Cellular Metabolism
Chapter 4
Cellular Metabolism
 Cellular metabolism refers to all of the
chemical processes that occur inside living
cells.
Energy
 Energy can exist in two states:
 Kinetic energy – energy of motion.
 Potential energy – stored energy.
 Chemical energy – potential energy stored in
bonds, released when bonds are broken.
 Energy can be transformed form one state to
another.
Energy
 The ultimate source
of energy for most
living things is the
sun.
Laws of Thermodynamics
 First law of thermodynamics – energy cannot
be created or destroyed – only transformed.
 Second law of thermodynamics – a closed
system moves toward entropy, increasing
disorder.
 Living systems are open systems that maintain
organization and increase it during development.
Free Energy
 Free energy – the energy available for doing
work.
 Most chemical reactions release free energy – they
are exergonic.
 Downhill
 Some reactions require the input of free energy –
they are endergonic.
 Uphill
Enzymes
 Bonds must be destabilized before any reaction
can occur – even exergonic.
 Activation energy must be supplied so that the
bond will break.
 Heat – increases rate at which molecules collide.
 Catalysts can lower activation energy.
Enzymes
 Catalysts are chemical substances that speed
up a reaction without affecting the products.
 Catalysts are not used up or changed in any
way during the reaction.
 Enzymes are important catalysts in living
organisms.
Enzymes
 Enzymes reduce the
amount of activation
energy required for a
reaction to proceed.
 Enzymes are not
used up or altered.
 Products are not
altered.
 Energy released is
the same.
Enzymes
 Enzymes may be pure proteins or proteins
plus cofactors such as metallic ions or
coenzymes, organic group that contain groups
derived from vitamins.
Enzyme Function
 An enzyme works by binding with its
substrate, the molecule whose reaction is
catalyzed.
 The active site is the location on the enzyme
where the substrate fits.
 Enzyme + Substrate = ES complex.
Enzyme Specificity
 Enzymes are highly specific.
 There is an exact molecular fit between enzyme and
substrate.
 Some enzymes work with only one substrate, others work
with a group of molecules.
 Succinic dehydrogenase oxidizes only succinic acid.
 Proteases will act on any protein, although they still
have a specific point of attack.
Enzyme-Catalyzed Reactions
 Enzyme-catalyzed reactions are reversible.
 Indicated by double arrows in reactions.
 Tend to go mostly in one direction.
 Reactions tend to be catalyzed by different
enzymes for each direction.
 Catabolic (degradation) reaction catalyzed by
enzyme A.
 Anabolic (synthesis) reaction catalyzed by enzyme
B.
Importance of ATP
 Endergonic reactions require energy to
proceed.
 Coupling an energy-requiring reaction with an
energy-yielding reaction can drive endergonic
reactions.
 ATP is the most common intermediate in
coupled reactions.
Importance of ATP
 ATP consists of
adenosine (adenine
+ ribose) and a
triphosphate group.
 The bonds between
the phosphate
groups are high
energy bonds.
 A-P~P~P
Importance of ATP
 Phosphates have
negative charges.
 Takes lots of energy
to hold 3 in a row!
 Ready to spring
apart.
 So, ATP is very
reactive.
Importance of ATP
 A coupled reaction
is a system of two
reactions linked by an
energy shuttle – ATP.
 Substrate B is a fuel
– like glucose or lipid.
 ATP is not a
storehouse of energy
– used as soon as
it’s available.
Oxidation – Reduction Redox
 An atom that loses
an electron has
been oxidized.
Oxygen is a
common electron
acceptor.
 An atom that gains
an electron has
been reduced.
Higher energy.
Redox Reactions
 Redox reactions always occur in pairs.
 One atom loses the electron, the other gains
the electron.
 Energy is transferred from one atom to another
via redox reactions.
Cellular Respiration
 Cellular respiration – the oxidation of food
molecules to obtain energy.
 Electrons are stripped away.
 Different from breathing (respiration).
Cellular Respiration
 Aerobic versus Anaerobic Metabolism
 Heterotrophs
 Aerobes: Use molecular oxygen as the final
electron acceptor
 Anaerobes: Use other molecules as final electron
acceptor
 Energy yield much lower ATP yield
Cellular Respiration
 When oxygen acts as the final electron
acceptor (aerobes):
 Almost 20 times more energy is released than if
another acceptor is used (anaerobes).
 Advantage of aerobic metabolism:
 Smaller quantity of food required to maintain
given rate of metabolism.
Aerobic Respiration
 In aerobic respiration, ATP forms as electrons are
harvested, transferred along the electron transport chain
and eventually donated to O2 gas.
 Oxygen is required!
 Glucose is completely oxidized.
 C6H12O6 + 6O2
Glucose
Oxygen
6CO2 + 6H2O + energy (heat
or ATP)
Carbon
Water
Dioxide
Cellular Respiration - 3
Stages
 Food is digested to break it into
smaller pieces – no energy
production here.
 Glycolysis – coupled reactions
used to make ATP.
 Occurs in cytoplasm
 Doesn’t require O2
 Oxidation – harvests electrons
and uses their energy to power
ATP production.
 Only in mitochondria
 More powerful
Anaerobic Respiration
 Anaerobic respiration occurs in the absence
of oxygen.
 Different electron acceptors are used instead of
oxygen (sulfur, or nitrate).
 Sugars are not completely oxidized, so it doesn’t
generate as much ATP.
Glycolysis
 Glycolysis – the first stage in cellular
respiration.
 A series of enzyme catalyzed reactions.
 Glucose converted to pyruvic acid.
 Small number of ATPs made (2 per glucose
molecule), but it is possible in the absence of
oxygen.
 All living organisms use glycolysis.
Glycolysis
 Uphill portion primes the fuel
with phosphates.
 Uses 2 ATPs
 Fuel is cleaved into 3-C
sugars which undergo
oxidation.
 NAD+ accepts e-s & 1 H+ to
produce NADH
 NADH serves as a carrier to
move high energy e-s to the
final electron transport chain.
 Downhill portion produces 2
ATPs per 3-C sugar (4 total).
 Net production of 2 ATPs per
glucose molecule.
Glycolysis
 Summary of the enzymatically catalyzed
reactions in glycolysis:
Glucose + 2ADP + 2Pi + 2 NAD+
2ATP
2 Pyruvic acid + 2 NADH +
http://www.youtube.com/watch?v=3GTjQTqUuOw&list=FL9N_Px072WuVorSwDfqf-9w&index=4&feature=plpp
Harvesting Electrons form
Chemical Bonds
 When oxygen is available, a second oxidative
stage of cellular respiration takes place.
 First step – oxidize the 3-carbon pyruvate in the
mitochondria forming Acetyl-CoA.
 Next, Acetyl-CoA is oxidized in the Krebs cycle.
Producing Acetyl-CoA
 The 3-carbon pyruvate
loses a carbon producing
an acetyl group.
 Electrons are transferred
to NAD+ forming NADH.
 The acetyl group
combines with CoA
forming Acetyl-CoA.
 Ready for use in Krebs
cycle.
The Krebs Cycle
 The Krebs cycle is the next stage in oxidative
respiration and takes place in the mitochondria.
 Acetyl-CoA joins cycle, binding to a 4-carbon molecule
to form a 6-carbon molecule.
 2 carbons removed as CO2, their electrons donated to
NAD+, 4-carbon molecules left.
 2 NADH produced.
 More electrons are extracted and the original 4-carbon
material is regenerated.
 1 ATP, 1 NADH, and 1 FADH2 produced.
The Krebs Cycle
 Each glucose provides 2 pyruvates, therefore 2
turns of the Krebs cycle.
 Glucose is completely consumed during
cellular respiration.
The Krebs Cycle
Acetyl unit + 3 NAD+ + FAD + ADP + Pi
NADH + FADH2 + ATP
http://www.youtube.com/watch?v=-cDFYXc9Wko
2 CO2 + 3
Using Electrons to Make ATP
 NADH & FADH2
contain energized
electrons.
 NADH molecules carry
their electrons to the
inner mitochondrial
membrane where they
transfer electrons to a
series of membrane
bound proteins – the
electron transport
chain.
Building an Electrochemical
Gradient
 In eukaryotes, aerobic metabolism takes place
in the mitochondria in virtually all cells.
 The Krebs cycle occurs in the matrix, or
internal compartment of the mitochondrion.
 Protons (H+) are pumped out of the matrix into
the intermembrane space.
Producing ATPChemiosmosis
 A strong gradient
with many protons
outside the matrix
and few inside is set
up.
 Protons are driven
back into the matrix.
 They must pass
through special
channels that will
drive synthesis of
ATP.
 Oxidative
phosphorylation
Electron Transport Review
http://www.youtube.com/watch?v=kN5MtqAB_Yc&list=FL9N_Px072WuVorSwDfqf-9w&index=2&feature=plpp
Review of Cellular Respiration
 1 ATP generated for each proton pump
activated by the electron transport chain.
 NADH activates 3 pumps.
 FADH2 activates 2 pumps.
 The 2 NADH produced during glycolysis must
be transported across the mitochondrial
membrane using 2 ATP.
 Net ATP production = 4
Glucose + 2 ATP + 36 ADP + 36 Pi + 6 O2
6CO2 + 2 ADP + 36 ATP + 6 H2O
Fermentation
 In the absence of oxygen, the end-product of
glycolysis, pyruvate, is used in fermentation.
 During glycolysis, all the NAD+ becomes saturated
with electrons (NADH). When this happens,
glycolysis will stop.
 2 NADH and 2 ATP produced.
 Pyruvate is used as the electron acceptor resetting
the NAD+ for use in glycolysis.
Fermentation – 2 Types
 Animals add extracted
electrons to pyruvate
forming lactate.
 Reversible when oxygen
becomes available.
 Muscle fatigue
 Yeasts, single-celled fungi,
produce ethanol.
 Present in wine & beer.
 Alcoholic fermentation
Metabolism of Lipids
 Triglycerides are broken down into glycerol and
3 fatty acid chains.
 Glycerol enters glycolysis.
 Fatty acids are oxidized and 2-C molecules
break off as acetyl-CoA.
 Oxidation of one 18-C stearic acid will net 146 ATP.
 Oxidation of three glucose (18 Cs) nets 108 ATP.
 Glycerol nets 22 ATP, so 1 triglyceride nets 462 ATP.
Metabolism of Proteins
 Proteins digested in the gut into amino acids
which are then absorbed into blood and
extracellular fluid.
 Excess proteins can serve as fuel like
carbohydrates and fats.
 Nitrogen is removed producing carbon skeletons
and ammonia.
 Carbon skeletons oxidized.
Metabolism of Proteins
 Ammonia is highly
toxic, but soluble.
 Can be excreted by
aquatic organisms
as ammonia.
 Terrestrial
organisms must
detoxify it first.
Regulating Cellular
Respiration
 Rate of cellular respiration slows down when
your cells have enough ATP.
 Enzymes that are important early in the
process have an allosteric (regulating) site that
will bind to ATP.
 When lots of ATP is present, it will bind to this
site, changing the shape of the enzyme, halting
cellular respiration.
Regulating Cellular
Respiration
 Enzyme activity is controlled by presence or
absence of metabolites that cause
conformational changes in enzymes.
 Improves or decreases effectiveness as catalyst.