DEFINITIONS OF TERMS Homeostasis The tendency toward a relatively stable equilibrium that is maintained by physiological processes. For a complete list of defined terms, see the Glossary. LESSON 2.2 WORKBOOK Metabolism: Glucose is the middleman for ATP In Lesson 2.1 we discussed the digestion and absorption of nutrients. After the nutrients are absorbed, what are they used for? In this lesson we will discuss the process by which energy is made from the macronutrients. We will identify the steps in glucose metabolism that are important in the production of ATP, and explore why it is so important to maintain blood glucose homeostasis. Metabolism makes the macronutrients useful In Lesson 2.1 we learned that the monomer forms of the macronutrients are the forms that can be absorbed in the small intestine. Once the macronutrients reach the liver, metabolism takes those monomers and breaks them down into even simpler forms that have two main functions: they can become building blocks for cellular structures or they can be used to make the ATP that cells use for energy. Macronutrients are metabolized in the liver Glucose After being absorbed in the small intestines, carbohydrates, lipids and proteins travel in the blood to the liver. Over the next series of lessons you will see that the liver is the master regulator of metabolism. The liver is often referred to as the biochemist of the body because it can perform chemical reactions that other tissues cannot. For example, the liver stores glucose as glycogen, repackages fatty acids for storage and makes new amino acids. The liver is also the primary organ for making new glucose, which is a very important jo In fact, the liver is the only organ that can make macronutrients from other macronutrients. Because of this, the liver plays a central role in maintaining a constant concentration of glucose in the blood stream. This is important because the brain and the red blood cells can only use glucose for energy! 72
DEFINITIONS OF TERMS ATP The molecular unit of energy used by all cells in the body. For a complete list of defined terms, see the Glossary. The energy in macronutrients is shuffled through many forms to generate ATP The release of energy from macronutrients involves breaking chemical bonds because that is where the energy is stored. Different types of bonds contain different amounts of energy, remember it is the combustible energy in the macronutrients that is used to derive calories. As bonds in the macronutrients are broken the energy is released and recaptured into a new chemical bond that is the universal fuel source of all cells, ATP. So, the end goal of metabolism is to put the energy from the bonds of the macronutrients into the energy in the bonds of ATP. ATP is the energy source for our cells For the energy in macronutrients to be used in cellular activities the combustible energy in the carboncarbon bonds is transferred to a compound called adenosine triphosphate (ATP). ATP and its related compounds ADP (adenosine diphosphate) and AMP (adenosine monophosphate) are the key energyproducing molecules used by cells. As shown in Figure 1, one molecule of ATP consists of the nucleoside adenosine and three phosphate groups. The phosphate groups are negatively charged and don t like being close to one another, therefore the bond connecting the phosphates has a lot of energy. You can think of this like magnets: if you have two strong magnets and hold them close to each other they ll either snap together or repel one another. Now imagine the strength it takes to force the magnets together while they are repelling each other this is what the bonds holding the phosphate groups together is doing. These bonds are extremely high energy, and when they break energy is released. Cells use the high energy in the phosphate bonds to catalyze vast numbers of enzymatic reactions required for life. For example, energy from ATP is required to make new amino acids in the liver. Adenosine! High Energy Phosphate Bonds! Phosphate Groups! Figure 1: The molecular structure of ATP: the energy of ATP is stored in the bonds connecting the phosphate groups. When those bonds are broken, energy is released. 1. The type of energy stored in chemical bonds is: Thermal energy. Electrical energy. c. Potential energy. d. Combustible energy. 73
DEFINITIONS OF TERMS Acetyl CoA A molecule that shuttles carbons to the citric acid cycle. Citric acid cycle A cycle of reactions used to generate energy that takes place in the mitochondri Any cell that has mitochondria can do this. Electron transport chain A chain of proteins that transfers protons from hydrogen across a membrane, keeping them separate from electrons. The energy released at the end of the chain is used to generate ATP. Glycolysis The breakdown of glucose to produce energy. NADH A molecule that shuttles hydrogens into the electron transport chain. Pyruvate A molecule created from glucose and some amino acids. For a complete list of defined terms, see the Glossary. From glucose to ATP How does the energy from ATP relate to the calories in a food? When the macronutrients we eat are digested down to sugars, amino acids and fatty acids they can then be used to produce ATP, which is maintained in all cells until needed. The homeostasis of blood glucose is important because ATP can t travel in the blood, so glucose is the middleman passed between cells and used to synthesize ATP. Every cell in the body conducts glycolysis, a process used to convert glucose to acetyl CoA, and almost every cell can then use the acetyl CoA in the citric acid cycle and the electron transport chain to make more ATP. Additionally, only particular organs can use fatty acids or amino acids to produce ATP, while all cell types use glucose. Don't be alarmed by all the new terms here, we will be seeing them again! Mitochondria are the energy factories of the cell You may remember that mitochondria are organelles located within cells. What is important about the mitochondrion is that it has two membranes: an inner membrane and an outer membrane. Because of this, the mitochondria can keep steps Inner Membrane! Outer Membrane! Figure 2: By having both an outer and an inner membrane, the mitochondria can keep certain molecules and reactions separate. Glycolysis: Converting glucose to acetyl CoA in metabolism separated. We will see how separating molecules is important more when we discuss the electron transport chain. The mitochondrion is the location for the citric acid cycle, the electron transport chain and breakdown of fatty acids for energy, meaning that every cell that has mitochondria can participate in those reactions. Red blood cells do not contain mitochondria, and therefore can only use glycolysis for energy. The first steps of glucose metabolism occur in the cytosol of cells, and are called glycolysis (glycolysis means breaking down glucose ). As you can see on the next page in Figure 3, the six carbons (shown in blue) of glucose are converted into two pyruvate molecules, each containing three carbons. Because energy is stored in the carbon bonds, breaking the glucose into two pieces releases energy, resulting in the production of two molecules of ATP. Glycolysis also produces a molecule called NADH, an energy intermediate used to make ATP in the electron transport chain. 2. ATP is: Generated in the mitochondri Used by all cells except the brain and red blood cells. c. Absorbed from the foods that we eat. d. Easily transported in the blood. 74
The two pyruvates are shuttled into the mitochondria where one more carbon is removed, resulting in acetyl CoA and another NADH. As we will see in this unit, acetyl CoA is an important molecule that is at the crossroads of glucose metabolism, fatty acid metabolism and amino acid metabolism. What does breathing have to do with metabolism? As carbons are removed in metabolism, they exit the cell as carbon dioxide (CO 2 ), and we eventually breathe it out. If we were to hold our breath for too long, the CO 2 would build up in our blood and become toxic. Additionally, breathing brings in fresh oxygen that is needed for metabolism to occur. The citric acid cycle and the electron transport chain are both aerobic processes, meaning that they require oxygen. In times when we are not breathing quickly enough, like in exercise, the citric acid cycle cannot occur. Our brain is the organ that is the most sensitive to oxygen deprivation, and even a short time without breathing in oxygen can have detrimental effects to our nervous system. Acetyl CoA is shuttled into the citric acid cycle in the mitochondria Figure 4: NADH and FADH 2 work like delivery trucks, shuttling hydrogen from water to the electron transport chain. Glycolysis! Carbon! Energy Released! Glucose! Pyruvate! Acetyl CoA! Citric Acid Cycle! Figure 3: In glycolysis glucose is converted into acetyl CoA, which is transported into the mitochondria where it goes into the citric acid cycle. The major way energy released by glucose metabolism intersects with the electron transport chain is via the citric acid cycle, which takes place in the mitochondri When acetyl CoA enters the mitochondria and participates in the citric acid cycle, it first joins with four other carbons (see Figure 3 above). Subsequently, the carbon bonds are broken down releasing energy. That energy is used to transfer a hydrogen ion from water to NAD +, forming NADH, as shown in Figure 5. Similarly, two hydrogen ions can be added to FAD to make FADH 2. NADH and FADH 2 work like delivery trucks, taking hydrogen from water and delivering to the next step in metabolism: The electron transport chain. 3. Acetyl CoA is: Made out of carbons and a B-vitamin. The molecule that connects glucose, amino acid, and fatty acid metabolism. c. The form in which energy is stored in cells. d. Both A and B. e. All of the above. 75
At the end of one turn of the citric acid cycle, 3 NADH, 1 FADH 2 and only 1 ATP are produced. The NADH and FADH 2 are then used in the electron transport chain to produce 9 additional ATP. The electron transport chain is critically important because it provides 90% of the ATP obtained from the metabolism of glucose. Inner Membrane! H + H + H + H + - Outer Membrane! - - - Figure 5: Energy is transferred from the bonds holding carbons together to the bond attaching hydrogen to NAD +. ADP! ATP! Figure 6: The positive protons of hydrogen (H + ) and the negative electrons ( - ) are kept separate in the electron transport chain until the end, where the protons go down their energy gradient to make ATP, using the enzyme ATP synthase. Final steps: The electron transport chain Each hydrogen that is added to NAD + and FAD contains one proton and one electron. The positively charged proton, and the negatively charge electron are highly attracted to one another, and prefer to be kept close. In the electron transport chain the protons and electrons from the hydrogen are split apart and kept separate by the membranes of the mitochondria, as shown in the figure to the left. Once separated the attraction of the proton and electron create a 'pressure'. We can again use the idea of magnets to think of the energy in the electron transport chain. The negative electrons and positive protons are like strong magnets that are attracted to one another. A great amount of effort is required to separate the two, and when you bring them near one another they snap back together. The inner mitochondrial membrane is what is used to keep the protons and electrons separated until the last step of the electron transport chain, where the protons are allowed to travel through a channel in an enzyme called ATP synthase. NAD 1) Carbons bonded together" 2) A bond is broken, releasing energy" 3) That energy is used to remove a hydrogen from water and place it on NAD+" 4. Which of the following is NOT true about the citric acid cycle? It takes place in the mitochondri It requires oxygen. c. It creates NADH and FADH 2. d. All cells can do it. 5. Which of the following would prevent the electron transport chain from functioning? If ATP levels were too low. If no glucose was available to the liver. c. If the protons were not kept seperate from the electrons. d. If too much NADH is produced. 76
DEFINITIONS OF TERMS Brown adipose tissue A type of adipose tissue that is abundant in newborns and hibernating mammals. It generates body heat in animals that do not shiver. For a complete list of defined terms, see the Glossary. Figure 7: Protons move though ATP synthase and drive the production of ATP like a turbine collects energy from water flowing through a hydroelectric power plant. ATP synthase is an enzyme that can harness the energy released as the protons and electrons are allowed to rejoin, and uses that energy to add a phosphate group onto ADP, creating new ATP. You can think of ATP synthase as a water turbine: the protons trapped in the inter-membrane space are like water trapped in a dammed off lake. Once the dam is opened, the water flows out of the lake and moves a water turbine. This movement of the water turbine generates electricity for us to use. Similarly, as the protons are allowed to flow through the ATP synthase channel the energy actually rotates a part of ATP synthase, generating energy to make new ATP. Brown fat tissue uses the electron transport chain to keep us warm! If the energy that is released as the protons flow back to their electrons is enough to maintain the energy needs of our cells, what would happen if the energy was not redirected into ATP? There is a type of fat tissue called brown adipose tissue that is highly concentrated with mitochondri In these mitochondria a separate channel exists other than ATP synthase that lets protons rejoin the electrons. Instead of producing ATP, this channel lets the energy be released as heat, raising body temperature. Babies have higher concentrations of brown adipose tissue than adults, and are used to keep the infants warm. Additionally, people living in colder climates may have more brown adipose tissue than people in warm climates. Figure 8: People living in cold climates have extra brown fat to keep the body warm. How the liver keeps blood glucose levels constant ATP cannot be transported in the blood, so each cell in your body must produce its own ATP. To do this, a constant supply of glucose must be available in the blood, especially for the brain and the red blood cells that can only use glucose for energy! The liver can supply new glucose to the blood, and the muscles and adipose tissue can provide amino acids and fatty acids. 77
Acetyl CoA shuttles carbons between cycles If glucose levels were low and no glucose became available in the blood your brain would shut down! The body must call upon other nutrients fats and proteins, to supply the glucose or glucose-like compounds it needs. Conversely, in times of plenty the body stores away excess nutrients as fat and as protein to call upon later. At the crossroads of this whole process is acetyl CoA. Glucose, fatty acids and amino acids all interact with acetyl CoA, the consequences of which will depend on the energy requirements of the cell at that time. Acetyl CoA can either direct metabolites into energy producing, or energy storing, pathways. Releasing energy from fat Just as cells release energy from carbohydrates and trap it as ATP, they also release and trap energy from triglycerides. Recall that triglycerides are molecules formed by the combination of fatty acids and glycerol. During periods of low calorie intake or fasting the triglycerides are broken down into fatty acids and glycerol. Carbons are cut off of fatty acids, two at a time, and can be made into acetyl CoA. Acetyl CoA then enters the citric acid cycle to produce NADH and FADH 2 to be used in the electron transport chain as discussed above. Because fatty Acetyl CoA! Citric Acid Cycle! Fatty Acid (Palmitate)! Amino Acid! x 8! Carbon! Amino Group! Energy Released! Figure 10: Fatty acids provide acetyl CoA for the citric acid cycle, but completing the cycle also required amino acids. Figure 9: Acetyl CoA is the crossroads between glucose, fatty acids and amino acids. acids have a lot more carbons than a molecule of glucose, they can be used to make more ATP than glucose can. Additionally, the process of breaking off the 2-carbon fragments from fatty acids releases one NADH and one FADH 2 molecule, so for every 2-carbon fragment that is shuttled through the citric acid cycle, the total yield is 14 ATP. Palmitate is a common fatty acid found in palm oil, and contains 16 carbons (Figure 10). Therefore, metabolism of palmitate would yield 108 molecules of ATP! 6. Glucose is the only type of nutrient used to release energy: True. False. 78
Releasing energy from amino acids Protein is the other energy-containing macronutrient, however it is rarely used to produce energy. Amino acids are usually used to produce the proteins the body needs. In starvation when glycogen stores are depleted, some amino acids can be used to make energy by breaking apart their carbon bonds to make pyruvate, acetyl CoA, or other intermediates in the citric acid cycle, depending on the shape of the amino acid. Figure 11 shows some amino acids being used in the citric acid cycle. How are the micronutrients involved? Carbon! Amino Group! Energy Released! Amino Acid! Citric Acid Cycle! Figure 11: Amino acids can enter the citric acid cycle once their amino groups are removed. Vitamins and minerals have numerous functions in the human body, ranging from bone structure to immune function. They also play an important role in metabolism. Vitamins and minerals are required for many metabolic pathways. For example, the B-vitamins (thiamin, riboflavin, niacin, pantothenic acid, biotin, vitamin B6, folate, and vitamin B-12) as well as iron and copper are required for acetyl CoA to function. Some of the key players in metabolism that we discussed today are made from the B-vitamins: the CoA part of acetyl CoA is synthesized from pantothenic acid, NADH is made from fiacin, and FADH 2 is made from riboflavin. People that have a deficiency in one or more of these vitamins cannot readily make ATP, and can become fatigued and have neurological disorders. 79
STUDENT RESPONSES Dinitrophenol (DNP) is a chemical that was sold as a diet pill in the first half of the 1900 s. DNP prevents the mitochondria from being able to separate protons and electrons. Why would this cause someone to lose weight? Can you guess why this drug is no longer allowed? Remember to identify your sources 80
TERMS For a complete list of defined terms, see the Glossary. TERM Acetyl CoA Brown Adipose Tissue Citric Acid Cycle Electron Transport Chain Glycolysis Homeostasis NADH Pyruvate DEFINITION A molecule that shuttles carbons to the citric acid cycle. A type of adipose tissue that is abundant in newborns and hibernating mammals. It generates body heat in animals that do not shiver. A cycle of reactions used to generate energy that takes place in the mitochondri Any cell that has mitochondria can do this. A chain of proteins that transfers protons from hydrogen across a membrane, keeping them separate from electrons. The energy released at the end of the chain is used to generate ATP. The breakdown of glucose to produce energy. The tendency toward a relatively stable equilibrium that is maintained by physiological processes. A molecule that shuttles hydrogens into the electron transport chain. A molecule created from glucose and some amino acids. 81