Cellular Respiration & Metabolism

Introduction to Cellular Respiration

Cellular respiration is a fundamental process by which cells extract energy from organic molecules, primarily glucose, to produce ATP, the universal energy currency. This process involves a series of biochemical reactions that occur in specific cellular locations and is essential for the survival of all aerobic organisms.

• Cellular respiration includes glycolysis, pyruvate processing, the citric acid cycle, and oxidative phosphorylation.

• ATP (Adenosine Triphosphate) is the main energy carrier in cells.

• Metabolic pathways are interconnected networks of chemical reactions that manage energy and matter in cells.

Structure and Function of ATP

ATP is ideally suited to its role as the cell's energy currency due to its molecular structure, which allows for rapid energy release upon hydrolysis.

• ATP Structure: Consists of adenine (a nitrogenous base), ribose (a five-carbon sugar), and three phosphate groups.

• High-energy bonds: The bonds between phosphate groups, especially the terminal phosphate, store significant potential energy.

• ATP Hydrolysis: When ATP is hydrolyzed to ADP and inorganic phosphate (), a large amount of energy is released for cellular work.

• Role in Metabolism: ATP provides energy for anabolic reactions (building macromolecules) and powers cellular processes.0

Example: Muscle contraction, active transport, and biosynthesis all require ATP.

Overview of Metabolic Pathways

Cells use metabolic pathways to break down macromolecules and extract energy, as well as to build complex molecules from simpler ones.

• Catabolism: Breakdown of large molecules (e.g., glucose) to release energy.

• Anabolism: Synthesis of complex molecules from smaller subunits, requiring energy input.

• Central Metabolism: Includes glycolysis, citric acid cycle, and oxidative phosphorylation.

Additional info: The provided schematic map illustrates the complexity and interconnectedness of cellular metabolic reactions.

Main Steps of Cellular Respiration

1. Glycolysis

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

• Location: Cytosol

• Inputs: Glucose, 2 NAD+, 2 ADP, 2 Pi

• Outputs: 2 Pyruvate, 2 NADH, 2 ATP (net)

• Purpose: To extract energy from glucose and produce intermediates for further metabolism.

Equation:

2. Pyruvate Processing (Link Reaction)

Pyruvate produced in glycolysis is transported into the mitochondria and converted to Acetyl-CoA, releasing CO2 and generating NADH.

• Location: Mitochondrial matrix

• Inputs: 2 Pyruvate, 2 NAD+, 2 CoA

• Outputs: 2 Acetyl-CoA, 2 NADH, 2 CO2

Equation:

3. Citric Acid Cycle (Krebs Cycle)

The citric acid cycle completes the oxidation of glucose derivatives, producing NADH, FADH2, ATP, and CO2.

• Location: Mitochondrial matrix

• Inputs: 2 Acetyl-CoA, 6 NAD+, 2 FAD, 2 ADP, 2 Pi

• Outputs: 4 CO2, 6 NADH, 2 FADH2, 2 ATP

Equation:

4. Electron Transport Chain & Chemiosmosis

NADH and FADH2 donate electrons to the electron transport chain (ETC) in the inner mitochondrial membrane, driving ATP synthesis via chemiosmosis.

• Location: Inner mitochondrial membrane (cristae)

• Inputs: NADH, FADH2, O2, ADP, Pi

• Outputs: ATP, H2O, NAD+, FAD

• Purpose: To generate a proton gradient that powers ATP synthase.

Equation:

Overall Reaction of Aerobic Cellular Respiration

The complete oxidation of glucose in the presence of oxygen yields carbon dioxide, water, and a large amount of ATP.

• Overall Equation:

Comparing Aerobic Respiration and Fermentation

Aerobic Respiration

Aerobic respiration uses oxygen as the terminal electron acceptor, allowing for complete oxidation of glucose and maximal ATP yield.

• Terminal Electron Acceptor: Oxygen (O2)

• ATP Yield: High (~30-32 ATP per glucose)

• Location: Cytosol and mitochondria

Fermentation

Fermentation occurs when oxygen is unavailable. It allows glycolysis to continue by regenerating NAD+, but yields much less ATP.

• Terminal Electron Acceptor: Organic molecules (e.g., pyruvate or acetaldehyde)

• ATP Yield: Low (2 ATP per glucose)

• Location: Cytosol

• Purpose: To regenerate NAD+ for glycolysis

Types of Fermentation

Example: Muscle cells perform lactic acid fermentation during intense exercise when oxygen is scarce.

Cellular Locations of Metabolic Processes

Key Locations in Eukaryotic Cells

Different steps of cellular respiration occur in distinct cellular compartments, optimizing efficiency and regulation.

• Glycolysis: Cytosol

• Pyruvate Processing & Citric Acid Cycle: Mitochondrial matrix

• Electron Transport Chain & Chemiosmosis: Inner mitochondrial membrane (cristae)

Additional info: The compartmentalization of metabolic pathways is crucial for cellular function and energy production.

Endosymbiotic Theory

Origin of Mitochondria and Chloroplasts

The endosymbiotic theory proposes that mitochondria and chloroplasts originated as free-living bacteria that were engulfed by ancestral eukaryotic cells.

• Mitochondria: Found in most eukaryotes; site of cellular respiration.

• Chloroplasts: Found in photosynthetic eukaryotes; site of photosynthesis.

• Evidence: Both organelles have their own DNA, double membranes, and similarities to bacteria.

Example: Mitochondria are essential for ATP production in animal cells, while chloroplasts enable plants to convert light energy into chemical energy.

Summary Table: Aerobic Respiration vs. Fermentation

Key Takeaways

• Cellular respiration is essential for energy production in cells.

• ATP is the primary energy carrier, and its structure enables rapid energy release.

• Aerobic respiration yields much more ATP than fermentation.

• Cellular compartmentalization enhances metabolic efficiency.

• The endosymbiotic theory explains the origin of mitochondria and chloroplasts.