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9: Lipids and membranes

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    • 9.1: Structure and Function - Lipids and Membranes
      Lipids are a diverse group of molecules that all share the characteristic that at least a portion of them is hydrophobic. Lipids play many roles in cells, including serving as energy storage (fats/oils), constituents of membranes (glycerophospholipids, sphingolipids, cholesterol), hormones (steroids), vitamins (fat soluble), oxygen/ electron carriers (heme), among others.
    • 9.2: Basic Concepts in Membranes
      The orderly movement of compounds through cell membranes is critical for the cell to be able to 1) get food for energy; 2) export materials; 3) maintain osmotic balance; 4) create gradients for secondary transport; 5) provide electromotive force for nerve signaling; and 6) store energy in electrochemical gradients for ATP production (oxidative phosphorylation or photosynthesis). In some cases, energy is required to move the substances (active transport).
    • 9.3: Transport in Membranes
      It is essential for cells to be able to uptake nutrients. This function along with movement of ions and other substances is provided by proteins/protein complexes that are highly specific for the compounds they move. Selective movement of ions by membrane proteins and the ions’ extremely low permeability across the lipid bilayer are important for helping to maintain the osmotic balance of the cell and also for providing for the most important mechanism for it to make ATP - the process of oxidat
    • 9.4: Metabolism of Fat
      Breakdown of fat in adipocytes requires catalytic action of three enzymes, hormone sensitive triacylglycerol lipase (called LIPE) to remove the first fatty acid from the fat, diglyceride lipase to remove the second one, and monoglyceride lipase to remove the third. Only LIPE is regulated and it appears to be the rate limiting reaction. Synthesis of fat starting with glycerol-3-phosphate requires action of acyl transferase enzymes to catalyze addition of fatty acids to the glycerol backbone.
    • 9.5: Fatty Acid Oxidation
      Breakdown of fats yields fatty acids and glycerol. Glycerol can be readily converted to DHAP for oxidation in glycolysis or synthesis into glucose in gluconeogenesis. Fatty acids are broken down in two carbon units of acetyl-CoA. To be oxidized, they must be transported through the cytoplasm attached to coenzyme A and moved into mitochondria. The latter step requires removal of the CoA and attachment of the fatty acid to a molecule of carnitine.
    • 9.6: Ketone body production and acidosis
      The primary catabolic pathway in the body is the citric acid cycle because it is here that oxidation to carbon dioxide occurs for breakdown products of the cell’s major building blocks - sugars, fatty acids, and amino acids. The pathway is cyclic and thus, does not really have a starting or ending point. All of the reactions occur in mitochondria, though one enzyme is embedded in the organelle’s inner membrane. Cells may use a subset of the reactions of the cycle to produce a desired molecule.

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