Where are we heading?

Unit 2: Where are we heading? Unit 2: Introduction Unit 1: What s in your food? Unit 2: How does your body use food? Unit 3: What is metabolic disease? Unit 4: How do I identify good and bad food? Unit 5: How does this knowledge apply to me? In Unit 2 we will explore the processes by which the nutrients in food are absorbed and utilized in the body. We will begin with digestion and absorption, and then use biochemistry to understand how the body shuttles energy and nutrients into and out of storage. We will see that the body is in a constant quest to maintain available and adequate levels of blood glucose to nourish the vital functions of the body. 61

Lumen The central cavity of a hollow structure in the body. During digestion, food passes through the lumen of each organ in the digestive tract. LESSON 2.1 WORKBOOK Digestion: From the mouth to the blood stream In Unit 2 we will explore the processes by which the nutrients in food are absorbed and utilized by the body. We will begin with digestion and absorption, and then use biochemistry to understand how the body shuttles energy and nutrients into and out of storage. We will see that the body is in a constant quest to maintain available and adequate levels of blood glucose to nourish the vital functions of our bodies. In this lesson we will review and expand upon your knowledge of digestion and absorption of nutrients. We will describe the process of digestion in each of the main organs, and compare and contrast digestion and absorption of macronutrient rich foods. Digestion breaks polymers into monomers In Lesson 1.5 we learned about the structures of the three macronutrients: carbohydrates, lipids and proteins. We also saw that in most foods macronutrients exist in a larger polymer made of attached smaller monomer units. The polymer forms of macronutrients are too large to be absorbed from the lumens of our intestines. When we eat macronutrients as polymers, a series of steps will break down the polymers to monomers so the macronutrients can be absorbed into our blood, where they become useful to our cells. This process of breaking apart the polymers to monomers is digestion. It is critical that all of the steps of digestion are working properly, otherwise the food that we eat won t do us any good! 1. Why do we need to digest macronutrients? a. So they can be easily swallowed. b. So they are in a small enough form to be absorbed. c. Monomers are larger than polymers and are not absorbable. d. Both B and C. 62

Why do we have to digest our food? Our digestive system prepares all of the nutrients in the food that we eat (macronutrients and micronutrients) so they can be absorbed by our bodies and used by our cells. It does this by performing mechanical and chemical processes that digest food, absorbing the nutrients from food and eliminating food waste. To absorb food the body has to do something remarkable: it needs to let nutrients into the body while keeping microbes out. This function is performed by the intestines, which keep microbes out of the body, but let nutrients into the body. One way to do this is to discriminate what gets in based on size, so the intestine only absorbs molecules under a certain size. Hence, you need to break down macronutrients into monomers in order for your body to extract the important nutrients and energy from food. Following food through the gastrointestinal tract Figure 1: Organs of the digestive tract. Our digestive tract is outside of our body! The digestive system is made up of the mouth, esophagus, stomach, and the small and large intestines. While it may seem counter intuitive, nutrients are still considered outside of our body until they are absorbed. This means, even after you swallow your food, it is still outside of your body as it passes through your esophagus, stomach, small and large intestines! Other organs that aid the digestive process, but do not directly interact with the food and nutrients are called accessory organs. These include the liver, gallbladder, pancreas and kidneys. Digestion begins before we even eat our food Food preparation, such as cooking, marinating, pounding and dicing starts the process of digestion by reducing the physical size of the food. Starch granules in food swell as they take up water during cooking, making them easier to digest. You may have seen this when grains like oatmeal or rice get bigger after you boil them in water. Cooking also softens tough connective tissues in meats and fibrous plants. As a result, the food is easier to chew, swallow, and break down. 2. Which of the following is not true about digestion? a. Monomers must be made into polymers to be absorbed. b. It makes mutrients more absorbable. c. It occurs outside of our body. d. It begins with cooking. 63

Amylase An enzyme that digests starch and glycogen. Bolus Chemical digestion Breaking up polymers of food into their respective monomers using chemicals and enzymes. Lysozyme An anti-bacterial enzyme that destroys the cell walls of certain bacteria. Mechanical digestion Breaking up food into smaller chunks by force, such as chewing. Peristalsis Involuntary contractions of the muscles of the esophagus and intestine that create wavelike movements that push the contents of the organ forward. Sphincter A ring of muscle that creates a one-way valve to guard or close an opening of an organ, such as in the esophagus, stomach and anus. terms, see the Glossary. Digestion in the mouth Throughout digestion there are two types of processing that break down our food: mechanical digestion and chemical digestion. Mechanical digestion happens when we physically grind our food so it becomes smaller. Chemical digestion is when enzymes or chemicals react with the food to break it apart. Both mechanical and chemical digestion occur in the mouth. The teeth tear and grind solid foods into smaller pieces and mix food with saliva. By chewing food, the large pieces that we eat will be broken apart, creating more surface area. This gives important enzymes access to the food to digest it quicker. Saliva contains several substances to aid in digestion, including mucus to lubricate the food, an enzyme called lysozyme to kill bacteria, and enzymes to begin the chemical digestion of food. For example, salivary amylase is the primary enzyme in saliva, which breaks starch amylose into smaller monosaccharides and disaccharides. When food is mixed with saliva, it is called a bolus. The bolus is then swallowed and enters the esophagus. Try this at home: You can test out your own salivary amylase by putting a food containing amylose, like a piece of bread or a cracker, in your mouth without chewing. Your saliva will cover the food and amylase will convert the starch into sugars. You will be able to tell that the amylase is working when the food tastes sweeter. The esophagus brings the bolus to the stomach The esophagus is the muscular tube that extends from the mouth to the stomach. The bolus is moved through the esophagus by gravity and muscular motion called peristalsis. Much like how the muscles in a snake moves food through its body, peristalsis pushes food down the esophagus into the stomach. You can watch a video demonstrating peristalsis through the digestive system online see this unit on the student website or click below: Video: What is Peristalsis? Peristal)c Wave Bolus Esophagus Stomach Figure 2: Peristalsis is the muscular movement in the esophagus that moves the bolus from the mouth to the stomach. Before the bolus enters the stomach, it must first pass through a muscular door called a sphincter. The opening and closing of sphincters is tightly controlled, and keeps segments of the gastrointestinal tract separated. You can think of sphincters as one way valves. The sphincter dividing the esophagus from the stomach is aptly called the esophageal sphincter. The esophageal sphincter usually prevents the 64

Chyme The acidic mix of food and gastric juices that passes from the stomach to the small intestine. Denaturation A process in which the structure of a protein is altered due to exposure to heat or specific chemicals or enzymes. Microvilli Pepsin The primary digestive enzyme in the stomach; breaks down proteins into smaller peptide chains. Peptidase A type of enzyme that breaks peptide chains down into amino acids. Phyloric Used to describe something that is in the region of the stomach that connects the lower stomach to the small intestine. Villi A small, elongated projection that increases surface area of the small intestine. acidic contents of the stomach from traveling back into the esophagus. If the esophageal sphincter does not function properly, the acidic juices from the stomach can burn the esophagus, causing the symptoms of heartburn. Under some circumstances of chronic heartburn, the acidic stomach contents can even cause lesions in the esophagus, or esophageal ulcers. This condition is called gastroesophageal reflux disease, or GERD, which might be treated with antacids. The stomach grinds and mixes The stomach is essentially a holding and mixing tank because little absorption occurs here only water and alcohol are absorbed from the stomach. Contractions of the muscular layers of the stomach thoroughly mix food with gastric secretions, transforming the solid bolus into a soupy, acidic mixture called chyme (pronounced 'kime'). Each day, the stomach secretes about 8 cups of gastric juices that aid in digestion. These gastric juices include hydrochloric acid and enzymes that break down proteins. Hydrochloric acid produced in the stomach is very important because it: Can inactivate hormones and enzymes in foods by denaturing them. This prevents those hormones and enzymes from affecting our bodies functions. Destroys most harmful bacteria and viruses in foods Breaks dietary minerals free from the foods so that they can be absorbed Activates an enzyme called pepsin, a peptidase that digests proteins into amino acids. In the name you might notice the ase which refers to an enzyme, and peptid, which refers to a peptide. So this is an enzyme that breaks down peptides / proteins. Figure 4: The villi and microvilli are small, fingerlike projections that line the small intestine. Figure 3: The stomach breaks the bolus into chyme. The stomach also secretes a mucus layer that protects the stomach from being digested by its own hydrochloric acid secretions. Heavy use of aspirin and other painkillers can damage the stomach wall because they inhibit the production of mucus. The reduced mucous barrier in the stomach means stomach acid may damage the stomach wall. Absorption of nutrients occurs in the small intestine with help from the accessory organs The acidic chyme leaves the stomach and enters the small intestine by passing through the pyloric sphincter. Most digestion and absorption of nutrients occurs in the small intestine. The inside of the small intestine has fingerlike projections called villi and microvilli (as shown in Figure 4). 3. What type of enzyme breaks down proteins? a. Amylase. b. Lipase. c. Peptidase. d. Sucrase. 65

Bile A fluid that is created in the liver, and stored in the gall bladder until needed. Bile aids in digestion by making hydrophobic lipids absorbable in water. Electrolytes Salts and minerals that can conduct electrical impulses in the body. Sodium and potassium are important electrolytes that must be consumed in the diet. Lipase An enzyme that breaks down triglycerides to fatty acids and glycerol. Pancreatic amlyase An enzyme that digests starch and glycogen that is made in the pancreas, and secreted into the small intestine. These projections increase the surface area of the intestinal epithelium so that the nutrients can be thoroughly digested and absorbed. Most digestion occurs in the first half of the small intestine, and requires many secretions from the small intestine itself, as well as the accessory organs, which are the pancreas, liver and gallbladder: The small intestine secretes enzymes that will break down disaccharides into monosaccharides. The pancreas secretes lipase, the enzyme that will break down lipids, pancreatic amylase to digest amylose, and peptidases to digest proteins. The pancreas also secretes an alkaline mixture to neutralize the acidic chyme so it does not harm the small intestine. The liver produces bile that is stored in the gallbladder until it is secreted into the small intestine. This acts like dish detergent helping to package lipids into hydrophilic droplets. The small intestine absorbs about 95% of our food energy as protein, carbohydrates, fat and alcohol. The small intestine is also the site of most micronutrient absorption. This absorption occurs by transferring nutrients from the lumen of the small intestine into the intestinal cells, then repackaging the nutrients and releasing them into the blood stream. As we will see later, the liver is the first stop for many nutrients. Final stages of digestion in the large intestine Figure 6: Large intestines absorb water and some minerals then expel waste as feces. Figure 5: Most nutrients are absorbed in the small intestines. After digestion and absorption occurs, normally only water, some minerals, and undigested food fibers and starches remain to be emptied from the small intestine into the large intestine. It takes about 12-24 hours for a meal to travel through the large intestine. The large intestines have three primary functions: housing bacteria in our microbiome, absorbing water and electrolytes such as sodium and potassium, and forming and expelling feces. Recall from Unit 1 that there are two types of dietary fiber: soluble and insoluble. In the large intestine, soluble fiber will absorb extra bile and expel it in our feces. Because bile is made of cholesterol, soluble fiber can actually lower cholesterol levels in our blood by decreasing reabsorption of 66

Lactase An enzyme that breaks down the disaccharide lactose into one glucose and one galactose monomer. Maltase An enzyme that breaks down the disaccharide maltose into two glucose monomers. Sucrase An enzyme that breaks down the disaccharide sucrose into one fructose and one glucose monomer. bile cholesterol. Both types of fiber become a food source for the bacteria living in our microbiome. These beneficial bacteria in our large intestine synthesize vitamin K, which may then be absorbed from the large intestine into our blood stream. Let's review by looking at digestion and absorption of macronutrient rich foods! Now that we have a general idea of the flow of food through the digestive system, let's go into more depth about how each of the three macronutrients are digested and absorbed. Remember, almost all absorption occurs in the small intestine. Carbohydrates are broken down to monosaccharides before absorption The goal of carbohydrate digestion is to break down starch and sugars into monosaccharides. Some carbohydrates begin enzymatic digestion in the mouth by salivary amylase. When food reaches the small intestine, polysaccharides are digested further by pancreatic amylase. Disaccharides are then broken down into monosaccharides by enzymes produced by the small intestines. The type of enzyme that breaks the disaccharide depends on the two types of monosaccharide in the disaccharide. For example: Maltase acts on maltose to produce two glucose monomers. Sucrase acts on sucrose (table sugar) to produce glucose and fructose. Lactase acts on lactose (sugar in dairy) to produce glucose and galactose. Carb- rich food Polysaccharides Disaccharides Amylase in saliva Amylase released from pancreas to small intes3nes Enzymes from panacreas to small intes3nes Monosaccharides (Glucose) Absorbed into the body Figure 7: Steps of digestion and absorption of carbohydrates. Once freed, the monosaccharides are absorbed from the intestinal lumen into the blood, where they are transported to the liver. In the liver fructose and galactose are converted into glucose, and glucose is then released into the blood where it is available to the cells of the body. The homeostasis of glucose levels in the blood is highly regulated, a system that we will lean about in great detail over the next lessons of this unit. 4. Carbohydrates are absorbed as: a. Fiber. b. Monosaccharides. c. Disaccharides. d. Glucose. 67

Chylomicron A lipoprotein that carries dietary fat through the lymphatic system to the blood. Gastric lipase An enzyme that breaks down triglycerides that is produced in the stomach. Lingual lipase An enzyme that breaks down triglycerides that is produced in the mouth. This enzyme is active in infants, but loses activity in adults. Lymphatic system A network of vessels through which fluid containing white blood cells is transported throughout the body. Micelle A vesicle used to transport lipids that has a hydrophilic exterior and a hydrophobic interior. Pancreatic lipase An enzyme that breaks down triglycerides that is produced in the pancreas and secreted into the small intestine. terms, see the Glossary. Lipids are emulsified by micelles and transported in the lymphatic system Dietary Fat Figure 8: Bile emulsifies fats into micelles, increasing the surface area of lipids and allowing pancreatic lipase to break triglycerides into free fatty acids. Similar to carbohydrates, digestion of lipids begins in the mouth by the activity of lingual lipase. This enzyme only plays a minor role in fat digestion in adults, but is active in infancy when it is used to break down the fats in breast milk. Some lipid digestion also occurs in the stomach by the enzyme gastric lipase, but the majority of lipid digestion occurs in the small intestine. The presence of fat in the small intestine stimulates the release of bile from the gallbladder and pancreatic lipase from the pancreas. Bile emulsifies fats, meaning that it breaks fat into many tiny droplets called micelles, and forms a shell around the micelles that keep the fat droplets suspended in water-based intestinal contents. This process increases the surface area of lipids and allows pancreatic lipase to efficiently break down triglycerides into free fatty acids (see Figure 8). The lipid portion of the micelles is absorbed by the intestinal cells of the small intestine, this is where about 95% of dietary fat is absorbed. Because lipids are large structures, they cannot be absorbed directly into the blood stream like amino acids or monosaccharides. Instead, lipids are absorbed into the lymphatic system in a lipoprotein called chylomicrons. Chylomicrons are similar to other lipoproteins like HDL and LDL cholesterol, and have a hydrophilic exterior and a hydrophobic interior so that they can transport lipids in the water-based blood and lymphatic system. The chylomicrons will eventually enter the blood stream, where they will be transported to the liver for repackaging. The fat-soluble vitamins are also absorbed from the small intestine with the lipids in these chylomicrons. Proteins are digested into amino acids Enzymatic digestion of protein begins in the stomach with the secretion of hydrochloric acid. This acid will denature, or unravel, proteins. As we already learned, pepsin is an enzyme secreted in the stomach and breaks down long polypeptide chains into shorter chains of amino acids. The partially digested proteins then move from the stomach into the small intestine, where the pancreas secretes other peptidases to further breakdown the peptide chains into amino acid monomers. The amino acids are absorbed into the cells of the small intestine, and then travel via the blood to the liver for use in protein synthesis, energy needs, conversion to carbohydrate or fat, or release into the blood for transport to other cells. 5. Micelles are: a. Made of bile. b. Hydrophilic. c. Hydrophobic. d. All of the above. 68

STUDENT RESPONSES Each step of digestion presents a potential complication. For example, someone that has had a stoke may have difficulty swallowing their food so they have to cut their foods into small pieces. What would be the consequences of having too little, or too much stomach acid? What sort of symptoms would this person have? How would the digestion and absorption of the macronutrients and micronutrients change? Remember to identify your sources 69

TERMS TERM DEFINITION Amylase An enzyme that digests starch and glycogen. Bile A fluid that is created in the liver, and stored in the gall bladder until needed. Bile aids in digestion by making hydrophobic lipids absorbable in water. Chemical Digestion Breaking up polymers of food into their respective monomers using chemicals and enzymes. Chylomicron A lipoprotein that carries dietary fat through the lymphatic system to the blood Chyme The acidic mix of food and gastric juices that passes from the stomach to the small intestine. Denaturation A process in which the structure of a protein is altered due to exposure to heat or specific chemicals or enzymes. Electrolytes Salts and minerals that can conduct electrical impulses in the body. Sodium and potassium are important electrolytes that must be consumed in the diet. Gastric Lipase An enzyme that breaks down triglycerides that is produced in the stomach. Lactase An enzyme that breaks down the disaccharide lactose into one glucose and one galactose monomer. Lingual Lipase An enzyme that breaks down triglycerides that is produced in the mouth. This enzyme is active in infants, but loses activity in adults. Lipase An enzyme that breaks down triglycerides to fatty acids and glycerol. Lumen The central cavity of a hollow structure in the body. During digestion, food passes through the lumen of each organ in the digestive tract. Lymphatic System A network of vessels through which fluid containing white blood cells is transported throughout the body. Lysozyme An anti-bacterial enzyme that destroys the cell walls of certain bacteria. Maltase Mechanical Digestion An enzyme that breaks down the disaccharide maltose into two glucose monomers. Breaking up food into smaller chunks by force, such as chewing. 70

TERMS TERM Micelle Microvilli Pancreatic Amylase Pancreatic Lipase Pepsin Peptidase Peristalsis Pyloric Sphincter Sucrase Villi DEFINITION A vesicle used to transport lipids that has a hydrophilic exterior and a hydrophobic interior. Even small than the villi, microvilli are projections that cover the villi An enzyme that digests starch and glycogen that is made in the pancreas, and secreted into the small intestine. An enzyme that breaks down triglycerides that is produced in the pancreas and secreted into the small intestine. The primary digestive enzyme in the stomach; breaks down proteins into smaller peptide chains. A type of enzyme that breaks peptide chains down into amino acids. Involuntary contractions of the muscles of the esophagus and intestine that create wavelike movements that push the contents of the organ forward. Used to describe something that is in the region of the stomach that connects the lower stomach to the small intestine. For example, the pyloric sphincter is the sphincter between the stomach and the small intestine. A ring of muscle that creates a one-way valve to guard or close an opening of an organ, such as in the esophagus, stomach and anus. An enzyme that breaks down the disaccharide sucrose into one fructose and one glucose monomer. A small, elongated projection that increases surface area of the small intestine. 71