6.1: Digestion and Absorption

List the source, substrate and product of amylases. Understanding: The pancreas secretes enzymes into the lumen of the small intestine.

Amylases are enzymes that hydrolyzes starch into maltose. Salivary Amylase Source: Salivary glands Substrate: starch Product: maltose Optimal pH: 7 Pancreatic Amylase Source: pancreas (secreted into small intestine) Substrate: starch Product: maltose Optimal pH: 7-8

Explain the use of dialysis tubing as a model for the small intestine. Application: Use of dialysis tubing to model absorption of digested food in the intestine.

Dialysis tubing is an artificial semipermeable membrane tubing that facilitates the flow of tiny molecules in solution based on differential size. Pores in the tubing allow water and small molecules or ions to pass through freely, but not large molecules. These properties mimic the wall of small intestine. Dialysis tubing can be used to model absorption by passive diffusion and osmosis.

List the source, substrate and product of lipase. Understanding: The pancreas secretes enzymes into the lumen of the small intestine.

Lipase is an enzyme that hydrolyzes fats to fatty acids and glycerol. Source: pancreas (secreted into small intestine) Substrate: triglyceride Product: glycerol and three fatty acids Optimal pH: 7-8

Outline the function of digestion of food. Understanding: Enzymes digest most macromolecules in food into monomers in the small intestine.

Many molecules are too large to be absorbed by the villi in the small intestine. Large food molecules are digested into small molecules that can be absorbed and moved into the cells.

Outline the function of the small intestine in digestion. Skill: Production of an annotated diagram of the digestive system.

Partially digested food from the stomach is mixed with bile from the liver and pancreatic juice from the pancreas to complete digestion. Then, the small intestine absorbs the nutrients and minerals from the food.

Outline the role of peristalsis in the digestive process. Understanding: The contraction of circular and longitudinal muscle of the small intestine mixes the food with enzymes and moves it along the gut.

Peristalsis is the involuntary, wave-like contraction of muscle layers of the small intestine. Peristalsis helps prevents backward movement of food and maintains the forward movement of the material. Peristalsis also mixes food with intestinal enzymes.

List materials absorbed by the epithelial cells of the villi in the small intestine. Understanding: Villi increase the surface area of epithelium over which absorption is carried out.

Products of digestion are absorbed into the epithelial cells in the jejunum, the midsection of the small intestine. These include: Monosaccharide carbohydrates such as glucose, fructose, and galactose. Any of the twenty amino acids used to make proteins. Components of fat molecules such as fatty acids, monoglycerides and glycerol. Nitrogenous bases from digested nucleic acid nucleotides. Other nutrients that are absorbed include: Mineral ions such as calcium, potassium and sodium. Vitamins such as ascorbic acid (vitamin C).

Outline the digestion and absorption of proteins in humans. Understanding: Enzymes digest most macromolecules in food into monomers in the small intestine.

Proteins must be digested into amino acids. The protease enzyme pepsin digests proteins into smaller polypeptides. Pepsin works in the stomach and requires an acidic pH (2) to function. In the small intestine lumen, the protease enzyme trypsin (from the pancreas) digest polypeptides into amino acids. Trypsin requires a basic pH (8) to function. The amino acids are absorbed by diffusion and active transport at the villus of the small intestine. The amino acids move into capillaries and blood carries them throughout the body. The amino acids then move into cells which then cells use the amino acids to build proteins.

Outline the digestion of starch into maltose. Application: Processes occurring in the small intestine that results in the digestion of starch and transport of the products of digestion to the liver.

Starch is a polymer of alpha-glucose monomers. Starch is both amylose (by 1,4 bonds) and amylopectin (by 1,4 bonds and occasional by 1,6 bonds). The enzyme that digests starch is called amylase. Saliva contains amylase and some starch digestion begins in the mouth. However, most starch digestion occurs in the small intestine, catalysed by pancreatic amylase. Amylase breaks the 1,4 bonds in starch molecules as long as there is a chain of at least four glucose monomers. The product of the digestion is the disaccharide maltose.

State the role of the digestive system. Skill: Production of an annotated diagram of the digestive system.

The digestive system is a collection of organs that collectively digest food, absorb nutrients and excrete waste.

Define "epithelium." Understanding: Villi increase the surface area of epithelium over which absorption is carried out.

The epithelium is the thin layer of cells that create a tissue covering the outer layer of a body's surface and lining the digestive tract and other hollow structures.

State the function of the villus capillary. Understanding: Villi increase the surface area of epithelium over which absorption is carried out.

The villus capillary network maintains a concentration gradient for absorption by rapidly transporting absorbed products away. Capillaries transport absorbed nutrients (sugars and amino acids) away from the small intestine.

State limitations of using model systems in physiology research. Nature of Science: Use models as representations of the real world- dialysis tubing can be used to model absorption in the intestine.

A good model is both as accurate as possible and as simple as possible. Models should have as much accuracy and predictive power as possible while being to be as simple as possible. Therefore, all models have limitations. Missing details: models can't incorporate all the details of complex natural phenomena. Approximations: approximations are not exact, so predictions based on them tend to be a little less accurate than what is actually observed

Explain the use of models in physiology research. Nature of Science: Use models as representations of the real world- dialysis tubing can be used to model absorption in the intestine.

A model is a representation of an idea, an object, a process or a system that is used to describe, explain or test phenomena that cannot be experienced directly.

Define "absorption". Understanding: Villi increase the surface area of epithelium over which absorption is carried out.

Absorption is the taking in of digested substances through the epithelial cell membrane from the lumen of the gut. Absorption occurs in the small intestine.

Describe the absorption of fats by villus epithelial cells. Understanding: Different methods of membrane transport are required to absorb different nutrients.

Fat molecules (triglycerides) are digested by pancreatic lipase within the lumen of small intestine. The products of the digestion are fatty acids, monoglycerides and glycerol, which are absorbed into the epithelial cells on villi. The fatty acid diffuse across the epithelial cell membrane. Within the epithelial cell, they are again converted into triglycerides. The triglycerides are coated with proteins to form chylomicrons (a lipoprotein) which then enter into lacteals by exocytosis.

Describe the role of hydrolysis in digestion. Understanding: Enzymes digest most macromolecules in food into monomers in the small intestine.

Hydrolysis is the chemical breakdown of a compound due to reaction with water. Large molecules are hydrolyzed into small molecules that can be absorbed and moved into the cells. Hydrolysis is catalyzed by enzymes. Polysaccharides are hydrolysed to disaccharides and monosaccharides. Proteins/polypeptides are hydrolysed to amino acids. Triglycerides are hydrolysed to fatty acids and glycerol.

Explain the reasons for starch being digested by the human digestive system. Application: Processes occurring in the small intestine that results in the digestion of starch and transport of the products of digestion to the liver.

Starch is a significant component of human diets. Starch is a large polysaccharide molecule that can not be absorbed by the epithelial cells of the small intestine. Additionally, starch is not directly used as an energy source by cells and is not soluble, so it could not be transported in the blood. The monosaccharide glucose is produce by the multistep digestion of starch. Glucose can be absorbed by the epithelial cells of the small intestine. Glucose is a useful source of energy for cells and can be transported in the blood.

State two examples of model systems used to study digestion. Nature of Science: Use models as representations of the real world- dialysis tubing can be used to model absorption in the intestine.

TNO (gastro-) Intestinal Models (“TIM”) are model systems mimicking the digestive tract. The models are dynamic computer controlled multicompartmental systems with adjustable parameters for the physiological conditions of the stomach and intestine. Temperature, peristalsis, bile secretion, secretion of saliva, stomach and pancreas enzymes are all fully adjustable. Dialysis tubing can model absorption by the small intestine. It is a selectively permeable membrane, this means it has pores in it which allow small molecules through but not larger molecules. At regular intervals the liquid inside and outside of the tubing can be sampled and tested for starch (using iodine) and glucose (using Benedict’s reagent).

Explain how the structure of the villus is adapted for absorption. Understanding: Villi increase the surface area of epithelium over which absorption is carried out.

Villi are finger-like projections of the small intestine mucosa that increase the surface area for better absorption of the products of digestion. Microvilli on the villi epithelial cells further increase surface area to improve absorption. The epithelium is a single layer thick, which allows fast diffusion of nutrients from the small intestine lumen into the blood. Capillary bed within the villi maintain a concentration gradient of nutrients (by constantly carrying away absorbed nutrients) so that the rate of diffusion is higher. Lacteal in villus to absorb fatty acids and carry them away from small intestine.

Describe absorption of glucose by villus epithelial cells. Understanding: Different methods of membrane transport are required to absorb different nutrients.

Carbohydrate molecules such as starch are digested by pancreatic enzymes (eg amylase) within the lumen of small intestine. Additional digestion of carbohydrates may occur through enzymes embedded within the villi epithelial cells. The products of the digestion are monosaccharides which are absorbed into the epithelial cells on villi. Glucose cannot pass through the plasma membrane by simple diffusion because it is polar and therefore hydrophilic. Glucose is transported into the epithelial cell coupled to the movement of sodium ions into the cell through a sodium-glucose cotransport protein. The cotransport protein couples transport of sodium down its concentration gradient (established by the active transport of sodium out of the cell by the sodium-potassium pump) into the cell with the transport of glucose against its concentration gradient into the cell. Active transport requires ATP (from many mitochondria within the cells).

Outline the digestion and absorption of lipids in humans. Understanding: Enzymes digest most macromolecules in food into monomers in the small intestine.

Fats (triglycerides) are digested into fatty acids and glycerol. Pancreatic lipase breaks down triglycerides into free fatty acids and monoglycerides. Pancreatic lipase works with the help of the salts from bile secreted by the liver and the gallbladder. Bile salts attach to triglycerides and help to emulsify them; this aids access by pancreatic lipase because the lipase is water-soluble, but the fatty triglycerides are hydrophobic and tend to orient toward each other and away from the watery intestinal surroundings. The bile salts act to hold the triglycerides in their watery surroundings until the lipase can digest them into the smaller fatty acid components that are able to enter the villi for absorption via diffusion.

Label the four layers of tissue found in the wall of the small intestine as viewed with a microscope or in a micrograph. Skill: Identification of tissue layers in transverse sections of the small intestine viewed with a microscope or in a micrograph.

From the inner surface of the tube moving outward, the layers are: (lumen) mucosa (epithelial and goblet cells) submucosa (villi capillary and lymph vessels) smooth muscle (circular and longitudinal) serosa (connective tissue)

Describe transport of glucose into and through villi capillaries. Application: Processes occurring in the small intestine that results in the digestion of starch and transport of the products of digestion to the liver.

Glucose cannot pass through the plasma membrane by simple diffusion because it is polar and therefore hydrophilic. Glucose is transported in to the epithelial cell from the small intestine lumen through a sodium-glucose cotransport protein. Then, the glucose moves out of the epithelial cell and moves into the villi capillary. Glucose channels allow the glucose to move by facilitated diffusion into blood capillaries in the villus. Glucose in the blood is then carried via the hepatic portal vein to the liver where excess glucose can be absorbed by liver cells and converted to glycogen for storage.

State the function of the villus goblet cell. Understanding: Villi increase the surface area of epithelium over which absorption is carried out.

Goblet cells are found scattered among the epithelial lining of the small intestine. These cells secrete mucus. Mucus is a slippery aqueous secretion that protects the epithelial cells and serves as a lubricant for the digested food material as it passes through the digestive system.

Draw a diagram of the human digestive system. Skill: Production of an annotated diagram of the digestive system.

Mouth - hollow cavity in the head. Esophagus – hollow tube connecting mouth to the top of the stomach. Stomach – a roughly crescent-shaped, hollow organ located just under the diaphragm in the left part of the abdominal cavity. Located between the esophagus and the small intestine. Small intestine - winds throughout the abdominal cavity inferior to the stomach. Large intestine - wraps around the border of the abdominal body cavity from the right side of the body, across the top of the abdomen, and finally down the left side. Liver - a roughly triangular organ that extends across the entire abdominal cavity just under to the diaphragm. Most of the liver’s mass is located on the right side of the body. Gall bladder - hollow, pear-shaped organ that sits under the liver. The end of the gallbladder narrows to join the common bile duct that extends to the wall of the small intestine. Pancreas – a narrow gland that lies inferior to the stomach on the left side of the abdominal cavity. The pancreas extends laterally across the abdomen. The head of the pancreas connects to the small intestine.

List four methods of membrane transport required to absorb nutrients. Understanding: Different methods of membrane transport are required to absorb different nutrients.

Products of digestion are absorbed into the epithelial cells via: Simple diffusion of nutrients down a concentration gradient (eg: fatty acids). Facilitated diffusion of nutrients through channel proteins (eg: fructose). Active transport of nutrients against a concentration gradient through protein pumps (eg: ions, glucose and amino acids). Endocytosis by means of vesicles of large molecules (eg: cholesterol in lipoprotein particles).

List the source, substrate and product of proteases. Understanding: The pancreas secretes enzymes into the lumen of the small intestine.

Proteases are enzymes that hydrolyzes proteins into smaller polypeptides, dipeptides and/or amino acids. Pepsin: Source: stomach Substrate: proteins Product: smaller polypeptides Optimal pH: 2 Trypsin: Source: pancreas (secreted into small intestine) Substrate: proteins and smaller polypeptides Product: dipeptides and/or amino acids Optimal pH: 7-8

Outline the digestion of maltose into glucose. Application: Processes occurring in the small intestine that results in the digestion of starch and transport of the products of digestion to the liver.

The enzyme maltase catalyzes the hydrolysis of the disaccharide maltose into glucose monomers. In humans, maltase is most often located embedded in the epithelial cell membranes of the small intestine.

Outline the structures and functions within a villus epithelial cell. Understanding: Villi increase the surface area of epithelium over which absorption is carried out.

The epithelium is the thin layer of cells that create a tissue covering the villi. The single layer allows fast diffusion of nutrients from the small intestine lumen into the blood. Each epithelial cell contains: Microvilli that further increase surface area to improve absorption. Protein pumps in the cell membrane to carry out active transport of nutrients into the cell. Channel proteins in the cell membrane carry out facilitated diffusion of nutrients into the cell. Embedded enzymes within the membrane to complete digestion. Large number of mitochondria provide ATP to fuel the active transport of nutrients into the cell. Tight junctions between the cells to create an impermeable barrier between the fluid of the intestinal lumen and the intercellular fluid. Pinocytotic vesicles formed by endocytosis of the fluid with the products of digestion.

Outline the function of the esophagus in digestion. Skill: Production of an annotated diagram of the digestive system.

The esophagus functions as the conduit for food and liquids that have been swallowed in the mouth to reach the stomach.

Outline the structure and function of enzymes immobilized in the cell membrane of small intestine epithelial cells. Understanding: Enzymes digest most macromolecules in food into monomers in the small intestine.

The final step in digestion of dietary carbohydrates and proteins occurs on the face of small intestinal epithelial cells, in the immediate vicinity of the protein transporters which will move the resulting sugars and amino acids into the cells. The enzymes responsible for this final stage of digestion are not free in the intestinal lumen, but rather, tethered as integral membrane proteins in the cell membrane of the epithelial cells.

Draw the villi as viewed in cross section. Understanding: Villi increase the surface area of epithelium over which absorption is carried out.

The following structures should be clearly drawn and labeled: Capillary Epithelial cell Lacteal Goblet cell

Outline the function of the gallbladder in digestion. Skill: Production of an annotated diagram of the digestive system.

The gallbladder stores bile produced in the liver until it is needed for digesting fat in the small intestine. Bile travels through the bile ducts and is released into the small intestine where it emulsifies large masses of fat. The emulsification of fats by bile turns the large clumps of fat into smaller pieces that have more surface area and are therefore easier for lipase to digest.

List adaptations that increase the surface area for absorption on the small intestine. Understanding: Villi increase the surface area of epithelium over which absorption is carried out.

The inner layer of the small intestine is covered by numerous folds. The surface of these folds contains villi which are finger-like projections that further increase the surface area for better absorption. Microvilli on the villi epithelial cells even further increase surface area to improve absorption.

State the function of the villus lacteal. Understanding: Villi increase the surface area of epithelium over which absorption is carried out.

The lacteal is a vessel of the lymphatic system with the villus that absorbs fats and transports them away from small intestine. The lacteals will merge to form larger lymphatic vessels that transport the fats to the thoracic duct where they are emptied into the bloodstream at the subclavian vein.

Outline the function of the large intestine in digestion. Skill: Production of an annotated diagram of the digestive system.

The large intestine absorbs water and vitamins (e.g. K and B12) while converting undigested food into feces.

Outline the function of the liver in digestion. Skill: Production of an annotated diagram of the digestive system.

The liver plays an active role in the process of digestion through the production of bile. Bile is a mixture of water, bile salts, cholesterol, and the pigment bilirubin. Hepatocytes in the liver produce bile, which then passes through the bile ducts to be stored in the gallbladder. When food containing fats reaches the small intestine, the gallbladder releases bile. Bile emulsifies large masses of fat. The emulsification of fats by bile turns the large clumps of fat into smaller pieces that have more surface area and are therefore easier for lipase to digest.

Define "lumen." Understanding: The pancreas secretes enzymes into the lumen of the small intestine.

The lumen is the inside space of a tubular body structure that is surrounded by body tissue known as an epithelial membrane. Examples of body structures that have a lumen include the large intestine, small intestine, veins, and arteries.

Outline the function of the mouth in digestion. Skill: Production of an annotated diagram of the digestive system.

The mouth contains many structure- such as the teeth, tongue, and the salivary glands - that work together to aid in the ingestion, digestion and swallowing of food. Teeth are hard structures specialized for the mechanical digestion (biting and grinding) of food. Salivary glands release saliva into the mouth through many tiny ducts. Saliva helps to moisten and chemically digest starch in the mouth before swallowing.

Outline the function of the pancreas in digestion. Skill: Production of an annotated diagram of the digestive system.

The pancreas serves as two glands in one: a digestive exocrine gland and a hormone-producing endocrine gland. Functioning as an exocrine gland, the pancreas secretes pancreatic juice into the lumen of the small intestine. Pancreatic juice has a basic pH in nature due to the high concentration of bicarbonate ions. Bicarbonate is useful in neutralizing the acidic gastric acid. Pancreatic juice also contains enzymes to digest down the proteins (protease), lipids (lipase), carbohydrates (amylase), and nucleic acids (nuclease) in food.

Outline the function of the submucosa layer of tissue found in the wall of the small intestine. Skill: Identification of tissue layers in transverse sections of the small intestine viewed with a microscope or in a micrograph.

The small intestine is made up of four layers of tissue. Beneath the mucosa is the submucosa layer that provides blood vessels, lymphatic vessels, and nerves.

Outline the function of the smooth muscle of tissue found in the wall of the small intestine. Skill: Identification of tissue layers in transverse sections of the small intestine viewed with a microscope or in a micrograph.

The small intestine is made up of four layers of tissue. Beneath the submucosa, several layers of smooth muscle tissue contracts and moves the small intestines. This "peristalsis" helps prevents backward movement of food material and maintains the forward movement of the material.

Outline the function of the mucosa layer of tissue found in the wall of the small intestine. Skill: Identification of tissue layers in transverse sections of the small intestine viewed with a microscope or in a micrograph.

The small intestine is made up of four layers of tissue. The mucosa forms the inner layer of epithelial tissue and is specialized for the absorption of nutrients.

Outline the function of the serosa layers of tissue found in the wall of the small intestine. Skill: Identification of tissue layers in transverse sections of the small intestine viewed with a microscope or in a micrograph.

The small intestine is made up of four layers of tissue. The serosa forms the outermost layer of small intestines and functions as connective tissue. It helps suspend the gut in abdominal cavity by attaching itself to surrounding structures.

Summarize the role of digestive enzymes within the small intestine. Understanding: Enzymes digest most macromolecules in food into monomers in the small intestine.

The small intestine is where most chemical digestion takes place. Most of the digestive enzymes in the small intestine are secreted by the pancreas and enter the small intestine via the pancreatic duct. Enzymes increase the rate of the digestive process by catalyzing the chemical breakdown of large molecules to form molecules that are small enough to be absorbed. Most digestive enzymes work outside the cells in a location within the digestive tract with specific conditions for each reaction. For example, variations in pH throughout digestive tract promote the activity of different digestive enzymes. Enzymes allow digestion to occur at body temperature.

Outline the function of the stomach in digestion. Skill: Production of an annotated diagram of the digestive system.

The stomach stores and mechanically digests food. The stomach also secretes a mixture of acid, mucus, and digestive enzymes that helps to chemically digest proteins and sanitize our food while it is being stored.