in the mitochondria. Long chain fatty acids (more than 14 carbon) need to be converted to fatty acyl-CoA in order to pass across the mitochondria membrane. Fatty acid catabolism begins in the cytoplasm of cells as acyl-CoA synthetase uses the energy from cleavage of an ATP to catalyze the addition of coenzyme A to the fatty acid. The resulting acyl-CoA cross the mitochondria membrane and enter the process of beta oxidation. The main products of the beta oxidation pathway are acetyl-CoA (which is used in the citric acid cycle to produce energy), NADH and FADH. The process of beta oxidation requires the following enzymes: acyl-CoA dehydrogenase, enoyl-CoA hydratase, 3-hydroxyacyl-CoA dehydrogenase, and 3-ketoacyl-CoA thiolase. The diagram to the left shows how fatty acids are converted into acetyl-CoA. The overall net reaction, using palmitoyl-CoA (16:0) as a model substrate is: 7 FAD + 7 NAD+ + 7 CoASH + 7 H2O + H(CH2CH2)7CH2CO-SCoA → 8 CH3CO-SCoA + 7 FADH2 + 7 NADH + 7 H+ Lipid biosynthesis In addition to dietary fats, storage lipids stored in the adipose tissues are one of the main sources of energy for living organisms. Triacylglycerols, lipid membrane and cholesterol can be synthesized by the organisms through various pathways. Membrane lipid biosynthesis There are two major classes of membrane lipids: glycerophospholipids and sphingolipids. Although many different membrane lipids are synthesized in our body, pathways share the same pattern. The first step is synthesizing the backbone (sphingosine or glycerol), the second step is the addition of fatty acids to the backbone to make phosphatidic acid. Phosphatidic acid is further modified with the attachment of different hydrophilic head groups to the backbone. Membrane lipid biosynthesis occurs in the endoplasmic reticulum membrane. Triglyceride biosynthesis The phosphatidic acid is also a precursor for triglyceride biosynthesis. Phosphatidic acid phosphotase catalyzes the conversion of phosphatidic acid to diacylglyceride, which will be converted to triglycerides by acyltransferase. Triglyceride biosynthesis occurs in the cytosol. Fatty acid biosynthesis The precursor for fatty acids is acetyl-CoA and it occurs in the cytosol of the cell. The overall net reaction, using palmitate (16:0) as a model substrate is: 8 Acetyl-coA + 7 ATP + 14 NADPH + 6H+ → palmitate + 14 NADP+ + 6H2O + 7ADP + 7P¡ Cholesterol biosynthesis Cholesterol can be made from acetyl-CoA through a multiple-step pathway known as isoprenoid pathway. Cholesterols are essential because they can be modified to form different hormones in