Flow Energy in the Life of a
Cell
Chapter 6
Free Radicals
Unbound molecular fragments w/ the
wrong number of electrons
Example 02(-1) [should be 02(-3)]
Highly reactive
Can disrupt structure of molecules
age spots as you get older is an ex.
These Keep Free Radicals
in Check
Superoxide Dismutase
Catalyzes the formation of hydrogen peroxide from oxygen free radicals
and hydrogen ions
Accumulation of hydrogen peroxide can be lethal to cells
O2(-) + 2H(+)>>H2O2
Catalase
Catalyzes the formation of oxygen and
water from hydrogen peroxide
2 H202>>>>>2 H2O
Overall effect
Red molecules = free radicals
= SOD action
Pink molecules = toxic to cells
= catalase action
Roundworm Experiments
Diet supplemented w/
SOD and catalase
increased life span of
normal worms
Diet also allowed worms
genetically engineered for
susceptibility to free
radicals to live normal life
spans
What Is Energy?
Energy: capacity to do work
2 different forms of energy
1. P.E. (stored energy)
chemical energy in bonds, electrical
charge in a battery, rock at top of hill
2. K.E. (active energy)
Light, heat, electricity, moving objects
What Can Cells Do
with Energy?
Energy imputs become coupled to
energy-requiring processes
Cells use energy for:
Mechanical work
Chemical work
Electrochemical work
The Laws of Thermodynamics describe
the basic properties of Energy
First Law of Thermodynamics
The total amount of energy in the
universe remains constant unless
energy is added or removed from the
system
Energy can undergo conversions, but it
cannot be created or destroyed
One-Way Flow of Energy
The sun is lifes primary energy source
Producers trap energy from the sun
and convert it into chemical bond
energy
All organisms use the energy stored in
the bonds of organic compounds to do
work: (the 3 phosphate molecules in
ATP can be cleaved off to release
Energy)
Second Law of
Thermodynamics
Energy conversion is never 100%
efficient
The total amount of energy is flowing
from high-energy forms to lowenergy forms
Entropy
Measure of degree of disorder in a
system
The living world can resist the flow
toward maximum entropy (max.
disorder) only because it is re-supplied
w/ energy from the sun
100 units
chemical
energy
(concentrated)
LOW
ENTROPY
75 units heat 25 units kinetic energy
+
(motion)
Energy
HIGH ENTROPY
Living Things Use the Energy of Sunlight
to Create the Low-Entropy Conditions
Characteristic of Life
Living things must gain external energy in order to counteract the increase in
their entropy
Photosynthetic organisms use external solar energy to maintain orderly
structure
Non-photosynthetic life uses stored chemical energy in other living things to
counter their increase in entropy
Energy Changes &
Cellular Work
Energy changes in cells tend to run spontaneously
in the direction that results in a decrease in usable
energy
Chemical reactions are process that form or break
chemical bonds between atoms
Chemical reactions convert reactants to products
Reactants
Products
Reactants contain more energy than products in
exergonic reactions
Reactions can be characterized as exergonic or
endergonic based on energy gain or loss
Exergonic reaction
energy
released
reactants
products
Example: the burning of glucose
Glucose + 6O2
(reactants)
6CO2 + 6H2O + released
energy
(products)
Exergonic Reaction
glucose energy-rich starting
substance
+
602
Energy out
6
6
products with less energy
Exergonic reaction
energy
released
reactants
products
Example: Photosynthesis
6CO2 + 6H2O + sunlight energy
glucose +
6O2
(reactants)
(products)
Endergonic Reaction
glucose - a product with
more energy
+ 602 and 6H2O
Energy in
energy-poor
starting substances
6
12
Coupled Reactions Link Exergonic
and Endergonic Reactions
Exergonic reactions drive endergonic reaction
The product of an energy-yielding reaction fuels an energyrequiring reaction in a coupled reaction
Exergonic reaction:
100 units
ADP
energy released
P
80 units energy ADP
released as heat
ATP
P
Endergonic reaction:
20 units
energy
contracted
muscle
relaxed
muscle
Coupled reaction:
ATP
relaxed
muscle
contracted
muscle
Coupled Reactions Link Exergonic
and Endergonic Reactions
Energy provided by exergonic reaction must exceed that needed by endergonic
reaction
Some energy is lost as heat during the transfer
Energy carrier molecules are used to transfer energy in cells
Energy Carrier Molecules
Food energy cant be sued directly to power energyrequiring reactions (muscle contraction)
Energy carrier molecules act as intermediates to carry
energy between exergonic and endergonic reactions
ATP is the most common energy carrying molecule
ATP is composed of an adenosine molecule and three
phosphates
Structure of ATP
nucleotide base (adenine)
three groups
sugar phosphate (ribose)
Adenosine triphosphate (ATP)
Adenosine diphosphate (ADP)
NH2
adenine
N CC
HC
NH2
2
"high-energy
bond
N
NCC
HC
N C N CH
O
O
ribose
H
H
H
OH
OH
Shorthand
representations
A
Energy
content
CH2
H
O
P
H
O
H
low
P
or
ADP
A
H
OH
phosphate groups
O
CH2
H
OH
P
O
O
O
P O
O
P
N
N C N CH
O
O
"high-energy
bonds
P
O
O
P
O
O
O
P O
O
phosphate groups
P
P
high
or
ATP
ATP: Main Energy Carrier
ATP couples energy inputs and outputs
ATP/ADP cycle regenerates ATP
Energy stored in the high energy bond extending to the last
phosphate
Heat is given off when ATP breaks into ADP (adenosine diphosphate)
and P (phosphate)
The energy is released when ATP>ADP+P is transferred to
endergonic reaction through coupling
ATP
energy
input
energy
output
ADP + Pi
ATP breakdown: Energy of ATP is released
Energy in
the form
of heat
A
P
P
P
ATP
A
P
ADP
P
P
phosphate
Electron Carriers Also Transport
Energy Within Cells
Energy can be transferred to electrons in glucose metabolism and photosynthesis
Electron carriers transport high-energy electrons
Oxidation- lose electron(s)
Reduction- gain electron(s)
LEO the lion goes GER
Central to the formation of ATP during photosynthesis and aerobic respiration
Overview of Metabolism
The sum of all the chemical reactions inside a cell is its
metabolism
Many cellular reactions are linked through metabolic pathways
Metabolic pathways proceed smoothly for 3 reasons:
1.
2.
3.
Chemical reactions are regulated through protein enzymes
Endergonic reactions are coupled w/ exergonic reactions
Energy-carrier molecules capture energy and transfer in
between endergonic and exergonic reactions
3 Types of Metabolic
Pathways
A
B
C
D
E
F
K
N
J
M
L
G
I
H
Participants in
Metabolic Pathways
Reactants
Intermediates
Products
Energy carriers
Enzymes
Cofactors
Which Way Will
a Reaction Run?
Nearly all chemical reactions are
reversible
Direction reaction runs depends upon
Energy content of participants
Reactant-to-product ratio
Chemical Equilibrium
RELATIVE
CONCENTRATION
OF REACTANT
RELATIVE
CONCENTRATION
OF PRODUCT
No Vanishing Atoms
Law of conservation of mass
Reactions rearrange atoms, but they never
destroy them
There are as many atoms of each element in
all the products as there were in all the
reactants
H2O2>>H2O + O2 (unbalanced)
Enzyme Structure and Function
Enzymes are catalytic molecules
They speed up to the rate of reactions, but
the enzyme molecule remains unchanged
during the reaction
For any chemical reaction to occur, an
energy barrier must be surmounted
Enzymes lower the activation energy and
make the energy barrier smaller
high
activation energy
without
catalyst
energy
content
of
molecules
activation energy
with catalyst
reactants
products
low
progress of reaction
Four Features of Enzymes
Enzymes dont make anything happen that
could not happen on its own. They just make
it happen much faster
3)
Reactions dont alter or use up enzyme
molecules
4)
The same enzyme usually works for both the
forward and reverse reactions
4)
Each type of enzyme recognized and binds
to only certain substrates
1)
Enzymes Are Biological Catalysts
Enzymes (proteins) differ from non-biological catalysts because:
1.
Enzymes are very specific for the molecules they catalyze
2.
Enzyme activity is often enhanced or suppressed by their reactants or
products
Enzymes have a pocket called an active site
Reactants (substrates) bind to the active site
Enzyme Structure Allows Them to
Catalyze Specific Reactions
Three steps of enzyme catalysis
1.
2.
3.
Substrates enter the active site in a specific
orientation
Upon binding, the substrates and enzyme
changes shape to promote a reaction
Products of the reaction leave the active site;
leaving the enzyme ready for another catalysis
substrates
1
active site
of enzyme
enzyme
3
2
Two examples of enzyme regulation include
Allosteric Activation
allosteric
activator
vacant
allosteric
binding
site
active site
altered,
can bind
substrate
enzyme active site
active site cannot
bind substrate
allosteric inhibitor
allosteric
binding site
vacant;
active site
can bind
substrate
active site altered,
cant bind substrate
Feedback Inhibition
enzyme 2
enzyme
1
enzyme 3
enzyme 4
A cellular change, caused by a
specific activity, shuts down the
activity that brought it about
+ vs. - feedback
SUBSTRATE
enzyme 5
END
PRODUCT
(tryptophan)
Factors Influencing
Enzyme Activity
Coenzymes and cofactors
Allosteric regulators
Temperature
pH
Salt concentration
Effect of Temperature
Small increase in
temperature
increases molecular
collisions, reaction
rates
High temp disrupts
bonds and destroy
the shape of the
active site
Effect of pH