Cellular Respiration & Fermentation

Overview of Energy Conversion in Cells

Cells require energy to perform essential functions, which they obtain by converting chemical energy stored in organic molecules. The two primary pathways for energy conversion are cellular respiration and fermentation. These processes allow cells to extract energy from glucose and other nutrients, storing it in the form of ATP for cellular work.

• Photosynthesis in plants converts CO2 and H2O into glucose and O2 using sunlight.

• Energy storage occurs as starch, glycogen, and fats synthesized from glucose.

• Energy use involves breaking down glucose to power cellular activities.

Glucose Oxidation: The Central Reaction

The breakdown of glucose is a key reaction in cellular respiration, releasing energy, carbon dioxide, and water.

• Overall reaction:

• Glucose (C6H12O6) is oxidized, releasing energy.

• Oxygen acts as the final electron acceptor.

• Products: Carbon dioxide, water, and released heat.

Pathways for ATP Generation

Cellular Respiration vs. Fermentation

Cells generate ATP through two main pathways: cellular respiration (aerobic) and fermentation (anaerobic). Both begin with glucose but differ in their final products and efficiency.

• Cellular respiration is more efficient, producing more ATP per glucose molecule.

• Fermentation occurs when oxygen is absent, yielding less ATP and producing organic byproducts (e.g., lactate or ethanol).

Redox Reactions in Cellular Respiration

Electron Carriers: NAD+ and FAD

Redox reactions transfer electrons between molecules, playing a central role in energy extraction. Two key electron carriers are nicotinamide adenine dinucleotide (NAD+) and flavin adenine dinucleotide (FAD).

• NAD+ is reduced to NADH by accepting electrons and a proton.

• FAD is reduced to FADH2 by accepting two electrons and two protons.

• These carriers shuttle electrons to the electron transport chain, facilitating ATP production.

Stages of Cellular Respiration

1. Glycolysis

Glycolysis is the first step in glucose breakdown, occurring in the cytoplasm of both prokaryotes and eukaryotes.

• Converts glucose to pyruvate.

• Produces ATP and NADH.

• Does not require oxygen.

2. Pyruvate Processing

Pyruvate produced in glycolysis is transported into the mitochondria (in eukaryotes) or remains in the cytoplasm (in prokaryotes), where it is converted to acetyl CoA.

• Releases CO2.

• Generates NADH.

• Prepares substrates for the citric acid cycle.

3. Citric Acid Cycle (Krebs Cycle)

The citric acid cycle occurs in the mitochondrial matrix (eukaryotes) or cytoplasm (prokaryotes). It completes the oxidation of glucose derivatives, producing electron carriers and ATP (or GTP).

• Acetyl CoA enters the cycle, combining with oxaloacetate.

• Produces CO2, NADH, FADH2, and ATP/GTP.

• Regulated by feedback inhibition (e.g., by NADH and ATP).

4. Electron Transport Chain & Oxidative Phosphorylation

Located in the inner mitochondrial membrane (eukaryotes) or plasma membrane (prokaryotes), the electron transport chain uses electrons from NADH and FADH2 to create a proton gradient, driving ATP synthesis.

• Electrons are transferred through protein complexes.

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

• ATP synthase uses the proton gradient to generate ATP.

Summary Table: Stages of Cellular Respiration

Key Terms and Concepts

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

• Substrate-level phosphorylation: Direct synthesis of ATP during glycolysis and the citric acid cycle.

• Oxidative phosphorylation: ATP synthesis driven by the electron transport chain and proton gradient.

• Feedback inhibition: Regulation of metabolic pathways by products such as ATP and NADH.

Example: Muscle Activity and Energy Use

During intense exercise, muscle cells rapidly consume ATP. When oxygen is limited, cells switch from cellular respiration to fermentation, producing less ATP and generating lactate as a byproduct.

Additional info:

• Fermentation pathways include lactic acid fermentation (in animals) and alcoholic fermentation (in yeast).

• Other macromolecules (fats, proteins) can also be metabolized for energy, entering cellular respiration at various points.