The Digestive System

• 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

• 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

• Absorption: small molecules absorbed by cells in small intestine

• Elimination: removal of undigestable materials (like cellulose)

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)

Your mouth does both!

•http://www.hns.org.uk/bio/index.php?option=com_content&view=article&id=123&Itemid=176

• 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

• 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

• When you swallow the epiglottis covers the windpipe so that you don’t choke!

Esophagus

• Tube connecting throat to stomach• Peristalsis: rhythmic contractions of smooth

muscles that move food through the digestive system

Stomach

• Large muscular sac• Gastric glands release pepsin and HCl• Chemical digestion:

– Proteins--> smaller polypeptide chains

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

• Mechanical digestion: smooth muscle churns and mixes food

– Chyme: mixture with an oatmeal-like consistency

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

• Pancreas: produces enzymes that break down carbohydrates, proteins, lipids and nucleic acids

– Also produces sodium bicarbonate…to neutralize stomach acid

• Liver and Gallbladder..help in the digestion of fats!

• Bile: emulsifies fats

• Bile is made in the liver and stored

(released) in the gallbladder

Absorption also occurs in SI

• Structure adapted for this function!

• Villi: fingerlike projections– Microvilli:tiny projections covering villi

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