Free Energy and ATP
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Transcript of Free Energy and ATP
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Free Energy and ATP
But I thought nothing in life is
free?!
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Spontaneous vs. Nonspontaneous
• Spontaneous processes: those that can occur without outside help– example: your room getting
messy!– increases stability of a system
• Nonspontaneous processes: those that can only occur if energy is added to a system– example: cleaning up your room!– decreases stability of a system
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Free Energy• Free energy provides a criterion for measuring
spontaneity of a system.
• Free energy is the portions of a system’s energy that is able to perform work when temperature is uniform throughout the system.
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Free Energy Examples• High Free Energy:
– compressed springs– separated charges
• These are unstable and tend to move toward a more stable state, one with less free energy.
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Free Energy Equation• Free energy = G
• Total energy = H
• Entropy = S
• Temperature (Kelvin) = T
G = H – TS
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Change(∆) in Free Energy
∆ G = G final state - G starting state
Or… ∆ G = ∆ H - T ∆ S
• For a system to be spontaneous, the system must either give up energy (decrease in H), give up order (decrease in S), or both.– ∆ G must be negative.– The more negative, means the more work can be done.– Nature runs “downhill.”
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Chemical Reactions• Chemical reactions can be classified based on
free energy:
– exergonic reaction: proceeds with a net release of free energy (∆G is negative)
– endergonic reaction: absorbs free energy from its surroundings (∆G is positive)
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Is this an endergonic or exergonic reaction?!
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Exergonic Reaction• ∆G is negative
• Example: breakdown of sugar– ∆G = -686
kcal/mol– Through this
reaction 686 kcal have been made available to do work in the cell.
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Endergonic Reaction• Endergonic
reactions store energy
• ∆G is positive• nonspontaneous• Example:
– Cleaning your room!!
– Photosynthesis making sugar =
+ 686 kcal
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Equilibrium
• A system at equilibrium is at maximum stability.• forward and backward reactions are equal • no change in the concentration of products or reactants
• At equilibrium ∆ G = 0 and the system can do no work.– Movements away from equilibrium are nonspontaneous
and require the addition of energy from an outside energy source (the surroundings).
– Reactions in closed systems eventually reach equilibrium and can do no work.
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Equilibrium in Cells
• A cell that has reached metabolic equilibrium has a ∆ G = 0 and is dead!– Metabolic disequilibrium is one of the defining
features of life.
• Cells maintain disequilibrium because they are open with a constant flow of material in and out of the cell.
• A cell continues to do work throughout its life.
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Cells have to work?!• A cell does three main kinds of work:
1. Mechanical work: beating of cilia, contraction of muscle cells, and movement of chromosomes.
2. Transport work: pumping substances across membranes against the direction of spontaneous movement.
3. Chemical work: driving endergonic reactions such as the synthesis of polymers from monomers.
• What powers all this work?
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ATP!• The energy that powers cellular work is ATP!
• ATP (adenosine triphosphate) is a type of nucleotide consisting of the nitrogenous base adenine, the sugar ribose, and a chain of three phosphate groups.
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How does ATP release energy?• The bonds between phosphate groups can
be broken by hydrolysis.– Hydrolysis of the end phosphate group forms
adenosine diphosphate [ATP -> ADP + Pi] and releases 7.3 kcal of energy per mole of ATP under standard conditions.
– ∆G is about -13 kcal/mol
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Why does this release energy?• Bonds are unstable… their hydrolysis yields
energy because the products are more stable.
• The phosphate bonds are weak because each of the three phosphate groups has a negative charge.
• Their repulsion contributes to the instability of this region of the ATP molecule.
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How is the energy harnessed?• the energy from
the hydrolysis of ATP is coupled directly to endergonic processes by transferring the phosphate group to another molecule.
• This molecule is phosphorylated.– now more
reactive.
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Where does the ATP come from?• ATP is continually regenerated by adding a
phosphate group to ADP.
• Energy for renewal comes from catabolic reactions in the cell (breakdown of sugar!).– In a working muscle cell the entire pool of ATP is
recycled once each minute, over 10 million ATP consumed and regenerated per second per cell.
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