Cellular Respiration and Fermentation

Introduction to Cellular Respiration

Cellular respiration is a fundamental metabolic process that allows cells to extract energy from glucose and other high-energy molecules. This energy is used to synthesize ATP, the primary energy currency of the cell. The process is central to metabolism and is essential for life.

• ATP fuels cellular work and is produced continuously due to its instability.

• Cells obtain glucose from photosynthesis (plants) or food (other organisms).

• Glucose is stored as glycogen (animals) or starch (plants).

Overview of Cellular Respiration

Cellular respiration consists of a series of redox reactions that oxidize glucose to carbon dioxide, releasing energy used to synthesize ATP. The process is divided into four main stages:

• Glycolysis: Glucose is broken down into two pyruvate molecules.

• Pyruvate Processing: Pyruvate is oxidized to form acetyl CoA.

• Citric Acid Cycle: Acetyl CoA is oxidized to CO2.

• Electron Transport and Oxidative Phosphorylation: Electrons are transferred through a chain, creating a proton gradient used to make ATP.

Fermentation is an alternative pathway that allows ATP production in the absence of oxygen, but is less efficient than cellular respiration.

Metabolic Pathways and Their Interactions

Cellular respiration interacts with other metabolic pathways, both catabolic and anabolic. Catabolic pathways break down molecules to harvest energy, while anabolic pathways use energy to build macromolecules.

• Carbohydrates, fats, and proteins can all be used as substrates for cellular respiration.

• Intermediates from cellular respiration are used to synthesize nucleotides, fatty acids, amino acids, and glucose.

• Metabolic regulation maintains homeostasis in cells.

Glycolysis: Oxidizing Glucose to Pyruvate

Discovery and Process of Glycolysis

Glycolysis is a sequence of 10 reactions occurring in the cytosol, discovered by Hans and Edward Buchner. It involves the breakdown of glucose and the production of ATP and NADH.

• Energy investment phase: 2 ATP are used (reactions 1–5).

• Energy payoff phase: NADH and ATP are produced by substrate-level phosphorylation (reactions 6–10).

• Net yield: 2 NADH, 2 ATP, and 2 pyruvate per glucose.

Substrate-level phosphorylation is the direct transfer of a phosphate group to ADP to form ATP.

Regulation of Glycolysis

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

• Phosphofructokinase has two ATP binding sites: active and regulatory.

• Low ATP: binds to active site, catalyzes reaction.

• High ATP: binds to regulatory site, inhibits enzyme.

Processing Pyruvate to Acetyl CoA

Pyruvate Transport and Processing

In eukaryotes, pyruvate is transported into mitochondria and processed by pyruvate dehydrogenase in the mitochondrial matrix. In prokaryotes, this occurs in the cytosol.

• One carbon is oxidized to CO2, NADH is produced.

• The remaining two-carbon unit is attached to coenzyme A, forming acetyl CoA.

Pyruvate processing is regulated by feedback inhibition: high levels of NADH, acetyl CoA, or ATP increase phosphorylation and inhibit the enzyme.

The Citric Acid Cycle: Oxidizing Acetyl CoA to CO2

Cycle Organization and Function

The citric acid cycle occurs in the mitochondrial matrix (eukaryotes) or cytosol (prokaryotes). Each acetyl CoA is oxidized to two CO2, and the cycle regenerates oxaloacetate.

• Reduces three NAD+ to NADH, one FAD to FADH2, and phosphorylates ADP (or GDP) to ATP (or GTP).

• Turns twice per glucose molecule.

The cycle is regulated by feedback inhibition: high ATP or NADH levels decrease reaction rates.

Electron Transport and Chemiosmosis

Electron Transport Chain (ETC)

The ETC is a series of protein complexes and molecules that transfer electrons from NADH and FADH2 to oxygen, forming water. The energy released pumps protons across the membrane, creating a proton gradient.

• ETC is organized into four protein complexes; ubiquinone (Q) and cytochrome c transfer electrons between complexes.

• Low-energy electrons are passed to oxygen at the end, forming water.

ATP Synthase and Chemiosmosis

ATP synthase is the enzyme responsible for synthesizing ATP from ADP and inorganic phosphate, powered by the proton-motive force generated by the ETC.

• ATP synthase consists of a knob (ATPase) and a membrane-bound base (proton channel).

• Proton flow causes the knob to rotate, changing its conformation and catalyzing ATP synthesis.

Oxidative phosphorylation produces about 29 ATP per glucose, making it the most efficient ATP production method.

Fermentation

Fermentation Pathways

Fermentation is a metabolic pathway that allows cells to regenerate NAD+ from NADH in the absence of oxygen, enabling glycolysis to continue producing ATP by substrate-level phosphorylation.

• Lactic acid fermentation: pyruvate accepts electrons from NADH, producing lactate.

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

• Fermentation is less efficient, yielding only 2 ATP per glucose.

• Facultative anaerobes can switch between fermentation and aerobic respiration depending on oxygen availability.

Summary Table: Cellular Respiration vs. Fermentation

Additional info: The study notes expand on the brief points in the original materials, providing definitions, examples, and context for each step of cellular respiration and fermentation. Images included are directly relevant to the explanation of metabolic pathways and ATP yield.