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Digestion and absorption

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Digestion is the process by which large biological molecules such as carbohydrates, lipids and proteins get hydrolysed into their smaller constituent molecules so they may be absorbed across cell membranes.

Digestion starts with food ingestion and even before the nutrients reach the stomach. A variety of enzymes carry out the breakdown of these nutrient molecules. The first, amylase, is present in saliva as well as secreted by the pancreas.


Carbohydrates

Starch is hydrolysed to disaccharides and trisaccharides before further reactions by other enzymes convert the products into glucose, the ultimate usable nutrient.

The hydrolysis of starch is catalysed by amylase. As this step of carbohydrate digestion begins in the mouth, initially non-sweet carbs like potatoes or rice gradually sweeten in taste before being swallowed for their digestion to continue.




Amylase thus breaks the glycosidic bonds between glucose monomers. It doesn't do so for each and every one of them, so the resulting disaccharide for example would be maltose (glucose-glucose).



At this stage, whether in the mouth or stomach, disaccharides or trisaccharides have yet to be hydrolysed further into glucose or their constituent monomer. This takes place just before absorption in the small intestine (ileum) and is catalysed by membrane-bound disaccharidases specific to each molecule.

This gives a maltase for maltose producing 2 glucose molecules, a lactase for lactose producing a molecule of glucose and a molecule of galactose, and a sucrase for sucrose producing one molecule of glucose and one molecule of fructose. These enzymes are also known as brush border hydrolases and they are located on the membrane of absorptive epithelial cell villi.


Upon breakdown into monosaccharides, the nutrients make their way into the bloodstream via co-transport, previously covered as the example of sodium ions (Na+) alongside glucose molecules being co-transported across the membrane.


Lipids

Lipids also have two steps in their digestion. The first is emulsification and the second is hydrolysis (again).

Emulsification is necessary because lipids are not water soluble. When they travel through the digestive tract from food, they maintain rather large aggregates of themselves. Emulsification is carried out by bile salts (in bile, produced by the liver and stored and concentrated in the gall bladder) which are amphipathic as they have both hydrophobic and hydrophilic parts.

This enables them to sequestrate lipids into smaller droplets.


Pancreatic lipase then hydrolyses these in further digestion into monoglycerides and free fatty acids. The smaller, emulsified droplets provide a much greater surface area for the enzyme to work, and hence the first step is necessary in the digestion of lipids.



Smaller yet droplets called micelles of both the lipid products as well as the bile salts still hanging onto them come into contact with the brush border. They are then absorbed in the intestine by simple diffusion into the epithelial cell, however there are also specific fatty acid transporters to help them cross the membrane.


Proteins

Proteins, too, need to be broken down in multiple steps. The first involves enzymes like endopeptidases and exopeptidases secreted by the pancreas. They break down long amino acid chains, polypeptides, into smaller ones like dipeptides. Endopeptidases break peptide bonds between amino acids, while exopeptidases break the peptide bonds of amino acids with terminal amino or carboxy groups either end of a polypeptide.


The second step is similar to that of carbohydrates. Membrane-bound enzymes on the brush border break down the small polypeptides left into free amino acids or short polypeptide chains of no more than 4 amino acids. The former can then be absorbed in the same way as carbohydrates via co-transport with sodium ions, while the short polypeptides can be absorbed via co-transport with hydrogen ions (H+).

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