AEROBIC

RESPIRATION Chapter 8

AEROBIC RESPIRATION Aerobic respiration is the next step after Glycolysis if the cell can

obtain oxygen.

We won’t need it until the last step…but we still need it.

Remember that the final product of Glycolysis is pyruvate.

Aerobic respiration takes place in the mitochondria

The first step is an intermediate reaction to prepare the pyruvate for

the citric acid cycle

(The intermediate step is only one step, and it’s not technically part

of Glycolysis OR aerobic respiration. It’s just a preparatory step.)

The pyruvate gives off a carbon as CO2, donates a H to form NADH, and the

final molecule is an acetyl-CoA molecule

CITRIC ACID CYCLE

The citric acid cycle takes place in the matrix of the

mitochondria

**Remember: each step in aerobic respiration happens

TWICE—once for each PGAL formed in Glycolysis

Step 1

Acetyl CoA (2-carbon molecule) bonds with an oxaloacetate (4-

carbon molecule) to form citric acid (6-carbon molecule)

CITRIC ACID CYCLE

Step 2

Citric acid gives off a CO2

The molecule now contains 5-carbons

CO2 is not needed by the cell, so it is expelled out into the blood

stream.

Citric acid donates a hydrogen to an NADH

Citric acid reforms to an alpha-ketoglutarate

CITRIC ACID CYCLE

Step 3

Alpha-ketogluterate gives off a CO2, a hydrogen for an

NADH, and a phosphate for ATP

The molecule then forms a succinate

The molecule is now back to the 4-carbon molecule that the

cycle started with

CITRIC ACID CYCLE

Step 4

Succinate donates a hydrogen for an FADH2 molecule.

The succinate then rearranges to form a molecule of

fumerate

CITRIC ACID CYCLE

Step 5

The fumerate rearranges to form a molecule called

malate

CITRIC ACID CYCLE

Step 6

The malate donates a hydrogen to form NADH

The malate then reforms to the original

oxaloacetate molecule

The oxaloacetate begins the cycle over again.

CITRIC ACID CYCLE SUMMARY

Inputs…per cycle, (per glucose)

Acetyl CoA…1 (2)

Outputs

ATP… 1 (2)

FADH2… 1 (2)

CO2… 2 (4)

NADH… 3 (6)

ELECTRON CARRIERS

A hydrogen is simply one proton and one electron. So, when a

“hydrogen” is donated, it is also appropriate to say an “electron” is

donated

Electron carriers are molecules that transport a hydrogen from one

location to another

Typically, the electron of the hydrogen will be used as a cofactor for an

enzyme

NADH and FADH2 have been synthesized multiple times so far in

cellular respiration.

All will finally be used in the electron transport chain as reactants

ELECTRON TRANSPORT CHAIN

The electron transport chain follows glycolysis and the citric

acid cycle.

It is taking place at the same time as glycolysis and citric acid cycle,

but it uses the products of glycolysis and the citric acid cycle as

reactants

The ETC takes place in the inner membrane of the

mitochondria.

The ETC is powered thanks to the concept of diffusion and

equilibrium

Important fact to remember: diffusion and osmosis naturally occur

in the universe, which means that these processes happen for free.

ELECTRON TRANSPORT CHAIN The ETC is a series of protein channels embedded in the cristae

(inner mitochondrial membrane).

The NADH and FADH2 give off their electron, which powers each

protein channel in sequence.*

The NAD+ and FAD+ then return to pick up another electron

*REMEMBER: If we can’t do this step, then the cell has to do fermentation instead.

These proteins move hydrogen atoms from inside the membrane to

outside the membrane, against the concentration gradient.

The energy for this comes from the NADH and FADH2 electrons.

The hydrogen that cross the membrane are already present. They never leave.

This creates an unequal ratio of hydrogen atoms along the membrane

(more are outside than inside). The membrane is NOT in equilibrium

ATP SYNTHASE The only way for the hydrogen atoms to get back across

the membrane (and reach equilibrium) is through a specific

channel enzyme called ATP synthase.

ATP synthase looks like an upside-down light bulb.

As the hydrogen atoms pass through the ATP synthase

from the outside of the membrane to the inside, they

provide kinetic energy to the enzyme.

With this energy, ATP synthase attaches phosphates to

ADP molecules in the “bulb” part, building an ATP

molecule.

ATP PRODUCTION Each molecule of NADH powers the ETC enough to build 3

molecules of ATP

FADH gives a little less power and can build only 2 ATP

This means the ETC can produce a total of 32-34 ATP per glucose

molecule.

Add that to the four ATP already produced in glycolysis and the citric

acid cycle, you have a maximum-possible net gain of 36-38 ATP

molecules from 1 molecule of glucose.

With fermentation: it’s two.

To remove the electron from the ETC, the cell bonds it with a

molecule of oxygen and forms H2O

This is why you need to breathe. This is what the oxygen is used

for.

ETC SUMMARY

Inputs (per molecule of glucose)

10 NADH

2 FADH2

O2

Outputs

28-30 ATP from NADH

4 ATP from FADH2

NAD+

FAD+

H2O

CELL RESPIRATION SUMMARY

C6H12O6 + 6 O2 6 CO2 + 6 H2O + Energy

C6H12O6 : For glycolysis

6 O2 : To collect the electron in the ETC

6 CO2 : Given off in intermediate step and Citric Acid Cycle

6 H2O : Given off in the ETC

Energy : In the form of ATP