The Digestive System

Guts, teeth and glands!

Images from: http://www.whfreeman.com/life/update/

Why Guts?

• Multicellular animals must have specialized structures for obtaining and breaking down their food.

• There are two processes: feeding and digestion.

• Animals are heterotrophs, they must absorb nutrients or ingest food sources.

How to dine...

• Ingestive eaters (us).

• Absorptive feeders (tapeworm)

• Filter feeders (clam)

• Substrate feeders (earthworms)

• Fluid feeders (mosquito)

Vertebrate Digestion

• The digestive system uses mechanical and chemical digestion to breakdown food.

Tube in a tube• Vertebrates have a tube-within-a-tube system.

• digestion occurs in the lumen with the nutrient molecules being transferred to the blood.

Stages of digestion

• Movement of food

• Secretion of digestive juices

• Digestion of food into molecules

• Absorption of molecules

• Elimination of undigested food and wastes

The Digestive System

• Mouth, pharynx,• esophagus, stomach• small intestine• large intestine• anus• salivary glands• pancreas• liver and gall bladder

The Start• In the mouth, teeth, jaws and the tongue begin

the mechanical breakdown of food.

• Chemical breakdown of starch by amylase

• Mucus moistens food and lubricates the esophagus.

• The chewed food and saliva is then pushed into the pharynx and esophagus.

• The esophagus uses peristalsis to send the food to the stomach.

Move the food

• Food is chewed and passed to the stomach through the esophagus. The name of the movement is peristalsis. (see video)

The Stomach

• Holds 1 to 2L (folds)

• The stomach secretes mucus, hydrochloric acid and pepsin. HCl lowers pH of the stomach to activate pepsin.

• Pepsin hydrolysis of proteins into peptides.

• The stomach also mechanically churns the food. Chyme, leaves the stomach and enters the small intestine.

• Ulcers occur when mucus lining is reduced.

The Small Intestine

• 3 m long tube with coils and folding plus villi. Very large surface area!

• Final digestion of all food and absorbtion.

• Villi produce enzymes which complete the digestion of peptides and sugars.

• The absorption process in the villi.

Villi

• Sugars and amino acids go into the bloodstream via capillaries in each villus.

• Glycerol and fatty acids go into the lymphatic system. Absorption is an active transport, requiring cellular energy.

Duodenum - busy place

• Secretions from the liver and pancreas are used for digestion in the duodenum.

• The pancreas secretes digestive enzymes and stomach acid-neutralizing bicarbonate.

• The liver produces bile, which is stored in the gall bladder before entering the bile duct into the duodenum.

Small intestine - more

• Digestion of carbohydrates, proteins, and fats continues in the small intestine. See table in text - page 166.

• Bile emulsifies fats so that lipases can completely digested lipids.

• Most absorption occurs in the ileum and jejeunum (second third of the small intestine).

Liver and Gall Bladder

• The liver produces bile and helps to detoxify of blood

• synthesis of blood proteins

• destruction of old erythrocytes

• storage of glucose as glycogen

• De-amination amino groups and ammonia. (this produces urea, less toxic)

Glycogen-Glucose

• Low glucose levels in the blood cause glucagon to stimulate breakdown of glycogen into glucose.

• Insulin helps store glucose and glycogen in the liver (see page 929)

• When no glucose or glycogen is available, amino acids are converted into glucose in the liver.

The Large Intestine what to do with left overs!

• The large intestine produces an alkaline mucus that neutralizes acids produced by bacterial metabolism.

• Water, salts, and vitamins are absorbed, the remaining contents in the lumen form feces (mostly cellulose, bacteria, bilirubin).

• Bacteria in the large intestine, such as E. coli, produce vitamins (including vitamin K) that are absorbed.

Nutrition

• See the basics of nutrition in your text.

• Have a hand out!