and the Electron Transport System.

The learning objectives of this module

are one, describe the basic steps in glycolysis, the TCA

cycle, and the electron transport system, and two,

summarize the energy yields of glycolysis

and cellular respiration.

Most cells generate ATP by breaking down carbohydrates,

especially glucose.

When cells consume oxygen, they can break down glucose

into carbon dioxide and water.

And by doing so, they provide a typical body cell

with 36 molecules of ATP.

The breakdown of glucose takes place

in a series of small steps.

The first steps begin in the cytosol of the cell.

Here in the cytosol, glucose is broken down

into smaller molecules in a process called glycolysis.

Glycolysis does not require oxygen.

And so the process is said to be anaerobic,

which means no oxygen.

The molecules produced through glycolysis

are small enough to be absorbed by the mitochondria.

Once these smaller molecules are in the mitochondria,

they will be involved in reactions that will use oxygen

and are considered aerobic.

The reactions inside the mitochondria

will create most of the ATP that the cell needs to function.

These reactions are collectively called aerobic metabolism

or cellular respiration.

Let's start with glycolysis.

Glycolysis is the breakdown of glucose to pyruvic acid.

In this process, glucose, which is a six carbon compound,

will break into two three carbon compounds of pyruvic acid.

Pyruvic acid then loses a hydrogen atom

to become pyruvate.

During the process of glycolysis,

the cells must use two ATP molecules

to complete this process but will also create four ATP

through phosphorylation.

Since glycolysis uses two ATP but creates four ATP,

the net ATP production during glycolysis

is two ATP for every glucose molecule catabolized.

This amount of ATP is not enough for the cell to survive.

But the pyruvate molecules hold much energy

that will be used in the mitochondria

to create more ATP.

In addition to the ATP that are produced,

the process of glycolysis will lose two hydrogen atoms,

which will be transferred to a hydrogen carrier called NAD.

When the two hydrogen atoms are transferred to NAD,

the molecule then becomes NADH.

The hydrogen will be carried to the electron transport

system in the mitochondria.

We will discuss later what happens

to the hydrogen in the NADH molecule.

Glycolysis is anaerobic because it does not require oxygen

and it occurs in the cytosol of the cell.

If oxygen is present in the cell,

the three carbon pyruvate molecules

will enter the mitochondria and continue being catabolized.

Once in the mitochondria, each pyruvate molecule

will lose a carbon in a process called decarboxylation.

This carbon will join with oxygen

to become carbon dioxide.

The carbon dioxide will diffuse out of the cell

and into the bloodstream, where it will be carried to the lungs

and be breathed out during expiration.

The new molecule formed is now a two carbon molecule

called acetyl coenzyme A. Acetyl coenzyme A, or acetyl CoA,

will participate in reactions in the matrix of the mitochondria.

An enzyme called coenzyme A, or CoA,

will carry acetyl CoA into the citric acid cycle in the matrix

of the mitochondria.

Recall that mitochondria are organelles

with double membranes.

That is, the mitochondria has two layers

of membrane, the inner membrane and the outer membrane.

The space in between the inner membrane and the outer membrane

is called the intermembrane space.

The space inside the inner membrane is called the matrix.

The citric acid cycle will take place

in the matrix of the mitochondria.

The function of the citric acid cycle

is to remove hydrogen atoms from organic molecules

and transfer them to coenzymes.

These coenzymes are NAD and FAD.

At the start of the citric acid cycle,

CoA will release the two carbon acetyl CoA

so that it may be transferred to a four carbon molecule.

Together, these two molecules form a six carbon

compound called citric acid.

After releasing the acetyl CoA, the CoA molecule

is now free to pick up another acetyl CoA.

At the end of the citric acid cycle,

two carbon atoms will have been removed

to recreate the original four carbon compound.

These carbons join with oxygen atoms

to form carbon dioxide, which is a waste product of the cell

and which will eventually diffuse out of the cell

into the blood and be transported to the lungs

to be breathed out.

More importantly, during the citric acid cycle,

hydrogen atoms will be transferred to coenzyme NAD

and to another related one called FAD.

NAD and FAD will be called NADH and FADH2 as they both pick up

two hydrogen atoms each.

NAD is a negatively charged compound.

And when it picks up two hydrogen ions,

it becomes NADH plus H plus.

However, it is commonly just referred to as NADH.

NADH and FADH2 will carry the hydrogen over

to another area in the mitochondria

to be used in the electron transport system.

NADH and FADH2 release their hydrogen

to the cytochromes of the electron transport system.

Without the hydrogen, NAD and FAD

then return to the citric acid cycle

to pick up more hydrogen atoms.

The only immediate energy benefit of the citric acid

cycle is that one GTP, or guanosine triphosphate,

will be created from one GDP, or guanosine diphosphate.

GTP is equivalent to ATP because it will readily

transfer a phosphate group to ADP to create ATP.

So we can say that for each acetyl CoA,

one ATP is directly created by the citric acid cycle.

Of course, with one glucose molecule,

two acetyl CoA are produced.

So ultimately, from one molecule of glucose,

the citric acid cycle produces two ATP.

The formation of GTP from GDP in the citric acid cycle

is another example of substrate phosphorylation.

In this process, a phosphate group

is transferred to a suitable acceptor molecule

using energy from a chemical process.

This occurs in many reactions in the cytosol

where a phosphate group is transferred to an ADP

to produce ATP.

For example, the ATP produced during glycolysis

is generated through substrate phosphorylation.

Normally, however, substrate phosphorylation

only provides a small amount of ATP

and isn't enough for the cell to function.

Most of the ATP that the cell needs

will be produced during oxidative phosphorylation,

which is what we'll talk about next.

Oxidative phosphorylation is the generation

of ATP within the mitochondria that

requires both coenzymes and consumes oxygen.

This process produces more than 90%

of the ATP used by body cells.

The key reactions take place in the electron transport system.

Oxidative phosphorylation also forms water, or H2O,

by combining two hydrogen atoms with one oxygen atom.

The oxygen is provided from the atmosphere during respiration.

And hydrogen is found in all organic molecules.

So both oxygen and hydrogen are readily

available for this reaction.

The reaction of creating water releases a tremendous amount

of energy.

There is so much energy released that this type of reaction

is used to launch space shuttles into orbit.

Obviously, cells can't handle this much energy.

So the energy released must be gradual.

Oxidative phosphorylation proceeds

in a series of small, controlled steps

so that energy can be captured safely

and ATP can be generated in the process.

During these steps, molecules lose electrons in a process

called oxidation.

When one molecule loses an electron,

another molecule will gain it in a process called reduction.

When electrons are passed from one molecule to another,

the electron donor is oxidized.

And the electron recipient is reduced.

Oxidation and reduction are important because electrons

carry chemical energy.

In a typical oxidation-reduction reaction,

the reduced molecule gains energy

at the expense of the oxidized molecule.

You can remember that molecules are oxidized

when they lose electrons and are reduced when they gain

electrons by remembering the words oil rig,

or oxidation is loss and reduction is gain.

In an oxidation-reduction exchange,

the reduced molecule does not gain all the energy released

by the oxidized molecule.

Some energy is always released as heat.

The remaining energy will be used to form ATP.

The electron transport system starts with NADH and FADH2

delivering hydrogen atoms to cytochromes.

The cytochromes are integral and peripheral proteins

that are embedded in the inner mitochondrial membrane.

These hydrogen atoms are from the citric acid cycle

and were delivered by NAD and FAD

in the form of NADH and FADH2.

If you remember, hydrogen atoms, they

consist of one electron and one proton.

The electron has a negative charge.

And the proton has a positive charge.

The electron carries energy with it.

NADH and FADH2 will both carry two hydrogen atoms.

The electrons from these atoms will

be passed from NADH and FADH2 to one of the cytochromes embedded

in the inner membrane.

The electron will then be passed from one cytochrome

to the next in small steps.

At the last cytochrome, at of the electron transport system,

an oxygen atom accepts the electron

and will use its energy to combine the oxygen and hydrogen

ions to form water.

Note that this sequence starts with the removal of two

hydrogen atoms from a substrate molecule, NADH or FADH2,

and ends with the formation of water

by combining two hydrogen with one oxygen.

This reaction occurred in several steps.

Had it occurred in only one step,

it would've been highly explosive.

The coenzymes NADH and FADH2 transfer the electrons

to the first cytochrome in the electron transport system.

And the electrons continue to be passed from one cytochrome

to the next.

As they are passed along, the electrons

themselves release energy.

This energy released causes hydrogen ion pumps

to start working.

These pumps move hydrogen ions from the matrix

across the inner membrane and into the intermembrane space.

This causes a large concentration gradient

for hydrogen ions between the matrix

and the intermembrane space.

Following the rules of diffusion,

hydrogen would diffuse from the high hydrogen concentration

in the intermembrane space to the lower concentration

in the matrix.

Hydrogen can't cross the inner membrane

because they are not lipid soluble.

However, in the inner membrane, an integral protein

with a channel called ATP synthase has the ability

to permit hydrogen ions to diffuse into the matrix.

This process is called chemiosmosis.

And it creates a kinetic energy that will

be used to convert ADP to ATP.

Both ADP and phosphate groups are already

in the matrix of the mitochondria.

The energy from chemiosmosis will

be used to phosphorylate ADP, which

means a phosphate group will combine with ADP to form ATP.

Because this process uses both coenzymes and oxygen,

the process of making ATP through this method

is called oxidative phosphorylation.

For every NADH molecule that transfers its hydrogen