Transcript Document

Energy
The capacity to do work or cause particular changes
Life is sustained by the trapping and use of energy
Use of energy is made possible by the action of enzymes
Energy and work
Chemical work
The synthesis of complex
biological molecules from
simpler precursors
Energy and work
Transport work
The ability to transport molecules
against a concentration gradient
(uptake of nutrients, elimination
of waste, maintenance of ion
balance)
Energy and work
Mechanical work
Changing the location of
organisms, cells and structures
within cells
The flow of carbon and energy
Source of most biological
energy is sunlight
Phototrophs trap light
energy during
photosynthesis
The flow of carbon and energy
Chemolithoautotrophs
derive energy from the
oxidation of inorganic
molecules
Energy from photosynthesis
and chemolithoautotrophy
can then be used to
transform CO2 into organic
molecules
The flow of carbon and energy
Chemoheterotrophs can use
organic molecules as carbon
and energy sources
Energy is released as organic
molecules are oxidized to
CO2
Oxidation and reduction
Loss of electrons is
oxidation (LEO)
Gain of electrons is
reduction (GER)
Aerobic respiration is when
O2 acts as the final electron
acceptor (O2  H2O)
Adenosine 5´-triphosphate (ATP)
ATP serves as the major
energy currency of cells
Contains 2 high energy
bonds
ATP  ADP + Pi + Energy
Energy + ADP +Pi  ATP
Pi = orthophosphate
Energy cycle
The laws of thermodynamics
1. Energy can neither be created or destroyed
The total amount of energy in the universe remains
constant (although it can be redistributed)
The laws of thermodynamics
2. Physical and chemical processes proceed in such a way that the
randomness of the universe increases to the maximum
possible
Entropy
A measure of the randomness or disorder of a system
The greater the disorder the greater the entropy
Free energy and reactions
G = H - T x S
G = change in free energy (amount of energy available to do work)
H = change in enthalpy (heat content)
T = temperature in Kelvin (C + 273)
S = change in entropy
Free energy and reactions
G = H - T x S
A reaction with a large positive change in entropy will result in a
negative G value and will occur spontaneously
The change in free energy has an effect on the direction of a
reaction
Free energy and reactions
G = H - T x S
When G is determined under standard conditions of, pressure,
pH and temperature the G is called the standard free energy
change (G )
If the pH is set to 7, the standard free energy change is indicated
by the symbol G ´
Free energy and reactions
The change in free energy has an effect on the direction of a
reaction
Keq = the equilibration constant
Free energy and reactions
When G ´ is negative, the Keq is greater than 1 and the
reaction goes to completion as written
The reaction is said to be exergonic
Free energy and reactions
When G ´ is positive, the equilibrium constant is less than 1
and the reaction is not favored
The reaction is said to endergonic
ATP and metabolism
A major role of ATP is to
drive endergonic reactions to
completion
ATP links energy-yielding
reactions with energy-using
reactions
Oxidation-reduction reactions
The release of energy normally involves oxidation reduction
reactions (redox reactions)
Electrons move from an electron donor to an electron acceptor
Acceptor +ne-  donor
(n = number of electrons transferred)
Oxidation-reduction reactions
2H+ + 2e-  H2
The equilibrium constant of a redox reaction is called the
standard reduction potential (E)
The reference standard for reduction potentials is the hydrogen
system with an E ´ of - 0.42 volts
Each hydrogen atom provides 2 protons and 2 electrons
Oxidation-reduction reactions
Redox couples with more
negative reduction potentials
will donate electrons to
couples with more positive
potentials (and a greater
affinity for electrons)
Oxidation-reduction reactions
Electron tower with most
negative reduction potentials
at the top
Electrons move from donors
to acceptors from more
negative to more positive
potentials
Electron carriers
Various carriers serve to transport electrons to different parts of
the cell
Example - Nicotinamide adenine dinucleotide
NADH + H+ + 1/2 O2  H2O + NAD+
NAD+/ NADH is more negative than 1/2 O2/ H2O, so electrons
will flow from NADH (donor) to O2 (acceptor)
Electron carriers
Structure of NAD
Flavin adenine dinucleotide (FAD)
Proteins bearing FAD (or
FMN) are referred to as
flavoproteins
Coenzyme Q (CoQ) or ubiquinone
Transports electrons and
protons in respiratory
electron transport chains
Cytochromes
Cytochromes use iron atoms
to transport electrons by
reversible oxidation and
reduction reactions
Iron atoms in cytochromes
are part of a heme group
Nonheme iron proteins carry
electrons but lack a heme
group (e.g. Ferrodoxin)
Enzymes
Enzymes can be defined as protein catalysts
Increase rate of reactions without being permanently altered
Reacting molecules = substrates
Substances formed = product
Structure of enzymes
Some enzymes are composed purely of protein
Some enzymes contain both a protein and a nonprotein
component
The protein component = apoenzyme
The nonprotein component = cofactor
Apoenzyme + cofactor = holoenzyme
Structure of enzymes
Cofactor tightly attached to apoenzyme = prosthetic group
Loosely bound cofactor = coenzyme
Classification of enzymes
Enzymes can be placed in one of six classes
Usually named in terms of substrates and reactions catalyzed
Mechanisms of enzyme activity
Enzymes serve to speed up the rate at which a reaction proceed
to equilibrium by lowering the activation energy
Activation energy required to from the transition state (AB)
Mechanisms of enzyme activity
The enzyme may be rigid
and shaped to precisely fit
the substrate
Binding to substrate
positions it properly for
reaction
Referred to as the lock-andkey model
Mechanisms of enzyme activity
Some enzymes change shape when they bind their substrate so
that the active site surrounds and precisely fits the substrate
Referred to as the induced fit model