Stages of Aerobic Respiration

Glycolysis

Glycolysis is the first stage of aerobic respiration, occurring in the cytoplasm. It involves the breakdown of glucose into pyruvate.

Example: Glucose $C₆$ is converted to two pyruvate $C₃$ molecules through a series of enzyme-catalyzed reactions.

Key points of glycolysis:

• Produces a net gain of 2 ATP through substrate-level phosphorylation
• Generates 2 NADH molecules
• Does not require oxygen

Vocabulary: Substrate-level phosphorylation is the direct transfer of a phosphate group from a high-energy molecule to ADP, forming ATP.

Bridge Reaction

The bridge reaction, also known as the link reaction, occurs in the mitochondrial matrix. It connects glycolysis to the Krebs cycle.

Key points of the bridge reaction:

• Converts pyruvate $C₃$ to acetyl-CoA $C₂$
• Releases CO₂
• Produces NADH

Highlight: The bridge reaction is a crucial step in preparing the products of glycolysis for entry into the Krebs cycle.

Krebs Cycle and Electron Transport Chain

Krebs Cycle

The Krebs cycle, also known as the citric acid cycle, takes place in the mitochondrial matrix. It is a series of chemical reactions that complete the oxidation of glucose.

Key points of the Krebs cycle:

• Acetyl-CoA enters the cycle and is completely oxidized
• Produces CO₂, which is released as a waste product
• Generates NADH and FADH₂, which carry electrons to the electron transport chain
• Produces one ATP per cycle through substrate-level phosphorylation

Example: The Krebs cycle involves intermediates such as citrate, α-ketoglutarate, and succinate.

Electron Transport Chain

The electron transport chain is the final stage of aerobic respiration, occurring in the inner mitochondrial membrane.

Key points of the electron transport chain:

• Electrons from NADH and FADH₂ are passed through a series of electron carriers
• Oxygen serves as the final electron acceptor, forming water
• Energy released during electron transfer is used to pump protons into the intermembrane space, creating a proton gradient
• ATP synthase uses the proton gradient to produce ATP through oxidative phosphorylation

Vocabulary: Oxidative phosphorylation is the process of ATP production driven by the electron transport chain and proton gradient.

Energy Yield of Aerobic Respiration

The total ATP yield from the complete oxidation of one glucose molecule through aerobic respiration is approximately 30-32 ATP.

Breakdown of ATP production:

• Glycolysis: 2 ATP $net$
• Krebs cycle: 2 ATP
• Electron transport chain: 26-28 ATP

Highlight: Aerobic respiration is highly efficient, producing significantly more ATP than anaerobic processes.

Anaerobic Respiration and Fermentation

Anaerobic respiration occurs in the absence of oxygen and is less efficient than aerobic respiration.

Types of anaerobic organisms:

• Obligate anaerobes: cannot tolerate oxygen
• Facultative anaerobes: can switch between aerobic and anaerobic respiration

Fermentation is a type of anaerobic respiration that occurs in the cytoplasm.

Key points of fermentation:

• Begins with glycolysis, producing 2 ATP
• Regenerates NAD+ to allow glycolysis to continue
• Two main types: alcoholic fermentation and lactic acid fermentation

Example: Alcoholic fermentation, used by yeast, produces ethanol and CO₂ as end products.

Comparison of Aerobic and Anaerobic Respiration

Aerobic Respiration:

• Occurs in most organisms
• Uses oxygen as the final electron acceptor
• Takes place in the cytoplasm and mitochondria
• Produces 30-32 ATP per glucose molecule

Anaerobic Respiration $Fermentation$:

• Occurs in some bacteria, fungi, and protists
• Uses organic molecules as the final electron acceptor
• Takes place in the cytoplasm
• Produces 2 ATP per glucose molecule

Highlight: While less efficient, anaerobic respiration allows organisms to survive in oxygen-poor environments and plays important roles in various industrial and biological processes.

Lactic Acid Fermentation

This section focuses on lactic acid fermentation, particularly in muscle tissue.

Example: Lactic acid fermentation occurs in muscle cells during intense exercise when oxygen is limited.

Highlight: The process helps regenerate NAD+ to allow continued glycolysis.

Other Metabolic Processes

The page covers additional metabolic pathways including gluconeogenesis and fat metabolism.

Definition: Gluconeogenesis is the formation of glucose from non-carbohydrate sources.

Highlight: Fat breakdown occurs when carbohydrate sources are depleted.

Aerobic Respiration Overview

Aerobic respiration is a multi-step process that breaks down glucose in the presence of oxygen to produce energy in the form of ATP. This process occurs in both the cytoplasm and mitochondria of cells.

Highlight: The four main stages of aerobic respiration are glycolysis, the bridge reaction, the Krebs cycle, and the electron transport chain.

The key components required for aerobic respiration include:

• Glucose as an energy substrate
• Appropriate enzymes
• ADP and inorganic phosphate
• Mitochondria
• Oxygen as the final electron acceptor

Vocabulary: Mitochondria are semi-autonomous organelles with their own DNA and ribosomes for protein synthesis.

Mitochondrial structure:

• Outer membrane: smooth and permeable
• Inner membrane: folded and impermeable, forming cristae
• Matrix: internal space containing enzymes and DNA

Definition: Cristae are the folded inner membranes of mitochondria that increase surface area for ATP production.