MCAT Biology Review

Chapter 9: The Digestive System

Conclusion

In this chapter, we have reviewed a lot of information about the digestive system that we can use to our advantage on Test Day. We began with an overview of the anatomy, keeping in mind that the system is designed to carry out extracellular digestion. Considering that all our foodstuffs are made up of fats, proteins, and carbohydrates, these compounds have to be broken down to their simplest molecular forms before they can be absorbed and distributed to the tissues and cells of the body. As we moved through the gastrointestinal tract, we discussed whether each organ was a site of absorption, digestion, or both. We spent a good bit of time discussing each of the enzymes involved in digestion and their specific purposes. While digestion occurs primarily in the oral cavity, stomach, and duodenum, absorption occurs primarily in the jejunum and ileum, where the method of transport into the circulatory system is slightly different depending on the compound. Finally, we discussed the three segments of the large intestine and their roles in water and salt absorption, as well as the temporary storage of waste products. Although the amount of information about the digestive system may seem overwhelming, the underlying concepts are relatively straightforward, and a systematic approach (like charts, tables, or flashcards) will help you manage this content.

In the end, the digestive system’s main purpose is to break down energy-containing compounds and get them into the circulation so they can be used by the rest of the body. Equally as important are the systems the body has for getting rid of compounds from the blood. Buildup of waste products like ammonia, urea, potassium, and hydrogen ions can lead to serious pathology. For instance, hyperammonemia (buildup of ammonia in the blood) can lead to severe, permanent neurological damage. Hyperkalemia (buildup of potassium in the blood) can quickly cause a fatal heart attack. Temperature regulation is similarly important; both hyperthermia and hypothermia can lead to organ dysfunction and, ultimately, death. In the next chapter, we turn our attention to these regulatory systems: the renal system and the skin.

Concept Summary

Anatomy of the Digestive System

·        Intracellular digestion involves the oxidation of glucose and fatty acids to make energy. Extracellular digestion occurs in the lumen of the alimentary canal.

o   Mechanical digestion is the physical breakdown of large food particles into smaller food particles.

o   Chemical digestion is the enzymatic cleavage of chemical bonds, such as the peptide bonds of proteins or the glycosidic bonds of starches.

·        The pathway of the digestive tract is: oral cavity → pharynx → esophagus → stomach → small intestine → large intestine → rectum

·        The accessory organs of digestion are the salivary glands, pancreas, liver, and gallbladder.

·        The enteric nervous system is in the wall of the alimentary canal and controls peristalsis. Its activity is upregulated by the parasympathetic nervous system and downregulated by the sympathetic nervous system.

Ingestion and Digestion

·        Multiple hormones regulate feeding behavior, including antidiuretic hormone (ADH or vasopressin) and aldosterone, which promote thirst; glucagon and ghrelin, which promote hunger; and leptin and cholecystokinin, which promote satiety.

·        In the oral cavitymastication starts the mechanical digestion of food, while salivary amylase and lipase start the chemical digestion of food. Food is formed into a bolus and swallowed.

·        The pharynx connects the mouth and posterior nasal cavity to the esophagus.

·        The esophagus propels food to the stomach using peristalsis. Food enters the stomach through the lower esophageal (cardiacsphincter.

·        The stomach has four parts: fundusbodyantrum, and pylorus. The stomach has a lesser and greater curvature and is thrown into folds called rugae. Numerous secretory cells line the stomach.

o   Mucous cells produce bicarbonate-rich mucus to protect the stomach.

o   Chief cells secrete pepsinogen, a protease activated by the acidic environment of the stomach.

o   Parietal cells secrete hydrochloric acid and intrinsic factor, which is needed for vitamin B12 absorption.

o   G cells secrete gastrin, a peptide hormone that increases HCl secretion and gastric motility.

·        After mechanical and chemical digestion in the stomach, the food particles are now called chyme. Food passes into the duodenum through the pyloric sphincter.

·        The duodenum is the first part of the small intestine and is primarily involved in chemical digestion.

o   Disaccharidases are brush-border enzymes that break down maltose, isomaltose, lactose, and sucrose into monosaccharides.

o   Brush-border peptidases include aminopeptidase and dipeptidases.

o   Enteropeptidase activates trypsinogen and procarboxypeptidases, initiating an activation cascade.

o   Secretin stimulates the release of pancreatic juices into the digestive tract and slows motility.

o   Cholecystokinin stimulates bile release from the gallbladder, release of pancreatic juices, and satiety.

Accessory Organs of Digestion

·        Acinar cells in the pancreas produce pancreatic juices that contain bicarbonate, pancreatic amylase, pancreatic peptidases (trypsinogenchymotrypsinogencarboxypeptidases A and B), and pancreatic lipase.

·        The liver synthesizes bile, which can be stored in the gallbladder or secreted into the duodenum directly.

o   Bile emulsifies fats, making them soluble and increasing their surface area.

o   The main components of bile are bile salts, pigments (especially bilirubin from the breakdown of hemoglobin), and cholesterol.

·        The liver also processes nutrients (through glycogenesis and glycogenolysis, storage and mobilization of fats, and gluconeogenesis), produces urea, detoxifies chemicals, activates or inactivates medications, produces bile, and synthesizes albumin and clotting factors.

·        The gallbladder stores and concentrates bile.

Absorption and Defecation

·        The jejunum and ileum of the small intestine are primarily involved in absorption.

o   The small intestine is lined with villi, which are covered with microvilli, increasing the surface area available for absorption.

o   Villi contain a capillary bed and a lacteal, a vessel of the lymphatic system.

o   Water-soluble compounds, such as monosaccharides, amino acids, water-soluble vitamins, small fatty acids, and water, enter the capillary bed.

o   Fat-soluble compounds, such as fats, cholesterol, and fat-soluble vitamins, enter the lacteal.

·        The large intestine absorbs water and salts, forming semisolid feces.

o   The cecum is an outpocketing that accepts fluid from the small intestine through the ileocecal valve and is the site of the appendix.

o   The colon is divided into ascending, transverse, descending, and sigmoid portions.

o   The rectum stores feces, which are then excreted through the anus.

o   Gut bacteria produce vitamin K and biotin (vitamin B7).

Answers to Concept Checks

·        9.1

1.    Mechanical digestion, such as chewing, physically breaks food into smaller pieces. Chemical digestion involves hydrolysis of bonds and breakdown of food into smaller biomolecules.

2.    Oral cavity (mouth) → pharynx → esophagus → stomach → small intestine → large intestine → rectum → anus

3.    The parasympathetic nervous system increases secretions from all of the glands of the digestive system and promotes peristalsis. The sympathetic nervous system slows peristalsis.

·        9.2

1.    Saliva contains salivary amylase (ptyalin), which digests starch into smaller sugars (maltose and dextrin), and lipase, which digests fats. 

2.     

Cell

Secretions

Functions

Mucous cell

Mucus

Protect lining of stomach, increase pH (bicarbonate)

Chief cell

Pepsinogen

Pepsinogen Digest proteins, once activated by H+

Parietal cell

HCl, intrinsic factor

HCl: decrease pH, kill microbes, denature proteins, some chemical digestion; intrinsic factor: absorption of vitamin B12

G cell

Gastrin

Increase HCl production, increase gastric motility

3.     

Substance

Enzyme or Hormone?

Functions

Sucrase

Enzyme

Brush-border enzyme; break down sucrose into monosaccharides

Secretin

Hormone

Increase pancreatic secretions, especially bicarbonate, reduce HCl secretion, decrease motility

Dipeptidase

Enzyme

Brush-border enzyme; break dipeptides into free amino acids

Cholecystokinin

Hormone

Recruit secretions from gallbladder and pancreas; promote satiety

Enteropeptidase

Enzyme

Activate trypsinogen, which initiates an activation cascade

4.    Bile accomplishes mechanical digestion of fats, emulsifying them and increasing their surface area. Pancreatic lipase accomplishes chemical digestion of fats, breaking their ester bonds.

·        9.3

1.    Carbohydrates: pancreatic amylase; proteins: trypsin, chymotrypsin, carboxy-peptidases A and B; fats: pancreatic lipase

2.    Bile is composed of bile salts (amphipathic molecules derived from cholesterol that emulsify fats), pigments (especially bilirubin from the breakdown of hemoglobin), and cholesterol.

3.    Bile is synthesized in the liver, stored in the gallbladder, and serves its function in the duodenum.

4.    The liver processes nutrients (through glycogenesis and glycogenolysis, storage and mobilization of fats, and gluconeogenesis), produces urea, detoxifies chemicals, activates or inactivates medications, produces bile, and synthesizes albumin and clotting factors.

5.    As outgrowths of the gut tube, the accessory organs of digestion arise from embryonic endoderm.

·        9.4

1.    The two circulatory vessels are capillaries and lacteals. The capillary absorbs water-soluble nutrients, like monosaccharides, amino acids, small fatty acids, water-soluble vitamins, and water itself. The lacteal absorbs fat-soluble nutrients, like fats, cholesterol, and fat-soluble vitamins.

2.    The fat-soluble vitamins are A, D, E, and K.

3.    The small intestine consists of the duodenum, jejunum, and ileum. The large intestine consists of the cecum, colon, and rectum.

4.    While the large intestine’s main function is to absorb water, the small intestine actually absorbs a much larger volume of water. Thus, massive volumes of watery diarrhea are more likely to arise from infections in the small intestine than the large intestine.

Shared Concepts

·        Biochemistry Chapter 2

o   Enzymes

·        Biochemistry Chapter 9

o   Carbohydrate Metabolism I

·        Biochemistry Chapter 11

o   Lipid and Amino Acid Metabolism

·        Biology Chapter 5

o   The Endocrine System

·        Biology Chapter 7

o   The Cardiovascular System

·        Biology Chapter 8

o   The Immune System