Essential idea: Digestion is the breaking down of food particles into smaller particles. It is controlled by nervous and hormonal mechanisms.

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Tyrone Banks
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It is controlled by nervous and hormonal mechanisms.

1 D2 Diges(on Essential idea: Digestion is the breaking down of food particles into smaller particles. It is controlled by nervous and hormonal mechanisms. This figure shows the main parts of the diges(ve system and the approximate volumes of each sec(on.

2 Understandings D.2.U1 Statement Nervous and hormonal mechanisms control the secretion of digestive juices. Guidance D.2.U2 D.2.U3 D.2.U4 D.2.U5 D.2.U6 D.2.U7 Exocrine glands secrete to the surface of the body or the lumen of the gut. The volume and content of gastric secretions are controlled by nervous and hormonal mechanisms. Acid conditions in the stomach favour some hydrolysis reactions and help to control pathogens in ingested food. The structure of cells of the epithelium of the villi is adapted to the absorption of food. The rate of transit of materials through the large intestine is positively correlated with their fibre content. Materials not absorbed are egested. Adaptations of villus epithelial cells include microvilli and mitochondria.

3 Applica(ons and Skills D.2.A1 Statement The reduction of stomach acid secretion by proton pump inhibitor drugs. Guidance D.2.A2 D.2.A3 D.2.S1 Dehydration due to cholera toxin. Helicobacter pylori infection as a cause of stomach ulcers. Identification of exocrine gland cells that secrete digestive juices and villus epithelium cells that absorb digested foods from electron micrographs.

4 D.2.U1 Nervous and hormonal mechanisms control the secretion of digestive juices. Bile The liver produces bile which is responsible for the emulsification of fats (separates fats globules to enhance lipase activity). Bile produced in the liver is commonly stored in the Gall Bladder until release. Pancreatic Juice Pancreatic juice is produced by the exocrine cells in the pancreas. In includes water, pancreatic amylase, trypsin (secreted as trypsinogen), pancreatic lipase, carboxypeptidase chymotrypsin, and hydrogen carbonate ions (HCO 3- ). Intestinal Juice Saliva Saliva contains water, mucus, and salivary amylase. Salivary glands in the mouth secrete salivary amylase, which assists in the breakdown of glucose polymers (e.g. starch). Gastric Juice Gastric juice is produced by the gastric glands in the stomach wall. The juice contains water, mucus, pepsin (secreted as pepsinogen), and hydrochloric acid. Intestinal juice is secreted from pits called crypts of Leiberkuhn, located between intestinal villi. It includes enterokinase (which will cleave trypsinogen into trypsin) and other digestive enzymes, hormones, and mucus.

5 D.2.U1 Nervous and hormonal mechanisms control the secretion of digestive juices. The nervous control is mainly provided by the autonomic nervous system (the part of the nervous system responsible for unconscious control of bodily func(ons like breathing, heartbeat, and diges(on). The sympathe(c system controls processes involved in danger. The parasympathe(c division controls homeostasis and processes related to rest and diges(on. The smell, taste, touch, or thought of food can s(mulate your parasympathe(c system.

6 D.2.U1 Nervous and hormonal mechanisms control the secretion of digestive juices. Know the chart!!!

D.2.U1 Nervous and hormonal mechanisms control the secretion of digestive juices. D.2.U3 The volume and content of gastric secretions are controlled by nervous and hormonal mechanisms.

7 D.2.U1 Nervous and hormonal mechanisms control the secretion of digestive juices. D.2.U3 The volume and content of gastric secretions are controlled by nervous and hormonal mechanisms. Nervous Mechanism: The sight and smell of food triggers an immediate response by which gastric juice is secreted by the stomach pre-ingestion. When food enters the stomach it causes distension, which is detected by stretch receptors in the stomach lining. Signals are sent to the brain, which triggers the release of digestive hormones to achieve sustained gastric stimulation. Hormonal Mechanism: Gastrin is secreted into the bloodstream from the gastric pits of the stomach and stimulates the release of stomach acids. If stomach ph drops too low (becomes too acidic), gastrin secretion is inhibited by gut hormones (secretin and somatostatin). When digested food (chyme) passes into the small intestine, the duodenum also releases digestive hormones: Secretin and cholecystokinin (CCK) stimulate the pancreas and liver to release digestive juices Pancreatic juices contain bicarbonate ions which neutralize stomach acids, while the liver produces bile to emulsify fats.

D.2.U2 Exocrine glands secrete to the surface of the body or the lumen of the gut. All exocrine glands secrete their products via ducts to where they are needed.

8 D.2.U2 Exocrine glands secrete to the surface of the body or the lumen of the gut. All exocrine glands secrete their products via ducts to where they are needed. Cells of an exocrine gland that produce diges(ve juices are arranged in a single layer around small ducts that are connected to the intes(ne. One group of exocrine cells arranged around a duct is called an acinus, and one exocrine gland contains many acini. These small ducts join together to form one larger duct, which carries the secre(ons to their des(na(on.

9 D.2.U2 Exocrine glands secrete to the surface of the body or the lumen of the gut. Know the func(ons! Exocrine glands secrete many enzymes. Rough endoplasmic re(culum (R-ER) synthesizes proteins, which is what enzymes are made of, which explains why exocrine glands contain so much R-ER. Golgi apparatus packages and processes the enzymes into vesicles. Vesicles store the enzymes before they are secreted by exocytosis into the duct of the gland. Many mitochondria produce ATP for the expensive protein/enzyme synthesis.

10 D.2.U2 Exocrine glands secrete to the surface of the body or the lumen of the gut. The stomach wall is lined with millions of gastric pits which release gastric juice into the stomach lumen Gastric juice is an acidic solution that functions to break down food into a creamy paste called chyme The gastric pits are lined by a number of different cell types which contribute to the overall function of the stomach: KNOW THESE! Goblet Cells secrete mucus to form a protective layer around the stomach lining Parietal Cells secrete hydrochloric acid which is responsible for creating a low ph environment in the stomach G cells secrete gastrin (stimulates release of stomach acids to increase stomach acidity) D cells secrete somatostatin (inhibits release of stomach acids to reduce stomach acidity) Chief cells secrete pepsinogen (inactive protease precursor which is activated by acidity to form active pepsin)

S1 Identification of exocrine gland cells that secrete digestive

11 D.2.U2 Exocrine glands secrete to the surface of the body or the lumen of the gut. D.2.S1 Identification of exocrine gland cells that secrete digestive juices and villus epithelium cells that absorb digested foods from electron micrographs. Exocrine Gland Gastric Pit

D.2.U1 Nervous and hormonal mechanisms control the secretion of digestive juices. Cephalic Phase: reflex response to visual, smell or thought stimulus Secretion of gastric juice.

12 D.2.U1 Nervous and hormonal mechanisms control the secretion of digestive juices. Cephalic Phase: reflex response to visual, smell or thought stimulus Secretion of gastric juice. Hypothalamus recieve the stimulus Medulla Oblongata ACh Vagus Nerve If ph gets too Low Somatostatin is an inhibitory hormone that directly inhibits acid producing cells. It also acts indirectly by preventing the release of gastrin, CCK and secretin, thus slowing down the digestive process. D Cell

13 D.2.U1 Nervous and hormonal mechanisms control the secretion of digestive juices. Medulla Oblongata mechanoreceptors Gastric Phase: mechanoreceptors (touch and stretch) and chemoreceptors (protein) stimulus medulla oblongata (brain) stimulation of gastric glands and production of the hormone gastrin Medulla Oblongata continued production of HCl - (ph will drop) D Cell Vagus Nerve If lipids are present, enterogasterone will be secreted. slow flow of gastric juice, slow exit of fats from stomach

14 D.2.U1 Nervous and hormonal mechanisms control the secretion of digestive juices. Medulla Oblongata If ph gets too Low Intestinal Phase: reducing secretions and pancreatic stimulation low ph of chyme stimulates activation of prosecretin into secretin presence of chyme stimulates secretion of cholecystokinin (CCK) Secretin will stimulate the pancreas to release hydrogen carbonate ions (HCO 3- ) to neutralize the ph. Cholecystokinin (CCK) stimulates release of bile from gall bladder and the release of enzymes from pancreas into duodenum. Secretin and CCK inhibit gastric secretions

0, which is the optimum ph for hydrolysis reactions by stomach enzymes.

15 D.2.U4 Acid conditions in the stomach favor some hydrolysis reactions and help to control pathogens in ingested food. The gastric glands that line the stomach wall secrete an acidic solution that creates a low ph environment within the stomach. The normal ph of the stomach is roughly , which is the optimum ph for hydrolysis reactions by stomach enzymes. The acid conditions in the stomach serve a number of functions: Assists in the digestion of food (by dissolving chemical bonds within food molecules). Activates stomach proteases (e.g. pepsin is activated in acid conditions). Prevents pathogenic infection (stomach acids destroy microorganisms in ingested food). The stomach wall is lined by a layer of mucus, which protects the stomach lining from being damaged by the acid conditions. The pancreas releases bicarbonate ions into the duodenum which neutralizes the stomach ph (intestinal ph ~ ). Certain foods (e.g. antacids) may also neutralize stomach acids, impairing digestion and increasing chances of infection.

16 D.2.A1 The reduction of stomach acid secretion by proton pump inhibitor drugs. Acidic conditions help to control pathogens in ingested food. The bacteria present in food cannot survive such acid conditions. The mucus cover of the stomach avoids self-damage. In some cases (either because the patient has eaten spicy foods or excess proteins, or is very stressed) the stomach will produce an excess of gastric acid. This can damage the mucus layer, producing gastric problems which might develop into a gastric ulcer. This is when the lining of the stomach is disrupted. The main symptoms of an ulcer are stomach pain, heartburn, nausea and in some cases presence of blood in the stools.

17 D.2.A1 The reduction of stomach acid secretion by proton pump inhibitor drugs. Parietal Cell Parietal Cell The low ph environment of the stomach is maintained by proton pumps in the parietal cells of the gastric pits. These proton pumps secrete H+ ions (via active transport), which combine with Cl ions to form hydrochloric acid. Certain medications and disease conditions can increase the secretion of H+ ions, lowering the ph in the stomach. Proton pump inhibitors (PPIs) are drugs which irreversibly bind to the proton pumps and prevent H+ ion secretion. This effectively raises the ph in the stomach to prevent gastric discomfort caused by high acidity (e.g. acid reflux). Individuals taking PPIs may have increased susceptibility to gastric infections due to the reduction of acid secretion

D.2.A3 Helicobacter pylori infection as a cause of stomach ulcers. Helicobacter pylori is a bacterium that can survive the acid conditions of the stomach by penetrating the mucus lining. H. pylori anchors to the epithelial lining of the stomach, underneath the mucus lining.

18 D.2.A3 Helicobacter pylori infection as a cause of stomach ulcers. Helicobacter pylori is a bacterium that can survive the acid conditions of the stomach by penetrating the mucus lining. H. pylori anchors to the epithelial lining of the stomach, underneath the mucus lining. An inflammatory immune response damages the epithelial cells of the stomach including the mucussecreting goblet cells. This results in the degradation of the protective mucus lining, exposing the stomach wall to gastric acids and causing ulcers. The prolonged presence of stomach ulcers may lead to the development of stomach cancer over many years (20 30 years). Ulcers due to H. pylori are treated with a combination of medications: 1. Amoxicillin: antibiotic that directly inhibits the synthesis of bacterial cell walls. 2. Clarithromycin: antibiotic that prevents bacteria from growing by inhibiting the translation of peptides in the ribosome, thus inhibiting their protein synthesis. 3. Proton Pump Inhibitor (PPI): inhibits acidification of stomach.

19 D.2.U5 The structure of cells of the epithelium of the villi is adapted to the absorption of food. Once digested food has passed through the stomach, it enters the small intestine for absorption into the blood. The small intestine also releases digestive enzymes to ensure the complete hydrolysis of food molecules. The inner epithelial lining of the small intestine is highly folded into finger-like projections called villi (singular: villus). Many villi will protrude into the intestinal lumen, greatly increasing the available surface area for material absorption

D.2.U5 The structure of cells of the epithelium of the villi is adapted to the absorption of food. Microvilli Ruffling of epithelial membrane further increases surface area.

20 D.2.U5 The structure of cells of the epithelium of the villi is adapted to the absorption of food. Microvilli Ruffling of epithelial membrane further increases surface area. Rich blood supply Dense capillary network rapidly transports absorbed products. Single layer epithelium Minimizes diffusion distance between lumen and blood. Lacteals Absorbs lipids from the intestine into the lymphatic system. Intestinal glands Exocrine pits (crypts of Lieberkuhn) release digestive juices. Membrane proteins Facilitates transport of digested materials into epithelial cells Mnemonic: MR SLIM

D.2.U5 The structure of cells of the epithelium of the villi is adapted to the absorption of food. D.2.S1 Identification of exocrine gland

The cross section of the wall of the small intestine consists of five layers: Mucosa: contains the epithelium formed by enterocytes,

Goblet cells produce mucus and endocrine cells secrete hormones. Submucosa: contains blood vessels and connective tissue.

21 D.2.U5 The structure of cells of the epithelium of the villi is adapted to the absorption of food. D.2.S1 Identification of exocrine gland cells that secrete digestive juices and villus epithelium cells that absorb digested foods from electron micrographs. The cross section of the wall of the small intestine consists of five layers: Mucosa: contains the epithelium formed by enterocytes, goblet cells and endocrine cells. Enterocytes are cells that have microvilli; they digest and absorb substances. Goblet cells produce mucus and endocrine cells secrete hormones. Submucosa: contains blood vessels and connective tissue. Circular muscle layer: smooth muscle arranged in a circular manner. Longitudinal muscle layer: smooth muscle arranged in a longitudinal manner. Serosa: single layer of epithelial cells with connective tissue.

D.2.U5 The structure of cells of the epithelium of the villi is adapted to the absorption of food. D.2.S1 Identification of exocrine gland cells that secrete digestive juices and villus epithelium cells that absorb digested foods from electron micrographs.

22 D.2.U5 The structure of cells of the epithelium of the villi is adapted to the absorption of food. D.2.S1 Identification of exocrine gland cells that secrete digestive juices and villus epithelium cells that absorb digested foods from electron micrographs. Tight Junctions Occluding associations between the plasma membrane of two adjacent cells, creating an impermeable barrier.

23 D.2.U5 The structure of cells of the epithelium of the villi is adapted to the absorption of food. D.2.S1 Identification of exocrine gland cells that secrete digestive juices and villus epithelium cells that absorb digested foods from electron micrographs. Microvilli Microvilli borders significantly increase surface area of the plasma membrane (>100 ), allowing for more absorption to occur.

24 D.2.U5 The structure of cells of the epithelium of the villi is adapted to the absorption of food. D.2.S1 Identification of exocrine gland cells that secrete digestive juices and villus epithelium cells that absorb digested foods from electron micrographs. Mitochondria Epithelial cells of intestinal villi will possess large numbers of mitochondria to provide ATP for active transport mechanisms.

Humans lack the necessary enzymes to break down certain plant matter (e.g. lack cellulase required to digest cellulose).

25 D.2.U6 The rate of transit of materials through the large intestine is positively correlated with their fibre content. Dietary fibre, or roughage, is the indigestible portion of food derived principally from plants and fungi (cellulose, chitin, etc.). Humans lack the necessary enzymes to break down certain plant matter (e.g. lack cellulase required to digest cellulose). Certain herbivores (ruminants) possess helpful bacteria in the digestive tract that can break down indigestible plant matter. The rate of transit of materials through the large intestine is positively correlated with their fiber content: Roughage provides bulk in the intestines to help keep materials moving through the gut. Roughage also absorbs water, which keeps bowel movements soft and easy to pass. There are several health benefits associated with diets rich in dietary fibre: It reduces the frequency of constipation and lowers the risk of colon and rectal cancer. It lowers blood cholesterol and regulates blood sugar levels (by slowing the rate of absorption). It aids in weight management (contributes few calories despite consisting of a large volume of ingested material).

D.2.U7 Materials not absorbed are egested. Materials that are not absorbed by the small and large intestines are ultimately egested from the body as faeces.

26 D.2.U7 Materials not absorbed are egested. Materials that are not absorbed by the small and large intestines are ultimately egested from the body as faeces. A large portion of human faeces consists of dietary fibre, such as cellulose and lignin. Also present in faeces are the remains of intestinal epithelial cells, bile pigments and human flora (intestinal bacteria). Summary of egested materials: Bile pigments Epithelial cells Lignin Cellulose Human flora (bacteria) Mnemonic: BELCH

D.2.A2 Dehydration due to cholera toxin. Vibrio cholerae is a bacterial pathogen that infects the intestines and causes acute diarrhoea and dehydration.

cholerae releases a toxin that binds to ganglioside receptors on the surface of intestinal epithelium cells.

27 D.2.A2 Dehydration due to cholera toxin. Vibrio cholerae is a bacterial pathogen that infects the intestines and causes acute diarrhoea and dehydration. The associated disease cholera can kill within hours unless treated with oral rehydration therapies. V. cholerae releases a toxin that binds to ganglioside receptors on the surface of intestinal epithelium cells. This toxin is internalized by endocytosis and triggers the production of cyclic AMP (a second messenger) within the cell. Cyclic AMP (camp) activates specific ion channels within the cell membrane, causing an efflux of ions from the cell. The buildup of ions in the intestinal lumen draws water from cells and tissues via osmosis causing acute diarrhoea. As water is being removed from body tissues, dehydration will result if left untreated

Understandings, Applications & Skills

D.2 Digestion Understandings, Applications & Skills Statement D.2.U1 Nervous and hormonal mechanisms control the secretion of digestive juices. D.2.U2 Exocrine glands secrete to the surface of the body