26.1 Digestive system processes  (Page 3/2)

Introduction

Every creature has its own food, and an appropriate alchemist with the task of dividing it ... The alchemist takes the food and changes it into a tincture which he sends through the body to become blood and flesh. This alchemist dwells in the stomach where he cooks and works. The man eats a piece of meat, in which is both bad and good. When the meat reaches the stomach, there is the alchemist who divides it. What does not belong to health he casts away to a special place, and sends the good wherever it is needed.

Obtaining nutrition and energy from food is a multi-step process that, contrary to the thinking of Paracelsus, does not involve alchemy or an alchemist. Many physical and biochemical processes are involved in digestion of food, and it is also a highly regulated process For true animals, the first step is ingestion, the act of taking in food. This is followed by digestion, absorption, and elimination. In the following sections, each of these steps will be discussed in detail.

Ingestion

The large polymeric molecules found in intact food cannot pass through cell membranes. So these polmers need to be broken into smaller monomers so that animal cells can absorb and metabolize them to produce energy. The first step in this process is ingestion . Ingestion is the process of taking in food through the mouth. In vertebrates, the teeth, saliva, and tongue play important roles in mastication (preparing the food into a bolus). While the food is being mechanically broken down, the enzymes in saliva begin to chemically process the food as well. The combined action of these processes modifies the food from large particles to a soft mass that can be swallowed and can travel the length of the esophagus.

Digestion and absorption

Digestion is the mechanical and chemical break down of food into small organic molecules. It is important to break down macromolecules into smaller monomers that are of suitable size for absorption across the digestive epithelium. Large, complex molecules (e.g., proteins, polysaccharides, nucleic acids, and lipids) must be hydrolyzed into monomers before they can be absorbed by the digestive epithelial cells. If this terminology seems a bithazy, ou may want to review hydrolysis reactions [link] before proceeding. Different organs play specific roles in the digestive process. The animal diet needs carbohydrates, protein, nucleic acids, and fat, as well as vitamins and inorganic components for nutritional balance. We will briefly discuss digestion and absorption of some of these in the sections below.

Carbohydrates

The digestion of carbohydrates begins in the mouth. The salivary enzyme amylase begins the breakdown of food starches into maltose, a disaccharide. No significant further digestion of carbohydrates takes place in the stomach. The esophagus produces no digestive enzymes but does produce mucous for lubrication. The acidic environment in the stomach inhibits the action of the salivary amylase enzyme.