Totalling the ATP: Yield, Efficiency, Other Fuels and Control

How good a deal is respiration?

A gasoline car engine converts only about a quarter to a third of the fuel's energy into motion; the rest is heat out the tailpipe and radiator. How does your body compare? In this lesson you will add up all the ATP from one glucose, calculate the efficiency of respiration, and see how the cell also burns fats and proteins and adjusts its rate to its needs.

Adding up the ATP

There is one complication. The 2 NADH made by glycolysis are in the cytosol, and the inner mitochondrial membrane doesn't let NADH through. Instead, shuttle systems pass their electrons in:

• In liver, kidney and heart cells, a shuttle passes the electrons to NAD⁺ inside the matrix, so each cytosolic NADH is worth about 2.5 ATP.
• In skeletal muscle and brain cells, a different shuttle passes them to FAD, so each is worth only about 1.5 ATP.

Putting it all together (modern estimate):

Why some textbooks say 36 or 38

Older textbooks (and some still used in Ontario) assumed 3 ATP per NADH and 2 ATP per FADH₂, which gives 36 ATP (with the muscle shuttle) or 38 ATP (with the liver shuttle, or in prokaryotes, which have no mitochondrial membrane to cross). Later measurements showed that the H⁺ gradient is also used to move ADP, Pᵢ and ATP across the membrane, and some H⁺ leaks back, so the real yield is lower: about 30 to 32. The ATP number is a theoretical maximum that depends on conditions; it is not a fixed count like the number of CO₂.

Use whichever values your teacher or exam asks for, and always state your assumption (for example, "assuming 2.5 ATP per NADH").

How efficient is it?

Efficiency = (energy captured in ATP ÷ energy released by glucose) × 100%

With 32 ATP × 30.5 kJ/mol = 976 kJ, and 2870 kJ released per mole of glucose:

efficiency = 976 ÷ 2870 × 100% ≈ 34%.

That compares well with a car engine. The other 66% or so is released as heat, exactly as the second law predicts. In birds and mammals, that heat isn't wasted: it maintains body temperature.

Other fuels: fats and proteins

Your cells don't run only on glucose. Fats and proteins enter the same pathways at different points.

• Fats (triglycerides) are split into glycerol and fatty acids. Glycerol is converted to G3P and enters glycolysis. Fatty acids are cut into two-carbon pieces by β-oxidation in the matrix; each cut makes 1 NADH, 1 FADH₂ and 1 acetyl-CoA, which enters the Krebs cycle.
• Proteins are digested into amino acids. Usually amino acids are used to build proteins, but when they are used as fuel, the amino group is removed (deamination). The ammonia is toxic, so the liver converts it to urea, which the kidneys excrete (you will study this in the Homeostasis unit). The remaining carbon skeleton enters as pyruvate, acetyl-CoA or a Krebs intermediate.

Why fat is so energy-dense: fatty acids are long chains of carbon with many C–H bonds: they are highly reduced (Lesson 3), so they provide many electrons to the ETC. Fat provides about 37 kJ/g, compared with about 17 kJ/g for carbohydrates or proteins. (Nutrition labels in Canada list energy in Calories: 1 Calorie = 1 kcal = 4.184 kJ.)

Matching supply to demand: control at PFK

A cell shouldn't burn fuel faster than it needs ATP. The main control point is the enzyme phosphofructokinase (PFK), step 3 of glycolysis, which commits the sugar to being split. PFK is an allosteric enzyme (from the Biochemistry unit):

• High ATP (plenty of energy) binds an allosteric site and inhibits PFK.
• High citrate (the Krebs cycle is backed up) also inhibits PFK.
• High AMP and ADP (energy is being used up) activate PFK.

This is feedback inhibition: products made later (ATP, citrate) slow down an early step. When you start running, ATP is used, ADP and AMP rise, PFK speeds up, and respiration accelerates within seconds.