Cellular Respiration
Cellular 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 your
laptop. 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
ATP is a nucleic acid. It has a ribose sugar attached to a nitrogenous base called adenine. However,
instead of the single phosphate group typical of 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,
creating energy and either adenosine diphosphate (ADP), which has two phosphates, or adenosine
monophosphate (AMP), which has one phosphate.
Cellular 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.
Aerobic and Anaerobic Respiration
There are actually two types of respiration – aerobic and anaerobic. Aerobic respiration occurs in the
presence of oxygen, while anaerobic respiration does not use oxygen. Both types of respiration begin
with the process of glycolysis. After glycolysis, these two types of respiration follow different chemical
paths.
Aerobic respiration (requiring oxygen) is referred to as cellular respiration.
Anaerobic respiration (not requiring oxygen) is referred to as fermentation.
Aerobic Respiration (Cellular Respiration)
Aerobic respiration (cellular respiration) is more efficient (produces more ATP molecules) 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 sugar
molecule:
6O2 + C6H12O6
6CO2 + 6H2O + ATP energy
This general equation for aerobic respiration is actually the product of three separate stages: glycolysis,
the Krebs cycle, and the electron transport chain. Glycolysis
1. Glycolysis
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 two three-carbon
compounds called pyruvate. The cell must use 2 already existing 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 to ATP. Unlike the rest of aerobic respiration,
glycolysis does not require oxygen.
 Glycolysis takes place outside the mitochondria, in the cytoplasm of the cell
 Glycolysis does not require oxygen
 Glycolysis produces a net gain of 2 ATP molecules
2. The Krebs Cycle
The Krebs Cycle is the second stage of aerobic respiration. 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 (the
Krebs cycle is the source of the carbon dioxide you exhale). 2 ATP molecules are produced during the
Krebs cycle.
 The Krebs cycle takes place in the mitochondria
 The Krebs cycle requires oxygen
 The Krebs cycle produces 2 ATP molecules
3. The Electron Transport Chain
A great deal of energy is stored in the molecules formed in glycolysis and the Krebs cycle. This energy is
converted into ATP in the final phase of respiration, the electron transport chain. The electron transport
chain produces 34 ATP molecules.
 The electron transport chain takes place in the mitochondria
 The electron transport chain requires oxygen
 The electron transport chain produces 34 ATP molecules.
Anaerobic Respiration (Fermentation)
Aerobic respiration (cellular 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 (fermentation) 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 the other two stages of cellular respiration do require oxygen.
Therefore, cells have another way of producing ATP from glucose when oxygen is not available. This
“other way” is called anaerobic respiration, or fermentation.
Fermentation
Fermentation basically keeps the process of glycolysis running. The Krebs cycle and the electron
transport chain are not used (they do not work unless oxygen is present). Therefore, only 2 ATP
molecules are produced during fermentation.
There are two principle forms of fermentation, lactic acid fermentation and alcoholic fermentation. No
matter what kind of fermentation occurs, anaerobic respiration only produces 2 net ATP in glycolysis.
Lactic Acid Fermentation
In lactic acid fermentation, glucose is converted into 2 molecules of lactic acid:
C6H12O6 → 2 CH3CHOHCOOH
(glucose)
(lactic acid)
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.
Alcoholic Fermentation
In alcoholic fermentation, glucose is converted into 2 molecules of alcohol (ethanol) and 2 molecules of
carbon dioxide:
C6H12O6 → 2 C2H5OH
(glucose)
(ethanol)
+
2 CO2
(carbon dioxide)
Alcoholic fermentation is the source of ethyl alcohol present in wine and beer. 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.
After completing the reading, fill in the chart below:
Is the reaction
aerobic or
anaerobic?
Does the
reaction
require
oxygen?
Does the
reaction go
though
glycolysis?
Does the
reaction go
though the
Krebs cycle?
cellular
respiration
lactic acid
fermentation
alcoholic
fermentation
Questions:
1. When do cells use fermentation instead of cellular respiration to produce ATP?
2. What are the two types of fermentation?
3. Which type of fermentation do humans use?
4. When would humans use this type of fermentation?
5. What are two advantages of cellular respiration versus fermentation?
6. What are two advantages of fermentation versus cellular respiration?
Does the
reaction go
though the
electron
transport
chain?
How much
ATP is
produced?
What are the
products of
this reaction?