Cellular Respiration

Overview of Cellular Respiration

Cellular respiration is the process by which plant and animal cells break down organic molecules to release energy, primarily in the form of ATP. This process occurs mainly in the mitochondria and involves a series of metabolic pathways that transform the chemical energy in food into usable energy for cellular work.

• ATP (Adenosine Triphosphate): The main energy currency of the cell, used to drive most cellular processes.

• Energy Transformation: Chemical energy in food is converted to ATP, with some energy lost as heat.

• Equation for Aerobic Respiration:

• Catabolic Pathways: Release stored energy by breaking down complex molecules.

• Electron Transfer: Central to energy release in these pathways.

Energy Flow and 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 maintain this balance.

• Photosynthesis: Uses CO2 and H2O to produce organic molecules and O2.

• Cellular Respiration: Uses O2 and organic molecules to produce ATP, CO2, and H2O.

Redox Reactions in Cellular Respiration

Oxidation and Reduction

Cellular respiration relies on oxidation-reduction (redox) reactions, which involve the transfer of electrons between molecules.

• Oxidation: Loss of electrons from a substance.

• Reduction: Gain of electrons by a substance (reduces positive charge).

• Reducing Agent: Electron donor; reduces the electron acceptor.

• Oxidizing Agent: Electron acceptor; oxidizes the electron donor.

• Partial Electron Transfer: In some reactions, electrons are not fully transferred but are shared unequally in covalent bonds (e.g., oxygen's high electronegativity).

Energy Release in Redox Reactions

Electrons moving toward more electronegative atoms (like oxygen) lose potential energy, releasing energy that can be harnessed for ATP synthesis.

• Glucose Oxidation: Transfers electrons from high-energy glucose to lower-energy oxygen, releasing energy.

Stages of Cellular Respiration

Stepwise Energy Harvest via NAD+ and the Electron Transport Chain

Cellular respiration occurs in a series of steps, with electrons often transferred to carriers before reaching oxygen.

• NAD+ (Nicotinamide Adenine Dinucleotide): A coenzyme that acts as an electron carrier and oxidizing agent.

• NADH: The reduced form of NAD+, stores energy for ATP synthesis.

• Dehydrogenases: Enzymes that remove hydrogen atoms from substrates, transferring electrons to NAD+.

• Electron Transport Chain: A series of molecules in the mitochondrial inner membrane (or plasma membrane in prokaryotes) that transfer electrons in a stepwise manner, releasing energy gradually.

Three Main Stages

• Glycolysis: Breaks down glucose into two pyruvate molecules in the cytoplasm.

• Pyruvate Oxidation and Citric Acid Cycle: Completes glucose breakdown to CO2 in the mitochondria.

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

ATP Production Methods

• Oxidative Phosphorylation: Powered by redox reactions; produces ~90% of ATP.

• Substrate-Level Phosphorylation: Enzyme transfers phosphate directly from substrate to ADP.

ATP Yield

• Up to 32 ATP molecules produced per glucose molecule degraded to CO2 and H2O.

Glycolysis

Phases of Glycolysis

Glycolysis occurs in the cytoplasm and consists of two phases:

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

• Energy Payoff Phase: 4 ATP are produced, 2 NAD+ are reduced to NADH, and sugars are oxidized to form 2 pyruvate and 2 H2O.

• Net ATP: 2 ATP produced by substrate-level phosphorylation.

Key Features

• All carbon from glucose is found in pyruvate.

• No CO2 released; occurs with or without O2.

• Most energy remains in pyruvate after glycolysis.

Pyruvate Oxidation and Citric Acid Cycle

Pyruvate Oxidation

Pyruvate is converted to acetyl CoA before entering the citric acid cycle.

• Pyruvate Dehydrogenase: Catalyzes oxidation, reduction of NAD+, and combination with coenzyme A.

• Products: CO2, NADH, acetyl CoA.

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle completes the breakdown of glucose, generating energy carriers and releasing CO2.

• Per Cycle: 1 ATP, 3 NADH, 1 FADH2, 2 CO2 (per pyruvate).

• Cycle Runs Twice: For each glucose molecule (2 pyruvate).

• Eight Steps: Each catalyzed by a specific enzyme; acetyl CoA combines with oxaloacetate to form citrate, which is decomposed back to oxaloacetate.

• NADH and FADH2: Carry electrons to the electron transport chain.

Oxidative Phosphorylation

Electron Transport Chain

The electron transport chain is embedded in the inner mitochondrial membrane (cristae) in eukaryotes and plasma membrane in prokaryotes.

• Structure: Multiprotein complexes, including cytochromes (proteins with heme groups).

• Function: Electrons from NADH and FADH2 are passed through carriers, dropping in free energy and finally reducing O2 to H2O.

• No Direct ATP: Chain itself does not produce ATP.

Chemiosmosis and ATP Synthase

Energy released by the electron transport chain pumps H+ into the intermembrane space, creating a proton gradient.

• ATP Synthase: H+ flows back into the matrix, spinning ATP synthase and catalyzing ADP phosphorylation.

• Chemiosmosis: Use of H+ gradient to drive ATP synthesis.

• Proton-Motive Force: The H+ gradient's capacity to do work.

Energy Flow and ATP Accounting

• Energy flows: glucose → NADH → electron transport chain → proton-motive force → ATP.

• About 34% of glucose energy is transferred to ATP (~32 ATP per glucose); rest lost as heat.

• Exact ATP yield varies due to coupling inefficiencies, electron carrier differences, and other uses of proton-motive force.

Fermentation and Anaerobic Respiration

ATP Production Without Oxygen

When oxygen is unavailable, cells use fermentation or anaerobic respiration to produce ATP.

• Anaerobic Respiration: Uses electron transport chain with a final electron acceptor other than O2 (e.g., sulfate ion SO42-).

• Fermentation: Extension of glycolysis; regenerates NAD+ by transferring electrons to pyruvate or derivatives.

Types of Fermentation

• Alcohol Fermentation: Pyruvate → ethanol; releases CO2, regenerates NAD+. Used in brewing, winemaking, baking.

• Lactic Acid Fermentation: Pyruvate → lactate; no CO2 released. Used in cheese, yogurt production.

Comparing Fermentation, Anaerobic, and Aerobic Respiration

Obligate vs. Facultative Anaerobes

• Obligate Anaerobes: Cannot survive in O2; use fermentation or anaerobic respiration.

• Facultative Anaerobes: Can use either fermentation or cellular respiration; pyruvate is a metabolic fork.

Evolutionary Significance and Metabolic Connections

Evolution of Glycolysis

Glycolysis is an ancient, widespread metabolic pathway, used by early prokaryotes before atmospheric oxygen was present.

• Functions in both fermentation and cellular respiration.

• Represents a metabolic heirloom from early cells.

Catabolic and Anabolic Pathways

Glycolysis and the citric acid cycle are central to many metabolic pathways, connecting catabolism and anabolism.

• Catabolic Pathways: Funnel electrons from various organic molecules into cellular respiration.

• Carbohydrates: Glycolysis can use starch, glycogen, and disaccharides.

• Proteins: Digested to amino acids; amino groups removed (deamination), nitrogenous waste excreted as ammonia, urea, etc.

• Fats: Digested to glycerol (for glycolysis) and fatty acids (beta oxidation yields acetyl CoA, NADH, FADH2); fats yield more ATP per gram than carbohydrates.

Biosynthesis (Anabolic Pathways)

• Small molecules from food, glycolysis, or citric acid cycle are used to build macromolecules (e.g., proteins from amino acids).

Regulation of Cellular Respiration

Feedback Mechanisms

Cellular respiration is regulated by feedback inhibition to prevent wasteful production and maintain energy balance.

• ATP Concentration: Low ATP speeds up respiration; high ATP slows it down.

• Enzyme Regulation: Catabolism is controlled by regulating enzyme activity at strategic points in the pathway.

Additional info: Figures referenced (e.g., 9.1b, 9.4, 9.7, etc.) are not included but would typically illustrate pathways, structures, and mechanisms described above.