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Lipid Metabolism

Topic Review on "Title":

Lipid digestion and absorption
Lipids play an important role in cell structure and metabolism. TAGs are the major storage form of energy. Cholesterol is a component of cell membranes and precursor of steroid hormones. Lipid digestion occurs at lipid water interfaces since TAG is insoluble in water and digestive enzymes are water soluble. Lipids are digested and absorbed with the help of bile salts. Products of lipid digestion aggregate to form mixed micelles and are absorbed into the small intestine. Lipids are transported in the form of lipoproteins.

Fatty acid oxidation
Fatty acids have to be activated prior to their entry into mitochondrial matrix where the enzymes of β-oxidation of fatty acids are located. Activated fatty acids are then transported from cytosol to the mitochondrial matrix with the help of carnitine transporter. Total net yield of ATP per molecule of palmitic acid is 129. Similarly oxidation of unsaturated and odd chain fatty acids also take place with additional reactions. Β-oxidation in peroxisomes involves three enzymatic reactions. Minor pathways of oxidation such as α-oxidation of branched chain fatty acids and ω-oxidation of medium and long chain fatty acids in microsomes do take place in our body.
 
Ketone body metabolism
Ketone bodies are acetoacetate, β-hydroxy butyrate and acetone. Ketone bodies are synthesized in the liver but they are utilized by extra hepatic tissues as fuels. Ketone bodies are accumulated in the blood if the rate of synthesis exceeds the ability of extra hepatic tissues to utilize them. This leads to excess ketone bodies in blood, excretion of ketone bodies in urine and smell of acetone in breath. All these three together are known as ketosis. In uncontrolled diabetes mellitus and starvation, ketone bodies are formed.

Fatty acid biosynthesis
The enzymes of synthesis are located in the cytoplasm. Acetyl CoA is the source of carbon units and NADPH provides reducing equivalents. ATP is the supplier of energy. Synthesis is not the reversal of oxidation. Dimer of fatty acid synthase takes part in fatty acid synthesis. TAGs are synthesized from fatty acyl CoA and glycerol 3-phosphate or dihydroxy acetone phosphate. TAG gets deposited in adipose tissue.

Cholesterol biosynthesis
80% of endogenous cholesterol is formed in the liver. Enzymes involved in the synthesis are partly located in the endoplasmic reticulum and partly in the cytoplasm. Acetyl CoA is the precursor. HMG CoA, mevalonate, isopentenyl pyrophosphate, squalene are some of the important intermediate compounds formed during cholesterol synthesis. Cholesterol is transported in lipoprotein complexes. Elevation of lipids in blood leads to the deposition of cholesterol on the arterial walls leading to atherosclerosis.

Eicosanoids
Prostaglandins are synthesized from arachidonic acid. Cycloxygenase enzyme catalyzes the reaction in which C20 fatty acid is converted to prostaglandins. Leukotrienes are also derived from arachidonic acid.

Synthesis of phospholipids and sphingolipids
Glycerol is the starting material in the synthesis of glycerophospholipids. CDP-diacylglycerol is the activated intermediate of this pathway. Sphingolipids are synthesized from palmitoyl CoA and serine, initially to form ceramide. Sphingomyelins and glycolipids are synthesized from ceramide. Various sphingolipids storage diseases are resulted due to hereditary absence of hydrolytic enzymes.


Rapid Study Kit for "Title":
Flash Movie Flash Game Flash Card
Core Concept Tutorial Problem Solving Drill Review Cheat Sheet

"Title" Tutorial Summary :

Lipids are digested and absorbed with the help of bile salts. Products of lipid digestion aggregate to form mixed micelles and are absorbed into the small intestine. Lipids are transported in the form of lipoproteins. Fatty acids are activated, transported across mitochondrial membrane with the help of carnitine transporter. β -oxidation of saturated fatty acids takes place in the mitochondrial matrix. Similarly oxidation of unsaturated and odd chain fatty acids also take place with additional reactions. Ketone bodies are formed in the liver but they are utilized by extra hepatic tissues. In uncontrolled diabetes mellitus and starvation, excessive ketone bodies are formed, leading to ketosis. Fatty acid biosynthesis takes place in the cytosol of cells. Fat gets deposited in the adipose tissue. Acetyl Coenzyme A is the precursor of fatty acid synthesis as well as cholesterol biosynthesis. Elevation of lipids in blood leads to deposition of cholesterol plaques in the arterial walls leading to atherosclerosis. Prostaglandins and leukotrienes are synthesized from twenty carbon unsaturated fatty acids. Phosphatidic acid is an important intermediate in the synthesis of glycerophospholipids. In sphingolipids, sphingosine is present as an alcohol.


Tutorial Features:
  • Concept map to explain the concept of lipid metabolism.
  • Lipid absorption concept is animated and explained.
  • Flowcharts for oxidation and synthesis in a simplified manner.
  • Normal range of important lipid parameters
  • Lipid storage diseases are listed.

"Title" Topic List:

Lipid Digestion and Absorption

  • Introduction
  • Digestion
  • Absorption
  • Lipid transport

Fatty Acid Oxidation

  • Fatty acid activation
  • Fatty acid transport across mitochondrial membrane
  • β -oxidation of saturated fatty acids
  • Oxidation of unsaturated and odd chain fatty acids
  • Peroxisomal β-oxidation
  • Minor pathways of fatty acid oxidation

Ketone Body Metabolism

  • Ketogenesis
  • Ketolysis
  • Ketosis

Fatty Acid Biosynthesis

  • Introduction
  • Fatty acid synthase
  • Transport of acetyl CoA to cytosol
  • Fatty acid biosynthesis
  • Synthesis of triacylglycerol

Cholesterol Metabolism

  • Cholesterol biosynthesis
  • Regulation of cholesterol synthesis
  • Cholesterol transport
  • Cholesterol utilization

Eicosanoids

  • Prostaglandin synthesis
  • Leukotriene synthesis

Synthesis of Phospholipids and Sphingolipids

  • Biosynthesis of  phospholipids
  • Biosynthesis of sphingolipids


See all 24 lessons in college chemistry, including concept tutorials, problem drills and cheat sheets:
Teach Yourself Biochemistry Visually in 24 Hours

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