UNIT 5 The Digestive System PDF

# UNIT 5: THE DIGESTIVE SYSTEM Digestion refers to the physical and chemical breakdown of food into substances (molecules) that can be absorbed and used by the body. **PHYSICAL** digestion involves the breakdown of large food particles into smaller ones, without changing their molecular make-up. It...

# UNIT 5: THE DIGESTIVE SYSTEM Digestion refers to the physical and chemical breakdown of food into substances (molecules) that can be absorbed and used by the body. **PHYSICAL** digestion involves the breakdown of large food particles into smaller ones, without changing their molecular make-up. It is designed to increase the surface area of the food so the efficiency of enzyme activity is maximized. Chewing, churning of the food in the stomach by peristalsis, and emulsification of fats by bile are examples of physical digestion. **CHEMICAL** digestion involves the breakdown of large, complex molecules into small, soluble molecules that can be absorbed and used by the body. This is accomplished by enzyme activity, e.g. breaking proteins down into the amino acids that make them up; breaking starch eventually into the glucose molecules that make it up. The term 'digestive system' refers to the collection of organs that work together to accomplish the digestion of food, including the alimentary canal AND all the other associated organs, e.g. teeth, liver, gallbladder and pancreas. The alimentary canal refers strictly to the tube that leads from the mouth to the anus, through which food passes as it is digesting - made up of the mouth, esophagus, stomach, small intestine, large intestine only. Peristalsis occurs in all parts of the alimentary canal except the mouth. ## The Digestive System **Label the structures of the digestive system.** 1. teeth 2. esophagus 3. liver 4. gall bladder 5. duodenum 6. large intestine (colon) 7. appendix 8. Salivary glands 9. stomach 10. pancreas 11. small intestine 12. rectum 13. anus ## WHAT HAPPENS DURING DIGESTION? ### 1. IN THE MOUTH - Ingestion occurs - this is the intake of food by eating/drinking - this takes place in the mouth. - Salivary glands (located in the roof of the mouth and under the tongue) produce saliva, which contains water and mucus to moisten food and help stick food particles together, and the enzyme, salivary amylase. - Amylase is the enzyme that begins the chemical digestion of starch, converting it into maltose. - The tongue assists with moving food around in the mouth for chewing, and forming the food into a bolus for swallowing. ### 2. IN THE ESOPHAGUS - Peristalsis occurs - this is the involuntary, rhythmic, wave-like contractions of smooth muscles in the esophagus, stomach, small and large intestines. This is how food moves through the alimentary canal. - Both circular muscles (which form 'rings' around the tube), as well as longitudinal muscles (which run up and down its length) are present. As they contract and relax, the bolus from the mouth is transferred to the stomach. ### 3. IN THE STOMACH - Gastric glands in its walls secrete gastric juice, which contains two main active ingredients: - Enzymes: pepsin, which chemically digests converts large protein molecules into smaller 'polypeptides', and rennin in infants, which clots/solidifies the protein in milk - Hydrochloric acid (HCI), to create the acidic conditions favoured by pepsin (about pH 2), and to kill bacteria. Teeth are used for biting and chewing food, physically breaking it down to smaller pieces. This makes food easier to swallow, and increases the surface area of the food so that the rate of enzyme activity can be maximized. Humans grow 2 sets of differentiated teeth during their lifetime: the milk set (20 teeth) and the permanent set (32 teeth). **Complete the table with details about human teeth.** | | | | | | | |---|---|---|---|---|---| | **Incisors** | **Canines** | **Premolars** | **Molars** | **Crown** | **Root** | | | | | | | | | | | | | **enamel** | | | | | | | **dentine** | | | | | | | **gum** | | | | | | | **pulp cavity** | | | | | | | **cementum** | | | | | | | | **jaw bone** | ### 4. IN THE SMALL INTESTINE - **DUODENUM** - A greenish fluid called bile (secreted by the liver and stored in the gallbladder) is added to the chyme. When food enters the duodenum, it stimulates the gallbladder to contract and squirt the bile out through the bile duct. The bile helps to neutralize the acidic chyme, and causes emulsification of fat – turning large liquid drops of fat into tiny droplets to increase the surface area. This is the effect that dish detergent has on greasy dishes. **NOTE:** This only physically changes the fats. - The pancreas adds pancreatic juice through the pancreatic duct. This contains 3 main enzymes: - pancreatic amylase (to digest any starch that was not converted before entering the stomach); - trypsin (to chemically digest proteins/polypeptides); - lipase (to chemically digest fats) - These pancreatic enzymes favour a slightly alkaline environment for optimal efficiency, so like bile, the digestive juice from the pancreas is basic in order to help raise the pH of the small intestine. - **ILEUM** - Glands in the wall of the ileum secrete intestinal juice, which contains various enzymes which act on any molecules that are not yet small enough to be absorbed. This completes the process of digestion. - The ileum is adapted for efficient absorption (movement of digested nutrients into the bloodstream, mostly by diffusion) - It has a very large surface area because of its great length (up to 5 meters) and the presence of numerous villi (one shown →) - It has a very thin epithelium to facilitate rapid diffusion; - It has an excellent blood supply to transport the absorbed nutrients; - It is coiled, which slows down the rate at which the food can move, giving more opportunity for maximum absorption. - **NOTES:** Fatty acids and glycerol enter into the lacteals, which are a part of the lymphatic system. These nutrients will enter the main blood circulation near to where the vena cava enters the heart. Glucose and amino acids, which pass into the capillaries of the villi, are transported directly to the liver in the hepatic portal vein. In the liver, the **ASSIMILATION** of food begins (the conversion of food nutrients into living tissue by the cells). A thick layer of mucus is secreted by cells in the stomach walls, which prevents the stomach from digesting itself and from the effects of hydrochloric acid. Smooth muscles the stomach walls carry out peristalsis. Food is churned until it becomes a thick liquid, called chyme; this can take 3 to 5 hours. The entry and exit to the stomach are kept closed by the cardiac (lower esophageal) and pyloric sphincter muscles. The chyme is released bit by bit into the duodenum through the pyloric sphincter. ### 5. IN THE LARGE INTESTINE - **COLON** - Substances that could not be digested (e.g. cellulose/fiber) due to lack of necessary enzymes, and some that did not need to be digested (because their molecules are small enough to be absorbed) enter the colon. The contents of the alimentary canal are still very watery at this point, due to the many digestive juices that have been added during the digestive process. - The majority of the water present is reabsorbed into the bloodstream through the intestinal walls (by osmosis) as the undigested matter moves along the colon by peristalsis. - Mineral salts are absorbed into the bloodstream. - Feces is formed from the indigestible matter by the action of bacteria present in the colon. - **RECTUM** - Feces is stored until it is expelled through the anus (by peristalsis). The removal of feces from the body is called defecation or egestion. ## Enzymes, Substrates and End Products of Digestion | Organ | Digestive juice secreted | Enzyme(s) present | Substrate (nutrient it acts on) | Converts it to... | Final end product(s) (which can be absorbed) | |---|---|---|---|---|---| | salivary glands | saliva | salivary amylase | starch | maltose | — | | stomach | gastric juice | pepsin | protein | polypeptides | — | | | | rennin (in infants) | | | | | liver | bile | NONE | lipids (fats) | tiny droplets of fat | — | | pancreas | pancreatic juice | amylase | starch | maltose | — | | | | trypsin | protein | polypeptides | — | | | | lipase | lipids | fatty acids, glycerol | **FATTY ACIDS, GLYCEROL** | | ileum | intestinal juice | maltase | maltose | glucose | **GLUCOSE** | | | | peptidase | polypeptides | amino acids | **AMINO ACIDS** | | | | sucrase | sucrose | glucose, fructose | **GLUCOSE, FRUCTOSE** | | | | lipase | lipids | fatty acids, glycerol | **FATTY ACIDS, GLYCEROL** | ## ROLE OF THE LIVER The liver is the largest internal organ and has been found to have hundreds of functions. Aside from producing bile, which has already been noted, some other major functions include: - Receiving all the blood that leaves the ileum in the hepatic portal vein and converting the nutrients into usable forms for the body (assimilation). - Storing nutrients such as iron, and lipid-soluble vitamins such as A, D and E. - Working with the pancreas to control blood glucose levels. When there is too much glucose in the bloodstream, insulin causes the liver to remove the excess and convert it to glycogen (a.k.a. animal starch), to be temporarily stored. When blood glucose drops too low, the stored glycogen will be converted back to glucose and re-enter the bloodstream. - Deamination, which is the chemical breakdown of excess amino acid (protein) molecules to remove the nitrogenous parts. This is the process that produces urea. - Removing toxins/poisons from the blood and converting them to less harmful substances that can more safely be excreted. This includes alcohol and all types of drugs, as well as things like pesticide residue and other chemicals we may unintentionally ingest. The process is called **detoxification**. The Liver | | | | |---|---|---| | **Right lobe** | **Left lobe** | **Gall bladder** | | | | **Hepatic artery - takes blood from the heart** | | | | **Hepatic portal vein - brings blood from the small intestine** | | | | **Common bile duct** |