glycolysis, Pyruvate Processing, Citric Acid Cycle, and Electron Transport & Oxidative Phosphorylation.

• Cells require both an energy source (to generate ATP) and a carbon source (for synthesizing macromolecules).

Catabolic and Anabolic Pathways

Metabolic pathways are classified as either catabolic or anabolic:

• Catabolic pathways break down molecules and harvest stored chemical energy to produce ATP.

• Anabolic pathways synthesize larger molecules from smaller components, often using ATP as an energy source.

Interconnection of Metabolic Pathways

Cellular respiration interacts with other metabolic pathways, allowing cells to use carbohydrates, fats, and proteins as substrates for ATP production. Fats are broken down into glycerol and fatty acids, proteins into amino acids, and carbohydrates into sugars. These intermediates can enter glycolysis or the citric acid cycle.

Cellular Respiration Intermediates in Anabolic Pathways

Intermediates from cellular respiration are used to synthesize macromolecules:

• Glycolysis intermediates can be used to make nucleotides for DNA and RNA synthesis.

• Acetyl CoA is used for synthesis of fatty acids, phospholipids, and fats.

• Amino acids can be synthesized from citric acid cycle molecules.

• Pyruvate can be used to make glucose for glycogen and starch production.

Summary of Glucose Oxidation

Glucose oxidation involves several steps, each producing specific products:

• Glycolysis: Glucose is broken down into two pyruvate molecules, producing 2 ATP and 2 NADH.

• Pyruvate Processing: Pyruvate is converted to acetyl CoA, producing NADH and CO2.

• Citric Acid Cycle: Acetyl CoA is oxidized, producing NADH, FADH2, ATP, and CO2.

Summary of Cellular Respiration

Cellular respiration maximizes ATP yield through oxidative phosphorylation:

• Electron transport chain and chemiosmosis generate a proton gradient used to produce ATP.

• Maximum yield is about 29 ATP per glucose molecule.

Glycolysis: Oxidizing Glucose to Pyruvate

Discovery and Process of Glycolysis

Glycolysis is a sequence of 10 reactions occurring in the cytosol. It starts with an energy investment phase (using 2 ATP) and ends with an energy payoff phase (producing 4 ATP and 2 NADH). The net yield is 2 ATP, 2 NADH, and 2 pyruvate per glucose molecule.

• ATP is produced by substrate-level phosphorylation during glycolysis.

Regulation of Glycolysis

Glycolysis is regulated by feedback inhibition, primarily at the enzyme phosphofructokinase. High ATP levels inhibit this enzyme, preventing excessive ATP production.

Processing Pyruvate to Acetyl CoA

Pyruvate Processing

Pyruvate produced by glycolysis is transported into mitochondria (in eukaryotes) and processed by pyruvate dehydrogenase. One carbon is oxidized to CO2, NADH is produced, and the remaining two-carbon unit is attached to coenzyme A, forming acetyl CoA.

Regulation of Pyruvate Processing

Pyruvate processing is regulated by feedback inhibition. High levels of NADH, acetyl CoA, or ATP increase phosphorylation and inhibit pyruvate dehydrogenase.

The Citric Acid Cycle: Oxidizing Acetyl CoA to CO2

Citric Acid Cycle Overview

The citric acid cycle oxidizes acetyl CoA to CO2 in a cyclic series of reactions. It occurs in the mitochondrial matrix (eukaryotes) or cytosol (prokaryotes). Each cycle produces NADH, FADH2, ATP (or GTP), and CO2.

Regulation of the Citric Acid Cycle

The cycle is regulated by feedback inhibition at multiple points. High ATP or NADH levels decrease reaction rates.

Electron Transport and Chemiosmosis

Electron Transport Chain (ETC)

The ETC consists of protein complexes and electron carriers (such as ubiquinone and cytochrome c) embedded in the inner mitochondrial membrane. Electrons from NADH and FADH2 are transferred through the chain, releasing energy used to pump protons and create a proton gradient.

ATP Synthase and Chemiosmosis

ATP synthase is a protein complex that synthesizes ATP from ADP and inorganic phosphate, powered by the proton-motive force generated by the ETC. This process is called oxidative phosphorylation.

Fermentation

Fermentation Pathways

When oxygen is unavailable, cells use fermentation to regenerate NAD+ from NADH, allowing glycolysis to continue. Fermentation produces much less ATP (2 per glucose) compared to cellular respiration (29 per glucose).

• Lactic acid fermentation: Pyruvate accepts electrons from NADH, forming lactate.

• Alcohol fermentation: Pyruvate is converted to acetaldehyde, which accepts electrons from NADH, forming ethanol.

Efficiency and Adaptation

Fermentation is less efficient than cellular respiration. Some organisms, called facultative anaerobes, can switch between fermentation and aerobic respiration depending on oxygen availability.

Summary Table: Comparison of Cellular Respiration and Fermentation

Key Equations

• Overall equation for aerobic cellular respiration:

• Substrate-level phosphorylation:

• Oxidative phosphorylation:

Additional info:

• Metabolic reprogramming in cancer cells can alter energy pathways, as seen in liver cancer where alanine is used as an alternative energy source.

• Regulation of metabolic pathways is essential for maintaining homeostasis.