Glucose is produced by plants, using water, carbon dioxide and the help of solar energy.
Animals and humans ingest glucose and use the chemical energy stored in these molecules as “fuel”. This energy is released through several steps, including glycolysis, Krebs cycle and the electron transport chain.
First, we have to introduce some terms:
ATP - Adenosine triphosphate:
Adenosine triphosphate (often called “molecular unit of currency”) is a molecule storing and transporting energy in our bodies. You can think of it as a truck delivering fuel (chemical energy) to a factory (your body cells). This fuel gets "burned” and the energy released is used to process other materials (e.g. synthesize proteins). Adenosine triphosphate releases energy through hydrolysis to ADP (adenosine diphosphate) and phosphate. Throughout the day an average human produces and breaks down about 70 kg (about 150 lbs) of ATP.
ATP.wal (23.2 KB)
Nicotinamide adenine dinucleotide (NAD) is also used to store energy, just like a battery. It gets “charged” (meaning storing energy in this context) by accepting a single proton (hydrogen core) and two electrons. Due to the additional hydrogen atom a “charged” NAD molecule (NAD + H) gets called NADH. By reversing this process the stored energy is released.
NAD.wal (32.8 KB), NADH.wal (33.7 KB)
Aerobic decomposition of glucose can be split into three parts:
Glycolysis: One glucose molecule is broken down throughout several steps into two molecules pyruvate. This also turns two molecules ADP into ATP. Many organisms like bacteria only use this mechanism to obtain energy, as oxygen is needed to break down pyruvate.
Glucose.wal (11.8 KB), Glycolysis.wal (5.8 KB)
Krebs cycle (citric acid cycle): In our bodies glycolysis is followed by the citric acid cycle. One molecule pyruvate combines with oxaloacetate and forms citrate. Citrate dissociates two carbon atoms oxidized to carbon dioxide and some hydrogen atoms. This turns it into succinate and later oxalacetate, which again combines with pyruvate. As you can see this reaction works like a cycle: “new” pyruvate is provided and processed to something that can again process pyruvate. Throughout this process energy is released. In conclusion one molecule pyruvate is turned into three molecules of carbon dioxide and eight hydrogen atoms which make up NADH.
Citrate.wal (10.3 KB), Oxalacetate.wal (7.3 KB)
Electron transport chain (respiratory chain): In the final step of the electron transport chain oxygen accepts hydrogen atoms from NADH. You may have learned about the combustion of oxyhydrogen in your chemistry course, the reaction is the same. Luckily for us, the “combustion” happens slower and in a more controlled matter. Carbon dioxide is released through exhalation, the resulting energy is once again stored in ATP.