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Cellular Respiration

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Title: Cellular Respiration


1
Cellular Respiration
2
  • Life is work, which requires E
  • In most ecosystems, E enters as sunlight
  • Light E trapped in O-molecules is available to
    both photosynthetic organisms and others that eat
    them

3
  • Cellular respiration and fermentation are
    catabolic, energy-yielding pathways
  • E is stored in molecules
  • Enzymes catalyze the hydrolysis of high E
    molecules to low E products
  • Some of the released E is used to do work the
    rest is lost as heat

4
  • Fermentation leads to the partial degradation
    of sugars in the absence of O2
  • Cellular respiration A more efficient and
    widespread catabolic process uses O2 as a
    reactant to complete the breakdown of a variety
    of organic molecules
  • Carbohydrates, fats, and proteins can all be used
    as the fuel

5
  • ATP pivotal molecule in cellular energetics
  • It is the chemical equivalent of a loaded spring
  • The close packing of 3 negatively-charged
    phosphate groups is an unstable, E-storing
    arrangement
  • Loss of the end phosphate group relaxes the
    spring

6
  • Most cellular work converts ATP to ADP and
    inorganic phosphate (Pi)
  • Animal cells regenerate ATP from ADP and Pi by
    the catabolism of organic molecules

7
  • The transfer of the last phosphate group from ATP
    to another molecule is phosphorylation
  • The receiving molecule changes shape, performing
    work (transport, mechanical, or chemical)
  • When the phosphate groups leaves the molecule,
    the molecule returns to its original shape

8
  • Catabolic pathways relocate e-s stored in food
    molecules, releasing E used to synthesize ATP
  • Redox rxns
  • Oxidation loss of e-s
  • Reduction addition of e-s
  • More generally Xe- Y ? X Ye-
  • X, the e- donor, is the reducing agent which is
    oxidized and reduces Y
  • Y, the e- recipient, is the oxidizing agent which
    is reduced and oxidizes X
  • Redox reactions require both a donor and acceptor

9
  • O2 very potent oxidizing agent
  • An electron loses E as it shifts from a less
    electronegative atom to a more electronegative
    one
  • A redox rxn that relocates e-s closer to O2
    releases chemical E that can do work
  • To reverse the process, E must be added to pull
    an e- away from an atom

10
  • e-s fall from organic molecules to O2 during
    cellular respiration
  • C6H12O6 6O2 ? 6CO2 6H2O
  • Glucose is oxidized, O2 is reduced, and e-s
    lose potential E
  • Molecules that have an abundance of hydrogen are
    excellent fuels because their bonds are a source
    of hilltop e-s that fall closer to O2

11
  • The cell has a rich reservoir of e-s associated
    with hydrogen, especially in carbohydrates and
    fats
  • However, these fuels dont spontaneously combine
    with O2 because they lack the activation E
  • Enzymes lower the barrier of activation E,
    allowing these fuels to be oxidized slowly
  • The fall of e-s during respiration is stepwise,
    via an ETC and NAD

12
  • Glucose and other fuels are broken down gradually
    in a series of steps, each catalyzed by a
    specific enzyme
  • At key steps, H atoms are stripped from glucose
    and passed first to a coenzyme, like NAD
    (nicotinamide adenine dinucleotide)
  • Dehydrogenase enzymes strip two H atoms from the
    fuel, pass two e-s and one proton to NAD and
    release H

13
  • O2 and H2 could either combust and lose all of
    the E as heat, or through a step-by-step process,
    the E could be harnessed to form ATP

14
Mitochondrion
  • Double membrane-bound organelle
  • Outer membrane
  • Inter- membrane space
  • Inner membrane
  • Matrix
  • Cristae folds of inner membrane

15
Cellular Respira-tion
  • 3 phases
  • Glyco- lysis
  • Krebs cycle
  • ETC

16
Substrate-level phosphorylation
  • Enzymes transfer phosphate from O-molecule to ADP
  • Occurs in glycolysis and the Krebs cycle

17
Oxidative phosphorylation
  • Occurs due to the ETC
  • The transfer of e-s down the ETC to O2 powers the
    phosphorylation of ADP to ATP
  • Produces almost 90 of the ATP generated by
    respiration

18
Glycolysis
  • Occurs in the cytoplasm
  • Glucose (6-C sugar) is split ultimately into two
    pyruvates
  • Process occurs regardless of the presence of O2
  • No CO2 is produced
  • Net E yield per glucose
  • 2 ATP
  • 2 NADH

19
Glycolysis
  • Each of the ten steps in glycolysis is catalyzed
    by a specific enzyme
  • Kinase phosphorylates
  • Isomerase rearranges molecules to form isomers
  • Dehydrogenase oxidizes O-molecules
  • 10 steps can be divided into two phases
  • an E investment phase
  • an E payoff phase

20
E investment phase
  • ATP provides activation E by phosphorylating
    glucose with 2 ATPs

21
E payoff phase
22
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23
Krebs Cycle
  • Pyruvate can now enter the mitochondrion
  • The Krebs cycle can only accept a 2-C molecule
  • So, a multi-enzyme complex breaks down the 3-C
    pyruvate to the 2-C acetate
  • This process is known as pyruvic acid breakdown

24
Pyruvic Acid Breakdown
  • An enzyme rips off a carboxyl group in the form
    of CO2 to make acetate
  • In the process, NAD is reduced to NADH
  • Coenzyme A then grabs the acetate, making acetyl
    CoA, and carries it off to the Krebs cycle

25
Pyruvic Acid Breakdown
26
Pyruvic Acid Breakdown
27
Krebs Cycle
  • Occurs in the matrix of the mitochondrion
  • The acetate from acetyl CoA bonds to oxaloacetate
    to form citrate
  • The cycle ultimately recycles oxaloacetate,
    releasing CO2, ATP, NADH, and FADH2 in the process

28
Krebs Cycle
29
  • Krebs cycle, per glucose, produces a total of
  • 8 NADH
  • 2 FADH2
  • 2 ATP
  • 6 CO2
  • Krebs cycle, per pyruvate, produces half of the
    above totals

30
ETC
  • Only 4 of the 38 ATPs that are formed in Cell
    Resp are formed by substrate-level
    phosphorylation
  • The other 34 come from the E from the e-s carried
    by NADH and FADH2
  • ETC is a chain of proteins found in the inner
    membrane of the mitochondrion
  • Its folded (cristae) to increase surface area

31
ETC
  • Thousands of ETCs are found on the cristae of a
    single mitochondrion
  • NADH and FADH2 are oxidized as they dump their
    e-s into the ETC
  • FADH2 has less free E ? it dumps its e-s later in
    the ETC ? fewer H move across the membrane

32
ETC
33
ETC
  • The e-s drop in free E as they are passed along
    the ETC
  • This loss of E drives H across the inner
    membrane from the matrix to the inter-membrane
    space (start of chemiosmosis)
  • The high H creates the proton-motive force
    ability of the proton gradient to do work

34
ETC
  • Each NADH contributes enough E to generate a
    maximum of 3 ATP
  • In some eukaryotic cells, NADH produced in the
    cytosol by glycolysis may only be worth 2 ATP
  • The e-s must be shuttled to the mitochondrion
  • In some shuttle systems, the e-s are passed to
    NAD, in others the e-s are passed to FAD
  • Each FADH2 can be used to generate about 2 ATP

35
ETC
36
ETC
  • The high proton concentration flows through ATP
    synthase from inter-membrane space to the matrix
  • ATP synthase makes ATP from ADP and Pi

37
ETC
38
ETC
  • Chemiosmosis is not unique to mitochondria
  • Plant cells use a very similar system in the
    chloroplasts
  • Prokaryotes generate H gradients across their
    plasma membrane

39
Anaerobic Respiration
  • Up to 38 ATPs can be generated when O2 is present
  • What happens when there is no O2?
  • Glucose can still be oxidized to make ATPmuch
    less ATP

40
Anaerobic Respiration
  • Glycolysis still takes place in the cytosol,
    which still produces
  • 2 (net) ATP
  • 2 NADH
  • 2 pyruvate
  • Pyruvate is harmful to cells and must be broken
    down

41
Anaerobic Respiration
  • The process is called fermentation
  • Alcoholic fermentation
  • Lactic acid fermentation

42
Alcoholic Fermentation
  • The NADH made in glycolysis is used to convert
    pyruvate into ethanol and CO2
  • Glucose ? pyruvate ? acetaldehyde CO2 ? ethanol

43
Alcoholic Fermentation
  • Utilized by yeast in the absence of O2
  • CO2 produced by fermentation allows bread to rise
  • Ethanol utilized in production of beer and wine

44
  • Alcoholic Fermentation

45
Lactic Acid Fermentation
  • The NADH made in glycolysis is used to convert
    pyruvate into lactate
  • Glucose ? pyruvate ? lactate

46
Lactic Acid Fermentation
  • Some fungi and bacteria are used to make cheese,
    yogurt, sour cream, sauerkraut, and pickles
  • Muscle cells switch from aerobic to anaerobic
    when O2 is depleted
  • NADH and FADH2 cannot dump their e-s into the ETC
  • Krebs cycle stops making NADH and FADH2, i.e.,
    the cycle stops
  • Pyruvate is not broken down

47
Lactic Acid Fermentation
  • Lactate may cause muscle fatigue, but it is
    ultimately converted back into pyruvate in the
    liver

48
Aer- vs. Anaerobic Respiration
  • Both perform glycolyis
  • Both generate ATP
  • Aerobic generates 38 ATP both substrate-level
    and oxidative phosphorylation
  • Anaerobic generates 2 ATP only substrate-level
    phosphorylation
  • Both use NADH as e- carrier

49
Facultative Anaerobes
  • Yeast and many bacteria (and humans at the
    cellular level) can perform either type

50
Obligates
  • Obligate aerobes must live where O2 is present
  • Obligate anaerobes must live where O2 is not
    present

51
Fuels for E
  • Glucose isnt the only fuel used to make ATP
  • Other O-molecules can be broken down and shoved
    into glycolysis, PAB, or the Krebs cycle
  • Other carbs, fats, and proteins can be used

52
Other carbs for E
  • Other monosaccharides can enter glycolysis just
    like glucose
  • Disaccharides are hydrolyzed into two
    monosaccharides
  • Polysaccharides are also hydrolyzed into their
    monomer constituents
  • Starch is digested into glucose in the digestive
    system
  • Glycogen is digested between meals

53
Fats for E
  • Fats store 2X the E as carbs
  • Broken into two parts glycerol and the fatty
    acids, which store most E
  • Glycerol converts to G3P (PGAL)
  • Fatty acids are broken down by beta oxidation
    into 2-C fragments that enter Krebs as acetyl-CoA

54
Proteins for E
  • Digested into the individual amino acids
  • aas must be daminated amino group removed
  • The NH3 waste is removed as either ammonia, urea,
    or other wastes
  • The remaining portion can enter as intermediates
    in either glycolysis, PAB, or Krebs

55
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56
Anabolic Pathways
  • Not all the O-molecules of food are completely
    oxidized to make ATP
  • Intermediaries in glycolysis and Krebs can be
    diverted to anabolic pathways
  • Ex a human cell can synthesize half the 20
    different aas by modifying compounds from the
    Krebs cycle

57
Anabolic Pathways
  • Glucose can be synthesized from pyruvate and
    fatty acids from acetyl-CoA
  • Excess carbs and proteins can be converted to
    fats through intermediaries of glycolysis and the
    Krebs cycle

58
Feedback Mechanisms
  • Supply and demand regulate metabolic economy
  • If a cell has an excess of a certain amino acid,
    it typically uses feedback inhibition to prevent
    the diversion of more intermediary molecules from
    the Krebs cycle to the synthesis pathway of that
    amino acid

59
Feedback Mechanisms
  • The rate of catabolism is also regulated,
    typically by the level of ATP in the cell
  • If ATP levels drop, catabolism speeds up to
    produce more ATP
  • High levels of ATP inhibit the enzyme
    phosphofructokinase (3rd step in glycol)
  • It is stimulated by high levels of AMP
  • Therefore, rate of glycolysis ? as ATP?
  • And rate of glycolysis ? as ATP?

60
Feedback Mechanisms
  • The rate of catabolism is also regulated by
    citrate
  • Citrate also inhibits phosphofructokinase
  • As citrate?, glycolysis?
  • As citrate?, glycolysis?
  • This synchronizes the rate of glycolysis and the
    Krebs cycle

61
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62
Feedback Mechanisms
  • Also, if intermediates from the Krebs cycle are
    diverted to other uses (Ex amino acid
    synthesis), glycolysis speeds up to replace these
    molecules
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