the First Energy-releasing Pathways

Chapter 7

The first energy-releasing pathways

evolved about 3.8 billion years ago and could run to completion without oxygen.

Many bacteria and protistans still make

ATP by anaerobic pathways, mainly fermentation and anaerobic electron transport.

Some cells in your own body can use an

anaerobic route for short periods, but they do so only when they don't receive enough oxygen. Your cells use mainly aerobic respiration.

With each breath you take, you provide your actively respiring cells with a fresh supply of oxygen. energy_releasing.swf

The main energy-releasing pathways all

start in the cytoplasm with glycolysis, a pathway in which enzymes cleave and rearrange each glucose molecule into two pyruvate molecules. Once this stage is over, the energy-releasing pathways differ.

When a glucose molecule is the starting

material, aerobic respiration can be summarized this way: C

6

H12O6 --------> 6CO2 + 6H2O

However, this summary equation tells us

only what the substances are at the start and finish of the pathway.

In between are three reaction stages. These

stages are briefly described in the clip

aerobic_stages.swf

Each glucose molecule has six carbon atoms,

twelve hydrogen atoms, and six oxygen atoms covalently bonded to one another. During glycolysis, glucose or some other

carbohydrate in the cytoplasm partially breaks down to pyruvate, a molecule with a backbone of three carbon atoms.

Glycolysis takes place in two major parts.

The first part requires energy input. The second part releases energy. The Figure shows a schematic of the

energy changes during glycolysis.

glycolysis_two_stages.swf

Glycolysis begins when ATP molecules each

transfer a phosphate group to glucose and so donate energy to it.

Such a transfer is known as phosphorylation.

In this case, phosphorylation raises the energy content of glucose to a level that is high enough to enable entry into the second part, the energy-releasing steps of glycolysis.

The first energy-releasing step cleaves the

activated glucose into two molecules, which we can call PGAL (phosphoglyceraldehyde). Each PGAL gives up two electrons and a

hydrogen to the coenzyme NAD+, reducing it to NADH.

The first energy-releasing step cleaves the

activated glucose into two molecules, which we can call PGAL (phosphoglyceraldehyde). Each PGAL gives up two electrons and a

hydrogen to the coenzyme NAD+, reducing it to NADH.

As a result of these donations, each PGAL

is converted to an unstable intermediate. This intermediate enables ATP to form by

giving up a phosphate group to ADP. The next intermediate in the sequence does the same thing.

A total of four ATP form by substrate-level

phosphorylation. This metabolic event is defined as the direct transfer of a phosphate group from a substrate of a reaction to some other molecule, such as ADP. Remember that two ATP were invested to start

the reactions, so the net energy yield from glycolysis is only two ATP. Two NADH also form.

The animation provides a step-by-step

look at the reactions of glycolysis.

glycolysis.swf

Suppose two pyruvate molecules, formed

by glycolysis, leave the cytoplasm and enter a mitochondrion. In this organelle, both the second and

third stages of the aerobic pathway run to completion. The Figure reviews the structure of a mitochondrion.

mitochondrion.swf

The second stage occurs in the inner

compartment of the mitochondrion. It starts with a few preparatory steps in which

an enzyme removes a carbon atom from each pyruvate molecule. A coenzyme, known as coenzyme A, becomes

acetyl-CoA when it combines with the remaining two-carbon fragment. preparatory_rx.swf

The second-stage reactions continue when

acetyl-CoA transfers the two-carbon fragment to oxaloacetate, the entry point of the Krebs cycle.

The reactions of the Krebs cycle in more detail The combination of acetyl-CoA and

oxaloacetate forms citrate. The Krebs cycle breaks down citrate into carbon dioxide and water in a stepwise fashion. All the carbon molecules of pyruvate eventually end up in carbon dioxide.

There are three other important points to

understand about the Krebs cycle:

1. Hydrogen and electrons are transferred

to the coenzymes NAD+ and FAD (flavin adenine dinucleotide, a different coenzyme) to produce NADH and FADH2.

2. Substrate-level phosphorylations produce

more ATP.

3. Oxaloacetate regenerates. (That's why

the process is called a cycle.)

The animation shows the reactions and

intermediates of the Krebs cycle in detail.

krebs_cycle_reactions.swf

In total, the second-stage reactions produce

two ATP, eight NADH, and two FADH2 for each molecule of glucose. The coenzymes go to the electron transport system for the final stage of the aerobic pathway. The animation will show a final overview of the

second-stage reactions

krebs_telecourse.mov

ATP production goes into high gear in the

third stage of the aerobic pathway, electron transport phosphorylation. During the earlier stages, hydrogen and

electrons were stripped from reactants and loaded onto the coenzymes NAD and FAD, reducing them to NADH and FADH2.

In the final stage, these coenzymes deliver

hydrogen ions and electrons to an electron transfer chain in the inner mitochondrial membrane. The electrons are transferred from one

molecule of the chain to the next molecule in line.

When certain molecules accept and then

donate electrons, they also pick up hydrogen ions in the inner compartment. Quickly afterward, they release the ions to

the outer compartment.

This shuttling action sets up H+

concentration and electric gradients across the inner mitochondrial membrane. Nearby in the membrane, H+ ions follow

the gradients and flow back to the inner compartment, through the interior of ATP synthases.

The H+ flow through these transport proteins

drives the formation of ATP from ADP and unbound phosphate.

Free oxygen keeps ATP production going. When

it withdraws electrons at the end of the transport systems and then combines with H+, water is the result.

This process is illustrated in the following

animations

electron_transport.mov

mito_chemiosmosis.swf

Thirty-two ATP typically form during the

third stage of aerobic respiration. Add these to the net yield from the

preceding stages, and the total harvest is thirty-six ATP from one glucose molecule. However, the exact yield varies depending

on the type of cell and prevailing conditions. See Figure. energy_harvest.swf

The human body has many alternative sources

of energy. Complex carbohydrates, fats, and proteins cannot enter the aerobic pathway directly. The digestive system and individual cells must first break apart these molecules into simpler degradable subunits. The Figure shows the reaction sites where a variety of organic compounds can enter the stage of aerobic respiration.

alt_energy_sources.swf

In common usage, the term fermentation is

used to describe the process by which alcoholic beverages such as beer and wine are produced. This is alcoholic fermentation, an energy-

releasing pathway. It is an example of an anaerobic pathway because, unlike aerobic respiration, it does not require oxygen.

Many microorganisms rely entirely on

fermentation pathways to meet their modest energy requirements. Other microorganisms switch back and forth

between aerobic and anaerobic pathways as the oxygen levels in their environments change.

The yeast used to brew alcoholic

beverages is an example. Human muscle cells also can switch back

and forth. They utilize a fermentation pathway (lactate fermentation) during periods of vigorous exercise, when they are short of oxygen.

Like all the main energy-releasing pathways,

fermentation begins with the breakdown of glucose by glycolysis.

However, the reactions do not completely

break down glucose to carbon dioxide and water, and produce no more ATP beyond the tiny yield from glycolysis.

The final steps serve only to regenerate NAD+,

a coenzyme with central roles in the breakdown reactions.

Exactly what happens after glycolysis

depends on what type of fermentation is taking place.

fermentation.swf

Summary of Cellular Respiration

CellRespiration.svg

The End