Cellular Respiration & Fermentation: Study Notes for General Biology

Cellular Respiration & Fermentation

Overview

Cellular respiration is a fundamental metabolic process in which cells convert biochemical energy from nutrients into adenosine triphosphate (ATP), and release waste products. This process is essential for the survival of most organisms and occurs in several stages, including glycolysis, pyruvate oxidation, the Krebs cycle, and oxidative phosphorylation. Fermentation is an alternative pathway used when oxygen is not available.

Oxidation-Reduction (Redox) Reactions

Definition and Importance

• Oxidation-reduction (redox) reactions involve the transfer of electrons between molecules, releasing energy that can be used to synthesize ATP.

• Oxidation: Loss of electrons from a substance.

• Reduction: Gain of electrons by a substance.

• Mnemonic: LEO goes GER (Lose Electrons = Oxidation, Gain Electrons = Reduction).

• Electrons are typically transferred with hydrogen atoms during redox reactions.

Oxidation vs. Reduction

• Oxidation:

  • Loses electrons

  • Proportion of oxygen increases

  • Proportion of hydrogen decreases

  • Example: Glucose is oxidized into carbon dioxide during cell respiration.

• Reduction:

  • Gains electrons

  • Proportion of oxygen decreases

  • Proportion of hydrogen increases

  • Example: Oxygen is reduced into water during cell respiration.

Redox in Cellular Respiration

• High-energy electrons are taken away from glucose and ultimately given to oxygen.

• Glucose loses electrons (oxidation) and forms carbon dioxide.

• Oxygen gains electrons (reduction) and forms water.

• Overall reaction: $C_6H_{12}O_6 + 6O_2 ightarrow 6CO_2 + 6H_2O + ext{Energy (ATP)}$

Electron Carrier Molecules

• During cellular respiration, hydrogen atoms and high-energy electrons are transferred to electron carrier molecules such as NAD+ and FAD.

• Key reactions: $\text{NAD}^+ + H^+ + 2e^- \rightarrow \text{NADH}$ $\text{FAD} + 2H^+ + 2e^- \rightarrow \text{FADH}_2$

• NAD+ and FAD are coenzymes that assist enzymes in catalyzing reactions and act as oxidizing agents.

• NADH and FADH2 carry high-energy electrons to the electron transport chain, where most ATP is produced.

Glycolysis, Pyruvate Oxidation & Krebs Cycle

Cellular Respiration Overview

• Aerobic cellular respiration is the complete oxidation of glucose, with oxygen as the final electron acceptor.

• Occurs in four stages:

  1. Glycolysis

  2. Pyruvate Oxidation

  3. Krebs Cycle (Citric Acid Cycle)

  4. Oxidative Phosphorylation

Glycolysis

• Consists of 10 enzyme-catalyzed reactions that split one glucose (6-carbon) into two pyruvate (3-carbon) molecules.

• Occurs in the cytoplasm and does not require oxygen.

• Two major phases:

  1. Energy investment phase: 2 ATP are used to phosphorylate glucose, which splits into two G3P molecules.

  2. Energy harvesting phase: Each G3P is oxidized into pyruvate, producing 2 NADH and 4 ATP (net gain: 2 ATP).

• Substrate-level phosphorylation: ATP is produced directly by enzyme-catalyzed transfer of phosphate from substrate to ADP.

Pyruvate Oxidation

• Occurs in the mitochondrial matrix if oxygen is present.

• Each pyruvate is oxidized to form Acetyl CoA (2-carbon molecule).

• For both pyruvates:

  • 2 CO2 released

  • 2 NADH formed

  • Coenzyme-A added to form 2 Acetyl CoA

Krebs Cycle (Citric Acid Cycle)

• Series of 8 reactions in the mitochondrial matrix.

• Starts and ends with oxaloacetate (4-carbon).

• Acetyl CoA binds to oxaloacetate to form citrate (6-carbon).

• Each cycle: citrate is oxidized and eventually regenerates oxaloacetate.

• Occurs twice per glucose (2 Acetyl CoA).

• Products from two cycles:

  • 4 CO2

  • 6 NADH

  • 2 FADH2

  • 2 ATP (substrate-level phosphorylation)

Oxidative Phosphorylation

Overview

• Only 4 ATP are made during glycolysis, pyruvate oxidation, and the Krebs cycle (via substrate-level phosphorylation).

• 10 NADH and 2 FADH2 are produced and used in oxidative phosphorylation to make much more ATP.

• Occurs in two steps:

  1. Electron Transport Chain (ETC)

  2. Chemiosmosis

Electron Transport Chain (ETC)

• Series of protein complexes in the inner mitochondrial membrane.

• NADH and FADH2 drop off electrons, which are transported through the complexes.

• Electrons lose free energy, which is used to pump protons (H+) from the matrix to the intermembrane space (active transport).

• Electron pairs from NADH pump out 3 H+; from FADH2, 2 H+.

• Oxygen acts as the final electron acceptor, forming water.

• 12 electron pairs (from 10 NADH and 2 FADH2) require 12 oxygen atoms (6 O2), creating 12 H2O.

Chemiosmosis

• Proton gradient (proton-motive force) is created by pumping H+ into the intermembrane space.

• ATP Synthase allows H+ to diffuse back into the matrix, using the exergonic flow to drive ATP production.

• Each NADH (3 protons) creates 3 ATP; each FADH2 (2 protons) creates 2 ATP.

Decoupling Oxidative Phosphorylation from ETC

• Uncoupling proteins allow protons to diffuse back into the matrix without passing through ATP synthase.

• This process generates heat instead of ATP, which can be used by endothermic organisms to regulate body temperature.

Important Things to Note

Aerobic Prokaryotes vs. Eukaryotes

• Aerobic cellular respiration occurs in both eukaryotes (mitochondria) and some prokaryotes.

• In prokaryotes, the electron transport chain is located on the cell membrane.

Versatility of Catabolism

• Catabolic pathways can use many types of organic molecules, not just glucose.

• Proteins are digested to amino acids; fats to glycerol and fatty acids.

• Fats produce more than twice as much ATP per gram as carbohydrates.

Regulation of Cell Respiration

• Feedback inhibition regulates respiration; high ATP slows the process, low ATP speeds it up.

• ATP can act as an allosteric inhibitor for enzymes in glycolysis.

Evolutionary Significance of Glycolysis

• Glycolysis can occur with or without oxygen and does not require mitochondria.

• It is the most common metabolic pathway among all domains of life, indicating its ancient evolutionary origin.

Anaerobic Pathways: Fermentation & Anaerobic Respiration

Fermentation

• Occurs in the absence of oxygen; couples glycolysis with reactions that regenerate NAD+.

• Two common types:

  1. Alcohol Fermentation: Pyruvate is converted to ethanol and CO2; NAD+ is regenerated.

  2. Lactic Acid Fermentation: Pyruvate is reduced to lactic acid by NADH; NAD+ is regenerated.

• Fermentation produces only 2 ATP per glucose.

• Examples: Yeast (alcohol fermentation), human muscle cells (lactic acid fermentation).

Anaerobic Respiration

• Some bacteria and archaea use molecules other than oxygen as the final electron acceptor (e.g., sulfate, nitrate, sulfur, carbon dioxide).

• Obligate anaerobes: Can only survive without oxygen.

• Facultative anaerobes: Can switch between aerobic and anaerobic pathways depending on oxygen availability.

Comparison Table: Fermentation vs. Anaerobic Respiration

Feature	Fermentation	Anaerobic Respiration
Final Electron Acceptor	Organic molecule (e.g., pyruvate)	Inorganic molecule (e.g., sulfate, nitrate)
ATP Yield	2 ATP per glucose	More than 2 ATP, but less than aerobic respiration
Location	Cytoplasm	Cell membrane (prokaryotes)
Examples	Yeast, muscle cells	Bacteria, archaea

Self-Assessment Questions

• Glycolysis is an exergonic reaction.

• Pyruvate is formed in the cytosol.

• Most energy entering the electron transport chain comes as NADH and FADH2.

• The immediate energy source for ATP synthesis by ATP synthase is the H+ concentration gradient across the inner mitochondrial membrane.

• The final electron acceptor in the electron transport chain is oxygen.

• Metabolic poisons affecting ATP synthase would increase the H+ gradient and decrease ATP synthesis.

• Most CO2 from catabolism is released during the Krebs cycle.

• Uncoupling agents reduce oxidative phosphorylation, decreasing ATP production and increasing heat.

• Cyanide blocks electron transport, halting ATP synthesis and proton gradient formation.

• Fermentation and cellular respiration both share glycolysis as a common pathway.

• Fermentation regenerates NAD+ so glycolysis can continue.

Additional info: These notes expand on the original slides and handwritten notes, providing definitions, examples, and a comparison table for clarity. All equations are presented in LaTeX format for academic accuracy.