Lipid Catabolism
In order for muscle, liver, and adipose tissue to oxidize the components derived from triglycerides (fatty acids) to produce ATP, the triglycerides must first be split by a process called Lipolysis . This process is catalyzed by enzymes called Lipases. The glycerol and fatty acids resulting from lipolysis are catabolized via different pathways. The glycerol is converted by many cells of the body to glyceraldehyde 3-phosphate (G3P), which can be converted into glucose. If ATP is high in the cell, G3P is converted into glucose. If ATP is low in the cell, G3P enters the catabolic pathway to pyruvic acid.
The fatty acids are catabolized differently than glycerol and yield more ATP.
Beta Oxidation is the first step and occurs in the matrix of the mitochondria ultimately forming Acetyl CoA. Fatty acids are powerful. A 16-carbon fatty acid such as palmitic acid can produce up to 129 ATP upon completion of beta oxidation, Krebs cycle, and ETC.
Lipid Anabolism
Liver cells and adipose cells can synthesize lipids from glucose or amino acids through a process called Lipogenesis which is stimulated by insulin. Lipogenesis occurs when we consume more calories than are needed to satisfy our ATP needs. Excess dietary carbs, proteins, and fats all have the same fate – converted into triglycerides.
Protein Metabolism
Proteins are broken down into amino acids during digestion. Proteins are NOT stored for future use like carbs and triglycerides. Instead, amino acids are either oxidized to produce ATP or used to synthesize new proteins for body growth and repair. Excess dietary amino acids are not excreted in urine or feces but are converted into glucose (gluconeogenesis) or triglycerides (lipogenesis).
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