Metabolism and Cellular Respiration

Overview of Metabolism

Metabolism refers to the sum of all chemical reactions occurring within an organism. These reactions are essential for maintaining life, providing energy, and building cellular components.

• Anabolic reactions: Build complex molecules from simpler ones (e.g., protein synthesis).

• Catabolic reactions: Break down complex molecules into simpler ones, releasing energy (e.g., digestion).

• Example of catabolism: Digestion involves breaking down food molecules to release energy.

Fermentation vs. Aerobic Respiration

Cells can produce ATP through different processes depending on oxygen availability. Fermentation and aerobic respiration are two major pathways.

• Fermentation: Occurs in anaerobic (no oxygen) environments; produces ATP less efficiently than aerobic respiration.

• End product of fermentation: Lactic acid, which can cause muscle cramps and acidify cells.

• Aerobic respiration: More efficient; uses oxygen to fully break down glucose, producing more ATP.

• Limitation of fermentation: Buildup of lactic acid and limited energy yield.

ATP: The Energy Currency of the Cell

Role and Production of ATP

Adenosine triphosphate (ATP) is the primary energy carrier in cells. It powers various cellular activities, including biosynthesis, transport, and mechanical work.

• ATP in anabolic reactions: Used to build complex molecules from simpler ones.

• ATP in transport: Powers active transport and movement within cells.

• ATP in mechanical work: Drives muscle contraction and other forms of cellular movement.

ATP Production Pathways

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

• Krebs Cycle (Citric Acid Cycle): Occurs in the mitochondrial matrix; further oxidizes pyruvate, producing CO2, ATP, NADH, and FADH2.

• Electron Transport Chain (ETC): Located in the inner mitochondrial membrane; uses electrons from NADH and FADH2 to produce ATP via oxidative phosphorylation.

Redox Reactions in Cellular Respiration

Oxidation and Reduction

Redox reactions are central to energy production in cells. They involve the transfer of electrons between molecules.

• Oxidation: Loss of electrons (or hydrogen atoms).

• Reduction: Gain of electrons (or hydrogen atoms).

• NAD+ to NADH: NAD+ is reduced to NADH during glycolysis and the Krebs cycle.

• FAD to FADH2: FAD is reduced to FADH2 in the Krebs cycle.

Electron Carriers

• NAD+, NADH, FAD, FADH2: These molecules shuttle electrons to the ETC, where their energy is used to produce ATP.

Glycolysis, Krebs Cycle, and Electron Transport Chain

Glycolysis

Glycolysis is the first step in cellular respiration, occurring in the cytoplasm. It breaks down glucose into pyruvate, generating ATP and NADH.

• Location: Cytoplasm

• Products: 2 ATP, 2 NADH, 2 pyruvate per glucose molecule

Krebs Cycle

The Krebs cycle further oxidizes pyruvate, producing electron carriers and CO2.

• Location: Mitochondrial matrix

• Products: CO2, ATP, NADH, FADH2

Electron Transport Chain (ETC)

The ETC uses electrons from NADH and FADH2 to create a proton gradient, driving ATP synthesis.

• Location: Inner mitochondrial membrane

• Process: Electrons are passed from carrier to carrier, pumping protons across the membrane.

• ATP synthesis: Protons flow back through ATP synthase, generating ATP.

• Potential maximum ATP: Not always reached due to proton leak and membrane inefficiencies.

Summary Table: Cellular Respiration Pathways

Key Equations

• Overall equation for aerobic respiration:

• ATP synthesis via oxidative phosphorylation:

• Redox reaction example:

Additional info:

• Proton leak across the mitochondrial membrane can reduce the efficiency of ATP production, resulting in heat generation (thermogenesis).

• Substrate-level phosphorylation occurs in glycolysis and the Krebs cycle, while oxidative phosphorylation occurs in the ETC.