Cellular Respiration

Cellular respiration is the aerobic process of oxidizing food molecules, like

glucose(monosaccharide) , to carbon dioxide and water. It harvests chem energy from organic
fuel molecules

C6H12O6 + 6O2 + 6H2O → 12H2O + 6 CO2 plus 32 atp

The energy released is trapped in the form of ATP for use by all the energy-consuming
activities of the cell.

Cellular respiration consistes of many steps………….three main stages……….transport.

1) Glycolysis:

Glycolysis takes place in the cytosol of a cell, and it can be

broken down into two main phases: the energy-requiring phase,
and the energy-releasing phase.

Energy-requiring phase. In this phase, the starting molecule

of glucose gets rearranged, and two phosphate groups are
attached to it. The phosphate groups make the modified
sugar—now called fructose-1,6-bisphosphate—unstable,
allowing it to split in half and form two phosphate-bearing
three-carbon sugars. Because the phosphates used in these
steps come from ATP, two ATP molecules get used
The three-carbon sugars formed when the unstable sugar
breaks down are different from each other. Only one—
glyceraldehyde-3-phosphate—can enter the following step.
However, the unfavorable sugar, DHAP, can be easily
converted into the favorable one, so both finish the pathway in
the end
Energy-releasing phase. In this phase, each three-carbon
sugar is converted into another three-carbon molecule,
pyruvate, through a series of reactions. In these reactions, two
ATP\text{ATP}ATP molecules and one
NADH\text{NADH}NADH molecule are made. Because this
phase takes place twice, once for each of the two three-carbon
sugars, it makes four ATP\text{ATP}ATP and two
NADH\text{NADH}NADH overall.
Overall, glycolysis converts one six-carbon molecule of glucose
into two three-carbon molecules of pyruvate. The net products
of this process are two molecules of ATP\text{ATP}ATP
(444 ATP\text{ATP}ATP produced −-− 222
ATP\text{ATP}ATP used up) and two molecules of
NADH\text{NADH}NAD

2)

tricarboxylic acid (TCA) cycle/Krebs cycle

of the citric acid cycle
In eukaryotes, the citric acid cycle takes place in the matrix of the
mitochondria, just like the conversion of pyruvate to acetyl CoA.:

In prokaryotes, these steps both take place in the cytoplasm. The

citric acid cycle is a closed loop; the last part of the pathway reforms
the molecule used in the first step. The cycle includes eight major
steps.

In the first step of the cycle, acetic acid combines with a four-carbon
acceptor molecule, oxaloacetate, to form a six-carbon molecule called
citrate. After a quick rearrangement, this six-carbon molecule releases
two of its carbons as carbon dioxide molecules in a pair of similar
reactions, producing a molecule of NADH each time1^11.

The remaining four-carbon molecule undergoes a series of additional

reactions, first making an ATP then reducing the electron carrier FAD
to FADH2, and finally generating another NADH from nadplus. This
set of reactions regenerates the starting molecule, oxaloacetate, so the
cycle can repeat.

Overall, one turn of the citric acid cycle releases two carbon dioxide
molecules and produces three NADH, one FADH2, and one ATp

. The citric acid cycle goes around twice for each molecule of glucose
that enters cellular respiration because there are two pyruvates—and
thus, two acetyl CoA—made per glucose. CoA is stripped and recycled.
Mitochondria are membrane-enclosed organelles distributed through the cytosol of most
eukaryotic cells. Their number within the cell ranges from a few hundred to, in very active
cells, thousands. Their main function is the conversion of the potential energy of food
molecules into ATP.

Mitochondria have:

 an outer membrane that encloses the entire structure

 an inner membrane that encloses a fluid-filled
matrix
 between the two is the intermembrane space
 the inner membrane is elaborately folded with
shelflike cristae projecting into the matrix.
 a small number (some 5–10) circular molecules of
DNA

This electron micrograph (courtesy of

Keith R. Porter) shows a single
mitochondrion from a bat pancreas cell.
Note the double membrane and the way
the inner membrane is folded into
cristae. The dark, membrane-bounded
objects above the mitochondrion are
lysosomes.

The number of mitochondria in a cell can

 increase by their fission (e.g. following mitosis);

 decrease by their fusing together.

(Defects in either process can produce serious, even fatal, illness.)

The Outer Membrane

The outer membrane contains many complexes of integral membrane proteins that form
channels through which a variety of molecules and ions move in and out of the
mitochondrion.

The Inner Membrane

The inner membrane contains 5 complexes of integral membrane proteins:

 NADH dehydrogenase (Complex I)

 succinate dehydrogenase (Complex II)
 cytochrome c reductase (Complex III; also known as the cytochrome b-c1 complex)
 cytochrome c oxidase (Complex IV)
 ATP synthase (Complex V)

The Matrix

The matrix contains a complex mixture of soluble enzymes that catalyze the respiration of
pyruvic acid and other small organic molecules.

Here pyruvic acid is

 oxidized by NAD+
producing NADH +
H+
 decarboxylated
producing a molecule
of
o carbon dioxide
(CO2) and
o a 2-carbon
fragment of
acetate bound
to coenzyme A
forming acetyl-
CoA

The Citric Acid

Cycle
 This 2-carbon
fragment is donated to
a molecule of oxaloacetic acid.
 The resulting molecule of citric acid (which gives its name to the process) undergoes
the series of enzymatic steps shown in the diagram.
 The final step regenerates a molecule of oxaloacetic acid and the cycle is ready to turn
again.

Summary:

 Each of the 3 carbon atoms present in the pyruvate that entered the mitochondrion
leaves as a molecule of carbon dioxide (CO2).
 At 4 steps, a pair of electrons (2e-) is removed and transferred to NAD+ reducing it to
NADH + H+.
 At one step, a pair of electrons is removed from succinic acid and reduces the
prosthetic group flavin adenine dinucleotide (FAD) to FADH2.
The electrons of NADH and FADH2 are transferred to the electron transport chain.

The Electron Transport Chain

The electron transport chain consists of 3 complexes of
integral membrane proteins

 the NADH dehydrogenase (complex I)

 the cytochrome c reductase (complex III)
 the cytochrome c oxidase (complex IV)

and two freely-diffusible molecules

 ubiquinone (also known as Coenzyme Q)

 cytochrome c

that shuttle electrons from one complex to the next.

The electron transport chain accomplishes:

 the stepwise transfer of electrons from NADH

(and FADH2) to oxygen molecules to form (with
the aid of protons) water molecules (H2O).

(Electrons from FADH2 enter the electron

transport chain through another integral membrane
protein — complex II — reducing ubiquinone
(see figure on right).
(Cytochrome c can only transfer one electron at a time, so cytochrome c oxidase must
wait until it has accumulated 4 of them before it can react with oxygen.)

 harnessing the energy released by this transfer to the pumping of protons (H+) from
the matrix to the intermembrane space.
 Approximately 20 protons are pumped into the intermembrane space as the 4
electrons needed to reduce oxygen to water pass through the respiratory chain.
 The gradient of protons formed across the inner membrane by this process of active
transport forms a miniature battery.
 The protons can flow back down this gradient only by reentering the matrix through
ATP synthase, another complex (complex V) of 16 integral membrane proteins in
the inner membrane. The process is called chemiosmosis.

Chemiosmosis in mitochondria
The energy released as electrons pass down the gradient from NADH to oxygen is harnessed
by three enzyme complexes of the respiratory chain (I, III, and IV) to pump protons (H+)
against their concentration gradient from the matrix of the mitochondrion into the
intermembrane space (an example of active transport).
As their concentration increases there (which is the same as saying that the pH decreases), a
strong diffusion gradient is set up. The only exit for these protons is through the ATP
synthase complex. As in chloroplasts, the energy released as these protons flow down their
gradient is harnessed to the synthesis of ATP. The process is called chemiosmosis and is an
example of facilitated diffusion.

One-half of the 1997 Nobel Prize in Chemistry was awarded to Paul D. Boyer and John E.
Walker for their discovery of how ATP synthase works. Link to some of the details.

External Link
Scroll down the left-hand column at this link to view animations of ATP synthase, the
electron transport chain and others on "cellular energy conversion".
Please let me know by e-mail if you find a broken link in my pages.)

How many ATPs?

It is tempting to try to view the synthesis of ATP as a simple matter of stoichiometry (the
fixed ratios of reactants to products in a chemical reaction). But (with 3 exceptions) it is not.

Most of the ATP is generated by the proton gradient that develops across the inner
mitochondrial membrane. The number of protons pumped out as electrons drop from NADH
through the respiratory chain to oxygen is theoretically large enough to generate, as they
return through ATP synthase, 3 ATPs per electron pair (but only 2 ATPs for each pair
donated by FADH2).

With 12 pairs of electrons removed from each glucose molecule,

 10 by NAD+ (so 10x3=30); and

 2 by FADH2 (so 2x2=4),

this could generate 34 ATPs.

Add to this the 4 ATPs that are generated by the 3 exceptions and one arrives at 38.

But
  The energy stored in the proton gradient is also used for the active transport of several
molecules and ions through the inner mitochondrial membrane into the matrix.
 NADH is also used as reducing agent for many cellular reactions.

So the actual yield of ATP as mitochondria respire varies with conditions. It probably seldom
exceeds 30.