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Physiology 2130
Anita Woods

Metabolism Section 16.1 – Objectives By the end of this section you should be able to: • List the building blocks of all the main food groups as well as their storage forms in the body. • Describe the three chemical reactions/metabolic pathways in the body that produceATP. List how muchATP each can form, which one requires oxygen, and where the reactions take place in the cell. • Describe where each fuel source enters the metabolic pathways. • Describe in detail the full metabolism of glucose by glycolysis and the citric acid cycle (in the presence of oxygen and without oxygen). • Describe in detail the full metabolism of fats and amino acids. • Explain all the processes involved in the fed state of metabolism and how insulin is involved in this process and all its functions. • Explain all the processes involved in the fasted state of metabolism and how glucagon is involved in this process and all its functions. • List all the other hormones that help to regulate metabolism and explain how they perform their function. • Explain what would happen to digestion and metabolism if you had not eaten for an extended period of time and were in a very stressful situation. List all the hormones that would be involved. This will require you to combine everything you know about digestion and metabolism. Section 16.2 – Introduction • In the last section, we saw how food is broken down and then absorbed into the circulatory system. It is now time to see what happens to it next. • Essentially, the fats, carbohydrates, and proteins are used for energy or to build structures. • Metabolism:  Defined as the chemical changes that occur within a cell to either manufacture energy, store energy, or build new cell structures.  The two types of metabolism are listed below • You should remember that the energy molecule used by the body is called adenosine triphosphate, or ATP. Section 16.3 – The Building Blocks 1 • Recall that all of the food we saw in the digestive module is made up of smaller building blocks. • Triglycerides:  Make up most of the fat found in the body  Consist of three fatty acid chains attached to one molecule of glycerol. • Proteins:  Consist of long chains of amino acids linked together  Carbohydrates are long chains of monosaccharides (like glucose) joined together. • We have seen how these larger molecules are broken down in the digestive tract and then absorbed. • Once in the body, some may partially reform while others stay as building blocks. • These building blocks are circulating in the body and can be used to create new structures in the cell or to formATP, or they can be stored for later use. • Their storage form may not be the same as their circulating form. • The carbohydrate glucose:  Stored inside muscle and liver cells in the form of glycogen. • Fats:  Stored as triglycerides inside fatty tissue called adipose tissue. • The amino acids become structural or functional proteins inside the muscle cells. Section 16.4 – The Building Blocks: Energy Production • All of the molecules we have just seen can be used to produce energy in the form of ATP. 2 • Glucose:  Contributes to 1% of the total energy requirements of the body, and its reserves (in the form of glycogen) can last for roughly a day. • Fatty acids:  Contribute to 77% of the total energy produced and can last for up to two months depending on the individual. • Amino acids:  Stored as proteins are not usually used to create energy but can account for 22% of the body’s fuel requirements if absolutely necessary. Section 16.5 – Metabolism:AGeneral Look • There are three primary chemical reactions that take place within the cell to produce energy from the breakdown of food molecules. • Glycolysis (or the glycolytic pathway):  Is a series of reactions that occurs in the cytoplasm  It does not require oxygen and is therefore considered an anaerobic reaction.  Glycolysis can produce two molecules ofATP very quickly from one molecule of glucose. • The citric acid cycle (CAC, or Krebs cycle):  This reaction, which takes place inside the mitochondria, requires the presence of oxygen and therefore is an aerobic reaction.  It can produce two molecules ofATP per molecule of glucose. 3 Section 16.6 – Metabolism:AGeneral Look (cont’d) • Oxidative phosphorylation:  Sometimes referred to as the electron transport chain  Can produce 34 molecules ofATP from one molecule of glucose.  It also occurs in the mitochondria of the cell and also requires oxygen. • Notice that all of these equations are linked together through various intermediate steps that we will examine shortly. 4 Section 16.7 – Metabolism:AGeneral Look (cont’d) • Each of the food molecules can enter one or more of these reactions to make energy (ATP). • For example: • Glucose (as well as its storage form glycogen):  Can enter glycolysis at the beginning of this reaction. • Amino acids (AA):  Can be converted to pyruvate to enter glycolysis or can be converted to acetyl coenzyme A (acetyl CoA) to enter the citric acid cycle (CAC) to produceATP. • Fats (triglycerides):  Can be broken down to glycerol and free fatty acids. • The glycerol molecules:  Can enter glycolysis, and the • Fatty acids:  Can be converted to acetyl CoAto enter the CAC. 5 Section 16.8 – The Metabolism of Glucose • As we saw in the digestive system, glucose is a monosaccharide, and is stored as glycogen in most cells of the body.  However, most glycogen stores can be found in the liver and skeletal muscle. • Glucose is a common fuel source for all the cells in the body, but it is one of the only fuel sources for the brain – unlike most other cells in the body, which can use fats and amino acids as well as glucose. • The liver stores glucose for the brain, which cannot store any for when blood glucose levels get low. 6 Section 16.9 – The Metabolism of Glucose: Glycolysis • Glucose will enter a cell and will be almost immediately converted to glucose-6- phosphate (G-6-P). • From here the G-6-P can enter glycolysis to produceATP or it can be converted to glycogen and then stored. • When necessary, glycogen can be converted back to G-6-P to enter glycolysis to produce energy. Section 16.10 – The Metabolism of Glucose: Glycolysis (cont’d) • In glycolysis, G-6-P will undergo a series of reactions that will result in the production of ATP and the end product of pyruvate. • The pyruvate can then undergo two possible reactions: 1. It can either enter the citric acid cycle (CAC) and produce lots ofATP (with the help of oxidative phosphorylation) or 2. It can enter another shorter reaction to produce a small amount ofATP and the by-product lactate (also called lactic acid). • Which reaction will pyruvate take?  That depends on whether oxygen is present or not.  If there is sufficient oxygen (O 2, then most of the pyruvate will enter the CAC.  Remember that since it is an aerobic pathway, it requires O .2  If there is not enough O p2esent, then the CAC will not be running at full capacity and the pyruvate will be converted to lactic acid. Section 16.11 – The Metabolism of Glucose: LacticAcid 7 • During strenuous exercise, your cardiovascular system may not be able to supply sufficient oxygen to your working muscle cells to produceATP via the citric acid cycle (CAC). • However, since glycolysis is anaerobic, this reaction can continue without oxygen to produceATP and the end product pyruvate. • With insufficient oxygen, the CAC will not work at full capacity and pyruvate will start to accumulate. o If too much pyruvate accumulates, then even glycolysis will be slowed. • In order to keep glycolysis working so it can produceATP for the muscles, the pyruvate will be converted to lactic acid. o The accumulation of lactic acid causes the "burning" sensation in the muscle and is believed to interfere with the contractile proteins causing fatigue. • You should recall that the accumulation of lactic acid will cause the blood vessels to dilate and the decreasing pH will unload more oxygen from hemoglobin. • These mechanisms will help to increase blood flow and increase oxygen delivery to this working tissue to reduce the buildup of lactic acid. Section 16.12 – The Metabolism of Glucose: LacticAcid (cont’d) • It is very important to note that most of the reactions we have seen so far are reversible – that is, they can work in both directions.  For example: If oxygen is restored, lactic acid can be converted back to pyruvate to enter the citric acid cycle.  Also, pyruvate can be turned back into glycogen, through G-6-P, and stored for later use. 8 Section 16.13 – The Metabolism of Glucose: The CitricAcid Cycle • If there is a sufficient supply of oxygen, then the citric acid cycle (CAC) can function at full capacity. • The pyruvate from glycolysis is converted to acetyl coenzymeA(acetyl CoA), which then enters the CAC. • As we mentioned earlier, the CAC in conjunction with oxidative phosphorylation together produce a total of 36 molecules ofATP from one molecule of glucose (2 ATP from CAC and 34 from oxidative phosphorylation). • They also produce the by-products CO2 and H2O.  The CO2 will diffuse into the blood and leaves the body at the lungs. 9 Section 16.14 – The Metabolism of Glucose: Review • Glucose enters the cell where it can be converted to its storage form of glycogen, or it can enter the glycolytic pathway where it will produceATP and pyruvate. • Without sufficient oxygen, the pyruvate will be converted to lactic acid while producingATP. • In the presence of oxygen, the pyruvate will enter the citric acid cycle to produce large quantities of ATP while giving off CO2 and H2O. Section 16.15 – The Metabolism of Fats andAminoAcids • Amino acids are stored in cells as proteins • Fats are mostly stored as triglycerides. • These molecules can also be metabolized to createATP by cells, but each one enters the metabolic pathways at different points. • Amino acids:  Can be converted to either pyruvate or acetyl CoA to enter glyc
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