Macromolecule Digestion

Chemical Digestion

Review

• What were the four macromolecules we studied back in chapter 2?– Proteins, carbohydrates, nucleic acids, and lipids

• Keep those in the back of your mind as we go through this– We will focus on proteins, carbs, and lipids

Question

• What are the building blocks of carbohyrates?– Monosaccharides

Carbohydrates

• 1) Mouth– All food journeys start here!– In our saliva, we have salivary amylase which

breaks down starch and disaccharides into simpler sugars

• 2) Esophagus• 3) Stomach

Just passes through

Carbohydrates

• 4) Small Intestine– When the small intestine

senses sugars, it sends a signal to the pancreas• The pancreas sends out

pancreatic amylase to break down the sugars even further• These enzymes act on the

bulk flow– The stuff on the inside of the

SI lumen

Carbohydrates

• At this point, the sugars are broken down into disaccharides

Carbohydrates

• Now, within the small intestine, the brush border enzymes break down the disaccharides into monosaccharides– These enzymes are found within the microvilla’s

plasma membrane of the epithelial cells (enterocytes)

Carbohydrates

• The monosaccharides are absorbed within the epithelial cells that make up the SI and are transported into the bloodstream (called a lacteal) via active transport

Carbohydrates

• All of the carbohydrates are absorbed at this point in the digestive process in the SI.

• The food stuff continue into the large intestine and then out the anus.

• The End (of carbohydrates)

Graphic

• Let’s draw a picture of the carbohydrate digestive process

Question

• Simple sugars, including glucose, are absorbed into the bloodstream

• What is the fate of those glucose molecules?– Cellular respiration!

Question

• What are the basic building blocks of proteins?– Amino acids

Proteins

• Unlike carbohydrates, proteins are not chemically digested until it hits the stomach

• 1) Mouth– Mechanical digestion

• 2) Esophagus – just passes through• 3) Stomach– Main organ of protein digestion

Recall

• There are gastric pits within the stomach that contains gastric glands and secrete gastric juice

Stomach and Protein Digestion

• The gastric juice within the stomach contains hormones and acid– The acid (hydrochloric acid)

• Helps to activate the various enzymes needed for protein digestion

• DOES NOT break up proteins– Functions to kill bacteria that may be on our food

» Our saliva does the same thing—it is slightly acidic

– Question: Why doesn’t our stomach get burned by the acid?• Answer: The mucous layer!

Question:

• What happens if the stomach mucosa is compromised?– Stomach ulcers

Question:

• What is “heartburn”?– Also known as acid-

reflux disease• Remember the cardiac

sphincter?• Sometimes the

sphincter (gatekeeper) doesn’t work OR the stomach is TOO acidic and burns at the lining of the esophagus

Stomach and Protein Digestion

• Involves many steps, hormones, and cells, but we will simplify it

• Here we go:– When the food enters the stomach, it stimulates the release

of gastrin from the cells within the stomach• This causes those gastric pits to release HCl (acid)• The gastric pits also secrete a hormone called pepsinogen

– -ogen = inactive form– The acid stimulates the pepsinogen to be converted into its active form,

pepsin

• The pepsin converts proteins into polypeptides (long chains of amino acids)

Let’s draw a comic strip to show this

Proteins, Small Intestine, and the Pancreas

• When the small intestine senses that food is there, the pancreas injects its digestive enzymes into the bulk flow– What enzyme did we already talk about that came

from the pancreas?• Pancreatic amylase (digested carbs)

– Along with the pancreatic amylase, the pancreas injects protein-digesting enzymes to further break down the polypeptides• I don’t expect you to know the names, just know that the

pancreas helps digest carbs (via pancreatic amylase) and proteins

• Protein-digesting enzymes act on the food in the bulk phase

Question

• Pancreatic amylase and the pancreatic protein digesting enzymes act on which part of the small intestine?A) Bulk flowB) Brush border

Proteins and the Small Intestine

• The chyme is squirted into the small intestine

• Brush border enzymes on the microvillus break down the polypeptides into amino acids– Called enteropeptidases

• Entero=gut; peptid=protein; -ase=enzyme

Proteins

• When the chyme leaves the stomach, it goes to the small intestine

• Various brush border enzymes break down the polypeptide further and is absorbed into the lacteal via the enterocytes

Proteins

• All of the proteins are digested by the time the foodstuff leaves the small intestine

• Indigestible substances continue through the large intestine and anus

• The End (of proteins)

Question

• What are the basic building blocks of fats?– Glycerides and fatty acids

Question

• What are the accessory organs that we talked about?– Pancreas, liver, and gall bladder

• The pancreas helps digest which two macromolecules?– Proteins and carbs

• Predict: What will the liver and gall bladder help digest?– Fats!

Liver and Gall Bladder

• The liver’s primary function is to produce something called bile

• The bile is stored in a gall bladder, just like our urinary bladder stores urine

Fats

• These are easy, peasy, lemon-squeezy• Here is how things go:– 1) Mouth: mechanical digestion– 2) esophagus: just passes though– 3) stomach: just passes through

Fats and the Small Intestine

• When the small intestine senses that there is food in its midst, It sends a signal to the gall bladder to release some of its stored bile into the SI

• This bile acts on the bulk flow of food material and breaks down fats– What were those again?

• Fatty acids and glycerol

• The fats are further broken down by pancreatic lipase from the pancreas

Fats and the Small Intestine

• The fatty components are then absorbed by the small intestine into the lacteal

• The indigestible matter then goes through the large intestine and out the anus

• THE END!!!!! (of fats!)

Whaaaaat?

• If fats are not properly digested OR if you have a high-fat meal, then there will be a lot of fat in your feces

• You will be able to tell if this happens if your feces floats!

• You’re welcome.

Wait…..What About Water?

• Don’t we need water to survive????

• Water is absorbed in the large intestine!