Cellular Respiration and Fermentation

Introduction to Cellular Energy Production

Cells require energy to perform essential functions, and this energy is primarily supplied by ATP. Cellular respiration and fermentation are metabolic pathways that harvest energy from high-energy molecules, such as glucose, to produce ATP. The energy released during these processes is used to add a phosphate group to ADP, forming ATP, which fuels cellular work.

Overview of Cellular Respiration

Cellular respiration is a series of metabolic reactions that oxidize glucose to produce ATP. Most cells continuously synthesize ATP, as it is unstable and only stored briefly. Glucose is obtained from photosynthesis in plants or from food in other organisms and is stored as glycogen or starch.

Key Points:

• ATP is unstable and must be synthesized constantly.

• Glucose is the primary molecule used to generate ATP.

• Organisms store glucose as glycogen (animals) or starch (plants).

Oxidation of Glucose: Cellular Respiration vs. Fermentation

Glucose can be oxidized fully via cellular respiration or partially via fermentation. Cellular respiration produces more energy because it fully oxidizes glucose to CO2, whereas fermentation leaves small, reduced organic molecules as waste.

Four Steps of Cellular Respiration

Cellular respiration consists of four main processes:

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

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 move through a transport chain, creating a proton gradient used to make ATP.

Catabolic and Anabolic Pathways

Metabolic pathways are classified as:

• Catabolic pathways: Break down molecules to harvest energy and produce ATP.

• Anabolic pathways: Synthesize larger molecules from smaller components, using ATP.

Glycolysis

Sequence and Regulation of Glycolysis

Glycolysis is a series of 10 reactions occurring in the cytosol. It starts with an energy investment phase (using 2 ATP) and ends with an energy payoff phase (producing 4 ATP and 2 NADH). The net yield is 2 ATP, 2 NADH, and 2 pyruvate per glucose.

Substrate-Level Phosphorylation

ATP is produced directly in glycolysis by substrate-level phosphorylation, where an enzyme transfers a phosphate group from a phosphorylated substrate to ADP.

Regulation of Glycolysis

Glycolysis is regulated by feedback inhibition, primarily at the enzyme phosphofructokinase. High ATP levels inhibit this enzyme, slowing glycolysis.

Pyruvate Processing

Conversion of Pyruvate to Acetyl CoA

Pyruvate produced by glycolysis is transported into mitochondria in eukaryotes, where it is processed by pyruvate dehydrogenase. One carbon is oxidized to CO2, NADH is produced, and the remaining two-carbon unit is attached to coenzyme A, forming acetyl CoA.

Regulation of Pyruvate Processing

• Pyruvate dehydrogenase is inhibited by phosphorylation when NADH, acetyl CoA, or ATP levels are high.

• It is activated by dephosphorylation when ADP, pyruvate, CoA, or NAD+ levels are high.

Citric Acid Cycle

Oxidation of Acetyl CoA to CO2

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. The cycle turns twice per glucose molecule.

Energy Yield

• Reduces three NAD+ to NADH

• Reduces one FAD to FADH2

• Phosphorylates ADP (or GDP) to form ATP (or GTP)

Regulation of the Citric Acid Cycle

Feedback inhibition regulates the cycle at multiple points. High ATP and NADH levels decrease reaction rates.

Electron Transport Chain and Oxidative Phosphorylation

Electron Transport Chain (ETC)

The ETC consists of four protein complexes and two mobile carriers (Q and cytochrome c). Electrons from NADH and FADH2 are transferred through the chain, releasing energy used to pump protons across the membrane, creating a proton gradient.

ATP Synthase and Chemiosmosis

ATP synthase is a protein complex that synthesizes ATP using the energy from the proton gradient. The F0 unit acts as a proton channel, and the F1 unit phosphorylates ADP to ATP as protons flow through.

Oxidative Phosphorylation

Most ATP is produced by oxidative phosphorylation, which depends on 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+ so glycolysis can continue. Fermentation is less efficient than cellular respiration, producing only 2 ATP per glucose.

Lactic Acid Fermentation

In muscle cells, pyruvate accepts electrons from NADH, forming lactate and regenerating NAD+.

Alcohol Fermentation

In yeast, pyruvate is converted to acetaldehyde and CO2. Acetaldehyde accepts electrons from NADH, forming ethanol and NAD+.

Efficiency and Facultative Anaerobes

• Fermentation yields much less ATP than cellular respiration.

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

Metabolic Pathways and Intermediates

Catabolic Pathways

Cells use carbohydrates, fats, and proteins for ATP production. Fats are broken down into glycerol and fatty acids, proteins into amino acids, and these intermediates enter glycolysis or the citric acid cycle.

Anabolic Pathways

Intermediates from cellular respiration are used to synthesize macromolecules, such as nucleotides, fatty acids, and amino acids. This integration allows cells to maintain homeostasis.

Summary Table: Cellular Respiration Steps

Key Terms and Concepts

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

• Glycolysis: The breakdown of glucose into pyruvate.

• Citric Acid Cycle: A series of reactions that oxidize acetyl CoA to CO2 and produce NADH and FADH2.

• Electron Transport Chain: A series of protein complexes that transfer electrons and pump protons to generate a proton gradient.

• Oxidative Phosphorylation: ATP synthesis driven by the proton gradient created by the ETC.

• Fermentation: An anaerobic pathway that regenerates NAD+ and allows glycolysis to continue.

• Feedback Inhibition: Regulation of metabolic pathways by the end products.

Equations

Overall Cellular Respiration Equation:

ATP Formation:

Fermentation (Lactic Acid):

Fermentation (Alcohol):

Summary

Cellular respiration and fermentation are central to cellular metabolism, providing energy for life processes. Cellular respiration is highly efficient, producing up to 29 ATP per glucose, while fermentation serves as an emergency backup when oxygen is unavailable. The integration of catabolic and anabolic pathways ensures cells can adapt to varying environmental conditions and maintain homeostasis.