Zoology Lecture 10 March 2025

Mechanical and Chemical Digestion

• Mechanical Digestion: Begins with chewing food to break it down.

• Chemical Digestion: Initiated in the mouth with saliva containing salivary amylase, an enzyme that starts carbohydrate digestion.

Stomach and Acid Chyme

• Upon reaching the stomach, food is mixed with gastric juice containing the enzyme pepsin to digest proteins.

• Peristalsis contributes to mechanical digestion.

• The mixture of gastric juice and digested food is known as acid chyme, characterized by a very acidic pH of around 2.

Small Intestine and Digestion

• The first part of the small intestine is called the duodenum, where digestion continues.

• A sphincter muscle regulates the passage of acid chyme from the stomach into the duodenum.

• Pancreas and liver secretions join the chyme in the duodenum to aid digestion:

  • Bicarbonate from the pancreas neutralizes the acidity of chyme to a pH of approximately 6, suitable for enzyme activity.

Digestion in the Duodenum

• Chemical Digestion in the Duodenum: Involves carbohydrates, proteins, nucleic acids, and fats.

  • Carbohydrates: Salivary amylase acts in the mouth, while pancreatic amylase completes digestion in the duodenum.

  • Proteins: Begins with pepsin in the stomach and continues with pancreatic proteases (trypsin and chymotrypsin) activated in the duodenum.

  • Nucleic Acids: Digested by nucleases, which break down RNA and DNA.

  • Fats: Digested by pancreatic lipase, requires emulsification by bile to increase surface area for enzyme action.

Structure of the Small Intestine

• Increased surface area for absorption is achieved through:

  • Large circular folds

  • Villi: Finger-like projections lined with epithelial cells, containing blood vessels and lacteals for fat absorption.

  • Microvilli: Tiny projections on individual epithelial cells forming the brush border.

Absorption of Nutrients

• Nutrient absorption occurs primarily in the small intestine.

  • Delimited products of digestion:

    • Amino acids and monosaccharides absorbed into blood vessels.

    • Fat digestion products enter the lacteals as chylomicrons.

• The extensive folding in the intestinal structure greatly increases the absorptive surface area (approx. size of a tennis court).

Large Intestine Function

• Connects to the small intestine and is responsible for water absorption, forming feces from unabsorbed material.

• Microbiome plays a crucial role in digestion within the large intestine, contributing to fecal composition (about 8% dead bacteria).

Digestive Adaptations in Herbivores

• Adaptations for Plant Digestion:

  • Long small intenstines provide more time for digestion/absorption.

  • Cecum: A pouch aiding in cellulose digestion with help of anaerobic microbes.

  • Some herbivores practice coprophagy (eating feces) to re-digest nutrients.

Ruminant Digestion

• Ruminants (e.g., cows) possess a specialized multi-chamber stomach:

  • Rumen: Contains microorganisms that digest cellulose.

  • Animals regurgitate food to chew (cud) for further processing.

Regulation of Digestion and Homeostasis

• Feedback Mechanisms: Maintain homeostasis in the digestive system as follows:

  • Positive Feedback: Amplifies responses (e.g., pepsin activation).

  • Negative Feedback: Dampens responses (e.g., regulation of blood glucose).

Hormonal Control of Blood Glucose

• Insulin and Glucagon: Hormones produced by the pancreas that regulate blood glucose levels.

  • Insulin lowers blood glucose by signaling cells to absorb glucose.

  • Glucagon raises blood glucose by promoting glucose release from liver cells.

  • Type 1 diabetes results from an autoimmune attack on insulin-producing beta cells.

  • Type 2 diabetes is characterized by insulin resistance.