Digestion and absorption of carbohydrates and proteins

Flashcards made using the lecture delivered by Dr Tim Green. Rao JN, Wang JY. Regulation of Gastrointestinal Mucosal Growth. San Rafael (CA): Morgan & Claypool Life Sciences; 2010. Role of GI Hormones on Gut Mucosal Growth. Available from: https://www.ncbi.nlm.nih.gov/books/NBK54093/. Heda R, Toro F, Tombazzi CR. Physiology, Pepsin. [Updated 2023 May 1]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK537005/. Paju A, Bjartell A, Zhang WM, Nordling S, Borgström A, Hansson J, Stenman UH. Expression and characterization of trypsinogen produced in the human male genital tract. Am J Pathol. 2000 Dec;157(6):2011-21. doi: 10.1016/S0002-9440(10)64840-7. PMID: 11106574; PMCID: PMC1885775.

What are the 5 cell types in the stomach?

Mucous cells, parietal cells, chief cells, enteroendocrine cells and stem cells.

What is the role of mucous cells?

They secrete mucus which is protective of the stomach lining against the acidic conditions of the stomach.

What is the role of parietal cells?

They secrete hydrochloric acid and intrinsic factor.

What is the role of chief cells?

They secrete pepsinogen.

What is the role of the enteroendocrine cells?

They release peptide hormones - for example gastrin.

What is the role of the stem cells found in the stomach?

Replace the old cells which are damaged due to the harsh conditions in the stomach.

Where is gastrin secreted into?

The stomach.

What is the main macromolecule stimulant of gastrin secretion?

Proteins.

What cells secrete gastrin?

G cells.

What does gastrin stimulate?

Gastrin stimulates gastric acid and pepsinogen secretion. It also stimulates gastric motility.

Where is secretin secreted into?

The duodenum.

What is secretin released in response to?

It is released in response to gastric acid.

What are the roles of secretin?

It inhibits gastric acid secretion and stimulates the secretion of increased pancreatic and biliary bicarbonate.

Where is cholecystokinin secreted?

The duodenum mainly in response to fats.

What are the roles of CCK?

Inhibits gastric acid secretion and motility.

Stimulates the secretion of bile, pancreatic bicarbonate secretion and enzyme secretion.

What cells secrete somatostatin?

D cells.

Where in the gastrointestinal tract is somatostatin released?

The stomach and the duodenum in response to gastric acid.

What is the function of somatostatin?

It inhibits gastric acid secretion.

What are the main types of drug used to treat peptic ulcers?

Proton Pump Inhibitors.

What feature of the small intestine is responsible for increasing the surface area for absorption?

Villi.

What are the two main types of cell found on the villi located in the small intestine?

Goblet cells that secrete mucus and absorptive enterocytes.

Where does the venous drainage of the villi of the stomach go?

Into the hepatic portal vein.

Why do intestinal cells have a short lifespan?

Because of the harsh environments they are in.

Where are the stem cells in the intestine located?

In the crypt.

Where are the new intestinal epithelial cells produced?

In the crypts.

What happens to the new cells that are produced by the stem cells in the intestine as they move up from the crypt to the tip of the villi?

They differentiate.

What is the role of the paneth cells?

They release antimicrobial factors.

Where are the paneth cells located?

In the crypts.

What is a consequence of the flattening of the mucosa?

Malabsorption.

Which digestive enzymes are present in the mouth?

Salivary amylase which begins the digestion of carbohydrates and lingual lipase which begins the digestion of triglycerides.

In the stomach, what is the role of gastric acid in digestion?

Denaturation of proteins.

In the stomach, what is the role of pepsin in digestion?

Protein digestion.

What is the role of gastric lipase?

Breakdown of triglycerides in the stomach.

What type of drugs are absorbed in the stomach?

Acidic drugs.

What common toxin is absorbed in the stomach?

Alcohol.

What are the breakdown products of carbohydrates and proteins before the intestinal stage?

Carbohydrates are broken down into disaccharides and proteins are broken down into short peptides and free amino acids.

What happens in terms of carbohydrate and protein digestion in the duodenum?

Starch and disaccharides are digested by pancreatic amylase and disaccharidases.

Proteins are digested by trypsin, chymotrypsin and elastase.

What is absorbed in the ileum?

Monosaccharides, amino acids, fatty acids, glycerol, some drugs and water.

What is the precursor for pepsin?

Pepsinogen.

What activates the precursor for pepsin?

Gastric acid.

What is the precursor for trypsin?

Trypsinogen.

What activates the precursor for trypsin?

Enteropeptidase.

What is the precursor for chymotrypsin?

Chymotrypsinogen.

What activates the precursor of chymotrypsin?

Trypsin.

What is the precursor for elastase?

Pro-elastase.

What activates the precursor for elastase?

Trypsin.

What is the precursor for carboxypeptidases?

Pro-carboxypeptidases.

What activates the precursor for carboxypeptidases?

Trypsin.

What is the precursor for aminopeptidases?

Pro-aminopeptidases.

What activates the precursor for aminopeptidases?

Trypsin.

Where is pepsinogen produced?

In the stomach from chief cells.

Where is trypsinogen produced?

In the pancreas.

What are proteins broken down into by protease enzymes?

One of three things - free amino acids, di/tripeptides and larger peptides.

What is the most common form that proteins end up being absorbed by the body?

Amino acids.

How are disaccharides broken down?

They are broken down by specific enzymes.

How are monosaccharides absorbed?

By glucose transporters via an active transport process.

Why are glucose transporters known as symporters?

Because they transport sodium ions alongside glucose into the endothelial cells from the lumen of the small intestine.

How does glucose move from the endothelial cells lining the gut to the bloodstream?

Passive transport.

How many sodium ions are moved into the endothelial cells lining the small intestine alongside the glucose being transported in?

2.

How is the sodium content of the endothelial cells regulated?

By the sodium/potassium ATPase pump which uses ATP to pump sodium ions out into the bloodstream and pump potassium ions into the endothelial cell.

What are non-starch polysaccharides?

It is another name for dietary fibre.

What is resistant starch?

Starch that cannot be broken down by the small intestine and therefore make it to the large intestine undigested.

What happens to the resistant starch once it makes it to the large intestine?

It is broken down by the microorganisms that are present in the large intestine into short chain fatty acids.

What is meant by glycaemic index?

It is the measure of how carbohydrate foods increase the blood glucose levels. For example, a food with a low glycaemic index means that it increases the blood glucose levels more slowly as it is digested and absorbed more slowly.

Which three fatty acids make up the colon’s short fatty acid chain content?

60% acetate, 20% butyrate and 20% propionate.

What is acetate used for once it is absorbed by the bloodstream?

It is used as an energy source.

What is a potential health benefit of butyrate?

It potentially protects against colorectal cancer.

What is the role of fibre in terms of cholesterol and bile acids?

Fibre absorbs bile acids and cholesterol. Increases the clearance of cholesterol from the body.