Transcript Document

Cells require a constant supply of energy, which they derive from chemical bond
energy in food molecules. Plants make their own sugars by photosynthesis.
Animal eat other organisms for food. Sugars are oxidized to carbon dioxide and
water in a stepwise fashion. Energy is saved as high-energy chemical bonds in
activated carrier molecules such as ATP and NADPH.
And enzymes
Cells use enzymes
to carry out
oxidation in a
controlled series of
reactions,
generating activated
carrier molecules by
coupled reactions
Enzymatic
breakdown
(digestion)
occurs in our
intestine outside
cells or in
lysosomes
inside cells
Glycolysis = a
chain of
reactions
which converts
glucose (6
carbons) into 2
pyruvate (3
carbons)
Large polymeric
molecules are broken
down into their
monomers by
enzymes. These
monomers enter the
cytosol of the cell
During glycolysis 2 ATP and
2 NADH are produced
Pyruvate enters the
mitochondria and is converted
to a 2 carbon acetyl group
which is attached to
coenzyme A --> acetyl CoA
Large amounts of acetyl CoA
are also produced by the
oxidation of fatty acids
which are carried in the
bloodstream, imported into
cells and moved into
mitochondria.
The acetyl group is linked
through a high-energy linkage
and is easily transferable to other
molecules.
1st it is transferred to
oxaloacetate, a 4 carbon
molecule. The acetyl group is
oxidized to carbon dioxide in the
citric acid cycle.
Large amounts of the electron carrier
NADH are generated. These electrons
are then passed along an electrontransport chain within the
mitochondrial inner membrane, where
ATP is generated by oxidative
phosporylation. About 106 ATP
molecules are found in a typical
cell, used up and replaced every
1-2 seconds!! About 50% of the
free energy released is useful - the
rest is lost as heat.
This process will be covered more
thoroughly in chapter 13.
Glycolysis takes
place in the cytosol
of most cells,
including many
anaerobic
microorganisms
No oxygen required
May have evolved early in the
history of life, before
photosynthetic organisms
produced enough oxygen for
oxidative phosphorylation.
Each of the ten steps is
catalyzed by a different
enzyme (--ase) and
produces a different sugar
intermediate.
Although no oxygen is
involved, oxidation occurs,
electrons are removed by
NAD+ (producing NADH)
from some of the carbons
derived originally from
glucose. Some of the energy
released drives the direct
synthesis of ATP.
A net gain of two molecules of ATP and two
NADH are produced. In aerobic organisms,
electrons are then passed along the electrontransport chain to oxygen, forming water.
Glucose is phosphorylated by ATP
This step traps glucose inside the cell.
Isomerization moves the carbonyl oxygen from
carbon 1 to carbon 2
The entry of sugars into glycolysis is controlled at this
step through regulation of the enzyme
phosphofructokinase
Cleaved to produce two three-carbon molecules.
Steps 6 and 7 will be covered in Figure 4-5.
In steps 6-10 ATP and NADH are generated along with pyruvate,
a three carbon sugar.
If oxygen is present, pyruvate and NADH enter the mitochondria
in eukaryotes. In prokaryotes, cellular respiration takes place in
the cytosol and cellular membrane.
In anaerobic conditions, pyruvate and NADH stay in the
cytosol rather than entering the mitochondria.
Muscle cells when
oxygen is limited.
Remember, aerobic prokaryotes (bacteria) do not have
mitochondria, but they do have cellular respiration = Kreb’s
cycle and oxidative phosphorylation (electron-transport
chain).
2
Fermentation generates NAD+, needed for step 6 of
glycolosis. Without the generation of NAD+, glycolysis
would be blocked at step 6. What intermediate would
accumulate?
This pathway was
studied in yeast, in
cell extracts, and
has been
understood for
more than 50 years
2
Oxydation of an aldehyde to a carboxylic acid is coupled to the
formation of ATP and NADH. Overall energetically favorable.
These reactions are
the only ones in
glycolysis that create
a high-energy
phosphate linkage
directly from
inorganic phosphate.
These reactions are
the only ones in
glycolysis that create
a high-energy
phosphate linkage
directly from
inorganic phosphate.
• Getting an energetically unfavorable
reaction to go
– couple it to an energetically favorable reaction
(total change in free-energy must be negative)
• ATP  ADP + P
• ATP  AMP + P-P  AMP + 2 P (Figure 3.34)
• activated intermediate 
use energy in the high energy bond to transfer
a chemical group (Figure 3-27)
• couple to a reaction that “uses up” the product of the
1st reaction to “pull the 1st reaction along” (Figure
3.22) Remember concentration of reactants and
products affects the free-energy change of a reaction
-11 to –13
kcal/mole
Total = 26
kcal/mole
• Getting an energetically unfavorable
reaction to go
– couple it to an energetically favorable reaction
(total change in free-energy must be negative)
• ATP  ADP + P
• ATP  AMP + P-P  AMP + 2 P (Figure 3.34)
• activated intermediate 
use energy in the high energy bond to transfer
a chemical group (Figure 3-27)
• couple to a reaction that “uses up” the product of the
1st reaction to “pull the 1st reaction along” (Figure
3.22) Remember concentration of reactants and
products affects the free-energy change of a reaction
• Getting an energetically unfavorable
reaction to go
– couple it to an energetically favorable reaction
(total change in free-energy must be negative)
• ATP  ADP + P
• ATP  AMP + P-P  AMP + 2 P (Figure 3.34)
• activated intermediate 
use energy in the high energy bond to transfer
a chemical group (Figure 3-27)
• couple to a reaction that “uses up” the product of the
1st reaction to “pull the 1st reaction along” (Figure
3.22) Remember concentration of reactants and
products affects the free-energy change of a reaction
• Arsenate (AsO43-) is chemically very similar
to phosphate (PO43-) and is used as an
alternative substrate by many phosphaterequiring enzymes. In contrast to
phosphate, however, the high energy
arsenate bond is quickly hydrolyzed in
water, requiring no enzyme. Why is
arsenate a compound of choice for
murderers, but not for cells? Question 4-2.
pyruvate
In the presence of oxygen,
pyruvate moves into the
mitochondria and is
decarboxylated to acetyl
CoA. Fatty acids are also
degrades to produce acetyl
CoA
In aerobic metabolism, the pyruvate produced by glycolysis is rapidly decarboxylated in the
mitochondria by a giant complex of three enzymes called pyruvate dehydrogenase complex.
CO2, NADH+ and acetyl
CoA are produced
Enzymatic
breakdown
(digestion)
occurs in our
intestine outside
cells or in
lysosomes
inside cells
Glycolysis = a
chain of
reactions
which converts
glucose (6
carbons) into 2
pyruvate (3
carbons)
Large polymeric
molecules are broken
down into their
monomers by
enzymes. These
monomers enter the
cytosol of the cell
During glycolysis 2 ATP and
2 NADH are produced
Pyruvate enters the
mitochondria and is converted
to a 2 carbon acetyl group
which is attached to
coenzyme A --> acetyl CoA
Large amounts of acetyl CoA
are also produced by the
oxidation of fatty acids
which are carried in the
bloodstream, imported into
cells and moved into
mitochondria.
Lipid droplets in the cytoplasm are
composed of triacylglycerols. These are
cleaved into glycerol and three fatty
acids.
The majority of useful energy extracted
from oxidation of food comes from
pyruvate (glucose) and fatty acids.
Enzymes in the mitochondria also
degrade fatty acids, trimming 2 carbons
at a time from its carboxyl end. Acetyl
CoA, NADH and FADH2 are produced.
Enzymes in the
mitochondria also degrade
fatty acids, trimming 2
carbons at a time from its
carboxyl end. Acetyl CoA,
NADH and FADH2 are
produced.
The majority of useful energy extracted from oxidation of
food comes from pyruvate (glucose) and fatty acids.
The citric acid cycle (tricarboxylic acid cycle)
(Krebs cycle) accounts for about two-thirds of
the total oxidation of carbon compounds in most
cells. CO2 and high-energy electrons in
NADH are the major products. These highenergy electrons are then passed to a membranebound electron-transport chain (oxidative
phosphorylation) and finally
accepted by O2 to produce
water – H2O.
NAD+ must be
regenerated,
therefore oxygen
is required as a
final electron
acceptor to keep
this cycle going.
GTP is a close relative of ATP and
transfer of its terminal phosphate
group to ADP produces ATP
FADH2 is
another activated
carrier molecule
produced in the
Krebs cycle. It is
a carrier of highenergy electrons
and hydrogen.
Oxygen atoms required to produce CO2 are supplied by water.
Three molecules of water are split each cycle.
Oxygen here
is from water.
• The citric acid cycle also produces vital
carbon-containing intermediates like
oxaloaacetate, which are transferred back
from the mitochondria into the cytosol
where they serve as precursors for synthesis
of many essential molecules, such as amino
acids.
The last step in the degradation of
food molecules is oxidative
phosphorylation or the electrontransport chain. The enzymes
involved are specialized electron
acceptor and donor molecules.
These enzymes are embedded in
the mytochondrial membrane. As
the high energy electrons are
passed from acceptor to donor,
hydrogen protons are pumped
across the membrane, setting up a
large concentration/electric
gradient. This gradient of H+ ions
is used to generate ATP by the
phosphorylation of ADP.
Total oxidation of a molecule of glucose produces
about 30 molecules of ATP.
Starch and glycogen differ only in the
frequency of branch points. Glycogen
has many more branches than starch.
Fat is far more important in energy storage than glycogen.
Oxidation of fat releases 2xs as much energy and takes far
less space since it doesn’t bind water. The average adult
stores enough glycogen for only about a day, but has
enough fat for nearly a month.
During periods of light, photosynthetic
cells convert some sugars made into
starch and fats.
Adipose Tissue = Fat cells
Plants produce NADPH and ATP by photosynthesis in the chloroplast. However, most of
the plants ATP needs are met by their mitochondria. Sugars are exported out of the
chloroplasts into the mitochondria.
During periods of light, photosynthetic cells convert some sugars made during
photosynthesis into starch and fats. (see figure 4.15) Plant fats are triacylglycerols, but
contain predominantely different fatty acids than animal cells (more unsaturated vs.
saturated).
Many biosynthetic
pathways begin with
glycolysis or the citric
acid cycle. Many of
the intermediates are
siphoned off by other
enzymes to produce
amino acids,
nucleotides, etc.
The complexity of this
network of metabolic
pathways is matched
only with the stringent
controls placed at each
branching.
The metabolic balance
of a cell is amazingly
stable. The cell can
adapt to starvation,
damage, or disease – to
a certain extent. The
cell survives due to an
elaborate network of
controls, and redundant
pathways.