Digestive System

6.1 Digestive Functions The food we eat is a combination of a huge assortment of molecules Our system Breaks these molecules down via physical and chemical digestive processes to obtain forms that can be absorbed and used in our body The parts that can not be used are released

The digestive functions are: Ingestion Physical and chemical digestion Absorption Defecation

6.2 Digestive Structures Imagine the digestive system as a long tube (around 7 m long) contained within the body Food enters one end, the mouth, is processed throughout its length; nutrients absorbed, and wastes are released from the other end, the anus

Much of the treatment that food is exposed to comes from the specializations of this tube, the alimentary canal (gastrointestinal tract) Its muscular walls control the movement of materials through it; glands along its walls release water, enzymes, and some hormones; specialized structures absorb nutrients

The pancreas and the liver are attached to the alimentary canal and produce secretions that are used in the digestive process Both of these secretions enter at the duodenum

Pancreas The pancreas is located behind the stomach It is specialized both as an endocrine and exocrine gland

It is an endocrine gland because it produces and secretes the hormones insulin and glucagon from cells called islets of Langerhans into the blood stream Insulin lowers blood sugar levels by increasing cell’s ability to use it and promoting its storage in the form of glycogen by the liver

Glucagon allows the liver to release glucose from glycogen when blood sugar levels drop Together, these hormones help regulate the blood’s level of glucose

The pancreas is an exocrine gland because it produces the digestive juice called pancreatic juice, which it releases to the duodenum through the pancreatic duct Pancreatic juice is a combination of water, sodium bicarbonate, lipase, trypsinogen (which becomes trypsin), nucleases, and pancreatic amylase

Liver The liver is the bodies largest internal organ It is located behind the diaphragm The liver is richly supplied with blood, and has the following functions:

1) Regulate the glucose level in blood Glucose should comprise about 0.1% of blood plasma If the level is too high, glucose will be converted to and stored as glycogen by the liver If it is too low, glycogen breaks down into glucose

2) Produce urea Body cells produce ammonia, a toxic by-product from the metabolism of amino acids and nucleotides It is transported though blood to the liver where it is converted into, the less toxic, urea Urea is removed from the blood by the kidneys during urine formation

3) Detoxify blood The liver destroys other toxins in the blood (like alcohol) The liver metabolizes alcohol into fatty acids, which increases the fatty nature of liver tissue Removes and stores fat-soluble vitamins, which have toxic effects on cells when in high concentrations

4) Produce bile Bile is stored in the gall bladder until a hormone signals its release Bile emulsifies fat clusters into smaller pieces This increases their surface area, increasing the efficiency of the lipase enzyme, which chemically breaks them down into fatty acids and glycerol

5) Produce blood proteins (globulins) Albumin (helps maintain osmotic pressure) Fibrinogen Prothrombin (needed for blood clotting)

6) Destroy old red blood cells RBC have an average life span of 4 months (millions die every day) The liver disassembles the dead RBC and recycles the useful components For example, iron from hemoglobin is returned to bone marrow for the manufacture of new RBC; the rest of the hemoglobin is worn out and added to bile to be excreted through the digestive system

Review The function of the gall bladder is to Store bile Synthesize lipids Produce enzymes Stimulate the release of pancreatic juice

Review The function of the gall bladder is to Store bile Synthesize lipids Produce enzymes Stimulate the release of pancreatic juice

Glucose is stored in the liver as Fat Starch Protein Glycogen

Glucose is stored in the liver as Fat Starch Protein Glycogen

The release of insulin is triggered by Low levels of blood sugar High levels of blood sugar Low amounts of sugar in one’s diet High amounts of sugar in one’s diet

The release of insulin is triggered by Low levels of blood sugar High levels of blood sugar Low amounts of sugar in one’s diet High amounts of sugar in one’s diet

One function of the liver is the production of Urea Lipase Mucus Hydrochloric acid

One function of the liver is the production of Urea Lipase Mucus Hydrochloric acid

Which of the following pancreatic secretions doesn’t enter the duodenum? Insulin Lipase Trypsin Bicarbonate ions

Which of the following pancreatic secretions doesn’t enter the duodenum? Insulin Lipase Trypsin Bicarbonate ions

Processes of Digestion Mouth Stomach Small intestines Colon (Large intestines)

6.3 Mouth First, food is chewed up (physical digestion), increasing the surface area so that chemical digestion can take place faster. Salivary glands release saliva (water and salivary amylase, also known as ptyalin) through ducts into the mouth Water is used as a lubricant to aid in swallowing food and as a reactant in hydrolytic reactions that occur during digestion

Salivary amylase (or ptyalin) breaks the bonds between the sugar molecules of starch to begin its chemical conversion to maltose

The tongue is a muscle that rolls chewed food into lumps (bolus) and pushes them to the back of the mouth for swallowing Swallowing is a reflexive action resulting from contractions of the two layers of smooth muscles that line the pharynx and esophagus

The pharynx is a common passageway for both food and air In order to prevent food materials from going down the trachea, the epiglottis covers it

The muscle contractions called peristalsis move each bolus down the esophagus There is a constriction called the cardiac sphincter at the base of the esophagus that must relax and open before the food materials can enter the stomach

6.4 Stomach The stomach is a large J-shaped organ with walls that have three layers of muscle that churn food material , causing the release of gastrin (a hormone that travels from the cells of the stomach walls into the blood stream As gastrin circulates the body, it affects the stomach and causes the release of gastric juices

Gastric juice contains water as well as HCl and pepsinogen At this point the bolus becomes known as acid chyme (runny) The HCl creates an environment with a low pH to kill any bacterial growth that may be in the food and reacts with pepsinogen, converting it to pepsin (which converts proteins into polypeptides)

Pepsin is secreted as pepsinogen because a cell would not survive the production of it in its active form The inner walls of the stomach also produce mucus to protect themselves from the acid chyme Bacterial infection of the mucosal lining by Helicobacter pylori can lead to an ulcer

Review Which of the following digestive enzymes is NOT correctly matched with its substrate? Lipase – fat Pepsin – protein Trypsin – nucleic acid Salivary amylase - starch

Review Which of the following digestive enzymes is NOT correctly matched with its substrate? Lipase – fat Pepsin – protein Trypsin – nucleic acid Salivary amylase - starch

The part of the digestive tract where starch first undergoes chemical digestion is the Mouth Stomach Small intestine Large intestine

The part of the digestive tract where starch first undergoes chemical digestion is the Mouth Stomach Small intestine Large intestine

When salivary amylase enters the stomach, it becomes Basic Acidic Buffered denatured

When salivary amylase enters the stomach, it becomes Basic Acidic Buffered denatured

Which region has openings for both the trachea and esophagus? Throat Pharynx Stomach epiglottis

Which region has openings for both the trachea and esophagus? Throat Pharynx Stomach epiglottis

Which of the following correctly matches a digestive enzyme with its source? Pepsin – pancreas Bile – gall bladder Trypsin – stomach Amylase - pancreas

Which of the following correctly matches a digestive enzyme with its source? Pepsin – pancreas Bile – gall bladder Trypsin – stomach Amylase - pancreas

6.5 Small Intestines The pyloric sphincter is similar to the cardiac sphincter, except it is located at the posterior of the stomach where it controls the passage of the liquid acid chyme into the duodenum (the first part of the small intestines) The duodenum is specialized by the presence of chemoreceptors (chemical sensitive nerve endings that are able to detect the different biochemicals in the food material), that regulate which secretions are released

When signaled to, the gall bladder releases the fat emulsifier bile and the pancreas releases pancreatic juice The biocarbonate ions (HCO3-) “over-neutralize” the acid chyme and buffer it at a pH of about 8.3 The enzymatic components of pancreatic juice are active at this alkaline pH

Lipase converts emulsified lipids into fatty acids and glycerol Trypsin breaks many peptide bonds to convert various lengths of polypeptides into shorter peptide units Pancreatic amylase breaks remaining starch into maltose Nuclease converts some of the nucleic acids into nucleotides and nucleotidases (which break nucleotides into their component molecules)

All of these enzymes are active in the duodenum The duodenum also produce and secrete its own enzymes Disaccharidases (maltase and lactase) Peptidase (to break remaining polypeptides into amino acids) nuclease

The longest part of the small intestines is called the ileum (absorbs the products of digestion) It has a large surface area and is lined with specialized structures for absorption by active trasnport These structures are called villi

The epithelial cells lining the villi are equipped with mitochondria to produce the required ATP Internally, each villus has a capillary bed and a lacteal (an absorptive end of the lymphatic system) The products of fat digestion are reconstructed into neutral fats by the epithelial cells before they enter the lacteals, the rest of the products of digestion enter the blood stream

6.6 Colon The undigested food move from the ileum through the ileo-caecal valve (another sphincter) to the large intestine (colon) The first segment of the colon is the caecum, then the appendix (a vestigial structure)

The major roles of the colon are to absorb a great amount of the water and store E. coli E. coli is a bacterium that metabolizes some of what our body cannot (waste material) Their metabolism releases minerals and manufactures some vitamins and amino acids

These nutrients (from E These nutrients (from E. coli) get absorbed along with water into the circulatory system The bacteria begin the decomposition of the waste materials and convert them to feces with a changed color, smell, and texture

The last part of the colon is the rectum that stores feces until defecation It ends with the anal sphincter, which controls defecation

Review The bacterium E. coli is normally abundant in Food Feces Bile salts Stomach juices

Review The bacterium E. coli is normally abundant in Food Feces Bile salts Stomach juices

Which of the following is absorbed into lacteals? Glucose Fatty acids Nucleotides Amino acids

Which of the following is absorbed into lacteals? Glucose Fatty acids Nucleotides Amino acids

Which digestive organ is the appendix attached to? The stomach The gall bladder The large intestine The small intestine

Which digestive organ is the appendix attached to? The stomach The gall bladder The large intestine The small intestine

The removal of a large section of the colon is likely to affect the Release of bile Digestion of fats Absorption of glucose Water balance in the body

The removal of a large section of the colon is likely to affect the Release of bile Digestion of fats Absorption of glucose Water balance in the body

Which of the following structures does NOT produce digestive enzymes? Stomach Pancreas Small intestine Large intestine

Which of the following structures does NOT produce digestive enzymes? Stomach Pancreas Small intestine Large intestine