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Eating is essential to life. Many of us look to eating as not only a necessity, but also a pleasure. Nevertheless, what does this all mean to our body and the physiological processes it carries out each day? Well, this paper will discuss the processes involved in digesting the food, product absorption, and the enzymatic processes that occur in the human stomach and intestines.

The digestive system is made of gastrointestinal (GI) tract also called the digestive tract, liver, pancreas, and gallbladder (as shown in the diagram below). According to Clegg and Mackean (2000), the easiest way to understand the digestive system is to divide its organs into two main categories. The first group is the organs that make up the digestive tract. The second is the accessory digestive organs and are critical for orchestrating the breakdown of food and the assimilation of its nutrients into the body. The main function of the organs of the digestive tract is to nourish the body. This tube begins at the mouth and terminates at the anus. Between those two points, the canal is modified as the pharynx, esophagus, stomach, and small and large intestines to fit the functional needs of the body. Digestion is mainly a chemical process and consists of breaking down large molecules to small molecules.

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The processes of digestion include six activities: ingestion, propulsion, mechanical, or physical digestion, chemical digestion, absorption, and defecation. The first of these processes, ingestion, refers to the entry of food into the digestive tract through the mouth. There, the food is chewed and mixed with saliva, which contains enzymes that begin breaking down the carbohydrates in the food plus some lipid digestion via lingual lipase. Food leaves the mouth when the tongue and pharyngeal muscles propel it into the esophagus. This act of swallowing, the last voluntary act until defecation, is an example of propulsion, which refers to the movement of food through the digestive tract. It includes both the voluntary process of swallowing and the involuntary process of peristalsis.
Furthermore, digestion includes both mechanical and chemical processes. Although there may be a tendency to think that mechanical digestion is limited to the first steps of the digestive process, it occurs after the food leaves the mouth, as well. The mechanical churning of food in the stomach serves to further break it apart and expose more of its surface area to digestive juices, creating an acidic soup called chyme. In chemical digestion, starting in the mouth, digestive secretions break down complex food molecules into their chemical building blocks (for example, proteins into separate amino acids). These secretions vary in composition, but typically contain water, various enzymes, acids, and salts. The process is completed in the small intestine. Food that has been broken down is of no value to the body unless it enters the bloodstream and its nutrients are put to work. This occurs through the process of absorption, which takes place primarily within the small intestine. ‘There, most nutrients are absorbed from the lumen of the digestive tract into the bloodstream through the epithelial cells that make up the mucosa’ (Cnx,org, 2013).
Chemical digestion in the small intestine cannot occur without the help of the liver and pancreas. The liver produces bile and delivers it to the common hepatic duct. Bile contains bile salts and phospholipids, which emulsify large lipid globules into tiny lipid droplets, a necessary step in lipid digestion and absorption. The gallbladder stores and concentrates bile, releasing it when the small intestine needs it. The pancreas produces the enzyme and bicarbonate-rich pancreatic juice and delivers it to the small intestine through ducts. Pancreatic juice buffers the acidic gastric juice in chyme, inactivates pepsin from the stomach, and enables the optimal functioning of digestive enzymes in the small intestine.

Within the mouth, the teeth and tongue begin mechanical digestion of carbohydrates, while the salivary glands begin chemical digestion. Glucose, galactose, and fructose are the three monosaccharaides that are readily absorbed. Although, they are not directly consumed as soluble nutrients as they occur in food as starch. Starch is first broken down in the mouth by salivary amylase which breaks it down into smaller fragments called maltose. In the small intestine, pancreatic amylase does the heavy lifting for starch and carbohydrate digestion.
‘The small intestine comprises of three main regions that are the duodenum, the jejunum, and the ileum’ (Dupree & Dupree, 2014). The small intestine is where digestion is completed and virtually all absorption occurs. Combined with pancreatic juice, intestinal juice provides the liquid medium needed to further digest and absorb substances from chyme. The small intestine is also the site of unique mechanical digestive movements. Moreover, the digestion of proteins which are polymers composed of amino acids linked by peptide bonds to form long chains begin in the stomach. Digestion reduces them to their constituent amino acids. The stomach continues the digestion of carbohydrates and fats, Hydrochloric acid (HCl) and pepsin breaks proteins into smaller polypeptides, which then travel to the small intestine. Clegg and Mackean (2000) showed that chemical digestion in the small intestine is continued by pancreatic enzymes, including chymotrypsin and trypsin, each of which act on specific bonds in amino acid sequences. At the same time, the cells of the brush border secrete enzymes such as amino peptidase and dipeptidase, which further break down peptide chains. This results in molecules small enough to enter the bloodstream.
Furthermore, when lipids arrive in the intestine largely undigested, so much of the focus there is on lipid digestion, which is facilitated by bile and the enzyme pancreatic lipase. Moreover, intestinal juice combines with pancreatic juice to provide a liquid medium that facilitates absorption. ‘The small intestine’s absorptive cells also synthesize digestive enzymes and then place them in the plasma membranes of the microvilli. This distinguishes the small intestine from the stomach; that is, enzymatic digestion occurs not only in the lumen, but also on the luminal surfaces of the mucosal cells’ (Mackean, 1995.) For optimal chemical digestion, chyme must be delivered from the stomach slowly and in small amounts. The most common dietary lipids are triglycerides, which are made up of a glycerol molecule bound to three fatty acid chains. In addition, the three lipases responsible for lipid digestion are lingual lipase, gastric lipase, and pancreatic lipase. However, because the pancreas is the only consequential source of lipase, virtually all lipid digestion occurs in the small intestine.
Finally yet importantly, the mechanical and digestive processes have one goal: to convert food into molecules small enough to be absorbed by the epithelial cells of the intestinal villi. The absorptive capacity of the digestive tract is almost endless. Although the entire small intestine is involved in the absorption of water and lipids, most absorption of carbohydrates and proteins occurs in the jejunum. By the time chyme passes from the ileum into the large intestine, it is essentially indigestible food residue (mainly plant fibers like cellulose), some water, and millions of bacteria.

According to Cnx.org (2013), absorption can occur through five mechanisms: active transport, passive diffusion, facilitated diffusion, cotransport (or secondary active transport), and endocytosis. Active transport refers to the movement of a substance across a cell membrane going from an area of lower concentration to an area of higher concentration (up the concentration gradient). In this type of transport, proteins within the cell membrane act as pumps, using cellular energy (ATP) to move the substance. Passive diffusion refers to the movement of substances from an area of higher concentration to an area of lower concentration, while facilitated diffusion refers to the movement of substances from an area of higher to an area of lower concentration using a carrier protein in the cell membrane. Co-transport uses the movement of one molecule through the membrane from higher to lower concentration to power the movement of another from lower to higher. Finally, endocytosis is a transportation process in which the cell membrane engulfs material. It requires energy, generally in the form of ATP. Because the cell’s plasma membrane is made up of hydrophobic phospholipids, water-soluble nutrients must use transport molecules embedded in the membrane to enter cells..
Firstly, all carbohydrates are absorbed in the form of monosaccharides. The small intestine is highly efficient at this, absorbing monosaccharides at an estimated rate of 120 grams per hour. All normally digested dietary carbohydrates are absorbed; indigestible fibers are eliminated in the faeces. The monosaccharides leave these cells via facilitated diffusion and enter the capillaries through intercellular clefts. The monosaccharide fructose (which is in fruit) is absorbed and transported by facilitated diffusion alone. The monosaccharides combine with the transport proteins immediately after the disaccharides are broken down.

Secondly, active transport mechanisms, primarily in the duodenum and jejunum, absorb most proteins as their breakdown products, amino acids. Almost all protein is digested and absorbed in the small intestine. The type of carrier that transports an amino acid varies. Most carriers are linked to the active transport of sodium. Short chains of two amino acids (dipeptides) or three amino acids (tripeptides) are also transported actively.
Lastly, about 95 percent of lipids are absorbed in the small intestine. Bile salts not only speed up lipid digestion, they are also essential to the absorption of the end-products of lipid digestion. Fatty acids are relatively water-soluble and can enter the absorptive cells (erythrocyte) directly. The small size of short-chain fatty acids enables them to be absorbed by erythrocyte via simple diffusion, and then take the same path as monosaccharides and amino acids into the blood capillary of a villus.

In the end, the digestive system’s main purpose is to break down energy containing compounds and get them into circulation so they can be used by the rest of the body. There are mechanical and chemical processes that work simultaneously to digest the food and provide energy to the body.

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