Transcript Slide 1

Metabolism and Energy
Mrs. Stahl
AP Biology
The Energy of Life
• The living cell is a miniature chemical factory
where thousands of reactions occur
• The cell extracts energy stored in sugars and
other fuels and applies energy to perform
work
• Some organisms even convert energy to light,
as in bioluminescence
Figure 8.1
Forms of Energy
• Energy is the capacity to cause change
• Energy exists in various forms, some of which
can perform work
• Kinetic energy is energy associated with
motion
• Heat (thermal energy) is kinetic energy
associated with random movement of atoms
or molecules
• Potential energy is energy that matter
possesses because of its location or structure
• Chemical energy is potential energy available
for release in a chemical reaction
• Energy can be converted from one form to
another
A diver has more potential
energy on the platform
than in the water.
Figure 8.2
Climbing up converts the kinetic
energy of muscle movement
to potential energy.
Diving converts
potential energy to
kinetic energy.
A diver has less potential
energy in the water
than on the platform.
The Laws of Energy Transformation
• Thermodynamics is the study of energy
transformations
• An isolated system, such as that approximated
by liquid in a thermos, is unable to exchange
energy or matter with its surroundings
• In an open system, energy and matter can
be transferred between the system and its
surroundings
• Organisms are open systems
The First Law of Thermodynamics
• According to the first law of thermodynamics,
the energy of the universe is constant
– Energy can be transferred and transformed,
but it cannot be created or destroyed
• The first law is also called the principle of
conservation of energy
• Plants do not produce energy, they
transform light energy to chemical energy.
During every transfer, some energy is
converted to heat -> a system can use heat
to do work only when there is a difference
that results in heat flowing from warmer
locations to cooler ones. If heat is uniform
as in a living cell, heat can only be used to
warm the organism.
Figure 8.3
Heat
H2O
Chemical
energy
(a) First law of thermodynamics
CO2
(b) Second law of thermodynamics
The Second Law of Thermodynamics
• During every energy transfer or
transformation, some energy is unusable, and
is often lost as heat
• According to the second law of
thermodynamics
– Every energy transfer or transformation increases
the entropy (disorder) of the universe
– Entropy- measure of disorder or randomness
– Increase entropy = increase heat
• Living cells unavoidably convert organized
forms of energy to heat
• Spontaneous processes occur without energy
input; they can happen quickly or slowly.
– Living systems create ordered structures from less
ordered starting materials.
– Ex- amino acids are ordered into polypeptide chains
– Ex- structure of a multicellular body is organized and
complex.
• For a process to occur without energy input, it
must increase the entropy of the universe
Highly Ordered Living Organisms Do
Not Violate the Second Law of
Thermodynamics
• Organisms also take in organized forms of
matter and energy from its surrounding and
replaces them with less ordered forms.
– Ex- animal consumes organic molecules as food
and catabolizes (breaks down -> metabolism)
them to low energy molecules such as carbon
dioxide and water.
• The evolution of more complex organisms does
not violate the second law of thermodynamics
because Earth and organisms are open systems.
We get our energy from the sun and we can
evolve and create order by increasing the
disorder of the universe.
• Entropy (disorder) may decrease in an organism,
but the universe’s total entropy increases
The free-energy change of a reaction
tells us whether or not the reaction
occurs spontaneously
• Biologists want to know which reactions occur
spontaneously and which require input of
energy
• To do so, they need to determine energy
changes that occur in chemical reactions
Free-Energy Change, G
• A living system’s free energy is energy that
can do work when temperature and pressure
are uniform, as in a living cell
• The change in free energy (∆G) during a process is related to
the change in enthalpy, or change in total energy (∆H),
change in entropy (∆S), and temperature in Kelvin units (T)
∆G = ∆H - T∆S
• ∆H= enthalpy, in a spontaneous reaction it gets smaller or
decreases because energy is released.
• ∆S= entropy, measure of the disorder . In a spontaneous
reaction entropy increases.
• T= temperature, when temperature increases the
spontaneous reaction is more likely to happen.
• What makes ∆G decrease?
– A decrease in ∆H
– An increase in ∆S
– An increase in T
• Only processes with a negative ∆G are spontaneous also
known as exergonic ( releases free energy as heat)
• ∆G > 0 = endergonic reaction, energy must be
added
• ∆G = 0 = equilibrium
• Increases in temperature amplify the entropy.
Think about a cherry bomb-> add heat and it
blows up.
• Not all the energy in a system is available for
work because the entropy component must be
subtracted from the enthalpy component. What
remains is the free energy, referred to as the
stability of the system.
Figure 8.5b
(a) Gravitational motion
(b) Diffusion
(c) Chemical reaction
Exergonic and Endergonic Reactions in
Metabolism
• Exergonic Reaction:
– ∆G is negative, Spontaneous
– The products of the reaction contain less free energy
than reactants. Bond is lower or disorder is higher or
both.
– Energy is released. Ex- Cellular Respiration
• Endergonic reaction:
– absorbs free energy from its surroundings and is
non-spontaneous
– Energy must be supplied, ex- photosynthesis
(a) Exergonic reaction: energy released,
spontaneous
Free energy
Reactants
Amount of
energy
released
(∆G < 0)
Energy
Products
Progress of the reaction
(b) Endergonic reaction: energy required,
nonspontaneous
Free energy
Products
Reactants
Energy
Progress of the reaction
Amount of
energy
required
(∆G > 0)
If all chemical reactions that release
free energy tend to occur
spontaneously, why haven’t all such
reactions already occurred?
• Most reactions require an input of energy to
get started such as endergonic reactions
(photosynthesis).
Activation Energy
• Before any bonds can form, they have to be broken
by energy input.
• Defined as the extra energy needed to destabilize
existing chemical bonds and initiate / start a
chemical reaction.
• Exergonic rate depends on the activation energy
required for the reaction to begin.
• Rates of reactions are increased by:
– Increasing the energy of reacting molecules
– Lowering activation energy
Catalysis / Catalysts
• A substance that lowers the activation
energy
– Ex- enzymes
– They cannot make an endergonic reaction
proceed spontaneously.
Adenosine Triphosphate- ATP
• Main energy currency in all living cells
• 1. Makes sugars
• 2. Supplying activation energy for chemical
reactions
• 3. Actively transporting substances across
membranes
• 4. Moving through the environment and
growing
The Structure and Hydrolysis of ATP
• ATP (adenosine triphosphate) is the cell’s
energy shuttle
• ATP is composed of ribose (a sugar), adenine
(a nitrogenous base), and three phosphate
groups
How does ATP store energy?
• The energy is stored in the bonds between the
triphosphates. These groups repel each other
due to their negative charges, and the
covalent bonds joining the phosphates are
unstable and can break.
• They are easily broken by hydrolysis and when
they break they release and transfer a large
amount of energy which can be used.
Figure 8.9a
Adenine
Triphosphate group
(3 phosphate groups)
(a) The structure of ATP
Ribose
Figure 8.9b
Adenosine triphosphate (ATP)
H 2O
Energy
Inorganic
phosphate
Adenosine diphosphate
(ADP)
(b) The hydrolysis of ATP
How does ATP become ADP?
• The bonds are broken on the third phosphate,
releasing energy.
• ATP -> ADP +Pi
• Energy = 7.3kcal / mol
• This release of energy comes from the
chemical change to a state of lower free
energy, not from the phosphate bonds
themselves
Why does the hydrolysis of ATP yield
so much energy?
• The release of energy initially comes from the
chemical change to a state of lower free
energy.
• Each phosphate group has a negative charge.
Three like charges are crowded together and
their mutual repulsion contributes to the
instability of the molecule.
How the Hydrolysis of ATP Performs
Work
• The three types of cellular work (mechanical,
transport, and chemical) are powered by the
hydrolysis of ATP
• In the cell, the energy from the exergonic
reaction of ATP hydrolysis can be used to drive
an endergonic reaction
• Overall, the coupled reactions are exergonic
• ATP drives endergonic reactions by
phosphorylation, transferring a
phosphate group to some other
molecule, such as a reactant
• The recipient molecule is now called a
phosphorylated intermediate
• Transport and mechanical work in the cell are
also powered by ATP hydrolysis
• ATP hydrolysis leads to a change in protein
shape and binding ability
Transport protein
Solute
Figure 8.11
ATP
ADP
P
Pi
Pi
Solute transported
(a) Transport work: ATP phosphorylates transport proteins.
Vesicle
ATP
Cytoskeletal track
ATP
Motor protein
ADP
Protein and vesicle moved
(b) Mechanical work: ATP binds noncovalently to motor
proteins and then is hydrolyzed.
Pi
What reactions does ATP Drive?
• Endergonic Reactions
Figure 8.12
ATP
Energy from
catabolism
(exergonic, energyreleasing processes)
ADP
H2O
Pi
Energy for cellular
work (endergonic
energy-consuming
processes)
The Regeneration of ATP
• ATP is a renewable resource that is regenerated
by addition of a phosphate group to adenosine
diphosphate (ADP)
• The energy to phosphorylate ADP comes from
catabolic reactions in the cell
• The ATP cycle is a revolving door through which
energy passes during its transfer from catabolic
to anabolic pathways
Enzymes speed up metabolic reactions
by lowering energy barriers
• A catalyst is a chemical agent that speeds up a
reaction without being consumed by the
reaction
• Substrate- The molecules that will undergo
the reaction
• An enzyme is a catalytic protein
• Hydrolysis of sucrose by the enzyme sucrase is
an example of an enzyme-catalyzed reaction
Figure 8.UN02
Sucrase
Sucrose
(C12H22O11)
Glucose
(C6H12O6)
Fructose
(C6H12O6)
Functions of Enzymes
• 1. Lowers the activation energy required for
new bonds to form
• 2. Speeds up the rate of reaction
• 3. Regulates metabolic pathways
What is the importance of carbonic
acid in vertebrate red blood cells?
• Vertebrate RBC’s have an enzyme called
carbonic anhydrase, which aids in breaking
down carbon dioxide in our blood. 600,000
molecules of carbonic acid form every second.
This enzyme increases the rate of reaction one
million times.
The Active Site
• They are pockets on the enzyme where the
substrates fit perfectly.
Catalysis in the Enzyme’s Active Site
• In an enzymatic reaction, the substrate binds
to the active site of the enzyme
• The active site can lower an EA barrier by
– Orienting substrates correctly
– Straining substrate bonds
– Providing a favorable microenvironment
– Covalently bonding to the substrate
Enzyme Substrate Complex
• A substrate molecule binds with an enzyme at
its active site. Chemical reactions occur and
bonds are either broken or new ones are
formed. The substrates have been changed
into products. The products leave the enzyme
and the process starts all over again.
Induced Fit
• When the active site changes its shape slightly
so that it can bind onto the substrate more
tightly.
Figure 8.15
Substrate
Active site
Enzyme
Enzyme-substrate
complex
Where are most enzymes found?
• Cytoplasm
• Cell membranes
• Organelles
Multienzyme Complexes and
Advantages
• They allow a plethora of chemical reactions to
occur-> molecular machine
• Advantages:
– 1. All reactions can be controlled as a unit
– 2. No unwanted side reactions
– 3. Rate of the enzyme is limited
The Activation Energy Barrier
• Every chemical reaction between molecules
involves bond breaking and bond forming
• The initial energy needed to start a chemical
reaction is called the free energy of activation,
or activation energy (EA)
• Activation energy is often supplied in the form
of thermal energy that the reactant molecules
absorb from their surroundings
A
B
Figure
8.13
C
D
Free energy
Transition state
A
B
C
D
EA
Reactants
A
B
∆G < O
C
D
Products
Progress of the reaction
Animation: How Enzymes Work
1 Substrates enter
Figure 8.16-1
2 Substrates are
active site.
Substrates
Enzyme-substrate
complex
held in active
site by weak
interactions.
1 Substrates enter
Figure 8.16-2
2 Substrates are
active site.
Substrates
held in active
site by weak
interactions.
Enzyme-substrate
complex
3 Substrates are
converted to
products.
1 Substrates enter
Figure 8.16-3
2 Substrates are
active site.
Substrates
held in active
site by weak
interactions.
Enzyme-substrate
complex
4 Products are
released.
3 Substrates are
Products
converted to
products.
1 Substrates enter
Figure 8.16-4
2 Substrates are
active site.
Substrates
held in active
site by weak
interactions.
Enzyme-substrate
complex
5 Active site
is available
for new
substrates.
Enzyme
4 Products are
released.
3 Substrates are
Products
converted to
products.
Effects of Local Conditions on Enzyme
Activity
• An enzyme’s activity can be affected by
– General environmental factors, such as
temperature and pH
– Chemicals that specifically influence the enzyme
Effects of Temperature and pH
• Each enzyme has an optimal temperature in
which it can function
• Each enzyme has an optimal pH in which it
can function
• Optimal conditions favor the most active
shape for the enzyme molecule
Temperature
• Above the optimal temperature= forces are
too weak to maintain the enzymes shape ->
denatures
• Below optimum temperatures= hydrogen
bonds and hydrophobic interactions that
determine the enzymes shape is not flexible
to allow induced fit.
• Humans= 35 to 40 Celcius
• Prokaryotes= 70 Celcius (hot springs)
pH
• Interactions are sensitive to the hydrogen ion
concentration of the fluid in which the
enzyme is dissolved, changing the
concentration. Shifts the balance between +/amino acids
• Optimum pH= 6 to 8
Optimal temperature for
typical human enzyme
(37C)
Optimal temperature for
enzyme of thermophilic
(heat-tolerant)
bacteria (77C)
Rate of reaction
Figure 8.17
0
20
40
60
80
Temperature (C)
(a) Optimal temperature for two enzymes
Rate of reaction
Optimal pH for pepsin
(stomach
enzyme)
0
1
2
3
4
100
120
Optimal pH for trypsin
(intestinal
enzyme)
5
6
pH
(b) Optimal pH for two enzymes
7
8
9
10
Enzyme Inhibitors- substance that binds to
an enzyme and decreases its activity
• Competitive inhibitors bind to the active site
of an enzyme, competing with the substrate
• Noncompetitive inhibitors bind to another
part of an enzyme, causing the enzyme to
change shape and making the active site less
effective. Binds at the allosteric site.
• Examples of inhibitors include toxins, poisons,
pesticides, and antibiotics
Figure 8.18
(a) Normal binding
Substrate
Active site
(b) Competitive inhibition
(c) Noncompetitive
inhibition
Competitive
inhibitor
Enzyme
Noncompetitive
inhibitor
• Allosteric enzymes- enzymes that exist as
either active or inactive
• Allosteric site- on /off chemical switches
• Allosteric inhibitors- binds to the allosteric site
and reduces enzyme activity
• Allosteric Activator- keeps the enzyme active ,
increases enzyme activity
Cofactors
• Cofactors are non-protein enzyme helpers that are
found around the active site to assist in catalysis.
They help weaken bonds and make them easier to
break.
• Cofactors may be inorganic (such as a metal in ionic
form) or organic
• An organic cofactor is called a coenzyme
• Coenzymes serve as an electron acceptor which
then transfers the electrons to a different enzyme,
which releases them to the substrates in another
reaction (Ex- NADP)
The Evolution of Enzymes
• Enzymes are proteins encoded by genes
• Changes (mutations) in genes lead to changes
in amino acid composition of an enzyme
• Altered amino acids in enzymes may result in
novel enzyme activity or altered substrate
specificity
• Under new environmental conditions a novel
form of an enzyme might be favored
– For example, six amino acid changes improved
substrate binding and breakdown in E. coli
Regulation of enzyme activity helps
control metabolism
• Chemical chaos would result if a cell’s
metabolic pathways were not tightly regulated
• A cell does this by switching on or off the
genes that encode specific enzymes or by
regulating the activity of enzymes
Localization of Enzymes Within the Cell
• Structures within the cell help bring order to
metabolic pathways
• Some enzymes act as structural components
of membranes
• In eukaryotic cells, some enzymes reside in
specific organelles; for example, enzymes for
cellular respiration are located in
mitochondria
Metabolism
• Totality of an organism’s chemical reaction.
Emergent property of life that arises from
interactions between molecules within the
orderly environment of the cell.
• Catabolic pathways release energy by
breaking down complex molecules into
simpler compounds
• Cellular respiration, the breakdown of
glucose
in the presence of oxygen, is an example
of a pathway of catabolism
• Anabolic pathways consume energy to
build complex molecules from simpler
ones
• The synthesis of protein from amino acids
is an example of anabolism
• Bioenergetics is the study of how energy
flows through living organisms
Biochemical Pathways
• Organizational units of metabolism. The
elements an organism needs / controls to
achieve metabolic activity.
• They think they came from early oceans,
creating an organic soup.
Feedback Inhibition
• In feedback inhibition, the end product of a
metabolic pathway shuts down the pathway
• Feedback inhibition prevents a cell from
wasting chemical resources by synthesizing
more product than is needed
Cells!
• Cells are not in equilibrium; they are open
systems experiencing a constant flow of
materials
• A defining feature of life is that metabolism is
never at equilibrium
• A catabolic pathway in a cell releases free
energy in a series of reactions