Bio 224, lecture 22, march 12 - Digestive physiology and metabolism

Major Functions of the Digestive System

• Mechanical and Chemical Breakdown of Food

  • Essential for digestion, involving physical and enzymatic processes to convert food into smaller, absorbable units.

• Food Storage

  • Stomach can hold food for certain periods, allowing for a gap between meals; this provides an evolutionary advantage by reducing the frequency of feeding.

• Absorption of Nutrients

  • Nutrients released during digestion are mostly absorbed in the small intestine, highlighting its absorptive functions.

• Elimination of Undigested Food

  • The digestive system plays a crucial role in expelling non-digestible food remnants from the body.

Classification of Nutrients

• Macronutrients

  • Nutrients needed in large amounts, including carbohydrates, proteins, and certain essential elements.

  • Examples: carbohydrates (providing energy), proteins (building blocks of body tissues).

• Micronutrients

  • Needed in smaller amounts but are still essential for survival.

  • The body usually cannot synthesize these, requiring absorption through food.

Essential Nutrients

• Essential Amino Acids

  • Nine amino acids that cannot be synthesized by the body; must be obtained from food.

• Essential Fatty Acids

  • Examples:

    • Linoleic Acid (Omega-6)

    • Alpha-Linolenic Acid (Omega-3)

  • Critical for various body functions and must be consumed as they cannot be produced by the body.

• Vitamins

  • Thirteen essential vitamins needed for various metabolic processes; most cannot be produced, requiring dietary sources.

  • Exception: Vitamin D can be synthesized by the skin with sunlight exposure.

• Essential Elements

  • Some are macronutrients like calcium, vital for bones and muscle function.

  • Others, like copper and iodine, are required in small amounts as micronutrients crucial for various enzymatic reactions.

Digestion of Macronutrients

• Carbohydrates

  • Mostly consumed as polysaccharides and disaccharides; need hydrolysis to break down into monosaccharides (e.g., glucose).

  • Lactose intolerance is due to the lack of the enzyme required to digest lactose.

  • Most carbohydrates consumed are in the form of starch and glycogen, which need breakdown for absorption.

• Proteins

  • Comprised of amino acids and digested by proteases that break peptide bonds.

  • The smallest absorbable units are amino acids.

• Lipids

  • Mainly in the form of triglycerides (three fatty acids and one glycerol); broken down into fatty acids and glycerol during digestion.

• Nucleic Acids

  • Digested into nucleotides which can further break down into nitrogen bases and sugars to be absorbed.

Hydrolysis and Enzymatic Action

• Hydrolysis

  • Describes the breakdown of food substances with the addition of water; this is essential for chemical digestion.

  • Digestion is facilitated by different enzymes (amylases, proteases, lipases, nucleases) that target specific types of nutrients.

Vitamin Solubility and Storage

• Fat-Soluble Vitamins

  • Stored in body fat; do not require daily intake since the body can retain them.

• Water-Soluble Vitamins

  • Cannot be stored; must be consumed regularly since they're excreted through urine.

Types of Digestion

• Intracellular Digestion

  • Occurs in simple organisms (e.g., sponges) where food is broken down within cells through endocytosis and lysosomal enzyme action.

• Extracellular Digestion

  • Found in more complex organisms, where digestion occurs in a system of tubes (gastrointestinal tract) connected to the external environment, allowing for more efficient nutrient absorption.

Evolution of Digestive Systems

• Primitive Animals

  • Simple digestion occurring within cells.

• Complex Animals

  • More advanced structures aiding in digestion, such as separate compartments for storage, absorption, and specialized accessory organs (e.g., liver, pancreas).

  • Examples include insects and mammals with intricate systems to optimize digestion and nutrient uptake.