The Fate of Amino Acids: Absorbed amino acids could be used for tissue protein, enzyme, and hormone synthesis and deamination or transamination, and the carbon skeleton can be used for energy.
Undigested proteins in the hindgut are subjected to microbial fermentation leading to the production of ammonia and other polyamines. Protein digestion in the ruminant animals can be divided into two phases: 1 digestion degradation in the reticulorumen and 2 digestion in the abomasum and small intestine. Therefore, in ruminant animals, dietary proteins are classified as rumen degradable and rumen undegradable proteins. Like monogastric animals, the main goal for protein supplementation is to provide amino acids to the animal.
However, in ruminants, proteins serve as a source of nitrogen for rumen microbes so they can make their own microbial protein from scratch. Protein entering the rumen may be degraded by both bacteria and protozoa, which produce proteolytic enzymes. The rumen microbes provide proteases and peptidases to cleave peptide bonds in polypeptides to release the free amino acids from proteins. Several factors such as solubility and the physical structure of protein can affect rumen degradation.
These rumen-degraded amino acids release NH3 and the C skeleton by a process called deamination. Along with volatile fatty acids from carbohydrates , rumen microbes synthesize their own microbial protein, which serves as a primary source of protein to the host ruminant animals.
Microbial protein is enough for maintenance and survival but not for high-producing animals. Ammonia absorbed from rumen is converted to urea and secreted into the blood as blood urea nitrogen BUN.
Urea can be filtered and recycled to the rumen via saliva or through the rumen wall. The concentration of BUN in ruminants reflects the efficiency of protein utilization. RUP enters the abomasum and small intestine of the ruminant animal for digestion and absorption.
Proteins reaching the small intestine could be RUP or those from microbial sources. The amino acid needs of the host animal are met by RUP and microbial proteins. Both ruminants and monogastrics require the essential amino acids in their diet, and amino acids cannot be stored within the body, so a constant dietary supply is necessary.
Some of the similarities and differences in monogastric and ruminant animals in protein digestion or degradation are shown in the table below. However, it should be noted that corn is deficient in essential amino acids such as lysine and methionine. Animal protein sources such as fish meal and meat meal have high bypass potential. Stomach Digestion of proteins begins in the stomach with pepsin which is secreted by gastric chief cells of oxyntic glands and is only active in the stomach's low pH environment.
Small Intestine Lumen Pancreatic digestive enzymes perform the majority of protein digestion. The major proteolytic enzymes include trypsin, chymotrypsin, elastase, and carboxypeptidase. These enzymes of the exocrine pancreas digest proteins down to short chains of a few amino acids, termed "Oligopeptides".
Small Intestine Epithelium The final stage of protein digestion occurs on the brush border of the small intestine epithelium. Here, membrane-bound peptidases complete digestion of oligopeptides to either single amino acids or dipeptides and tripeptides. Chemical Digestion of Proteins Proteins consist of polypeptides, which must be broken down into their constituent amino acids before they can be absorbed.
Chemical Digestion of Lipids The chemical digestion of lipids begins in the mouth. Chemical Digestion of Nucleic Acids Nucleic acids DNA and RNA in foods are digested in the small intestine with the help of both pancreatic enzymes and enzymes produced by the small intestine itself. Chemical Digestion by Gut Flora The human gastrointestinal tract is normally inhabited by trillions of bacteria, some of which contribute to digestion.
Here are just two of dozens of examples: The most common carbohydrate in plants, which is cellulose, cannot be digested by the human digestive system. However, tiny amounts of cellulose are digested by bacteria in the large intestine. Certain bacteria in the small intestine help digest lactose, which many adults cannot otherwise digest.
As a byproduct of this process, the bacteria produce lactic acid, which increases the release of digestive enzymes and the absorption of minerals such as calcium and iron. Absorption When digestion is finished, it results in many simple nutrient molecules that must go through the process of absorption from the GI tract by blood or lymph so they can be used by cells throughout the body. Note that each cell in the thin surface layer of the villus is actually covered with microvilli that greatly increase the surface area for absorption.
Feature: My Human Body The process of digestion does not always go as it should. Review Define digestion. Where does it occur? Identify two organ systems that control the process of digestion by the digestive system.
What is mechanical digestion? Describe chemical digestion. What is the role of enzymes in chemical digestion? What is absorption? When does it occur? Where does most absorption occur in the digestive system? Name two digestive enzymes found in saliva and identify which type of molecule they digest. Where is bile produced? What are some functions of bile? True or False.
Pepsin digests cellulose. Glucose can be absorbed by the body without being further broken down. Attributions Dyspepsia wafers , Public Domain via Flickr. Salivary Glands, Pancreas. Amylose Polysaccharide. Small Intestine. Sucrose Disaccharide. This is followed by digestion, absorption, and elimination. In the following sections, each of these steps will be discussed in detail. The large molecules found in intact food cannot pass through the cell membranes. Food needs to be broken into smaller particles so that animals can harness the nutrients and organic molecules.
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 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 is the mechanical and chemical break down of food into small organic fragments. It is important to break down macromolecules into smaller fragments that are of suitable size for absorption across the digestive epithelium.
Large, complex molecules of proteins, polysaccharides, and lipids must be reduced to simpler particles such as simple sugar before they can be absorbed by the digestive epithelial cells. Different organs play specific roles in the digestive process.
The animal diet needs carbohydrates, protein, and fat, as well as vitamins and inorganic components for nutritional balance. How each of these components is digested is discussed in the following sections. The digestion of carbohydrates begins in the mouth. The salivary enzyme amylase begins the breakdown of food starches into maltose, a disaccharide.
As the bolus of food travels through the esophagus to the stomach, no significant digestion of carbohydrates takes place. The esophagus produces no digestive enzymes but does produce mucous for lubrication. The acidic environment in the stomach stops the action of the amylase enzyme. The next step of carbohydrate digestion takes place in the duodenum. Recall that the chyme from the stomach enters the duodenum and mixes with the digestive secretion from the pancreas, liver, and gallbladder.
Pancreatic juices also contain amylase, which continues the breakdown of starch and glycogen into maltose, a disaccharide. The disaccharides are broken down into monosaccharides by enzymes called maltases , sucrases , and lactases , which are also present in the brush border of the small intestinal wall.
Maltase breaks down maltose into glucose. Other disaccharides, such as sucrose and lactose are broken down by sucrase and lactase, respectively. The monosaccharides glucose thus produced are absorbed and then can be used in metabolic pathways to harness energy.
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