Chapter 23: Metabolism and Nutrition

Module 23.1 Overview of Metabolism and Nutrition

Introduction to Metabolism and Nutrition

Metabolism and nutrition are foundational concepts in human physiology, describing how the body acquires, transforms, and utilizes energy and nutrients from food. The energy content in the human diet is measured in terms of heat.

• Calorie (C): A measure of heat; one calorie is the amount of heat required to raise the temperature of one gram of water by one degree Celsius.

• Kilocalorie (kcal): The actual unit used to measure human diet; one kilocalorie is the amount of heat required to raise the temperature of one kilogram of water by one degree Celsius. (1 kcal = 1000 calories)

Definition and Functions of Metabolism

Metabolism is the sum of all chemical reactions in the body, organized into enzyme-catalyzed sequences called metabolic pathways. These reactions serve several key purposes:

• Harnessing energy from chemical bonds in nutrients to make adenosine triphosphate (ATP).

• Converting molecules for cellular synthesis reactions.

• Synthesizing macromolecules such as proteins, polysaccharides, nucleic acids, and lipids.

• Breaking down macromolecules into monomers or smaller molecules for energy or recycling.

Types of Metabolic Reactions

• Catabolism: Series of reactions in which substances are broken down into smaller parts, releasing energy that the cell can harness for other processes.

  • Glucose catabolism: Breakdown of dietary and stored carbohydrates; glucose is a preferred fuel for many cells, especially the brain and liver.

  • Fatty acid catabolism: Breakdown of triglycerides into free fatty acids and glycerol; these can enter separate catabolic pathways or be used for other purposes.

  • Amino acid catabolism: Proteins are broken down into amino acids, which can be further catabolized or used for other cellular functions.

• Anabolism: Series of reactions in which smaller molecules are combined to form larger molecules. Cells use these reactions to build macromolecules such as:

  • Proteins

  • Nucleic acids

  • Lipids

  • Carbohydrates

Energy Requirements of Metabolic Reactions

Metabolic reactions are usually paired so that energy released from one (exergonic, usually catabolic) feeds into another (endergonic, usually anabolic). This is governed by the law of conservation of energy:

• Energy cannot be created or destroyed; it can only change forms.

• Energy present before a reaction (in reactants) must equal energy present after the reaction (in products or as liberated energy).

Types of reactions:

• Exergonic reactions: Release energy, leaving products with less energy than the original reactants. Most catabolic reactions are exergonic.

• Endergonic reactions: Require input of energy to proceed; products possess more energy than reactants. Most anabolic reactions are endergonic.

Energy from exergonic reactions is often used to drive endergonic reactions, such as the synthesis of ATP from ADP and inorganic phosphate.

Key Terms and Concepts

• ATP (Adenosine Triphosphate): The primary energy currency of the cell, produced by catabolic pathways and used in anabolic reactions.

• Enzyme-catalyzed reactions: All metabolic pathways are facilitated by enzymes, which lower the activation energy and increase the rate of reactions.

Example: Coupling of Reactions

In the cell, the energy released from the breakdown of glucose (catabolic, exergonic) is used to synthesize ATP (anabolic, endergonic). The hydrolysis of ATP then provides energy for various cellular processes.

Additional info: The concepts of catabolism and anabolism are central to understanding how the body maintains energy balance and supports growth, repair, and maintenance of tissues.