Metabolic Pathways and Energy Production: Study Guide

Metabolism Basics

Definition of Metabolism

Metabolism refers to the sum of all chemical reactions that occur within a living organism to maintain life. These reactions are responsible for converting food into energy and building blocks for growth, as well as eliminating waste products.

• Anabolic reactions are metabolic pathways that construct molecules from smaller units. These processes require energy input. Example: Synthesis of proteins from amino acids.

• Catabolic reactions are metabolic pathways that break down molecules into smaller units, releasing energy. Example: Breakdown of glucose during glycolysis.

• ATP (Adenosine Triphosphate) hydrolysis provides the energy needed for cellular activities. The reaction is: $\mathrm{ATP} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{ADP} + \mathrm{P}_i + \text{energy}$

Digestion and Catabolism

Stages and Products of Catabolism

Catabolism is the process by which complex molecules are broken down into simpler ones, releasing energy. It occurs in three main stages:

1. Digestion: Breakdown of large molecules into smaller units (monomers) via hydrolysis, primarily in the digestive tract.

2. Conversion to Acetyl-CoA: Small molecules are further processed into acetyl-CoA, a key metabolic intermediate.

3. Citric Acid Cycle and Electron Transport: Acetyl-CoA is oxidized to CO2 and H2O, producing ATP.

• Products of Digestion:

  • Carbohydrates → Monosaccharides (e.g., glucose)

  • Proteins → Amino acids

  • Triacylglycerols (fats) → Fatty acids and glycerol

• Location of Digestion Initiation:

  • Carbohydrates: Mouth (salivary amylase)

  • Proteins: Stomach (pepsin)

  • Triacylglycerols: Small intestine (pancreatic lipase)

• Hydrolysis is the first step in the digestion of all three macronutrients, breaking chemical bonds with the addition of water.

Cellular Respiration Overview

Purpose and Major Steps

Cellular respiration is the process by which cells extract energy from nutrients, primarily glucose, to produce ATP. It consists of three major steps:

1. Glycolysis: Occurs in the cytoplasm; glucose is converted to pyruvate.

2. Krebs Cycle (Citric Acid Cycle/Tricarboxylic Acid Cycle): Occurs in the mitochondrial matrix; acetyl-CoA is oxidized to CO2.

3. Electron Transport Chain (ETC) and Oxidative Phosphorylation: Occurs in the inner mitochondrial membrane; electrons are transferred to oxygen, producing ATP.

• Glycolysis is the major anaerobic pathway for energy production (does not require oxygen).

• Fermentation is an anaerobic process that occurs when oxygen is limited, converting pyruvate to lactate (lactic acid) in animals.

Krebs Cycle and Oxidative Phosphorylation

Key Features and ATP Yield

• The Krebs cycle is also known as the Citric Acid Cycle and the Tricarboxylic Acid (TCA) Cycle.

• The chemiosmotic model of oxidative phosphorylation explains how ATP is produced as electrons move through the electron transport chain, creating a proton gradient that drives ATP synthesis.

• ATP Production by Pathway:

  Pathway	ATP Produced (per glucose)
  Glycolysis	2
  Krebs Cycle	2
  Electron Transport Chain	~28-34

  Additional info: The total ATP yield per glucose molecule is typically about 32-38, depending on cell type and conditions.

• Electron transport is the major aerobic pathway for ATP production during cellular respiration.