Erika Veidis
AP Biology – Pd. 1
October 28, 2009
CELL RESPIRATION HOMEWORK
1. Cellular respiration and fermentation are catabolic pathways. Cellular respiration – aerobic
respiration – requires oxygen to work. Contrarily, fermentation does not need oxygen.
2. C6H12O6 + 6O2  6CO2 + 6H20 + ATP
C6H12O6  2 pyruvate + 2 NADH + 2 ATP
3. After ATP is used for cellular work, it is broken down back into ADP and a phosphate group. This
will get put back together when there is an investment of energy, such as when chemiosmosis
fuels the ATP synthase, which spins very quickly to provide energy for the assembling of ATP.
ATP is assembled through phosphorylation, whether that be substrate level phosphorylation (as
in glycolysis and the Krebs cycle) or oxidative phosphorylation (as in the electron transport
chain).
4. Oxidation is the loss of electrons. Reduction is the gaining of electrons.
5. Organic molecules that have a large amount of hydrogen are excellent cellular fuels, because
they provide more hydrogens to pump through the cytochromes in the electron transport chain,
which will make a larger hydrogen gradient. The larger the hydrogen gradient is, the more
energy will be released through chemiosmosis, when the hydrogens rush through the ATP
synthase which consequently assembles the ATP.
6. NAD+ accepts the electrons released by the oxidation of glucose (resulting in NADH) and carries
them to the electron transport chain. Here, the hydrogen proton and electron separate from
NADH. The electron bounces along the electron transport chain (the cytochromes), providing
energy to pump the H+s into the intermembrane space. The force pulling the electrons down
the chain is the electronegative oxygen, the final electron acceptor, at the bottom of the chain.
7. Glycolysis occurs in the cytoplasm. The Krebs cycle occurs in the mitochondrial matrix. The
electron transport chain occurs on the inner membrane of the cristae of the mitochondria.
8. The carbon skeleton of glucose has 6 carbons. In the energy investment phase of glycolysis, this
is broken down into 2 G3P, which have 3 carbons each. In the energy payoff phase, it is
converted into 2 pyruvate, which also have 3 carbons each. In pyruvate oxidation, 2 carbons are
lost as CO2, and the remaining 4 are converted into 2 acetyl coA molecules, which have two
carbons each. Glucose is then completely oxidized in the Krebs cycle and 4 CO2 are released.
9. ATP is required to change glucose to 2 G3P in the early steps of glycolysis. 2 ATP are invested
for this to happen. The 2 G3P (each has 3 carbons) can then be converted into 2 pyruvate. This
ATP investment oxidizes the glucose, removing the electrons to break it up.
10. Sugar oxidation occurs in the first step of glycolysis, in the energy investment phase, when the
energy from ATP breaks glucose into 2 G3P molecules. Substrate level phosphorylation occurs
in the energy payoff phase as 4 ATP (2 ATP net) are created. The reduction of NAD+ occurs in
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the energy payoff phase of glycolysis as 2 NAD+ are reduced to 2 NADH. The electrons gained
by the NAD+ come from the breakdown of glucose.
Pyruvate oxidation occurs in the mitochondrial matrix. The molecules produced in this reaction
are 2 acetyl coA, 2 CO2, and 2 NADH. The 2 acetyl coA then enter the Krebs cycle to be
completely oxidized.
Glucose is completely oxidized during the Krebs cycle in cellular respiration.
The electrons bouncing down the cytochromes in the electron transport chain (an exergonic
process) provides the energy needed to be invested in the endergonic process of pumping H+s
into the intermembrane space, building up an H+ gradient.
Chemiosmosis is the flow of H+s through the ATP synthase. This flow is caused by the high H+
gradient, which diffuses back into the mitochondria through the ATP synthase. The flow of H+s
through the ATP synthase provides the energy to assemble ATP through oxidative
phosphorylation.
The cytochromes of the mitochondrial inner membrane serve as hydrogen pumps to build up an
H+ gradient in the intermembrane space. This membrane is impermeable to protons. These
H+s then diffuse back through an ATP synthase in the membrane (down the concentration
gradient).
Fermentation is necessary to take NADH and convert it back to NAD+. This NAD+ can then
perpetuate glycolysis.
In lactic acid fermentation, pyruvate remains a 3 carbon molecule. In alcohol fermentation, it is
broken down to a 2 carbon molecule, and one of the carbons is lost as CO2.
Fermentation produces no ATP, and occurs without oxygen. The goal of fermentation is to
convert NADH back to NAD+ to further perpetuate glycolysis. Cellular respiration produces 34
ATP total and requires oxygen to work, as the oxygen is the final electron acceptor in the
electron transport chain. The goal of cellular respiration is to make ATP.
Protein must first be digested to individual amino acids before entering the respiratory
pathways. This is converted to G3P, which can then enter glycolysis. Fats must be digested to
glycerol and fatty acids. The glycerol can be converted to glyceraldehydes phosphate, which is
an intermediate of glycolysis. The fatty acids are broken down into 2 carbon fragments in a
process known as beta oxidation. These 2 carbon fragments are similar to acetyl coA. They can
enter the Krebs cycle.
ATP production is regulated by the cell through allosteric enzymes, which can be turned on or
off by activators or inhibitors. The end products inhibit enzymes that catalyze early steps.
Phosphofructokinase, which is used in glycolysis, is inhibited by ATP. Glycolysis can’t occur
when ATP inhibits it. This is an example of negative feedback regulation – there is too much
product, so the product inhibits the reaction.