What are the three partsstages of cellular respiration What

What are the three parts/stages of cellular respiration? What is the (energy) investment? What is the energy payoff for each part and for the entire process per glucose molecule?

Solution

The three parts of cellular respiration are

In glycolysis, each glucose molecule gives two pyruvate molecules. Then, pyruvate molecules are oxidized to acetyl CoA which will enter TCA cycle.

Acetyl CoA then combines with oxaloacetate to form six carbons molecule citrate. This citrate undergoes a series of enzymatic reactions to ultimately form oxaloacetate and in this process looses two carbons as CO₂.

Glycolysis and TCA cycle produce ATP and electron carriers such as NADH and FADH2. These will deposit their electron into the electron transport chain to drive synthesis of ATP molecules through oxidative phosphorylation.

Energy expenditure and yield:

Glycolysis:

Each molecule of glucose consumes two ATP when it enters glycolysis. In the energy yielding phase, at total of 4 ATPs and two NADH are produced. So, the net gain from glycolysis is 2 ATP and 2 NADH

Pyruvate to acetyl CoA:

Two molecules of pyruvate produce two molecules of acetyl CoA and two NADH.

TCA cycle:

For each cycle of TCA, three molecules of NADH and one molecule of FADH2 and one molecule of ATP are produced.

So, from one glucose molecule, two molecules of acetyl CoA enter TCA cycle.

So, a total of 6 NADH, 2 FADH2 and 2 ATP are generated.

Electron transport:

In this phase, NADH yields about 2.5 ATP where as each FADH2 yield 1.5 ATP

So, if we assume that NADH and FADH2 are equivalent to 2.5 and 1.5 ATP,

In glycolysis, net yield of ATP is 7.

In pyruvate oxidation, net yield of ATP is 5.

In TCA cycle, net yield of ATP is 20.

So, cellular respiration yields 32 ATP for each glucose molecule.