Cellular Metabolism Biology 105 Lecture 6 Chapter 3 (pages 56-61) Metabolism Consists of all the chemical reactions that take place in a cell! Cellular metabolism: Aerobic cellular respiration requires oxygen, produces carbon dioxide Anaerobic fermentation does not require oxygen Summary of Cellular Respiration Electrons transferred by NADH Blood vessel Electrons transferred by NADH Cytoplasm Carrier protein Glucose Plasma membrane Electrons transferred by NADH and FADH 2 Glycolysis Transition Reaction Citric Acid Cycle Electron Transport Chain glucose pyruvate Oxygen Mitochondrion Extracellular fluid +2 ATP +2 ATP +32 ATP 36 ATP Figure 3.27 1
Aerobic Cellular Respiration Aerobic cellular respiration: Cells take sugar (glucose) and break it down into carbon dioxide and water. This requires oxygen! This process produces energy in the form of ATP. C 6 H 12 O 6 + 6O 2 6CO 2 +6H 2 O + energy Aerobic Cellular Respiration There are four steps in aerobic cellular respiration: 1. Glycolysis 2. Transition Reaction 3. Citric Acid Cycle (Krebs Cycle) 4. Electron Transport Chain 2
NADH and FADH 2 are important carriers of electrons Cellular Respiration Phase 1: Glycolysis Occurs in the cytoplasm Splits 1 glucose into 2 pyruvate molecules Generates a net gain of 2 ATP and 2 NADH molecules Does not require oxygen Starts with glucose Ends with 2 ATP, 2 NADH, 2 pyruvate Glycolysis Glycolysis (in cytoplasm) Cytoplasm During the first steps, two molecules of ATP are consumed in preparing glucose for splitting. Glucose Energyinvestment phase 2 ADP 2 ATP During the remaining steps, four molecules of ATP are produced. 4 ADP 4 ATP Energy- The two molecules of 2 NAD + yielding pyruvate then diffuse phase from the cytoplasm into the inner compartment of the mitochondrion, where they pass through 2 NADH a few preparatory steps (the transition reaction) before entering the citric acid cycle. Two molecules of nicotine adenine dinucleotide 2 Pyruvate (NADH), a carrier of high-energy electrons, also are produced. Figure 3.23 3
Cellular Respiration Phase 2: Transition Reaction Occurs within the mitochondria Coenzyme A combines with pyruvate, and CO 2 is removed from each pyruvate Forms 2 acetyl CoA molecules Produces 2 NADH Transition Reaction Starts with: 2 pyruvate (which are 3-carbon molecules) 2 Coenzyme A Ends with: 2 CO 2 2 NADH 2 Acetyl CoA (which are 2-carbon molecules) Transition Reaction Transition Reaction (in mitochondrion) Pyruvate (from glycolysis) A molecule of NADH is formed when NAD + gains two electrons and one proton. NAD + One carbon (in the form of CO 2) is removed from pyruvate. CO 2 NADH (electron passes to electron transport chain) CoA Acetyl CoA Coenzyme A The two-carbon molecule, called an acetyl group, binds to coenzyme A (CoA), forming acetyl CoA, which enters the citric acid cycle. Citric Acid Cycle Figure 3.24 4
Cellular Respiration Phase 3: Citric Acid Cycle Also known as the Krebs Cycle Occurs in the mitochondria Acetyl CoA enters the citric acid cycle Releases 2 ATP, 2 FADH 2, and 6 NADH Generates 4 CO 2 molecules Requires oxygen Citric Acid Cycle Starts with 2 Acetyl CoA Ends with: 4 CO 2 2 ATP 6 NADH and 2 FADH 2 Citric Acid Cycle Citric Acid Cycle (in mitochondrion) The citric acid cycle also yields several molecules of FADH 2 and NADH, carriers of high-energy electrons that enter the electron transport chain. Oxaloacetate NADH Acetyl CoA, the two-carbon compound formed during the transition reaction, enters the citric acid cycle. Acetyl CoA CoA CoA Citrate NAD + Malate Citric Acid Cycle CO 2 leaves cycle NAD + FADH 2 FAD ATP ADP + Pi NADH -Ketoglutarate Succinate NADH NAD + CO 2 leaves cycle The citric acid cycle yields one ATP from each acetyl CoA that enters the cycle, for a net gain of two ATP. Figure 3.25 5
Potential energy 6/20/2015 Cellular Respiration Phase 4: Electron Transport Chain Electrons of FADH 2 and NADH are transferred from one protein to another, until they reach oxygen Releases energy that results in 32 ATP Requires oxygen The Big Pay Off Electron Transport Chain! NADH and FADH 2 are important electron carriers. They donate electrons to the electron transport chain. At the end of the chain, oxygen accepts the electrons. The electron transport chain produces ATP using the ATP synthase molecule (a protein). The electron transport chain produces 32 ATP! Electron Transport Chain Electron Transport Chain (inner membrane of mitochondrion) High The molecules of NADH and FADH 2 produced by earlier phases of cellular respiration pass their electrons to a series of protein molecules embedded in the inner membrane of the mitochondrion. NAD + NADH 2e As the electrons are transferred from one protein to the next, energy is released and used to make ATP. FADH 2 FAD 2e 2e Membrane proteins 2e Eventually, the electrons are passed to oxygen, which combines with two hydrogens to form water. Low Energy released is used for synthesis of ATP 2e H 2O 2 H + 1 + 2 O 2 Figure 3.26 6
How is ATP made using the ETC? 1. In the mitochondria, the NADH and FADH 2 donate electrons to the electron transport chain (ETC). 2. Oxygen is the final electron acceptor from the ETC. 3. The ETC uses the energy from the electrons to transport H + against its concentration gradient, transporting the H + from the lumen of the mitochondria to the intermembrane space (from the inner compartment to the outer compartment). How is ATP made using the ETC? 4. The ATP synthase transports the H + back to the lumen (inner compartment) of the mitochondria. 5. The H + falling through the ATP synthase provides the energy for ATP synthase to catalyze the reaction of: ADP + P ATP 7
Summary of Cellular Respiration Table 3.5 Summary of Cellular Respiration: The Players One molecule of glucose is broken down and 36 ATP are generated. Oxygen is used by the electron transport chain it accepts electrons from the ETC. Carbon dioxide is produced by the Transition Reaction and the Citric Acid Cycle. Summary of Cellular Respiration: The Steps Glycolysis: kicks off the process by taking in one glucose produces 2 ATP Transition Reaction: produces CO 2 and NADH Citric Acid Cycle: produces 2 ATP, lots of NADH and FADH 2, and some CO 2 8
Summary of Cellular Respiration: The Steps Electron Transport Chain Takes electrons from NADH and FADH 2 and uses them to produce ATP using the ATP synthase molecule. Requires oxygen! Oxygen is the final electron acceptor on the electron transport chain. One glucose can produce a total of 36 ATP!!! Complex carbohydrates must first be broken down into glucose before entering glycolysis. Fats and proteins enter the process at different steps. A few words about oxygen Cellular respiration requires oxygen this is why it is called aerobic cellular respiration. Sometimes organisms, including humans, need to produce energy without using oxygen. When you need energy quickly, or if there is not enough O 2, then the cell will use only glycolysis. 9
Anaerobic Fermentation Breakdown of glucose without oxygen. Takes place entirely in the cytoplasm. It is very inefficient and results in only two ATP. Anaerobic Fermentation: anaerobic pathway to produce ATP from glycolysis, without the Citric Acid Cycle and the ETC. Fermentation in Animals When cells need energy quickly, they will use this pathway for a short time. 2 pyruvic acid + 2 NADH 2 lactate and 2 NAD + End result = lactate and 2 ATP produced (from glycolysis) and NAD + is regenerated. 10
Q: What is the starting molecule of glycolysis? 1. Acetyl CoA 2. Protein 3. Glucose 4. Pyruvate (pyruvic acid) Q: Which stage produces CO 2? 1. Glycolysis 2. Electron Transport Chain 3. Transition 4. Citric Acid Cycle 5. Both 3 and 4 Q: Which stage uses O 2? 1. Glycolysis 2. Krebs Cycle 3. Electron Transport Chain Q: Which stage produces the most NADHs? 1. Glycolysis 2. Krebs Cycle 3. Electron Transport Chain Q: Which stage produces the most ATP? 1. Glycolysis 2. Krebs Cycle 3. Electron Transport Chain 11
Important Concepts Read chapter 3, pages 56-61 What are aerobic cellular respiration and anaerobic fermentation? What are the differences between them? What are the four steps of aerobic cellular respiration, what happens in each step, what are the starting molecules, what comes out of each step, where in the cell does each step occur, how many ATP and NADH/FADH 2 are produced in each step? Important Concepts Describe in detail how ATP is made using the electron transport chain. What are the roles of ATP synthase, H +, O 2, NADH and FADH 2, and the electron transport chain in ATP production? Know the overall picture of cellular respiration (summary slides). Important Concepts What is the role of oxygen in cellular respiration, and which steps produce carbon dioxide? What is anaerobic fermentation, what steps are involved in fermentation, and what end products are produced in humans? Is oxygen required? If so, when is it used? 12
Definitions Aerobic cellular respiration, anaerobic fermentation, ATP synthase, metabolism 13