Cellular Respiration: Harvesting Energy

Learning Objectives

Differentiate between aerobic and anaerobic respiration.

Outline the three main stages of aerobic respiration: glycolysis, the Krebs cycle, and oxidative phosphorylation.

Explain the roles of ATP, NADH, and FADH2 as energy carriers.

Identify the cellular locations for each major stage.

Harvesting Energy from Glucose

Cellular respiration is the set of metabolic reactions and processes that take place in the cells of organisms to convert chemical energy from nutrients into adenosine triphosphate (ATP), and then release waste products. The overall reaction for aerobic respiration is:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP

Stage 1: Glycolysis

This is the initial, anaerobic (does not require oxygen) breakdown of glucose.

Location: Cytoplasm.

Process: One molecule of glucose (a 6-carbon sugar) is split into two molecules of pyruvate (a 3-carbon compound).

Net Products: 2 ATP and 2 NADH.

NADH is a high-energy electron carrier that will be used later to generate more ATP.

Stage 2: The Krebs Cycle (Citric Acid Cycle)

If oxygen is present, the pyruvate from glycolysis enters the mitochondria.

Location: Mitochondrial matrix.

Process: Before the cycle begins, each pyruvate is converted into Acetyl-CoA, releasing one molecule of CO₂ and producing one NADH. The Acetyl-CoA then enters the Krebs Cycle, a series of reactions that completely oxidizes the carbon atoms.

Products (per turn, from one Acetyl-CoA): 2 CO₂, 1 ATP, 3 NADH, and 1 FADH₂.

Since one glucose molecule yields two pyruvates, the cycle turns twice per glucose.

Stage 3: Oxidative Phosphorylation

This is the final stage and where the vast majority of ATP is produced. It consists of two parts: the Electron Transport Chain (ETC) and Chemiosmosis.

Location: Inner mitochondrial membrane.

Process:

1.ETC: High-energy electrons from NADH and FADH₂ are passed down a series of protein complexes. As electrons move, they release energy, which is used to pump protons (H⁺) from the matrix into the intermembrane space, creating a steep electrochemical gradient. Oxygen acts as the final electron acceptor, combining with protons to form water.

2.Chemiosmosis: The protons flow back down their gradient through an enzyme called ATP Synthase. This flow powers the enzyme, which phosphorylates ADP to create large amounts of ATP.

Net Product: Approximately 28-32 ATP molecules.

Anaerobic Respiration (Fermentation)

When oxygen is not available, cells can undergo fermentation to regenerate the NAD⁺ needed for glycolysis to continue. This produces only the 2 ATP from glycolysis. Common types are lactic acid fermentation (in muscle cells) and alcoholic fermentation (in yeast).

Key Terms

Glycolysis: The anaerobic metabolic pathway in the cytoplasm that breaks down one molecule of glucose into two molecules of pyruvate, producing a net of 2 ATP and 2 NADH.
Krebs Cycle (Citric Acid Cycle): A series of chemical reactions in the mitochondrial matrix that oxidizes Acetyl-CoA to carbon dioxide, generating ATP, NADH, and FADH₂.
Oxidative Phosphorylation: The final stage of aerobic respiration, occurring on the inner mitochondrial membrane, where the energy from NADH and FADH₂ is used to create a proton gradient that powers ATP synthase to produce the majority of a cell's ATP.
ATP Synthase: A membrane-embedded enzyme that uses the energy of a proton gradient (chemiosmosis) to synthesize ATP from ADP and inorganic phosphate.