How Cells Harvest Chemical Energy

1
How Cells Harvest Chemical Energy

• Chapter 6

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Overview

• Photosynthesis
• Aerobic respiration
• Anaerobic respiration
• Alternate sources of energy

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Components of a Reaction

• Reactants
• Intermediates
• Products

A
C
B
4
Endergonic vs. Exergonic Reactions

• Endergonic
• Energy-requiring
• Exergonic
• Energy-releasing

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Redox Reactions

• One molecule gives up electrons oxidized
• One molecule gains electrons reduced
• H atoms released simultaneously
• (are attracted to negative charge of electrons)
• Coenzymes pick up e-s H from substrates
  deliver to e- transfer chains

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Electron Transfer Chains

• Membrane-bound groups of enzymes/molecules
• Accept give up e-s in sequence
• E-s enter chain at higher energy level than when
  they leave it
• (lose energy at each descending step of chain)

e-s
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Substrate-Level Phosphorylation

• Formation of ATP by direct transfer of Pi group
  to ADP from intermediate

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NAD FAD

• Coenzymes
• 1. Accept e-s H from intermediates that form
  during glucose catabolism
• Become reduced NADH FADH2
• NADH and FADH2 give up e-s H to e- transfer
  chains during final stages of aerobic respiration
• Become oxidized NAD FAD

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Autotrophs

• Self-nourishing
• Synthesize own food
• Obtain energy organic compounds (e.g. C) from
  the physical environment

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Chemoautotrophs

• Have no enzymes to allow for complex metabolic
  reactions
• Obtain energy C from simple inorganic organic
  compounds e.g. CH4, H2S

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Photoautotrophs

• Contain light-sensitive molecules
• Can split H2O use electrons
• Process releases lots of oxygen, which reacts
  rapidly with metals creates toxic free radicals

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• Early photoautotrophs existed when there was
  lots of Fe metals everywhere
• Released O2 oxidized these metals rusted them
  out
• O2 could then be released freely

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• Over a few thousand years, O2 levels in sea
  atmosphere increased
• Survival of the fittest
• Most anaerobes died out because couldnt
  neutralize toxic O2 radicals
• Chemoautotrophs with little or no O2 tolerance
  restricted to extreme anoxic environments

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• As O2 accumulated in the atmosphere, O atoms
  combined to form O3
• ozone layer
• (protects against lethal UV radiation from sun)
• Life was able to move out from the darks
  live under open sky
• diversification
• evolution

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Photosynthesis

• The process by which photoautotrophs use light
  energy from the sun to make glucose, which can
  then be converted into ATP
• 12H2O 6CO2 ? 6O2 C6H12O6 6H2O

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• Respiration
• breathing
• Cellular respiration
• getting energy from food

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• Organisms need usable energy in order to survive
• Obtained energy is converted into ATP chemical
  bond energy
• Can be used to do work e.g. metabolism

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Anaerobes

• Cant tolerate O2
• Make ATP via fermentation
• 1 glucose ? 2 ATP
• e.g. first organisms, some prokaryotes
  eukaryotes

Clostridium difficile
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Aerobes

• Require O2
• Make ATP via aerobic respiration
• (many also use anaerobic pathways)
• 1 glucose ? 36 ATP
• (vital for survival of large organisms)
• e.g. most eukaryotes, some prokaryotes

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Facultative Anaerobes

• Normally use aerobic pathways (i.e. use O2)
• Can switch to anaerobic pathways when O2 levels
  are low

Entamoeba histolytica
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Mitochondria

• Membrane-bound organelles in most eukaryotic
  cells
• ( differs depending on cell type)
• Power source of cells
• Production of ATP in presence of O2
• Convert NADH and FADH2 into ATP energy via
  oxidative phosphorylation
• Allow cell to produce lots of ATP simultaneously
• Without mitochondria, complex animals wouldnt
  exist

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Mitochondrion Structure
Outer membrane Selectively permeable Inner
membrane Highly impermeable
Contains ATP synthase Has
membrane potential Cristae ? surface area of
inner membrane, which ? capacity to
generate ATP Matrix Contains 100s of enzymes
which oxidize pyruvate and fatty acids, and
control the Krebs cycle
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Cellular Respiration

• The oxidation of food molecules (e.g. glucose)
  into CO2 H2O
• Energy released is captured as ATP
• Used for all endergonic activities of cell
• Enzymes catalyze each step
• Intermediates formed at one step become
  substrates for enzyme at next step

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2 phases of cellular respiration

• Glycolysis
• Glucose ? 2 pyruvates
• Occurs in all cells
• Oxidation of pyruvate into CO2 and H2O
• Energy-releasing pathways differ depending on
  cell its needs

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• 40 of energy from glucose is harvested
• Rest (60) is lost as heat
• A working muscle uses 10 million ATP per second!

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Aerobic Respiration

• C6H1206 6O2 ? 6CO2 6H2O
• Breakdown of glucose in presence of O2
• 3 stages of reactions
• Glycolysis
• Krebs Cycle
• Electron Transfer Phosphorylation

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• Glycolysis
• Glucose ? 2 pyruvates
• Occurs in cytosol
• Krebs Cycle
• Pyruvate ? CO2 H2O e-s
• Occurs in mitochondria
• Electron Transfer Phosphorylation
• Formation of lots of ATP

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Stage I Glycolysis

• Glucose ? 2 pyruvates
• Universal energy-harvesting process of life
• Initial energy-releasing mechanism for all cells
• Occurs in cytosol
• Coupled endergonic exergonic reactions

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Endergonic Steps of Glycolysis

• Requires input of 2 ATP
• ATP 1 phosphorylates glucose
• Glucose ? intermediate
• ATP 2 transfers Pi to intermediate
• Intermediate ? PGAL DHAP
• DHAP converts into PGAL
• 2 PGAL enter next stage

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Exergonic Steps of Glycolysis

• Each PGAL gives 2 e-s H to NAD
• 2 NAD ? 2 NADH
• Intermediates each give Pi to ADP
• 2 ADP ? 2 ATP
• (substrate-level phosphorylation)
• Pays back 2 ATP used in endergonic steps
• Intermediates each release H OH
• 2 intermediates ? 2 PEP
• Each PEP gives Pi to ADP
• 2 ADP ? 2 ATP
• (substrate-level phosphorylation)
• 2 PEP ? 2 pyruvate

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Sum Total of Glycolysis

• Glucose ? 2 pyruvate 2 NADH 2 ATP
• From here, pyruvate can enter
• Aerobic pathway (Krebs cycle)
• Anaerobic pathway (fermentation)
• (depends on cell environmental conditions)

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Stage II Krebs Cycle

• Pyruvate ? CO2 H2O ( e-s)
• a.k.a. citric acid cycle
• Occurs in mitochondria
• Main function is to supply Stage III with e-s
• (in order to reduce NAD FAD in stage III)

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• Mitochondrial membrane proteins transport
  pyruvate into inner compartment
• Enzymes take 1 C from pyruvate
• C O2 ? CO2
• Intermediates coenzyme A ? acetyl-CoA
• NAD is reduced into NADH

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• Acetyl-CoA enters Krebs cycle
• Transfers 2 Cs to oxaloacetate ? citrate
• Rearrangement of intermediates occurs
• 2 C released ? 2CO2
• 3 NAD H e-s ? 3 NADH
• ADP Pi ? ATP
• FAD H e-s ? FADH2
• Oxaloacetate regenerates so that cycle can run
  again

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• In total, one turn of the cycle
• 3 NADH 1 FADH2 1 ATP
• Cycle repeats again for 2nd pyruvate molecule
• Remember 1 glucose ? 2 pyruvates
• After both pyruvates are broken down
• 6 NADH 2 FADH2 2 ATP

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Sum Total of the Krebs Cycle

• With 2 NADH from acetyl-CoA formation
• 2 pyruvate ? 8 NADH 2 FADH2 2 ATP 6CO2
• CO2 released into surroundings
• NADH FADH2 deliver e-s and H to 3rd stage

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Stage III Electron Transfer Phosphorylation

• H e-s ? H2O ATP
• E-s delivered to electron transfer chains (ETCs)
  in inner mitochondrial membrane
• E- flow in ETCs drives phosphorylation of ADP ?
  ATP (lots of it!)

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• a.k.a. oxidative phosphorylation
• ATP formed by oxidation of NADH FADH2
• Responsible for high ATP yield

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• NADH FADH2 give e-s to ETCs
• Simultaneous release of H
• Energy released at each transfer of ETC
• At 3 transfers, released energy pumps H across
  mitochondrial membrane into outer compartment
• Concentration electric gradients result across
  inner membrane
• membrane potential

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• H re-enters inner compartment by flowing down
  concentration gradient through ATP synthases
• Causes reversible change in shape of ATP
  synthases
• ADP Pi ? ATP
• (oxidative phosphorylation)
• At end of ETCs, O2 picks up e-s H? H2O

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Sum Total of ET Phosphorylation

• H e-s ? H2O 32 ATP
• In liver, heart, and kidney
• e-s from NADH delivered to different ETC entry
  point
• H gradient makes 3 ATP (instead of 1)
• Results in 34 ATP total

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Animation of ET Phosphorylation

• http//vcell.ndsu.nodak.edu/animations/etc/movie.h
  tm
• http//highered.mcgraw-hill.com/sites/0072437316/s
  tudent_view0/chapter9/animations.html

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• In oxygen-starved cells, e-s have nowhere to go
  so get gridlocked
• No e- flow no H gradients no ATP forms
• Results in cell death because not enough ATP to
  sustain metabolic processes

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• 3 different categories of poisons interfere with
  cellular respiration
• ETC Blockers
• Inhibitors
• Uncouplers

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ETC Blockers

• Block ETC at various steps of chain
• Starves cells of energy by prohibiting ATP
  synthesis

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Inhibitors

• Inhibit ATP synthase
• No passage of H through ATP synthase
• no ATP generation

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Uncouplers

• Make mitochondrial membrane leaky to H
• Electron transport O2 consumption continue but
  lack of H gradient no ATP synthesis

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Sum Total of Aerobic Respiration

• Glycolysis
• Glucose ? 2 pyruvate 2 NADH 2 ATP
• Krebs Cycle
• 2 pyruvate ? 8 NADH 2 FADH2 2 ATP 6CO2
• ET Phosphorylation
• H e-s (from coenzymes) ? 32 ATP 6H2O

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• Glucose 6O2 ? 6CO2 6H2O 36 ATP
• (38 ATP in liver, heart, kidney)

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Cellular Respiration Video

• http//video.google.com/videoplay?docid1463788471
  587082686qrespirationninjatotal2start0num
  10so0typesearchplindex0

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Anaerobic Respiration

• Oxidation of molecules in absence of O2
• Requires different electron acceptor at the end
  of it
• 2 stages
• Glycolysis
• Various Energy-Releasing Pathways
• Alcoholic Fermentation
• Lactate Fermentation
• Anaerobic Electron Transfers

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Stage I Glycolysis

• Glucose ? 2 pyruvate 2 NADH 2 ATP
• Glucose is not broken down any further into CO2
  or H2O
• All ATP comes from glycolysis
• (not enough energy to sustain large multicellular
  organisms)

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After Glycolysis

• Final stages in fermentation pathways do not
  generate more ATP
• Instead, they regenerate NAD so that it can act
  as an electron acceptor

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• Pyruvate not moved into mitochondria
• (stays in cytosol is converted into waste
  products that can be transported out of cells)
• Waste product depends on type of cell
• e.g. ethanol in yeast
• e.g. lactate in skeletal muscles, bacteria

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Alcoholic Fermentation

• 2 pyruvates ? 2 acetylaldehydes 2CO2
• NADH gives up e-s H to acetaldehyde ? ethanol
• e.g. yeasts (Saccharomyces spp.) that ferment
  wine, bread, etc.

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Lactate Fermentation

• NADH gives e-s and H to pyruvate ? lactate
• e.g. bacteria (Lactobacillus spp. and others) in
  cheese, yoghurt, etc.

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• Certain organisms can couple aerobic anaerobic
  respiration or switch from one mode to the other
• e.g. skeletal muscles associated with bones
• Variety of cell types within muscle fibres

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Slow-Twitch Muscle Fibres

• Lots of mitochondria myoglobin
• Make ATP via aerobic respiration
• Used for steady, prolonged activity
• e.g. long-distance running, migration, etc.
• e.g. dark meat of birds

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Fast-Twitch Muscle Fibres

• Few mitochondria no myoglobin
• Make ATP via lactate fermentation
• Used for short bursts of intense activity
• e.g. sprints, weight-training
• e.g. white meat of birds

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Anaerobic Energy Transfers

• Pathway of some archaeans bacteria
• Inorganic compounds used as final e- acceptors
  rather than O2
• Aids in cycling of elements through biosphere
• Energy yield varies but is small

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Alternative Energy Sources

• Glycogen Stores
• Lipids
• Proteins

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The Fate of Glucose

• When food is ingested, glucose is absorbed into
  the bloodstream
• Pancreas secretes insulin to make cells take up
  glucose faster
• Cells convert glucose to glucose-6-phosphate
  (intermediate of glycolysis)
• Cant leave cell once phosphorylated

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Glycogen Stores

• When more glucose than necessary is taken in, is
  biosynthesized into glycogen
• (stored in liver and muscles)
• Only 1 of total energy stores
• Glycogen stores used up within 12 hours if
  regular meals arent eaten

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• When blood glucose drops, pancreas secretes
  glucagon
• Liver cells convert stored glycogen ? glucose
  send back to blood
• Can then enter glycolysis pathway

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• If excess carbs are eaten
• Glucose ? pyruvate ? acetyl-CoA
• (aerobic respiration)
• Acetyl-CoA doesnt enter Krebs cycle if excess
  glucose
• Enters lipid biosynthesis pathway instead
• ??? carbs fat

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Fat Stores

• Fat stored as triglycerides in adipose cells
• Triglyceride glycerol 3 fatty acid tails
• Between meals or during sustained exercise,
  fatty acids yield half of ATP needed by muscle,
  liver, kidney cells

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• When blood glucose ?, enzymes in adipose cells
  separate glycerol fatty acids and release into
  blood
• Glycerol ? PGAL
• Used in glycolysis
• Fatty acids ? acetyl-CoA
• Used in Krebs cycle

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Protein Stores

• When proteins ingested, are broken down into
  amino acids
• Absorbed into bloodstream and taken up by cells
  to make more proteins, etc.
• If excess protein eaten, amino acids broken down
• Form acetyl-CoA, pyruvate, or intermediates of
  Krebs cycle

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• Reverse pathways also exist
• Food molecules used for biosynthesis
• Requires ATP

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Summary

• Energy is required by all living organisms to
  sustain life
• Because energy flow is one-directional, constant
  energy inputs are needed

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Interpreting Data
0

• This graph illustrates the free energy relative
  to oxygen of the electron transport chain. The
  solid blue circles are electron carrier
  molecules, and the light blue ovals represent
  protein complexes. From an energy standpoint,
  are these reactions endergonic or exergonic?
• Endergonic
• Exergonic
• Some are exergonic and others are endergonic.
• There is not enough information.

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Interpreting Data
0

• What would happen to the flow of electrons if
  oxygen were not present?
• The flow of electrons would continue but at a
  slower rate.
• The flow would cease and ATP production would
  stop.
• The presence of oxygen would have no effect.