Question

The maximum yield of ATP per molecule of glucose in eukaryotic cells is 36 or 38 ATP, depending on cell type and conditions. Why is the calculated ATP yield referred to as "Maximal Theoretical Yield" in eukaryotic and in prokaryotic cells?

Answer 1

ATP - Maximal Theoretical Yield :

Cellular respiration is the set of the metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate (ATP), and then release waste products.

The reactions involved in respiration are catabolic reactions, which break large molecules into smaller ones, releasing energy in the process as weak so-called "high-energy" bonds are replaced by stronger bonds in the products.

Respiration is one of the key ways a cell gains useful energy to fuel cellular activity.

Cellular respiration is considered an exothermic redox reaction.

The overall reaction is broken into many smaller ones when it occurs in the body, most of which are redox reactions themselves.

Although technically, cellular respiration is a combustion reaction, it clearly does not resemble one when it occurs in a living cell.

This difference is because it occurs in many separate steps.

While the overall reaction is a combustion reaction, no single reaction that comprises it is a combustion reaction.

Nutrients that are commonly used by animal and plant cells in respiration include sugar, amino acids and fatty acids, and a common oxidizing agent is molecular oxygene.

The energy stored in ATP , its third phosphate group is weakly bonded to the rest of the molecule and is cheaply broken allowing stronger bonds to form, thereby transferring energy for use by the cell.

ATP can then be used to drive processes requiring energy, including biosynthesis, locomotion or transportation of molecules across cell membranes.

The ATP molecule contains three phosphate groups linked together and bonded to a molecule of adenine, which is also found in DNA.

Although there is a theoretical yield of 38 ATP molecules per glucose during cellular respiration, such conditions are generally not realized due to losses such as the cost of moving pyruvate (from glycolysis), phosphate, and ADP (substrates for ATP synthesis) into the mitochondria.

All are actively transported using carriers that utilise the stored energy in the proton electrochemical gradient.

The outcome of these transport processes using the proton electrochemical gradient is that more than 3 H+ are needed to make 1 ATP.

Obviously this reduces the theoretical efficiency of the whole process and the likely maximum is closer to 28–30 ATP molecules.

In practice the efficiency may be even lower due to the innermembrane of the mitochondria being slightly leaky to protons.

Other factors may also dissipate the proton gradient creating an apparently leaky mitochondria.

An uncoupling protein known as thermogenin is expressed in some cell types and is a channel that can transport protons.

When this protein is active in the inner membrane it short circuits the coupling between the electron transport chain and ATP synthesis.

The potential energy from the proton gradient is not used to make ATP but generates heat.

This is particularly important in brown fat thermogenesis of newborn and hibernating mammals.