I have divided the second major topic for unit II into two subtopics. Each subtopic is handled separately. This review is organized in the following format: subtopic outline, discussion of the subtopic, and finally references for that subtopic.
Subtopics Discussed in this Review:
1. Digestion . . . From Food to Fuel
2. Food and Nutrition
Images above: www.gesa.org.au/digestive-system/mouth_throat.cfm, www.technabob.com, nutrition.about.com/.../ss/explorefoodpyr.htm, msclipart
Outline
Subtopic 1 - Digestion and Energy Production
1. Organs of the Digestive Tract
2. The Process of Digestion
3. Energy Production: Cellular Respiration
4. Digestive Health
Discussion 1.1 Organs of the Digestive Tract
There are a number of organs and structures that contribute to the process of digestion. Below is an outline of the primary organs involved in digestion and their function.
http://www.enchantedlearning.com/subjects/anatomy/digestive/
Mouth
Chemical and mechanical digestion begins when food enters the mouth. The teeth manually tear and cut food into pieces during chewing (assisted by the tongue which moves food around in the mouth). At the same time, saliva is produced by the salivary glands. Saliva is a combination of mucus, water, bicarbonate, and salivary amylase. Salivary amylase is an enzyme that initiates the digestion of starch.
Stomach 
The stomach found on the left side of the abdominal cavity, is a sac like organ that is a continuation of the pharynx and esophagus. The stomach is responsible for beginning protein digestion and for storing food before the food passes into the small intestine. The wall of the stomach contains three layers of smooth muscle. There is a circular layer, a longitudinal layer, and third layer that runs in a slanted fashion to the other two layers. This third smooth muscle layer is what enables the stomach to stretch and also assists the stomach in manual digestion of food. The fourth layer of tissue the mucosa has wrinkles called rugae. These wrinkles allow the mucosa to stretch as the stomach fills. The mucosa also has gastric pits that contain gastric glands that produce gastric juices. The gastric juices are composed of the enzyme pepsin that is responsible for protein digestion and hydrochloric acid. The presence of HCl is important because it kills any bacteria that enters the body in our food. (Image Right) http://healthguide.howstuffworks.com/stomach-picture.htm
Small Intestine
The small intestine is found below the stomach and is a continuation of the stomach. It is a long folded tube like organ that averages 18 ft long. the small intestine (and most of the digestive tract) is composed of four different layers. The outermost layer is the serosa which is continuous with the peritoneum. The next layer is the muscularis wich is composed of two layers of smooth muscle. Contractions of the muscularis help move food through the digestive tract. The next layer as you move inward is the submucosa. The submucosa is composed of loose connective tissue and contains blood vessels and nerves. The innermost layer is the mucosa. The mucosa or mucous membrane produces mucous to protect the digestive tract, secretes or receives digestive enzymes (depending on the location). The inner tube or open portion on the digestive tract is referred to as the lumen. The mucosa of the small intestine is also modified to absorb nutrients from food (described below). The small intestine is responsible for continuing the digestion of proteins that started in the stomach but also has enzymes for all other food as well. These enzymes are produced by the pancreas and follows into the small intestine from a duct in the duodenum. The duodenum is the first part of the small intestine that is also the site of another duct that delivers bile for the liver and gallbladder. (Image Above) http://www.healinglightseries.com/tutorialdigestion.html
Pancreas
The pancreas is a secondary or accessory organ in digestion. (Secondary because food does not actually pass through the pancreas). The pancreases produces pancreatic juices which enter the small intestine through a duct in the duodenum. Pancreatic juice contains enzymes needed to digest all food and sodium bicarbonate that neutralizes the acid that enters the small intestine from the stomach. The pancreas is also responsible for the production of insulin that helps to control the level of glucose in the blood. (Image Right) http://www.csupomona.edu/~jlbath/Lab%20Pics/pancreas.htm
Liver
The liver is another secondary digestive organ and is responsible for a number of metabolic functions. The liver purifies the blood of toxins that may have entered the body via the digestive tract. The liver also stores and removes iron and vitamins (A, D, E, K, B12). The liver is also responsible for storing glucose in the form of glycogen. It also will break glycogen down when the glucose level in the blood drops. If there is not glucose the liver can also convert glycerol from fat into glucose. The liver helps to regulate the amount of cholesterol in the blood by using cholesterol to create bile salts. Finally the liver is responsible for producing plasma proteins. (Image Right) http://www.livercancer.com/liver_anatomy.html
The gallbladder is found just below the liver and is where the bile produced by the liver is stored until it is sent to the duodenum. (Image Right) http://www.nlm.nih.gov/medlineplus/ency/imagepages/8789.htm
The large intestine is the final stop for food in the digestive process. The large intestine does not contain digestive enzymes, rather digestion in the large intestine takes place by the action of bacteria found in the large intestine. These intestinal flora produce vitamins (B complex vitamins and vitamin K) through their efforts that the large intestine absorbs. The large intestine is where indigestible material is formed into feces and is expelled from the body during defecation.
Discussion 1.2 The process of digestion
The Mouth
As mentioned above the process of digestion begins when food enters the mouth. This is called ingestion. Enzymes in the saliva begin chemical digestion of starches and the food is chewed and broken apart by the teeth. During chewing the tongue helps to form food into a mass that can be easily swallowed. This mass of food or bolus then passes from the mouth into the pharynx and then the esophagus. This is accomplished by swallowing. Swallowing is at first voluntary and then is involuntary once food is moved to the pharynx. When swallowing is initiated, the epiglottis moves over the trachea to prevent food from entering the airway. Once food is in the esophagus rhythmic contraction of the muscles called peristalsis helps move food down the esophagus. The lower portion of the esophagus is closed off from the stomach by the lower esophageal sphincter. This sphincter opens to allow food to pass from the esophagus and closes to prevent stomach acid from entering the esophagus. The images below illustrate the process of ingestion and swallowing.
Digestion in the Stomach
Once food enters the stomach gastric juices produced by the gastric glands ( as discussed above) begins to digest proteins (with the enzyme pepsin) and kill any bacteria that may be in the food (HCl). The connective tissue of the food is broken down by the hydrochloric acid found in the gastric juices. The food is converted to a thick liquid referred to as chyme that is expelled into the small intestine in squirts. This is accomplished by peristaltic waves of the stomach and the pyloric sphincter with only allows a little chyme into the small intestine at a time. It typically takes 2 to 6 hours for the stomach to empty out into the small intestine.
The small intestine continues chemical digestion through the use enzymes secreted by the pancreas and bile produced by the gallbladder. The chart below shows how the different components of food are digested and the enzymes responsible for their digestion.
Nutrient Absorption in the Small Intestine
The mucosa of the small intestine then absorbs many of the products of digestion, including sugars, amino acids, fatty acids, and glycerol. This is possible because of the unique structure of the mucosa of the small intestine (see images below). The villi or projections of the mucosa have capillaries that allow for absorption of sugars and amino acids into the blood stream. The villi also have small lymphatic capillaries that are called lacteal that are involved in the absorption of fatty acids and glycerol. The glycerol and fatty acids that are produced by the digestion of fats are absorbed into the epithelial cells of the villi. There they are formed into lipoprotein droplets that are referred to as chylomicrons. These chylomicrons then enter the lacteal.
Control of Digestive Secretions
As mentioned in the previous discussion, the liver, gallbladder, pancreas are accessory organs that assist in digestion. The production of digestive juices by these organs is regulated by the nervous system and also by digestive hormones.
The sight or smell of food stimulates the parasympathetic nervous system to initiate gastric secretions.
The hormone gastrin is produced by the stomach which will increase the production of gastric juices in the stomach when a large amount of protein has been ingested.
Other hormones involved in the regulation of digestion are CCK (cholecystokinin) and secretin. These hormones are produced by the wall of the duodenum. Secretin is released in response to the presence of HCl in chyme. CCK is produced when fat and protein enter the small intestine. The presence of seretin and CCK in the blood causes the pancreas to increase the production of pancreatic juice. CCK also stimulates the liver to increase bile production and causes the gallbladder to contract releasing stored bile.
The Large Intestine
After passing through the small intestine what remains of the food that was ingested moves into the large intestine, The large intestine, as mentioned above, absorbs vitamins produced by the intestinal flora breaking down the food as well as excess water. The large intestine also expels indigestible materials from the body. The process of defecation begins with peristalsis of the large intestine that forces feces into the rectum. Contraction of the rectal muscles and relaxation of the anal sphincters allow the feces to then exit the body through the anus.
Discussion 1.3 Cellular Respiration and the Production of Energy
As mentioned previously, when we ingest carbohydrates, they are broken down into glucose molecules. Eventually, these glucose molecules find their way to the cells to be used for cellular energy. Glucose is the fuel the body needs to run. The excerpt below is from my unit I compendium and explains the process of cellular respiration in more detail.
Cellular Respiration
- Cellular respiration begins after the blood has brought glucose and oxygen into the cells. Glucose molecules are broken down slowly. This means that ATP molecules are produced slowly so that the cells can use all the energy produced. If the energy from glucose were released all at once then a lot of energy would be wasted.
Glycolysis
- Glycolysis takes a glucose molecule (containing 6-carbon atoms) and converts it into a two pyruvate molecules (3-carbon atoms each).
- This process takes place in the cytoplasm of almost every different type of cell. Glycolysis doesn't require oxygen and is said to be anaerobic.
- Glycolysis produces 2 ATP per glucose molecule. It also produces a hydrogen atom and two electrons that join with NAD to produce NADH (as discussed above).
Citric Acid Cycle
- Next the pyruvate molecules are taken into the matrix of the mitochandria where enzymes break them down (this step occurs only when oxygen is available . . . if oxygen is not available then fermentation occurs.) The breakdown of pyruvate produces carbon dioxide, 2 ATP (per glucose molecule), a hydrogen atom, and two electrons. Again, the hydrogen and electrons joins with NAD to form NADH and is transported to the electron transport chain.
http://highered.mcgraw-hill.com/sites/dl/free/0072986867/391204/chapt03_lecture.ppt
Electron Transport Chain
- The electron transport chain is composed of complexes of carrier proteins located in the cristae of the the mitochandria.
- The electrons (delivered by NADH) enter the electron transport chain two at a time and are then passed on to the next carrier in the chain. With each exchange the electrons lose energy. This energy is used by the inner mitochandrial membrane to create ATP. This is accomplished by adding an inorganic phosphate molecule to ADP. 32 ATP per glucose molecule are created.
- At the end of the transport chain the electrons that have completed the electron transport chain and hydrogen (that was delivered by NADH)join with oxygen and a water molecule is produced.
- Glucose is not the only substance that can be used to fuel cellular respiration. Fats and proteins can also be used when there is not enough glucose available. This is possible because fatty acids can be broken down into C2 molecules and proteins are broken down (by the liver) into amino acids.
Fermentation
- Fermentation differs from cellular respiration in that it is an anaerobic process. Because of this fermentation is used by the cell for energy production when oxygen is not available.
- In fermentation the citric acid cycle and the electron transport chain are not used. Instead, the NADH formed in glycolysis transfers its hydrogen and electrons to the pyruvate also created by glycolysis. This converts the pyruvate to lactate.
- Fermentation is only good for supplying energy short-term. This is because fermentation only produces 2 ATP (vs 36 for cellular respiration). Also, the lactate created during fermentation can not be broken down directly and starts to build up in the cells. This causes fatigue, muscle cramps, and (if enough lactate builds up) death.
- Once oxygen is available to the muscles again then lactate can be converted back into pyruvate and can continue to the citric acid cycle as normal.
Discussion 1.4 Digestive Health
There are a number of diseases and disorders that involve the digestive system and the organs of the digestive tract. The table below lists some of these disorders and diseases.
| Disease/ Disorder | Organ/ Location | Symptoms/ Treatment |
| Heartburn | Esophagus | Burning sensation in the esophagus caused by acid from the stomach getting into the esophagus. Can become a chronic condition. Diet modification, weight loss, exercise, OTC medications or prescription medications are all used for treatment. |
| Lactose Intolerance | Stomach | Caused by a lack of the enzyme needed to digest lactose in milk products. Causes diarrhea, gas, bloating, and cramping. Can be avoided by avoided by taking supplements or consuming lactose free milk products. |
| Gastric Ulcer | Stomach | Caused by the Helicobacter pyloris a bacterium that is able to survive the acidic environment of the stomach. Can be treated with medication. |
| Diabetes Mellitus | Pancreas | The pancreas does not produce insulin needed for the cells to be able to take up glucose. If untreated a number of chronic conditions and death can result. Treated by injection of manufactured insulin. |
| Gallstones | Gallbladder | Painful condition that is caused by the fluid in the gallbladder hardening into masses that block the ducts of the gallbladder. |
| Jaundice | Liver | Characterized by a yellow tint to the skin and white of the eyes caused when bile pigments get into the blood. A form of jaundice is often seen in newborns and those with more serious liver conditions. Treatment depends on the cause of jaundice. |
| Hepatitis | Liver | Inflammation of the liver caused by either the hepatitis A, B, or C virus. Treatment depends on the type of virus acquired. |
| Cirrhosis | Liver | A condition where the liver tissue becomes fatty and fibrous and unable to perform its functions. Caused by alcohol consumption, a high fat diet, and other diseases of the liver such as hepatitis c. |
| Diverticulosis/ Diverticulitis | Colon | Caused when pieces of the mucosa that lines the large intestine or colon are pushed in forming pockets that food particles collect in and become infected. |
| Irritable Bowel Syndrome | Colon | The muscluaris of the colon contracts causing severe cramps and can cause more severe bowel symptoms. |
| Inflammatory Bowel Disease | Colon/ Rectum | IBD refers to a number of disorders including Crohn’s disease and colitis that involve inflammation of the colon and rectum. |
| Polyps/ Cancer | Colon | Polyps are growths of epithelial tissue. Polyps can be cancerous or non cancerous. Typically they are removed and evaluated for cancer. They are believed to be caused by a diet high in fat. A diet high in fiber may serve as prevention of polyps and colon cancer. |
References
Image References
Images obtained from Aris site for Human Biology by Sylvia Mader (http://highered.mcgraw-hill.com/classware/selfstudy.do?isbn=0072986867 chapter resources - power point presentation), unless otherwise cited under image.
Subtopic II - Nutrition
1. Nutrients
2. Vitamins and Minerals
3. Food Pyramid and Nutrition Guidelines
4. Nutritional and Eating Disorders
Discussion 2.1 Nutrients
The food we eat (just like our bodies) is made up of the building blocks of life. Since RNA & DNA are found in abundance in everything we eat, RNA and DNA are typically not discussed as nutrients in the diet. The remaining building blocks of life (carbohydrates, proteins, lipids) in the context of diet and nutrition are referred to as nutrients. Nutrients are required by the body in order for it to perform its functions properly.
Carbohydrates
Carbohydrates are required for cellular energy (as seen above in cellular respiration). Carbohydrates are found in grains, beans, fruits, nuts, and peas. The body breaks these foods down to get glucose for cellular energy. Complex carbohydrates are those that contain many sugars linked together, while simple carbohydrates contain only one or two sugars linked together. Foods that contain simple sugars (like table sugar or fruit) are easily broken down by the body and provide quick energy. Complex carbohydrates (like grains and nuts) take longer to break down and provide longer lasting energy for the body.
Proteins
Proteins are made out of amino acids. Amino acids are used by the body to build proteins needed for cellular metabolism. As discussed in unit I, there are twenty amino acids, the body can make many of these amino acids but there are eight amino acids that the body is unable to create and must be obtained in the diet. These are referred to as essential amino acids. While there is a certain amount of protein found in foods like grains and nuts, they are termed "incomplete" because they do not contain all of the eight essential amino acids. Dairy products, meat, and poultry, are better sources of protein because they are "complete" containing all eight of the essential amino acids.
Lipids
The body uses lipids (in the subunit form of fatty acids) to build the structure of the cells of the body. Fats, oils, and cholesterol are all lipids. Fats are typically obtained from animal products while oils are obtained from plants. Cholesterol comes from animal products like mean, poultry, and dairy. For the most part fats are solids at room temperature and are called saturated fats, while oils are liquid at room temperature and are called unsaturated fats. Fats are further categorized as polyunsaturated or monounsaturated. Polyunsaturated oils, like corn oil and safflower oils, have a high amounts of polyunsaturated fats and are essential to the diet because they contain two essential fatty acids (linolenic acid and linoleic acid). Monounsaturated fats are found in olive oil and canola oil. Omega-3 fatty acids (which are believed to be crucial to brain development and protective against heart disease) are found in some cold water fish like salmon and in flaxseed oil. The chart to the right shows common fats and their composition in relation to saturated fat, unsaturated fat, monounsaturated fat, and cholesterol. Another categorization of fats is "transfat" or trans-fatty acids. These are not naturally occurring and are found in processed foods that contain partially hydrogenated oils.
Discussion 2.2 Vitamins and Minerals
Minerals
Minerals are categorized by the amount contained in the body. If the body contains more than 5g they are said to be major minerals. Those that are found in quantities less than 5g are said to be trace minerals. Minerals perform varied functions in the body from serving as components of body fluids and tissues to building blocks of larger molecules. The chart below lists the major and trace minerals, their function in body, the food they are found in, and conditions that may result from too little or too much of each mineral.
Vitamins
Vitamins are compounds found in food that the body needs to perform its various functions. Many vitamins are parts of coenzymes that are necessary to help enzymes catalyze reactions. There are thirteen different vitamins which are divided into water soluble vitamins and fat soluble vitamins. The two tables below show the different vitamins, their functions in the body, the foods they can be found in, and conditions that result from too little or too much of each vitamin.
Discussion 2.3 The Food Pyramid and Nutrition Guidelines
The food pyramid (seen below) outlines the recommend amount of foods from each food group need each day. It is believed that following these guidelines will provide the body with adequate amounts of nutrients, vitamins, and minerals need to keep the body functioning properly. In addition, to following diet that covers the food groups in adequate amounts one must be cautious not to eat more calories than the body requires which can lead to obesity (which will be discussed in the next section).
The chart below itemizes other suggestions for good nutrition.
| Carbohydrates/ Grains | Eat mostly whole grains because refined grains have lost much of their nutritional content. Whole grains are also a good source of fiber which is beneficial to the heart by helping remove cholesterol from the body and is beneficial for intestinal health helping prevent colon cancer. |
| Protein/ Meat | It is recommended that meat be consumed in the appropriate portions and that lean meat and skinless poultry be selected over more fatty options. |
| Protein/ Dairy | Adults (and older children) should choose fat free or low fat dairy products over full fat versions. |
| Fats/ Cooking Oils | Oils should be chosen over fats for cooking. Those that contain a greater amount of polyunsaturated fats are best. |
| Fats/ Omega-3 Fatty Acids | Cold water fish like Salmon and trout or flaxseed oil should be incorporated into the diet to provide omega-3 fatty acids which help protect against cardiovascular disease. Fish intake should be monitored however for mercury content. |
| Transfats/ Processed Foods | Transfats should be avoided and are usually present in processed and packaged foods like cookies and cakes. Processed foods should also be avoided or consumed in extreme moderation because of they typically are high in sugar content and low in nutritional value. |
| Minerals/ Calcium | Individuals should be conscious of getting adequate calcium intake to prevent osteoporosis. Drinking milk is the easiest way to get calcium in the diet but other foods also contain calcium. Men should get 1,000 mg a day of calcium and women should get 1,300 mg. It may be difficult to get the proper amount of calcium through diet alone so many recommend that a calcium supplement be taken. |
| Minerals/ Iron | Iron is necessary to form the hemoglobin molecule found in the blood. Women need more iron than men because of menstruation. If anemia is still a problem with adequate protein intake iron supplements can be taken. |
| Minerals/ Sodium | It is recommended that only 500 mg of sodium be consumed in a day. Many people consume much more sodium than is necessary which can make hypertension worse. Ways to limit sodium intake are to reduce the use of sodium added to food during cooking or at the table or substitute other seasonings. Also, processed and canned foods are very high in sodium. Eliminating these from the diet can go a long way to reducing sodium intake. |
| Vitamins/ Antioxidants | Several vitamins (C,E, and A) are classified as antioxidants because they help to protect the body from free radicals (O2- and OH- ions created during cellular metabolism that damage cells and are believed to contribute to cancer. Vitamin C, E, and A intake can be assured by eating the recommended daily amount of fruits and vegetables and by consuming a wide variety of different fruits and vegetables. |
| Water | Most Americans are technically dehydrated. The body needs water to perform at its best and maintain health. It is recommended that individuals consume at least 64 ounces of water per day. A more individualized calculation is 1 ounce of water for every 2 pounds of body weight. If beverages containing caffeine are consumed (which tends to dehydrate) they should be replaced with 2 oz of water for every ounce of caffeinated beverage. Exercise also increases the body’s need for water above and beyond this calculation. (1) http://www.mayoclinic.com/health/water/NU00283 |
| Exercise | Physical activity is necessary for good health. A minimum of thirty minutes of brisk physical activity is recommended daily to maintain heart health and prevent obesity (if already at a healthy weight). |
Discussion 2.4 Nutritional and Eating Disorders
Obesity
Obesity is one of the most pervasive problems in our society today. About one third of adults in this country today are categorized as obese. Obesity itself is defined as having a body mass index that is over 32 (or roughly 30% over the recommended body weight for height). Obesity is such a problem because it can (and most often does) lead to other health concerns. Individuals who are over weight are at risk for type 2 diabetes, cardiovascular disease, hypertension, stroke, gallbladder disease, respiratory diseases, osteoarthritis, and cancer.
As a rule, the best way to reverse obesity and its associated health risks is to increase activity, eat fewer calories, and make sure the calories you do ingest come from nutritious foods. There are other ways people attack obesity such as crash diets and weight loss surgery but both of these approaches pose potential health risks of their own.
Type 2 Diabetes
In type two diabetes (unlike diabetes mellitus) the pancreas produces adequate insulin but the glucose control mechanism of the body doesn't function adequately. This is believed to be caused by a diet high in simple carbohydrates which causes the pancreas to repeatedly release large amounts of insulin. Eventually, the cells of the body begin to become resistance to the insulin and thus the cells are not triggered to take up the glucose. High blood sugar is the result.
Vitamin Deficiency
A number of problems are caused when the body is deficient in certain vitamins. For example, rickets (malformation of leg bones) is caused in children who get inadequate vitamin D in the diet. Niacin deficiency can cause a sensitivity of the skin to light. Insufficient vitamin C intake can cause bleeding gums.
Eating Disorders
There are a number of eating disorders that individuals suffer from. These disorders typically have emotional as well as biological components. Although, discussed in its own section above, obesity could also be described as an eating disorder. Even though, an individual can have a genetic tendency toward obesity it is still necessary for them to consume more calories than they burn before gaining weight.
Anorexia Nervosa
Anorexia nervosa is characterized by a distorted body image and the perception or fear of getting fat. Individuals suffering from this illness will starve themselves, induce vomiting when they do eat, and exercise excessively to keep themselves from gaining weight. Primarily, women suffer from anorexia nervosa.
Bulimia Nervosa
Bulimia is much like anorexia in that individuals have a distorted body image and a fear of weight gain. Those suffering from bulimia typically are of normal weight but will undergo periods of binge eating and then purge to rid themselves of the excess calories.
Binge-Eating Disorder
In this disorder, individuals will binge eat but do not follow the periods of binge eating with purging. These periods of binge eating are usually triggered by emotional stress.
Muscle Dysmorphia
Unlike, the other disorders muscle dysmorphia typically affects men. They think their bodies are underdeveloped and will spend a great deal of time working out or body building.
References
Image References
Images obtained from Aris site for Human Biology by Sylvia Mader (http://highered.mcgraw-hill.com/classware/selfstudy.do?isbn=0072986867 chapter resources - power point presentation), unless otherwise cited under image.
Topic References
(1) http://www.mayoclinic.com/health/water/NU00283
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