Cellular Respiration and Fermentation (Chapter 9): Study Notes

Cellular Respiration and Fermentation

Overview and Ecosystem Linkages

Cellular respiration and fermentation are essential metabolic pathways that allow cells to extract energy from organic molecules. These processes are tightly linked to photosynthesis in the ecosystem, forming a cycle of energy transformation and matter recycling.

• Photosynthesis in plants, algae, and some bacteria converts light energy into chemical energy, producing organic molecules (like glucose) and O2 from CO2 and H2O.

• Cellular respiration in plants, animals, and many microbes breaks down these organic molecules, consuming O2 and releasing CO2, H2O, and energy (as ATP and heat).

• These processes are performed by different types of organisms but are interconnected, maintaining the flow of energy and cycling of matter in ecosystems.

Example: Plants produce glucose and oxygen via photosynthesis; animals consume these and release carbon dioxide and water via cellular respiration.

Catabolic Pathways

Types of Catabolic Pathways

Catabolic pathways break down complex molecules into simpler ones, releasing energy.

• Fermentation: Partial degradation of sugars without oxygen (anaerobic).

• Aerobic respiration: Consumes organic molecules and oxygen, yielding ATP.

• Anaerobic respiration: Similar to aerobic, but uses electron acceptors other than oxygen.

Definition: Exergonic reactions release energy; cellular respiration is a major exergonic process in cells.

Glucose Catabolism

Overall Reaction

Although carbohydrates, fats, and proteins can all be used as fuel, glucose is the primary molecule traced in cellular respiration.

• Overall equation:

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

This reaction is highly exergonic, releasing energy stored in glucose.

Redox Reactions in Cellular Respiration

Oxidation and Reduction

Cellular respiration involves a series of oxidation-reduction (redox) reactions that transfer electrons and release energy.

• Oxidation: Loss of electrons (or hydrogen atoms); the substance becomes oxidized.

• Reduction: Gain of electrons (or hydrogen atoms); the substance becomes reduced.

Example: In the reaction $Na + Cl \rightarrow Na^+ + Cl^-$, sodium is oxidized and chlorine is reduced.

Some redox reactions involve changes in electron sharing in covalent bonds rather than complete electron transfer.

Redox in Respiration

• Glucose is oxidized to carbon dioxide.

• Oxygen is reduced to water.

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Energy}$

Electron Carriers: NAD+ and FAD

• Electrons from organic molecules are first transferred to NAD+ (nicotinamide adenine dinucleotide), forming NADH.

• Each NADH stores energy used to synthesize ATP via the electron transport chain and chemiosmosis.

• FAD (flavin adenine dinucleotide) is another electron carrier, reduced to FADH2.

Stages of Cellular Respiration

Three Main Stages

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

2. Citric Acid Cycle (Krebs Cycle): Completes the breakdown of glucose by oxidizing pyruvate to CO2 (in mitochondrial matrix).

3. Oxidative Phosphorylation: Includes the electron transport chain and chemiosmosis; produces most ATP (in inner mitochondrial membrane).

Summary Table: ATP, NADH, and FADH2 Production

Pathway	ATP Produced	NADH Produced	FADH2 Produced
Glycolysis	2	2	0
Pyruvate Oxidation	0	2	0
Citric Acid Cycle	2	6	2
Total	4	10	2

Phosphorylation Events

ATP Formation

• Oxidative phosphorylation: Powered by redox reactions; accounts for ~90% of ATP generated by cellular respiration.

• Substrate-level phosphorylation: Direct transfer of phosphate from a substrate to ADP to form ATP; occurs in glycolysis and the citric acid cycle.

$\text{ADP} + P_i \xrightarrow{\text{Energy}} \text{ATP}$

Types of Reactions in Cellular Respiration

• Dehydrogenations: Removal of hydrogen atoms (transferred to NAD+ or FAD).

• Decarboxylations: Removal of carboxyl groups (released as CO2).

• Preparative reactions: Substrate undergoes rearrangements.

• Substrate-level phosphorylation: Phosphate group transferred from intermediate to ADP.

Glycolysis

Process and Phases

Glycolysis is the splitting of glucose (6C) into two molecules of pyruvate (3C each). It occurs in the cytoplasm and is common to both aerobic and anaerobic pathways.

• Energy investment phase: 2 ATP are used to phosphorylate glucose and its intermediates.

• Energy payoff phase: 4 ATP and 2 NADH are produced; net gain is 2 ATP and 2 NADH per glucose.

Glycolysis: Net Inputs and Outputs

Input	Output
Glucose	2 Pyruvate + 2 H2O
2 NAD+ + 4 e- + 4 H+	2 NADH + 2 H+
4 ADP + 4 Pi	4 ATP (2 net ATP)

Oxidation of Pyruvate to Acetyl CoA

Before entering the citric acid cycle, pyruvate is converted to acetyl CoA by a multienzyme complex. This links glycolysis to the citric acid cycle.

• Reactions: Decarboxylation (CO2 released), dehydrogenation (NADH produced), and addition of Coenzyme A.

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle completes the breakdown of pyruvate to CO2. Each turn generates 1 ATP, 3 NADH, and 1 FADH2 per acetyl CoA.

• Key reactions: Dehydrogenations, decarboxylations, substrate-level phosphorylation, and preparative steps.

Electron Transport Chain and Chemiosmosis

Electron Transport Chain (ETC)

• Located in the inner mitochondrial membrane (eukaryotes) or plasma membrane (prokaryotes).

• Electrons from NADH and FADH2 are transferred through a series of protein complexes, releasing energy in steps.

• O2 is the final electron acceptor, forming water.

Chemiosmosis and ATP Synthesis

• Electron transfer in the ETC pumps protons (H+) into the intermembrane space, creating a proton gradient.

• ATP synthase uses the flow of H+ back into the matrix to drive ATP production (chemiosmosis).

Accounting of ATP Production

About 30–32 ATP are produced per glucose molecule in aerobic respiration. Some energy is lost as heat or when electrons are transferred to mitochondria.

Pathway	By Substrate-Level Phosphorylation	By Oxidative Phosphorylation (NADH)	By Oxidative Phosphorylation (FADH2)
Glycolysis	2	5	0
Intermediate step	0	5	0
Citric acid cycle	2	15	3
Total	4	25	3

Each NADH yields ~2.5 ATP; each FADH2 yields ~1.5 ATP.

Key Terms and Concepts

• ATP (Adenosine Triphosphate): The main energy currency of the cell.

• Substrate-level phosphorylation: Direct formation of ATP by transfer of a phosphate group to ADP.

• Oxidative phosphorylation: ATP formation driven by the transfer of electrons through the ETC and chemiosmosis.

• Redox reaction: Chemical reaction involving the transfer of electrons.

• Electron carrier: Molecule that transports electrons during cellular respiration (e.g., NAD+, FAD).

Summary

Cellular respiration is a multi-step process that efficiently extracts energy from glucose, producing ATP through glycolysis, the citric acid cycle, and oxidative phosphorylation. Redox reactions and electron carriers play central roles in energy transfer, and the process is tightly regulated to meet cellular energy demands.