Question

Which of the following statements regarding the vertebrate stomach is false? a. Its cells secrete the protease pepsin in its active form. b. It is a saclike organ that may have evolved to store food. c. Its cells secrete hydrochloric acid. d. It is the initial site of protein digestion. e. Little or no absorption of nutrients occurs there.

Short answer

The false statement is A - Its cells secrete the protease pepsin in its active form. The stomach secretes pepsinogen, which is the inactive precursor to the protease pepsin, and it becomes activated in the acidic environment of the stomach.

Step-by-step solution

Statement A Analysis

Statement A says that the cells of the vertebrate stomach secrete the protease pepsin in its active form. This statement is false because the stomach cells secrete pepsinogen, which is an inactive form of pepsin. Pepsinogen is later activated to pepsin in the stomach's acidic environment.

Statement B Analysis

Statement B says that the stomach is a saclike organ that may have evolved to store food. This is true because the stomach serves as a temporary holding receptacle for swallowed food and controls the rate at which food enters the small intestine for absorption.

Statement C Analysis

Statement C states that the cells of the vertebrate stomach secrete hydrochloric acid. This is true as parietal cells in the stomach lining secrete hydrochloric acid. The acid lowers the pH of the stomach, helping to unravel proteins and activate pepsinogen.

Statement D Analysis

Statement D indicates that the stomach is the initial site of protein digestion. This statement is true. When food enters the stomach, it is mixed with stomach acid and enzymes, including pepsin, which begins the breakdown of proteins into amino acids.

Statement E Analysis

Statement E suggests that little or no absorption of nutrients occurs in the stomach. This statement is indeed true. While some substances such as alcohol and certain drugs can be absorbed through the stomach lining, the majority of nutrient absorption happens in the small intestine.

Key concepts

Pepsinogen

The vertebrate stomach uses pepsinogen, a precursor to pepsin, as an essential component in digestion. Pepsinogen is secreted by the chief cells in the stomach lining in an inactive form. This is crucial to prevent the digestion of the stomach itself.
Once secreted, pepsinogen encounters hydrochloric acid in the stomach, which activates it by cleaving away inhibitory segments. This activation process transforms pepsinogen into pepsin, the active enzyme that digests proteins.
Understanding pepsinogen and its activation gives insight into the sophisticated mechanisms the stomach uses to initiate digestion safely.

Protein Digestion

Protein digestion begins in the stomach, marking a critical step in the digestive process. Once proteins enter the stomach, they are subjected to a highly acidic environment fostered by gastric secretions.
In this setting, the active enzyme pepsin starts breaking down complex protein structures into smaller polypeptides. This breakdown is essential as it converts proteins into amino acids, making them easier to absorb later in the small intestine.

• Initial step: Proteins are denatured by stomach acid.
• Key enzyme: Pepsin cleaves proteins into peptides.

The process ensures that proteins, a vital dietary component, are effectively prepared for absorption and utilization by the body.

Hydrochloric Acid

Hydrochloric acid (HCl) in the gastric fluid serves multiple purposes in the vertebrate stomach. It is secreted by parietal cells lining the stomach and contributes to creating an acidic environment, with a pH ranging from 1.5 to 3.5.
This acidity plays a vital role in not only activating pepsinogen to become pepsin but also in chemically starting the breakdown of food. The acidic conditions unravel complex food molecules, especially proteins, making them more accessible for digestive enzymes.

• Acidic environment: Assists in protein denaturation.
• Defense mechanism: Destroys potentially harmful pathogens.

Without hydrochloric acid, the stomach would be unable to efficiently begin the digestion of proteins or protect the body from ingested pathogens.

Nutrient Absorption

Despite its pivotal role in digestion, the stomach is not primarily responsible for nutrient absorption. Its main function relates to the breakdown of food material, leaving most nutrient absorption tasks to the small intestine.
The stomach contributes to the digestion process by turning meals into a semi-liquid substance called chyme, which is then passed into the small intestine.

• Minimal absorption: Limited to certain small molecules and drugs.
• Preparation: Converts ingested food into a form suitable for absorption.

Although minimal, the capacity to absorb certain substances indicates the stomach’s selective conservation role in the digestive system.

Stomach Evolution

The vertebrate stomach has undergone significant changes throughout evolution. Initially, it likely served primarily as a storage organ for ingested food, allowing early animals to consume large quantities swiftly and digest slowly over time.
Over many generations, it evolved mechanisms for acid secretion and enzyme production, enhancing its role in the digestive system. The stomach’s ability to regulate the controlled release of digested food into the intestine showcases evolutionary adaptations to optimize digestion and nutrient uptake.

• Storage function: Temporary holding site for food intake.
• Digestive enhancements: Development of acidic and enzymatic processing.

Understanding the evolutionary history of the stomach highlights its complex integration within vertebrate biology and its ongoing adaptations in response to dietary changes.