HOW CELLS RELEASE ENERGY

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HOW CELLS RELEASE ENERGY
The last chapter was concerned with how certain
cells cyanobacteria, algae, plants) capture
energy (photosynthesis). This chapter is
concerned with how cells release that energy.

• Chapter 08

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• All cells (prokaryotic eukaryotic) require
  energy to
• combat entropy
• carry out day-to-day functions
• repair/replace worn out organelles
• reproduce
• What form of energy do cells use?

ATP
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• How do cells obtain ATP?
• All cells must make their own ATP from nutrients
  they have either synthesized (autotrophs) or
  consumed (heterotrophs).
• Most cells break down nutrients to make ATP by
• Cellular respiration (aerobic process)
• Fermentation (anaerobic process)
• Aerobic - requiring oxygen (O2)
• Anaerobic - lacking or not requiring oxygen (O2)

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• A. Cellular Respiration (aka. Aerobic
  Respiration)
• Biochemical pathways that extract energy from
  nutrients, in the presence of oxygen.
• Occurs in cells of most eukaryotes some
  prokaryotes.
• Eukaryotes - protists, fungi, plants animals
• Prokaryotes - bacteria archaea
• General equation for cellular respiration of
  glucose
• C6H12O6 6O2 ? 6CO2 6H2O 30 ATP

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• Cellular respiration occurs in 3 stages

Eukaryotic cells
Prokaryotic cells
Glycolysis Krebs Cycle Electron Transport Chain
Cytoplasm
Cytoplasm
Mitochondria
Cell membrane
Glycolysis, Krebs cycle electron transport
chain occur in different locations within cells.
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• 1. Glycolysis (glucose-splitting)
• Glucose (6C) is split into two pyruvate (3C)
  molecules. Pyruvate pyruvic acid
• does not require oxygen
• energy harvested/glucose
• 2 ATP (via substrate-level phosphorylation
  )
• 2 NADH (actively transported into
  mitochondria of eukaryotic cells)
• Eukaryotic cells have to use 2 ATP molecules to
  shuttle these 2 NADHs into mitochondria. Energy
  stored in the bonds of NADH is used by electron
  transport chain to produce ATP.

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• First half of glycolysis activates glucose.
• Note two ATP molecules have to be used to get
  the reaction going.

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• Second half of glycolysis extracts energy.
• Total energy yield during the 2nd half of
  glycolysis is 4 ATPs and 2 NADHs. Since 2 ATPs
  were consumed in the first half of glycolysis,
  there is a net gain of only 2 ATPs.
• Note ATP is synthesized in glycolysis by
  substrate-level phosphorylation. This means that
  an enzyme transfers a phosphate group from an
  organic molecule (substrate) to ADP, forming ATP.

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• Pyruvic acid must be converted to Acetyl CoA
  before it can enter Krebs cycle.
• This transition step takes place in the
  mitochondria of eukaryotic cells cytoplasm of
  bacterial cells.
• During the conversion, CO2 is released NAD is
  reduced to NADH.
• For every glucose molecule that enters
  glycolysis, 2 pyruvates are produced converted
  into 2 acetyl CoA molecules.

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• 2. Krebs Cycle (aka. citric acid cycle)
• Acetyl CoA is broken down completely to CO2.
• cells use carbon skeletons of intermediates to
  produce other organic molecules (amino acids).
• energy harvested per acetyl CoA
• 1 ATP (via substrate-level phosphorylati
  on)
• 3 NADH
• 1 FADH2
• Known as citric acid cycle because first molecule
  formed after acetyl CoA enters is citric acid
  (citrate).

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• Thus far, how much useable energy has been
  produced from the breakdown of 1 glucose
  molecule?
• 4 ATPs
• Need the electron transport chain to harvest
  potential energy in NADHs FADH2s.

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• 3. Electron Transport Chain (ETC)
• Series of proteins electron carriers embedded
  in the inner mitochondrial membrane (eukaryotes)
  or cell membrane (prokaryotes).
• O2 is the final electron acceptor
• H2O is the final product
• energy harvested/NADH 2.5 ATPs (via
  chemiosmotic phosphorylation)
• energy harvested/FADH2 1.5 ATPs (via
  chemiosmotic phosphorylation)

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• Chemiosmotic phosphorylation
• occurring in mitochondria is essentially the same
  as that occurring in chloroplasts.
• NADH FADH2
• molecules pass electrons to the ETC. As
  electrons move down the chain, they release
  energy which is used to pump protons (H) out of
  the mitochondrial matrix into the intermembrane
  space. A proton gradient is established.
  Gradient drives ATP synthesis (protons pass
  through ATP synthase channels from the
  intermembrane space to the matrix ADP is
  phosphorylated, forming ATP).
• NOTE
• Some insecticides, like 2,4-dinitrophenol
• kill by making the inner mitochondrial membrane
  permeable to protons (destroys the proton
  gradient). Insect dies when it runs out of ATP.
• Cyanide carbon monoxide
• kill because they block the transfer of electrons
  to oxygen. Organism dies when it runs out of
  ATP.

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• How many ATPs can 1 glucose yield?

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• Most of the ATP generated from cellular
  respiration comes from chemiosmotic
  phosphorylation.
• NADH molecules yield more ATPs than FADH2
  molecules (2.5 vs. 1.5/molecule) because their
  electrons enter the chain a step earlier than
  FADH2 electrons.
• How efficient is cellular respiration? About 32
  of the energy in glucose is passed to ATP.
  Doesnt seem very efficient until you compare it
  to the efficiency of an automobile (20-25).
• NOTE 30 ATPs per glucose is an estimate. ATP
  yield is variable for several reasons
• highly active cells (muscle, liver) tend to
  generate more ATPs.
• not all NADHs produced in cellular respiration
  are used to produce ATP.
• not all protons pumped by ETC flow back through
  ATP synthase.
• many of the intermediates of glycolysis Krebs
  cycle may be used to form other organic
  molecules.

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• Can cells use proteins lipids to produce
  energy?
• Most cells use carbohydrates as the primary
  source of energy (ATP) however, many cells can
  use monomers of proteins lipids to produce
  energy.
• Neurons cannot utilize energy in proteins
  lipids. They must break down carbohydrates to
  obtain energy.
• Plant cells use energy derived from lipids to
  fuel activities such as seed germination.

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• Proteins are digested to amino acids. To produce
  ATP from amino acids, cells must convert them
  into pyruvic acid, acetyl CoA or intermediates of
  Krebs cycle.
• Fats are digested to glycerol fatty acids.
  Glycerol is converted to pyruvic acid. Fatty
  acids are converted to acetyl CoA.

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• B. Fermentation
• Biochemical pathways that extract energy from
  nutrients, in the absence of oxygen.
• 1. Alcoholic fermentation
• Pyruvic acid is broken down to ethanol and carbon
  dioxide.
• Ex. yeast (used in production of baked goods
  alcoholic beverages)

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Notice that alcoholic fermentation yields only 2
ATPs (from glycolysis).
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• 2. Lactic acid fermentation
• Pyruvic acid is broken down to lactic acid.
• Examples
• certain bacteria (used in production of cheese
  yogurt)
• human muscle cells in oxygen debt
• Under oxygen-debt conditions (cells are working
  so strenuously that their production of pyruvic
  acid exceeds the oxygen supply), human muscle
  cells revert to lactic acid fermentation to
  extract energy. If enough lactic acid
  accumulates, the muscle fatigues cramps.

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Notice that lactic acid fermentation yields only
2 ATPs (from glycolysis).
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• Photosynthesis, glycolysis cellular respiration
  are interrelated.

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• Products of photosynthesis (O2 glucose) are
  reactants in cellular respiration.
• Products of cellular respiration (CO2 H2O) are
  reactants in photosynthesis.
• Glycolysis is probably the most ancient of the
  energy pathways because it is common to nearly
  all cells (bacteria, archaea, protists, fungi,
  plants, animals).