Download Cellular Respiration Explained

Cellular Respiration Explained
by Brandon D Stiller
Remember the notes on the way in which ATP is made. What is ATP? Where did we learn it was
made? The answer is in the mitochondria of cells. The overall reaction is
C6H12O6 + 6O2→6CO2+ 6H2O+ Energy (ATP+ Heat).
Notice that oxygen is required. When oxygen is used, it is called aerobic respiration.
ANAEROBIC Respiration is called fermentation. No O2 used in fermentation. Without O2 there is
not as much ATP made (more later on this later).
Fermentation is only a partial degradation of sugars. BOTH Fermentation and Cell Respiration
(CR) use the products of the process of GLYCOLYSIS to make energy, although as we said, CR is
much more efficient.
❋GLYCOLYSIS means the “splitting of sugar,” and takes place in the cytoplasm! The overall
process is just that: Glucose, a six carbon sugar is split into two separate three carbon
molecules. These are then changed into another type of molecule called pyruvate, AKA
pyruvic acid.
But pyruvate is not the only product of glycolysis that is important! Energy in the form of ATP is
produced and another molecule that is needed to get more energy later is formed by NAD+
changing to NADH (NADH is derived from Vitamin B2 or Niacin).
NAD+ can be thought of as a carrier molecule. It carries 2 H+ ions and 2 e- every time it gets
loaded (one H on NADH, and one free H+) NAD+ +2e- + 2H+→NADH, H+
To recap…Glycolysis takes place in the cytoplasm and changes one molecule of glucose into two
molecules of Pyruvate, a net of 2 ATP, and 2 NADH, H+. The overall reaction of glycolysis is:
We know that the ATP produced is used right away for energy in the cytoplasm, and that NADH
carries H+ and e-. But what happens to pyruvate? This is a good question that you really need
to know the answer to…
There are 2 possibilities or the fate of pyruvate. It can be used in Cell Respiration (CR) or
Fermentation.
Let’s deal with CR first, more specifically the Krebs Cycle, and the Electron Transport Chain.
❋PRE-KREBS IN THE MATRIX OF THE MITOCHONDRION: Pyruvate can enter what is called
the Krebs cycle, where more ATP and NADH is produced, as well as another carrier called
FADH2. The Krebs cycle takes place in the mitochondrial matrix, so the pyruvic acid has to
move from the cytoplasm into the mitochondrion. To make the ATP, NADH, and FADH2, the
pyruvic acid is catalyzed by many enzymes in the Krebs Cycle. After the Krebs Cycle
comes the ETC (more later).
Before the Krebs cycle can begin, an enzyme called pyruvate dehydrogenase (PDH) adds
Coenzyme A to a pyruvate forming what is called Acetyl-CoA and gives off CO2. The diagram
below should help you understand that one of the Cs on pyruvate is ripped off (turning into
CO2), another NADH, H+ is produced and the rest of pyruvate (acetyl) is hooked up with a coenzyme called CoA to make acetyl-CoA. Word.
❋Now comes the KREBS CYCLE which begins with the unloading Acetyl-CoA by attaching it to
a small molecule called Oxaloacetate (OAA) which makes a molecule of citric acid (the reason
why the Krebs cycle is also called the citric acid cycle). Then through a series of reactions (8),
1ATP (from GTP), 3NADH, H+, 1FADH2, and 2CO2 are produced from each acetyl-CoA.
The ATP is used for cellular energy where it is needed, CO2 passively diffuses out of the
mitochondrion, and then out of the cell (humans eventually breath it out), and the NADH and
FADH2 hang around to unload the e- they are carrying on the ETC. The Krebs cycle ends
when oxaloacetate is regenerated and is able to bond to another acetyl-CoA!!!! Word booty.
❋The next step in CR is the ELECTRON TRANSPORT CHAIN (ETC). This is how the proton
gradient is produced that provides the gradient for ATP synthase to make ATP, the final product,
and purpose of all of these reactions.
First, the ETC unloads the NADH and FADH2 that were the products of glycolysis and the
Krebs cycle, and uses the e- that they were carrying to provide E for a proton pump. All of the
e- were carried by NADH and FADH2!!!! By the way, this all happens on the inner membrane of
the mitochondria. Lining the inner membrane of mitochondria are several membrane proteins
(NADH reductase, coenzyme Q, cytochrome B, cytochrome C, and cytochrome oxidase).
This is where O2 finally comes in. It takes the e- off cytochrome oxidase and produces water.
Extra info: The poison cyanide works at the ETC by permanently binding to cytochrome oxidase.
Cyanide (CN-) poisoning makes an organism run out of ATP really fast, and they usually die.
So now there is a proton gradient across the inner membrane, because there are more
protons in the intermembrane space than in the matrix. Those protons will go down their
gradient through ATP Synthase to produce a lot of the ATP (between 32 and 34 ATP!).
The overall CR process is depicted below. Get it? If not read, study, look at other diagrams, or
ask me, all good ideas!
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Really bad stuff can happen when there is no O2 to unload the ETC, namely death. The
NADH that provides electrons and their energy to the ETC cannot unload, so the NAD+ in the cell
gets used up. Remember that NAD+ plus electrons makes NADH. Well if the NADH can’t lose
electrons, then their is no NAD+.
Plus, if there is no way to get NADH unloaded and use the energy in the electrons to pump H+
and make ATP, then the cell is about to die since there will be no ATP. So what can it do?
Solution to no oxygen in cell: Fermentation
Glycolysis can run without O2 as long as there is some NAD+. And glycolysis can make 2 ATP
per glucose that enters it. But it can’t run if there is no NAD+. This is where Fermentation
comes in.
Lactic Acid Fermentation
Alcohol Fermentation
Fermentation allows the cell to make ATP by doing reactions that unload NADH
without O2.
There are two types of fermentation:
Lactic Acid Fermentation, which animals and many bacteria do.
Alcohol Fermentation, which yeast and some bacteria do.