SAS' Cell Biology Page

Cellular Respiration

Energy, it is all a matter of releasing energy from stored fuel, such as glucose (C6H12O6). Cellular Respiration delivers the energy that cells need to live, from synthesizing proteins, to building microtubules and microfiliments for structure. In multicellular organisms, like ourselves, cellular respiration gives us energy to run, swim, a slow movie, or even staying awake for a boring lecture (I'll keep number of the boring ones down). But how do cells release energy from glucose?

For most organisms, cellular respiration takes place in two steps, one without O2, followed by one the needs O2. When combined, cellular respiration releases 36ATP molecules per O2 and C6H12O6 used. That is enough energy for Michael Johnson to win the 100m sprint. Cellular respiration using oxygen can be summarized with the following chemical equation:

But let's start by looking at anaerobic respiration, also called glycolysis, releasing energy without using oxygen. This half of the process occurs in the cytoplasm.

Just like a car engine that needs a spark energized from the car's battery before it can convert gasoline into energy, anaerobic respiration (glycolysis), also needs a boast of energy to get started. So in the first step, glucose becomes charged by 2ATP with a little help from an enzyme, receiving two organic phosphates from the 2ATPs. Glucose becomes Fructose-1,6-phosphate, for the phosphate has been attached to the 1st carbon and the 6th carbon on glucose.

This charge from ATP pushes the process along, like being pushed uphill, reaching the top, and down you have a fast coast downhill. After this step, Fructose 1,6-diphosphate is split in half by the enzyme Triose phosphate isomerase, forming two glyceraldehyde-3-phosphate, the phosphate being attached to the third carbon of glyceraldehyde.

Now its time to get a little of the energy back. In the next two steps, energy is released from Glyceraldehyde-3-phosphate, producing NADH and ATP. Since we have two Glyceraldehyde-3-phosphates, the 2ATP used earlier to start the reaction has been returned, plus an extra 2NADPH have been made.

Notice that the big change to glyceraldehyde is the addition of another organic phosphate. That sets up the next step, where ATP is made.

Now, lets go after that last organic phosphate group, and lets make some more ATP.

So at the end of anaerobic respiration (glycolysis), 2ATP were used to start the reaction, but 4ATP and 2NADH were made, a net product of 2ATP and 2 NADH. Energy has been released from glucose now, which could be used for that run, or the boring lecture.

Ahh, but you are yawning now. Need a little oxygen to pull more energy from that molecule of glucose. Now we are entering the pathways of aerobic respiration.