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 redox reactions and occurs in multiple stages within different regions of the cell.

• Cellular respiration includes glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation.

• ATP is generated through substrate-level and oxidative phosphorylation.

• Cells require a constant supply of ATP for cellular work.

• Energy is released as heat during these processes.

• Cellular respiration occurs in the mitochondria of eukaryotic cells.

• Fermentation is an alternative pathway that allows ATP production without oxygen.

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 utilize these processes.

Redox Reactions: Oxidation and Reduction

Principles of Redox Reactions

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 are also called oxidation-reduction reactions.

Examples of Redox Reactions

Redox reactions can be illustrated by the transfer of electrons between sodium and chlorine, forming NaCl, or between other generic atoms.

• In the reaction: , sodium is oxidized and chlorine is reduced.

• Generic example: , where Xe is oxidized and Y is reduced.

Mechanism of Redox Reactions in Cells

Enzymes facilitate redox reactions by transferring electrons and protons to electron carriers such as NAD+. NADH, the reduced form, stores energy and can donate electrons to other molecules.

• Dehydrogenases remove hydrogen atoms from substrates.

• NAD+ acts as an oxidizing agent, accepting electrons and protons.

• NADH represents stored energy for ATP synthesis.

Stages of Cellular Respiration

Stepwise Energy Harvest and Electron Transport Chain

Cellular respiration occurs in three main stages: glycolysis, pyruvate oxidation and the citric acid cycle, and oxidative phosphorylation. Each stage takes place in specific regions of the cell and involves distinct mechanisms for ATP production.

• Glycolysis: Occurs in the cytosol, breaks down glucose into pyruvate.

• Pyruvate oxidation and citric acid cycle: Occur in the mitochondria, further oxidize pyruvate to CO2 and generate electron carriers.

• Oxidative phosphorylation: Electron transport chain and chemiosmosis produce most ATP.

• Substrate-level phosphorylation occurs in glycolysis and the citric acid cycle.

• Oxidative phosphorylation is powered by redox reactions.

Substrate-Level Phosphorylation

Substrate-level phosphorylation is a process where an enzyme transfers a phosphate group directly from a substrate to ADP, forming ATP. This occurs during glycolysis and the citric acid cycle.

• Direct transfer of phosphate group to ADP.

• Produces a small amount of ATP compared to oxidative phosphorylation.

Glycolysis: Harvesting Energy from Glucose

Phases of Glycolysis

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

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

• Energy payoff phase: 4 ATP are synthesized, NAD+ is reduced to NADH, and pyruvate is formed.

• Net gain: 2 ATP and 2 NADH per glucose molecule.

Detailed Steps of Glycolysis

Glycolysis involves a series of enzyme-catalyzed reactions that convert glucose to pyruvate. The process does not require oxygen and occurs in all living cells.

• All carbon from glucose is accounted for in pyruvate.

• ATP is required for preparatory steps.

• Substrate-level phosphorylation and NAD+ reduction occur during glycolysis.

Pyruvate Oxidation and the Citric Acid Cycle

Oxidation of Pyruvate to Acetyl CoA

After glycolysis, pyruvate is transported into the mitochondria and converted to acetyl CoA. This process links glycolysis to the citric acid cycle and involves the release of CO2 and reduction of NAD+ to NADH.

• Pyruvate dehydrogenase catalyzes the conversion.

• Acetyl CoA enters the citric acid cycle.

• CO2 is released as a waste product.

The Citric Acid Cycle (Krebs Cycle)

The citric acid cycle completes the oxidation of organic molecules, generating ATP, NADH, and FADH2 per turn. The cycle runs twice for each glucose molecule consumed.

• Eight steps, each catalyzed by a specific enzyme.

• Acetyl group combines with oxaloacetate to form citrate.

• Citrate is decomposed back to oxaloacetate, making the process cyclical.

• 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 chain, creating a proton gradient that drives ATP synthesis.

• Electron transport chain is embedded in the inner mitochondrial membrane.

• Electrons are passed through carrier molecules, including cytochromes.

• Proton gradient is established, referred to as proton-motive force.

• ATP synthase uses the gradient to catalyze ATP formation.

*Additional info: The exact number of ATP produced per glucose varies due to differences in shuttle mechanisms and membrane permeability.*

Fermentation and Anaerobic Respiration

ATP Production Without Oxygen

When oxygen is unavailable, cells can produce ATP through fermentation or anaerobic respiration. Fermentation extends glycolysis and regenerates NAD+ by transferring electrons to pyruvate or its derivatives.

• Alcohol fermentation: Pyruvate is converted to ethanol, releasing CO2.

• Lactic acid fermentation: Pyruvate is reduced to lactate without releasing CO2.

• Fermentation produces 2 ATP per glucose, much less than aerobic respiration.

*Additional info: Anaerobic respiration uses electron transport chains with final electron acceptors other than oxygen, such as sulfate ions.*

Catabolic Pathways and Regulation

Versatility of Catabolism

Cells can oxidize various food molecules, including carbohydrates, proteins, and fats, to generate ATP. Glycolysis and the citric acid cycle connect to many other metabolic pathways.

• Proteins are digested to amino acids and deaminated.

• Fats are broken down by beta oxidation, yielding acetyl CoA, NADH, and FADH2.

• An oxidized gram of fat produces more than twice as much ATP as a gram of carbohydrate.

Regulation of Cellular Respiration

Cellular respiration is regulated by feedback mechanisms, primarily through the activity of enzymes such as phosphofructokinase. This ensures efficient ATP production and prevents wasteful metabolism.

• Feedback inhibition controls metabolic pathways.

• ATP concentration influences the rate of respiration.

• Enzyme activity is regulated at strategic points in the pathway.

Summary Equation for Cellular Respiration: