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MCAT > MCAT Biology > Digestive System > Flashcards

Digestive System Flashcards

From pepsin and trypsin to the role of lacteals, use these cards to master the digestive system as tested on the Biological & Biochemical Foundations section of the MCAT.

1
Q

Define:

digestion

A

The mechanical and chemical process in which food is broken down into smaller components.

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2
Q

What is the difference between extracellular and intracellular digestion?

A
  • Extracellular digestion occurs outside of cells, as in the chemical digestion of food in the digestive tract.
  • Intracellular digestion is the breakdown of molecules within the cytoplasm or lysosomes of cells.
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3
Q

Is the breakdown of proteins by trypsin in the small intestine an example of intracellular or extracellular digestion?

A

extracellular chemical digestion

It takes place outside the cells, in the lumen of the duodenum.

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4
Q

What is the difference between mechanical and chemical digestion?

A
  • Mechanical digestion is the physical fragmenting of food into pieces.
  • Chemical digestion splits food into smaller chemical units by breaking bonds.
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5
Q

The churning of food in the stomach is an example of what kind of digestion?

A

Extracellular mechanical digestion

This activity physically mixes food but does not break any chemical bonds.

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6
Q

What is the alimentary canal?

A

Also called the digestive tract, it is the tube through which food enters the body and exits as waste.

The digestive tract consists of the mouth, pharynx, esophagus, stomach, small intestine, large intestine, and anus.

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7
Q

Define:

absorption

A

The process by which molecules are taken up from the digestive tract into the cells.

Absorption of nutrients occurs in the small intestine, while absorption of water occurs in the large intestine.

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8
Q

What name refers to the ball of food formed by the action of the tongue during ingestion?

A

bolus

Once formed, the bolus is forced to the back of the oropharynx and swallowed.

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9
Q

Why is mastication a useful precursor to the later chemical digestion of food?

A

Also known as chewing, it provides an increase in the surface area available for digestion.

The action of digestive enzymes, and thus digestion, is able to proceed more rapidly when the surface area-to-volume ratio of the ingested food particles is increased.

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10
Q

Which class of macromolecules is digested by salivary amylase?

A

Salivary amylase acts on starch, which is a carbohydrate.

This enzyme cleaves certain glycosidic linkages, producing the disaccharide maltose from starch, a polysaccharide.

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11
Q

What flap of elastic cartilage, if dysfunctional, could potentially lead to difficulty swallowing?

A

the epiglottis

One of the cartilaginous structures that constitute the voice box, the epiglottis is attached to the larynx. When elevated during swallowing, it blocks food from entering the trachea and directs it instead into the esophagus.

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12
Q

If the epiglottis does not close properly when a person is eating, what can happen?

A

Food can enter the trachea.

The epiglottis closes off the airway when a person is eating or drinking.

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13
Q

In the digestive tract, which structure is positioned between the oral cavity and the esophagus?

A

the pharynx

The pharnyx is part of both the respiratory and digestive systems. It is divided into three components: the nasopharynx, the oropharynx, and the laryngopharynx.

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14
Q

The upper portion of the esophagus is composed of what type of muscle?

A

striated muscle

The esophagus is a muscular tube through which food passes. The superior portion of the esophagus is surrounded by striated, or skeletal, muscle.

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15
Q

What term describes the coordinated muscular action that helps food pass through the digestive tract?

A

peristalsis

Peristalsis involves a series of muscular contractions throughout the gastrointestinal tract. These contractions are undergone by smooth muscle, meaning that the process is involuntary.

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16
Q

The pyloric sphincter separates the stomach from the duodenum. Which similar muscular structure serves to keep the contents of the stomach contained?

A

the cardiac sphincter

The digestive system contains two main sphincters, or muscular rings composed of smooth muscle. The cardiac sphincter separates the esophagus from the stomach.

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17
Q

What name is given to the semi-solid, partially digested food that exists in the stomach?

A

chyme

Chyme results from the mechanical and chemical breakdown of food in the stomach, and is composed of water, stomach acid, and digestive enzymes.

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18
Q

What are the key structural characteristics of the stomach?

A

It is a muscular and elastic sac sealed on both ends by sphincters. Due to a densely folded inner membrane, the stomach can easily stretch.

This organ is the main site of digestion. It secretes hydrochloric acid that facilitates the activity of certain enzymes.

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19
Q

Chyme is a product of which type(s) of digestion?

A

Both mechanical and chemical digestion.

Chyme is the mixture of food and fluid that forms in the stomach. Here, mechanical digestion is performed by the stomach musculature, while chemical digestion involves enzymes such as pepsin and gastric lipase.

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20
Q

What compounds do chief cells and parietal cells secrete into the stomach?

A
  • Chief cells: pepsinogen
  • Parietal cells: HCl and intrinsic factor

Both cell types are located in the gastric glands of the stomach.

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21
Q

How does the pH of the stomach lumen change in response to secretion of hydrochloric acid (HCl)?

A

decreases

HCl, sometimes referred to as gastric acid, is a strong acid. Its secretion from parietal cells keeps the pH of the stomach lumen around 2. Note that the lumen always refers to the inside cavity of an organ.

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22
Q

What are zymogens, and what function do they serve in the digestive system?

A

It is the inactive version of an enzyme, and can usually be activated via cleavage.

Zymogens are also known as proenzymes.

Zymogens are especially prevalent among proteases, or protein-digesting enzymes. The presence of an inactive form prevents these enzymes from breaking down the digestive cells themselves.

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23
Q

What zymogen responsible for digestion of proteins in the stomach is secreted by gastric chief cells?

A

pepsinogen

Pepsinogen is the inactive precursor of the proteolytic enzyme pepsin. When activated by the acidic conditions of the stomach, pepsin acts to digest protein by cleaving specific peptide bonds.

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24
Q

In what layer of the stomach are gastric glands located?

A

In the gastric mucosa

These glands contain chief and parietal cells, which secrete HCl, mucus, pepsinogen, and hormones. The mucosa is the uppermost layer of the stomach; in other words, it physically contacts the stomach contents.

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25
Q

How does the stomach’s mucous membrane work to protect the stomach?

A

It secretes mucus, an alkaline covering that protects the stomach from the acidity of its contents.

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26
Q

Through which structure does food leave the stomach when it passes into the duodenum?

A

pyloric sphincter

Sphincters are muscular valves. Upon parasympathetic stimulation, the pyloric sphincter opens to allow chyme into the duodenum of the small intestine.

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27
Q

Other than gastrin itself, what chemical or hormone would be over-secreted as a result of a gastrinoma, a gastrin-secreting tumor?

A

Hydrochloric acid

(HCl)

Gastrin is a hormone that stimulates HCl release and increases stomach motility. Hypersecretion of gastrin would thus lead to a decrease in pH and promotion of peristalsis.

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28
Q

Which blood vessels supply the liver?

A
  • hepatic portal vein
  • hepatic arteries

Portal circulation carries nutrient-rich blood from the digestive tract for processing, while the hepatic arteries supply oxygenated blood as part of the systemic circulation.

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29
Q

In what organ is bile produced?

A

In the liver

Bile is an alkaline emulsifying agent that aids in the digestion of fats. Note that while bile is synthesized in the liver, it is stored in the gall bladder.

30
Q

Xenobiotics, or foreign compounds, are modified during hepatic metabolism. What is the ultimate fate of these substances?

A

They will be excreted.

A main role of the liver involves processing of lipid-soluble drugs and toxins, which arrive from the hepatic portal vein. This modification aids in their removal from the body via excretion.

31
Q

Name the process of glucose synthesis that takes place in the liver and is stimulated by glucagon.

A

Gluconeogenesis, or the synthesis of glucose from amino acids and other non-carbohydrate substrates, occurs primarily in the liver.

This process also takes place in the kidneys.

Glucagon, a pancreatic hormone, acts to increase blood glucose levels. Since gluconeogenesis produces glucose, glucagon would act to stimulate it.

32
Q

Vitamins stored in the liver display what level of affinity for water?

A

Hydrophobic or hydrophilic, meaning that their affinity for water varies.

While the liver is the primary storage site for hydrophobic, fat-soluble vitamins, including A, D, and K, the water-soluble vitamin B12 is stored there as well.

33
Q

In a portal system, one capillary bed drains directly into another without going through the heart. What does hepatic portal circulation accomplish?

A

It drains blood from the gastrointestinal tract directly into blood vessels that feed the liver.

Another portal system often mentioned on the MCAT is the hypophyseal portal system, which connects the hypothalamus and the anterior pituitary.

34
Q

Digestion of what type of macromolecule is aided by bile?

A

lipids

Amphipathic bile salts serve to emulsify the hydrophobic component of dietary fats. While emulsification does not break chemical bonds, it does increase the surface area of fats available for contact with pancreatic lipase.

35
Q

Liver dysfunction can cause decreased synthesis of albumin. What effect would this abnormality have on peripheral tissue?

A

Edema, or swelling, would occur.

Albumin, the most abundant human blood protein, acts mainly to maintain osmotic pressure in the capillaries. A decrease in albumin concentration would cause circulatory osmotic (or more specifically, oncotic) pressure to become lower as well. With decreased fluid resportion into capillaries, fluid would remain in the tissue, causing edema.

36
Q

What substance does the gallbladder release, and in which part of the digestive tract does it act?

A

Bile is stored and concentrated in the gallbladder. When released, it acts on the duodenum of the small intestine.

Remember that the gallbladder stores, but does not produce, bile. When stimulated by the hormone cholecystokinin (CCK), this bile is released into the small intestine, where it emulsifies fats to assist in their digestion.

37
Q

What effect will a malfunctioning gallbladder have on digestion?

A

It will lead to improper digestion of fats.

The gallbladder stores and releases bile, which emulsifies fats, increasing the rate of their digestion. An impaired gallbladder would result in slow or incomplete lipid breakdown.

38
Q

Which structure of the digestive system serves both an endocrine and an exocrine function?

A

pancreas

Endocrine organs release hormones into the blood, while exocrine structures secrete enzymes or other substances through ducts. The pancreatic hormones insulin, glucagon, and somatostatin contribute to its endocrine function, while digestive enzymes such as pancreatic amylase are part of its exocrine role.

39
Q

What chemical bonds does carboxypeptidase cleave?

A

carboxy-terminal peptide bonds

Carboxypeptidase is one of a number of pancreatic digestive enzymes that selectively hydrolyzes certain chemical bonds. Since it assists in the breakdown of proteins, it is classified as a protease.

40
Q

Name three proteases produced by the pancreas.

A
  1. Trypsinogen
  2. Chymotrypsinogen
  3. Carboxypeptidase
    .

Note that both trypsinogen and chymotrypsinogen are zymogens, or inactive forms. Trypsinogen is converted to active trypsin, and chymotrypsinogen is converted to active chymotrypsin.

41
Q

Which pancreatic secretion neutralizes acidic chyme?

A

Bicarbonate

HCO3-

It neutralizes stomach acid in the small intestine. Its release is promoted by the hormone secretin.

As a base, bicarbonate raises the pH of the duodenum to around 6, in comparison to the stomach pH of 2.

42
Q

How does increased insulin secretion affect blood glucose levels in a healthy individual?

A

Insulin release decreases blood glucose concentration.

Insulin, a peptide hormone, is released in response to elevated blood glucose levels (after meals). By increasing uptake of glucose into the cells and promoting its storage as glycogen, this activity lowers blood glucose concentrations to normal levels.

43
Q

What is the most likely treatment for type I diabetes, an autoimmune destruction of pancreatic beta islet cells?

A

insulin administration

The loss of beta islet cell function results in reduced production of insulin. Administration of exogenous insulin is required to manage blood glucose concentrations in affected individuals. This differs from type II diabetes, in which cells become less responsive to insulin.

44
Q

What part of the brain, in addition to its role in regulating endocrine function, is involved in the hunger response?

A

The hypothalamus is involved in hunger, as well as hormone synthesis and thermoregulation.

Leptin, an appetite-inhibiting hormone, is produced by adipose cells in response to increased body fat. This hormone stimulates the production of neuropeptide Y, which in turn prevents the release of hunger-inducing orexins from the hypothalamus.

45
Q

What digestive organ synthesizes cholecystokinin (CCK)?

A

small intestine

This peptide hormone facilitates digestion in the duodenum. Its effects include inhibited gastric acid production, reduced or slowed gastric motility, and increased release of pancreative digestive enzymes and bile.

46
Q

What hormone is secreted in response to the acidity of the contents entering the small intestine?

A

Secretin, a pancreatic hormone, responds to excessive acidity in the duodenum.

Secretin slows gastric emptying, decreases HCl secretion in the stomach, increases the release of stomach bicarbonate, and promotes the secretion of enzymes through the pancreatic duct.

47
Q

In what organ does most absorption of dietary nutrients occur?

A

The small intestine

Remember that both digestion and absorption occur in the small intestine, while only digestion occurs in the stomach.

48
Q

Which segment of the small intestine precedes the jejunum?

A

The duodenum, or first segment

It is the site of most chemical digestion.

In the order that material passes through them, the segments of the small intestine are the duodenum, jejunum, and ileum.

49
Q

The majority of ingested ions and water are absorbed in which segment(s) of the small intestine?

A
  • jejunum
  • ileum

These segments contain enterocytes that are specialized for, among other things, absorption of ions, water, and vitamin B12, and resorption of bile salts.

50
Q

What term refers to the small, finger-like protrusions from the small intestinal wall that aid in digestion?

A

Intestinal villi

In the digestive system, villi are tube-shaped projections of the intestinal epithelium. These structures increase the total surface area available for nutrient absorption.

51
Q

What is the purpose of microvilli?

A

To further increase the surface area of the small intestine, allowing for more effective absorption of nutrients.

Microvilli are small projections that extend from the surface of larger villi.

52
Q

What is the digestive role of a lacteal?

A

This is a small lymphatic vessel within a villus, which absorbs lipids from the small intestine in the form of chylomicrons.

After briefly entering the lymphatic system, chylomicrons are brought to venous circulation by passing through the thoracic duct.

53
Q

In the gastrointestinal tract, which type of muscle is responsible for the mixing and propulsion of digestive contents?

A

Smooth muscle lines the human digestive tract. Its contraction propels the tract’s contents via peristalsis and mixes these contents within the stomach.

Since smooth muscle is innervated by the autonomic nervous system, this digestive activity is involuntary.

54
Q

What function of the small intestine would be most impaired by blockage of the pancreatic duct?

A

chemical digestion

The small intestine aids in both digestion and absorption. However, the pancreatic duct is only involved in the release of pancreatic enzymes, which contribute to the chemical digestion of lipids, proteins, and carbohydates.

55
Q

Which digestive organ is primarily involved in the absorption of water?

A

large intestine, or colon

The large intestine also absorbs salt, but all other biologically useful molecules are absorbed earlier, in the small intestine.

56
Q

What organisms are responsible for the production of flatus (gas) within the large intestine?

A

Bacteria

Gut flora, or bacteria that reside in the large intestine, act symbiotically to ferment undigested polysaccharides that arrive in the large intestine. This process produces small amounts of several gases.

57
Q

What pressure gradient drives absorption of water from the lumen of the colon?

A

The osmotic pressure gradient

Waste within the colon possesses relatively large amounts of both sodium and water. Epithelial cells actively absorb sodium, which establishes an osmotic gradient; water then follows sodium out of the intestinal lumen.

58
Q

What segment of the gastrointestinal tract immediately precedes the anus?

A

The rectum

After waste passes through the rest of the large intestine, it moves to the rectum, or final segment. From the rectum, waste enters the anal canal and is expelled.

59
Q

Into which segment of the large intestine does chyme from the ileum first pass?

A

Chyme first enters the cecum, the initial segment of the large intestine.

In order, the remaining segments of the large intestine are the ascending, transverse, descending, and sigmoid colon, followed by the last structure, the rectum.

60
Q

Relaxation of which two structures permits feces stored in the rectum to exit the anal canal?

A

The internal and external anal sphincters.

Though minor anatomical structures such as these are rarely tested on the MCAT, it is important to know that sphincters regulate release of digested contents, and that they are composed of smooth muscle.

61
Q

What small structure attached to the cecum has little, if any, current function?

A

The appendix

The appendix is a functionless, blind pouch found at the intersection of the large and small intestine. It is a frequent site of bacterial infection.

62
Q

Which vitamins are fat-soluble and which are water-soluble?

A
  • Fat-soluble: Vitamins A, D, E, and K
  • Water-soluble: Vitamins B and C

Water-soluble vitamins can easily be excreted in the urine, while fat-soluble vitamins tend to persist in the body’s adipose cells.

63
Q

Name the common enzymes that digest carbohydrates.

A
  • salivary amylase
  • pancreatic amylase
  • maltase
  • sucrase
  • lactase

In biology, the “-ase” ending generally denotes an enzyme. With digestive enzymes, the part of the word that precedes “-ase” often refers to the substance that is broken down. For example, lactase breaks down lactose into glucose and galactose.

64
Q

Name the common enzymes that digest proteins.

A
  • pepsin
  • trypsin
  • chymotrypsin
  • carboxypeptidase

Aminopeptidase, dipeptidase, and enterokinase also cleave peptide bonds but are less commonly tested.

Pepsin is found in the stomach, while trypsin, chymotrypsin, and carboxypeptidase act in the small intestine.

65
Q

Name the enzyme class that digests fats.

A

lipase

Lipases hydrolyze triglycerides, usually at a specific site on the glycerol backbone. Note that although bile helps to emulsify fats, it is not an enzyme; it breaks lipid particles into smaller droplets but does not perform catalysis.

66
Q

A patient is found to have impaired water absorption in his GI tract. Damage to which structure(s) could potentially produce this effect?

A

Impairment of either the small or large intestine could affect water absorption.

The small intestine absorbs both nutrients and water, while the large intestine absorbs the water that remains.

67
Q

How does blood flow in intestinal capillary beds promote absorption of simple sugars and amino acids?

A

Capillary flow promotes absorption by establishing a concentration gradient.

Due to the constant influx of blood to capillaries in the villi, the plasma concentration of amino acids and simple carbohydrates remains low compared to their concentration in intestinal cells. This establishes a concentration gradient that promotes nutrient diffusion from the intestinal epithelium into the bloodstream.

68
Q

A condition causes obstruction of the lymphatic vessels that drain the small intestine. Absorption of which macromolecule would be most altered by this disorder?

A

Absorption of fats would be most impaired.

Lymphatic vessels within villi, called lacteals, are only vital to the absorption of fats. Fatty acids are absorbed directly into intestinal cells. From there, triglycerides and esterified cholesterol molecules, in the form of chylomicrons, enter lymphatic circulation through lacteals.

69
Q

Digestion of which macromolecule would be most affected by a significant increase in the pH of the stomach?

A

An increased pH would most strongly affect protein digestion.

Breakdown of proteins in the stomach is carried out by pepsin. Pepsin activity, as well as its activation from pepsinogen, is optimized for the extremely low pH of the stomach. Any increase in pH that significantly exceeds 2 would result in lowered enzyme activity.

70
Q

Impaired secretion from which type of cell could potentially lead to erosion of the stomach’s mucosal lining?

A

Reduced secretion from goblet cells could damage the stomach lining.

Gastric goblet cells secrete mucus, which protects the stomach epithelium from the acidic contents of the lumen. When mucus is absent or reduced, damage to the epithelium can occur, leading to an erosive gastric ulcer.

71
Q

What property of amino acids requires their absorption from the small intestine via transport proteins?

A

They are polar molecules.

Polar molecules are unable to pass through the hydrophobic plasma membrane of intestinal cells. As a result, their absorption from the lumen of the small intestine requires specific transporter proteins.

MCAT Biology (14 decks)

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