Chapter 9: Cellular Respiration: Harvesting Chemical Energy

1. Chapter 9: Cellular Respiration: Harvesting Chemical Energy

2. Metabolism includes all of an organism’s chemical reactions.  Metabolism is the sum of all anabolic pathways and catabolic pathways.             - Anabolicpathways result in the formation of complex compounds from simpler ones.  For example, the metabolic pathways that result in growth are anabolic pathways.             - Catabolicpathways release energy by breaking down complex compounds into simpler ones.  The energy stored in organic molecules is made available to do work.  Thus, the metabolic pathways that create the energy for growth are catabolic pathways.

3. A.  Principles of Energy Harvest The most common form of energy for humans is an organic molecule, glucose (starches in pasta, potatoes, bread, etc. are merely long chains of glucose molecules). Glucose (and other organic molecules) is produced by plants as an outcome of photosynthesis.   Plants, animals, fungi, and many bacteria use glucose as the primary organic molecule for energy.  See Fig. 9.2 for the overall concept of energy production and harvest.  Note that CO2 and H2O are used to make glucose and are the products of glucose catabolism.

5. There are two types of Energy-Yielding (catabolic) pathways:  respiration and fermentation.             1.  Fermentation is a partial degradation of organic fuel (glucose) without using oxygen.  Partial degradation results in the creation of acids and alcohols (hence the term "fermentation").  When you exercise and your muscles hurt, this is because your body was not able to get enough oxygen to the muscles and glucose was fermented into acids that cause the pain.

6.   2.  During respiration, energy is gained from the consumption of oxygen and organic fuel (This is the process denoted in Fig. 9.2).  The process can be summarized as: •                         Organic compound + Oxygen  Carbon dioxide + Water + Energy •                         Example: * C6H12O6 + 6 O2 6 CO2 + 6 H2O + Energy (ATP + Heat) • C6H12O6 is the chemical formula for glucose.

7. Note the following:                         a.  The energy gained from this reaction is used to make ATP (useful to the cell) and some heat is given off (wasted energy).                         b.  ATP (Adenosine triphosphate) is the energy source for cellular work.  The main objective of cellular respiration and fermentation is to produce ATP.  This ATP is then later used to drive other cellular reactions.  (e.g. when you shake when you are cold, ATP is being broken down to create heat for your body.)

8. c.  In order to create ATP from food molecules, cells need to catabolize these organic molecules.  Catabolism usually is a process that transfers electrons from organic molecules to O2.  The result is the production of H2O.   During this process, energy stored in the electrons is used to create ATP. Processes in which electrons are transferred from one molecule (called the reductant) to another molecule (called the oxidant) are termed redoxreactions (short for reduction-oxidation).  Note that during catabolism of glucose, glucose is the reductant because it has energetic electrons and that oxygen is the oxidant to which these electrons are passed. 

9. Thus, - Oxidation is the loss of electrons from a substance.   - Reduction is the gain of electrons to a substance.           Generalization:  Xe- + Y  X + Ye-           Example:  Na + Cl  Na+ + Cl- In this example, sodium (Na) was oxidized (lost electrons); chlorine (Cl) was reduced (gained electrons). Which is the oxidant? Which is the reductant?

10. Remember that: • - Transfer of electrons in redox reactions releases energy that can be used to produce ATP. • Transfer of electrons from glucose to oxygen is an example of a redox reaction.  During cellular respiration, electrons move away from an organic compound toward oxygen.  • * C6H12O6 + 6 O2 6 CO2 + 6 H2O + Energy (ATP + Heat) •  d.  NAD+ serves as an oxidant (gains electrons) and as an electron carrier during cellular respiration.  Upon gaining two electrons, NAD+ becomes NADH and is electrically neutral. • - NADH acts as an electron “escort.”  It carries electrons from an organic compound (food) to the electron transport chain, where the electrons can be used to produce ATP.

11. B.  The Process of Cellular Respiration             Cellular respiration involves three metabolic stages:             1.  Glycolysis             2.  The Krebs Cycle (Citric Acid Cycle)             3.  Electron Transport Chain and Oxidative Phosphorylation             Figure 9.6 (p. 164) – An overview of cellular respiration. Note that the organelle in which “2” and “3” occur is the mitochondrion.

14. 1.  Glycolysis.  Glucose is brought across the cell's plasma membrane as the first step in its catabolism.  Next, it is cleaved in two, a process that is called Glycolysis (glyco = glucose, lysis = cleave, hence the term glycolysis), during which glucose is converted into two molecules of pyruvate.This occurs in the cytosol. Note: The cell uses two molecules of ATP to start the process of glycolysis.  But that four ATP are produced, thus the cell gains two ATP (Fig. 9.8). Note: There are also two molecules of NADH produced that will be used later to make more ATP.

17. 2.  TheKrebsCycle breaks down pyruvate into carbon dioxide (CO2). - Note, this part of the catabolism of glucose occurs in the mitochondria.  Figure 9.11 (p. 168) – A summary of the Krebs cycle.

20. The main events in the Kreb's Cycle to know are: a.  If oxygen is present, the pyruvate from glycolysis enters a mitochondrion. b.  Pyruvate is converted to acetyl coenzyme A (acetyl CoA). c.  Acetyl CoA enters the Kreb's Cycle. The cycle results in the production of carbon dioxide (CO2) as exhaust.  This CO2 is produced from the original molecule of glucose. d.  During the cycle, a total of six (6) electrons are transferred to NAD+ (forming NADH), two (2) electrons are transferred to another electron carrier, FAD (forming FADH2), and one molecule of ATP is formed from each pyruvate. e.  Two (2) molecules of CO2 are released from the system.

22. 3.  Most of the energy from the original glucose molecule is stored in the molecule's electrons.  After the Kreb's Cycle is done, these electrons are now present on the electron carriers, NADH and FADH2.  The cell's next big concern is converting this energy into more ATP.  How does it do it?  By using the ElectronTransportChain (ETP).  This process is also in the mitochondrion; the chain is in the mitochondrial membrane as we will see below. The electrons can be thought of as possessing energy that can be used to create ATP.  This energy is an electro-potential, pretty much like the flow of electrons that is used to light bulbs and used for other processes in the home.  

24. The ETP consists of multiple molecules and the electrons pass from one to another, much like an electric current.  During this passage, the electrons give off energy into multiple energy-releasing steps.  The beauty of the ETP is shown in Figure 9.5 (p. 163) – An introduction to electron transport chains.  Note that if one mixes H2 and O2 the energy is given off as an explosion.  But if the electrons are passed through the ETP, the energy can be captured in smaller, controlled quantities to create ATP.

26. Note the following:                           a.  NADH and FADH2 “escort” electrons from the Krebs cycle to the first protein in the electron transport chain.                          b.  Electrons are passed from one protein to the next until they reach an oxygen molecule.  Oxygen then accepts the electrons and bonds with two (2) hydrogen ions to form water.

28. c.  So how does ATP get made?  By passing electrons to the outside of the membrane, we have set up a hydrogen ion gradient across a membrane that can be used to perform work; in this case, the gradient is across the mitochondrial membrane and the work is the synthesis of ATP.  The work is carried out by allowing the hydrogen ions to flow back into the cell, much like water will flow over a dam or through a water mill, during which the flow can be used to do useful work.  The gradient that results is known as the proton-motiveforce.

29. d.  Relatively high concentrations of H+ in the intermembrane space lead to the flow of H+ ions into the mitochondrial matrix through a channel; this channel is formed by a special enzyme called an ATP synthase (i.e. synthesizer of ATP).  e.  ATPsynthase, the enzyme that makes ATP, is located on the inner membrane of the mitochondrion.  This process of making ATP is known as oxidative phosphorylation. The entire process of using the proton motive force to make ATP is called chemiosmosis. Figure 9.14 (p. 167) – ATP synthase, a molecular mill.

31. C.  A review of cellular respiration             Figure 9.16 (p. 173) – Review:  how each molecule of glucose yields many ATP molecules during cellular respiration.

33.             Glucose → NADH → electron transport chain → proton-motive force → ATP             1.  Glycolysis:                                     +2 ATP             2.  Krebs Cycle:                                 +2 ATP             3.  Electron Transport Chain:           +34 ATP                                     Total Gain:                 + 38 ATP             - Energy stored in glucose that is not consumed during cellular respiration is lost as heat.  We use that to maintain a high body temperature; excess is dissipated by sweating, etc.