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Cellular respiration
Cellular respiration is a kind of catabolic reaction by wich chemical bond energy of
organic molecule is released as ATP, the fuel used by all living things and heat energy.
Cellular respiration is braking down glucose (sugars) wich are made in process of
photosynthesis. There are two main types of cellular respiration
1. Aerobic-when oxygen is present
2. Anaerobic where there is no oxygen present
Aerobic cellular respiration:
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O +38 ATP
There are four main steps in aerobic respiration:
1. Glycolysis is a series of enzyme catalyzed reaction by which glucose molecule is
converted into two molecules of pyruvate. The products are NADH, ATP, and
pyruvate. NADH and ATP are two form of chemical energy that is going to put in
one “energy bank”. There also create something what we called pyruvate.
Pyruvate will be reactant in next step aerobic cellular respiration. In glycolysis we
use 2ATP and we create 4ATP.
2. Pyruvate oxidation-pyruvic acid that is produced in glycoysis diffuses across
double membrane of a mitochondrion and enters in mitochondrion matrix. When
pyruvic acid enters into mitochondrial matrix, it reacts with a molecule called
coenzyme A to form acetyl CoA. CO2, NADH are produced in this reaction.
3. Krebs cycle- reactant is acetyl CoA. Acetyl CoA will be degraded to produce
ATP, CO2, FADH2, and NADH. Krebs cycle occurs in the matrix of
mitochondrion. This step doesn’t require any ATP. So we don’t have to invest
any ATP but we create 2ATP.
4. Electron transport system (ETS)- the reactants are NADH and FADH2 which are
created in Krebs cycle. In this cycle we created more ATP. The final product is
water. The reason why we create water is O2. Oxygen will accept two electrons
and create water. Here bulk of ATP is created- 34 molecules of ATP.
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1. Glycolysis Is a series of enzyme catalyzed reaction Glycolysis starts with
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glucose. There are two phases in glycolysis.
 Preparatory (investment) phases – it is called investment because it uses
2ATP. By the end of these phases two molecules of ATP have been
consumed in the production of 2 molecules of PGAL from one glucose
molecule.
Phosphorylation of glucose – this step requires investment of one ATP
molecule. The addition of a phosphate group on the 6 carbon is catalyzed by
enzyme hexokinase.
Conversion of Glucose-6-Phosphate into Fructose-6-Phosphate
Phosphorylation of Fructose-6-Phosphate into Fructose-1,6-Diphosphate.
Fructose-6-phosphate is phosphorylated by addition of a second phosphate
group, this time to the 1 st carbon of the fructose molecule. Another ATP
molecule is consumed in this reaction.
Cleavage of Fructose-1,6-Diphosphate: Fructose-1,6-Diphosphate is cleaved
by enzyme aldase to produce two molecule containing 3 carbon atom and one
phosphate group. One is Glyceraldehyde-3-Phosphate (PGAL), the other is
dihydroxyaceton phosphate
Conversion of Dihydroxyaceton phosphate into PGAL.
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 Payoff phase – PGAL enter in payoff phase. Chemical bonds are broken
NAD pick up electron and hydrogen ions forming NADH. The energy is
released to produce ATP.
Oxidation of Glyceraldehyde 3-phosphate 1,3 diphosphoglycerate:
glyceraldehide 3- phosphate is converted in1,3 diphosphoglycerate by
addition of inorganic phosphate and removal of hydrogen. The hydrogen
molecules are then accepted by NAD and two molecules of NADH are
synthesized.
Synthesis of ATP from 1,3 diphosphoglycerate: The phosphate on the 1st
carbon of the 1,3 diphosphoglycerate molecule is transferred to ADP. Thus
two molecules of ATP are synthetised.
Conversion of 3-phosphoglycerate to 2-phosphoglicerate:the number - prefix
before the name of the molecule indicates the position of the phosphate group.
Ex. 3-phosphoglycerate has a phosphate group attached on 3rd carbon
phosphoglycerate molecule.
Removal water from 2-Phosphoglycerate: two molecules water are
synthesized from molecule phosphoglycerate with gain two molecule
phosphoenolpyruvate.
Synthesis ATP from phosphoenolpyruvate. The product of these reactions is
pyruvate.
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Explanation of process glycolysis:
There are two phases of glycolysis. One, I call them investment phase because uses 2
ATP.
C-C-C-C-C-C
glucose
↓2ATP
P-C-C-C-C-C-C-P
And than I am essentially going to break up the glucose into two molecules containing 3carbon compound.
P-C-C-C
C-C-C-P
Pyruvate
Pyruvate
They have phosphate group on them. The phosphate group comes from these ATPs. This
is called PGAL or phosphoglyceraldehyde. So we use 2ATPs. That’s why called
investment phase is. Each of these PGAL can go in payoff phase and each of these PGAL
can turn into pyruvate which is another 3-carbon.
Payoff phase: when we go from PGAL to pyruvate we produce two things. Each of these
PGAL produces two ATPs and each produce NADH. They start of with raw material
NAD and they reduce by adding hydrogen. NAD is reduced in NADH and later these
NADH are used in ETC to produce ATPs. Net of glycolysis is 2ATPs, 2NADH, and 2
pyruvate molecules.
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2. Pyruvate Oxidation
The inner compartment of mitochondria is called matrix. But you have these pyruvate
and they are quite not ready to enter in Krebs cycle. So there is something like
preparation step. We have to oxidize them. Reaction is called pyruvate oxidation and we
end up with two carbon compound call Acetyl CoA. We generate something what is two
carbon compound and we reduce NADH. Pyruvate is with three carbon atom, but Acetyl
CoA is with two carbon atom. One carbon cleavage of and it pick up some O2 and it is
released in atmosphere like CO2.
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3. Krebs cycle
Now you have acetyl CoA ready to enter in Crebs cycle. All of these are catalyzed by
enzymes. Acetyl CoA merges with oxalacetate wich is four carbon atoms. These two
react together and they form Citrate or Citric acid wich is six carbon molecules and Citric
acid oxidise to get back to oxalacetate acid (four carbon molecule). Two carbon atom
cleavage off and forms CO2. This whole idea is generate ATP, NADH, and FADH2.
These are input in ETC.
The first “step” citrate is isomerased to isocitrate, oxidized and decarboxylated to form aketoglutarate (5C) yieliding NADH, H and CO2. a-ketoglutarate is converted into
succinate (4C) yielding NADH, H, and ATP and CO2. Succinate is converted into
fumarate and one molecule FADH2 is produced. Fumarate is converted into malate and
molecule of NADH is produced. Maltase is converted into oxalacetate.
Neto of Krebs cycle: 6 molecules of NADH, 2molecules of FADH2, and 2ATP.
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4. ETS (ETC)
After we have done with glycolysis and Krebs cycle we have left 10 NADH and 2
FADH2. These are going to be input in ETC in order to generate ATP. Each NADH will
be indirectly responsible for the production 3 ATPs, and each FADH2 is going to be
responsible for production 2 ATPs. In general NADH gets oxidized (oxidation) losing H
ions.
NADH → NAD + H + 2e
Now these electrons are popping out and they are in very high energy state. They are
transported to series transmission molecules. And these electrons when travel from one
molecule to another they are going into slightly lower energy state.
In the last step of ETC these electrons are used to reduce water. 2e + H + O2→ H2O
Every times when electrons go from high to lower energy states, it releases energy. So
energy is released when you go from high to low energy state. Energy is used to pump
protons across the inner membrane of mitochondria.
So energy is released when you go from high to low energy state. Energy is used to pump
protons across the inner membrane of mitochondria.
Krebs cycle occurs in matrix. So in matrix you have a lot of NADH and FADH2.When
they oxidize (lose electrons) to NAD and electrons keep transferring from one molecule
to another. As they from high to low energy state they release energy. That energy is used
to pump proton (H ions) across membrane of mitochondria into other compartment. But
that is not ATP yet. It’s just a gradient where we have a lot of H proton concentration in
outer compartment become more acidic than the matrix. So there is electric gradient
between outer and inner compartment. These protons are pumped from matrix into outer
compartment and they want to get back into the matrix. And that is where ATP is formed.
On the end ETC is some protein that is called ATP synthetase.
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H ions want to back but they can not. The cristae are impermeable to do that. They have
to found special way. They are going to beck trough ATP synthetase. ADP molecules are
little attached on one part of protein (ATP synthetase). As the protein enter in ATP
synthetase they make electrical gradient and the inner axle of ATP synthetase turns and
its is going to squeeze ADP and phosphate group together to form actually ATP. So ATP
synthetase used this energy from proton gradient to drive axle its essentially pushes these
ATP together.
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Calculation of ATP production:
1NADH= 3 ATPs
1FADH2= 2 ATPs
10 NADH= 30 ATPs
2FADH2= 4 ATPs
2ATP ( glycolysis)
+ 2 ATP (Krebs cycle) + 34 ( ETS)= 38 ATP
The 2 NADH+H from glycolysis are transferred from the cytoplasm to the mitochondria.
During their transfer across the mitochondrial membrane, some of their energy is lost.
Instead of each NADH+H yielding 3 ATP molecules, during this stage they only produce
2 ATPs.
2ATP ( glycolysis)
+ 2 ATP (Krebs cycle) + 32 ( ETS)= 36 ATP
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Catabolism of lipids
Digestion converts the foods we eat to a form that the body can use for energy or store for
future needs as fat. Digestion is a catalyzed process - chemical reactions take place in the
body that would not occur without the presence of catalysts called enzymes. The specific
enzymes that operate to catalyze fat digestion are called lipases.
A lipid molecule is composed of a glycerol molecule bonded to fatty acid.
The first step in lipid metabolism is the hydrolysis of the lipid in the cytoplasm to
produce glycerol and fatty acids.
In the next step glycerol (3C) is converted into pyruvate (3C) and it goes trough
glycolysis pathway to make energy.
Excess lipids are stored in adipose tissue. Hormone glucagon stimulates hormone
sensitive lipase. It’s help get fatty acid into blood.
We have short and long chain fatty acid. Short chain is 6C long and it can easily cross
mitochondrial membrane. Long chain is greater than 12C. These require transport
mechanism. On both side mitochondrial membrane you have CAT (carantine acyl
transferase), and these will help long fatty acid chain to get into mitochondria.
Fatty acids are going to enter in process β oxidation. β oxidation just break down carbon
chain, clip two carbon on a time. What we do in β oxidation is create Acetyl CoA.
Let’s pretend we deal with 16 carbon molecule.
C-C-C-C -C-C-C-C- C -C- C-C- -C-C- C-C
We have divided them into two, and get eight round. Each 2C is acety CoA.
Each section will give us 1 NADH and one FADH2.
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How many sections? Seven so we have got seven FADH2 and NADH.
8 Acetyl CoA will participate to the Krebs cycle and release 24 NADH and 8 FADH2 an
8 ATP. Totally 31 NADH and 15 FADH2 are produced from breakdown of this fatty acid
molecule. This provides a total of 131 ATP molecules from one fatty acid chain when
NADH and FADH2 are used in electron transport chain.
A lipid contains three fatty acids attached on one glycerol molecule. 3x131 + 19 an ATP
molecule from glycerols gives the huge total 412 ATP molecules from the catabolism the
single lipid molecule.
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