How does the emulsion of fats occur through the bile of the gallbladder?

Answer

Hello Evelyn,

Bile is an aqueous solution of various molecules: cholesterol (esters), phospholipids and bile acids/bile salts. If you look at these molecules you will see that they all have a part apolar (not water soluble, hydrophobic, nothing but C and H in the structure) and a part polar (water soluble, hydrophilic, charges due to dissociation, dipole moments due to of a lot of electronegativity difference between 2 partners of a covalent bond, e.g. C and O or O and H). This makes these molecules called amphiphilic: they like the 2 (water-polar – and apolar solvents or environments).

When bile is mixed with these molecules through the food mass (in the small intestine), these molecules will associate with the dietary fats. This association is not a real chemical reaction (with exchange of ions or electrons, or with changing molecular structures), but a bonding with each other with ‘weak’ bonds. Weak bonds are all kinds of bonds (ion bonds, dipole-dipole bonds, Van Der Waals bonds or apolar bonds, hydrogen bonds) with the exception of the covalent bond. This last bond is much stronger and in watery medium it cannot just break, unlike the other weak bonds.

The association bonds form an emulsion: the non-polar parts of the bile components will bind with the fatty substances (triglycerides in particular, which are almost completely non-polar), while the polar parts of the bile components remain ‘outward’, in contact with the aqueous environment in the intestine (water from food, saliva, intestinal juice, gastric juice and from bile). In this way you get more or less spherical aggregates with the apolar triglycerides from the food in the center and the amphiphilic molecules from bile (also some from food) around it as a mantle.

Bile acids will release a proton in the slightly basic environment of the intestine and carry a negative charge: this means that the emulsion spheres all carry the same charge and start to repel each other. Together with an intense muscle action in the intestine (movement, mixing effect), a stable emulsion is formed. You can compare this with mayonnaise: by mixing and using polar and non-polar you can get a heterogeneous mass with finely divided emulsion droplets in it. Real farm butter and whipped cream are also emulsions.

If you enter bile acids, cholesterol and phospholipids or phosphoglycerides in Google or Wikipedia, you will get the structure of the molecules.

The lipase you speak of is an enzyme that is secreted by the pancreas and thus reaches the food mass in the intestine: this protein (enzymes are proteins) crawls into the emulsion globules and will then split the fats with water.

Kind regards,