Cellular RespirationBIO100 Chp 7

• Cellular respiration• Mitochondria glycolysis• Inside the mitochondria• Fermentation• metabolism

Cellular Respiration• Overall process– Oxidation of glucose to carbon dioxide, water, and energy– Exergonic reaction used to drive ATP synthesis – 4 phases of respiration required for complete oxidation of glucose– Oxidation involves the removal of hydrogen atoms from substrates by redox coenzymes NAD+ and FAD

• Phases of cellular respiration– Glycolysis• Breakdown of glucose to 2 molecules of pyruvate• Oxidation by removal of hydrogens releases enough energy to make 2 ATP– Preparatory reaction• Pyruvate oxidized to acetyl CoA and carbon dioxide is removed• Prep reaction occurs twice because glycolysis produces 2 pyruvates– Citric acid cycle• Acetyl CoA is converted to citric acid and enters the cycle• Cyclical series of oxidation reactions that produces 1 ATP and carbon dioxide • Citric acid cycle turns twice because 2 acetyl CoA’s are produced per glucose– Electron transport chain• Series of electron carrier molecules• Electrons passed from one carrier to another• As the electrons move from a higher energy state to a lower one, energy is released to make ATP• Under aerobic conditions 32-34 ATP per glucose molecule can be produced– Pyruvate • Pivotal metabolite in cellular respiration• If no oxygen is available, pyruvate is reduced to lactate (in animals) or ethanol and carbon dioxide (in plants) in a process called fermentation

Cellular respirationOutside the mitochondria: glycolysis

• Energy-investment steps– Energy from 2 ATP is used to activate glucose– Glucose is split into 2 3-carbon G3P molecules

• Energy-harvesting steps– Oxidation of G3P by removal of hydrogens– Hydrogens are picked up by NAD+ to form NADH– Oxidation of G3P and further substrates yields enough energy to produce 4 ATP by direct substrate phosphorylation

• Glycolysis yields:– 4 ATP by direct substrate phosphorylation• 2 ATP were consumed in the investments steps• Net gain of ATP from glycolysis is therefore 2 ATP– 2 NADH which will carry electrons to the electron transport chain• When oxygen is available pyruvate will enter the mitochondria for further oxidation• If no oxygen is available, pyruvate will enter the fermentation pathway

• Breathing, eating, and cellular respiration– Oxygen is taken in by breathing– Digested food contains glucose– Oxygen and glucose are carried to cells by the bloodstream– Glucose and oxygen enter cells where respiration occurs– Carbon dioxide is taken by the bloodstream to the lungs

Relationship between breathing, eating, and cell respiration

• Preparatory reaction– Produces the molecule that will enter the citric acid cycle– 3C pyruvate is converted to 2C acetyl CoA– Carbon dioxide is produced – Hydrogen atoms are removed from pyruvate and picked up to form NADH– This reaction occurs twice per glucose

• Citric acid cycle– 2C acetyl group from prep reaction combines with a 4C molecule to produce 6C citrate– Oxidation of citrate by removal of hydrogens– Produces 3 NADH and 1 FADH2

– Produces 1 ATP by direct substrate phosphorylation– Cycle turns twice per glucose– Total yield: 6 NADH, 2 FADH2, 2 ATP, 4 CO

• Electron transport chain (ETC)– 2 electrons per NADH and FADH2 enter ETC– Electrons are passed to series of electron carriers called cytochromes– Energy is captured and stored as a hydrogen ion concentration gradient– For each NADH enough energy is released to form 3 ATP– For each FADH2 there are 2 ATP produced– the final electron acceptor is oxygen– After receiving electrons oxygen combines with hydrogen ions to form water as an end product ½ O2+ 2 e- + 2H+ H2O– NAD+ and FAD recycle back to pick up more electrons from glycolysis, prep reaction, and citric acid cycle• The Efficiency of cellular respiration– The difference in energy content of reactants (glucose and oxygen) and products (carbon dioxide and water) is 686 kcal– ATP phosphate bond has 7.3 kcal of energy– 36 ATP are produced in respiration 36 X 7.3 = 263 kcal– 263/686 = 39% efficiency of energy capture– The rest of the energy is lost as heat

Fermentation• Fermentation– Occurs when O2 is not available– Animal cells convert pyruvate to lactate– Plant cells, yeasts convert pyruvate to ethanol and CO2

– Fermentation regenerates NAD+ which keeps glycolysis going• Advantages and Disadvantages of fermentation– Provides a low but continuous supply of ATP when oxygen is limited and only glycolysis can function– Lactate is potentially toxic to muscles, lowering pH and causing fatigue– Transported to liver where it is converted to pyruvate• This process requires oxygen• During exercise an oxygen debt is built up• Oxygen debt is the amount of oxygen “owed” to the liver to convert accumulated lactic acid to pyruvate

• Energy yield of fermentation

– Produces only a net of 2 ATP per glucose through direct substrate phosphorylation by allowing glycolysis to continue– Following fermentation most of the potential energy from glucose is still waiting to be released– Fermentation is a way to continue an ATP supply to cells when oxygen is in short supply

Metabolism• Catabolism-break down reactions– Carbohydrates-digested to glucose for cell respiration– Fats-digested to glycerol and fatty acids• Glycerol can enter glycolytic pathway• Fatty acids metabolized to acetyl CoA which enters citric acid cycle– Proteins- deamination• Amino acids can enter pathway at different points

• Anabolism- synthesis reactions– Substrates of glycolysis and citric acid cycle can be substrates for synthesis of macromolecules• G3P can be converted to glycerol• Acetyl groups can be converted to fatty acids• Some citric acid intermediates can be converted to amino acids– Anabolic reactions require the input of energy in the form of ATP generated in catabolic reactions