Human Digestion & Biomolecules: Essential Biology

Posted on Aug 10, 2025 in Biology

Macromolecules and Their Functions

The Human Digestive System

Mouth

Saliva moistens food and contains the enzyme *amylase*, which begins starch breakdown.

Esophagus

Transports chewed food to the stomach.

Stomach

Food is held here for several hours, where initial protein digestion occurs. The stomach wall secretes hydrochloric acid, which helps kill bacteria ingested with food, protecting against food poisoning. The protease enzyme, *pepsin*, produced in the stomach, is adapted to function in these acidic conditions.

Small Intestine

Duodenum

The first part of the small intestine, where food breakdown continues with enzymes.

Ileum

The final part of the small intestine, where digestion is completed and digested food is absorbed.

Large Intestine

Colon

Absorbs water from undigested remains and stores feces before elimination.

Rectum

Holds feces until they are expelled from the body through the anus.

Pancreas

Produces pancreatic enzymes that break down food. These enzymes travel to the duodenum.

Enzyme Classes and Actions

Carbohydrases

• *Amylase* breaks down starch into maltose.
  • Source: Salivary glands
  • Location: Mouth
• *Amylase* breaks down starch into maltose.
  • Source: Pancreas
  • Location: Small intestine
• *Maltase* converts maltose into glucose.
  • Source: Small intestine wall
  • Location: Small intestine

Proteases

• *Pepsin* breaks down proteins into peptides.
  • Source: Stomach wall
  • Location: Stomach
• *Trypsin* converts proteins into peptides.
  • Source: Pancreas
  • Location: Small intestine
• *Peptidases* break peptides into amino acids.
  • Source: Small intestine wall
  • Location: Small intestine

Lipase

• Breaks down lipids into glycerol and fatty acids.
• Source: Pancreas
• Location: Small intestine

The Intestinal Villus

The villus absorbs small, soluble, digested molecules.

Villus Adaptations for Absorption

• Finger-like shape increases surface area for absorption.
• Microvilli further increase surface area for absorption.
• Rich capillary network ensures a short diffusion distance.
• Thin epithelial layer (one cell thick) provides a short diffusion distance.

Consequences of Celiac Disease

Celiac disease damages the villi, leading to impaired nutrient absorption and various health issues:

• Reduced absorption of vitamins A & C.
• Iron deficiency, leading to anemia.
• Reduced calcium absorption.
• Impaired red blood cell (hemoglobin) repair.
• Issues with insulation and cell membrane integrity.
• Protein deficiency.
• Reduced efficiency of cellular respiration.

Key Biological Concepts

Stomach Acid Functions

• Kills bacteria in food, protecting against pathogens.
• Activates stomach enzymes (e.g., pepsin) in acidic conditions.

Emulsification

The process of breaking down large fat globules into smaller droplets, increasing their surface area. This allows lipase enzymes to digest fats more quickly and efficiently.

Role of Bile in Digestion

Bile is produced by the liver and stored in the gallbladder. Its functions include:

• Neutralization of stomach acid in the small intestine.
• Emulsification of fats.

Balanced Diet

A balanced diet consists of the correct proportions of all essential nutrients, such as proteins, carbohydrates, fats, vitamins, and minerals.

Digestion as an Organ System

Digestion is an example of an organ system where several organs work together to digest and absorb nutrients from the food we consume.

Effect of pH on Enzymes

Enzyme Denaturation

*Denaturation* occurs when a protein, such as an enzyme, loses its specific three-dimensional shape. Extreme temperatures or pH levels outside an enzyme’s optimum range cause denaturation, rendering the enzyme inactive.

Example: Oxygen Production and pH

If the volume of oxygen produced decreases above pH 7 (or any non-optimal pH), it is because the enzyme (catalyst) denatures. This means:

• The enzyme’s active site changes shape.
• The substrate can no longer fit into the active site.
• Consequently, there is less breakdown of the substrate (e.g., hydrogen peroxide), leading to reduced oxygen production.