International School Bangkok Digestion in Humans IB Biology SL International School Bangkok

It All Starts Here

What Happens to Food in the Mouth? It gets chewed to small bits by the jaws, teeth and tongue. It gets moistened. It gets tasted. It gets mixed with saliva and forms a “bolus” – ready for swallowing. A little starch is digested.

Chemical Digestion in the Mouth Enzyme Where is it made? Where does it work? Substrate Product Special Salivary glands (3 pairs) In the Mouth Salivary Amylase Maltose (glucose + glucose) Neutral pH required Starch

Salivary Glands are “common” glands. Their secretions travel in ducts.

Features of the STOMACH The esophagus carries food from mouth to stomach This valve is also called the CARDIAC SPHINCTER Notice the “expandable” RUGAE (folds) in the stomach wall. There are muscle layers in the stomach wall which allow it to “churn” the food This valve is also called the PYLORIC SPHINCTER FACTOID: Some say the stomach is “J” shaped!

What Happens to Food in the Stomach? It gets liquefied. (Carbs quickly; proteins take longer) It gets warmed. Bacteria are killed. Very little gets absorbed – some salts, monosaccharides, water, alcohol. It leaves as a liquid called CHYME.

Chemical Digestion in the Stomach Enzyme Where is it made? Where does it work? Substrate Product Special Gastric glands of stomach pH 2 HCl activated Pepsin In the Stomach Proteins Peptides

The Role of HCl Turns pepsinogen into pepsin (or, it “activates” pepsin Kills bacteria Gives the enzyme pepsin its preferred environment (pH 2) Denatures proteins -- it unfolds the polypeptide chains

Features of the Small Intestine Is about 6 meters in length. Consists of the duodenum, jejunum, and ileum. Is lined with millions of villi giving it a “velvety” appearance. Produces some of its own enzymes in the intestinal glands. Receives some enzymes from the pancreas. Receives bile from the liver. Operates at a slightly alkaline pH

What Happens to Food in the Small Intestine? All type of foods (proteins, carbohydrates, triglycerides, nucleic acids) are digested. The products of digestion are absorbed. The products of digestion enter circulation – some through the lymphatic system.

LIVER FEATURES The liver produces a secretion which is called “bile”. Bile contains bile pigments and bile salts. Bile does NOT contain enzymes! Bile salts help to emulsify (liquefy) fats – giving a larger surface area. Bile pigments are recycled hemoglobin and most will be reasborbed in the intestines. Bile is stored, temporarily, in the gall bladder. (36 mL capacity) FACTOID: bile salts sometimes start to crystallize = GALLSTONES!

PANCREAS FEATURES The pancreas is a “compound” gland. FACTOID: Compound glands are both “common” and endocrine”! “Common” secretions -- enzymes. They enter a duct of some sort. Endocrine secretions – hormones (insulin and glucagon). They enter directly into the bloodstream The pancreas looks like a cluster of grapes and is located in the fold between the stomach and the duodenum

2 Problems 1. How do the gall bladder and pancreas “know” when food is in the small intestine and their secretions are needed? Answer: Secretagogues from the duodenum! Secretin stimulates the pancreas and CCK (cholecystokinin) stimulates the liver and gall bladder! 2. How do the secretions of the gall bladder and pancreas get to the small intestine? Answer: Secretions enter into 2 ducts that join to become the “common bile duct” This enters the duodenum!

CARBOHYDRATE DIGESTION IN THE SMALL INTESTINE Enzyme Where is it made? Where does it work? Substrate Product Special Pancreatic amylase Pancreas Small intestine Starch Maltose Alkaline pH Maltase Small Intestine Glucose (2) Sucrase Sucrose Glucose and Fructose Lactase Lactose Glucose and Galactose

PROTEIN DIGESTION IN THE SMALL INTESTINE Enzyme Substrate Product Where is it made? Where does it work? Special Trypsin Protein Peptides Pancreas Small Intestine Alkaline pH Peptidase Amino Acids

TRIGLYCERIDE DIGESTION IN THE SMALL INTESTINE Enzyme Substrate Product Where is it made? Where does it work? Special Pancreatic Lipase Triglyceride Glycerol and Fatty Acids Pancreas Small Intestine Alkaline pH, Fat Droplets

PROBLEM: How do all of those products get absorbed? ANSWER: The villi that line the small intestine will absorb them! FACTOID: The total surface area of the villi approach that of a tennis court!

Lacteal (a branch of the lymphatic system Arteriole from the heart Features of a VILLUS Lacteal (a branch of the lymphatic system Capillaries Arteriole from the heart Venule to the liver

VILLUS – another view Notice that a villus is multicellular! Arteriole from the heart Lacteal – to bloodstream eventually! Venule to the liver

The Lymphatic System – Your “other” circulatory system! The lacteals in the villi empty into the Lymphatic System Skeletal muscle contraction forces lymph through the system Lymph enters the blood stream at the left subclavian vein FACTOID: Lymph is the clear fluid that is forced from the capillaries – very similar to plasma

VILLUS – another view Capillaries absorb monosaccharides, amino acids, A, T, G, C, phosphates, vitamins Remember! These travel through the bloodstream to the liver. Lacteals absorb glycerol and fatty acids – and reassemble them into small triglycerides called “chylomicrons” Remember! These travel the lymphatic system and enter the bloodstream eventually

PROBLEMS: 1) How does the water get removed from the undigestable materials and 2) what if it doesn't? 1) The large intestine absorbs the water! 2) Diarrhea will occur and the dehydration could cause death! FACTOID: There are NO villi in the large intestine!

The large intestine is about 1.5 meters long Bacteria, particularly E. coli, are at work here Fiber (undigestable cellulose) in the diet helps to keep things moving along! FACTOID #1: E. coli provides us with histidine (one of the essential amino acids) and therefore has a MUTUALISTIC relationship with us! FACTOID #2: Darwin identified the appendix as a vestigial structure in humans. It may, at one time in our past, been a place where cellulose was worked on by bacteria to help with its breakdown.

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