Proteins consist of polypeptides, which must be broken down into their constituent amino acids before they can be absorbed. Protein digestion occurs in the stomach and small intestine through the action of three primary enzymes: pepsin, secreted by the stomach; and trypsin and chymotrypsin secreted by the pancreas. The stomach also secretes hydrochloric acid, making the contents highly acidic, which is required for pepsin to work.
Trypsin and chymotrypsin in the small intestine require an alkaline environment to work. Bile from the liver and bicarbonate from the pancreas neutralize the acidic chyme as it empties into the small intestine. After pepsin, trypsin, and chymotrypsin break down proteins into peptides, these are further broken down into amino acids by other enzymes called peptidases, also secreted by the pancreas.
The chemical digestion of lipids begins in the mouth. The salivary glands secrete the digestive enzyme lipase, which breaks down short-chain lipids into molecules consisting of two fatty acids. A tiny amount of lipid digestion may take place in the stomach, but most lipid digestion occurs in the small intestine. Digestion of lipids in the small intestine occurs with the help of another lipase enzyme from the pancreas as well as bile secreted by the liver.
Bile is required for the digestion of lipids because lipids are oily and do not dissolve in the watery chyme. Bile emulsifies, or breaks up, large globules of food lipids into much smaller ones, called micelles, much as dish detergent breaks up grease. Lipase can then access and break down the micelles into individual fatty acid molecules.
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. Pancreatic enzymes called ribonuclease and deoxyribonuclease break down RNA and DNA, respectively, into smaller nucleic acids. These, in turn, are further broken down into nitrogen bases and sugars by small intestine enzymes called nucleases. 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:. 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. A few substances are absorbed in the stomach and large intestine. For example, water is absorbed in both of these organs, and some minerals and vitamins are also absorbed in the large intestine.
However, about 95 percent of nutrient molecules are absorbed in the small intestine. The absorption of the majority of these molecules takes place in the second part of the small intestine, called the jejunum. However, there are a few exceptions. For example, iron is absorbed in the duodenum, and vitamin B12 is absorbed in the last part of the small intestine, called the ileum.
After being absorbed in the small intestine, nutrient molecules are transported to other parts of the body for storage or further chemical modification. For example, amino acids are transported to the liver to be used for protein synthesis.
The epithelial tissue lining the small intestine is specialized for absorption. It has many wrinkles and is covered with villi and microvilli, creating an enormous surface area for absorption. The thin surface layer of epithelial cells of the villi transports nutrients from the lumen of the small intestine into these capillaries and lacteals.
Blood in the capillaries absorbs most of the molecules, including simple sugars, amino acids, glycerol, salts, and water-soluble vitamins vitamin C and the many B vitamins. Additionally, microscopic finger-like processes called villi and microvilli are present in the folds with each having a network of capillaries.
These capillaries transfer nutrients from food into the bloodstream. Jejunum and ileum absorb most of the nutrients while large intestine does the majority of the water absorption.
Finally, after absorption at the large intestine, the undigested and unabsorbed portion is ready for defecation. Digestion and absorption are two of the processes that occur in our digestive system. Ingestion is the first process, then digestion and the absorption follows digestion. Digestion is the process that breaks food into small pieces and then into molecules. Absorption is the process of absorbing nutrients in the form of molecules into the blood.
Therefore, this is the key difference between digestion and absorption. Digestion starts in the mouth while absorption starts in the stomach. Moreover, digestion occurs from mouth to intestine while absorption mostly occurs from the stomach to intestine. So, this is another difference between digestion and absorption. The below infographic illustrates more details on the difference between digestion and absorption.
Once we ingest foods, they undergo digestion and absorption. A variety of digestive enzymes are secreted into the stomach to dissolve and break down carbohydrates , proteins , and lipids in food. The small intestine also secretes digestive enzymes for the further break down of the materials. Ultimately, carbohydrates, proteins, lipids, and nucleic acids in the food are broken down into monosaccharides , disaccharides , oligosaccharides, amino acids , triglycerides , and nucleotides in the small intestine.
These small nutrients are absorbed by the stomach, small intestine, and the large intestine. The components of the human digestive system and their functions are shown in figure 1.
Figure 1: Human Digestive System. However, fungi do not ingest food. Instead, they secrete digestive enzymes on the external food materials. Therefore, their digestion is called the external digestion. Absorption refers to the assimilation of substances into the blood stream from the digestive tract. The small intestine is the major component of the animal digestive system where absorption takes place. The components of the digested food such as simple sugars, amino acids, fatty acids, triglycerides, and nucleotides move across the wall of the small intestine into the blood stream.
In order to increase the efficiency of absorption in the small intestine, the epithelium of it contains small projections called villi. The epithelium of the villi consists of microvilli.
Two types of blood vessels are involved in absorbing nutrients from the small intestine. They are the capillaries and lacteals. The salivary glands secrete the digestive enzyme lipase , which breaks down short-chain lipids into molecules consisting of two fatty acids. A tiny amount of lipid digestion may take place in the stomach, but most lipid digestion occurs in the small intestine.
Digestion of lipids in the small intestine occurs with the help of another lipase enzyme from the pancreas, as well as bile secreted by the liver. As shown in the diagram below Figure Bile emulsifies or breaks up large globules of food lipids into much smaller ones, called micelles, much as dish detergent breaks up grease.
Lipase can then access and break down the micelles into individual fatty acid molecules. 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. Pancreatic enzymes called ribonuclease and deoxyribonuclease break down RNA and DNA, respectively, into smaller nucleic acids. These, in turn, are further broken down into nitrogen bases and sugars by small intestine enzymes called nucleases.
Your large intestine is not just made up of cells. Friendly bacteria live mostly in the large intestine and part of the small intestine. The acidic environment of the stomach does not allow bacterial growth. Gut bacteria have several roles in the body. For example, intestinal bacteria:. A wide range of friendly bacteria live in the gut. Bacteria begin to populate the human digestive system right after birth. Gut bacteria include Lactobacillus , the bacteria commonly used in probiotic foods such as yogurt, and E.
About a third of all bacteria in the gut are members of the Bacteroides species. Bacteroides are key in helping us digest plant food. It is estimated that trillion bacteria live in the gut. This is more than the human cells that make up you. It has also been estimated that there are more bacteria in your mouth than people on the planet — there are over 7 billion people on the planet!
The bacteria in your digestive system are from anywhere between and 1, species. As these bacteria are helpful, your body does not attack them. The bacteria actually cover themselves with sugar molecules removed from the actual cells of the digestive system. This disguises the bacteria and protects them from the immune system.
As the bacteria that live in the human gut are beneficial to us, and as the bacteria enjoy a safe environment to live, the relationship that we have with these tiny organisms is described as mutualism, a type of symbiotic relationship. Lastly, keep in mind the small size of bacteria.
Together, all the bacteria in your gut may weigh just about two pounds. The process of digestion is controlled by both hormones and nerves. Hormonal control is mainly by endocrine hormones secreted by cells in the lining of the stomach and small intestine.
These hormones stimulate the production of digestive enzymes, bicarbonate, and bile. The hormone secretin, for example, is produced by endocrine cells lining the duodenum of the small intestine. Acidic chyme entering the duodenum from the stomach triggers the release of secretin into the bloodstream. When the secretin returns via the circulation to the digestive system, it signals the release of bicarbonate from the pancreas.
The bicarbonate neutralizes the acidic chyme. See Table Gallbladder Partially digested fat and protein in duodenum Release lipase, trypsin, release bile Nerves involved in digestion include those that connect digestive organs to the central nervous system , as well as nerves inside the walls of the digestive organs. Nerves connecting the digestive organs to the central nervous system cause smooth muscles in the walls of digestive organs to contract or relax as needed, depending on whether or not there is food to be digested.
Nerves within digestive organs are stimulated when food enters the organs and stretches their walls. These nerves trigger the release of substances that speed up or slow down the movement of food through the GI tract and the secretion of digestive enzymes. When digestion is finished, it results in many simple nutrient molecules that must go through the process of absorption from the lumen of the GI tract to blood or lymph vessels, so they can be transported to and used by cells throughout the body.
A few substances are absorbed in the stomach and large intestine. Absorption of the majority of these molecules takes place in the second part of the small intestine, called the jejunum. There are, however, a few exceptions — for example, iron is absorbed in the duodenum , and vitamin B12 is absorbed in the last part of the small intestine, called the ileum.
After being absorbed in the small intestine, nutrient molecules are transported to other parts of the body for storage or further chemical modification. Amino acids, for instance, are transported to the liver to be used for protein synthesis. The epithelial tissue lining the small intestine is specialized for absorption. It is highly enfolded and is covered with villi and microvilli , creating an enormous surface area for absorption.
As shown in Figure The thin surface layer of epithelial cells of the villi transports nutrients from the lumen of the small intestine into these capillaries and lacteals. Blood in the capillaries absorbs most of the molecules, including simple sugars, amino acids, glycerol, salts, and water-soluble vitamins vitamin C and the many B vitamins.
Lymph in the lacteals absorbs fatty acids and fat-soluble vitamins vitamins A, D, E, and K.
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