# Structural Biochemistry/Cell Signaling Pathways/Digestive System
### Summary of the Digestive System

The digestive system deals primarily with the breakdown of food polymers into smaller molecules to provide energy for the body.
1.) The entire digestive process can be simplified through the individual processes of ingestion, digestion, absorption, and elimination. Through analysis of each of these processes, an effective comprehension of the organs that make up the digestive system can be observed. During the process of ingestion, an organism utilizes mechanical digestion by chewing the particular food of interest. In this situation, the mouth, also known as the oral cavity, acts as the source of increasing the food’s surface area by creating it into a bolus, a smaller and more circular version of the food that will subsequently be swallowed down the esophagus. Besides these organs, there are also accessory glands that contribute to ingestion as well such as the tongue and the teeth. Through observing the digestion portion of the digestive system, enzymatic hydrolysis can be witnessed in which the esophagus transfers food down from the oral cavity and towards the stomach in which hydrochloric acid and pepsin allow the chemical breakdown of the food. Next, in the absorption portion of the digestive system, nutrient molecules enter body cells within the small intestine. Lastly, the elimination process of the digestive system is closely connected with the excretory system in which the undigested material of an organism becomes eliminated from the body.
2.) With the general idea and characteristics about the digestive system recently observed, the structure and function of the digestive system of three different types of organisms can be examined: a human, an invertebrate, and a non-mammal vertebrate.
* Firstly, in the human digestive system, there is an alimentary canal, also known as a complete digestive tract, which represents a digestive tube extending between two openings, a mouth and an anus. The mouth acts as the initiating factor of the digestive tract and digestion initiates with the very first bite of any particular food type in which subsequent chewing breaks the food into pieces that are more easily digested. Simultaneously, saliva mixes with food to begin the process of breakdown into a form that the body can more readily absorb. Generally, humans are considered to be omnivorous, consuming both meat and plant-like objects.
* Secondly, the digestive system of the invertebrate octopus will be observed. An octopus also possesses a two way digestive system consisting of a mouth and an anus. In contrast the typically omnivorous human diet, the octopus is a carnivorous creature with a diet generally consisting of lobster, crab, and shrimp. Utilizing their incredibly strong beaks to kill and consume their prey, the food will be digested in the stomach and digestive sac and then released from the anus.
* Thirdly, as for a non-mammal vertebrate, a turtle’s digestive system can be observed. The digestive system of turtles is similar to that of other vertebrates. Unlike amphibians, they do not have mucous glands, but do have salivary glands. Turtles swallow their food whole or in large chunks and as this occurs, the salivary glands empty into the mouth and moisten the food to aid in swallowing. Although omnivorous in general, most turtles are at least partly carnivorous which cause them to possess strong digestive enzymes. The turtle's tongue is broad and flat while being firmly attached to the bottom of the mouth, preventing the tongue from moving. Alligator snapping turtles have small worm-like appendages on their tongues. These appendages can be filled with blood and are allowed to wiggle. With the turtle's mouth open and appendage wiggling, the alligator snapper can attract small fish to catch more effectively. The walls of the turtle's digestive tract are made of smooth muscle tissue and the muscles contract, pushing food down the esophagus and intestines towards the stomach.

### References

* ↑ , Digestive System & Personal BILD2 Essay.

## In-depth Steps in the Digestive System

Ingestion The act of ingestion begins the process of the whole digestive system. Ingestion begins when an organism puts a consumable in their mouth, and mechanical digestion begins in the form of chewing and breaking down the food. Performing mechanical digestion allows for the consumed food to increase in surface area, which allows for easier digestion and absorption of nutrients. During this time, the food turns into a bolus to allow for easier swallowing. Also during mechanical digestion, saliva is secreted in order to protect the lining of the mouth from the mechanical digestion. Saliva contains several enzymes, including amylase which protects the lining of the mouth, as well as mucin, which aids in swallowing. After mechanical digestion by chewing, and chemical digestion with saliva, the bolus of food is pushed into the esophagus, and moves down the throat by peristalsis.
Digestion There are two important forms of digestion where molecules are broken into smaller pieces for absorption, known as mechanical and chemical digestion. Mechanical digestion occurs first in the oral cavity and begins with the chewing of food. Polysaccharides and sugars are only broken down in the oral cavity. Chemical digestion then follows as the salivary glands produce saliva and brings it through ducts to the oral cavity where it helps the breakdown of food. There is an enzyme in saliva called amylase that hydrolyzes polysaccharides and disaccharides into simple sugars in a process known as enzymatic hydrolysis. The tongue helps shape the food into a bolus and aid with swallowing. Once swallowing has occurred, the bolus of food reaches the pharynx, a region in that throat that opens to up to either the trachea (which leads to the lungs) or the esophagus (which leads to the stomach). During swallowing, there is a cartilage flap called the epiglottis that covers the glottis, an entryway that leads to the trachea so that food will not go down the wrong way and end up blocking the windpipe. Because this entryway is blocked, the larynx is able to guide the bolus of food to the esophagus entryway. There is an esophageal sphincter that regulates the movement of material in and out of the esophagus. This sphincter contracts when person is not swallowing and contrastingly, it relaxes when swallowing occurs so that the bolus of food can travel down the esophagus to the stomach. Chemical digestion is also prevalent in the stomach. The bolus of food is further broken down by the stomach’s gastric juices, which is composed of HCl and pepsin. The strongly acidic HCl helps to break down the extracellular matrix in the bolus of food, while the pepsin, being a protease, aids in breaking down peptide bonds and digesting proteins in the food. As the bolus is mixed with the gastric juices, the stomach constantly churns until the bolus turns into chime. Various other organs assist in the digestion of food, with the liver aiding in lipid breakdown, and the pancreas secreting bicarbonate to neutralize the HCl in gastric juice.
Chemical Digestion in the Stomach The stomach utilizes strong chemicals and enzymes that would normally destroy organic matter easily. The stomach is able to protect itself from its own gastric juices by carefully synthesizing the chemicals together to efficiently digest the bolus of food and turn it into chyme. The entire inside of the stomach is lined with a layer of mucus, as well as other layers of epithelial cells that are replaced every few days. The stomach utilizes hydrochloric acid as a means to destroy harmful bacteria as well as to denature the proteins in the bolus. Because hydrochloric acid can cause high damage to the stomach, the H+ ions and Cl- ions are kept separate until it is needed for digestion. Separate parietal cells in the stomach keep the ions separate, until they use ATP to drive the H+ and Cl- ions out, where they react in the stomach lumen to form the HCl. HCl is also able to activate pepsinogen into pepsin, a useful enzyme that specializes in protein cleaving and digestion. Stomachs that do not have the proper defensive ways to utilize the HCl and pepsin within the gastric juice often get damaged stomachs, leading to gastric ulcers.
Absorption Absorption of nutrients begins in the small intestine. The small intestine has massive surface area lined with villi and microvilli, which help aid in absorbing the nutrients from the chyme. Afterwards, the chyme goes through the large intestine, which is divided into three parts: the colon, cecum, and appendix. The cecum aids in fermenting the ingested food, and the colon reabsorbs water and other nutrients that the body might’ve used during the digestive process. As the ingested material passes through the intestines by peristalsis, usable nutrients are absorbed, while waste is left behind in the form of feces.
Excretion The final stage in the digestive system is excretion, in which the ingested food is now formed as a (usually) solid waste known as feces. The feces is excreted out of the body via the anus, ending the digestive process for the particular ingested food.

## Gastric Juice Production

There are gastric glands located on the interior surface of the stomach. The gastric glands have three different types of cells that are responsible for secreting the parts that make up the gastric juice such as mucus, pepsin and hydrochloric acid. The parietal cells in the gastric gland secrete hydrochloric acid (HCl). The chief cells secrete an inactivate form of pepsinogen that is not activated to pepsin until it comes into contact with hydrochloric acid in the lumen of the stomach. Lastly are the mucus cells that secrete mucus, a substance that protects the lining of the stomach. Gastric juice does not destroy stomach cells that make it because all these ingredients remain inactive until they are released into the lumen. Pepsinogen and HCl are first secreted into the lumen of the stomach. Hydrochloric acid converts the pepsinogen (inactive form) into pepsin. And pepsin, the activated form begins a chain reaction, activating more pepsinogen. Gastric juice begins the process of chemical digestion of proteins in the stomach.

## Digestion in the Small Intestine

The small intestine is the longest compartment of the alimentary canal and is the major organ of digestion and absorption. This is the site where the rest of the digestion and enzymatic hydrolysis processes takes place, followed by absorption. First portion of the small intestine is the duodenum, where acid chyme combines with the gastric juices of the pancreas, liver, gall bladder.

## Hormonal Control of Digestion

* When food arrives at the stomach, it causes the walls to stretch and this stimulates the release of the gastrin hormone from the stomach. Gastrin will travel through the bloodstream and come back to target the stomach, causing the stomach to release gastric juices.
* Once chyme passes from stomach to duodenum, it triggers the release of hormones, secretin and cystokinin (CKK) because of the fatty acids and amino acids present in the chyme. The secretin works positively on the pancreas to stimulate the release of bicarbonate, which will help break down fats into fatty acids and neutralize acid chyme. Meanwhile CCK stimulates the gallbladder to release bile, which is a detergent that breaks down fats into smaller fats. CCK also stimulates the pancreas to release more digestive enzymes that can hydrolyze fats.
* The secretin hormone and CKK both have inhibitory responses and act negatively on the stomach to slow down digestion and the production of chyme.

## Absorption in the Small Intestine

The jejunum and ileum are parts of the small intestine that are strictly responsible for absorption. The large surface area of the small intestine is due to the villi and microvilli that are exposed to the intestinal lumen. This large microvillar surface increases the rate at which nutrients are absorbed. The villi and small epithelial cells make up and line the border of the small intestine. Nutrients can be transported across the epithelial cell through passive or active transport. Sugar moves through passive diffusion down the concentration gradient into the epithelial cell, it then gets collected in small blood capillaries and travels through the circulatory system. Meanwhile amino acids, peptides, vitamins and glucose are pumped actively against the concentration gradient by the epithelial cells and then circulated throughout the bloodstream by capillaries. Fats, on the other hand, need to be reassembled in the lymphatic system and transported differently.
Absorption of Fats In the lumen, the fat molecules, called triglycerides undergo enzymatic hydrolysis during which the enzyme, lipase breaks down the fat molecules into fatty acids and monoglycerides. The smaller molecules made up of fatty acids and monoglycerides diffuse into the epithelial cell. In the epithelial cell, the small molecules revert back to being triglycerides which are mixed with cholesterol and coated with protein to form chylomicrons. These molecules leave the epithelial cells and are transported into the lacteals.

## Absorption in the Large Intestine

While the primary function of the small intestine is to absorb nutrients and water, the large intestine functions to absorb mainly water. The colon is a part of the large intestine that connects to the small intestine and leads to the rectum and anus. The colon functions to recover the water that enters the alimentary canal. The cecum connects where the small and large intestine meet and helps in the fermentation of plant material. In humans, the cecum also has an extension known as the appendix. The appendix does not play a large part in immunity. Feces are any wastes from the digestive tract. As they move through the colon, they become more solid and ultimately pass through the rectum and leave out through the anus.

## Appetite Regulating Hormones

There are hormones secreted by tissues and organs in the body that then are transported through the bloodstream to the satiety center, a region in the brain that triggers impulses that give us feelings of hunger or aid in suppressing our appetite. Ghrelin is a hormone that is released by the stomach and targets the pituitary, signaling to the body that it needs to eat. PYY is a hormone that is released by the small intestine and counters ghrelin. It is released by the hypothalamus and signals that you have just eaten and helps to suppress our appetite. The pancreas releases the hormone insulin, which targets the hypothalamus and also aids in suppressing our appetite after we have just eaten and there is a rise in blood glucose levels. The last hormone is leptin which also helps to suppress appetite. Leptin is produced by adipose fat tissue and targets the hypothalamus.

## Mucin's Role in Gastrointestinal Tract Diseases

The intestinal barrier is composed of a mucin (glycoproteins) layer and tight junctions between epithelial cells as well as by regular epithelial cell replacement which protects the intestine from mechanical stress produced by the passage of digested food through the gastro intestinal (GI) tract, molecular breakdown from digestive enzymes, and foreign invaders such as bacteria and viruses. While providing protection, the mucin layer must allow the passage of digested molecules through to be absorbed by epithelial cells. Unraveling the depth of protection by the mucin layer to all damaging vectors can develop a comprehensive understanding of mucin’s potential role in many intestinal diseases.
Up to 20 mucin genes have been identified and are broadly classified as either secretory or membrane-associated large glycoproteins. Recent studies have shown that the mucin layers in the GI tract vary in mucin protein composition, mucin layers, and mucin thickness. The mucin layer has been shown to prevent digestive enzymes from entering the wall of the intestines. The mucin layer can be compromised if there is a structural abnormality or production of the glycoprotein is altered. This results in poor protection of the intestinal wall and can lead to systemic damage to the body.
For example, shock is a physiological process that still kills many individuals through multiple organ failure. Recent studies have shed light on the role the intestine plays in this process. The intestinal mucosal barrier becomes compromised during shock and no longer performs as a barrier. When this occurs, the luminal contents of the intestine, including cytotoxic free fatty acids (FFAs), may enter the wall of the intestine causing damage both to it and other organs.
A complete comprehension of mucin’s role as a protective agent can lead to a greater understanding of many GI diseases and complications and the role mucin has in each. Diseases such as necrotizing enterocolitis, Crohn’s Disease, and Irritable Bowel Syndrome could be treated more efficiently since mucin plays a role in each. Shock victims would also benefit since many of the complications arise from the breakdown of the intestinal barrier.

### References

Reece, Jane B., and Neil A. Campbell. Biology. Boston, MA: Cummings, 2011. Print.
* 1 2 3 Malin E. V. Johansson, Daniel Ambort, Thaher Pelaseyed, André Schütte, Jenny K. Gustafsson, Anna Ermund, Durai B. Subramani, Jessica M. Holmén-Larsson, Kristina A. Thomsson and Joakim H. Bergström, et al. Composition and functional role of the mucus layers in the intestine. Cellular and Molecular Life Sciences, 2011.
* ↑ Atuma, C., V. Strugala, A. Allen, and L. Holm: The adherent gastrointestinal mucus gel layer: thickness and physical state in vivo. Am J Physiol Gastrointest Liver Physiol 280: G922–G929, 2001.
* 1 2 Marisol Chang, Tom Alsaigh, Erik B. Kistler, and Geert W. Schmid-Schönbein: Breakdown of Mucin as Barrier to Digestive Enzymes in the Ischemic Rat Small Intestine. PLoS ONE 7(6): e40087. doi:10.1371/journal.pone.0040087.
* ↑ Chang M, Kistler EB, Schmid-Schönbein GW. Disruption of the mucosal barrier during gut ischemia allows entry of digestive enzymes into the intestinal wall. Shock. 37(3):297-305, March 2012.
* ↑ Alexander H. Penn and Geert W. Schmid-Schönbein: The intestine as source of cytotoxic mediators in shock: free fatty acids and degradation of lipid-binding proteins. Am J Physiol Heart Circ Physiol 294: H1779–H1792, 2008.