BIOL 103
6O2
L
AB
#5 – F
ERMENTATION AND
R
ESPIRATION
: B
ACKGROUND
ATP is the energy currency of the cell. It is produced through a process called respiration. The
energy molecules (ATP) generated through respiration, are available to fuel the processes of the
cell as needed. When ATP levels become too low a special protein signals the cell to begin
respiration. As long as all the critical components for the reaction are available, this cycle
provides a constant source of energy for the cell.
Respiration
Respiration harvests biological energy from fuel molecules, such as carbohydrates, and stores it
as ATP. Together with oxygen, the cell converts carbohydrates to carbon dioxide, water and most
importantly energy. Respiration is a controlled, multistep process which slowly releases the
energy stored in glucose and converts it to ATP. If all of this energy from glucose were released
at once, most would be lost as heat and light.
Carbohydrates contain high energy bonds that, when broken, release electrons. The first stage,
glycolysis, breaks carbohydrates (glucose) into pyruvate molecules. Though the bonds holding
pyruvate together contain a great deal of potential energy, this step yields little energy.
Glycolysis occurs with or without oxygen and takes place in the cytoplasm outside the
mitochondria. Interestingly, it is a pathway found in all living things.
C6H12O6 + 6CO2 + 6H2O + energy
glucose oxygen carbon dioxide water ATP
Yeast has been used to make leavened bread for centuries. When yeast undergoes
fermentation, CO2 is trapped between gluten and causes the bread to rise. Ethanol,
another byproduct of yeast fermentation, generates the alcohol content in beer, and
the CO2 provides effervescence. What ingredients must be present in order for this
process to occur?
BIOL 103
yeppp
Aerobic Respiration
Anaerobic vs. Aerobic Respiration
Aerobic respiration takes place in the mitochondria (a specialized organelle) of the cell and uses
oxygen as the final electron acceptor in the electron transport pathway. Pyruvate is oxidized to
generate energy. Special molecules shuttle electrons to the ATP production site. Since oxygen has
a very high affinity for
electrons, aerobic respiration is the most efficient means of producing ATP (36 per reaction).
Anaerobic Respiration
Anaerobic respiration takes place in the cytoplasm of the cell and uses other, less efficient,
molecules to transport electrons. If the final transfer molecule is organic (contains a carbon), the
process is called fermentation. Fermentation is an anaerobic process that reduce regenerate
NAD+ so that glycolysis can
continue. Because it cannot fully break down the glucose molecule, fermentation is far less
efficient than aerobic respiration, generating only two ATP molecules.
During physical activity, cells require more energy. As long as enough oxygen can be
delivered to cells, aerobic respiration dominates. When energy consumption exceeds
the oxygen supply,
anaerobic respiration starts. Lactic acid is a byproduct, and is what causes muscle
soreness after a hard workout!
