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The Digestive System
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• Purpose of digestive system:– Converts food into small molecules that can
be used by cells!!– Why do the food molecules need to be
small??
– Four phases:• Ingestiondigestion absorption elimination
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• Ingestion: process of putting food into your mouth
• Digestion: process of breaking food down– Mechanical: physical breakdown of large
pieces; increases surface area– Chemical: enzymes are used to break food
into smaller molecules
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• Absorption: small molecules absorbed by cells in small intestine
• Elimination: removal of undigestable materials (like cellulose)
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Mechanical vs Chemical
• Mechanical: grinding, tearing, peristalsis (churning action)….increases surface area for enzyme action
• Chemical: enzymes work to make larger molecules into smaller molecules (ex: starch-->simple sugars)
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Your mouth does both!
•http://www.hns.org.uk/bio/index.php?option=com_content&view=article&id=123&Itemid=176
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• Mechanical digestion is accomplished by chewing– Your teeth cut, tear, grind, and crush food
– Your tongue moves the food around so the tongue can work on it
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• Chemical digestion of mouth– Enzyme=amylase
– Amylase begins to break down long starches forming sugars
– Saliva also has lysozyme….an enzyme that protects us against bacteria
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• When you swallow the epiglottis covers the windpipe so that you don’t choke!
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Esophagus
• Tube connecting throat to stomach• Peristalsis: rhythmic contractions of smooth
muscles that move food through the digestive system
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Stomach
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• Large muscular sac• Gastric glands release pepsin and HCl• Chemical digestion:
– Proteins--> smaller polypeptide chains
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Another gland produces ______mucus_____________– o This lubricates so food can pass easier and
protects the stomach wall– o If acids erode the stomach wall and create a
sore it is called a peptic ulcer– o Now scientists know peptic ulcers caused by
Helicobacter pylori
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• Mechanical digestion: smooth muscle churns and mixes food
– Chyme: mixture with an oatmeal-like consistency
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Small Intestine
• Most digestion and all absorption• First part of SI= duodenum
– Mixes with fluids from the pancreas, the liver and the lining of the SI
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• Pancreas: produces enzymes that break down carbohydrates, proteins, lipids and nucleic acids
– Also produces sodium bicarbonate…to neutralize stomach acid
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• Liver and Gallbladder..help in the digestion of fats!
• Bile: emulsifies fats
• Bile is made in the liver and stored
(released) in the gallbladder
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Absorption also occurs in SI
• Structure adapted for this function!
• Villi: fingerlike projections– Microvilli:tiny projections covering villi
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Large Intestine
• Absorbs water from undigested material
• Bacteria found here help to produce Vitamin K• Concentrated waste material: feces
• Too much water absorbed: constipation
• If not enough water is absorbed: diarrhea
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