An investigation into the effect of lipase concentration on the rate of lipid digestion in milk, in the presence and absence of bile salts.
An investigation into the effect of lipase concentration on the rate of lipid digestion in milk, in the presence and absence of bile salts.
Background knowledge
Lipase is an enzyme which catalyses the hydrolysis of triglycerides (lipids) into fatty acids and glycerol. In humans, lipid digestion begins in the stomach with lipase in the gastric juice, but mostly occurs in the duodenum and ileum upon the addition of pancreatic juice (containing more lipase) and bile (containing bile salts). Lipid digestion is illustrated in this investigation by the fact that when the lipids in milk are digested, fatty acids begin to accumulate, and the pH of the solution is lowered due to this build-up (this may be observed by the use of an indicator or pH meter).
To expand on the point earlier that bile is released into the duodenum, it should be explained that the bile salts which it contains do not hydrolyse lipids as lipase does, but emulsify fat droplets into tiny globules, providing a greater surface area for lipase action; they aid lipase, as opposed to replicating its function. Bile salts are derivatives of cholesterol, and consist mostly of sodium glycochlolate, and sodium taurocholate. 1
2 In an enzyme controlled reaction such as that of lipid digestion in milk, an increased enzyme concentration will lead to an increased rate of reaction. This is thanks to the fact that, when more enzyme molecules are present, there is a greater chance of the substrate (the lipid) colliding with the enzyme to form an enzyme-substrate complex. When an enzyme-substrate complex is formed, the shape of the
substrate is altered slightly by interactions with the R-groups in the amino acids of the active site of the enzyme. This makes it easier for the making or breaking of bonds in the substrate molecule to occur, leading to the easier formation of the given products. Enzymes also lower the activation energy which a substrate requires to react, which in conjunction with an increased enzyme concentration, leads to more reactions occurring with greater ease. 3
My prediction for this investigation is that increasing the enzyme concentration will result in an increased rate of lipid digestion, and that the addition of bile salts also, when in the presence of lipase, will increase the rate further still.
Preliminary work
Within our class, we undertook an experiment to investigate the action of lipase in the presence and absence of bile salts, using 4 test tubes with the following set up:
