Glucose Essay, Research Paper
Extracting energy from glucose
Two different pathways are involved in the metabolism of glucose: one anaerobic
and one aerobic. The anaerobic process occurs in the cytoplasm and is only
moderately efficient. The aerobic cycle takes place in the mitochondria and is
results in the greatest release of energy. As the name implies, though, it
requires oxygen.
Anaerobic metabolism
Glucose in the bloodstream diffuses into the cytoplasm and is locked there by
phosphorylation. A glucose molecule is then rearranged slightly to fructose and
phosphorylated again to fructose diphosphate. These steps actually require
energy, in the form of two ATPs per glucose. The fructose is then cleaved to
yield two glyceraldehyde phosphates (GPs). In the next steps, energy is finally
released, in the form of two ATPs and two NADHs, as the GPs are oxidized to
phosphoglycerates. One of the key enzymes in this process is glyceraldehyde
phosphate dehydrogenase (GPDH), which transfers a hydrogen atom from the GP to
NAD to yield the energetic NADH. Due to its key position in the glycolytic
pathway, biochemical assays of GPDH are often used to estimate the glycolytic
capacity of a muscle cell. Finally, two more ATPs are produced as the
phosphoglycerate
Aerobic metabolism
Pyruvate is the starting molecule for oxidative phosphorylation via the Krebb’s
or citric acid cycle. In this process, all of the C-C and C-H bonds of the
pyruvate will be transferred to oxygen. The pathway can be seen in the figure
below. Basically, the pyruvate is oxidized to acetyl coenzyme A, which can then
bind with the four carbon oxaloacetate to generate a six carbon citrate. Carbons
and hydrogens are gradually cleaved from this citrate until all that remains is
the four carbon oxaloacetate we started with. In the process, four NADHs, one
FADH and one GTP are generated for each starting pyruvate.
Energy accounting
Each NADH will be oxidized to NAD, generating three ATPs (although it “costs”
one ATP to transfer the NADHs generated during anaerobic metabolism into the
mitochondria for reduction). For each molecule of glucose we can calculate the
useable energy produced:
Anaerobic
Consumed: 2 ATP
Produced: 8 ATP
_____________________________
Net: 6 ATP
Aerobic
Consumed: 0 ATP
Produced: 2x 15 ATP
_____________________________
Net: 30 ATP
Thus, for each glucose that enters the muscle, up to 36 ATPs can be generated.