Cellular Respiration: Obtaining Energy from Food

Introduction to Cellular Respiration

Cellular respiration is a fundamental metabolic process by which cells extract energy from organic molecules, primarily glucose, to produce adenosine triphosphate (ATP), the energy currency of the cell. This process is essential for the survival of almost all living organisms.

• Definition: Cellular respiration is the aerobic harvesting of chemical energy from organic fuel molecules.

• Main Purpose: To convert the chemical energy stored in food into ATP, which powers cellular activities.

• Location: In eukaryotic cells, most steps occur in the mitochondria.

Energy Flow and Chemical Cycling in Ecosystems

Life on Earth depends on the flow of energy and cycling of chemicals, with most energy originating from the sun.

• Photosynthesis: Plants (autotrophs) convert sunlight into chemical energy stored in sugars.

• Producers vs. Consumers:

  • Producers (Autotrophs): Organisms that make their own organic matter from inorganic nutrients (e.g., plants).

  • Consumers (Heterotrophs): Organisms that obtain food by eating other organisms (e.g., animals, humans).

• Chemical Cycling: The products and reactants of photosynthesis and cellular respiration cycle between plants and animals.

Overview of Cellular Respiration

Cellular respiration consists of a series of enzyme-catalyzed reactions that break down glucose and other organic molecules to produce ATP.

• Overall Equation:

• Main Stages:

  1. Glycolysis

  2. Citric Acid Cycle (Krebs Cycle)

  3. Electron Transport Chain

• ATP Yield: Up to 32 ATP molecules per glucose molecule.

Stage 1: Glycolysis

Glycolysis is the first step in cellular respiration, occurring in the cytoplasm. It splits one glucose molecule (6 carbons) into two molecules of pyruvic acid (3 carbons each).

• Energy Investment: Requires 2 ATP molecules to start.

• Energy Payoff: Produces 4 ATP molecules (net gain of 2 ATP) and 2 NADH molecules.

• Does not require oxygen (anaerobic).

Stage 2: The Citric Acid Cycle (Krebs Cycle)

The citric acid cycle completes the breakdown of glucose, releasing CO2 and transferring electrons to carrier molecules.

• Preparation: Pyruvic acid is converted to acetic acid, which combines with coenzyme A to form acetyl CoA.

• Cycle Steps: Acetyl CoA enters the cycle, joining a 4-carbon molecule to form citric acid. The cycle releases 2 CO2 per acetyl CoA and generates NADH, FADH2, and a small amount of ATP.

• Location: Mitochondrial matrix.

Stage 3: Electron Transport Chain and ATP Synthesis

The electron transport chain (ETC) is a series of proteins embedded in the inner mitochondrial membrane. Electrons from NADH and FADH2 are transferred through the chain, releasing energy used to produce ATP.

• Oxygen's Role: Final electron acceptor, forming water.

• ATP Synthase: Enzyme complex that uses the energy from the ETC to synthesize ATP from ADP and inorganic phosphate.

• Most ATP is produced in this stage.

Summary Table: Stages of Cellular Respiration

Fermentation: Anaerobic Harvest of Food Energy

When oxygen is not available, cells can produce ATP through fermentation, which relies on glycolysis.

• Lactic Acid Fermentation: In human muscle cells, pyruvic acid is converted to lactic acid, regenerating NAD+ for glycolysis.

• Alcoholic Fermentation: In yeast, pyruvic acid is converted to ethyl alcohol and CO2.

• ATP Yield: Only 2 ATP per glucose (from glycolysis).

Comparing Aerobic and Anaerobic Respiration

Evolutionary Perspective: Glycolysis as an Ancient Pathway

Glycolysis is found in nearly all living organisms and does not require membrane-bound organelles, suggesting it evolved early in the history of life, before significant oxygen was present in Earth's atmosphere.

• Significance: Indicates glycolysis is a universal and ancient energy-harvesting process.

Key Terms and Definitions

• ATP (Adenosine Triphosphate): The main energy carrier in cells.

• NAD+ (Nicotinamide Adenine Dinucleotide): Electron carrier that becomes NADH when reduced.

• FAD (Flavin Adenine Dinucleotide): Electron carrier that becomes FADH2 when reduced.

• Pyruvic Acid: Three-carbon compound produced by glycolysis.

• Acetyl CoA: Two-carbon molecule attached to coenzyme A, enters the citric acid cycle.

• Fermentation: Anaerobic process that allows glycolysis to continue by regenerating NAD+.

Applications and Examples

• Muscle Fatigue: During intense exercise, muscles may switch to lactic acid fermentation, leading to lactic acid buildup.

• Baking and Brewing: Yeast fermentation produces CO2 and alcohol, used in bread and alcoholic beverages.

Major Themes Illustrated

• Pathways that Transform Energy and Matter: Cellular respiration and photosynthesis are linked cycles in ecosystems.

• Structure and Function: Mitochondrial structure (folded membranes) supports efficient ATP production.

• Interactions within Biological Systems: Metabolic pathways intersect to maintain balanced metabolism.