Digestive System:



Digestive System:

Digestion is the breakdown of complex food molecules into smaller components that are used by the organism. This may occur outside of the organism as in bacteria and fungi, or the organism may have extracellular digestion. Most organisms take food into cells.

Definitions:

Holotrophs: Organisms that ingest other organisms, dead or alive, whole or by the piece, or absorb organic molecules directly.

Below are the types of holotrophs:

Herbivores: Organisms that eat plants or algae.

Carnivores: Organisms that ingest other animals.

Omnivores: Organisms that ingest both plant/algae and animals.

Suspension Feeders: Organisms that sift small food particles from the water.

Substrate Feeders: Organisms that live in or on the food source.

Deposit Feeders: A type of substrate feeder—they eat through dirt (earthworms).

Fluid Feeders: Organisms that suck nutrient rich fluids of the living host.

Bulk Feeders: Organisms that ingest relatively large pieces of food.

Digestion is a series of chemical reactions that use hydrolytic enzymes. The chemical digestion can be preceded by mechanical fragmentation of food into smaller pieces. Breaking down the food mechanically increases the surface area of the food and makes the digestive enzymes more effective. Once the food is digested the organic molecules can cross the plasma membrane and enter cells. The undigested materials are defecated. Ingestion is the act of eating. Digestion is the breaking down of food into useful organic molecules.

Comparative Digestion:

The simplest compartment for digestion is a food vacuole. These can digest food without the enzymes mixing with the cytoplasm. Protists take food in through endocytosis and then perform intracellular digestion.

Protists: Those that are heterotrophic take in their food by phagocytosis and form a food vacuole. A vacuole containing digestive enzymes (lysosome) binds to the food vacuole and breaks down the food. Food molecules move through the cytoplasm.

Fungus: Extracellular digestion. Fungi secrete enzymes into their surroundings. These enzymes breakdown the food into organic molecules that are absorbed by the hyphae.

Sponge: Food particles enter the choanocytes via phagocytosis. Digestion occurs in food vacuoles. The food vacuoles are transferred to amebocytes, which distribute the food to other cells.

Gastrovascular Cavities are digestive sacs with a single opening. This cavity functions in both the digestion and distribution of nutrients through the body.

For example, hydra catches food using cnidocytes on the tentacles, stuff food into a mouth. In the gastrovascular cavity specialized cells produce digestive enzymes that break the food down, flagellated cells spread the food particles in the cavity, and particles are taken into cells by phagocytosis.

Planarians also have a gastrovascular cavity with one opening.

An alimentary canal is a tube with two openings: mouth and anus. Parts of the tube are specialized, ie. Some parts digest the food and other parts absorb the food. In earthworms, food is taken in by the mouth and pharynx, passes to the esophagus, the crop (stores food and moistens it), the gizzard (grinds the food which is mechanical digestion. The crop and gizzard combine to form a stomach), the intestine (absorb molecules), on to the anus. This is similar to the foraging and digestive structures of fish, amphibians, reptiles and birds.

The lining of the digest tract can also play a protective role for the surrounding tissues. It protects the tissues from digestive enzymes, mechanical stress of food passing through, and from the bacteria associated with the digestive tract.

Digestive organs and Peritoneum:

The digestive organs and glands are lined with serous membrane. There are two parts to the serous membrane: visceral membrane that lines the organ and the parietal membrane that lines the inner surface of the body wall. The serous membrane produces peritoneal fluid that reduces friction. About 7 liters of fluid is produced and reabsorbed daily.

Mesenteries:

Portions of the digestive tract are suspended in the peritoneal cavity by sheets of serous membranes that connect the parietal and visceral peritoneum. These mesenteries are double sheets of peritoneal membrane. Blood vessels, nerves, and lymph vessels pass through these mesenteries to the digestive tract. The mesenteries also stabilize the organ positions and prevent the intestines from becoming entangled during digestive movements or sudden changes in body position.

Organization of the Digestive System: There are 4 major layers of the digestive system: 1) mucosa, 2) submucosa, 3) muscularis externa and 4) serosa.

Mucosa: the inner lining of the digestive system. This includes the epithelium and a lamina propria of areolar tissue.

• Epithelium: simple or stratified depending on location and stress. Oral cavity, pharynx, esophagus have stratified squamous. Stomach, small intestine, large intestine have simple columnar. Scattered among the epithelial cells, there are enteroendocrine glands that secrete hormones that affect the digestive tract and accessory glands. Often times, the epithelium is folded, which increases the surface area to volume ration.

• Lamina Propria: layer of areolar tissue that contains flood vessels, lymph vessels, nerves, stretch muscle cells, and areas of lymph tissue. Mucous glands are also present here.

Submucosa: dense irregular connective tissue. This layer has nerves, large blood vessels, lymph vessels, and sometimes exocrine glands. These glands secrete enzymes and buffers into the digestive tract.

Muscularis externa: smooth muscle layer. This layer helps process food by helping with the mechanical breakdown of food. The smooth muscles also move food along by peristalsis.

Serosa: serous membrane.

Movement of food: Peristalsis. The muscularis externa contracts, which pushes food along in a series of waves—the muscle contracts behind the bolus.

Nerves, hormones, and muscle contractions coordinate the movement of the food. The motor neurons that control the smooth muscle and glandular secretions are located at the myenteric plexus. This set of ganglia controls the peristalsis and controls digestive glands. This is called the short reflex—impulses are not controlled by the central nervous system. The CNS triggers the long reflex—higher level of control over the digestive and glandular actions.

There are, at least, 18 different hormones may play a role in the digestive function. Local mechanisms—histamine and prostaglandins also play a small role in digestion. We’ll take about some of these at the appropriate time.

Path of Food:

Foraging and Digestive Structures in Mammals: Human Digestive System is a good representative of the mammalian system.

Teeth grow in sockets in the jaw. Usually mammals begin with temporary milk teeth that are replaced by the adult teeth. There are 4 types of teeth in mammals.

1) Incisor: Chisel shaped for cutting (4 on each jaw)

2) Canines: Tearing food and defense (absent in herbivores—2 on each jaw).

3) Premolars and 4) Molars: grinding food. In herbivores these are large and flat (4 and 6, respectively on each jaw).

Humans have teeth that are not specialized for any particular type of diet.

Oral Cavity and Esophagus:

The lips are essential in eating.

1) Must close lips to swallow.

2) Help food keep inside the mouth.

Tongue:

1) Moves food into position for chewing.

2) Tells us when our food can be swallowed.

3) Prevents us from swallowing.

4) Contains chemoreceptor (taste buds) which can distinguish five different types of taste: salt, sweet, sour, bitter, and umami (meat taste—found in parmasean cheese and portabello mushrooms). These receptors are in a specific concentration on specific sites of the tongue. The stimulation of the taste buds can also enhance saliva flow.

The lingual frenulum is the fold of mucous membrane that connects the underside of the tongue body to the floor mucosa.

The tongue’s mucosa contains small glands in the lamina propria. These glands secrete water, mucin and lingual lipase—begins lipid digestion. The two muscles that control the tongue: the large extrinsic tongue muscle and the smaller intrinsic tongue muscle.

Oral Mucosa: the oral cavity is lined with squamous epithelium. The surface of the tongue is covered with keratinized cells other surfaces are covered with unkeratinized cells. The mucosa under the tongue is very thin. Substances can enter the blood through this part of the oral cavity.

Gingivae are ridges of oral mucosa that surround the teeth.

The roof of the mouth is composed of the hard and soft palate. The tongue dominates the floor of the mouth.

The uvula dangles in the pharynx. The uvula helps stop food from entering the esophagus prematurely and helps prevent the food from entering the nasal cavity.

Three pairs of Salivary Glands: Parotids, Submaxillaries, and Sublinguals.

Parotids: Located in the front and below the ears at the angle of the jaw.

Submaxillaries: found below the angle of the jaw.

Sublinguals: found below the tongue.

Saliva: 95% water, ions, lubricating mucus, and starch splitting enzyme amylase, which starts starch digestion and digests the starch caught between the teeth. Dissolved in the saliva is a slippery glycoprotein called MUCIN. Mucin protects the mouth from abrasions and lubricates the food. Saliva also contains buffers, which help prevent dental cavities by neutralizing acid in the mouth, and antibacterial agents.

70% of the saliva originates in the submandibular gland, 25% is formed in the parotids, and the sublingual glands produce 5%.

As you chew, the food forms a bolus at the back of the throat (pharynx). A bolus is a ball of chewed food and saliva.

Pharynx: Near the rear of the oral cavity and forms a common passageway to the nasal cavity. Below the tongue the pharynx divides into the larynx and laryngopharynx. When you swallow the top of the larynx moves so that a cartilaginous flap called the epiglottis blocks the air passageway, by flapping down over the glottis like a trash can cover. This ensures that the bolus (ball of food) stays out of the respiratory system.

Esophagus:

Moistens the food and moves it to the stomach and is made up of smooth muscle.

The esophagus serves two functions:

1) Secretes mucus into the lumen, cavity, of the digestive tract.

2) Moves food along- compressing food into a bolus- muscles contract behind the food and push it along (peristalsis action).

The esophagus is lined with unkeratinized cells. The submucosa contains esophageal glands that produce mucin.

The Stomach: Located on the left side of the abdominal cavity just below the diaphragm. This temporarily stores and helps digest the food.

The stomach has four functions:

1) Storage of food

2) Mechanical breakdown of food with muscle contractions

3) Chemical breakdown of food with enzymes and acid

4) Produces intrinsic factor, a glycoprotein necessary for vitamin B12 absorption in the small intestine.

The stomach stretches easily (to accommodate about 2 liters of food and fluid) and any resistance to the stretching will cause cramps. The stomach is closed off at either end by two sphincter muscles that lock the food in. The pyloric sphincter, which is the bottom sphincter, that opens into the small intestine. The cardiac sphincter is the top sphincter that opens up into the esophagus. Food is churned without being pushed into the esophagus (heartburn) or intestine (ulcer). Ulcers occur when the digestive system digests its own stomach (thought to be caused by a bacterial infection by H. pylori) or small intestine. The vomit center in the brain allows for the cardiac sphincter to relax, the pyloric sphincter to tighten, and the stomach to contract.

A complex layering of muscles (three layers that go in different directions) allows the twisting action and wringing and shortening of the stomach that grinds up the food.

After the food has been ground up, the pyloric sphincter relaxes a bit and the food enters the small intestine.

The stomach is lined with mucosa, mucus secreting tissue, and tubular glands, which secrete gastric juices. Chief cells secrete hydrochloric acid, which activates pepsinogen to be converted to pepsin. Pepsin hydrolyzes proteins and works well in the low pH generated by the HCl. (Parietal cells of the stomach have carbonic anhydrase, which will convert CO2 and H2O into carbonic acid H2CO3. H2CO3 will dissociate into H+ and HCO3-. The H+ will join with Cl- to form HCl and the bicarbonate ion will go to the small intestine and increase the pH of the acid chyme.) Other glands secrete water, mucus, rennin, which digests milk and gastric lipases. A low pH in the stomach, 1.6-2.4, kills bacteria. Mucin lines the stomachs so that it doesn't digest itself.

The nervous system and hormones control gastric secretions. When we see and smell food, the brain stimulates the stomach to secrete gastric juices. Certain substances will cause the stomach wall to release a hormone called GASTRIN. Gastrin travels through the circulatory system to the stomach, causing the stomach to secrete more gastric juices.

About every 20 seconds the churning of the smooth muscles mixes the stomach contents. When the stomach is empty, the stomach churns and hunger pangs are felt. After food and gastric juices mix, ACID CHYME is produced. The pyloric sphincter opens and the stomach squirts some chyme into the small intestine. It takes about 2-6 hours for the stomach to empty after a meal.

Small Intestine:

Food in liquid form enters the small intestine.

Functions of the small intestine:

1) Chemical digestion

2) Absorption: The small intestine is about 20 feet long (6 meters), and is broken up into the three regions: duodenum, jejunum, and ileum. It is lined with mucin and adapted for absorption of food through a highly folded inner surface. Each fold is called a villus (pl. villi).

Villi: the epithelium is simple columnar. This epithelium is covered with microvilli and looks like a brush—called a brush border. This will increase the surface area to volume ratio to 2 million cm2 (2,200 ft2).

The lamina propria of each villus contains capillaries that originate from the submucosa. Nutrients enter the capillary and travel to the liver via the hepatic-portal circuit. The liver will adjust the concentration of nutrients of the blood. Nerves are also found in each villus with a lacteal. The lacteal is part of the lymph system. Fatty acids that are too large to enter the capillary will enter the lacteal.

The food moves along by peristalsis. The myenteric reflex stimulates peristalsis in the small intestine. The squeezing of the muscles will also squeeze the lacteal and move the lymph fluid.

Enzyme Producing Organs: Secrete the enzymes into the small intestine.

The small intestine contains glands—crypts of Liberkuln that extend into the lamina propria. The brush border cells can produce the enterokinase. Enteroendocrine cells produce gastrin, cholecystokinin and secretin. Duodenal glands produce mucus and a buffer to protect it from the acid chyme (to increase the pH from 1.5 to 7 or 8). These glands also produce urogastrone that will inhibit the production of acid chyme and to increase the cell division in the digestive tract.

The liver secretes bile into the duodenum.

Bile is a fat emulsifier. Bile is stored in the gall bladder and reaches the duodenum of the small intestine via the bile duct. Where basic fluids from the pancreatic duct join it. Bile reduces the size of fat globules so lipases, enzymes that break down lipids, can break them down. If the bile duct is blocked, one becomes jaundiced because bile ends up in the blood stream.

The liver cells are called hepatocytes. These cells adjust the circulating nutrient levels. The liver is responsible for 1) metabolic regulation and 2) bile production.

Metabolic regulation: Liver cells will extract nutrients and toxins/drugs from the blood. The excess nutrients are stored and the drugs/toxins are neutralized. The cells also add nutrients to the blood, if there are deficient levels of nutrients in the blood, or the cells can synthesize compounds for the blood.

• Carbohydrate metabolism: liver stabilizes blood glucose levels at about 90 mg/dl. If too low the hepatocytes will bread down glycogen and put the glucose in the blood. If glucose is too high, the hepatocytes will remove the glucose and store as glycogen or fat. Insulin and glycogen help with this regulation.

• Lipid metabolism: Liver regulates circulating levels of triglycerides and fatty acids. If too low, hepatocytes will break down fat reserves. If too high, the lipids are removed and stored in adipose cells.

• Amino acid metabolism: the hepatocytes will add or remove amino acids from the blood. If there are too much amino acids, then they are stored as glucose or lipids.

• Produce urea. In changing amino acids to lipids or carbohydrates, the amino group is removed as ammonia. The ammonia is toxic. The liver takes the ammonia and forms urea that is sent to the kidney.

• Fat-soluble vitamins and vitamin B12 are stored in the liver.

• Mineral storage: the excess iron is changed to ferritin.

• Drug inactivation: the liver removes and breaks down circulating drugs.

The pancreas is a glandular organ that lies in the first turn of the small intestine. The pancreas secretes bicarbonate, which neutralizes stomach acid. The pancreas also secretes enzymes that break down carbohydrates, proteins, fats and nucleic acids. These enzymes from the pancreas carry out much of the digestive process. The acid of the chyme activates the stomach to produce SECRETIN, a hormone that stimulates the pancreas to release the bicarbonate. (The liver and pancreas are exocrine glands in this capacity. The products are secreted by the liver and pancreas and carried to another location by a duct (tube).)

The digestion of starch, begun in the mouth, is continued in the small intestine by the addition of pancreatic amylase, which hydrolyzes starch into the disaccharide maltose. Maltase will break down maltose into glucose monomers. Sucrase breaks down sucrose, lactase breaks down lactose…

Proteins are broken down by trypsin and chymotrypsin. These break down the polypeptides into smaller polypeptides. Carboxypeptidases split one amino acid at a time off of the carboxyl end while the aminopeptidases work in the opposite direction. The pancreas secretes these enzymes as inactive ZYMOGENS. ENTEROKINASE activates zymogens in the small intestine.

Nucleases hydrolyze DNA and RNA.

All this occurs in the duodenum of the small intestine (the first 25 cm of the small intestine). The two remaining regions, the jejunum and ileum, are specialized for absorption.

Absorption in the small intestine:

Each villus is covered with epithelial cells that contain microvilli (many more small folds-- called the BRUSH BORDER). These projections increase the surface area of the small intestine. Within each villus is a capillary bed and a lacteal (lymphatic vessel). The nutrients enter directly into the blood stream from the small intestine, which feeds into the hepatic portal vessel that goes directly to the liver to process the nutrients.

The small intestine is capable of vigorous movement, which mixes food and enzymes together. This movement also helps with absorption.

Nutrients are absorbed across the epithelial layer into the capillaries or lacteals. Sometimes the transport is passive while other times the transport is active. Amino acids, vitamins, and glucose are pumped against the gradient by the epithelial membranes. The absorption of some nutrients appears to be coupled with the active transport of sodium across the membrane. The sodium is actively transported out of the cell; nutrients are co-transported.

Large Intestine:

The small intestine joins the large intestine on the right side of the body. Here, the cecum forms a pouch, which has a finger like projection called the appendix. There is a one-way valve at the junction of the large and small intestine called the ileocecal valve, which insures that there is no backflow into the small intestine.

The large intestine consists of several parts: cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, anal canal and anus.

The functions of the large intestine include the absorption of water and minerals into the blood, preparation the feces and the housing of bacteria (usually E. coli), which produce vitamin K, Vitamin B’s, biotin, folic acid and methane. Approximately 2/3 of fecal matter is dead bacteria. Feces is usually dead bacteria, cellulose (fiber), and other undigested material.

The large intestine secretes mucus to lubricate for easier export. The mucus is secreted in response to local stimuli—like irritation. Large lymphoid nodules are scattered through the lamina propria and submucosa.

Less than 10% of the nutrients absorbed is done in the large intestine. The most important function is the reabsorption of water. About 1500 ml of materials enter the colon, and about 200 ml of feces is ejected. In the average feces, 75% is water, 5% is bacteria, and 20% is indigestible material and epithelial cells.

The large intestine absorbs bile salts in the cecum. The bacteria in the colon produce Vitamin K, which is needed for blood clotting. In fact, the bacteria produce ½ of your daily vitamin K requirement. The biotin produced is needed for a variety of reactions, especially in glucose metabolism. Vitamin B5 (pantothenic acid) is needed in steroid production and some neurotransmitters.

The rectal valves support the feces until defecation. The wastes go through the anal sphincter, under voluntary control, and out the anus.

It takes from 12-24 hours for material to travel the length of the large intestine.

Digestion in herbivores: Ruminants.

These organisms must digest cellulose. However, mammals don't produce the necessary enzymes. In order to digest the cellulose, mammals must have large flat teeth for grinding cellulose. Cellulose must be broken down to release plant nutrients. Once broken down the food goes through the esophagus to a four-chambered stomach of such ruminant herbivores such as cattle, deer, giraffes, antelopes, and buffalo. The stomachs harbor protozoans and bacteria that break down cellulose.

Integration and control of the digestive process:

The release of digestive enzymes must be precisely timed. They are under three controls.

1) Mechanical 2) Neural 3) Hormonal

For example, the thought of chocolate stimulates saliva flow. Chewing can also stimulate the saliva flow. Neural and mechanical stimulants can stimulate saliva flow.

Gastric secretions can be stimulated by the presence of food. The neural message is sent along the vagus nerve from the brain to the stomach lining. Food in the stomach stimulates sensory neurons in the stomach (mechanical) and a hormone gastrin is released from stomach to the blood. Other hormones are collectively called ENTEROGASTRONES and are secreted by the cells in the duodenum. The pH of the acid chyme causes the cells to release secretin, which signals the pancreas to release bicarbonate. Another enterogastrone is cholecytokinin (CCK), which is secreted in the presence of amino acids or fatty acids. This causes the gall bladder to release bile.

Chemistry of Digestion:

Chemical digestion and absorption:

Carbohydrates are broken down into simple sugars.

Fats are broken down into fatty acids and glycerol.

DNA and RNA are broken down into free nucleotides.

Proteins are broken down into various amino acids.

Carbohydrate Digestion:

Starch digestion begins in the mouth with amylase that is found in the saliva. The breakdown of carbohydrates is stalled in the stomach because of the acidity and resumes in the small intestine where amylase from the pancreas converts all starch into maltose. Maltose is broken down by maltase into glucose, which is absorbed.

Sucrose, which is glucose and fructose, is broken down by sucrase.

Lactose, milk sugar, is broken down in the gut by lactase. Lactase is absent in most blacks and Asians and in some whites preventing them from breaking down lactose; if milk is ingested, diarrhea and gas will follow. Glucose and Fructose are transported into the capillaries of the villi and go to the liver to form glycogen chains.

Fat Digestion:

Fat digestion begins in the mouth with lingual lipase. The enzyme will break off two fatty acids leaving a monoglyceride. Fats reach the small intestine with little chemical change. Even though the stomach secretes a lipase.

Bile separates fats into tiny droplets that are broken down further by lipases into fatty acids and glycerol. These cross the membrane and reform in the villi. Longer chains of fatty acids, more than 12 carbons, enter the lymphatic vessels of the villi. Fatty acids with fewer than 12 carbons go into the capillaries and are carried to the liver. The ones in the lymph system follow the lymphatic system to the thoracic duct near the heart and enter the blood stream. If there is too much fat in the blood after the meal, the blood will appear milky. Animals need to ingest unsaturated fats (usually from plant material). We can produce our own saturated fats.

Protein Digestion:

Proteins are the most complex food molecules and their digestion is complex. Pepsinogen, secreted by the chief cells is changed by hydrochloric acid into pepsin. Pepsin is a nonspecific endopeptidase and hydrolyzes proteins into smaller peptides.

Specific enzymes (see small intestine notes) will break peptides down further until individual amino acids are left. Trypsin and chymotrypsin are active in the small intestine. The pancreas secretes trypsinogen, which is secreted to the duodenum. Once in the duodenum, the trypsinogen is acted upon by enterokinase, which changes the inactive enzyme to the active trypsin. Trypsin breaks the peptide bonds involving the amino acids arginine or lysine. Chymotrypsin will break the peptide bonds between tyrosine or phenylalanine. The peptides are broken down further by carboxypeptidases and aminopeptidases. These amino acids are absorbed into the blood stream, by facilitated transport, and sent to the liver. In the liver, the amino acids are used for energy and to build proteins.

Nucleic Acids:

Almost everything we eat contains some nucleic acids. Nucleases produced in the pancreas break nucleic acid bonds. They break down the nucleic acids into small units of either single bases or small chains.

Water Absorption: water passes into the blood and tissue by osmosis.

Mineral/Ion Absorption: To maintain a water balance, osmotic balance, must be maintained by ion gradient.

Na+ enters the blood and tissues by diffusion, facilitated transport, and active transport.

Ca2+ enters the blood and tissues by active transport.

K+ enters the blood and tissues by diffusion.

Mg2+ and Fe2+ enter the blood and tissues by active transport.

Cl-, I-, HCO3-, and NO3- enter the blood and tissues by diffusion.

PO43-, and SO42- enter the blood and tissues by active transport.

Nutrition:

Carbohydrates are the most common source of energy used in cellular respiration. Blood glucose must remain constant. Glucose is stored in the muscles as glycogen and used for energy. If not used, carbohydrates are stored as fat.

Fats: Unsaturated fats are necessary for cell membrane synthesis. Fats are sources of energy and insulators. Fats also serves as a source for vitamins A,D,E, and K.

Proteins can be stored in the liver and in muscle tissue. There is a constant turnover of body protein. Amino acids can build proteins, form nitrogen bases, can be oxidized for energy, and can be converted into fats and carbohydrates. When amino acids are used, they leave behind nitrogenous wastes, which are poisonous and must be removed (via the excretory system). Humans can produce 12 out of the 20 amino acids. There are eight amino acids that humans must obtain from food (babies must obtain histadine from food). Brain development requires proteins.

Vitamins and Minerals:

Vitamins: Function in common enzymatic reactions as co-enzymes. There are two types of vitamins: fat-soluble (A, D, E, and K). The fat-soluble vitamins need lipids to coat the vitamins. Water-soluble (all others) enter the blood and cells by diffusion. Except B12, they need the intrinsic factors to enter the blood and cells.

Minerals: Simple inorganic ions. They may be used in the formation of gross structures, such as bones or may become an active part of functional molecules. Some are necessary for enzymatic actions.

Food as Fuel:

The energy content of food is measured in CALORIES (kilocalories). The energy content of fat is 9 kcal/ 1 gram. Protein is 4 kcal/ 1 gram. Carbohydrates is 4 kcal/1 gram.

Several processes must occur continually in higher animals alive, for example, breathing, heart beating. The number of kcal a resting animal needs at a give time is called a Basal Metabolism Rate or BMR.

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