Digestion and food sources

The process by which food is converted into substances that can be absorbed and assimilated by the body. Mechanical & chemical breakdown of food into nutrients are called digestion.

Four main macromolecules in food: nucleic acids, proteins, fats, and carbohydrates are broken down into smaller molecules of nucleotides, amino acids, fatty acids and glycerol, and simple sugars. Being absorbed important minerals, vitamins, and water are also extract from food during the process of digestion.

Food Sources

Biologists categorize animals into three groups based on their food source.

• Herbivores obtain all food from plants. Examples: cows, horses, and nearly all rodents.
• Carnivores obtain all food from meat. Examples: cats, eagles, wolves, and frogs.
• Omnivores obtain food from both plants and meat.      

Examples: humans and bears. An animal’s digestive system is specifically suited to processing food obtained from its food source. For example, herbivores’ digestive systems are equipped to break down plant material, and carnivores’ are not.

What is a digestive system and how does its structure reflect its function?

Digestive systems mechanically and chemically degrade food into small molecules that can be absorbed, along with water, into the internal environment. These systems also expel the undigested residues from the body. Incomplete digestive systems are a saclike cavity with one opening. Complete digestive systems are a tube with two openings and regional specializations in between. Structural variations in bills, teeth, and regions of the gut are adaptations that allow an animal to exploit a particular type or types of foods.

Digestive system (Tract)

The digestive tract is the area where digestion takes place.

•         The digestive tract varies widely in complexity from species to species.

•         The most primitive form found in nematodes is simply a tubular gut with no specialized features.

•         The slightly more advanced digestive tract found in earthworms includes specialized areas for ingestion, storage, fragmentation, digestion, and absorption.

•         The most advanced digestive tracts, found among the vertebrates, exhibit specialization on a much greater scale.

What are the components of the human digestive system?

What are the components of the human digestive system?

Humans have a complete digestive system. Swallowing forces food and water from the mouth into the pharynx. Food continues through an esophagus to the stomach. Food processing starts in the mouth. Most digestion and absorption occurs in the small intestine. The colon absorbs most of the remaining water and ions, which causes the wastes to compact. The rectum briefly stores the wastes before they are expelled through the anus.The liver and pancreas have an accessory role in digestion. They produce substances that are secreted into the small intestine.

Human Digestive system

 

The human alimentary canal: The alimentary canal begins at the mouth and the pharynx.

From here, food travels down a muscular tube called the esophagus and into the stomach. The stomach connects to the small intestine, which in turn connects to the large intestine. Some form of mechanical and chemical digestion breaks down food at every step along this tube and Nutrients are absorbed across the walls of both intestines. All products remaining after food travels through the large intestine are waste, which exit the body through the anus.

Saliva: Contains water, salt and an enzyme called salivary amylase. Salivary amylase breaks down starch into smaller sugars. Contains buffers, substances that neutralize acidic foods. Antibacterial substances that kill bacteria in the food. Also eases the passage of food through the pharynx and esophagus.

The Esophagus The esophagus shuttles food from the pharynx (part of mouth) to the stomach. The muscles surrounding the esophagus perform peristalsis, the rhythmic and stepwise contraction of muscle that forces food to move along the esophageal passage. Sphincter muscles at each end prevent backflow.

The Stomach A saclike organ. Inner surface is highly convoluted, allowing it to fold up when empty and expand when full. Digestion occurs in the stomach as gastric juice, is secreted by gastric glands located in the stomach lining. Gastric juice is an acidic mixture of enzymes and mucus. Gastric glands are composed of two types of cells: 

Parietal cells: secrete hydrochloric acid (HCl)

Chief cells: secrete pepsinogen, the precursor to the enzyme pepsin.

Protein digestion is initiated by the acidic environment of the stomach. The HCl, lowers the acidity of the gastric juice to a pH level of 2. Low pH levels are necessary for pepsinogen to be converted into the active digestive enzyme pepsin, which further breaks down food proteins. If chief cells were to secrete pepsin directly into the stomach, the pepsin would digest the cells themselves. Additionally, mucus lines the stomach walls and prevents it from being digested. A mixture of gastric acid and partially digested food called chyme passes out of the stomach.

The Upper Digestive Tract

What happens to food in the mouth?

Teeth mechanically break food into smaller particles. Enzymes in saliva begin the chemical digestion of carbohydrates and fats.

What are the functions of the stomach?

The stomach receives food from the esophagus and stretches to store it.Stomach contractions break up food and mix it with gastric fluid. They also move the resulting mixture (the chyme) into the small intestine.Chemical digestion of proteins begins in the stomach.

The oral cavity (Digestion in Mouth):  Mouth, the teeth. the tongue, Salivary Glands.

 The teeth: break food into smaller pieces to expose more of the food’s surface area to digestive enzymes.

 The tongue: a muscle used to manipulate food and form it into a bolus, a round, easy-to swallow ball.

Salivary glands: secrete mucous saliva.

Most Chemical Digestion Occurs in the Small Intestine

How does the structure of the small intestine affect its function?

 The surface of the small intestine is highly folded and each fold has many projections (villi). Brush border cells at the surface of a villus have tiny projections (microvilli) at their surface. The many folds and projections greatly increase the surface area for the two functions of the small intestine—digestion and absorption.

Dudenum :It’s a tube of about 30 cm long :

–        Receives bile via bile duct from the liver

–        Receives pancreatic juice from the pancreas, through the pancreatic duct

–        Releases digestive juice from its walls. This intestinal juice contains protease and the enzyme lipase for fat digestion

The Small Intestine

Duodenum: Controls the release of food into the small intestine

Most of the chemical breakdown (Digestion) and absorption of Food takes place here. Bile salts break down fats in the chyme while pancreatic fluid, composed of bicarbonate, neutralizes the acid. The small intestine also contains a host of enzymes that help digest various food molecules. The enzymes present in the small intestine and their functions are outline in the table below.

Pancreatic amylase, maltase, sucrase, lactase ------Starch

Trypsin, chymotrypsin, aminopeptidase,carboxypeptidase ------Protein 

Nucleases------ Nucleic acids 

Bile salts, lipase ------Fats

What are the roles of the small intestine?

Chemical digestion is completed in the small intestine. Enzymes from the pancreas and enzymes embedded in the membrane of brush border cells break large molecules into smaller absorbable subunits. Small subunits (monosaccharides, amino acids, fatty acids, and monoglycerides) enter the internal environment when they are absorbed into the intersitial fluid in a villus. Most fluid that enters the gut is also absorbed across the wall of the small intestine.

The Small Intestine

• The small intestine is split into three sections: the duodenum (30 Cm), jejunum (2.5m), and the ileum(2-4m).

• Chemical digestion takes place in the duodenum and the jejunum.

• The ileum, the final section of the small intestine, is where most absorption of the nutrients takes place.

• Small projections called villi cover the ileum walls.

• Cells lining the villi are covered in folds of plasma membrane that form even smaller projections called microvilli.

• Villi and microvilli increase the ileum’s surface area, providing more surface across which nutrients can be absorbed.

• Nutrients pass through capillaries in the lining of the villi and into the bloodstream, where they circulate first to the liver, then throughout the rest of the body.

• Villi and microvilli projections also contain digestive enzymes to further digest food. 

Carbohydrate Digestion and Absorption

Figure description

Enzymes break polysaccharides down to simple sugars, or monosaccharides. Monosaccharides are actively transported into brush border cells, then out into interstitial fluid. Proteins are broken into polypeptides, then amino acids. Amino acids are actively transported into brush border cells, then out into interstitial fluid.

1.      In the small intestine, carbohydrates are broken down into monosaccharides, or simple sugars .

2.      This process began in the mouth, where salivary amylase broke polysaccharides into disaccharides (two-unit sugars). A pancreatic amylase carries out the same reaction in the small intestine. Disaccharides are substrates for enzymes embedded in the plasma membrane of the brush border cells. The enzymes split disaccharides into monosaccharides. For example, sucrase breaks sucrose into glucose and fructose subunits. Lactase splits lactose into glucose and galactose. Monosaccharides are actively transported into a brush border cell, then out into the interstitial fluid inside a villus. From here they enter the blood.

Protein Digestion and Absorption

3.      Protein digestion began in the stomach, where pepsin broke proteins into polypeptides. It is completed in the small intestine.

4.      The pancreas secretes proteases such as trypsin and chymotrypsin that break polypeptides into peptide fragments. Enzymes at the surface of the brush border cell break these fragments into amino acids. Like monosaccharides, amino acids are actively transported into brush border cells, then out into the interstitial fluid. From here they enter the blood.

5.       Movements of the intestinal wall break up fat globules into small droplets. Bile salts coat the droplets, so that globules cannot form again.
 

6.       Pancreatic enzymes digest the droplets to fatty acids and monoglycerides.

7.      Monoglycerides and fatty acids diffuse across the plasma membrane's lipid bilayer, into brush border cells.
8.      A In a brush border cell, the products of fat digestion form triglycerides, which associate with proteins.  I he resulting lipoproteins are then expelled by exocytosis into the interstitial fluid inside the villus.

The Large Intestine

Food from the small intestine empties into the large intestine, or colon. No digestion and only a small percentage of absorption take place in the large intestine. The large intestine primarily functions to concentrate waste material into a form called feces. Movements of the large intestine compact feces and move it into the rectum, where it is exited from the body through either the anus. Feces is expelled from the body through the voluntary movements of muscles surrounding the rectum.

Rectum: Muscular storage chamber where the undigested food (feces) is held and molded before being pushed through anus during expulsion.

Anus: The exit of alimentary canal, it is closed by a ring of muscle (the anal sphincter). The ring is relaxed during expulsion of feces.

What are the functions of the large intestine?

By absorbing water and mineral ions, the colon compacts undigested residues and other wastes as feces, which are siored briefly in the rectum before expulsion.

Accessory organs

•         Produce and store digestive chemicals

•         Food does not pass through these

•         Liver - Produces bile; Stores glucose as glycogen & releases glucose into blood stream

•         Gallbladder - Stores bile, Squirts bile into small intestine when fat arrives

•         Pancreas - Makes enzymes. Empties into small intestine

•         Pancreatic juice: enzymes, bicarbonate

•         Insulin and glucagon secreted directly into blood

•         Insulin: Enables cells to use glucose (glucose enters cells) & promotes formation of glycogen in liver     

•         Glucagons: Signals to the liver to convert glycogen into glucose & release it into the bloodstream

Liver Functions

1. Forms bile (assists fat digestion), rids body of excess cholesterol and blood's respiratory pigments
2. Controls amino acid levels in the blood; converts potentially toxic ammonia to urea
3. Controls glucose level in blood; major reservoir for glycogen
4. Removes hormones that served their functions from blood
5. Removes ingested toxins, such as alcohol, from blood
6. Breaks down worn-out and dead red blood cells, and stores iron
7. Stores some vitamins                

Pancreas

•         Lies between the stomach and duodenum

•         It secretes pancreatic juice which passes to the duodenum to help in digestion

–        Amylase – for digesting starch

–        Protease – protein

–        Lipase – fat

Stomach Disorders

Stomach Disorders

Gastroesophageal Refiux In some people, the gastroesophageal sphincter at the entrance to the stomach does not close properly or opens when it should not. The result is gastroesophageal reflux. Acidic chyme splashes back into the esophagus, causing a burning pain commonly called heartburn or acid indigestion. Occasional acid reflux can be treated with over-the-counter antacids, but a chronic problem should be discussed with a doctor. Repeated exposure to acid can damage the tissue of the esophagus and raise the risk of esophageal cancer.

Stomach Ulcers When something disrupts the stomach's protective mucus layer, gastric fluid and enzymes can erode the stomach lining, causing an ulcer. Most ulcers occur after an acid-loving species of bacteria (Helicobacter pylori) makes its way through gastric mucus and infects cells of the stomach lining. H. pylori releases chemicals that increase gastrin secretion, thus making the stomach more acid. The extra acidity stresses cells of the stomach lining so the infection spreads more easily. Antibiotics halt the infection and allow healing to occur.

Continual use of nonsteroidal anti-inflammatory drugs such as ibuprofen or aspirin can also cause a stomach ulcer. These drugs interfere with chemical signals that maintain the health of the stomach lining.

Disorders That Affect Digestion in the Small Intestine

Lactose Intolerance To be able to digest lactose, a person must have functional lactase at the surface of their brush border cells. In many people, including most Asians and African Americans, expression of the gene for lactase declines in adulthood. As a result, lactose is not broken down in the small intestine. When lactose enters the large intestine, resident bacteria break it down in reactions that produce hydrogen gas as a by-product. The result is flatulence, a feeling of bloating, and cramps.

Disorders That Affect Digestion in the Large Intestine

Appendicitis—an inflammation of the appendix— requires prompt treatment. It often occurs after a bit of feces lodges in the appendix and infection sets in. Removing an inflamed appendix prevents it from bursting and releasing bacteria into the abdominal cavity. Such a rupture could cause a lifc-threatcninc infection.

Human Nutritional Requirements

Energy-Rich Carbohydrates

Fresh fruits, whole grains, and vegetables—especially legumes such as peas and beans—provide abundant complex carbohydrates . The body breaks the starch in these foods into glucose, your primary source of energy. These foods also provide essential vitamins and fiber. Eating foods high in soluble fiber helps lower one's cholesterol level and may reduce the risk of heart disease. A diet high in insoluble fiber helps prevent constipation.

Foods rich in processed carbohydrates such as white flour, refined sugar, and corn syrup are sometimes said to be lull of "empty calories/' This is a way of saying that these foods provide little in the way of vitamins or fiber.

You may have noticed breads and other grain-based foods labeled as "gluten-free." Gluten is a protein found in  wheat and many other grains. An estimated 1 percent of the population has a genetic disorder called celiac disease, in which gluten causes an autoimmune reaction that harms the small intestine's villi. Celiac disease is treated by eliminating gluten from the diet.

Good Fat, Bad Fat

Your body uses lipids to build cell membranes, as energy stores, and as a reservoir for fat-soluble vitamins.

Linoleic acid and alpha-linolenic acid arc essential fatty acids, meaning the human body needs them but cannot make them, so they are required in the diet. Both are polyunsaturated fats; their long carbon tails include two or more double bonds. Unsaturated fats are liquid at room temperature.

We divide the polyunsaturated fatty acids into two categories: omega-3 fatly acids and omega-6 fatty acids. Omega-3 fatty acids, the main fat in oily fish such as sar dines, seem to have special health benefits. Studies suggest that a diet high in omega-3 fatty acids can reduce the risk of cardiovascular disease, lessen the inflammation associated with rheumatoid arthritis, and help diabetics control their blood glucose. Oleic acid, the main fat in olive oil. may also have health benefits. It is monounsaturated, which means its carbon tails have only one double bond. A diet in which olive oil is substituted for saturated fats helps prevent heart disease.

Dairy products and meats are rich in saturated fats and cholesterol. Overindulging in these foods increases one's risk for heart disease, stroke, and some cancers.

Trans fatty acids, or trans fats, are manufactured from vegetable oils. However, they have a molecular structure that makes them even worse for the heart than saturated fats. All food labels are now required to show the amounts of trans fats, saturated fats, and cholesterol per serving.

Body-Building Proteins

Amino acids are building blocks of proteins. Your cells can make some amino acids but you must get eight essential amino acids from food.Most proteins in meat are "complete," meaning their amino acid ratios match a human's nutritional needs. By contrast, most plant proteins are "incomplete," meaning they lack one or more amino acids essential for the human diet.

The American Dietetic Association states that, with careful planning, a vegetarian diet can provide all essential nutrients for people in any stage of life. To obtain all the required amino acids from plant sources alone, one must combine foods so that the amino acids missing from one component are present in some oth ers. As an example, rice and beans together provide all necessary amino acids, but rice alone or beans alone do not. You do not have to eat the two complementary foods at the same meal, but both should be consumed within a 24-hour period.

Vitamins and Minerals

Vitamins are organic substances that are essential in very small amounts; no other substance can carry out their metabolic functions. At a minimum, human cells require the thirteen vitamins. Minerals are inorganic substances that are essential for growth and survival because no other substance can serve their metabolic functions. Red blood cells require iron to make oxygen-transporting hemoglobin. Iodine is essential for development of a healthy nervous system and to make thyroid hormone.