work ≠ free gibbs free energy (Δg) otherwise known as the “available energy” if Δg is...
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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?