Chapter 7: How Do We Get Energy from the Food We Ingest?

Overview of Energy Flow in Biological Systems

Energy enters ecosystems as sunlight and exits as heat, while essential elements are recycled. Organisms extract energy from organic molecules through metabolic processes, primarily cellular respiration, to produce ATP, the universal energy currency.

• Photosynthesis in chloroplasts converts sunlight, water, and carbon dioxide into organic molecules and oxygen.

• Cellular respiration in mitochondria breaks down organic molecules, releasing energy as ATP and heat, and producing water and carbon dioxide.

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

Metabolism: Catabolic and Anabolic Pathways

Metabolism encompasses all chemical reactions in living organisms, including breaking down molecules to release energy (catabolism) and building new molecules (anabolism).

• Catabolism: Breakdown of complex molecules into simpler ones, releasing energy.

• Anabolism: Synthesis of complex molecules from simpler ones, requiring energy.

• Organic molecules (carbohydrates, proteins, fats) are digested and their components used for energy or biosynthesis.

Key Metabolic Pathways

• Carbohydrates (e.g., sucrose) are hydrolyzed to glucose, which enters glycolysis.

• Amino acids (from proteins) can be converted to intermediates of glycolysis and the citric acid cycle.

• Fats are digested to glycerol and fatty acids; glycerol enters glycolysis, fatty acids are broken into 2-carbon fragments for the citric acid cycle.

ATP: The Energy Carrier

ATP (Adenosine Triphosphate) is the primary molecule for storing and transferring energy in cells.

• Structure: ATP consists of an adenine base, a ribose sugar, and three phosphate groups.

• Energy is stored in the high-energy phosphate bonds; breaking these bonds releases energy for cellular work.

ATP Structure:

• Adenine: Nitrogenous base

• Ribose: Five-carbon sugar

• Phosphate groups: Three linked phosphates

Cellular Respiration: Extracting Energy from Food

Cellular respiration is the process by which cells release energy from organic molecules to produce ATP. It involves a series of metabolic reactions, including glycolysis, the citric acid cycle, and oxidative phosphorylation.

• General Equation for Cellular Respiration:

• Glucose is oxidized to carbon dioxide and water, releasing energy.

• Energy is harnessed through stepwise electron transfers (REDOX reactions).

• Electrons move from high-energy C-H bonds to lower-energy C-O bonds.

REDOX Reactions in Cellular Respiration

• Oxidation: Loss of electrons (LEO: Lose Electrons = Oxidation)

• Reduction: Gain of electrons (GER: Gain Electrons = Reduction)

• Electron carriers (e.g., NAD+, FAD) shuttle electrons between reactions.

Stages of Cellular Respiration

Cellular respiration occurs in both prokaryotic and eukaryotic cells, with some steps localized in the cytoplasm and others in mitochondria.

• Glycolysis: Occurs in the cytosol; breaks down glucose into pyruvate.

• Pyruvate Oxidation: Converts pyruvate to acetyl CoA.

• Citric Acid Cycle (Krebs Cycle): Completes the breakdown of glucose, producing NADH and FADH2.

• Oxidative Phosphorylation: Uses electron transport chain and chemiosmosis to generate ATP.

Electron Transport Chain and Chemiosmosis

• Electrons from NADH and FADH2 are transferred through protein complexes in the inner mitochondrial membrane.

• Proton gradient is established across the membrane.

• ATP synthase uses the proton-motive force to synthesize ATP from ADP and inorganic phosphate.

Aerobic vs Anaerobic Respiration

Cellular respiration can occur with or without oxygen.

• Aerobic respiration: Oxygen is the final electron acceptor; produces the most ATP.

• Anaerobic respiration: Uses alternative electron acceptors (e.g., sulfur); less efficient.

• Fermentation: Occurs when oxygen is absent; produces ATP via glycolysis only, followed by conversion of pyruvate to lactic acid or alcohol.

Comparison of Metabolic Pathways

Key Terms and Concepts

• Metabolism: All chemical reactions in a cell.

• Catabolism: Breakdown of molecules for energy.

• Anabolism: Synthesis of new molecules.

• ATP/ADP: Energy carrier molecules.

• NAD+/NADH, FAD/FADH2: Electron carriers.

• Glycolysis: First step in glucose breakdown.

• Pyruvate: End product of glycolysis.

• Acetyl CoA: Entry molecule for citric acid cycle.

• Citric Acid Cycle: Completes glucose oxidation.

• Electron Transport Chain: Series of proteins transferring electrons.

• Oxidative Phosphorylation: ATP production using electron transport and chemiosmosis.

• ATP Synthase: Enzyme that synthesizes ATP.

• Aerobic Respiration: Uses oxygen.

• Anaerobic Respiration: Does not use oxygen.

• Fermentation: Anaerobic process producing lactic acid or alcohol.

Example: Muscle cells perform lactic acid fermentation during intense exercise when oxygen is limited, producing ATP and lactic acid.

Additional info: The actual ATP yield in eukaryotic cells is often lower than the theoretical maximum due to energy costs of transporting molecules and use of NADH/FADH2 for biosynthesis.