Tutor profile: Austin H.

Describe the pathway of lipid digestion and absorption.

Lipid digestion begins in the small intestine where bile salts are released from the gallbladder into the duodenum (first portion of small intestine). Lipids, due to their hydrophobicity, will form micelles in an aqueous environment, such as the small intestine. The bile salts emulsify the fat droplets, i.e. they disrupt the hydrophobic interactions between lipid molecules and break the micelles into smaller droplets. The now free triacylglycerols are degraded into individual fatty acids by the enzyme lipase The individual fatty acids can now be absorbed by the intestinal mucosal cells where they are then formed back into triacylglycerols. The triacylglycerols combine with apolipoproteins into structures known as chylomicrons, which then enter the lacteal, a vessel of the lymphatic system. The lymphatic system ultimately drains into the circulatory system where the chylomicrons are transported to tissues. The triacylglycerols are broken down again into free fatty acids and glycerol for transport through the capillary walls into tissues for oxidation and storage.

What are stereoisomers? List the two major categories of stereoisomers and what they are.

Stereoisomers are molecules that have the same number and type of atoms (i.e. the same molecular formula) connected in the same sequence, but differ in their spatial arrangement. The two major categories are enantiomers and diastereomers. Enantiomers are molecules that are non-superimposable mirror image stereoisomers, similar to a right and left hand. Diastereomers are non-superimposable non-mirror image stereoisomers, i.e. they differ in their spatial arrangement but they are not mirror images.

What is the approximate net number of ATP generated from the complete oxidation of one molecule of glucose? Describe the major components of the pathway and how many ATP and electron carriers are generated at each step as well as the ATP the carriers are ultimately converted to (don’t worry about the exact steps and mechanisms of generation, just the major component of the cycle).

The number of ATP generated from one molecule of glucose is estimated to be about 32 (though this is just an estimate). The oxidation of glucose begins with glycolysis, where one glucose is split into two pyruvate. Glycolysis requires 2 ATP and produces 4 for a net gain of 2. Also, 2 NADH are produced. Each pyruvate is converted to acetyl CoA, which generates 1 NADH each (2 total then). Two acetyl CoA then enter the citric acid cycle. For one turn of the cycle 3 NADH, 1 FADH2, and 1 ATP (in the form of GTP) are produced. This happens for each of the acetyl CoA, so 6 NADH, 2 FADH2, and 2 ATP total. The electron carriers then enter the electron transport chain where each NADH is converted to the equivalent of 2.5 ATP and each FADH2 is converted to 1.5 ATP. So the total electron carriers entering the ETC is 10 NADH and 3 FADH2 which produce 10*2.5+2*1.5=28 ATP plus the 4 already generated. This brings the total ATP generated from the complete oxidation of one glucose to 32 ATP.