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Winter 2015
Cell Respiration
Introduction
Cellular respiration is the process by which the chemical energy of "food" molecules is
released and partially captured in the form of ATP. Carbohydrates, fats, and proteins
can all be used as fuels in cellular respiration, but glucose is most commonly used as an
example to examine the reactions and pathways involved.
We can divide cellular respiration into three metabolic processes: glycolysis, the
Krebs cycle, and oxidative phosphorylation. Each of these occurs in a specific
region of the cell.
1. Glycolysis occurs in the cytosol (cytoplasm of the
cell).
2. The Krebs cycle takes place in the matrix of the
mitochondria.
3. Oxidative phosphorylation via the electon transport
chain is carried out on the inner mitochondrial
membrane.
In the absence of oxygen, respiration consists of two metabolic pathways: glycolysis
and fermentation. Both of these occur in the cytosol. We will talk about this anaerobic
respiration later in this booklet.
Overview of the processes
C6H12O6 + 6O2 -------------------> 6CO2 + 6H2O + ~38 ATP
Glucose + oxygen → carbon dioxide + water + ATP (energy)
Glycolysis
Glycolysis literally means "splitting sugars." In glycolysis, the 6-carbon sugar,
glucose, is broken down into 2 molecules of a 3-carbon molecule called pyruvate. In
the process, two molecules of ATP, two molecules of pyruvic acid and two "high
energy" electron carrying molecules of NADH are produced.
Glycolysis can occur with or
without oxygen. In the presence
of oxygen, glycolysis is the first
stage of cellular respiration.
Without oxygen, glycolysis
allows cells to make small
amounts of ATP. This process
is called fermentation.
Krebs Cycle
The Krebs cycle occurs in the mitochondrial matrix and generates a pool of chemical
energy (ATP, NADH, and FADH2) from the oxidation of pyruvate.
Pyruvate moves into the mitochondria. In an intermediate transition step, the 2 pyruvate
molecules each combine with Coenzyme A and lose carbon dioxide to become 2
molecules of Acetyl-CoA. This is a 2-carbon molecule.
In the presence of Oxygen gas (O2), all the hydrogens (H2) are stripped off the Acetyl
CoA, two by two, to extract the electrons for making ATP, until there are no hydrogens
left - and all that is left of the sugar is CO2 - a waste product - and H2O (exhaled). The
Krebs cycle results in the production of only ~4 ATPs, but produces a lot of NADH,
which will go on to the next step... Hans Krebs won the Nobel Prize in 1953 for his
discovery of the Citric Acid Cycle, which we now call the Krebs Cycle.
When acetyl-CoA is oxidized to carbon dioxide in the Krebs cycle, chemical energy is
released and captured in the form of NADH, FADH2, and ATP.
The Electron Transport Chain - Oxidative Phosphorylation
The electron transport chain allows the release of the
large amount of chemical energy stored in NADH and
FADH2. The energy released is captured in the form of
ATP (3 ATP per NADH and 2 ATP per FADH2).
NADH + H+ + 3 ADP + 3 Pi + ½ O2 → NAD+ + H2O + 3 ATP
FADH2 + 2 ADP + 2 Pi + ½ O2 → FAD+ + H2O + 2 ATP
The electron transport chain (ETC) consists of a series of molecules, mostly proteins,
embedded in the inner mitochondrial membrane; the mitochondrial cristae.
http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter25/animation__electron_transport_system_and_atp_synthesis__quiz_1_.html
http://www.sumanasinc.com/webcontent/animations/content/cellularrespiration.html
To Recap
Cellular Respiration is the reverse process of photosynthesis
Cellular Respiratin consists of 3 basic steps:
Glycolysis
takes place in the cytosol of the cell (outside the mitochondria)
It involves the splitting of a glucose molecue into 2 pyruvate colecules
In the process 2 NADs are charged to become 2 NADH and 4 ADPs gain another phosphorus
atom to become 4 ATPs (the high energy storage molecules) - only 2 in total since 2 ATPs were
needed to begin the process. Two water molecules are also released.
Don't stop now though since each pyruvate contains lots more energy! Bring on the Krebb Cycle...
The Krebb Cycle
The pyruvates 1st move from the cytosol into the mitochondrial matrix.
On the way the 2 pyruvates undergo a transformation to become 2 acetyl-CoA
molecules when they interact with 2 co-enzyme A molecules.
Note: 2 more NAD molecules are charged to become NADH.
Carbon dioxide is also released in this step.
Next, the acetyl-CoA molecules undergo a series of reactions the charge a lot more
NAD molecules into NADH. Another type of "energy carrier" called FAD+ becomes
charged into FADH2.
Although some ATP are produced in this cycle, it is actually the NADH and FADH2 that
are the important part of this phase of the story. These two highly charged particles
must move on to the electron transport chain to give up their electrons.
When these molecules release their electrons in the presence of oxygen, loads of ATP
are produces.
Let's move on and look at how this happens in the electron transport chain...
The Electron Transport Chain
This step takes place in the mitochondrial cristae (the folds in
the inned membrane of the mitochondria).
In this step, the NADH and FADH2 will be used to
produce many, many ATP molecules.
The NADH and FADH2 molecules give their electrons in order to pump hydrogen atoms across
the cristae membrane.
Oxygen atoms MUST be present to snatch the electrons away at the bottom of the transport
chain process. When they do, they
create 2 water molecules.
The hydrogen concentration builds up
inside the cristae. As they flow back to
their normal concentration, they
convert ADP into ATP!