Transcript Metabolism: the chemical reactions of a cell

Metabolism: the chemical reactions of a cell
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• All organisms need two things with which to grow:
– Raw materials (especially carbon atoms)
– Energy.
• Types of metabolic reactions:
– Anabolism: biosynthesis; reactions that create
large/complex molecules from smaller, simpler ones. Use
raw materials and energy.
– Catabolism: degradation; reactions that break down
large/complex molecules, used to generate energy for use
and to produce smaller, building block molecules.
Energy: where from? What for?
• Chemotrophs vs. phototrophs
– Chemotrophs get energy from molecules
• Chemolithotrophs get energy from oxidation of
inorganic substances.
• Chemoorganotrophs get energy from oxidation of
organic compounds (like we do).
– Phototrophs get energy from sunlight
• Energy is needed to power the cell
– Biosynthesis to respond to environment, to grow
– Active transport, motility, etc.
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Bacteria obtain energy through
oxidation/reduction reactions
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• Oxidation: molecule gives up electrons
• Reduction: molecule accepts electrons
• Oxidation/reduction (redox) reactions always occur
in pairs; if electrons are removed, they must go
somewhere!
• Biological redox reactions usually involve PAIRS of
electrons.
• Biological redox reactions often involve entire
hydrogen atoms, not just the electrons (so called
dehydrogenation reactions).
Redox reactions release energy for use
• Depends on concentration, redox potential, etc.
• XH2 + Y
X + YH2 shows oxidation of X, reduction of Y
• Note that 2 H atoms are transferred, not just electrons
• Familiar redox reaction that releases energy:
• CH4 + 2O2
CO2 + 2H2O natural gas burning.
• Biological reactions release energy gradually, trap it as
ATP
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Is it good to eat?
• Reduced molecules have
lots of energy.
• Have lots of H, few O
• Oxidized molecules have
little energy;
• lots of O or few H.
Carbon dioxide
glucose
Redox Calculations
• One can assign an oxidation number to the carbon
atoms in a molecule to determine how much energy
an organic molecule has.
• Oxidation numbers:
H = +1
O = -2
– For an uncharged organic molecule, all the redox
numbers must add up to 0
– in methane (CH4): oxidation number C is -4.
– For CO2, 2 x -2 = -4; no net charge, then C must be = +4
– In going from CO2 to CH4 although a carbon has gained
electrons, the oxidation number for the carbon is lower,
thus “reduced”.
http://www.chem.vt.edu/RVGS/ACT/notes/oxidation_numbers.html
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Introduction to important molecules in metabolism
• Biological reactions release energy from redox reactions
gradually, trap it as ATP
• ATP is the energy molecule that cells use to power
most of their activities. “energy currency”
• ATP is a molecule under stress:
– too many negative charges in one place. Release of 1
phosphate: ATP → ADP + Pi relieves that stress,
releases energy which can be used for:
– cellular activities such as transport, motility,
biosynthesis, etc.
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Structure of ATP
http://www.ustboniface.mb.ca/cusb/abernier/Biologie/Module1/Image
s/atp.jpg
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The ATP cycle
ATP is hydrolyzed to
ADP to release
energy.
Energy is used to
reattach the
phosphate to ADP to
regenerate ATP.
Other molecules used for energy
include GTP and PEP.
www.cat.cc.md.us/.../ metabolism/energy/fg1.html
Important molecules: the electron carriers -1
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• The energy released in redox reactions is often
thought of as the energy in the bonds between the H
and the C; when a molecule is reduced by transfer of
the H, the energy is conserved in that reduced
molecule.
Important molecules: the electron carriers -2
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• The most common electron carrier in biological
redox reactions is NAD:
• NAD + XH2 X + NADH + H+
– where NAD carries 2 e-, 1 H+
– Reduced NAD (NADH) is like poker chips,
energy that can’t be spent, but can be “cashed in”
later to make ATP (which can be “spent”, i.e.
used as an energy source for cell activities).
Structure of NAD
http://www.bact.wisc.edu/themicrobialworld/NAD.jpg
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Other electron carriers
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• NADP
– Reduced during some reactions such as those in the
Pentose Phosphate pathway, Photosynthesis
– NADPH used to donate H for biosynthesis reactions such
as the Calvin Cycle, amino acid biosynthesis.
• FAD
– Reduced to FADH2
– Used in the Krebs Cycle, other reactions
Oxidations for energy
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If this reaction is reversible, and every oxidation is coupled to
a reduction, how can the oxidation of XH2 yield energy?
Go = - R T ln K
Describes the tendency of a reaction to occur spontaneously, to
release energy. Two major factors are the tendencies of X and
Y to give up or accept electrons and their concentrations.
In this example, we expect that YH2 will subsequently be
oxidized, driving the reaction to the right and that XH2 has a
greater tendency to give up electrons than YH2 .