CELLULAR RESPIRATION
GLYCOLYSIS: 6C sugar which breaks down into Pyruvate which is (2) 3C sugar. ATP is made
during this reaction & NAD+ is converted to NADH. Glycolysis can take place without oxygen
during Fermentation. Glycolysis takes place in the cytosol. Phosphofructokinase is the most
important enzyme which either speeds up or slows down the glycolysis pathway.
Energy investment phase: in this phase glucose gets rearranged & two phosphates attach to it.
Phosphate makes the now modified sugar fructose-1 & 6-bisphosphate unstable allowing them
to split further into 2 phosphates bearing 3 carbons each. This step uses 2 ATP. The 3 carbon
sugars that break down are different from each other and the unfavorable one the DHAP easily
converts to finish the process with the favorable 3 carbon sugar. This phase uses ATP to
phosphorylate glucose to create G3P
Energy release phase or payoff phase: in this phase each 3 carbon sugar is converted to 3
carbon pyruvates through a series of reactions. Because it happens for each pyruvate it makes
4 ATP & 2 NADH. This phase harnesses ADP energy in G3P to produce 4ATP, 2NADH,
2pyruvate.
Net energy produced is 2 ATP & 2 NADH
Overall one 6 carbon molecule of sugar(glucose) converts into 2 (3) carbon molecules of
Pyruvate
PYRUVATE OXIDATION: each Pyruvate from Glycolysis goes in to mitochondrial matrix in
inner most part of mitochondria in eukaryotes ( in prokaryotes it happens in the cytoplasm)
where it is converted in a 2C molecule bound to Coenzyme A known as acetyl CoA. CO2 is
released(carbon dioxide) & NADH is generated. Requires oxygen & cant run without oxidative
phosphorylation. At the end of glycolysis we have 2 pyruvate molecules that still contain
extractable energy. This step captures remaining energy in the form of ATP although no ATP is
directly made in this step. CoA acts as fuel for the citric acid cycle in the next step of cellular
respiration.
Step1- carboxyl group is removed from pyruvate releasing carbon dioxide ( leaves behind a 2
carbon molecule)
Step2- NAD+ is reduced to NADH (picking up electrons lost in the oxidation of the of the 2
carbon molecule left behind from step 1)
Step3- an acetyl group is transferred to coenzyme A, resulting in acetyl CoA. (CoA job is to
carry the acetyl group to the citric acid cycle) NAD+ makes 2 NADH
CITRIC ACID, KREBS, TCA cycle: the acetyl CoA made in the last step combines with a 4
carbon molecule & goes through a cycle of reactions regenerating the four carbon
molecules(oxaloacetate). ATP, NADH, FADH2 are produced & carbon is released. Requires
oxygen & cant run without oxidative phosphorylation. The reduced electron carriers NADH &
FADH2 generated in the TCA cycle will pass their electrons into the electron transport chain and
through oxidative phosphorylation will generate most of the ATP produced in cellular respiration.
Overall one turn of the citric acid cycle releases 2 carbon dioxide molecules and produces 3
NADH, 1 FADH2, 1 ATP or 1 GTP. The citric acid cycle goes around twice for each molecule of
glucose that enters cellular respiration because there are 2 pyruvates equalling 2 acetyl CoA
made per glucose. The citric acid cycle does not produce much ATP directly but it can make a
lot of ATP indirectly by NADH & FADH2 which deposit their electrons into the ETC to drive
synthesis of ATP through oxidative phosphorylation.
OXIDATIVE PHOS: NADH & FADH2 made in other steps deposit electrons in the ETC turning
them back into there empty forms (NAD+ & FAD) as electrons move down the chain energy is
released & used to pump protons out of matrix to form gradient. Protons flow back in through
the enzyme ATP synthase, making ATP. The end of the ETC oxygen accepts electrons and
takes up Protons from water. Requires oxygen & uses it directly. Without the presence of
oxygen the ETC will stop running and ATP will no longer be produced by chemiosmosis and
cells can not function.
FERMENTATION: anaerobic pathway for breaking down glucose when oxygen isn't available to
act as an acceptor at the end of the ETC. Fermentation begins with glycolysis just like cellular
respiration. In fermentation the pyruvate made in glycolysis does not continue through oxidation
and the citric cycle & the ETC does not run. Cause the ETC does not run the NADH made in
glycolysis can not drop off electrons to turn back into NAD+. Purpose of fermentation is to
regenerate NAD+ from NADH produced in glycolysis. This is accomplished by allowing NADH
drop off its electrons with pyruvate allowing glycolysis to keep running by ensuring a steady
supply of NAD+