Cellular Respiration and Energy Metabolism

Introduction to Cellular Respiration

Cellular respiration is a fundamental process in biology, occurring in the mitochondria of cells in animals, plants, and other organisms. It involves breaking down organic molecules to generate ATP, carbon dioxide, water, and heat. This process is essential for providing energy required for cellular activities.

• Cellular respiration: Occurs in mitochondria; breaks down organic molecules to produce ATP.

• Energy flow: Unidirectional; chemicals are recycled in cycles involving photosynthesis and cellular respiration.

• Photosynthesis: Uses light energy to produce organic molecules and oxygen, which are consumed by organisms.

• Cycle of photosynthesis and respiration: Photosynthesis generates organic molecules and oxygen; cellular respiration uses these to produce CO2 and water, which are then used again in photosynthesis.

Key Concepts

• Catabolic pathways: Yield energy by oxidizing organic fuels.

• Glycolysis: Harvests chemical energy by oxidizing glucose to pyruvate.

• Citric acid cycle: Completes the energy-yielding oxidation of organic molecules after pyruvate is oxidized.

• Oxidative phosphorylation: Chemiosmosis couples electron transport to ATP synthesis.

• Fermentation and anaerobic respiration: Enable cells to produce ATP without oxygen.

• Glycolysis and citric acid cycle: Connect to many other metabolic pathways.

Exergonic and Endergonic Reactions

Energy in Chemical Reactions

Exergonic reactions release free energy, while endergonic reactions require an input of energy. ATP acts as an intermediary to store and transfer energy for cellular work.

• Exergonic reactions: Net release of free energy.

• Endergonic reactions: Require energy input from surroundings.

• ATP in metabolism: ATP stores and releases energy by adding or removing phosphate groups.

Electron Transfer and Redox Reactions

Potential Energy and Electronegativity

Electrons farther from the nucleus have greater potential energy. Electronegativity differences between atoms determine the type of covalent bond formed.

• Nonpolar covalent bonds: Atoms have similar electronegativities; electrons are shared equally.

• Polar covalent bonds: Atoms have different electronegativities; electrons are shared unequally.

• Redox reactions: Involve the transfer of electrons from one reactant to another.

Redox Reaction Example

• Oxidized compound: Loses electrons.

• Reduced compound: Gains electrons.

• Example: Sodium (Na) and chlorine (Cl) form table salt through electron transfer.

Catabolic Pathways and ATP Production

Overview of Catabolic Pathways

Catabolic pathways break down organic molecules, releasing energy stored in chemical bonds. This energy is used to synthesize ATP, the universal energy currency of the cell.

• Exergonic reactions: Compounds act as fuels.

• Enzymes: Facilitate degradation of complex molecules.

• Fermentation: Partial degradation of sugars without oxygen.

• Aerobic respiration: Consumes oxygen and organic fuel; most efficient catabolic pathway.

• Anaerobic respiration: Uses substances other than oxygen as electron acceptors.

• Combustion analogy: Similar to gasoline combustion in engines.

Fuel Sources and Glucose Degradation

• Fuel sources: Carbohydrates, fats, proteins.

• Glucose degradation: Breakdown of glucose is exergonic.

• ATP regeneration: ATP is regenerated from ADP and Pi.

• Oxidation and reduction: Fundamental to cellular respiration.

Stages of Cellular Respiration

Overview of Stages

Cellular respiration consists of three main stages: glycolysis, pyruvate oxidation and the citric acid cycle, and oxidative phosphorylation.

• Glycolysis: Occurs in the cytosol; breaks down glucose into two pyruvate molecules, producing ATP and NADH.

• Pyruvate oxidation: Occurs in mitochondria; pyruvate is converted to acetyl-CoA.

• Citric acid cycle (Krebs cycle): Completes breakdown of glucose to CO2, producing ATP, NADH, and FADH2.

• Oxidative phosphorylation: Electron transport chain and chemiosmosis synthesize ATP.

ATP Yield and Substrate-Level Phosphorylation

• Substrate-level phosphorylation: Direct transfer of phosphate group to ADP to form ATP.

• Oxidative phosphorylation: Produces about 90% of ATP in cellular respiration.

• Each glucose molecule yields approximately 32 molecules of ATP.

Glycolysis

Phases and Enzymes of Glycolysis

Glycolysis is the process of splitting glucose into two three-carbon sugars, which are then oxidized to form two molecules of pyruvate. It occurs in the cytoplasm and does not require oxygen.

• Energy Investment Phase: 2 ATP are used.

• Energy Payoff Phase: 4 ATP and 2 NADH are produced.

• Net Energy Yield: 2 ATP and 2 NADH per glucose.

Key Steps and Enzymes

• Hexokinase: Transfers phosphate from ATP to glucose.

• Phosphoglucose isomerase: Converts glucose-6-phosphate to fructose-6-phosphate.

• Phosphofructokinase: Transfers phosphate from ATP to fructose-6-phosphate.

• Aldolase: Splits fructose-1,6-bisphosphate into two three-carbon sugars.

• Triose phosphate dehydrogenase: Catalyzes oxidation of G3P to 1,3-bisphosphoglycerate, reducing NAD+ to NADH.

• Phosphoglycerokinase: Transfers phosphate from 1,3-bisphosphoglycerate to ADP.

• Phosphoglyceromutase: Converts 3-phosphoglycerate to 2-phosphoglycerate.

• Enolase: Converts 2-phosphoglycerate to phosphoenolpyruvate (PEP).

• Pyruvate kinase: Transfers phosphate from PEP to ADP, forming ATP and pyruvate.

Inputs and Outputs

• Inputs: Glucose, 2 ATP, 2 NAD+

• Outputs: 2 pyruvate, 2 H2O, 2 ATP (net), 2 NADH

Pyruvate Oxidation and Citric Acid Cycle

Oxidation of Pyruvate to Acetyl CoA

Pyruvate enters the mitochondrion via active transport and is converted to acetyl-CoA, which enters the citric acid cycle. This conversion involves decarboxylation, oxidation, and transfer of the acetyl group to CoA.

• Step 1: Pyruvate is decarboxylated, releasing CO2.

• Step 2: Remaining two-carbon fragment is oxidized, reducing NAD+ to NADH.

• Step 3: Coenzyme A attaches to the two-carbon fragment, forming acetyl-CoA.

The Citric Acid Cycle (Krebs Cycle)

The citric acid cycle further oxidizes acetyl-CoA, releasing CO2, and transferring electrons to NAD+ and FAD. It produces ATP (or GTP), NADH, and FADH2 for use in the electron transport chain.

• For each acetyl-CoA entering the cycle:

• Cycle steps: Acetyl-CoA combines with oxaloacetate to form citrate; citrate is converted to isocitrate, then oxidized to alpha-ketoglutarate, succinyl-CoA, fumarate, malate, and back to oxaloacetate.

Oxidative Phosphorylation and Electron Transport Chain

Electron Transport Chain (ETC)

The ETC is located in the inner mitochondrial membrane (eukaryotes) or plasma membrane (prokaryotes). It consists of protein complexes that transfer electrons from NADH and FADH2 to oxygen, generating a proton gradient used to synthesize ATP.

• Electrons are shuttled by NADH to the higher-energy end of the chain.

• At the lower-energy end, electrons and hydrogen combine to form water.

• ATP synthase uses the proton gradient to generate ATP from ADP and Pi.

ATP Yield from Cellular Respiration

• Each glucose molecule yields up to 32 ATP molecules.

• Most ATP is produced during oxidative phosphorylation.

Fermentation and Anaerobic Respiration

Fermentation

Fermentation enables cells to produce ATP without oxygen by using glycolysis and regenerating NAD+ from NADH. It yields less ATP than aerobic respiration.

• Lactic acid fermentation: Pyruvate is reduced to lactate.

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

• Used by yeast in brewing, winemaking, and baking.

Anaerobic Respiration

• Uses electron acceptors other than oxygen (e.g., sulfate, nitrate).

• Occurs in some prokaryotes.

Summary Table: Comparison of Aerobic Respiration, Anaerobic Respiration, and Fermentation

Additional info:

• Some details about enzyme names and steps in glycolysis and the citric acid cycle were expanded for clarity.

• ATP yield numbers are approximate and may vary depending on cell type and conditions.