Respiration is the process by which organisms burn food
to produce energy. The starting material of cellular respiration
is the sugar glucose, which has energy stored in its
chemical bonds. You can think of glucose as a kind of cellular piece
of coal: chock-full of energy, but useless when you want to power
a stereo. Just as burning coal produces heat and energy in the form
of electricity, the chemical processes of respiration convert the
energy in glucose into usable form.
Adenosine triphosphate (ATP) is the usable
form of energy produced by respiration. ATP is like electricity:
it contains the same energy as coal, but it’s easier to transport
and is just what’s needed when the cell needs some power to carry
out a task.
ATP is a nucleic acid similar to RNA. It has a ribose
sugar attached to the nitrogenous base adenine. However, instead
of the single phosphate group typical of RNA nucleotides, ATP has
three phosphate groups. Each of the ATP phosphate groups carries
a negative charge. In order to hold the three negative charges in
such proximity, the bonds holding the phosphate groups have to be
quite powerful. If one or two of the bonds are broken and the additional
phosphates are freed, the energy stored in the bonds is released
and can be used to fuel other chemical reactions. When the cell
needs energy, it removes phosphates from ATP by hydrolysis, creating
energy and either adenosine diphosphate (ADP), which has two phosphates,
or adenosine monophosphate (AMP), which has one phosphate.
Respiration is the process of making ATP rather than breaking
it down. To make ATP, the cell burns glucose and adds new phosphate
groups to AMP or ADP, creating new power molecules.
There are actually two general types of respiration, aerobic
and anaerobic. Aerobic respiration occurs in the presence of oxygen,
while anaerobic respiration does not use oxygen. Both types of cell
respiration begin with the process of glycolysis, after which the
two diverge. We’ll first discuss aerobic respiration and then move
Aerobic Cell Respiration
Aerobic respiration is more efficient and more complicated
than anaerobic respiration. Aerobic respiration uses oxygen and
glucose to produce carbon dioxide, water, and ATP. More
precisely, this process involves six oxygen molecules for every
6O2 + C6H12O66CO2 +
6H2O + ATP energy
This general equation for aerobic respiration (which you
should know for the test) is actually the product of three separate
stages: glycolysis, the Krebs cycle, and the electron transport
chain. Typically, the SAT II Biology only asks questions about the
starting and ending products of each stage and the location where
each takes place. Understanding the internal details of stages will
help you remember these key facts and prepare you in case the testers
throw in a more difficult question, but the details of all the complex
reactions will probably not be tested by the SAT II.
Glycolysis is the first stage of aerobic (and anaerobic)
respiration. It takes place in the cytoplasm of the cell. In glycolysis
(“glucose breaking”), ATP is used to split glucose molecules
into a three-carbon compound called pyruvate. This
splitting produces energy that is stored in ATP and a molecule called NADH.
The chemical formula for glycolysis is:
2ATP + 2NAD+2pyruvate
+ 4ATP + 2NADH
As the formula indicates, the cell must invest 2 ATP
molecules in order to get glycolysis going. But by the time glycolysis
is complete, the cell has produced 4 new ATP, creating
a net gain of 2 ATP. The 2 NADH molecules
travel to the mitochondria, where, in the next two
stages of aerobic respiration, the energy stored in them is converted
The most important things to remember about glycolysis
- Glycolysis is part of both aerobic and anaerobic
- Glycolysis splits glucose, a six-carbon compound, into
two pyruvate molecules, each of which has three carbons.
- In glycolysis, a 2 ATP investment results
in a 4 ATP payoff.
- Unlike the rest of aerobic respiration, which takes place
in the mitochondria, glycolysis takes place in the cytoplasm of
- Unlike the rest of aerobic respiration, glycolysis does
not require oxygen.
The Krebs Cycle
After glycolysis, the pyruvate sugars are transported
to the mitochondria. During this transport, the three-carbon pyruvate
is converted into the two-carbon molecule called acetate. The extra
carbon from the pyruvate is released as carbon dioxide, producing
another NADH molecule that heads off to the electron transport chain
to help create more ATP. The acetate attaches to a coenzyme called
coenzyme A to form the compound acetyl-CoA. The acetyl-CoA
then enters the Krebs cycle. The Krebs cycle is called a cycle because
one of the molecules it starts with, the four-carbon oxaloacetate,
is regenerated by the end of the cycle to start the cycle over again.
The Krebs cycle begins when acetyl-CoA and oxaloacetate
interact to form the six-carbon compound citric acid. (The Krebs
cycle is also sometimes called the citric acid cycle.) This
citric acid molecule then undergoes a series of eight chemical reactions
that strip carbons to produce a new oxaloacetate molecule. The extra
carbon atoms are expelled as CO2 (the
Krebs cycle is the source of the carbon dioxide you exhale). In
the process of breaking up citric acid, energy is produced. It is
stored in ATP, NADH, and FADH2.
The NADH and FADH2 proceed
on to the electron transport chain.
The entire Krebs cycle is shown in the figure below. For
the SAT II Biology, you don’t have to know the intricacies of this
figure, but you should be able to recognize that it shows the Krebs
It is also important to remember that each glucose molecule
that enters glycolysis is split into two pyruvate molecules, which
are then converted into the acetyl-CoA that moves through the Krebs
cycle. This means that for every glucose molecule that enters glycolysis, the
Krebs cycle runs twice. Therefore, for one glucose molecule running
through aerobic cell respiration, the equation for the Krebs cycle
2acetyl-CoA + 2oxaloacetate4CO2 +
6NADH + 2FADH2 + 2ATP + 2oxaloacetate
For the SAT II Biology, the most important things
to remember about the Krebs cycle are:
- The Krebs cycle results in 2 ATP molecules
for each glucose molecule run through glycolysis.
- The Krebs cycle sends energy-laden NADH and FADH2 molecules
on to the next step in respiration, the electron transport chain.
It does not export carbon molecules for further processing.
- The Krebs cycle takes place in the mitochondrial matrix,
the innermost compartment of the mitochondria.
- Though the Krebs cycle does not directly require oxygen,
it can only take place when oxygen is present because it relies
on by-products from the electron transport chain, which requires
oxygen. The Krebs cycle is therefore an aerobic process.
The Electron Transport Chain
A great deal of energy is stored in the NADH and FADH2 molecules
formed in glycolysis and the Krebs cycle. This energy is converted
to ATP in the final phase of respiration, the electron transport
10NADH + 2FADH234ATP
The electron transport chain consists of a set of three
protein pumps embedded in the inner membrane of the mitochondria. FADH2 and NADH are
used to power these pumps. Using the energy in NADH and FADH2,
these pumps move positive hydrogen ions (H+) from
the mitochondrial matrix to the intermembrane space. This creates
a concentration gradient over the membrane.
In a process called oxidative phosphorylation, H+ ions
flow back into the matrix through a membrane protein called an ATP
synthase. This channel is the opposite of the standard membrane
pumps that burns ATP to transport molecules against their concentration
gradient: ATP synthase uses the natural movement of ions along their
concentration gradient to make ATP. All told, the flow of ions through
this channel produces 34 ATP molecules. The waste products from
the powering of the electron transport chain protein pumps combine
with oxygen to produce water molecules. By accepting these waste
products, oxygen frees NAD+ and
FAD to play their roles in the Krebs cycle and the electron transport
chain. Without oxygen, these vital energy carrier molecules would
not perform their roles and the processes of aerobic respiration
could not occur.
For the SAT II Biology, the most important things to remember
about the electron transport chain and oxidative phosphorylation
- Four ATP molecules are produced by glycolysis
and the Krebs cycle combined. The electron transport chain produces 34 ATP.
- The electron transport chain occurs across the inner membrane
of the mitochondria.
- The electron transport chain requires oxygen.
Aerobic respiration requires oxygen. However,
some organisms live in places where oxygen is not always present.
Similarly, under extreme exertion, muscle cells may run out of oxygen. Anaerobic
respiration is a form of respiration that can function without oxygen.
In the absence of oxygen, organisms continue to carry
out glycolysis, since glycolysis does not use oxygen in its chemical
process. But glycolysis does require NAD+.
In aerobic respiration, the electron transport chain turns NADH back
to NAD+ with the
aid of oxygen, thereby averting any NAD+ shortage
and allowing glycolysis to take place. In anaerobic respiration,
cells must find another way to turn NADH back
This “other way” is called fermentation.
Fermentation’s goal is not to produce additional energy, but merely
to replenish NAD+ supplies
so that glycolysis can continue churning out its slow but steady
stream of ATP. Because pyruvates are not needed in anaerobic respiration,
fermentation uses them to help regenerate NAD+.
While employing the pyruvates in this way does allow glycolysis
to continue, it also results in the loss of the considerable energy
contained in the pyruvate sugars.
There are two principle forms of fermentation, lactic
acid fermentation and alcoholic fermentation.
For the SAT II Biology, remember that no matter what kind of fermentation occurs,
anaerobic respiration only produces 2 net ATP in glycolysis.
Lactic Acid Fermentation
In lactic acid fermentation, pyruvate is converted to
a three-carbon compound called lactic acid:
pyruvate + NADHlactic
acid + NAD+
In this reaction, the hydrogen from the NADH molecule
is transferred to the pyruvate molecule.
Lactic acid fermentation is common in fungi and bacteria.
Lactic acid fermentation also takes place in human muscle cells
when strenuous exercise causes temporary oxygen shortages. Since
lactic acid is a toxic substance, its buildup in the muscles produces
fatigue and soreness.
Another route to NAD+ produces
alcohol (ethanol) as a by-product:
pyruvate + NADHethyl
alcohol + NAD+ + CO2
Alcoholic fermentation is the source of ethyl alcohol
present in wines and liquors. It also accounts for the bubbles in
bread. When yeast in bread dough runs out of oxygen, it goes through
alcoholic fermentation, producing carbon dioxide. These carbon dioxide
bubbles create spaces in the dough and cause it to rise.
Like lactic acid, the ethanol produced by alcoholic fermentation
is toxic. When ethanol levels rise to about 12 percent, the yeast