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Glycolysis - Advanced Douglas Wilkin, Ph.D. Barbara Akre Say Thanks to the Authors Click http://www.ck12.org/saythanks (No sign in required) To access a customizable version of this book, as well as other interactive content, visit www.ck12.org CK-12 Foundation is a non-profit organization with a mission to reduce the cost of textbook materials for the K-12 market both in the U.S. and worldwide. Using an open-source, collaborative, and web-based compilation model, CK-12 pioneers and promotes the creation and distribution of high-quality, adaptive online textbooks that can be mixed, modified and printed (i.e., the FlexBook® textbooks). Copyright © 2016 CK-12 Foundation, www.ck12.org The names “CK-12” and “CK12” and associated logos and the terms “FlexBook®” and “FlexBook Platform®” (collectively “CK-12 Marks”) are trademarks and service marks of CK-12 Foundation and are protected by federal, state, and international laws. Any form of reproduction of this book in any format or medium, in whole or in sections must include the referral attribution link http://www.ck12.org/saythanks (placed in a visible location) in addition to the following terms. Except as otherwise noted, all CK-12 Content (including CK-12 Curriculum Material) is made available to Users in accordance with the Creative Commons Attribution-Non-Commercial 3.0 Unported (CC BY-NC 3.0) License (http://creativecommons.org/ licenses/by-nc/3.0/), as amended and updated by Creative Commons from time to time (the “CC License”), which is incorporated herein by this reference. Complete terms can be found at http://www.ck12.org/about/ terms-of-use. Printed: October 6, 2016 AUTHORS Douglas Wilkin, Ph.D. Barbara Akre www.ck12.org C HAPTER Chapter 1. Glycolysis - Advanced 1 Glycolysis - Advanced • Recognize that glycolysis is the first and most universal of three stages in cellular respiration. • Explain why biologists consider glycolysis to be one of the oldest energy production pathways. • Describe how some of the energy in glucose is transferred to ATP in the cytoplasm, without oxygen. How do you slice a molecule of glucose in half? With sharp knives? Not really. But you lyse it with enzymes during a process named glycolysis. Glucose is sliced right in half from a 6-carbon molecule to two 3-carbon molecules. This is the first step and an extremely important part of cellular respiration. It happens all the time, both with and without oxygen. And in the process, transfers some energy to ATP. Glycolysis: A Universal and Ancient Pathway for Making ATP When was the last time you enjoyed yogurt on your breakfast cereal, or had a tetanus shot? These experiences may appear unconnected, but both relate to bacteria which do not use oxygen to make ATP. In fact, tetanus bacteria cannot 1 www.ck12.org survive if oxygen is present. However, Lactobacillus acidophilus (bacteria which make yogurt) and Clostridium tetani (bacteria which cause tetanus or lockjaw) share with nearly all organisms the first stage of cellular respiration, glycolysis (Figure 1.1). Because glycolysis is universal, whereas aerobic (oxygen-requiring) cellular respiration is not, most biologists consider it to be the most fundamental and primitive pathway for making ATP. FIGURE 1.1 Clostridium tetani bacteria are obligate anaerobes, which cannot grow in the presence of oxygen and use a variation of glycolysis to make ATP. Because they can grow in deep puncture wounds and secrete a toxin, which can cause muscle spasms, seizures, and death, most people receive tetanus vaccinations at least every ten years throughout life. Return to the overall equation for cellular respiration: Like photosynthesis, the process represented by this equation is actually many small, individual chemical reactions. We grouped the reactions of photosynthesis into two stages, the light reactions and the Calvin Cycle. We will divide the reactions of cellular respiration into three stages: glycolysis, the Krebs Cycle, and the electron transport chain (Figure 1.2). In this concept, Stage 1, glycolysis, the oldest and most widespread pathway for making ATP, is discussed. Before diving into the details, we must note that this first stage of cellular respiration is unique among the three stages: it does not require oxygen, and it does not take place in the mitochondrion. The chemical reactions of glycolysis occur without oxygen in the cytosol of the cell (Figure 1.3). The name for Stage 1 clearly indicates what happens during that stage: glyco- refers to glucose, and -lysis means "splitting." In glycolysis, within the cytosol of the cell, a minimum of eight different enzymes break apart glucose into two 3-carbon molecules. The energy released in breaking those bonds is transferred to carrier molecules, ATP and NADH. NADH temporarily holds small amounts of energy which can be used later to build ATP. The 3-carbon product of glycolysis is pyruvate, or pyruvic acid (Figure 1.4). (The difference between them is actually a sole hydrogen atom. Pyruvic acid: CH3 COCOOH, pyruvate: CH3 COCOO− .) Overall, glycolysis can be represented as: C6 H12 O6 + 2NAD+ + 2Pi + 2ADP → 2 pyruvate + 2NADH + 2ATP However, even this equation is deceiving. Just the splitting of glucose requires many steps, each transferring or capturing small amounts of energy. Individual steps appear in Figure 1.6. Studying the pathway in detail reveals that cells must "spend" or "invest" two ATP in order to begin the process of breaking glucose apart. Note that the phosphates produced by breaking apart ATP join with glucose, making it unstable and more likely to break apart. Later steps harness the energy released when glucose splits, and use it to build "hot hydrogens" (NAD+ is reduced to NADH) and ATP (ADP + Pi → ATP). If you count the ATP produced, you will find a net yield of two ATP per glucose (4 produced - 2 spent). Remember to double the second set of reactions to account for the two 2 www.ck12.org Chapter 1. Glycolysis - Advanced FIGURE 1.2 The many steps in the process of aerobic cellular respiration can be divided into three stages. The first stage, glycolysis, produces ATP without oxygen. Because this part of the cellular respiration pathway is universal, biologists consider it the oldest segment. Note that glycogen and fats can also enter the glycolysis pathway. The second stage is the Krebs Cycle, and the third stage is the electron transport chain. It is during the third stage that chemiosmosis produces numerous ATP molecules. 3-carbon molecules which follow that pathway! The "hot hydrogens" can power other metabolic pathways, or in many organisms, provide energy for further ATP synthesis. MEDIA Click image to the left or use the URL below. URL: https://www.ck12.org/flx/render/embeddedobject/184592 To summarize: In the cytosol of the cell, glycolysis transfers some of the chemical energy stored in one molecule of glucose to two molecules of ATP and two NADH. This makes (some of) the energy in glucose, a universal fuel molecule for cells, available to use in cellular work - moving organelles, transporting molecules across membranes, or building large organic molecules. Although glycolysis is universal, pathways leading away from glycolysis vary among species depending on the avail3 www.ck12.org FIGURE 1.3 Glycolysis, unlike the latter two stages of cellular respiration, takes place without oxygen in the cytosol (blue) of the cell. For many organisms, aerobic respiration continues with the Krebs cycle and the electron transport chain in the mitochondria (green). To enter the mitochondria, glucose must first be lysed into smaller molecules. ability of oxygen. If oxygen is unavailable, pyruvate may be converted to lactic acid or ethanol and carbon dioxide in order to regenerate NAD+ , ending anaerobic respiration. Anaerobic respiration is also called fermentation, which will be discussed in another concept. If oxygen is present, pyruvate enters the mitochondria for further breakdown, releasing far more energy and producing many additional molecules of ATP in the latter two stages of aerobic respiration - the Krebs cycle and electron transport chain. We will explore these, too, in a later section. Summary • The process of cellular respiration is actually many separate reactions, which can be divided into three stages: glycolysis, the Krebs Cycle, and the electron transport chain. • During glycolysis, glucose is split into two 3-carbon pyruvate molecules, using 2 ATP but generating 4 ATP, for a net gain of 2 ATP. • During glycolysis, 2 NADH are also produced. Review 1. List the three stages of cellular respiration, and contrast the first stage with the other two in terms of distribution throughout the living world, location within the cell, and use of oxygen. 2. Summarize the overall process of glycolysis, following both the path of carbon atoms and chemical energy. 3. What molecules can enter the glycolysis pathway, besides glucose? 4 www.ck12.org Chapter 1. Glycolysis - Advanced FIGURE 1.4 In glycolysis, glucose (C6) is split into two 3-carbon (C3) pyruvate molecules. This releases energy, which is transferred to ATP. How many ATP molecules are made during this stage of cellular respiration? FIGURE 1.5 Glycolysis breaks the 6-carbon molecule glucose into two 3-carbon pyruvate molecules, releasing some of the chemical energy which had been stored in glucose. References 1. Courtesy of the Centers for Disease Control and Prevention. http://commons.wikimedia.org/wiki/File:Clost ridium_tetani_01.png . Public Domain 2. User:Mikm/Wikipedia. http://commons.wikimedia.org/wiki/File:Cellular_respiration_flowchart_%28en%29.s vg . Public Domain 5 www.ck12.org FIGURE 1.6 3. Image copyright somersault1824, 2014. Illustration of an animal cell in cross section . Used under license from Shutterstock.com 4. Hana Zavadska. CK-12 Foundation . CC BY-NC 3.0 5. Laura Guerin. CK-12 Foundation . CC BY-NC 3.0 6