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Jim’s Condensed Notes on Cell Respiration
One gram of the sugar glucose (C6H12O6) produces 3811 calories when burned in the presence of O2.
A calorie is the energy necessary to raise 1g H2O 1 degree C. The Calorie from our food labels is
1000 of these calories.
6 O2 + C6H12O6
6 CO2 + 6H2O + Energy
It doesn’t happen all at once – it’s a gradual energy release.
Glycolosis is the first part of this process. This releases just a small amount of energy (in the form
of ATP). It takes place in the cytoplasm.
If O2 is present, the next step is the Krebs Cycle and the Electron Transport Chain.
If O2 is absent, then you have fermentation.
Glycolosis is the process in which one molecule of glucose (6 carbons) is broken in half, producing 2
molecules of pyruvic acid (each has 3 carbons).
Glycolosis uses 2 ATP but produces 4ATP, for a net of 2 ATP.
It also transfers 4 high-energy electrons to NAD+ (Nicotinamide adenine dinucleotide). Each
NAD+ molecule can take two electrons.
Although it produces only a small amount of ATP, Glycolosis happens so fast that it can produce
thousands of ATP molecules in small fractions of a second. But it quickly uses up all the available
NAD+ molecules.
Nevertheless, at the end of glycolysis, nearly 90 percent of the energy stored in the glucose molecule
is still unused. It's locked in the pyruvic acid molecules,
Fermentation releases energy from food molecules in the absence of oxygen (anaerobic).
Two main types: Alcoholic and Lactic Acid Fermentation
Alcoholic Fermentation forms ethyl alcohol and CO2 from the products of glycolosis. It also
produces NAD+ !
pyruvic acid + NADH
alcohol +CO2 + NAD+
This causes bread to rise, with CO2 bubbles producing the spaces in the bread. The alcohol
evaporates during baking.
Lactic Acid Fermentation also regenerates NAD+ so glycolosis can continue. In this type of
fermentation,
pyruvic acid + NADH
lactic acid + NAD+
Lactic acid is produced in your muscles when your muscles do not get enough oxygen. This is really
only improved by long-term conditioning. That’s why long distance and endurance athletes are
usually older.
With oxygen present, instead of fermentation, the next step is the Krebs Cycle and the Electron
Transport Chain. With oxygen, the process is considered to be aerobic.
In the Krebs Cycle,
Uncle Jim's Easy Bread Recipe
This recipe is versatile enough to make pizza, garlic bread, focaccia, and even dog bicuits or pita
bread if you experiment.
Materials: Here's what you are going to need:
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Active Yeast (in a jar or packets)
Salt
Warm water (warm, not boiling)
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Sugar
Flour (any white baking flour)
Olive Oil
Here is where you can get experimental. I encourage you to follow these directions, and then in a
second batch, make a significant change in the amount of yeast or the amount of sugar (independent
variables).
Procedure:
1. In a small bowl put in two cups of hot-warm water from the tap (careful, water more than 49oC will kill the
yeast - think easily bearable to your fingers but no hotter - like a perfect bath).
2. Add your packet of yeast (or two teaspoons of yeast from a jar)
3. Add three teaspoons of sugar.
4. Stir it up and leave it to sit for a few minutes until the yeast starts to froth up (activate).
5. While waiting, add four cups of flour into a large mixing bowl.
6. Add about a couple of teaspoons of salt or other spices for flavor.
7. When your yeast is nice and foamy tip it into the bowl with the flour.
8. Add about a tablespoon or two of olive oil and mix it together with a fork..
9. Gradually add more flour until it comes together into a doughy blob.
10. Tip it out onto a flour-covered board or counter top.
11. This is important - fold it over on itself squish down and turn it, fold and turn, fold and turn...if it is
too sticky add more flour.
After 5-10 minutes you should have a nice blob of dough. The key is the gradual addition of flour. If you
do it this way there's no way it can be too dry. Leave it on the bench while you wash out your larger
bowl.
12. Oil the sides of the bowl and put the dough back in it.
13. Rub a little oil over the dough too so it doesn't dry out and go all hard and crusty. Cover it with a
clean tea-towel.
14. Put it in a warm place to rise. A sunny place in the kitchen is good or maybe you could preheat my
oven a little then turn it off and put it in there.
After about an hour it will be all nice and puffy. Then you get to decide what you want to make (pizza,
bread, rolls, etc.).
Krebs Cycle
First, notice why this is a "cycle".
We start with oxaloacetate (OAA) and add the incoming acetyl CoA (derived from the pyruvate
endproduct of glycolysis) to form citrate. We proceed through 7 steps, and eventually regenerate OAA.
This OAA once again joins with acetyl CoA, and we begin the process once again. Hence, we have a
cycle.
You may also have heard the Krebs cycle called the Citric Acid Cycle -- now the reason should be
obvious! Notice the spelling has no apostrophe - Hans Krebs was the German-British scientist who, in
the 1930's, contributed significantly to the understanding of this cycle.
Remember that the cycle turns TWICE for each glucose molecule that enters glycolysis. This is
because each molecule of glucose (6 carbon) is split into two molecules of pyruvate (3 carbon). Each
molecule of pyruvate, in turn, is converted to acetyl CoA, which enters the Krebs cycle.
We can also keep track of the carbons easily:
Acetyl CoA is a 2 carbon compound (remember that it is generated when 1 carbon dioxide molecule is
removed from pyruvate).
It enters the Krebs cycle by combining with oxaloacetate (4 carbon) to form citrate (6 carbon).
Now, we know that we have to get back to OAA in order for the cycle to repeat itself. So, we must have
to LOSE 2 carbons along the way as we convert citrate (6 carbon) back to OAA (4 carbon).
Notice that we lose those 2 carbons in the form of two molecules of carbon dioxide that are given off
when isocitrate is converted to a-ketoglutarate, and when a-ketoglutarate is converted to succinyl CoA.
You know that when we EXHALE, we breathe out carbon dioxide. This is where it comes from!!!!
The Krebs cycle occurs entirely in the matrix of the mitochondrion (inside the inner membrane).
Let's do a check of how much we've spent and what we've gained:
1 molecule of acetyl CoA + 3 NAD+ + 1 FAD + 1 ADP yielded 2 CO2 + 1 FADH2+ 3 NADH +
3 H+
You'll notice that we have now only got ATP, FADH2, and NADH as products (CO2 is exhaled). The
organic glucose was broken down to 2 molecules of pyruvate in glycolysis. Pyruvate was converted to
acetyl CoA (with the net loss of ONE carbon in the form of carbon dioxide per pyruvate, a total of TWO
carbon dioxides per glucose molecule), and acetyl CoA entered the Krebs cycle.
During the Krebs cycle, we lost 2 carbons in the form of carbon dioxide, which is equivalent to the loss
of the original acetyl CoA that we invested (so for each molecule of glucose, the cycle turns twice and
we lose a total of 4 carbons).
This means that we have lost, by the end of glycolysis and the Krebs cycle, a total of 6 carbons -- the
amount we put in when we started with glucose. So, now we're ready to harvest the energy stored in
FADH2 and NADH.
Electron Transport Chain
The electron transport chain is located in the inner membrane of the mitochondrion. There are
thousands of copies of the electron transport chain per mitochondrion. The key is that one side of the
chain faces the matrix and the other side faces the intermembrane space.
Remember that we have generated some molecules of ATP already (through a process called substratelevel phosphorylation, whereby phosphate groups are transferred directly from some intermediate in
glycolysis or the Krebs cycle to ADP to form ATP!). We have also generated NADH and FADH2.
Now, we need to harvest the energy that is stored in the electrons of NADH and FADH2, the reduced
forms of NAD+ and FAD. We do this through a series of redox reactions that are carried out along the
electron transport chain. Remember our discussion at the beginning of this tutorial: as electrons move
from a less electronegative atom towards a more electronegative atom (favorable)
energy is released.
Most of the components along the electron transport chain are proteins. These proteins have nonprotein
groups attached to them (called prosthetic groups) that can be reduced as electrons are passed to them,
and can be oxidized as electrons are removed from them. The electrons "fall" down the electron
transport chain because it is energetically favorable for them to do so! Why is it favorable for the
electrons to fall down the chain? Well, from our previous discussion, you might infer that each
successive component of the electron transport chain is slightly MORE electronegative than the
component before it. And we know that electrons like to be close to electronegative atoms! So,
electrons move down the transport chain because they are successively moving closer and closer to more
and more electronegative atoms. The final carrier in the electron transport chain is oxygen. This makes
sense since oxygen is an EXTREMELY electronegative atom.
Electrons start out bound to NADH. Remember that NADH retains the potential energy that electrons
had when they were in food. When NADH encounters the first component of the electron transport
chain (FMN), it gives its electrons to FMN. In other words, NADH is an electron donor (it gets oxidized
back to NAD+) and FMN accepts electrons (it gets reduced). The electrons are then passed from FMN
to an iron-sulfur containing protein (i.e. FMN gets oxidized and the Fe-S protein gets reduced). And so
on... all the way down the electron transport chain until the final carrier (cytochrome a3) becomes reoxidized after it passes a pair of electrons to oxygen (which gets reduced!). FADH2 acts
like NADH, except that its electrons have less energy associated with them than the electrons of NADH.
Therefore, FADH2 dumps its electrons further down the electron transport chain (they don't have as far
to fall to reach the bottom).
So now all of the electrons have passed successively lost energy as they have fallen down the electron
transport chain, but how does this yield energy??? The answer is chemiosmosis.
Chemiosmosis
You already know that electrons "fall" down the electron transport chain because it is energetically
favorable. As it turns out, when electrons are passed to some members of the electron transport chain,
those specific members of the electron transport chain not only pass the electron on to the next
component in the chain, but they also pick up a H+ from the solution of the mitochondrial matrix and
pump it into the intermembrane space! In this way, as electrons fall down the electron transport chain,
protons get pumped from the matrix to the intermembrane space, thereby establishing a concentration
GRADIENT.
We know that substances like to move from where they're more concentrated to where they're less
concentrated (they move down their concentration gradients). So the H+ that build up in the
intermembrane space during the transport of electrons down the electron transport chain want to come
back into the matrix to re-establish equilibrium. However, we also know that lipid bilayers are not
permeable to ions! It takes energy to continue to push H+ into the intermembrane space because this
movement represents a movement AGAINST the concentration gradient. The energy to pump the H+
from the matrix to the intermembrane space comes from the passage of electrons down the electron
transport chain.
IN OTHER WORDS, the cell couples the exergonic "fall" of electrons down the electron transport chain
with the
ENDERGONIC pumping of H+ from the matrix to the intermembrane space against their concentration
gradient!
Once the cell has established a H+ concentration gradient (called a proton-motive force), it allows the
H+ to flow back into the mitochondrial matrix through specialized proteins called ATP synthases. The
flow of H+ back into the mitochondrial matrix (DOWN their concentration gradient) is EXERGONIC.
The ATP synthase captures the energy being released from this flow of H+ and couples it with the
synthesis of ATP!. This process of coupling the redox reactions of electron transport to ATP synthesis
via the establishment of a H+ gradient is called chemiosmosis.
Summary of Energy Release
glycolysis (net): 2 ATP + 2 NADH
conversion of pyruvate to acetyl CoA: 1 NADH for each pyruvate = 2 NADH
Krebs cycle: 1 ATP + 1 FADH2+ 3 NADH times two = 2 ATP + 2 FADH2+ 6 NADH
TOTAL = 4 ATP (by substrate level phosphorylation) + 2 FADH2+ 10 NADH
1 NADH has enough energy stored to make 3 ATP by oxidative phosphorylation
1 FADH2 has enough energy stored to make 2 ATP by oxidative phosphorylation
So, the total amount of ATP possible = 38 ATP