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Cellular Respiration Chapter 7 Energy All life requires energy Energy is required for: - growth - active transport - reproduction - synthesis of carbohydrates, lipids, proteins etc - movement The source of energy for cells is energy stored in the chemical bonds of organic molecules Ex. Food molecules, especially carbohydrates (also lipids) Most common source of energy is glucose Cellular respiration the process by which the chemical energy of "food" molecules is released and partially captured in the form of ATP. Carbohydrates, fats, and proteins can all be used as fuels in cellular respiration glucose is often used as an example to examine the reactions and pathways involved. Overview of Respiration We can divide cellular respiration into three metabolic processes: 1. Glycolysis 2. the Krebs cycle 3. oxidative phosphorylation. Each of these occurs in a specific region of the cell. Where? 1. Glycolysis occurs in the cytosol. 2. The Krebs cycle takes place in the matrix of the mitochondria. 3. Oxidative phosphorylation via the electon transport chain is carried out on the inner mitochondrial membrane. Not enough oxygen to run 5 miles? In the absence of oxygen, respiration consists of two metabolic pathways: 1. Glycolysis 2. Fermentation Both of these occur in the cytosol. Adenosine triphosphate (ATP) Cells store energy in chemical bonds of sugar, but cannot use it directly To use energy, the cell must transfer energy in its sugar molecules to ATP ATP is the energy currency of cells ATP (contd.) ATP = base (adenine) + sugar (ribose) + 3 phosphate groups Phosphate groups are connected by energy rich bonds ATP is like a rechargeable battery If a cell needs energy, it breaks the last phosphate bond of the ATP to release energy ATP ADP + Pi + energy (12 kcal) Where does ATP come from? What is a kcal? Kcal is an abbreviation for kilocalorie measurement for energy (for example for the amount of energy obtained from burning food). 1 kcal is the same as 1 Calorie www.emulsifiers.org/ViewDocument.asp Cellular Respiration The process by which cells break the energy containing bonds of glucose to produce ATP Occurs in the mitochondria Summary equation: Glucose Oxygen Carbon dioxide Water Energy Cell Respiration vs. “Respiration” In Respiration (as in “respiratory system” or breathing) the exchange of gases is between blood and the atmosphere (oxygen in, carbon dioxide out) In Cellular Respiration (aerobic*) the exchange of gases is between blood and the cell (oxygen in, carbon dioxide out). * means in the presence of oxygen Breathing Lungs Muscle cells Cellular respiration Digestion vs. Cellular Respiration Digestion: Enzymes break down large organic molecules into smaller ones Cellular Respiration: Smaller organic molecules (namely glucose) are broken down to make ATP Cellular Respiration vs. Photosynthesis Photosynthesis: 6CO2 + 6H2O + SOLAR ENERGY C6H12O6 + 6O2 Cellular Respiration: C6H12O6 + 6O2 6CO2 + 6H2O + ATP Recycling Energy Cellular Respiration vs. Photosynthesis continued Photosynthesis: takes place in autotrophs (plants, algae, cyanobacteria) Cellular Respiration: takes place in ALL organisms Glucose: C6H12O6 Most foods contain a variety of carbohydrates, proteins, and lipids. We will examine cellular respiration by following the breakdown of glucose, one of the most abundant organic molecules. But remember that cellular respiration happens with other organic compounds too. What is a “biochemical pathway”? A chemical reaction that occurs in sequences where the product of one reaction becomes the reactant for the next reaction Summary equation for cellular respiration: C6H12O6 + 6O2 6CO2 + 6H2O + ATP In an explosion, fuel is burned in one step Energy released as heat and light In cellular respiration, glucose is broken down in multiple steps Glucose C6H12O6 + 6O2 NAD+ NADH FAD FADH2 NAD+ NADH + H+ + H+ ADP + Pi 6CO2 + 6H2O ATP ADP + Pi ATP NAD: Nicotinamide Adenine Dinucleotide FAD: Flavin Adenine Dinucleotide Two stages of cellular respiration Anaerobic stage – without oxygen - Strict anaerobes; lack the enzymes needed to break down energy rich molecules with O2 - Facultative anaerobes; can use O2 to break down energy rich molecules when it is available Aerobic stage – with oxygen CELLULAR RESPIRATION: OVERVIEW Organic Compounds (Glucose) Glycolysis O2 absent Anaerobic Respiration 2 ATP O2 present Aerobic Respiration ATP 1. Glycolysis (anaerobic stage) Occurs in the cytoplasm Glucose (6C) splits up into 2 pyruvic acid (3C) Two molecules of ATP are required to split glucose Splitting of glucose releases enough energy to produce 4 molecules of ATP Net gain of 2 ATP More about Glycolysis During conversion of glucose to pyruvic acid, hydrogen is released This hydrogen is picked up by a coenzyme, nicotinamide adenine dinucleotide (NAD) When it accepts hydrogen, it becomes NADH NADH is an energy rich compound that will be used in the aerobic stage of respiration Glycolysis: Harvests energy by splitting glucose to pyruvic acid 2 Pyruvic acid Glucose Summary of Glycolysis Cytoplasm Glucose Pyruvic acid Fermentation Fermentation = breakdown of glucose, yielding ATP, without O2 Many types of bacteria and other single celled organisms still use anaerobic processes to convert energy Types of Fermentation 2 kinds: Alcoholic fermentation: occurs in micro-organisms such as yeast Lactic acid fermentation: occurs in bacteria and animal cells More Fermentation The first living organisms were single cells that existed without O2 Anaerobic Lack the enzymes needed to break down energy molecules with O2 Fermentation in the Cytosol Fermentation occurs in the cytosol It produces lactic acid or alcohol Fermentation begins with the process of glycolysis, which is also part of aerobic respiration. Alcoholic Fermentation in Microorganisms Various types of microorganisms perform fermentation Yeast cells carry out a slightly different type of fermentation pathway This pathway produces CO2 and ethyl alcohol Yeast Yeast are a tiny form of fungi or plant-like microorganism that exist in or on all living matter i.e. water, soil, plants, air, animals, etc. A common example of a yeast is the bloom (fuzz) we can observe on grapes. http://home.earthlink.net/~ggda/biology_of_yeast_cells_simplified.htm As a living organism yeast needs sugars, water and warmth to stay alive. In addition, albumen or nitrogenous material are also necessary for yeast to thrive. Alcoholic fermentation 2ADP + 2 2 CO2 released Glycolysis 2NAD Glucose 2 Pyruvic acid 2NAD + 2H 2 Ethyl alcohol In alcoholic fermentation, pyruvic acid is converted to CO2 and ethanol This recycles NAD+ to keep glycolysis working Simple Sugar → Ethyl Alcohol + Carbon Dioxide C6 H12 O6 → 2C H3 CH2 OH + 2CO2 The food industry uses yeast to produce various food products Lactic Acid Fermentation – Pyruvic acid is converted to lactic acid – Lactic acid fermentation is used to make cheese and yogurt Fermentation in Human Muscle Cells Human muscle cells can make ATP with and without oxygen They have enough ATP to support activities such as quick sprinting for about 5 seconds A secondary supply of energy (creatine phosphate) can keep muscle cells going for another 10 seconds To keep running, your muscles must generate ATP by the anaerobic process of fermentation Lactic acid fermentation 2ADP+ 2 Glycolysis 2 NAD Glucose 2 NAD + 2 H 2 Pyruvic acid 2 Lactic acid Uses of Lactic Acid Fermentation During strenuous exercise glycolysis occurs at a high rate Pyruvic acid is produced rapidly Muscle cells may not receive enough O2 to process pyruvic acid through aerobic respiration Therefore muscles produce lactic acid which permits glycolysis to continue to supply ATP to your muscles What happens when you don’t have enough oxygen to undergo glycolysis? When lactic acid builds up, your muscles ache O2 you take in from heavy breathing helps convert lactic acid back to pyruvic acid http://www.scientificamerican.com/article.cf m?id=why-does-lactic-acid-buil Cellular Respiration Most cells produce ATP by breaking the energy containing bonds of glucose in the presence of oxygen Production of ATP this way = Respiration Uses O2 to break sugars down to CO2 & H2O Not the same as breathing breathing provides O2, but otherwise quite different Aerobic respiration occurs in the many mitochondria of each cell Breathing Lungs Muscle cells Cellular respiration The Process of Cellular Respiration C6H12O6 + 6 O2 6 CO2 + 6 H2O + energy (sugar) Two stages of Cellular Respiration: Anaerobic without oxygen Aerobic with oxygen (ATP) Cellular Respiration Overview Anaerobic Stage: Glycolysis The anaerobic stage of cellular respiration is glycolysis, the same pathway used in fermentation This part of cellular respiration occurs in the cytoplasm Recall the energy budget for glycolysis: One molecule of glucose is split into two molecules of a three carbon compound called pyruvic acid 2 molecules of ATP provide the energy to split the glucose molecule When glucose splits, it releases enough energy to form 4 molecules of ATP from ADP + P Therefore 2 molecules of ATP are gained Energy of Glycolysis Aerobic Stage After glycolysis, the chemical bonds of pyruvic acid are broken down in a series of chemical reactions These occur in the mitochondria and require O2 The aerobic stage has two main parts: - The Citric Acid Cycle (Krebs cycle) The Electron Transport Chain These two stages are preceded by an intermediate step in which pyruvic acid is converted to acetyl-CoA Pyruvate Forms Acetyl CoA in the Intermediate Step Citric Acid (Krebs ) Cycle In aerobic organisms, the citric acid cycle is a metabolic pathway that forms part of the breakdown of carbohydrates, fats and proteins into carbon dioxide and water in order to generate energy. It is one of three metabolic pathways that are involved in fuel molecule catabolism and ATP production the other two being glycolysis and oxidative phosphorylation. The citric acid cycle also provides precursors for many compounds, such as certain amino acids, and some of its reactions are important in cells performing fermentation reactions in the absence of oxygen. The citric acid cycle is also known as the Krebs Cycle in honor of Sir Hans Adolf Krebs (1900 - 1981), who proposed the key elements of this pathway in 1937, and was awarded the Nobel Prize in Medicine for its discovery in 1953. http://www.newworldencyclopedia.org/entry/Citric_aci d_cycle The Citric Acid (Krebs) Cycle Steps to break down pyruvic acid: In the presence of O2, pyruvic acid breaks down to acetic acid and CO2 CO2 is released as waste Acetic acid combines with coenzyme A acetyl CoA This step also forms NADH2 from NAD Acetyl CoA enters the citric acid cycle and combines with a 4 carbon compound to produce citric acid As the cycle continues, citric acid is broken down in a series of steps, back to the original 4 carbon compound Energy from the Citric Acid Cycle For each molecule of acetyl CoA that enters the cycle, 8 atoms of H are released. These hydrogen atoms are trapped by NAD and FAD, forming NADH and FADH2 Therefore, each turn of the cycle yields 3NADH and one FADH2 Intermediate step The Electron Transport Chain NADH and FADH2 releases the hydrogen atoms trapped during glycolysis & the citric acid cycle Thus, NADH/FADH2 becomes NAD/FAD again Electrons contained in the H atoms pass through a series of coenzymes which are electron acceptors. Each time an electron moves from one acceptor to another, energy is released The energy released is used by chemiosmosis to form molecules of ATP from ADP + Pi This whole process = electron transport chain Oxygen & the Electron Transport Chain The last part of the chain is the electron acceptor, oxygen Electrons combine with oxygen & hydrogen to form H2O, which is released as a byproduct Picturing Chemiosmosis FADH2 FAD + 2H+ Chemiosmosis The process of ATP formation from the ETC of aerobic respiration when a pH gradient forms across the membrane of the cristae in the mitochondria = chemiosmosis Steps: H+ ions from the matrix are pumped into the space between the cristae and the outer membrane A H+ gradient develops between the inside and outside of the cristae This pH differential creates free energy H+ pass back across the membrane through ATP synthase, producing ATP from ADP + Pi O2 is the final H+/electron acceptor producing H2O Cellular Respiration Summary Thus for every molecule of glucose that is broken down by glycolysis and respiration, 38 molecules of ATP are formed