What Helps Break Down Food Into Nutrients During Chemical Digestion? | LIVESTRONG.COM What Helps Break Down Food Into Nutrients During Chemical Digestion? by MATTHEW FOX, MD Last Updated: Aug 10, 2015 Matthew Fox, MD Dr. Matthew Fox graduated from the University of California with a Bachelor of Arts in molecular, cell and developmental biology and received a M.D. from the University of Virginia. He is a pathologist and has experience in internal medicine and cancer research. Taking a forkful of food. Photo Credit milla1974/iStock/Getty Images Food has several general components. Macronutrients include carbohydrates, proteins and fats, each with subcategories. Micronutrients include vitamins, minerals and other chemicals needed in small amounts. Nutrients are typically bound together in large chemical compounds. These large molecules must be broken down by the digestive system into smaller units to be absorbed. The oral cavity, stomach and intestines -- along with other organs such as the pancreas -- are designed to break down and absorb nutrients. After absorption, the cells of the body can utilize the nutrients. Oral Cavity The process of breaking down food into usable nutrients begins in the oral cavity.

The tongue, lips and cheeks move the food around in the mouth, exposing it to the teeth. The teeth mash and grind the food. The saliva moistens the food and exposes it to enzymes such as salivary amylase, which breaks down the bonds between long branching carbohydrate molecules. Saliva also causes the chewed food to stick together, forming a bolus. Stomach The stomach contains hydrochloric acid. This kills many bacteria and it also helps the enzymes in the stomach to work. Stomach enzymes such as pepsin break down protein into amino acids. Other types of molecules are not as thoroughly digested as protein until reaching the small intestine. After digestion in the stomach, the food moves through the pylorus into the small intestine. Intestines The liver releases bile and the pancreas releases digestive enzymes into the first part of the small intestine, called the duodenum. The duodenum also has its own enzymes. Bile helps to dissolve fats and neutralize stomach acid. Enzymes break down the remaining protein into amino acids, and carbohydrates and fats into individual molecules. Then, the cells of the intestine absorb the nutrients and pass them into the blood stream, where they are processed by the liver.

Cellular Metabolism Once in the body, fats, carbohydrates and amino acids can be stored by such tissues as the skeletal muscle and the liver. Alternatively, they can be biochemically broken down by the cells in order to form the energy-carrying molecules of the cell, such as adenosine triphosphate, or ATP. Related Searches Lose Weight. Feel Great Change your life with MyPlate by LIVESTRONG.COM GOAL Gain 2 pounds per week Gain 1.5 pounds per week Gain 1 pound per week Gain 0.5 pound per week Maintain my current weight Lose 0.5 pound per week Lose 1 pound per week Lose 1.5 pounds per week Lose 2 pounds per week GENDER What Happens to Nutrients When Food Is Digested? | Healthy Eating | SF Gate What Happens to Nutrients When Food Is Digested? What Happens to Nutrients When Food Is Digested? Digestion unlocks the nutrients in the foods you eat. Your diet provides the nutrients you need to maintain health. Some of these nutrients are too large to be useful to your body without further processing, and your digestive system accomplishes this by breaking them down to a size small enough to enter your bloodstream.

Once in your blood, the digested nutrients travel throughout your body to the cells that need them. Carbohydrates The sugars and starches that make up carbohydrates are the main source of energy for your cells. In their simplest form, carbohydrates are monosaccharides, or single sugar units. They can also exist as disaccharides, made up of two monosaccharides, or starches, composed of many monosaccharides – specifically glucose – linked together into a large molecule. The digestive enzyme amylase, secreted from your salivary glands and pancreas, reduces starch molecules to single glucose units, while sugar-specific enzymes from your small intestine break apart disaccharides into individual sugars. Once these carbohydrates are fully digested, the cells lining your small intestine absorb them and transport them to your bloodstream. Fats Your digestive system breaks down your dietary fats to fatty acids and monoglycerides. Because your digestive juices are water-based, the fats you eat can’t mix well with them until they combine with a fluid called bile. Bile, produced in your liver and released into your small intestine, separates fat particles into tiny droplets that can mix with water.

In this way, the digestive enzyme lipase can act on the fat and separate it into its components. The resultant fatty acids and monoglycerides are small enough for your intestine to absorb and pass along to your circulatory system. Proteins Your small intestine cannot absorb a molecule as large as protein. Your gastrointestinal tract therefore breaks it down to its individual building blocks called amino acids. The process begins in your stomach, where hydrochloric acid uncoils the protein molecule and the acid-activated enzyme pepsin cuts it into smaller pieces. The pieces move to your small intestine, where additional digestive enzymes clip off amino acids one by one. Absorptive cells in this part of your gut then take in the single amino acids and carry them to your bloodstream. Considerations Not all the foods you eat undergo digestion. Some – vitamins, minerals and monosaccharides, for instance – are already small enough for absorption by your intestines without further processing. Other food components cannot be digested because you lack the necessary digestive enzymes. Dietary fiber falls into this category, and, even though it doesn’t add a specific nutrient to your diet, it can help improve your digestive health by adding bulk to your waste products.

In contrast, the inability to digest a certain nutrient can cause digestive distress, as in the case of lactose. If you don’t produce the enzyme lactase, undigested lactose passes through your system and can lead to stomach cramps or diarrhea. Digestive enzymes: Help your body break down the nutrients from food - Nutrition Express Articles Digestive enzymes: Help your body break down the nutrients from food You are what you digest—not just what you eat by Lindberg You are what you digest—not just what you eat The old saying, “You are what you eat,” is only partially true. In reality, you are what you digest and assimilate. Eating the healthiest, most nutritious foods can provide great benefits for your body, but only if the foods can be broken down and assimilated properly. We produce fewer digestive enzymes as we age As we age, our bodies lose the ability to produce enough digestive enzymes to ensure complete digestion. Also, eating a diet mainly of cooked and processed foods does not provide us with the enzymes naturally present in raw foods. Without enough digestive enzymes to break down food, we may experience indigestion, gas and bloating, and miss out on optimum health.

Supports the stomach, pancreas, liver and gall bladder Having food undergo the appropriate breakdown at each stage of digestion is key. Protein, carbohydrates and fats break down at different stages and by different enzymes and processes.* Lindberg Complete Digestant contains a wide spectrum for all types of food. Take with meals You can take it daily with meals or only when you need digestive help, such as with a large, heavy meal. It's also ideal for those on a high-protein diet. The older you are, the more likely you may need digestive help We've found that older people, particularly those over the age of 40, can benefit from supplemental enzymes. Bloating, gas, indigestion or upset stomach could point to a need to improve your digestive system. Comprehensive enzyme formula If your digestive system needs a little help, try this complete digestive enzyme formula to ensure your digestive system is functioning optimally. Complete Digestant supports several stages of digestion Stage 1 – Stomach The stomach secretes gastric juices to initiate the break down of food.

Betaine Hydrochloride – Supports the stomach’s production of hydrochloric acid to lower the pH level so enzymes including pepsin can be activated.* Pepsin – Helps break down certain protein bonds.* Papain – An enzyme from papaya fruit that helps break down protein.* Bromelain – An enzyme from pineapple that helps break down protein.* Lactase - Helps break down milk sugar (lactose).* Ideal for those who eat dairy or drink protein shakes.* Alpha-Galactosidase - Helps break down complex carbs such as legumes (beans), cruciferous vegetables (cauliflower, broccoli), and grains.* Stage 2 – Pancreas The pancreas secretes a mixture of enzymes into the small intestine. Pancreatin 10X – Consists of three enzymes: amylase to break down carbohydrates, protease to break down protein and lipase to break down fat.* Stage 3 – Liver and gall bladder The liver and gallbladder make and secrete bile into the small intestine to emulsify fat so it can be digested. Ox Bile Extract – Supports the liver and gall bladder’s production of bile.* How Does Your Digestive System Break Down Food?

- dummies How Does Your Digestive System Break Down Food? How Does Your Digestive System Break Down Food? How Does Your Digestive System Break Down Food? By David Terfera, Shereen Jegtvig The function of the digestive system is to take food into your body, break it down into individual macronutrients (proteins, fats, and carbohydrates) and micronutrients (vitamins and minerals), and then absorb those small bits and pieces of nutrients so the rest of your body can use them as fuel and raw materials for building tissues and structures. The digestive system includes a very long tube called the digestive tract and a couple additional organs that lend a little help along the way: Digestion begins in the mouth where the salivary glands make saliva (often before you start eating — just smelling a pumpkin pie can make your mouth water). Saliva is a fluid that contains mucin to lubricate foods, buffering agents that help to reduce acidity, antibacterial agents to kill germs, and a digestive enzyme called salivary amylase that begins the job of digesting complex carbohydrates. Who knew saliva contained so much stuff?

The mouth also contains teeth, which are used to chew food into smaller bits that make the process of digestion easier later on, and a tongue, which helps move foods around in your mouth. After you chew food in your mouth, it travels through your esophagus, a simple tube that connects the pharynx (throat) with the stomach. Between the esophagus and the stomach is a sphincter (a ring-like muscle that surrounds an opening or passageway) called the lower esophageal sphincter that only opens when you swallow foods or liquids or when you need to belch. The rest of the time it stays shut so the delicate lining of the esophagus isn’t damaged by the strong acid found in the digestive juices of the stomach. Gastroesophageal reflux disease (GERD) occurs when stomach acid backs up into the esophagus often enough to cause damage to the lining. The stomach is a muscular bag located in the upper part of the abdomen, toward the left side. It squeezes and mixes food with digestive juices that are released from specialized cells in the wall of the stomach.

Digestive juices include hydrochloric acid, which kills microorganisms and activates an enzyme called pepsin that helps break down proteins, and intrinsic factor, which is a substance that helps the small intestine absorb vitamin B12. Its lining contains cells that produce mucus to help protect the walls of the stomach from damage due to the acid. Your stomach slowly empties its contents into the small intestine, which is the longest portion of the digestive tract. It’s divided into three sections: the duodenum, the jejunum, and the ileum. Digestion finishes in the small intestine, and absorption of nutrients begins with a little assistance from a few other organs. Pancreas: This organ is located behind the stomach. It produces digestive enzymes and releases them into the duodenum. These enzymes include lipase, which breaks down fats, pancreatic amylase, which breaks down carbohydrates, proteases, which break down proteins. Gallbladder: This sac-like organ is located near the liver and stores bile, a substance that helps emulsify fats so the lipases have an easier time digesting them. Liver: The liver is located on the right side of the abdomen.

It makes the bile that helps emulsify fats, and it processes the nutrients that are absorbed in the small intestine. After the nutrients are digested, they’re absorbed into the cells of the small intestine. From there, they’re absorbed into the blood and carried away to be processed and metabolized or stored. The material that’s left after digestion and absorption in the small intestine travels into the large intestine, or colon. It travels up the right side of the abdomen, across the top, and down again on the left. From there it moves toward the midline of the body and terminates at the rectum with the anus that opens to the outside. The main function of the colon is to absorb water, leaving more solid material that’s passed through the rectum as stool. The appendix is a tube-like pouch of lymphoid tissue that hangs from the large intestine in the right lower quadrant of the abdomen. BBC - KS3 Bitesize Science - Diet, drugs and health : Revision, Page 5 Diet, drugs and health Next Digestion and enzymes Our teeth break food down into small pieces when we chew. This is only a start to the process of digestion, as chewed pieces of food are still too large to be absorbed by the body.

Food has to be broken down chemically into really small particles before it can be absorbed. Enzymes are needed so that this break-down happens quickly enough to be useful. Enzymes Take care - enzymes are not living things. They are just special proteins that can break large molecules into small molecules. Different types of enzymes can break down different nutrients: carbohydrase or amylase enzymes break down starch into sugar protease enzymes break down proteins into amino acids lipase enzymes break down fats into fatty acids and glycerol. Carbohydrates Carbohydrates are digested in the mouth, stomach and small intestine. Carbohydrase enzymes break down starch into sugars. The saliva in your mouth contains amylase, which is another starch digesting enzyme. If you chew a piece of bread for long enough, the starch it contains is digested to sugar, and it begins to taste sweet. Proteins Proteins are digested in the stomach and small intestine. Protease enzymes break down proteins into amino acids. Digestion of proteins in the stomach is helped by stomach acid, which is strong hydrochloric acid. This also kills harmful micro-organisms that may be in the food.

Fats Lipase enzymes break down fat into fatty acids and glycerol. Digestion of fat in the small intestine is helped by bile, made in the liver. Bile breaks the fat into small droplets that are easier for the lipase enzymes to work on. Things that are not digested Minerals, vitamins and water are already small enough to be absorbed by the body without being broken down, so they are not digested. Digestive enzymes cannot break down fibre, which is why it cannot be absorbed by the body. Page Digestion Big Ideas Digestion Digestion is the breakdown of food into simpler molecules that can be absorbed by the body. The digestive system is actually a long hollow tube called the gastrointestinal tract, or GI tract. The digestive system includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. Several major glands include the salivary glands, the pancreas, and the liver. The first task of the digestive system is to break down food into a fine pulp (mechanical digestion). When the food is physical broken down, digestive chemicals break the food down into small molecules (chemical digestion). The mouth is the organ in which the process of digestion begins.

It starts the process of mechanical digestion by grinding the food with teeth. Also in the mouth, an enzyme called salivary amylase begins to break down long starch molecules into maltose. Once the food is ready to be swallowed it is gathered into a ball called a bolus and sends to the part of the bolus called the pharynx. Once in the pharynx, the bolus next passes down the esophagus. The bolus is moved down the esophagus by rhythmic muscles contracts called peristalsis. At the end of the esophagus is a thick ring of a muscle called a sphincter which allows food to enter the stomach but not from the stomach back into the esophagus. Once the food enters the stomach it is mixed with the digestive juices. The stomach produces stomach acid; hydrochloric acid, and pepsin. Stomach acid activates the pepsin that digests protein into smaller proteins. Proteins are long chains of amino acids that are essential for cell growth and repair. Next, from the stomach the food moves into the small intestine where it continues to be digested by enzymes produced in the pancreas.

In the small intestine, the pancreas empties digestive juices that break down carbohydrates, fats, and protein. The liver produces bile that is stored in the gallbladder between meals and emptied into the small intestine. The small intestine is where most of the nutrients are absorbed and thus is the most important organ in the digestive system. The undigested materials continue to the large intestine where the remaining water is absorbed and eventually the waste material is expelled through the rectum. Also, fats (lipids) are molecules that are stored for future energy uses mainly because they are difficult to digest. EPE Vitamins to Help Break Down Carbs | Healthy Eating | SF Gate Vitamins to Help Break Down Carbs Vitamins to Help Break Down Carbs B vitamins break down the carbs found in pasta. Carbohydrates are energy-giving foods that fuel your body cells to carry out metabolism, while vitamins are organic food components that help your body use proteins, carbohydrates and fats. You get carbohydrates by eating foods like honey, potatoes and sugar. To get different types of vitamins, eat different types of foods, such as fruits and animal products . Each type serves a different purpose in your body. B vitamins are the main workhorses when it comes to breaking down carbohydrates.

Vitamin B1 To convert carbohydrates to simple sugars, such as glucose and galactose, you need vitamin B1. Eat foods rich in this vitamin to help you convert the blood sugars into energy, while also building and ensuring that your nervous system functions properly and optimally, since you require a properly functioning nervous system for an efficient communication between your brain and body organs during the breakdown of carbohydrates. Sources of vitamin B1 include whole-grain cereals and wheat flour. Vitamins B2 and B3 Obtain vitamins B2 and B3, or riboflavin and niacin, from leafy green vegetables, such as cabbage and kale, and beef. These two vitamins help break down carbohydrates into absorbable products by processing their calories. This happens before absorption to ensure that you obtain maximum energy by metabolizing them. Niacin also supports the digestive system by ensuring that almost all carbohydrates are broken down. Vitamin B5 and B6 Pantothenic and pyridoxine acid are the respective scientific names of vitamins B5 and B6. You need vitamin B5 to break down all essential nutrients like carbohydrates, fats and proteins to energy, which is required for cell metabolism.

Eat foods rich in pantothenic acid to maintain a healthy nervous system. You need a healthy nervous system to enable carbohydrate-breaking enzymes, such as amylase and cellulase, to be secreted once they are present in the alimentary canal. Include pyridoxine in your meals to assists with brain functioning and recognizing the presence of carbohydrates in your body and to stimulate the production of enzymes that will break the carbohydrates down. Vitamins B9 and B12 Create a fresh medium for a carbohydrate breakdown in your body by consuming foods rich in vitamin B9. This vitamin is responsible for the production of new cells where carbohydrates are broken down to produce energy. Vitamins B9 and B12 are also known as folic acid and cobalamin, respectively. Eat whole grains, mushrooms and animal products, such as eggs and milk, to enrich your diet with these vitamins. Animal products specifically supply your body with folic acid.

the digestive system THE DIGESTIVE SYSTEM Components of the Digestive System | Regulation of Appetite | Nutrition | Learning Objectives | Links Animals, for the most part, ingest their food as large, complex molecules that must be broken down into smaller molecules (monomers) that can then be distributed throughout the body of every cell. This vital function is accpomplished by a series of specialized organs that comprise the digestive system. Representative digestive systems are shown in Figure 1. Digestive System | Back to Top Single-celled organisms can directly take in nutrients from their outside environment. Multicellular animals, with most of their cells removed from contact directly with the outside environment, have developed specialized structures for obtaining and breaking down their food. Animals depend on two processes: feeding and digestion. Animals are heterotrophs , they must absorb nutrients or ingest food sources. Ingestive eaters , the majority of animals, use a mouth to ingest food. Absorptive feeders , such as tapeworms, live in a digestive system of another animal and absorb nutrients from that animal directly through their body wall. Filter feeders , such as oysters and mussels, collect small organisms and particles from the surrounding water.

Substrate feeders , such as earthworms and termites, eat the material (dirt or wood) they burrow through. Fluid feeders , such as aphids, pierce the body of a plant or animal and withdraw fluids. Figure 1. The digestive systems of representative animals. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Plans and Locations | Back to Top The digestive system uses mechanical and chemical methods to break food down into nutrient molecules that can be absorbed into the blood. Once in the blood, the food molecules are routed to every cell in the animal's body. There are two types of animal body plans as well as two locations fordigestion to occur. Sac-like plans are found in many invertebrates, who have a single opening for food intake and the discharge of wastes. Vertebrates, the animal group humans belong to, use the more efficient tube-within-a-tube plan with food entering through one opening (the mouth) and wastes leaving through another (the anus). Where the digestion of the food happens is also variable.

Some animals use intracellular digestion , where food is taken into cells by phagocytosis with digestive enzymes being secreted into the phagocytic vesicles . This type of digestion occurs in sponges, coelenterates (corals, hydras and their relatives) and most protozoans. Extracellular digestion occurs in the lumen (or opening) of a digestive system, with the nutrient molecules being transferred to the blood or some other body fluid. This more advanced type of digestion occurs in chordates, annelids, and crustaceans. Stages in the Digestive Process | Back to Top Food for the most part consists of various organic macromolecules such as starch, proteins, and fats. These molecules are polymers made of individual monomer units (as discussed in an earlier chapter). Breaking these large molecules into smaller components involves: movement: propels food through the digestive system secretion : release of digestive juices in response to a specific stimulus digestion : breakdown of food into molecular components small enough to cross the plasma membrane absorption : passage of the molecules into the body's interior and their passage throughout the body elimination: removal of undigested food and wastes Three processes occur during what we loosely refer to as "digestion".

Digestion proper, which is the mechanical and chemical breakdown of food into particles/molecules small enough to pass into the blood. Absorption is the passage of food monomers into the blood stream. Assimilation is the passage of the food molecules into body cells. Components of the Digestive System | Back to Top The human digestive system, as shown in Figure 2, is a coiled, muscular tube (6-9 meters long when fully extended) stretching from the mouth to the anus. Several specialized compartments occur along this length: mouth, pharynx, esophagus, stomach, small intestine, large intestine, and anus. Accessory digestive organs are connected to the main system by a series of ducts: salivary glands, parts of the pancreas, and the liver and gall bladder ( bilary system ). Figure 2. The human digestive system. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. The Mouth and Pharynx Mechanical breakdown begins in the mouth by chewing (teeth) and actions of the tongue. Chemical breakdown of starch by production of salivary amylase from the salivary glands .

This mixture of food and saliva is then pushed into the pharynx and esophagus . The esophagus is a muscular tube whose muscular contractions ( peristalsis ) propel food to the stomach. In the mouth, teeth, jaws and the tongue begin the mechanical breakdown of food into smaller particles, as shown in Figure 3. Most vertebrates, except birds (who have lost their teeth to a hardened bill), have teeth for tearing, grinding and chewing food. The tongue manipulates food during chewing and swallowing; mammals have tastebuds clustered on their tongues. Salivary glands secrete salivary amylase, an enzyme that begins the breakdown of starch into glucose. Mucus moistens food and lubricates the esophagus. Bicarbonate ions in saliva neutralize the acids in foods. Swallowing moves food from the mouth through the pharynx into the esophagus and then to the stomach. Step 1: A mass of chewed, moistened food, a bolus, is moved to the back of the moth by the tongue. In the pharynx, the bolus triggers an involuntary swallowing reflex that prevents food from entering the lungs, and directs the bolus into the esophagus.

Step 2: Muscles in the esophagus propel the bolus by waves of involuntary muscular contractions (peristalsis) of smooth muscle lining the esophagus. Peristalsis is shown in Figure 4. Step 3: The bolus passes through the gastroesophogeal sphincter, into the stomach. Heartburn results from irritation of the esophagus by gastric juices that leak through this sphincter. Figure 3. Structure of the throat and the mechanics of swallowing. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Figure 4. Peristalsis and the movement of food from the mouth to the stomach. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. The Stomach (or Churn, Churn, Churn) During a meal, the stomach gradually fills to a capacity of 1 liter, from an empty capacity of 50-100 milliliters.

At a price of discomfort, the stomach can distend to hold 2 liters or more. Epithelial cells line inner surface of the stomach, as shown in Figure 5, and secrete about 2 liters of gastric juices per day. Gastric juice contains hydrochloric acid, pepsinogen , and mucus; ingredients important in digestion. Secretions are controlled by nervous (smells, thoughts, and caffeine) and endocrine signals. The stomach secretes hydrochloric acid and pepsin . Hydrochloric acid (HCl) lowers pH of the stomach so pepsin is activated. Pepsin is an enzyme that controls the hydrolysis of proteins into peptides. The stomach also mechanically churns the food. Chyme, the mix of acid and food in the stomach, leaves the stomach and enters the small intestine. Figure 5. Scanning electron micrograph of the stomach lining of a mammal, X2000. This image is from , used by permission of Nanoworld. Hydrochloric acid does not directly function in digestion: it kills microorganisms, lowers the stomach pH to between 1.5 and 2.5; and activates pepsinogen.

Pepsinogen is an enzyme that starts protein digestion. Pepsinogen is produced in cells that line the gastric pits . It is activated by cleaving off a portion of the molecule, producing the enzyme pepsin that splits off fragments of peptides from a protein molecule during digestion in the stomach. Carbohydrate digestion, begun by salivary amylase in the mouth, continues in the bolus as it passes to the stomach. The bolus is broken down into acid chyme in the lower third of the stomach, allowing the stomach's acidity to inhibit further carbohydrate breakdown. Protein digestion by pepsin begins. Alcohol and aspirin are absorbed through the stomach lining into the blood. Epithelial cells secrete mucus that forms a protective barrier between the cells and the stomach acids. Pepsin is inactivated when it comes into contact with the mucus. Bicarbonate ions reduce acidity near the cells lining the stomach. Tight junctions link the epithelial stomach-lining cells together, further reducing or preventing stomach acids from passing. Ulcers Peptic ulcers result when these protective mechanisms fail. Bleeding ulcers result when tissue damage is so severe that bleeding occurs into the stomach.

Perforated ulcers are life-threatening situations where a hole has formed in the stomach wall. At least 90% of all peptic ulcers are caused by Helicobacter pylori. Other factors, including stress and aspirin, can also produce ulcers. The Small Intestine The small intestine , shown in Figure 6, is where final digestion and absorption occur. The small intestine is a coiled tube over 3 meters long. Coils and folding plus villi give this 3m tube the surface area of a 500-600m long tube. Final digestion of proteins and carbohydrates must occur, and fats have not yet been digested. Villi have cells that produce intestinal enzymes which complete the digestion of peptides and sugars. The absorption process also occurs in the small intestine. Food has been broken down into particles small enough to pass into the small intestine. Sugars and amino acids go into the bloodstream via capillaries in each villus. Glycerol and fatty acids go into the lymphatic system. Absorption is an active transport, requiring cellular energy. Figure 6. Structure and details of the small intestine.

Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Food is mixed in the lower part of the stomach by peristaltic waves that also propel the acid-chyme mixture against the pyloric sphincter. Increased contractions of the stomach push the food through the sphincter and into the small intestine as the stomach eempties over a 1 to 2 hour period. High fat diets significantly increase this time period. The small intestine is the major site for digestion and absorption of nutrients. The small intestine is up to 6 meters long and is 2-3 centimeters wide. The upper part, the duodenum , is the most active in digestion. Secretions from the liver and pancreas are used for digestion in the duodenum. Epithelial cells of the duodenum secrete a watery mucus. The pancreas secretes digestive enzymes and stomach acid-neutralizing bicarbonate. The liver produces bile, which is stored in the gall bladder before entering the bile duct into the duodenum. Digestion of carbohydrates, proteins, and fats continues in the small intestine.

Starch and glycogen are broken down into maltose by small intestine enzymes. Proteases are enzymes secreted by the pancreas that continue the breakdown of protein into small peptide fragments and amino acids. Bile emulsifies fats, facilitating their breakdown into progressively smaller fat globules until they can be acted upon by lipases . Bile contains cholesterol, phospholipids, bilirubin, and a mix of salts. Fats are completely digested in the small intestine, unlike carbohydrates and proteins. Most absorption occurs in the duodenum and jejeunum (second third of the small intestine). The inner surface of the intestine has circular folds that more than triple the surface area for absorption. Villi covered with epithelial cells increase the surface area by another factor of 10. The epithelial cells are lined with microvilli that further increase the surface area; a 6 meter long tube has a surface area of 300 square meters. Each villus has a surface that is adjacent to the inside of the small intestinal opening covered in microvilli that form on top of an epithelial cell known as a brush border . Each villus has a capillary network supplied by a small arteriole.

Absorbed substances pass through the brush border into the capillary, usually by passive transport. Maltose, sucrose, and lactose are the main carbohydrates present in the small intestine; they are absorbed by the microvilli. Starch is broken down into two-glucose units (maltose) elsewhere. Enzymes in the cells convert these disaccharides into monosaccharides that then leave the cell and enter the capillary. Lactose intolerance results from the genetic lack of the enzyme lactase produced by the intestinal cells. Peptide fragments and amino acids cross the epithelial cell membranes by active transport . Inside the cell they are broken into amino acids that then enter the capillary. Gluten enteropathy is the inability to absorb gluten, a protein found in wheat. Digested fats are not very soluble. Bile salts surround fats to form micelles , as shown in Figure 7, that can pass into the epithelial cells. The bile salts return to the lumen to repeat the process. Fat digestion is usually completed by the time the food reaches the ileum (lower third) of the small intestine. Bile salts are in turn absorbed in the ileum and are recycled by the liver and gall bladder.

Fats pass from the epithelial cells to the small lymph vessel that also runs through the villus. Figure 7. Absorption of lipids by cells in the small intestine. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. The Liver and Gall Bladder The liver produces and sends bile to the small intestine via the hepatic duct, as illustrated in Figure 8. Bile contains bile salts, which emulsify fats, making them susceptible to enzymatic breakdown. In addition to digestive functions, the liver plays several other roles: 1) detoxification of blood; 2) synthesis of blood proteins; 3) destruction of old erythrocytes and conversion of hemoglobin into a component of bile; 4) production of bile; 5) storage of glucose as glycogen , and its release when blood sugar levels drop; and 6) production of urea from amino groups and ammonia. Figure 8. The liver and associated organs and their connections to the digestive system.

Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. The gall bladder stores excess bile for release at a later time. We can live without our gall bladders, in fact many people have had theirs removed. The drawback, however, is a need to be aware of the amount of fats in the food they eat since the stored bile of the gall bladder is no longer available. Glycogen is a polysaccharide made of chains of glucose molecules, as shown in Figure 9. In plants starch is the storage form of glucose, while animals use glycogen for the same purpose. Low glucose levels in the blood cause the release of hormones, such as glucagon , that travel to the liver and stimulate the breakdown of glycogen into glucose, which is then released into the blood(raising blood glucose levels). When no glucose or glycogen is available, amino acids are converted into glucose in the liver. The process of deamination removes the amino groups from amino acids. Urea is formed and passed through the blood to the kidney for export from the body.

Conversely, the hormone insulin promotes the take-up of glusose into liver cells and its formation into glycogen. Figure 9. Glycogen structure. Note the individual glucose molecules that are linked to form glycogen. Image from . Liver diseases Jaundice occurs when the characteristic yellow tint to the skin is caused by excess hemoglobin breakdown products in the blood, a sign that the liver is not properly functioning. Jaundice may occur when liver function has been impaired by obstruction of the bile duct and by damage caused by hepatitis . Hepatitis A, B, and C are all viral diseases that can cause liver damage. Like any viral disease, the major treatment efforts focus on treatment of symptoms, not removal of the viral cause. Hepatitis A is usually mild malady indicated by a sudden fever, malaise, nausea, anorexia, and abdominal discomfort. Jaundice follows up for several days. The virus causing Hepatitis A is primarilly transmitted by fecal contamination, although contaminated food and water also can promote transmission. A rare disease in the United States, hepatitis B is endemic in parts of Asia where hundreds of millions of individuals are possibly infected.

Hepatitis B may be transmitted by blood and blood products as well as sexual contact. The blood supply in developed countries has been screened for the virus that causes this disease for many years and transmission by blood transfusion is rare. The risk of HBV infection is high among promiscuous homosexual men although it is also transmitted hetereosexually. Correct use of condoms is thought to reduce or eliminate the risk of transmission. Effective vaccines are available for the prevention of Hepatitis B infection. Some individuals with chronic hepatitis B may develop cirrhosis of the liver. Individuals with chronic hepatitis B are at an increased risk of developing primary liver cancer. Although this type of cancer is relatively rare in the United States, it is the leading cause of cancer death in the world, primarily because the virus causing it is endemic in eastern Asia. Hepatitis C affects approximately 170 million people worldwide and 4 million in the United States. The virus is transmitted primarily by blood and blood products. Most infected individuals have either received blood transfusions prior to 1990 (when screening of the blood supply for the Hepatitis C virus began) or have used intravenous drugs.

Sexual transmission can occur between monogamous couples (rare) but infection is far more common in those who are promiscuous. In rare cases, Hepatitis C causes acute disease and even liver failure. About twenty percent of individuals with Hepatitis C who develop cirrhosis of the liver will also develop severe liver disease. Cirrhosis caused by Hepatitis C is presently the leading cause of the need for liver transplants in the United States. Individuals with cirrhosis from Hepatitis C also bear increased chances of developing primary liver cancer. All current treatments for Hepatitis C employ of various preparations of the potent antiviral interferon alpha. However, not all patients who have the disease are good candidates for treatment, so infected individuals are urged to regularly consult their physician. Cirrhosis of the liver commonly occurs in alcoholics, who place the liver in a stress situation due to the amount of alcohol to be broken down. Cirrhosis can cause the liver to become unable to perform its biochemical functions. Chemicals responsible for blood clotting are synthesized in the liver, as is albumin, the major protein in blood. The liver also makes or modifies bile components.

Blood from the circulatory system passes through the liver, so many of the body's metabolic functions occur primarily there including the metabolism of cholesterol and the conversion of proteins and fats into glucose. Cirrhosis is a disease resulting from damage to liver cells due to toxins, inflammation, and other causes. Liver cells regenerate in an abnormal pattern primarily forming nodules that are surrounded by fibrous tissue. Changes in the structure of the liver can decrease blood flow, leading to secondary complications. Cirrhosis has many cuses, including alcoholic liver disease, severe forms of some viral hepatitis, congestive heart failure, parasitic infections (for example schistosomiasis), and long term exposure to toxins or drugs. The Pancreas The pancreas sends pancreatic juice, which neutralizes the chyme, to the small intestive through the pancreatic duct. In addition to this digestive function, the pancrease is the site of production of several hormones, such as glucagon and insulin. The pancreas contains exocrine cells that secrete digestive enzymes into the small intestine and clusters of endocrine cells (the pancreatic islets ). The islets secrete the hormones insulin and glucagon , which regulate blood glucose levels.

After a meal, blood glucose levels rise, prompting the release of insulin, which causes cells to take up glucose, and liver and skeletal muscle cells to form the carbohydrate glycogen . As glucose levels in the blood fall, further insulin production is inhibited. Glucagon causes the breakdown of glycogen into glucose, which in turn is released into the blood to maintain glucose levels within a homeostatic range. Glucagon production is stimulated when blood glucose levels fall, and inhibited when they rise. Diabetes results from inadequate levels of insulin. Type I diabetes is characterized by inadequate levels of insulin secretion, often due to a genetic cause. Type II usually develops in adults from both genetic and environmental causes. Loss of response of targets to insulin rather than lack of insulin causes this type of diabetes. Diabetes may cause impairment in the functioning of the eyes, circulatory system, nervous system, and failure of the kidneys. Diabetes is the second leading cause of blindness in the United States. Treatments might involve daily injections of insulin, oral medications such as metformin, monitoring of blood glucose levels, and a controlled diet. Type I diabetes may one day be cured by advances in gene therapy/stem cell research.

On recently recognized condition is known as prediabetes, in which the body gradually loses its sensitivity to insulin, leading eventually to Type II diabetes. Ora; medications, diet and behavior (in other words EXERCISE!!!) changes are thought to delay if not outright postpone the onset of diabetes if corrected soon enough. The fifth leading cause of cancer death in the United States is from pancreatic cancer, which is nearly always fatal. Scientists estimate that 25,000 people may die from this disease each year. Standard treatments are ineffective, although some promising avenues may open with advances in genomics and molecular biology of cancer cells. The Large Intestine The large intestine is made up by the colon, cecum, appendix , and rectum. Material in the large intestine is mostly indigestible residue and liquid. Movements are due to involuntary contractions that shuffle contents back and forth and propulsive contractions that move material through the large intestine. The large intestine performs three basic functions in vertebrates: 1) recovery of water and electrolytes from digested food; 2) formation and storage of feces; and 3) microbial fermentation: The large intestine supports an amazing flora of microbes.

Those microbes produce enzymes that can digest many of molecules indigestible by vertebrates. Secretions in the large intestine are an alkaline mucus that protects epithelial tissues and neutralizes acids produced by bacterial metabolism. Water, salts, and vitamins are absorbed, the remaining contents in the lumen form feces (mostly cellulose, bacteria, bilirubin). Bacteria in the large intestine, such as E. coli, produce vitamins (including vitamin K) that are absorbed. Regulation of Appetite | Back to Top The hypothalamus in the brain has two centers controlling hunger. One is the appetite center, the other the satiety center. Gastrin , secretin , and cholecystokinin are hormones that regulate various stages of digestion. The presence of protein in the stomach stimulates secretion of gastrin, which in turn will cause increased stomach acid secretion and mobility of the digestive tract to move food. Food passing into the duodenum causes the production of secretin, which in turn promotes release of alkaline secretions from the pancreas, stops further passage of food into the intestine until the acid is neutralized.

Cholecystokinin (CCK) is released from intestinal epithelium in response to fats, and causes the release of bile from the gall bladder and lipase (a fat digesting enzyme) from the pancreas. Nutrition | Back to Top Nutrition deals with the composition of food, its energy content, and slowly (or not at all) synthesized organic molecules. Chemotrophs are organisms (mostly bacteria) deriving their energy from inorganic chemical reactions. Phototrophs convert sunlight energy into sugar or other organic molecules. Heterotrophs eat to obtain energy from the breakdown of organic molecules in their food. Macronutrients are foods required on a large scale each day. These include carbohydrates, lipids, and amino acids. Water is essential, correct water balance is a must for proper functioning of the body. About 60% of the diet should be carbohydrates, obtained from foods such as milk, meat, vegetables, grains and grain products. The diet should contain at least 100 grams of carbohydrate every day. Recently, however, new recommendations have been developed that suggest a lowering of the amount of carbohydrate. A more detailed presentation of this topic may be fount at .

Proteins are polymers composed of amino acids. Proteins are found in meat, milk, poultry, fish, cereal grains and beans. They are needed for cellular growth and repair. Twenty amino acids are found in proteins, of which humans can make eleven. The remaining nine are the essential amino acids which must be supplied in the diet. Normally proteins are not used for energy, however during starvation (or a low-carb diet) muscle proteins are broken down for energy. Excess protein can be used for energy or converted to fats. Lipids and fats generate the greatest energy yield, so a large number of plants and animals store excess food energy as fats. Lipids and fats are present in oils, meats, butter, and plants (such as avocado and peanuts). Some fatty acids, such as linoleic acid, are essential and must be included in the diet. When present in the intestine, lipids promote the uptake of vitamins A, D, E, and K. Vitamins are organic molecules required for metabolic reactions. They usually cannot be made by the body and are needed in trace amounts. Vitamins may act as enzyme cofactors or coenzymes . Some vitamins are soluble in fats, some in water.

Minerals are trace elements required for normal metabolism, as components of cells and tissues, and for nerve conduction and muscle contraction. They can only be obtained from the diet. Iron (for hemoglobin), iodine (for thyroxin), calcium (for bones), and sodium (nerve message transmission) are examples of minerals. There is a quantitative relationship between nutrients and health. Imbalances can cause disease. Many studies have concluded nutrition is a major factor in cardiovascular disease, hypertension, and cancer. List all parts (in order) of the human digestive system through which food actually passes. Then list the auxiliary organs that contribute one or more substances to the digestive process. Explain how, during digestion, food is mechanically broken down. Then explain how it is chemically broken down. Describe how the digestion and absorption of fats differ from the digestion and absorption of carbohydrates and proteins. Describe the cross-sectional structure of the small intestine and explain how its structure is related to its function. List four functions of the liver.

Question: What bodily process breaks food down into nutrients?
Answer: Digestion