Cellular Respiration and Fermentation: Structured Study Notes

Cellular Respiration and Fermentation

Redox Reactions in Cellular Respiration

Redox reactions are fundamental to cellular respiration, involving the transfer of electrons between molecules. These reactions drive the energy transformations necessary for ATP production.

• Oxidation: The loss of one or more electrons from a molecule.

• Reduction: The gain of one or more electrons by a molecule, resulting in a reduced overall charge.

• Oxidation and reduction always occur together (simultaneously).

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

• Example: When glucose donates electrons to NAD+, glucose is oxidized and NAD+ is reduced.

Electron Carriers: NADH & FADH2

Electron carriers are molecules that transport electrons during cellular respiration, facilitating energy transfer.

• NADH and FADH2 are reduced forms that carry electrons to the electron transport chain.

• NAD+ and FAD are oxidized forms, ready to accept electrons.

• These carriers act like "electron taxis," shuttling electrons to different cellular locations.

• During cellular respiration, electrons carried by NADH and FADH2 are used to generate ATP.

Overview of Aerobic Cellular Respiration

Aerobic cellular respiration is the process of breaking down glucose in the presence of oxygen to produce ATP, water, and carbon dioxide. It is a multi-stage redox reaction.

• Requires oxygen gas (O2).

• Occurs in several stages, mostly within the mitochondria.

• Overall equation: $\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy (ATP)}}$

• Glucose is oxidized; oxygen is reduced.

Stages of Aerobic Cellular Respiration

The process consists of four main metabolic pathways, each contributing to the complete oxidation of glucose.

• Glycolysis: Occurs in the cytoplasm, breaks down glucose into pyruvate.

• Pyruvate Oxidation: Converts pyruvate to acetyl-CoA in the mitochondrial matrix.

• Krebs Cycle (Citric Acid Cycle): Oxidizes acetyl-CoA, producing NADH, FADH2, ATP, and CO2.

• Electron Transport Chain & Chemiosmosis: Uses electrons from NADH and FADH2 to generate ATP.

Types of Phosphorylation

ATP is synthesized by two main mechanisms during cellular respiration.

• Substrate-Level Phosphorylation: Direct transfer of a phosphate group to ADP by an enzyme, occurs in glycolysis and Krebs cycle.

• Oxidative Phosphorylation: Uses energy from redox reactions in the electron transport chain to phosphorylate ADP, producing most ATP.

• Chemiosmosis: Diffusion of ions (H+) across a membrane, driving ATP synthesis via ATP synthase.

Glycolysis

Glycolysis is the first step of cellular respiration, breaking down glucose into two pyruvate molecules in the cytoplasm. It does not require oxygen.

• "Glyco" = sugar; "lysis" = to split.

• Glucose (6 carbons) is converted to 2 pyruvate (3 carbons each).

• Consists of 10 reactions grouped into two phases:

  • Energy Investment Phase: Uses 2 ATP.

  • Energy Harvest Phase: Produces 2 NADH and 4 ATP (net gain: 2 ATP).

• Net products: 2 pyruvate, 2 NADH, 2 ATP.

Pyruvate Oxidation

Pyruvate oxidation connects glycolysis to the Krebs cycle, converting pyruvate into acetyl-CoA in the mitochondrial matrix.

• Each pyruvate yields 1 acetyl-CoA, 1 NADH, and 1 CO2 (per glucose: 2 acetyl-CoA, 2 NADH, 2 CO2).

• Acetyl-CoA enters the Krebs cycle.

Krebs Cycle (Citric Acid Cycle)

The Krebs cycle is the third stage, fully oxidizing acetyl-CoA to produce ATP, NADH, FADH2, and CO2.

• Occurs in the mitochondrial matrix.

• Each acetyl-CoA enters the cycle, producing:

  • 3 NADH

  • 1 FADH2

  • 1 ATP

  • 2 CO2

• Two turns per glucose molecule (since 2 acetyl-CoA are produced).

• Key intermediates: citrate, oxaloacetate.

Electron Transport Chain (ETC) and Chemiosmosis

The ETC is the final stage, consisting of protein complexes in the mitochondrial inner membrane. It harnesses energy from electrons carried by NADH and FADH2 to create a proton gradient, which powers ATP synthesis.

• Electrons are transferred through a series of redox reactions.

• H+ ions are pumped into the intermembrane space, creating a gradient.

• Oxygen is the final electron acceptor, forming water.

• ATP synthase uses the H+ gradient to synthesize ATP (chemiosmosis).

Fermentation and Anaerobic Respiration

When oxygen is unavailable, cells can perform fermentation or anaerobic respiration to regenerate NAD+ and produce ATP.

• Fermentation: Uses electrons from NADH to reduce pyruvate, regenerating NAD+.

• Lactic Acid Fermentation: Pyruvate is reduced to lactic acid (occurs in muscle cells and some bacteria).

• Alcohol Fermentation: Pyruvate is reduced to ethanol and CO2 (occurs in yeast and some plants).

• Anaerobic Respiration: Uses alternative final electron acceptors (e.g., nitrate, sulfate, CO2), producing more ATP than fermentation but less than aerobic respiration.

Summary Table: Products of Aerobic Cellular Respiration

The following table summarizes the total products generated from one molecule of glucose during each stage of aerobic cellular respiration.

Stage	ATP	NADH	FADH2	CO2
Glycolysis	2	2	0	0
Pyruvate Oxidation	0	2	0	2
Krebs Cycle	2	6	2	4
ETC & Chemiosmosis	~26-34	0	0	0
Total	~30-38	10	2	6

Key Equations

• Overall cellular respiration: $\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy (ATP)}}$

• ATP yield: $\mathrm{\text{Total ATP} \approx 30-38}$ per glucose molecule

Additional info:

• Some images and mnemonic devices are used to reinforce memory and understanding of complex pathways.

• Tables and diagrams are inferred and reconstructed for clarity and completeness.