23.7 Chemical Digestion and Absorption: A Closer Look

what is chemical digestion

is a complex process that reduces food into its chemical building blocks, which are then absorbed to nourish the cells of the body

what are large molecules

proteins, lipids, nucleic acids, and starches

what must large food molecules be broken down into

subunits that are small enough to be absorbed by the lining of the alimentary canal

how are large food molecules broken down into subunits

by enzymes through hydrolysis

what is the source of lingual lipase

lingual glands

what is the substrate of lingual lipase and product

Triglycerides: Free fatty acids, and mono- and diglycerides

what is the source of salivary amylase

Salivary glands

what is the substrate of salivary amylase and product

Polysaccharides: Disaccharides and trisaccharides

what is the source of gastric lipase and pepsin

chief cells

what is the substrate of gastric lipase and product

triglycerides: Fatty acids and monoacylglycerides

what is the substrate and product of pepsin, chymotrypsin, elastase, and trypsin

proteins: Peptides

what is the source of a-dextrinase, enteropeptidase, lactase, maltase, nucleosidases and phosphatases, peptidases, and sucrase

small intestine

what is the substrate of a-dextrinase and product

a-dextrins: Glucose

what is the substrate of enteropeptidase and product

Trypsinogen: Trypsin

what is the substrate of lactase and product

lactose: Glucose and galactose

what is the substrate of maltase and product

maltose: glucose

what is the substrate of nucleosidases and phosphatases and their products

nucleotides: Phosphates, nitrogenous bases, and pentoses

what is the substrate of peptidases

aminopeptidase (amino acids at the amino end of peptides) and dipeptidase (dipeptides)

what is the substrate of sucrase and its products

sucrose: glucose and fructose

where is the source of carboxy-peptidase, chymotrypsin, elastase, nucleases, pancreatic amylase, pancreatic lipase, and trypsin from

pancreatic acinar cells

what is the product of peptidases

aminopeptidase (amino acids and peptides) and dipeptidase (amino acids)

what is the substate and product of carboxy-peptidase

amino acids at the carboxyl end of peptide: amino acids and peptides

what is the substrate and product of nucleases

ribonuclease (ribonucleic acids), deoxyribonuclease (deoxyribonucleic acids): nucleotides

what is the substrate and product of pancreatic amylase

polysaccharides (starches): α-Dextrins, disaccharides (maltose), trisaccharides (maltotriose)

what is the substate and product of pancreatic lipase

Triglycerides that have been emulsified by bile salts: Fatty acids and monoacylglycerides

which enzymes are activated by other substances

pepsin, chymotrypsin, elastase, and trypsin

What percentage of the average American diet is made up of carbohydrates

50%

How are carbohydrates classified

They are classified based on the number of sugar monomers into simple sugars and complex sugars

What are examples of simple sugars

Monosaccharides and disaccharides

What are the three common monosaccharides in the diet

Glucose, galactose, and fructose

Are monosaccharides readily absorbed by the body

Yes they are readily absorbed

What is sucrose made of

Sucrose is made of glucose and fructose

what is the common name of sucrose

table sugar

What is lactose made of and what is its common source

Lactose is made of glucose and galactose and is found in milk

What is maltose made of and what is its common source

Maltose is made of two glucose molecules and is found in grains

What are examples of polysaccharides that the body can digest

Glycogen and starch

What types of carbohydrates can the body not digest

Most fibrous polysaccharides such as cellulose

Do indigestible polysaccharides have nutritional value

No they do not provide nutritional value

What benefit do indigestible polysaccharides provide

They provide dietary fiber which helps move food through the alimentary canal

Where does the chemical digestion of starches begin

It begins in the mouth

what is the digestive system able to breakdown

disaccharide sucrose, lactose, maltose, polysaccharides glycogen, and starch

what is pancreatic amylase

enzyme secreted by the pancreas that completes the chemical digestion of carbohydrates in the small intestine

What enzyme acts on the fragments produced by pancreatic amylase in the small intestine

The brush border enzyme a-dextrinase

What is a-dextrinase

brush border enzyme that acts on a-dextrins

what does a-dextrinase do

breaks a-dextrin into single glucose units

what is a-dextrin

breakdown product of starch

what is sucrase

brush border enzyme that breaks down sucrose into 1 glucose and 1 fructose

what is maltase

brush border enzyme that breaks down maltose and maltotriose into two and three molecules of glucose

what is lactase

brush border enzyme that breaks down lactose into 1 glucose and 1 galactose

What happens if the body does not produce enough lactase

Insufficient lactase can lead to lactose intolerance

whaat are proteins

polymers composed of amino acids linked by peptide bonds to form long chains

What are proteins made of

made of amino acids linked by peptide bonds

What does protein digestion reduce proteins into

It reduces them into their individual amino acids

What percentage of a person’s total calorie intake typically comes from protein

About 15 to 20 percent

Where does protein digestion begin

It begins in the stomach

What substances in the stomach start protein digestion

Hydrochloric acid or HCl and the enzyme pepsin

What do HCl and pepsin do to proteins

They break proteins into smaller polypeptides

Where do the polypeptides go after leaving the stomach

They travel to the small intestine

What pancreatic enzymes continue protein digestion in the small intestine

Trypsin and chymotrypsin

What do trypsin and chymotrypsin do

They act on specific peptide bonds in amino acid chains to further break down proteins

What enzymes do brush border cells secrete to assist in protein digestion

They secrete aminopeptidase and dipeptidase

What is aminopeptidase and what does it do

Aminopeptidase is a brush border enzyme that removes single amino acids from the amino end of peptides

What is dipeptidase and what does it do

Dipeptidase is a brush border enzyme that splits dipeptides into two separate amino acids

What is the final result of protein digestion

Proteins are broken down into molecules small enough to enter the bloodstream

how much does a healthy diet that limits lipids intake?

35% of total calorie intake

what is the most common dietary lipids

triglycerides, cholesterol, and phospholipids

what are triglycerides made up of

a glycerol molecule bound to 3 fatty acid chains

what 3 lipases are responsible for lipid digestion

lingual lipase, gastric lipase, and pancreatic lipase

what is pancreatic lipase

enzymes released from the pancreas that digest lipids in the diet

which organ is the only consequential (major, resulting in, important) source of lipase

the pancreas

where does all lipid digestion occur

all lipid digestion occurs in the small intestine

how does pancreatic lipase break down triglycerides

into two free fatty acids and a monoglyceride

what does the fatty acids of triglycerides contain

both short chain (less than 10-12 carbons) and long chain fatty acids

where are nucleic acids DNA and RNA found

in most of the foods you eat

which two types of pancreatic nuclease are responsible for DNA and RNA digestion

deoxyribonuclease and ribonuclease

what is pancreatic nuclease

enzyme secreted by the pancreas that participates in nucleic acid digestion

what is deoxyribonuclease

pancreatic enzyme that digests DNA

what is ribonuclease

pancreatic enzyme that digests RNA

deoxyribonuclease and ribonuclease are further broken down into two intestinal brush border enzymes:

nucleosidase and phosphatase

what is nucleosidase

brush border enzyme that digests nucleotides

what is phosphatase

brush border enzyme that digests nucleotides

what do nucleosidase and phosphatase further break down into

pentoses, phosphates, and nitrogenous bases

what does pentose, phosphate, and nitrogenous bases get absorbed through

the alimentary canal wall

what must carbohydrates (large food molecule) be broken down into to be able to absorbed

Monosaccharides: glucose, galactose, and fructose

what must proteins (large food molecule) be broken down into to be able to absorbed

Single amino acids, dipeptides, and tripeptides

what must triglycerides (large food molecule) be broken down into to be able to absorbed

Monoacylglycerides, glycerol, and free fatty acids

what must nucleic acids (large food molecule) be broken down into to be able to absorbed

Pentose sugars, phosphates, and nitrogenous bases

what is the goal of mechanical and digestive processes

to convert food into molecules small enough to be absorbed by the epithelial cells of the intestinal villi

what is the absorptive capacity of the alimentary canal

almost endless

how many liters of food, liquid, and Gl secretions can the alimentary canal process

up to 10 L

how many L of food enters the large intestine other than the alimentary canal (10L)

1 L

how much electrolytes and water are absorbed in the small intestine

80% of electrolytes and 90% of water

what intestine is involved in the absorption of water and lipids

the small intestine is involved in the absorption of water and lipids

where does most absorption of carbs and proteins occur

in the jejunum, the middle part of the small intestine

what does the terminal ileum absorb

bile salts and vitamin B12

what happens by the time chyme passes food from ileum into the large intest

it is essentially indigestible food residue (mainly plant fibers like cellulose), some water, and millions of bacteria

what are the 5 mechanisms where absorption can occur

active transport, passive diffusion, facilitated diffusion, co-transport (secondary active transport), and endocytosis

what is active transport

the movement of a substance across a cell membrane going from an area of lower concentration to an area of higher concentration using ATP (cellular energy)

what is passive diffusion

movement of substances from an area of higher concentration to an area of lower concentration

what is facilitated diffusion

the movement of substances from an area of higher to an area of lower concentration using a carrier protein in the cell membrane