Work ≠ Free

Gibbs Free Energy (ΔG)

• Otherwise known as the “available energy”

• If ΔG is negative, the products of the reaction are less complex (have less potential energy) and energy is available for cellular work

• If ΔG is positive, products have more potential energy than reactants and energy must be paid in

• Example: The “cost” of making glucose via photosynthesis is +686 kcal/mol

How do cells “pay” for cellular work?

The Rai of Yap

How is glucose analogous to the Rai or the $100,000 Bill?

Overpaying Cellular Currency

• Average Activation Energy required by cellular reactions

• 1.6 kcal/mol

• Average Potential Energy of Glucose

• 686 kcal/mol Using glucose to meet the activation energy needs of cells is wasteful and inherently unstable

The solution: Make “change” by transferring the energy of glucose to molecules of

Adenosine Triphosphate (ATP)

ATP Form & Function

• Form: ATP is a polymer• Function: transfer a more

appropriate amount of energy to power cellular reactions

• ΔG = -7.3 kcal/mol (vs. -686 kcal/mol from glucose)

• Like enzymes, ATP may be re-used

Polymer Structure of ATP

• Ribose (5 pentose sugar)

• Adenine (nitrogenous base)

• 3 Phosphate (PO4) groups

• Since all three phosphate groups are negatively charged, high potential energy bonds are required to keep them attached to the molecule

Transfer of Energy: Phosphorylation

• The energy stored in ATP is “used” by breaking off one of the phosphate groups

• ATP ADP + Pi + -7.3 kcal/mol (ΔG)

• The high kinetic energy of the phosphate group is transferred to the substrate, allowing it to exceed EA

How can ATP be “recharged”?

ATP-ADP Cycle

Recycling of ATP

• While only around 50 grams of ATP are in the body at any given time, 2 x 1026 molecules or >160kg of ATP is formed in the human body daily

• A cell recycles around 10 million ATP/sec/cell

ADP• The “leftover”

molecule is Adenosine Diphosphate (ADP) and has less energy than ATP

• Like a mousetrap that has been “sprung”, it may be reset by increasing its potential energy

Catabolism: Recharging ATP

• Energy is liberated from the bonds of glucose during cellular respiration in the mitochondria

• The enzyme ATP SYNTHETASE reduces the EA required to put a phosphate group back onto ADP

In Summary…

• Chemical reactions need to be “paid for”, even if it is just the activation energy

• Paying for chemical reactions with glucose is too wasteful/destructive

• ATP is a much less wasteful way to transfer the necessary energy to a reaction

• ADP can be recharged to ATP via cellular respiration– Glucose + O2 -> CO2 + H2O + 36 ATP

Creatine Phosphate (CP)• Creatine Phosphate

(CP) gives up its phosphate to ADP such that ADP can be recharged into ATP

• Creatine supplements are popular among some athletes because they allow the potential for more ADP to be recharged

How does the body’s use of energy-storing compounds vary in different

athletic events?