Download Discussion Points: Cellular Respiration

Discussion Points: Cellular Respiration
Cells, and therefore organisms, require energy to perform all of their different functions and
activities. For example, we already know that energy is required for active transport. Cell replication
is also an example of a process that requires a lot of energy in order to replicate DNA and organelles,
double the cytoplasm, divide the nucleus and synthesize new membranes in order to form two
daughter cells. Muscle contraction is another good example.
Cell use chemical energy to carry out their activities. This energy is stored in the bonds that join
atoms together in molecules. If a molecule is broken apart, the energy in these bonds is released.
This is one very important function of macromolecules, glucose, fats, and protein have bonds that
contain a lot of energy that can be released for the cell to use.
ATP/ADP Cycle
All living cells rely on one source of chemical
energy to do everything, ATP (adenosine
triphosphate). ATP has a high energy terminal
phosphate bond that when broken releases
energy. This energy is then used to carry out
the cells processes. Once ATP has released its
energy it becomes ADP (adenosine
diphosphate). ADP can be recharged by the
addition of a phosphate molecule making it
ready to use again. The recharging process
also requires some energy which is derived
from the breakdown of glucose molecules
from the food we eat. However, this requires
much less energy than making a whole new ATP molecule. The constant transformation between
ATP and ADP molecules is called the ATP/ADP cycle. Although glucose is the main energy source for
energy for cells, other macromolecules such as fats and proteins can be used to release energy to
make ATP.
Providing Energy in Cells
The first stage of the breakdown of glucose to produce ATP is called glycolysis. Glycolysis occurs in
the cytosol of the cell and splits the glucose molecule into two pyruvate molecules plus two ATP. In
reality glycolysis releases energy from glucose in a series of steps that involve a number of enzymes.
In humans glycolysis provides energy for things that require short bursts of energy such as 100 m
dashes or weight lifting because these activities happen so quickly that cells don’t get a chance to
get oxygen. As such glycolysis is an anaerobic process meaning it does not require oxygen.
The process that occurs next depends on whether oxygen is available or not:
 Aerobic Respiration: If oxygen is available, the pyruvate molecules are further broken down
into carbon dioxide and water and an additional 36-38 ATP. This is called aerobic respiration
(because it requires oxygen) and and it occurs in the mitochondria. The simplified formula for
aerobic breakdown of glucose through cellular respiration is:
Glucose + Oxygen
C6H12O6 + 6O2
Carbon Dioxide + Water + Energy
6CO2 + 6H20 + 36-38 ATP
The mitochondria are large organelle with two cell
membranes. The inner membrane is folded over on
itself many times. The folds of the inner membrane
are called cristae which has large surface area for
aerobic respiration to take place. Aerobic respiration
actually involves two linked processes, called the
Kreb’s cycle and the electron transport chain. The
number of mitochondria in a cell is related to the cell’s
energy requirements. As such you would expect cell
that require a lot of energy, such as your heart muscle
cells, to have thousands of mitochondria. A good
example of your body performing aerobic respiration
is how heavily you breath after going for a run. This is because you’re the cells in your
muscles and are trying to get as much oxygen as they can to produce energy for the activity.
 Anaerobic respiration: if oxygen isn’t available, then anaerobic respiration continues to occur
in the cytosol. The products of anaerobic respiration depend on what the organism is. In
plants, and microorganisms such as yeasts and bacteria the pyruvate molecules are broken
down into carbon dioxide and ethanol. This is otherwise known as fermentation and is how the
alcohol is made in wine and beer. In most animals the pyruvate molecules are broken down into
lactic acid. This can also be reffered to as homolactic fermentation. No ATP is produced from this
stage of cellular respiration.