If you put the following events in the order they occur in the human digestive system, the third event in the series would be a. Cells in gastric pits secrete protons. b. Pepsin activates pepsinogen. c. HCl activates pepsinogen. d. Partially digested food enters the small intestine.

Verified step by step guidance

1. Step 1: The first event in the human digestive system is the secretion of protons by cells in the gastric pits. This is because the stomach lining contains gastric pits, which are small pockets that secrete gastric juices. These juices contain hydrochloric acid (HCl) and the enzyme pepsinogen. The protons are part of the HCl that is secreted.
2. Step 2: The second event is the activation of pepsinogen by HCl. The acidic environment created by the HCl in the stomach triggers the conversion of pepsinogen into its active form, pepsin.
3. Step 3: The third event is the activation of pepsinogen by pepsin. This is a process known as autocatalysis, where pepsin itself activates more pepsinogen, amplifying the digestive process.
4. Step 4: The fourth and final event is the entry of partially digested food into the small intestine. After being broken down by pepsin in the stomach, the partially digested food, now called chyme, moves into the small intestine for further digestion and absorption.

Key Concepts

Gastric Secretion

Gastric secretion involves the release of digestive substances from the stomach lining, primarily hydrochloric acid (HCl) and digestive enzymes. HCl creates an acidic environment that is crucial for activating pepsinogen, an inactive enzyme secreted by gastric cells. This process is essential for protein digestion and prepares the food for further breakdown in the small intestine.

Pepsinogen Activation

Pepsinogen is an inactive precursor of the enzyme pepsin, which is responsible for breaking down proteins into smaller peptides. The activation of pepsinogen occurs in the acidic environment of the stomach, where HCl converts it into active pepsin. This step is critical for initiating protein digestion before the food moves into the small intestine.

Digestive Process Sequence

The digestive process follows a specific sequence of events that begins in the mouth and continues through the stomach and intestines. Understanding this sequence is vital for identifying the order of events in digestion. In this case, the sequence includes gastric secretion, activation of digestive enzymes, and the movement of partially digested food into the small intestine for further digestion and nutrient absorption.