Krebs Cycle (Citric Acid Cycle) & Quick and Long Energy
GlycolysisGlucose 2 molecules of pyruvate To the electron transport chain
Energy accounting of glycolysis 2 ATP 2 ADP glucose pyruvate 6C 2x 3C 4 ADP 4 ATP All that work! And that’s all I get? 2 NAD+ 2 But glucose has so much more• Net gain = 2 ATP + 2 NADH to give! – some energy investment (-2 ATP) – small energy return (4 ATP + 2 NADH)• 1 6C sugar 2 3C sugars
Krebs Cycle• British biochemist Hans Krebs, discovered 1937• During Krebs cycle, PYRUVATE is broken down into CARBON DIOXIDE in a series of energy extracting reactions• AKA the CITRIC ACID CYCLE because citrate (also called citric acid) is the first product of the cycle
• Citric Acid Production Step 1 – Pyruvate ( 3-carbons) enter mitochondria Matrix – One carbon is removed as CO2 (WASTE product) and electrons are removed by NAD+ (making NADH which goes to the ETC) – Co-enzyme A joins the 2-carbon molecule (that used to be pyruvate) making Acetyl-CoA – Now Acetyl-CoA can enter the Krebs cycle – Acetyl Co-A combines with 4- carbon molecule called OXALOACETATE , making citrate (citric acid), a 6-carbon molecule
Step 2• Citric acid (6 carbon molecule) is broken down into a few different 5-carbon compounds, then into a few different 4-carbon compounds.• Each step releases CO2, NADH and FADH2, and ATP – CO2 is a waste product (breath out!) – NADH and FADH2 (taxi cabs) goes onto the ETC (where the party is at) – ATP is used for cell to do work (mechanical, chemical, or transport)
Count the carbons! pyruvate acetyl CoA 3C 2C citrate 4C 6C 4C oxidation 6CThis happenstwice for each of sugars CO2glucosemolecule x2 4C 5C 4C 4C CO2
Count the electron carriers! CO2 pyruvate acetyl CoA 3C 2C NADH citrate NADH 4C 6C 4C reduction 6CThis happens of electrontwice for each carriers CO2glucose NADHmolecule x2 5C 4C FADH2 CO2 4C 4C NADH ATP
So we fullyoxidized(broke down)glucoseC6H12O6CO2& ended upwith 4 ATP! What’s the point?
Electron Carriers = Hydrogen Carriers H+ H+ Krebs cycle H+ H+ + H+ H+ H H+ produces large quantities of electron carriers ADP + Pi NADH ATP H+ FADH2 go to Electron Transport Chain! What’s so important about electron carriers?
Energy accounting of Krebs cycle 4 NAD + 1 FAD 4 NADH + 1 FADH22x pyruvate CO2 3C 3x 1C 1 ADP 1 ATP ATP Net gain = 2 ATP = 8 NADH + 2 FADH2
Value of Krebs cycle?• If the yield is only 2 ATP then how was the Krebs cycle an adaptation? – value of NADH & FADH2 • electron carriers & H carriers • to be used in the Electron Transport Chain like $$ in the bank
Summary• In one turn of the Krebs • For one Glucose Cycle: molecule how many – 3 CO2 (1 from right times does the Krebs before Krebs) Cycle turn? • Released when we exhale • What are the totals – 1 ATP (E for cell work) from the Krebs Cycle – 1 NADH from right before Krebs for one Glucose – 3 NADH from Krebs (to molecule? ETC) – 6 CO2s – 1 FADH2 (to ETC) – 2 ATPs – Water leaves and then – 8 NADHS reenters so we don’t – 2 FADH2 count it in the products
So we use Krebs if we have oxygen….what if there is NO oxygen???• Then we can’t even enter the mitochondria and go to the Krebs cycle…• We are stuck using Glycolysis…• Anaerobic respiration (NO oxygen) – 2 types • Lactic acid fermentation • Alcohol fermentation
Pyruvate is a branching point Pyruvate O2 O2fermentationanaerobicrespiration mitochondria Krebs cycle aerobic respiration
• Cells cannot get enough oxygen• Build up of pyruvic acid and NADH and no oxygen to break it down• Cells begin fermentation – Lactic Acid fermentation • Pyruvic acid + NADH lactic acid + NAD+ • Get about 90 seconds of energy without having to use oxygen • HOWEVER, oxygen will be paid back double when you are done (think heavy breathing) • Occurs in muscle cells, and microorganisms, such as the ones that turn milk into cheese and yogurt • Lactic acid causes muscle cramping and burning sensation • Oxygen is required to break down lactic acid and get it out of body – Alcohol fermentation • Pyruvic acid + NADH ethyl alcohol + NAD+ + CO2 • Occurs in yeast cells and other microorganisms, such as the ones involved in the production of bread and wine
How is NADH recycled to NAD+? with oxygen without oxygenAnother molecule aerobic respiration anaerobic respirationmust accept H from pyruvate “fermentation”NADH H2O NAD+ CO2 O2 NADH NADH acetaldehyde recycle acetyl-CoA NADH NAD+ NADH lactate NAD+ lactic acid fermentationwhich path you Krebs cycle ethanoluse depends on alcoholwho you are… fermentation
Fermentation (anaerobic)• Bacteria, yeast pyruvate ethanol + CO2 3C 2C 1C NADH NAD+ back to glycolysis beer, wine, bread Animals, some fungi pyruvate lactic acid 3C 3C NADH NAD+ back to glycolysis cheese, anaerobic exercise (no O2)
Alcohol Fermentation bacteria yeast recycle pyruvate ethanol + CO2 NADH 3C 2C 1C NADH NAD+ back to glycolysis Dead end process at ~12% ethanol, kills yeast can’t reverse the reaction Count the carbons!
animals some fungi Lactic Acid Fermentation recycle O2 pyruvate lactic acid NADH 3C 3C NADH NAD+ back to glycolysis Reversible process once O2 is available, lactate is converted back to pyruvate by the liver Count the carbons!
Quick Energy• 3 ways to obtain energy – ATP stored in muscles (glycogen) (short) – ATP from lactic acid (short) – ATP from cellular respiration (long)• Cells initially have small amount of ATP from cell resp. and glycolysis• Think of running a 200 m sprint – Gun goes off • Muscles of runner contract, turning glycogen in muscle cells into glucose, but this only provides for a few seconds of intense activity – You pass the 50m mark • most ATP the was initially stored is now gone • Muscle cells are producing ATP from lactic acid fermentation • This lasts about 90 seconds – End of Race • Lots of lactic acid build up • Only way to get rid of lactic acid is a chemical pathway that requires oxygen • Thus, at the end of the race, you are breathing heavily and you should follow an intense work out with a slow jog
Long Term• Energy for running long races or other endurance sports• Cellular respiration is the only way to get enough ATP to last the length of the race• Cellular respiration makes ATP more slowly than lactic acid fermentation• Athletes must pace themselves• Glycogen an important molecule – Carbohydrate – Polysaccharide (monosaccharide is glucose) – Muscle and liver cells store E as glycogen – Glycogen is broken down by the hormone Glucagon – Glycogen break down is also stimulated by muscle contraction • When you work out, muscles contract and they can use energy stored in glycogen – Increase glycogen storage, increase the duration of exhaustive work your muscles can do – Stores of glycogen last about 15-20 minutes• After glycogen is used up, body starts to break down other molecules to get energy – Fats and proteins – Fatty acids are broken down and carried to mitochondrial matrix and enter the membrane in fragments as acetyl-CoA – Proteins are broken down into aa and the these modified aa’s are fed back into the Krebs cycle (NAD+ and FAD)• Aerobic exercise is good for weight control because it leads to break down of fats
Training to Improve Function of ATP production• Anaerobic training – Increase levels of glycogen in muscle cells and increase tolerance of lactic acid build up• Aerobic training – Increases size and number of mitochondria in muscle cells and increase delivery of oxygen to muscle cells by improving heart and lung efficiency
What’s the point? The point is to make ATP! ATP2007-2008
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