The Digestive System

An Accelerated Review of the Digestive System

Extracellular Digestion Explained

1. What is digestion?

Digestion is the breaking down of larger organic molecules obtained from the diet, such as carbohydrates, fats and proteins, into smaller ones, such as glucose, fatty acids, glycerol and amino acids.

The Digestive System - Biology Questions and Answers

More Bite-Sized Q&As Below

2. How different are intracellular and extracellular digestion? What is the evolutionary advantage of extracellular digestion?

During intracellular digestion, the breaking down of macromolecules takes place within the cell. During extracellular digestion, macromolecules are broken down in places outside the cell (in the extracellular space, in the surrounding area, in the lumen of digestive tracts, etc.)

The evolutionary development of extracellular digestion allowed organisms to benefit from a greater variety of foods. The breaking down of larger molecules into smaller ones outside the cell permitted the use of other foods that, due the size of their molecules, could not be interiorized by diffusion, phagocytosis or pinocytosis.

3. How is extracellular digestion related to cell and tissue specialization?

A variety of specialized cells and tissues appeared as a result of extracellular digestion to provide enzymes and special structures for the breaking down of dietary macromolecules.

This phenomenon allowed other cells to be used for other tasks and differentiations while benefiting from nutrients distributed via circulation.

Complete Digestive Systems

4. What is the difference between a complete digestive system and an incomplete digestive system? How are these types of digestive systems related (or not) to extracellular digestion?

Animals with an incomplete digestive system are those in which the digestive tract has only one opening (cnidarians, platyhelminthes). Animals with a complete digestive system are those in which the digestive tract has two openings, a mouth and an anus (including all other animal phyla, with the exception of poriferans, which do not have any digestive tract).

In animals with incomplete digestive tracts, digestion is mixed. It begins in the extracellular space and finishes in the intracellular space. In animals with complete digestive systems, extracellular digestion within the digestive tract predominates.

5. What are some of the evolutionary advantages among animals with a complete digestive tract?

A complete digestive tract allows animals to continuously feed without waiting for waste to be eliminated before beginning to digest new foods. In this way, the absorption of larger amounts of nutrients is possible and therefore bigger and more complex species can develop. Digestive tracts with two openings also make digestion more efficient, since they provide different sites with different physical and chemical conditions (mouth, stomach, bowels) for the action of different complementary digestive enzyme systems.

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Mechanical Digestion

6. What is mechanical digestion? In general, in molluscs, arthropods, earthworms, birds and vertebrates, which organs are involved in this type of digestion?

Mechanical digestion is the fragmentation of food aided by specialized physical structures, such as teeth, prior to extracellular digestion. The mechanical fragmentation of food helps digestive enzyme reactions, because it provides a larger total area for the contact between enzymes and their substrates.

In some molluscs, mechanical fragmentation is carried out by the radula (a tooth-like structure). Some arthropods, such as lobsters and dragonflies, have mouthparts that carry out the mechanical digestion of food. In earthworms and birds, mechanical digestion is carried out by an internal muscular organ. In mandibulate vertebrates, mandibles and chewing muscles exist to triturate food prior to chemical digestion.

Chemical Digestion

7. Concerning extracellular digestion, what is meant by chemical digestion?

Chemical digestion is the series of enzymatic reactions used to break down macromolecules into smaller ones. 

8. Which type of chemical reaction is the breaking down of macromolecules into smaller ones that occurs during digestion? What are the enzymes that participate in this process called?

The reactions of extracellular digestion are hydrolysis reactions; or rather, the breaking down of molecules with the help of water. The enzymes that participate in digestion are hydrolytic enzymes.

Human Digestive Tissues and Organs

9. Which organs of the body are part of the human digestive system?

The digestive system, also known as “systema digestorium”, or the gastrointestinal system, is composed of the digestive tract organs plus the digestive adnexal glands. The digestive tract is composed of the mouth, pharynx, esophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (caecum, colon, rectum) and anus. 

10. What are peristaltic movements? What is their role in human digestion?

Peristalsis is the process of synchronized contractions of the muscular wall of the digestive tract. Peristaltic movements may occur starting at the esophagus up until and including the bowels.

Peristaltic movements are involuntary and they have the function of moving and mixing food along the digestive tube. Peristaltic movement deficiency (in the event of injuries of the innervation of the muscular wall of the digestive tract caused by Chagas’ disease, for example) can lead to the interruption of food traffic inside the bowels, leading to severe clinical consequences such as megacolon (abnormal enlargement of the colon) and megaesophagus (enlargement of the esophagus). 

11. From the lumen to the external surface, what tissues form the digestive tract wall?

From the internal surface to the external surface, the digestive tract wall is made of mucosa (epithelial tissue responsible for intestinal absorption), submucosa (connective tissue beneath the mucous membrane where blood, lymphatic vessels and neural fibers are located), muscle layers (smooth muscle tissue, two layers, one interior circular layer and another exterior longitudinal layer, structures responsible for peristaltic movements), and the serous membrane (epithelial and connective tissue which form the external surface of the organ). In the bowels, the serous membrane extends to form the mesentery, a serosa that encloses blood vessels and supports the bowels within the abdominal cavity. 

The Mouth and Salivary Glands

12. Where are salivary glands located in humans?

There are 6 major salivary glands in humans, one of which located in each parotid gland, two beneath the jaws (submandibular) and two at the base of the tongue (sublingual). More than 700 other minor salivary glands exist dispersed on the lip mucosa, gums, palate and pharynx.

13. What is the approximate pH of saliva secretion? Is it an acidic or alkaline fluid? What are the main functions of saliva?

The pH of saliva is approximately 6.8. Therefore, it is a slightly acidic pH.

Saliva lubricates the food and starts its enzymatic extracellular digestion. It also works as a buffer for the pH of the mouth, as well as plays an important role in protecting the body against pathogens, due to the presence of IgA antibodies in it (also present in tears, the colostrum, mother’s milk and in the mucosae of the intestine and airways).

14. What digestive enzyme is contained in saliva? Which type of food does it digest and into which smaller molecules does it break down the food?

Salivary hydrolase is known as salivary amylase, or ptyalin. Ptyalin digests carbohydrates by breaking down starch and glycogen, glucose polymers, into maltose (a glucose disaccharide) and dextrin.

The Esophagus

15. Why doesn't food enter the trachea instead the esophagus?

When food is swallowed, the swallow reflex is activated and the larynx elevates and closes to stop food particles from entering the trachea, preventing the aspiration of foreign materials into the bronchi.

  • Digestive System Review - Image Diversity: esophagus

16. Is the esophagus a muscular organ? Why can food reach the stomach even if someone is lying down flat on a bed?

The esophagus is a predominantly muscular organ. It is a muscular tube, which consists of striated muscle tissue in its upper third, mixed muscle tissue (striated and smooth) in its middle third and smooth muscle tissue in its lower third. The peristalsis of the esophagus causes the food to move towards the stomach even without the help of gravity.

The Stomach

17. What route does ingested food follow, from the time of swallowing until it reaches the duodenum?

Until reaching the duodenum, food enters the mouth, passes through the pharynx, goes down the esophagus and passes through the stomach.

18. What is the valve that separates the stomach from the esophagus called? What is its function?

The valve that separates the stomach from the esophagus is called the cardia. It has the function of preventing the acidic contents of the stomach from entering back the esophagus once again. The improper functioning of this valve causes gastroesophageal reflux, a disease in which patients complain of bloating and heartburn (retrosternal burning).

  • Digestive System Review - Image Diversity: stomach

19. What is the valve that separates the duodenum from the stomach called? What is its function?

The valve that separates the stomach from the duodenum is the pylorus. It has the function of keeping food within the gastric cavity for enough time to allow gastric digestion to take place. It also has the function of preventing intestinal contents from going back into the stomach.

20. What is the pH inside the stomach? Why is it necessary to maintain that pH level? How is it maintained? What cells produce that pH?

The normal pH of gastric juice is around 2. Therefore, it is an acidic pH.

It is necessary for the gastric pH to be maintained acidic for the transformation of pepsinogen (a proenzyme secreted by gastric chief cells) into pepsin, the digestive enzyme that acts only under low pHs. This pH level is attained through the secretion of hydrochloric acid (HCl) by parietal cells. 

21. Besides being necessary for the activation of the main gastric digestive enzyme, how is HCl directly involved in digestion?

With its corrosive effect, HCl also helps rupture the bond between food particles, facilitating the digestive process.

22. How are the gastric mucosa protected from the acidic pH of the stomach?

The gastric epithelium is mucus secretory tissue, meaning that it produces mucus. The mucus covers the stomach wall, preventing its corrosion by gastric juice.

23. What digestive enzyme acts within the stomach? Which type of food does it digest? What cells produce that enzyme?

The digestive enzyme that acts in the stomach is pepsin. Pepsin has the function of breaking proteins down into smaller peptides. The gastric cells that produce pepsinogen (the zymogen precursor of pepsin) are chief cells.

The Small Intestine

25. What are the three parts of the small intestine?

The small intestine is divided into three parts: the duodenum, jejunum and ileum. 

26. Which of carbohydrates, fats or proteins have already undergone chemical digestion upon reaching the pylorus (upon exiting the stomach)?

Upon exiting the stomach, carbohydrates have already undergone chemical digestion in the mouth and proteins have already undergone the chemical digestion process from the enzymes in the stomach. Carbohydrates are changed under the effects of salivary amylase (ptyalin) and proteins are changed under the action of the enzyme pepsin in gastric juice. Fats do not undergo chemical digestion until reaching the duodenum.

The Liver and Gallbladder

27. What substance produced in the liver is involved in digestion in the small intestine? What is the role of this substance in the digestive process?

Bile, an emulsifier liquid, is made by the liver and later stored in the gallbladder and released in the duodenum.

Bile is composed of bile salts, cholesterol and bile pigments. Bile salts are detergents, amphiphilic molecules, or rather, molecules with a polar water-soluble portion and a non-polar fat-soluble portion. This feature allows bile salts to enclose fats inside water-soluble micelles in a process called emulsification. Through this process, fats come into contact with intestinal lipases, enzymes that break them down into simpler fatty acids and glycerol.

28. What is the adnexal organ of the digestive system in which bile is stored? How does this organ react to the ingestion foods high in fat?

Bile is concentrated and stored in the gallbladder.

When foods high in fat are ingested, the gallbladder contracts to release bile into the duodenum. (This is the reason why patients with gallstones must not ingest fatty foods, as the reactive contraction of the gallbladder may move some of the stones to the point of blocking the duct that drains bile into the duodenum, causing pain and possible severe complications.) 

29. What are the digestive functions of the liver?

Besides making bile for release in the duodenum, the liver has other digestive functions.

The network of veins that absorbs nutrients from the intestines, called mesenteric circulation, drains its blood content almost entirely to the hepatic portal vein. This vein irrigates the liver with materials absorbed from digestion. Therefore, the liver is involved in storing, processing and inactivating nutrients.

Glucose is polymerized into glycogen in the liver. This organ also stores many vitamins and the iron absorbed in the intestine. Some important metabolic molecules, such as albumin and clotting factors, are made in the liver from dietary amino acids. In the liver, ingested toxic substances, such as alcohol and drugs, are also inactivated. 

  • Digestive System Review - Image Diversity: liver

The Pancreas

30. In addition to the liver, what other adnexal gland of the digestive system releases substances involved in extracellular digestion into the duodenum?

The other adnexal gland of the digestive system is the pancreas. This organ produces the digestive enzymes that digest proteins (proteases), lipids (lipases) and carbohydrates (pancreatic amylases). Other digestive enzymes, such as gelatinase, elastase, carboxypeptidase, ribonuclease and deoxyribonuclease are also secreted by the pancreas.

  • Digestive System Review - Image Diversity: pancreas

31. How does pancreatic juice participate in the digestion of proteins? What enzymes are involved?

The pancreas secretes trypsinogen which, upon being subject to the action of the enzyme enterokinase, which is secreted by the duodenum, is transformed into trypsin. Trypsin in turn catalyzes the activation of pancreatic chymotrypsinogen into chymotrypsin. Trypsin and chymotrypsin are proteases that break proteins down into smaller peptides. The smaller peptides are then broken down into amino acids by the enzyme carboxypeptidase (also secreted by the pancreas in zymogen form and activated by trypsin) with the help of the enzyme aminopeptidase, which is produced in the intestinal mucous membrane.

32. How does pancreatic juice proceed with the digestion of carbohydrates? What enzyme is involved?

Carbohydrate digestion begins with the action of salivary amylase (ptyalin) in the mouth and continues in the duodenum through the action of pancreatic juice. This juice contains the enzyme pancreatic amylase, or amylopsin, which breaks down starch (amylum) into maltose (a disaccharide made of two glucose molecules).

33. How does pancreatic juice help digest lipids? What enzyme is involved?

The enzyme pancreatic lipase is present in pancreatic juice. This enzyme breaks down triacylglycerol (triglyceride) into fatty acids and glycerol.

Digestive Enzymes, Digestive Secretions and pH

34. In addition to pancreatic juice in the intestine, enteric juice containing digestive enzymes is also secreted. What are these enzymes and which type of molecule do each of these enzymes break down?

Enteric juice is secreted by the small intestine mucosa. The enzymes of enteric juice and their respective functions are described as follows:

Enterokinase: enzyme that activates trypsinogen into trypsin. Saccharase: enzyme that breaks down sucrose (saccharose) into glucose and fructose. Maltase: enzyme that breaks down maltose into two glucose molecules. Lactase: enzyme that breaks down  lactose into glucose and galactose. Peptidases: enzymes that break down oligopeptides into amino acids. Nucleotidases: Enzymes that break down nucleotides into their components (nitrogenous bases, phosphates and pentoses).

35. Coming from the acidic pH of the stomach, what pH is present when chyme enters the duodenum? Why is it necessary to maintain that pH level in the small intestine? What organs are responsible for that pH level and how is it maintained?

Upon entering the duodenum, chyme comes into contact with pancreatic juice with a pH of approximately 8.5. The neutralization of the acidity of the chyme is necessary to maintain the adequate pH level for the functioning of the digestive enzymes that act in the duodenum. Without the neutralization of the acidity of the chyme, mucous membrane of the intestine would be damaged.

When stimulated by the acidity of the chyme, the duodenum produces a hormone called secretin. Secretin stimulates the pancreas to release pancreatic juice and also signals the gallbladder to expel bile in the duodenum. The pancreatic secretion, rich in bicarbonate ions, is released in the duodenum and neutralizes the chyme acidity; this acidity is also neutralized by the secretion of bile in the duodenal lumen.

36. What are the five human digestive secretions? Which of them is the only one that does not contain digestive enzymes?

The human digestive secretions are: saliva, gastric juice, bile, pancreatic juice and enteric juice. Among these secretions, only bile does not contain digestive enzymes.

37. Why do protease-producing cells of the stomach and of the pancreas produce only the precursors to active proteolytic enzymes?

The stomach and the pancreas make zymogens of the proteases pepsin, chymotrypsin and trypsin and these zymogens are released into the gastric or duodenal lumen for activation. This is to prevent the digestion of these organs' (stomach and pancreas) own cells and tissues by the active form of the enzymes. Therefore, the production of zymogens is a protective strategy against the natural effects of proteolytic enzymes.

Intestinal Villi and Microvilli

38. After digestion, the next step is absorption by cells of the mucous membrane of the intestine. For this to happen, a large absorption surface is an advantage. How is it possible for the small internal space of the body of a pluricellular organism to contain a large intestinal surface?

Evolution tried to solve this problem in two ways. The simplest way is the long and tubular shape of the bowels (approximately eight meters in length), made possible by the closely folded and numerous loops of the small intestine. More efficient solutions are intestinal villi and the microvilli of the mucosal membrane cells.

The intestinal wall is not smooth. The mucous membrane, together with its submucosa, projects into the gut lumen like gloved fingers, forming invaginations and villi that multiply the available surface for absorption. In addition, the epithelial cells that cover these villi contain numerous hair-like projections called microvilli on the external surface (lumen surface) of their plasma membrane. The absorptive area of the intestines is thus increased hundreds of times through these solutions.

The jejunum and ileum contain folds that also have the function of increasing the absorption surface.

The Colon

39. In which part of the digestive tract is water mainly absorbed? What about mineral ions and vitamins?

The majority of water, vitamins and mineral ions are absorbed by the small intestine. The large intestine, however, is responsible for the reabsorption of nearly 10% of ingested water, a significant amount that gives consistency to feces (colon diseases can cause diarrhea).

  • Digestive System Review - Image Diversity: the colon

The Route from Digestion to Tissues

40. From the intestinal lumen to tissues, what is the route of nutrients after digestion?

Monosaccharides, amino acids, mineral salts and water are absorbed by the intestinal epithelium and collected by the capillary vessels of the intestinal villi. From the capillaries, nutrients go to the mesenteric circulation, a system of blood vessels that drains the intestinal loops. The blood from the mesenteric circulation is drained to the hepatic portal vein and some nutrients are processed by the liver. From the liver, nutrients are gathered by the hepatic veins, which discharge their blood content into the inferior vena cava. Blood from the inferior vena cava then gains the right chambers of the heart and is pumped to the lungs for oxygenation. From the lungs, the blood then returns to the heart, where it is pumped to tissues, thus distributing nutrients and oxygen.

Chylomicrons and Cholesterol

41. What is the special route that lipids follow during digestion? What are chylomicrons?

Triglycerides emulsified by bile within micelles are subject to the action of lipases, which break them down into fatty acids and glycerol. Fatty acids, glycerol and cholesterol are absorbed by the intestinal mucosa. In the interior of the mucosal cells, fatty acids and glycerol form triglycerides once again, which, along with cholesterol and phospholipids, are packed in small vesicles covered by proteins called chylomicrons. The chylomicrons are released in minuscule lymphatic vessels as opposed to blood vessels, and enter into lymphatic circulation. Therefore, the lymphatic system plays an important role in the absorption of lipids.

Lymphatic circulation drains its content into venous blood circulation. In that way, chylomicrons reach the liver, where their lipid content is processed and released into the blood in the form of protein-containing complexes called lipoproteins, such as HDL, VLDL and LDL.

42. What are the so-called “good” and “bad” types of cholesterol?

Lipoproteins are complexes made of lipids (triglycerides and cholesterol) and proteins. Lipoproteins present different densities according to the ratio of their protein to lipid quantities, since lipids are less dense than proteins. Low-density lipoproteins (LDL) are those with a low protein/lipid ratio; high-density lipoproteins (HDL) have a high protein/lipid ratio; another group is very low-density lipoproteins (VLDL) with a very low protein/lipid ratio.

LDL is known as “bad cholesterol” because it transports cholesterol from the liver to tissues and, as a result, contributes to the formation of atheroma plaques inside blood vessels, a condition called atherosclerosis (not to be confused with arteriosclerosis), which can lead to severe circulatory obstructions such as acute myocardial infarction, cerebrovascular accidents and thrombosis. HDL is known as “good cholesterol” because it transports cholesterol from tissues to the liver (to be eliminated with bile). A high amount of HDL in the blood reduces the risk of atherosclerosis. (VLDL transforms into LDL after losing triglycerides in the blood).

The Digestive function of Vegetable Fibers

43. Why does the ingestion of vegetable fibers improve the regularity of the bowel movements in people who suffer from hard stool?

Some types of plant fibers are not absorbed by the intestine but play an important role in the functioning of the organ. They retain water inside the bowels and therefore contribute to the softening of the feces. Softer feces are easier to eliminate during defecation. People who eat less dietary fiber may suffer from hard stool and constipation.

Intestinal Bacterial Flora

44. What are the main functions of the bacterial flora within the human gut?

Bacteria that live inside the gut play an important role in digestion. Some polysaccharides such as cellulose, hemicellulose and pectin are not digested by digestive enzymes secreted by the body; instead, they are broken down by enzymes released by bacteria in the gastrointestinal tract. Intestinal bacterial flora also produce substances vital to the functioning of the bowels, facilitating or blocking the absorption of nutrients and stimulating or reducing peristalsis. Some gut bacteria are the main source of vitamin K for the body and, as a result, they are essential for the blood clotting process.

The intestinal flora contains useful but also potentially harmful bacteria. It is estimated that more than 100 trillion bacteria live in a human gut. Some bacteria are useful because they compete with other species, preventing the excessive proliferation of other bacteria.

Digestive Hormones

45. The release of digestive secretions is controlled by hormones. What hormones participate in this regulation?

The hormones that participate in the regulation of digestion are gastrin, secretin, cholecystokinin and enterogastrone.

46. How is gastrin produced and what is its function in the digestive process?

The presence of food in the stomach stimulates the secretion of gastrin, which in turn triggers the release of gastric juice.

47. Where is secretin produced and what is its  function in the digestive process?

Secretin is produced in the duodenum. The acidity of  chyme causes the duodenum to release this hormone, which in turn stimulates the secretion of pancreatic juice.

48. How is cholecystokinin produced and what is its function in the digestive process?

The fat level of the chyme detected in the duodenum stimulates the secretion of cholecystokinin (CCK). CCK acts by stimulating the secretion of pancreatic juice and the releasing of bile by the gallbladder.

49. Where is enterogastrone produced and what is its function in the digestive process?

When chyme is too fatty, the duodenum secretes enterogastrone. This hormone reduces the peristalsis of the stomach, thus slowing the entrance of food into the duodenum (as the digestion of fats takes more time).

Avian and Ruminant Digestive Systems

50. What are the special structures of the avian digestive tract and their respective functions?

The digestive tract of birds contains special structures, which occurs in this order: the crop, the proventriculus and the gizzard.

The crop has the function of the temporary storage of ingested food and is a more dilated area of the avian esophagus. The proventriculus is the chemical stomach of birds, in which food is mixed with digestive enzymes. The gizzard is a muscular pouch that serves as a mechanical stomach, in which food is ground up to increase the exposure area of the food particles to digestive enzymes. 

51. Compared to mammals, do birds absorb more or less water in their digestive system? Why is this phenomenon an adaptation to flight?

Bird feces are more liquid than mammal feces, meaning that less water is absorbed by the avian digestive system. The more frequent elimination of feces in birds due to their less solid feces is an adaptation to flight, since their body weight is maintained lower.

52. What is meant by “the mutualistic digestion of cellulose”, a phenomenon that occurs in some mammals and insects?

Herbivorous animals eat large amounts of cellulose, a substance not digested by their digestive enzymes. In these animals, regions of the digestive tract are colonized by microorganisms that digest cellulose. As a result, a mutualistic ecological interaction between animals and microorganisms occurs. This interaction is present in horses, cows, rabbits and in some insects, such as termites.

53. Cows swallow their food once and then this food goes back to the mouth to be chewed again. How can this phenomenon be explained?

The food ingested by cows and other ruminant animals first passes through two compartments of the digestive tract called the rumen and the reticulum. Within them, the food is subject to the action of digestive enzymes released by microorganisms that live there in a mutualistic ecological interaction. In the reticulum, the food is broken down. After passing through reticulum, the food (cud) is regurgitated to the mouth to be chewed and swallowed once again in a process called rumination. The food then enters the omasum, where it is mechanically mixed. After that, the food goes to the abomasum, the organ where chemical digestion takes place. After leaving the abomasum (the true stomach). the food gains the intestine.

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