Biochemistry of digestion

Question 1

what are the two types of carbohydrates that humans consume?

Answer 1

starch and cellulose

Question 2

describe the difference between starch and cellulose

Answer 2

starch is an oligosaccharide with its monosaccharide units arranged in the alpha 1-4 or 1-6 conformation.

Cellulose however has its oligosaccharides in the beta 1-4 conformation-which cannot be broken down by humans.

Question 3

what is the first stage of digestion?

Answer 3

the cephalic (head) stage

Question 4

what biochemical processes occur in the cephalic stage of digestion?

Answer 4

The brain anticipates food from the sensory input of sight, smell, thought and taste of food.
the sensory input of food stimulates the parasympathetic system headquarters, the vegas nerve.
The vegas nerve initiates digestion by secreting acetylcholine, which causes the pancreas, stomach and mouth to secrete more digestive enzymes.

Question 5

what is the second stage of digestion?

Answer 5

the mouth/upper tract stage

Question 6

what biochemical processes occur in the mouth stage of digestion?

Answer 6

maceration, or chewing of food occurs, this is mechanical digestion though.
Chewing and tasting the food further activates the vegas nerve and more saliva, lingual lipase and salivary amylase are released.

Question 7

what is the third stage of digestion?

Answer 7

The gastric stage, in the stomach

Question 8

what biochemical processes occur in the gastric phase of digestion?

Answer 8

chewed up food arrives in the stomach lumen where it undergoes both physical and chemical digestion.
HCL is secreted here and pepsinogen is converted to pepsin by the low pH. Salivary amylase from the 2nd mouth stage is denatured and is no longer active.
The duodenum facilitates the release of the stomach chyme.

Question 9

what role does the cardia of the stomach play in stomach digestion?

Answer 9

It is home to the esophageal sphincter

Question 10

what role does the antrum of the stomach play?

Answer 10

This is where stomach churning occurs

Question 11

what role does the pyloric sphincter play?

Answer 11

The pyloric sphincter when instructed by the duodenum releases chyme into the small intestine.

Question 12

describe the secretory cells found in the gastric gland

Answer 12

there are the mucous neck cells that secrete stomatostatin.
then there are the parietal cells that secrete HCl stomach acid.
chief cells secrete the zymogen pepsinogen that becomes pepsin.

Question 13

Name the different factors that the duodenum uses to control gastric emptying

Answer 13

Fat- high fat chyme delays gastric emptying
pH- low pH chyme delays gastric emptying because it is too acidic.
osmolarity and distension also affect gastric emptying

Question 14

what is the name of the detection and regulation of gastric emptying by the duodenum?

Answer 14

the enterogastric reflex

Question 15

what is the fourth stage of digestion?

Answer 15

the duodenal stage (small intestine)

Question 16

what biochemical processes occur during the duodenal stage?

Answer 16

The major digestive enzymes are all secreted into the small intestine from the pancreas, gallbladder or the small intestine lumen itself.
The small intestinal lumen is where the macromolecules are absorbed into the blood stream ti be used by the body.

Question 17

briefly explain the structure of the small intestine’s lumen

Answer 17

The small intestine surface is covered in little folds that project into the lumen, called villi. The individual villi themselves are made of specialised epithelial cells that themselves have microvilli. The absorption of nutrients and macromolecules occurs through these epithelial cells.

Question 18

What is the collective term given to the layer of microvilli that form the innermost small intestine layer?

Answer 18

The brushborder

Question 19

There are regulatory digestive enzymes that control the release of functional digestive enzymes. What are they called?

Answer 19

Secretagogues.

Question 20

what does the secretagogue secretin do to assist with digestion regulation?

Answer 20

secretin is a hormone produce in the duodenal mucosa that increases secretion of bicarbonate and pancreatic digestive enzymes.

Question 21

what does the secretagogue cholecystokinin do to assist with digestion regulation?

Answer 21

Cholecystokinin is a peptide hormone produced in the duodenum.
It increases the release of bile, pancreatic enzymes and insulin from the pancreas.
it increases feeling of satiety and fullness.
works by binding to g-coupled protein receptors on pancreas and inducing a calcium influx.

Question 22

does the combination of both secretin and cholecystokinin secretagogues have a different effect?

Answer 22

Yes, together they delay gastric emptying.

Question 23

The pancreas secretes which digestive enzymes?

Answer 23

the pancreas secretes zymogens for all 3 nutrient groups (lipase, protease and amylase) and bicarbonate for buffering the duodenum.

Question 24

The gallbladder secretes which digestive enzymes?

Answer 24

Bile, for fat emulsification and bicarbonate for buffering the duodenum.

Question 25

The duodenum secretes which digestive enzymes?

Answer 25

enterokinase, which activates proteinases like the trypsin which becomes chymotrypsin.

Question 26

How does pancreatic amylase break down starch?

Answer 26

It is an endosaccharidase that works by cleaving the alpha 1-4 bonds at the end of the polysaccharide chain.

Question 27

how does maltase, sucrase and lactase breakdown their corresponding disaccharides?

Answer 27

These 3 are exosaccharides that cleave the chain from the ends. They work at the brushborder of the small intestine and the liberated monosaccharides are absorbed directly into the blood.

Question 28

how are the liberated monosaccharides transported accross the brushborder?

Answer 28

fructose diffuses accross the brushborder into the blood with the help of the GLUT 5 transporter.
Glucose needs to be actively transported accross by a costransporter called the SGLT1, that works by pumping sodium inside the epithelial cell for every glucose pumped out.
Both monosaccharides are then transported via the GLUT 2 transporter accross the mucose into the blood.

Question 29

what are the digestive effects of lactose intolerance?

Answer 29

the absence of the exosaccharidase lactase results in high lactose levels inthe S.I lumen. The bacterial colonies there utilise this lactase and convert it to free fatty acids and gases. This results in increased osmolarity and gas in the colon which causes diarrhea.

Question 30

How is protein digested in the lumen?

Answer 30

pepsinogen is activated by HCl and converted to pepsin.
pepsin digest and cleaves the protein into smaller polypeptide chains. Then, other aa specific cleavers break the peptide bits into their individual amino acids.

Question 31

which enzymes cleave the polypeptides into shorter peptidases after pepsin in the stomach?

Answer 31

trypsin and chymotrypsin, then there is elastase and the carboxypeptidases A and B.

Question 32

How are oligopeptidases in the brushborder absorbed into the blood?

Answer 32

Amino acid transporters work with sodium and potassium pumps to actively transport them.

Question 33

what is cystenuria?

Answer 33

Is a fault, the absence or deficiency in the amino acid transporter responsible for cysteine and other basic aa transport.

Question 34

what is Hartnup’s disease?

Answer 34

Is a fault, the absence or deficiency of the amino acid transporter that carries the large, aromatic and hydrophobic aa’s accross. The symptoms are like pallagra ones because essential aa’s are not transported.

Question 35

what is celiac disease?

Answer 35

Intolerance of the major protein found in wheat and grains , known as the ‘gluten’ protein family. It results in a build up of toxic amino acids in the colon, which leads to chronic inflammation of the colon.