Metabolism = anabolism + catabolism
In a reaction, bonds between reactants are broken down and bonds between products are formed
\
Build complex substances from smaller subunits
Overall require the input of energy to occur
Endergonic reactions: a need for energy
E.g. photosynthesis
\
Breakdown of complex substances
Overall release energy
Exergonic reactions: release of energy
Energy can be used for other jobs
E.g. cellular respiration
\
\
Enthalpy (∆H): a measure of the energy in a system, related to the amount of heat released or absorbed by a reaction
Exothermic reactions: ∆H < 0 (negative) – releases more thermal energy than they absorb
Endothermic reactions: ∆H > 0 (positive) – absorbs more thermal energy than it releases
Chemical reactions need activation energy for a reaction to happen
\
Second law of thermodynamics: in every energy conversion, some energy becomes unusable, thus increasing the entropy of the universe
Heat is the typical form of loss of usual energy – it contributes to the increasing disorder in the surroundings
Entropy (∆S): measure of randomness/disorder
\
\
The most useful kind of energy
In a chemical change, since some energy is lost to entropy, the usable remainder is called Gibbs Free Energy
Exergonic reactions release free energy (-∆G) and are spontaneous – i.e. don’t need certain conditions to occur, will continue as long as there is sufficient reactant w/o a continuous input of free energy
Endergonic reactions absorb free energy (+∆G) and are not spontaneous – i.e. they absorb more free energy than it produces and it won’t happen with a continual source because of this
Reactions that release free energy (-∆G) are spontaneous
Spontaneous reactions are those that will continue on their own once started (after activation energy)
Total energy change/enthalpy(∆H) and total disorder change/entropy (∆S) both play a key role in determining whether or not a reaction will be spontaneous (along with temperature)
∆G = ∆H --- T∆S
\
Catabolic pathways
Anabolic pathways
In order for non spontaneous reactions to occur, they must be coupled with spontaneous ones
Energy for endergonic reactions in cells is mostly provided by coupling the reaction with the hydrolysis of ATP
\