Biology ACST 112 Chapter 6: How Cells Make ATP PDF

Biology ACST 112 Chapter 6: How Cells Make ATP: Energy-Releasing Pathways  Learning objectives By the end of this chapter, the student will be able to: Write a summary reaction for aerobic respiration that shows which reactant becomes oxidized and which...

Biology ACST 112 Chapter 6: How Cells Make ATP: Energy-Releasing Pathways  Learning objectives By the end of this chapter, the student will be able to: Write a summary reaction for aerobic respiration that shows which reactant becomes oxidized and which becomes reduced. List and give a brief overview of the four stages of aerobic respiration. Indicate where each stage of aerobic respiration takes place in a eukaryotic cell. Define chemiosmosis and explain how a gradient of protons is established across the inner mitochondrial membrane. Compare and contrast anaerobic respiration and fermentation. Include the mechanism of ATP formation, the final electron acceptor, and the end products.  Overview Living cells require energy from outside sources Some animals, obtain energy by eating plants, and some animals feed on other organisms that eat plants. Energy flows into the ecosystem as sunlight and leave as heat Photosynthesis generates O2 and organic molecules. Cells use chemical energy stored in organic molecules to regenerate ATP, which power all cellular work  Catabolism and ATP production The breakdown of organic molecules is exergonic (releases energy that stored in ATP). This occurs through a process called cellular respiration Cellular respiration is either aerobic or anaerobic Aerobic respiration require O2 Anaerobic respiration does not require O2 and are either: Anaerobic respiration Fermentation Aerobic respiration consumes organic molecules and O2 and yields ATP. Anaerobic respiration is similar to aerobic respiration but consumes compounds other than oxygen. Fermentation is a partial degradation of sugars that occurs without O2  Redox reactions Reactions in which electron are transferred from one molecule to another are called redox reaction. In oxidation, a substance loses electron, or is oxidized. In reduction, a substance gains electrons, or is reduced.  The Four Stages of Aerobic Respiration During cellular respiration, the fuel (such as glucose) is oxidized and O2 is reduced. The chemical reactions of the aerobic respiration of glucose are grouped into four stages Glycolysis (takes place in the cytosol) Formation of acetyl coenzyme A (takes place inside the mitochondria) Citric acid cycle (takes place inside the mitochondria) Electron transport chain and chemiosmosis (takes place inside the mitochondria) Because prokaryotes lack mitochondria, these process occur the reactions of in the cytosol and in association with the plasma membrane.  Glycolysis (glucose splitting) It’s the 1st step of cellular respiration Glycolysis is a series of 10 biochemical reactions that extract energy from glucose by splitting it into 2 three- carbon molecule called pyruvate Glycolysis occurs in cytosol of every living cells Glycolysis produces 2 ATP and 2 NADH and 2 pyruvate molecules  Formation of acetyl coenzyme Each pyruvate enters a mitochondrion and is oxidized to a 2-carbon group (acetyl coenzyme A). NADH is produced, and carbon dioxide is released as a waste product.  The citric acid cycle The acetyl group of acetyl coenzyme A combines with a 4-carbon molecule (oxaloacetate) to form a 6-carbon molecule (citrate). In the course of the cycle, citrate is recycled to oxaloacetate, and carbon dioxide is released as a waste product. Energy is captured as ATP and the reduced, high energy compounds NADH and FADH2  Electron transport and chemiosmosis The electrons removed from glucose during the glycolysis, formation of acetyl coenzyme A and citric acid cycle are transferred from NADH and FADH2 to a chain of electron acceptor compounds. These electrons are ultimately passed to the final electron acceptor, oxygen and water is formed. As the electrons are passed from one electron acceptor to another, some of their energy is used to transport hydrogen ions (protons) across the inner mitochondrial membrane, forming a proton gradient. In a process known as chemiosmosis, the energy of this proton gradient is used to produce ATP. Summary of glucose aerobic respiration  Fermentation and anaerobic respiration Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen Glycolysis can produce ATP with (aerobic ) or without O2 (anaerobic). In the absence of O2, glycolysis couples with either fermentation or anaerobic respiration to produce ATP. Anaerobic respiration uses an electron transport chain with an electron acceptor other than O2 (for example Sulfate (SO42-)) In Fermentation there no electron transport chain and organic molecules is used to re-oxidize NADH  Types of fermentation Fermentation consists of glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysis Two common types of fermentation are Alcohol fermentation Lactic acid fermentation In alcohol fermentation, pyruvate is converted to ethanol in two steps Pyruvate is converted to acetaldehyde with release of CO2 Acetaldehyde is reduced to ethanol Alcohol fermentation by yeast is used in brewing, winemaking, and baking  In lactic acid fermentation, pyruvate is reduced by NADH, forming lactate as end product, with no release of CO2. Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt Human muscle cells use lactic acid fermentation to generate ATP when O2 is scare. Lactic acid fermentation is used by Human RBCs to generate ATP.  Anaerobic Respiration Anaerobic respiration does not use oxygen as the final electron acceptor Anaerobic respiration is performed by some prokaryotes that live in anaerobic environments As in aerobic respiration, electrons are transferred in anaerobic respiration from glucose to NADH; they then pass down an electron transport chain that is coupled to ATP synthesis by chemiosmosis. However, an inorganic substance such as nitrate (NO3- ) 2 or sulfate (SO42- ) replaces molecular oxygen as the terminal electron acceptor. The end products of this type of anaerobic respiration are carbon dioxide, one or more reduced inorganic substances, and ATP.  Fermentation vs. aerobic respiration Both processes used glycolysis to oxidize glucose and other organic fuels to pyruvate. The processes have different final electron acceptors: An organic molecule (such as pyruvate or acetaldehyde) in fermentation. O2 in aerobic respiration. Aerobic respiration produces 30-32 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule  Energy Yield of Nutrients Other Than Glucose Many organisms, including humans, depend on nutrients other than glucose as a source of energy. Fatty acids derived from fat digestion and amino acid derived from protein digestion are also used as fuel molecules to generate ATP Such nutrients are transformed into one of the metabolic intermediates that are fed into glycolysis or the citric acid cycle