Cellular Respiration
9th grade – Biology
Miss Alexandra Martínez
GCI 2012-2013
What will we learn?
We will learn how organic compounds are broken
down in ATP, and the basic events of Cellular
Respiration (Glycolisis, Krebs Cycle, Electron
Transport Chain); as well as alternate energy
pathways that take place in the absence of oxygen.
Cellular Energy
• Cellular respiration is the process by which the energy in
organic compounds, specially glucose, is stored in ATP.
• Just as our cars need fuel to function, we need energy for
our everyday activities.
• For these activities, our bodies need to breakdown food
molecules and extract that energy. So we relay on this
process “Cellular Respiration” to live.
• We harvest energy by breaking down “energy-rich”
molecules (such as glucose), and as this happens, energy is
captured in the bonds of ATP molecules.
Cellular Respiration
INPUT
Glucose + Oxygen
C6H12O6 + O6
OUTPUT
Carbon Dioxide + Water + ATP
CO2 + H2O + ATP
Stage 1: Glycolisis
Glycolisis is a series of chemical reactions inside the cytoplasm
in which glucose is essentially splited in halves, and where a
little amount of energy is released.
Stage 1: Glycolisis
1. Two molecules of ATP are added to the glucose, attaching
two phosphates to each side, making the molecule
unstable, and vulnerable to break down.
2. Three chemical reactions yield the production of energy:
 Each half is added to another phosphate group and each half
donates High Energy Electrons to “electron carrier molecules”
called NAD+ , and protons (hydrogen ions H+) forming two
NADH molecules.
 As glycolisis continues, two phosphate bonds from the two
halves of glucose are broken, and the energy released is quickly
recaptured by ADP molecules, making two ATP molecules.
Stage 1: Glycolisis
• Two molecules of water are also produced during glycolisis. And by
the end two ADP molecules take the remaining phosphate groups,
forming two additional ATP molecules.
2. By the end of glycolisis, a molecule of glucose has been
broken down into two molecules of Pyruvate.
 The final output of glycosis results in the production of 2
NADH, and 4 ATP molecules. However, two ATP molecules
were originally invested at the beginning of the process, so
the net yield is 2 ATP and 2 NADH.
Stage 1: Glycolisis
Stage 2: Krebs Cycle
 The two pyruvate molecules move on to the inner sack of
the mitochondria, where the Krebs Cycle takes place.
 To prepare for the Krebs Cycle, the pyruvate is modified in
three quick steps :
1. With oxygen present, a molecule of CO2 is removed from
the pyruvate, and eventually exhaled from the body.
2. A coenzyme present in the mitochondria (Coenzyme-A) is
attached to each pyruvate, releasing electrons and protons,
which are donated to NAD+, making two more NADH
molecules.
3. The two pyruvates become Acetyl-CoA
Stage 2: Krebs Cycle
AFTER THE 3
QUICK
MODIFICATIONS
Stage 2: Krebs Cycle
 The steps of the Krebs Cycle are as follows:
1.
The first step of this pathway is when Acetyl-CoA combines
with water and a 4-carbon compound (oxaloacetate), to be
rearranged into a 6-carbon molecule, releasing the CoenzymeA.
2. This resulting molecule of 6-carbons is reorganized by the
addition and then removal of water, giving away electrons
and protons, forming two NADH and releasing two CO2
3. Energy is drawn from the remaining molecule, to join ADP and
a phosphate to produce a molecule of ATP.
4. With the addition of water, this remaining molecules donates
more electrons and protons to another energy carrier
molecule FADH+, becoming FADH2, and another molecule of
NADH is also formed.
Stage 2: Krebs Cycle
 The final outcome of the Krebs Cycle is the re-formation of
the 4-carbon molecule (oxaloacetate).
 The entire cycle happens again with the other Acetyl-CoA.
 As the second round continues, more NADH, more CO2,
more ATP and more FADH2 is formed.
 Considering the molecules formed during the modifications
of pyruvates, we are able to realize that the 6-carbons
contained in the original glucose molecule, are all released
as CO2 molecules.
 The final outcome of this cycle is then: 6 CO2, 6 NADH, 2
ATP, and 2 FADH2
Stage 2: Krebs Cycle
Although a small
amount of ATP has
been formed in the
Krebs Cycle, it is the
NADH, and FADH2,
that represents the
most energy for the
cell. Because these
molecules are going to
be used in the next
stage to produce a
large amount of ATP.
Stage 3: Electron Transport
Chain
 The last step in cellular respiration, the Electron Transport
Chain (ETC), takes place in the membrane of the inner sack
of the mitochondria.
 This step is essential to produce enough ATP for animals and
many other living organisms to survive.
 Through the ETC the cell can now use the energy
temporarily stored in NADH and FADH2, to produce ATP.
These molecules donate their electrons to the ETC.
 As electrons move from one protein to another in the ETC,
they transfer their energy to these proteins to pump H+
across the membrane.
Stage 3: Electron Transport
Chain
 With each transfer, electrons loose energy, which is used to
pump more protons.
 The process of electron transfers, results in a difference of
concentration of H+, on the two sides of the membrane,
creating a concentration gradient.
 The oxygen that we breath is essential to ETC. This oxygen
grabs the electrons at the end of the ETC, and together with
H+, form molecules of water.
 A complex in the membrane, provides the pathway for H+,
to move out of the membrane and produce ATP.
 The H+ will tend to flow in this direction until the
concentration gradient disappears
Stage 3: Electron Transport
Chain
Stage 3: Electron Transport
Chain
Oxygen grabs the
electrons
Stage 3: Electron Transport
Chain
Fermentation
 If oxygen disappears, the Kreb Cycle and the ETC, will shut
down.
 Fermentation is an alternative pathway when oxygen is not
present, and it follows glycolisis.
 There are two types of fermentation:
1. Lactic Fermentation
2. Alcoholic Fermentation
Lactic Acid Fermentation
 Under anaerobic (absence of oxygen), the Krebs Cycle and
the ETC cannot happen.
 So each of the two molecules of pyruvates produced during
glycolisis use a molecule of NADH to form two molecules of
Lactic Acid.
 This chemical reaction causes the NADH, become again
NAD+, which is recycled to glycolisis again.
 Lactic acid builds up in muscles, causing a burning pain,
blood pH drops causing muscle fatigue.
 At rest, lactate, is converted back to pyruvate.
 So we end up, with only the two molecules of ATP,
produced during glycolisis.
Alcoholic Fermentation
 In other organisms, the 3-carbon pyruvate is broken down
to ethanol, through alcoholic fermentation.
 First, the pyruvate is converted to a 2-carbon compound,
releasing CO2.
 Second, electrons and H+ are transferred from NADH to the
2-carbon compound to form Ethanol (Ethyl Alcohol).
 The molecule of NAD+ is recycled back to glycolisis.
 The outcome of this process will then be: CO2 and Ethyl
Alcohol, and yielding only the production of two molecules
of ATP, produced during glycolisis.
 Alcoholic fermentation by yeast, a fungus, has been used in
the preparation of many foods and beverages.
Production of ATP
Glucose
Glycolisis
Without O2
Fermentation
Lactate
Ethanol
and CO2
Anaerobic Processes
2 ATP
With O2
Pyruvate
Krebs Cycle
ETC
Aerobic Respiration
2 ATP
34 ATP