The complexity of metabolism
- This schematic diagram traces only a few hundred of the thousands of metabolic reaction that
occur in a cell.
- The dots represent molecules, and lines represent the chemical reaction that transforms them.
- The reaction proceeds in stepwise sequences called metabolic pathways, each step catalyzed by
a specific enzyme.
Metabolism, Energy and Life
- The chemistry of life is organized into metabolic pathways
o Metabolism totality of an organisms chemical processes
- Metabolic reactions are organized into pathways that are orderly series of enzymatcially
- Metabolic pathways are generally two types:
o Catabolic pathways- release energy by breaking down complex molecules to simpler
compounds (cellular respiration which degrades glucose to carbon dioxide and water;
provides energy for cellular work)
o Anabolic pathways- consume energy to build complicated molecules from simpler ones
(photosynthesis which synthesizes glucose from CO 2nd H O2 ad synthesis of a
macromolecule from it monomers.
Organisms transform energy
- Energy is a capacity to do work
o Kinetic energy – in the process of doing work(energy of motion)
o Thermal- (heat) is kinetic energy expressed in random movement of molecules
o Potential energy- energy that matter possesses because of its location or arrangement.
o Chemical energy- is potential energy stored in molecules because of the arrangement of
nuclei and electrons in its atoms
- Energy can be transformed from one form to another
o Kinetic energy of sunlight can be transformed to the potential energy of the chemical
bonds during photosynthesis
o Potential energy in the chemical bonds of gasoline can be transformed into kinetic
mechanical energy which pushes the pistons of the engine. The energy transformations of life are subject to two laws of thermodynamics
- Thermodynamics- study of energy transformations
- First law of thermodynamics- energy can be transferred and transformed, but cannot be created
nor destroyed (energy of the universe is constant)
- Second law of thermodynamics- every energy transfer or transformation makes the universe
more disordered (every process increases the entropy of the universe)
- Entropy- qualitative measure of disorder that is proportional to randomness
- Closed system- collection of matter under study which is isolated from its surroundings
- Open system- system of which energy can be transferred between system and its surroundings
- The entropy of a system may decrease, but the entropy of the system plus its surroundings must
- Highly ordered living organisms do not violate the second law because they are open systems.
o For example:
Maintain highly ordered structure at the expense of increased entropy of their
Take in complex energy molecules as food and extract chemical energy to create
and maintain order
The complexity of metabolism
- An unstable system is rich in free energy
- It has a tendency to change spontaneously to more stable state
o a) in this case, free energy is proportional to the girl’s
o b) The free-energy concept also applies on the molecular
scale, in this case the physical movement of molecules is known as diffusion.
o C) Chemical reactions also involve free energy.
The sugar molecule is more stable than the simpler molecules below
When catabolic pathways break down complex organic molecules, a cell can
harness the free energy stored in the molecules to perform work.
Organisms live at the expense of free energy
- Free energy: a criterion for spontaneous change.
o It is the amount of energy that is available to do work
- Free energy (G) is related to the systems total energy (H) and its entropy (S) in the following
way: G= H-TS
o G=Gibbs free energy
o H+ enthalpy of total energy
o T=Temperature in degrees K
o S=Entropy - Free energy (G)= portion of a systems energy available to do work; is the difference between the
total energy (enthalpy) and the energy not available for doing work (TS)
- The maximum amount of usable energy that can be harvested from a particular reaction is the
systems free energy change from the initial to the final state.
- This change in free energy (ΔG) is given by the Gibbs-Helmholtz equation at constant
temperature and pressure:
- ΔG= change in free energy
- ΔH= change in total energy (enthalpy)
- ΔS=change in entropy
- T= absolute temp in degrees Kalvin (degrees Celcius+273)
Free energy and equilibrium
- There is a relationship between chemical equilibrium and the free energy change (ΔG) of a
o As a reaction approaches equilibrium, the free energy of the systems decreases
(spontaneous and exergonic reaction)
o When a reaction is pushed away from equilibrium, the free energy of the system
increases (non-spontaneous and endergonic reaction).
o When a reaction reaches equilibrium, ΔG=0, because there is no net change in the
- Reactions can be classified based upon their free energy changes:
o Exergonic reaction- a reaction that proceeds with a net loss of energy
o Endergonic reaction- an energy-requiring reaction that proceeds with a net gain of frees
energy; a reaction absorbs free energy from its surroundings.
- Exergonic Reaction - Energonic Reaction
- Chemical products have less energy - Products store more free energy than
than the reactant molecules reactants
- Reaction is energetically downhill - Reaction is energetically uphill
- Spontaneous reaction - Non-spontaneous reaction( requires