Cellular Respiration and Fermentation

Overview of Cellular Respiration

Cellular respiration is a fundamental process by which cells extract energy from organic molecules, primarily glucose, to produce ATP, the energy currency of the cell. This process involves a series of metabolic pathways that occur in different regions of the cell and is essential for both plant and animal life.

• Cellular respiration occurs in the mitochondria of eukaryotic cells.

• It transforms chemical energy stored in food into ATP, releasing some energy as heat.

• Oxygen is required for aerobic respiration, and carbon dioxide is released as a waste product.

• Glucose is the primary fuel, but other molecules like fats and proteins can also be used.

• ATP produced powers most cellular work.

• Cellular respiration is tightly linked to breathing and eating in animals.

Energy Flow and Chemical Recycling in Ecosystems

Energy enters ecosystems as light and exits as heat, while essential chemical elements are recycled. Photosynthesis and cellular respiration are interconnected processes that sustain life.

• Photosynthesis uses light energy to convert CO2 and H2O into organic molecules and O2.

• Cellular respiration breaks down organic molecules, generating ATP and releasing CO2 and H2O.

• Both plant and animal cells perform cellular respiration.

Redox Reactions in Cellular Respiration

Oxidation and Reduction

Redox reactions are central to cellular respiration, involving the transfer of electrons between molecules. These reactions release energy stored in organic molecules, which is used to synthesize ATP.

• Oxidation: Loss of electrons from a substance.

• Reduction: Gain of electrons by a substance.

• Redox reactions often involve the transfer of hydrogen atoms (electron + proton).

• Dehydrogenation: Removal of hydrogen atoms (electrons and protons).

• Decarboxylation: Removal of carbon, releasing CO2.

Examples of Redox Reactions

Redox reactions can be illustrated with simple chemical equations, such as the formation of NaCl from sodium and chlorine.

• Na becomes oxidized (loses electron), Cl becomes reduced (gains electron).

• Generalized redox reaction: Xe- + Y → X + Ye-

Redox Reactions and Electron Carriers

Electron carriers such as NAD+ play a crucial role in cellular respiration by accepting and donating electrons during redox reactions.

• NAD+ functions as an oxidizing agent, accepting electrons and becoming NADH.

• Enzymes called dehydrogenases facilitate these reactions.

• NADH stores energy that is later used to synthesize ATP.

Stages of Cellular Respiration

Three Main Stages

Cellular respiration consists of three main stages, each occurring in specific regions of the cell and contributing to ATP production.

• Glycolysis: Occurs in the cytosol; breaks down glucose into two molecules of pyruvate.

• Pyruvate Oxidation and Citric Acid Cycle: Occurs in the mitochondrion; completes the breakdown of glucose.

• Oxidative Phosphorylation: Occurs in the mitochondrion; electron transport chain and chemiosmosis produce most ATP.

• ATP is produced by substrate-level phosphorylation (direct transfer of phosphate) and oxidative phosphorylation (via electron transport chain).

Substrate-Level Phosphorylation

Substrate-level phosphorylation is a process where an enzyme transfers a phosphate group directly from a substrate to ADP, forming ATP.

• Occurs during glycolysis and the citric acid cycle.

Glycolysis: Harvesting Energy from Glucose

Phases of Glycolysis

Glycolysis is the first stage of cellular respiration, occurring in the cytoplasm. It consists of two major phases: energy investment and energy payoff.

• Energy Investment Phase: 2 ATP are used to split glucose into two three-carbon molecules.

• Energy Payoff Phase: 4 ATP are synthesized, 2 NAD+ are reduced to NADH, and 2 pyruvate are formed.

• Net yield: 2 ATP and 2 NADH per glucose.

Steps of Glycolysis

Glycolysis involves a series of enzyme-catalyzed reactions that convert glucose to pyruvate.

• All carbon from glucose is accounted for in pyruvate.

• Glycolysis does not require oxygen and occurs in both aerobic and anaerobic conditions.

Pyruvate Oxidation and the Citric Acid Cycle

Oxidation of Pyruvate

After glycolysis, pyruvate is transported into the mitochondrion and converted to acetyl CoA, linking glycolysis to the citric acid cycle.

• Pyruvate dehydrogenase catalyzes the conversion.

• CO2 is released, NAD+ is reduced to NADH, and acetyl CoA is formed.

The Citric Acid Cycle (Krebs Cycle)

The citric acid cycle completes the oxidation of organic molecules, generating ATP, NADH, and FADH2 per turn. It is called a cycle because the starting molecule, oxaloacetate, is regenerated.

• Each turn produces 1 ATP, 3 NADH, 1 FADH2, and 2 CO2.

• Two turns per glucose molecule (since glycolysis produces two pyruvate).

• NADH and FADH2 carry electrons to the electron transport chain.

Oxidative Phosphorylation and Chemiosmosis

Electron Transport Chain and ATP Synthesis

Oxidative phosphorylation is the final stage of cellular respiration, where the electron transport chain and chemiosmosis produce most of the cell's ATP.

• NADH and FADH2 donate electrons to the electron transport chain.

• Electrons are passed through a series of protein complexes, releasing energy.

• Energy is used to pump protons across the mitochondrial membrane, creating a proton gradient (proton-motive force).

• ATP synthase uses this gradient to synthesize ATP from ADP and phosphate.

Equation for Cellular Respiration:

Summary Equation for Glucose Degradation:

ATP Yield: Up to 32 ATP per glucose molecule.

Efficiency: About 34% of the energy in glucose is transferred to ATP; the rest is lost as heat.

*Additional info: The electron transport chain is embedded in the inner mitochondrial membrane in eukaryotes and the plasma membrane in prokaryotes. Cytochromes are key protein carriers in the chain.*

Fermentation and Anaerobic Respiration

Fermentation

Fermentation is an anaerobic process that allows cells to produce ATP without oxygen. It extends glycolysis by regenerating NAD+ through the transfer of electrons to pyruvate or its derivatives.

• Alcohol fermentation: Pyruvate is converted to ethanol and CO2.

• Lactic acid fermentation: Pyruvate is reduced to lactate; no CO2 is released.

• Fermentation produces 2 ATP per glucose by substrate-level phosphorylation.

Anaerobic Respiration

Anaerobic respiration uses an electron transport chain with a final electron acceptor other than oxygen (e.g., sulfate ion), producing different by-products such as hydrogen sulfide.

• Both fermentation and anaerobic respiration allow ATP production in the absence of oxygen.

Comparison of Fermentation and Cellular Respiration

Catabolic Pathways and Regulation

Versatility of Catabolism

Cells can oxidize various food molecules to make ATP, including carbohydrates, proteins, and fats.

• Proteins are digested to amino acids and deaminated; nitrogenous waste is excreted.

• Fats are broken down to glycerol and fatty acids; fatty acids undergo beta oxidation to yield acetyl CoA, NADH, and FADH2.

• Fats produce more ATP per gram than carbohydrates.

Regulation of Cellular Respiration

Cellular respiration is regulated by feedback mechanisms, primarily through the activity of enzymes such as phosphofructokinase.

• Feedback inhibition prevents wasteful ATP production.

• When ATP levels are high, respiration slows; when ATP is low, respiration speeds up.

*Additional info: Phosphofructokinase is an allosteric enzyme that acts as a key regulatory point in glycolysis.*