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The 4 Phases of Cellular Respiration
Biology Group Project #2
Jennifer Vanderveen 3 points
Pam Furniss 3 Points
Cherlynne Davis 3 Points
Vanessa Churchill 3 Points
Glycolysis
Glycolysis occurs in the cytosol and is a source of ATP and NADH, which begins with glucose entering the
cell. There are a series of enzyme reactions with expenditure as well as yielding of ATP and formation of
NADH that ends with pyruvate.
Priming- Through enzymatic reactions the glucose is “primed” to be cleaved. Two ATP’s are spent in the
priming reaction.
Cleaving- The 6-carbon sugar is then split into two 3-carbon phosphate sugars, glyceraldehyde 3phosphate or G3P.
Oxidation and ATP Formation- Then the G3Ps are oxidized, transferring two electrons to NAD+ thus
forming NADH. A phosphate is added to G3P to make a bisphosphoglycerate or BPG. The phosphate
added to G3P can be transferred to ADP by phosphorylation to create a yield in ATP. Through more
reactions the BPG converts to pyruvate, which the phosphates transfer to the ADP to yield two ATP per
G3P. Overall, glycolysis uses 2ATP in the priming phase and produced 4ATP in the oxidation and ATP
formation phase for a net gain of 2 ATP and 2 NADH. Glucose has been broken down and oxidized to 2
molecules of pyruvate for the end product of the glycolysis pathway.
Linker stepThis is the step right after glycolysis. It starts with the pryuvate and converts that into a 2C Acetyl coA ,
CO2, and one NADH all to help get the Citric acid cycle going. This step is the first place that CO2 is
formed and it also forms NADH that will help make ATP in the citric acid cycle. The reason why this is the
linker step is because it connects the glycolysis through the membrane to the Citric acid cycle inside the
mitrochondra.
Citric Acid cycleThe citric acid cycle begins right after the linker step. After the Glucose has been broken down so there
are only 2 carbons remaining it enter the citric cycle. The Acytyl CoA joins an Oxaloacetate making it a 6
carbon group again, then through condensation with the Kinases enzyme is turned into Citrate. It is
then rearranged by an Isomerase enzyme and becomes Isocitrate. This leads to the first Oxidation, the
Isoctirate undergoes an oxidative decorboxylation reaction. It is oxidized and yields its first NADH, it is
decaboxylated and the central carboxyl group breaks off leaving another CO2 and a 5-carbon a-
Ketogluarate. Next the a-Ketogluarate is decorboxylated by a multi enzyme similar to Pyruvate
dehydrogenase, another CO2 is removed and the 4 carbon group is joined by CoA it is now a SuccinylCoA, during this process 2 electrons are cleaved off and reduce 2 NAD+ to NADH. The CoA, and the
Succinal are linked by a high energy bond, this bond is cleaved forming GTP which in turn gives up its
phosphate to ADT to form another ATP, changing the carbon group to Succinate. The third oxidation
comes next, Succinte is oxidized to Fumarate, the free energy change in this reaction is not big enough
for NAD so a lesser molecule FAD comes along and picks it up forming and FADH2. This reduced form
can only contribute electrons to the electron chain in the membrane. In the last reaction, H2O is added
to Fumarate forming Malate which in turn is oxidized yielding a 4 –carbon molecule of Oxaloacetate and
2 final electrons to reduce NAD+ to NADH, and the cycle repeats.
Electron Transport Chain/Oxidative Phosphorylation
The NADH and FADH2 that are produced in the 3 initial steps of cellular respiration (Glycolysis,
Pyruvate oxidation, and Krebs cycle) are transported to the inner mitochondrial membrane. Once at the
membrane the NADH and FADH2 deliver their high energy electrons into the electron transport chain;
with NADH going to NAD+ and delivering its electrons to protein complex I (NADH dehydrogenase), and
FADH2 to FAD2+and delivering its electrons to protein complex II. The electrons from NADH travel
through protein complex Iinducing a conformational change that allows the protein complex to pump 2
Hydrogen ions across the inner membrane into the intermembrane space, while protein complex II,
which received electrons from FADH2, is not capable of pumping hydrogen ions across the
membrane.The electrons from protein complex I and II are then carried to protein complex III through
the use of ubiquinone, where they travel through the complex allowing an additional 2 hydrogen ions to
be pumped across the membrane. The electrons are then carried to complex IV through cytochrome C,
where they allow the complex to pump 2 more hydrogens across the membrane, and finally reduce the
terminal electron carrier, O2. The reduced oxygen then combines with hydrogen to from H2O.
The pumping of hydrogens into the intermembrane space by the protein complexes allow for
the creation of a hydrogen ion gradient, with a higher concentration of hydrogen ions in the
intermembrane space compared to the mitochondrial matrix. The hydrogens then are allowed to flow
down their gradient back into the matrix, through ATP synthase, which uses the energy of the gradient
to catalyze the synthesis of ATP from ADP and Pi, through a process called chemiosmosis.
Misconceptions
Misconception #1 One of us forgot to cover the location of the linker step in her explanation of the
linker step. The Linker step is the process of moving the pyruvate from the cytosol through the outer
membrane of the mitochondria, and then the inner membrane of the mitochondria to the source of the
Citric/Krebs cycle. All while breaking down the pyruvate and changing it into Acytol CoA.
Misconception #2 One of the misconceptions a member of the group had was that the electrons on the
Electron Transport Chain went all the way to the ATP synthase protein, and that the electron helped
power the pump. We now know that the electrons end their journey as the exit the IV protein and are
then joined with a ½ O2 to form water. The ATP Is produced by the high Proton motive force of
Hydrogen+ ions in the internal membrane space, that forces them down through the ATP Synthase.
Misconception #3 Another one of the misconceptions one of us had, was understanding exactly how
many Hydrogen+ were moved across the proteins in the electron transport chain, per electron. It was
thought that only 2 hydrogen+ were pumped across the protein for each electron. But later was revealed
that it is 1 hydrogen+ per electron that is pumped through.
Misconception #4 Last misconceptions was that NADH cannot be reused to make more ATP is false.
NADH can be reused to make more ATP. NADH must be recycled to continue respiration. A cell does not
contain a large amount of NAD+ and for glycolysis to continue, NADH must be recycled into NAD+.
Photosynthesis
We need a constant input of sunlight to keep life on earth going because life is a cyclical process
and each process depends on each other to continue the cycle. The sun is the direct source of these
cycles’ operations.
Photosynthesis uses light energy to make food or chemical energy, which follows the 1st law of
thermodynamics, which is the transfer of energy.
In cellular respiration, the process of breaking down sugars to form ATP, some energy is lost in the form
of heat. Heat lost or entropy follows the 2nd Law of thermodynamics.
This cycle continues as the food chain is traced back to photosynthesis, which is powered by the sun.
Therefore, we need constant energy from the sun, as sunlight is vital to our existence.