Cellular Respiration and Fermentation

Introduction to Cellular Energy Production

Cells require energy to perform essential functions, and this energy is primarily supplied by adenosine triphosphate (ATP). ATP is generated through metabolic pathways that harvest energy from high-energy molecules such as glucose. The process of cellular respiration allows cells to efficiently convert glucose into ATP, while fermentation provides an alternative pathway when oxygen is absent.

ATP: The Cellular Energy Currency

ATP fuels cellular work by transferring energy released from catabolic reactions to endergonic processes. The ATP–ADP cycle involves phosphorylation and hydrolysis, linking energy release and consumption.

• ATP (Adenosine Triphosphate): A molecule that stores and transfers energy within cells.

• ADP (Adenosine Diphosphate): Formed when ATP loses a phosphate group, releasing energy.

• Phosphorylation: The addition of a phosphate group to ADP to regenerate ATP.

Overview of Cellular Respiration

Glucose as the Central Energy Source

Cells obtain glucose from photosynthesis (in plants) or food (in other organisms). Glucose is stored as glycogen or starch and serves as the hub for energy processing in cells.

Four Major Steps of Cellular Respiration

Cellular respiration is a series of four interconnected processes that oxidize glucose to produce ATP:

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

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

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

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

Glycolysis

Key Features of Glycolysis

Glycolysis is a sequence of 10 reactions occurring in the cytosol. It consists of two phases:

• Energy Investment Phase: Uses 2 ATP to activate glucose.

• Energy Payoff Phase: Produces 4 ATP (net gain of 2 ATP), 2 NADH, and 2 pyruvate via substrate-level phosphorylation.

Regulation of Glycolysis

Glycolysis is regulated by feedback inhibition, primarily through the enzyme phosphofructokinase. High ATP levels inhibit the enzyme, slowing glycolysis when energy is abundant.

• Phosphofructokinase: Has two ATP binding sites—active and regulatory.

• ATP binding at the regulatory site inhibits the enzyme.

Processing Pyruvate to Acetyl CoA

Mitochondrial Structure and Pyruvate Transport

In eukaryotes, pyruvate is transported into mitochondria, which have inner and outer membranes and cristae. The mitochondrial matrix is the site of pyruvate processing.

Pyruvate Dehydrogenase Reaction

Pyruvate is processed by pyruvate dehydrogenase, producing acetyl CoA, CO2, and NADH. This reaction is regulated by phosphorylation and feedback inhibition.

• One carbon from pyruvate is released as CO2.

• The remaining two-carbon unit is attached to coenzyme A.

The Citric Acid Cycle

Cycle Organization and Function

The citric acid cycle (Krebs cycle) oxidizes acetyl CoA to CO2 in the mitochondrial matrix (eukaryotes) or cytosol (prokaryotes). It is a cyclic pathway, regenerating oxaloacetate at the end of each turn.

• Each acetyl CoA yields 2 CO2, 3 NADH, 1 FADH2, and 1 ATP (or GTP).

• The cycle turns twice per glucose molecule.

Regulation of the Citric Acid Cycle

Feedback inhibition regulates the cycle at multiple points. High ATP or NADH levels decrease reaction rates, while scarcity increases them.

Glucose Oxidation: Summary and Energy Yield

Products of Glucose Oxidation

For each glucose molecule oxidized, the cell produces:

• 6 CO2

• 10 NADH

• 2 FADH2

• 4 ATP

Free Energy Changes

Most of the energy from glucose is transferred to NADH and FADH2. Electrons are ultimately transferred to oxygen, forming water. The energy released is used to synthesize ATP.

Electron Transport Chain and Chemiosmosis

Electron Transport Chain (ETC) Organization

The ETC consists of four protein complexes and mobile carriers (Q and cytochrome c). Electrons from NADH and FADH2 are transferred through the chain, releasing energy and forming water at the end.

ATP Synthase and Chemiosmosis

ATP synthase is a protein complex that synthesizes ATP using the proton gradient generated by the ETC. The F0 unit acts as a proton channel, and the F1 unit phosphorylates ADP.

Oxidative Phosphorylation

Most ATP is produced by oxidative phosphorylation, which uses the proton-motive force generated by the ETC. Approximately 29 ATP are produced per glucose molecule.

Fermentation

Fermentation Pathways

When oxygen is unavailable, cells use fermentation to regenerate NAD+ and allow glycolysis to continue. Fermentation is less efficient than cellular respiration, producing only 2 ATP per glucose.

Lactic Acid and Alcohol Fermentation

• Lactic Acid Fermentation: Pyruvate accepts electrons from NADH, forming lactate and NAD+ (occurs in muscle cells).

• Alcohol Fermentation: Pyruvate is converted to acetaldehyde and CO2; acetaldehyde accepts electrons from NADH, forming ethanol and NAD+ (occurs in yeast).

Efficiency and Facultative Anaerobes

Fermentation yields much less ATP than cellular respiration. Some organisms can switch between fermentation and aerobic respiration, depending on oxygen availability.

Metabolic Pathways and Interactions

Catabolic and Anabolic Pathways

Cellular respiration interacts with other metabolic pathways. Cells use carbohydrates, fats, and proteins as substrates for ATP production. Intermediates from these pathways are also used for biosynthesis.

Homeostasis and Metabolic Regulation

Metabolism consists of thousands of chemical reactions organized into pathways. Regulation of these pathways maintains homeostasis, ensuring a stable internal environment.

Summary Table: Cellular Respiration Steps

Key Equations

Overall Cellular Respiration Equation

The complete oxidation of glucose in aerobic respiration can be summarized as:

ATP Yield

Maximum theoretical ATP yield per glucose:

Conclusion

Cellular respiration is a central metabolic pathway that enables cells to efficiently harvest energy from glucose. Fermentation provides an alternative when oxygen is unavailable, though it is less efficient. The integration and regulation of these pathways are essential for cellular function and homeostasis.