Energetics
Chapter 6
Adapted by G. Cornwall, Ph.D.
From Raven’s Biology, McGraw Hill Publishing
1
Flow of Energy
•
Thermodynamics
–
•
Branch of chemistry concerned with energy
changes
Cells are governed by the laws of
physics and chemistry
2
• Energy – capacity to do work
– 2 states
1. Kinetic – energy of motion
2. Potential – stored energy
– Many forms – mechanical, heat, sound,
electric current, light, or radioactivity
3
• Heat the most convenient way of
measuring energy.
– 1 calorie = heat required to raise 1 gram of water 1ºC
– calorie or Calorie?
• Energy flows into the biological world from
the sun
• Photosynthetic organisms capture this
energy
• Stored as potential energy in chemical
bonds
4
• Oxidation
Redox reactions
– Atom or molecule loses an electron
• Reduction
– Atom or molecule gains an electron
– Higher level of energy than oxidized form
• Oxidation-reduction reactions (redox)
– Reactions always paired
5
Laws of thermodynamics
• First law of
thermodynamics
– Energy cannot be created
or destroyed
– Can only change from one
form to another
– Total amount of energy in
the universe remains
constant
– During each conversion,
some energy is lost as
heat
6
• Second law of thermodynamics
– Entropy (disorder) is continuously increasing
– Energy transformations proceed
spontaneously to convert matter from a more
ordered/ less stable form to a less ordered/
more stable form
7
Free energy
• G = Energy available to do work
• G = H – TS
 H = enthalpy, energy in a molecule’s
chemical bonds
 T = absolute temperature
 S = entropy, unavailable energy
ΔG = ΔH - TS
8
• ΔG = change in free energy
• Positive ΔG
– Products have more free energy
than reactants
– H is higher or S is lower
– Not spontaneous, Requires input
of energy
– Endergonic
• Neagtive ΔG
– Products have less free energy
than reactants
– H is lower or S is higher or both
– Spontaneous (may not be
instantaneous)
– Exergonic
9
Activation energy
• Extra energy required to destabilize existing
bonds and initiate a chemical reaction
• Exergonic reaction’s rate depends on the
activation energy required
– Larger activation energy proceeds more slowly
10
• Rate can be increased 2 ways
1. Increasing energy of reacting molecules (heating)
2. Lowering activation energy
11
Catalysts
• Substances that influence chemical bonds
in a way that lowers activation energy
• Cannot violate laws of thermodynamics
– Cannot make an endergonic reaction
spontaneous
• Do not alter the proportion of reactant
turned into product
12
ATP
• Adenosine triphosphate
• Chief “currency” all cells
use
• Composed of
– Ribose – 5 carbon sugar
– Adenine
– Chain of 3 phosphates
•
•
•
•
Key to energy storage
Bonds are unstable
ADP – 2 phosphates
AMP – 1 phosphate –
lowest energy form
13
ATP cycle
• ATP hydrolysis drives endergonic reactions
– Coupled reaction results in net –ΔG (exergonic and
spontaneous)
• ATP not suitable for long term energy storage
– Fats and carbohydrates better
– Cells store only a few seconds worth of ATP
14
Card Quiz A
Oxidation is the ____________ and reduction is the
________.
 loss of electrons, gain of electrons
 gain of protons, loss of protons
 loss of protons, gain of protons
 loss of electrons, gain of protons
Card Quiz A
The first law of thermodynamics states that energy can
be –
 Created
 Destroyed
 Converted
 Lost
Card Quiz A
The energy in a system that is able to do work.
 Enthalpy
 Entropy
 Free energy
 Kinetic energy
Card Quiz A
A reaction with a positive ∆G is –
 Exergonic
 Endergonic
 Enthalpic
 Energertic
Card Quiz A
Spending ATP involves hydrolyzing it into ADP and
inorganic P. The energy released can drive other
chemical reactions.
 This is true
 This is false
Card Quiz A
A catalyst speeds up chemical reactions. How do
catalysts do this?
 Decreasing entropy
 Altering ∆G
 Consuming reactants
 Lowering activation energy
Card Quiz Answers






Green
Red
Red
Red
Green
Red
Enzymes
• Most enzymes are protein
– Some are RNA
• Shape of enzyme stabilizes a temporary
association between substrates
• Enzyme not changed or consumed in reaction
• Carbonic anhydrase
– 200 molecules of carbonic acid per hour made without enzyme
– 600,000 molecules formed per second with enzyme
22
Active site
•
•
•
•
Pockets or clefts for substrate binding
Forms enzyme-substare complex
Precise fit of substrate into active site
Applies stress to distort particular bond to lower
activation energy
– Induced fit
23
• Enzymes may be suspended in the
cytoplasm or attached to cell membranes
and organelles
• Multienzyme complexes – subunits work
together to form molecular machine
– Product can be delivered easily to next
enzyme
– Unwanted side reactions prevented
– All reactions an be controlled as a unit
24
Nonprotein enzymes
• Ribozymes
• 1981 discovery that certain reactions
catalyzed in cells by RNA molecule itself
2 kinds
1. Intramolecular catalysis – catalyze reaction
on RNA molecule itself
2. Intermolecular catalysis – RNA acts on
another molecule
25
Enzyme function
• Rate of enzyme-catlyzed reaction depends on
concentrations of substrate and enzyme
• Any chemical or physical condition that affects
the enzyme’s 3 dimensional shape can change
rate
– Optimum temperature
– Optimum pH
26
Inhibitors
• Inhibitor – substance
that binds to enzyme
and decreases its
activity
• Competitive inhibitor
– Competes with substrate
for active site
27
• Noncompetitive inhibitor
– Binds to enzyme at a site
other than active site
– Causes shape change
that makes enzyme
unable to bind substrate
28
Cofactors & Coenzymes
• Cofactors: assist in enzyme function
– Often metal ions found in active site
– Zinc in corboxypeptidase
– Required in the diet in small amounts
• Coenzymes: a cofactor that is an organic
molecule (non-protein)
– Many are vitamins essential in our diets
• Often coenzymes and cofactors are involved in
capturing and transferring electrons
29
Allostery
• Allosteric enzymes – enzymes exist in active
and inactive forms
• Most noncompetitive inhibitors bind to allosteric
site – chemical on/off switch
• Allosteric inhibitor – binds to allosteric site and
reduces enzyme activity
• Allosteric activator – binds to allosteric site and
increases enzyme activity
30
Metabolism
• Total of all chemical reactions carried out
by an organism
• Anabolic reactions / anabolism
– Expend energy to build up molecules
• Catabolic reactions/ catabolism
– Harvest energy by breaking down molecules
31
Biochemical pathways
• Reactions occur in a
sequence
• Product of one reaction
is the substrate for the
next
• Many steps take place
in organelles
32
Feedback inhibition
• End-product of pathway binds to an
allosteric site on enzyme that catalyses
first reaction in pathway
• Shuts down pathway so raw materials and
energy are not wasted
33
Card Quiz B
Small organic molecules that assist in enzymatic
functions are –
 Coprolites
 Cofactors
 Activators
 Coenzymes
Card Quiz B
Feedback inhibition occurs when the final product acts
as an inhibitor to the first enzyme in the pathway.
 This is true
 This is false
Card Quiz B
Carbon monoxide (CO) binds to the hemoglobin protein
at the oxygen binding site. Once the CO binds, the
oxygen can no longer be transported. What does this
describe?
 Non-competitive inhibition
 Feedback inhibition
 Allosteric inhibition
 Competitive inhibition
Card Quiz B
Increasing the temperature increases the rate of an
enzyme-catalyzed reaction. Once a critical temperature
is reached, the reaction stops. Why does this happen?
 The concentration of reactants drop
 The enzymes have all been consumed in the
reaction
 The increase in temperature alters the pH
 The polypeptide chains in the enzyme denature
Card Quiz Answers
 Yellow
 Blue
 Red
 Red