For every 1 g of glycogen, 2 grams of water is H-bonded to it. Hence it would take 3 times more weight to store the equivalent amount of energy in carbohydrates as is stored in triacylglyceride, which are stored in anhydrous lipid "drops" within cells. The rest of this unit on lipids will focus not on triacylglycerides, whose main function is energy storage, but on fatty acids and phospholipids, and the structures they form in aqueous solution.
The structure of fatty acids and phospholipids show them to amphiphilic - i. Fatty acids can be represented in "cartoon-form" as single chain amphiphiles with a circular polar head group and a single acyl non-polar tail extending from the head.
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Likewise, phospholipids can be shown as double chain amphiphiles. Even through there are a very large number of fatty acids which can be esterified to C1 and C2 of phospholipids and a variety of P-X groups at C3, making the phospholipids and fatty acids extremely heterogeneous groups of molecules, their role in biological structures can be understood to a first approximation by modeling them either as single or double chain amphiphiles. In addition, they, in contrast to carbohydrates, amino acids, and nucleotides, do not form covalent polymers.
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Hence we will start our studies of biological molecules with simple lipids fatty acids, glycerophospholipids and sphingolipids and then apply our understanding of lipids to the more complex systems of biological polymers. We will see that glycerophospholipids and sphingolipids are essential components of membrane structure. Cholesterol is also found in membranes and is a precursor of steroid hormones. John's University. Properties of Lipids The structure of lipids determines their function. Contributors Prof. Although lipogenesis occurs in the cytoplasm, the necessary acetyl CoA is created in the mitochondria and cannot be transported across the mitochondrial membrane.
To solve this problem, pyruvate is converted into both oxaloacetate and acetyl CoA. Two different enzymes are required for these conversions. Oxaloacetate forms via the action of pyruvate carboxylase, whereas the action of pyruvate dehydrogenase creates acetyl CoA.
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Oxaloacetate and acetyl CoA combine to form citrate, which can cross the mitochondrial membrane and enter the cytoplasm. In the cytoplasm, citrate is converted back into oxaloacetate and acetyl CoA. Oxaloacetate is converted into malate and then into pyruvate. Pyruvate crosses back across the mitochondrial membrane to wait for the next cycle of lipogenesis.
The acetyl CoA is converted into malonyl CoA that is used to synthesize fatty acids.
Figure 6 summarizes the pathways of lipid metabolism. Lipids are available to the body from three sources.
They can be ingested in the diet, stored in the adipose tissue of the body, or synthesized in the liver. Fats ingested in the diet are digested in the small intestine.
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The triglycerides are broken down into monoglycerides and free fatty acids, then imported across the intestinal mucosa. Once across, the triglycerides are resynthesized and transported to the liver or adipose tissue. If excess acetyl CoA is created and overloads the capacity of the Krebs cycle, the acetyl CoA can be used to synthesize ketone bodies. When glucose is limited, ketone bodies can be oxidized and used for fuel. Excess acetyl CoA generated from excess glucose or carbohydrate ingestion can be used for fatty acid synthesis or lipogenesis.
Acetyl CoA is used to create lipids, triglycerides, steroid hormones, cholesterol, and bile salts. Lipolysis is the breakdown of triglycerides into glycerol and fatty acids, making them easier for the body to process.
Skip to content Increase Font Size. Chapter Metabolism and Nutrition. Learning Objectives By the end of this section, you will be able to: Explain how energy can be derived from fat Explain the purpose and process of ketogenesis Describe the process of ketone body oxidation Explain the purpose and the process of lipogenesis. Review Questions 1. Which molecule produces the most ATP?
Introduction to Fatty Acids and Lipids - Abstract - Intravenous Lipid Emulsions - Karger Publishers
Which molecules can enter the Krebs cycle? Critical Thinking Questions 1. Lipid , any of a diverse group of organic compounds including fats , oils , hormones , and certain components of membranes that are grouped together because they do not interact appreciably with water. One type of lipid, the triglycerides , is sequestered as fat in adipose cells , which serve as the energy-storage depot for organisms and also provide thermal insulation.
Some lipids such as steroid hormones serve as chemical messengers between cells , tissues , and organs , and others communicate signals between biochemical systems within a single cell. The membranes of cells and organelles structures within cells are microscopically thin structures formed from two layers of phospholipid molecules. Membranes function to separate individual cells from their environments and to compartmentalize the cell interior into structures that carry out special functions.
So important is this compartmentalizing function that membranes, and the lipids that form them, must have been essential to the origin of life itself. Water is the biological milieu—the substance that makes life possible—and almost all the molecular components of living cells, whether they be found in animals, plants, or microorganisms, are soluble in water. Some lipids are amphipathic—part of their structure is hydrophilic and another part, usually a larger section, is hydrophobic. Amphipathic lipids exhibit a unique behaviour in water: they spontaneously form ordered molecular aggregates , with their hydrophilic ends on the outside, in contact with the water, and their hydrophobic parts on the inside, shielded from the water.
This property is key to their role as the fundamental components of cellular and organelle membranes.