Question

4. Tell the story of digesting a cheeseburger

The hamburger bun: full of carbohydrates

The burger: full of protein

The cheese: full of lipids (remember the role of various important molecules, e.g. bile salts, chylomicrons, etc.)

For each of the parts of the burger, answer the following questions.

Identify any physical digestion processes

Identify the enzyme(s) used to chemically break down the macromolecule

Identify which organ or cell(s) release that enzyme

Identify where in the alimentary canal each enzyme is secreted into.

Describe how the respective monomer is finally absorbed into the blood stream.

Answer 1

carbohydrates

• The dietary carbohydrates in the Hamburger are mainly polysaccharides,disaccharides and monosaccharides.Starch is the polysaccharide that is usually digested in human GI tract.
• Digestion of carbohydrates begins in the mouth by the activation of salivary amylase which catalyses of alpha 1 to 4 linkage to form alpha dextrins.
• The pancreatic amylase also does the same function.The product of amylase digestion are maltose,maltotriose and alpha dextrins.
• The further digestion of starch is carried out by oligosaccharides that are present in the brush border of the intestine.These membrane enzymes are alpha dextrinase that causes hydrolysis of alpha 1,6 linkage,maltase that causes hydrolysis of maltotriose and maltose lactase that causes hydrolysis of lactose into glucose and galactose,glucoamylase that splits malto-oligo-saccharides into single glucose molecules and sucrase that causes hydrolysis of sucrose into fructose and glucose.
• The end product of these membrane digestion are glucose,galactose,and fructose.

Protein

• 10 to 30 grams of protein is added to the intestinal content from the GI secretion and the exfoliated intestinal cells.
• The digsetion of protein starts in the stomach.Pepsinogen secreted from chief cells is converted into pepsin by the action of HCL.Pepsin hydrolyzes proteins into pepetides and amino acids.
• In the duodenum and small intestine,the proteins are digested by proteases secreted in the pancreatic secretion.
• The enterokinase secreted from mucosal cells of duodenum and jejunum converts trypsinogen into trypsin,which then acts as an enzyme to convert other proteases.
• Peptidases are also present in the brush border of enterocytes.These enzyme hydrolyze the peptides produced by the pancreatic proteases into oligopeptides and amino acids.The brush border of peptidases are aminopeptidases and dipeptidases.
• The products of protein digetsion are small peptidases and amino acids.

​​​​​​​Absorotion of proteins : - There are many transport system(atleast seven such system) for absorption of amino acids and peptides.The five transport system require Na+ like that of Na+ -glucose co-transporter.The other two transporters use Cl-.

Lipids

• Digestion of lipids starts in the mouth.In oral cavity lingual lipase is secreted from Ebners glands in the tongue.
• Gastric lipase also helps in the digestion of fat.
• Principal fat digestion starts in the duodenum by the pancreatic lipase,which hydrolyses 1 and 3 bonds of triglycerides that results in formation of free fatty acid and 2-monoglycerides.Pancreatic lipase acts on lipids that have been emulsified.
• Fat digestion is facilitated by pancreatic co-lipase.
• There is another lipase secreted from pancreas called as bile salt activated lipase,which also assists in lipid digestion.
• The dietary cholesterol is hydrolyzed by cholesteryl ester hydrolase.
• Lipids are absorbed by passive diffusion and carrier mediated transport.As soon as lipids enter the cell they are esterified,therefore a gradient is maintained for their entry into the cell.