Download Biological energy

An Introduction to Metabolism
ENERGY AND ENERGY
TRANSFER
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Overview
• Metabolism
• Exothermic/Endothermic reactions
• ATP
• Energy pyramids and ecosystems
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• Overview: The Energy of Life
• The living cell
– Is a miniature factory where thousands of
reactions occur
– Converts energy in many ways
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• Metabolism
– Is the totality of an organism’s chemical
reactions
– Arises from interactions between molecules
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Organization of the Chemistry of Life into
Metabolic Pathways
• A metabolic pathway has many steps
– That begin with a specific molecule and end
with a product
– That are each catalyzed by a specific enzyme
Enzyme 1
A
Enzyme 2
D
C
B
Reaction 1
Enzyme 3
Reaction 2
Starting
molecule
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Reaction 3
Product
• Catabolic pathways
– Break down complex molecules into simpler
compounds
– Release energy
• Anabolic pathways
– Build complicated molecules from simpler ones
– Consume energy
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• Thermodynamics
– Is the study of energy transformations
An organism’s metabolism transforms matter and
energy, subject to the laws of thermodynamics
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The First and Second Laws of Thermodynamics
• According to the first law of thermodynamics
– Energy can be transferred and transformed
– Energy cannot be created or destroyed
• According to the second law of
thermodynamics
–
Spontaneous changes that do not require outside
energy increase the entropy, or disorder, of the
universe
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• Concept 8.2: The free-energy change of a
reaction tells us whether the reaction occurs
spontaneously
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Free-Energy Change, G
• A living system’s free energy
– Is energy that can do work under cellular
conditions
• The change in free energy, ∆G during a
biological process
– Is related directly to the enthalpy change (∆H)
and the change in entropy
– ∆G = ∆H – T∆S
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Free Energy, Stability, and Equilibrium
• Organisms live at the expense of free energy
• During a spontaneous change
– Free energy decreases and the stability of a
system increases
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Exergonic and Endergonic Reactions in Metabolism
• An exergonic reaction
– Proceeds with a net release of free energy and
is spontaneous
Reactants
Free energy
Amount of
energy
released
(∆G <0)
Energy
Products
Progress of the reaction
Figure 8.6
(a) Exergonic reaction: energy released
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• An endergonic reaction
– Is one that absorbs free energy from its
surroundings and is nonspontaneous
Free energy
Products
Energy
Reactants
Progress of the reaction
Figure 8.6
(b) Endergonic reaction: energy required
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Amount of
energy
released
(∆G>0)
Equilibrium and Metabolism
• Reactions in a closed system
– Eventually reach equilibrium
∆G < 0
Figure 8.7 A
∆G = 0
(a) A closed hydroelectric system. Water flowing downhill turns a turbine
that drives a generator providing electricity to a light bulb, but only until
the system reaches equilibrium.
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• Cells in our body
– Experience a constant flow of materials in and
out, preventing metabolic pathways from
reaching equilibrium
(b) An open hydroelectric
system. Flowing water
keeps driving the generator
because intake and outflow
of water keep the system
from reaching equlibrium.
Figure 8.7
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∆G < 0
• An analogy for cellular respiration
∆G < 0
∆G < 0
∆G < 0
Figure 8.7
(c) A multistep open hydroelectric system. Cellular respiration is
analogous to this system: Glucoce is brocken down in a series
of exergonic reactions that power the work of the cell. The product
of each reaction becomes the reactant for the next, so no reaction
reaches equilibrium.
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• Concept 8.3: ATP powers cellular work by
coupling exergonic reactions to endergonic
reactions
• A cell does three main kinds of work
– Mechanical
– Transport
– Chemical
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The Structure and Hydrolysis of ATP
• ATP (adenosine triphosphate)
– Is the cell’s energy shuttle
– Provides energy for cellular functions
Adenine
N
O
O
-O
O
-
O
-
O
O
C
C
N
HC
O
O
O
NH2
-
Phosphate groups
Figure 8.8
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N
CH2
O
H
N
H
H
H
OH
CH
C
OH
Ribose
Structure of ATP
• Phosphate – phosphate bonds
– Negative charges repel, unstable
– “high transferable energy”
– C-C ~400 KJ/mol while P-P 7.3 KJ/mol
– Right amount for most chemical reactions
• Each cell contains one billion ATP
• Short term storage
• Controlled production
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• Energy is released from ATP
– When the terminal phosphate bond is broken
P
P
P
Adenosine triphosphate (ATP)
H2O
P
i
+
Figure 8.9 Inorganic phosphate
P
P
Adenosine diphosphate (ADP)
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Energy
• The three types of cellular work
– Are powered by the hydrolysis of ATP
P
i
P
Motor protein
Protein moved
(a) Mechanical work: ATP phosphorylates motor proteins
Membrane
protein
ADP
+
ATP
P
P
Solute
P
i
Solute transported
(b) Transport work: ATP phosphorylates transport proteins
P
Glu + NH3
Reactants: Glutamic acid
and ammonia
Figure 8.11
NH2
Glu
+
P
i
Product (glutamine)
made
(c) Chemical work: ATP phosphorylates key reactants
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i
The Regeneration of ATP
• Catabolic pathways
– Drive the regeneration of ATP from ADP and
phosphate
ATP hydrolysis to
ADP + P i yields energy
ATP synthesis from
ADP + P i requires energy
ATP
Energy from catabolism
(exergonic, energy yielding
processes)
Figure 8.12
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Energy for cellular work
(endergonic, energyconsuming processes)
ADP + P
i
• Concept 8.4: 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
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• An enzyme
– Is a catalytic protein
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• The activation energy, EA
– Is the initial amount of energy needed to start a
chemical reaction
– Is often supplied in the form of heat from the
surroundings in a system
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• The effect of enzymes on reaction rate
Course of
reaction
without
enzyme
EA
without
enzyme
Free energy
EA with
enzyme
is lower
Reactants
∆G is unaffected
by enzyme
Course of
reaction
with enzyme
Products
Progress of the reaction
Figure 8.15
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• The active site can lower an EA barrier by
– Orienting substrates correctly
– Straining substrate bonds
– Providing a favorable microenvironment
– Covalently bonding to the substrate
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• Concept 8.5: Regulation of enzyme activity
helps control metabolism
• A cell’s metabolic pathways
– Must be tightly regulated
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• Energy
– Flows into an ecosystem as sunlight and
leaves as heat
Light energy
ECOSYSTEM
Photosynthesis
in chloroplasts
Organic
CO2 + H2O
+ O2
Cellular
molecules
respiration
in mitochondria
ATP
powers most cellular work
Figure 9.2
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Heat
energy
• Some redox reactions
– Do not completely exchange electrons
– Change the degree of electron sharing in
covalent bonds
Products
Reactants
becomes oxidized
+
CH4
CO
2O2
2
Energy
+
2 H2O
becomes reduced
O
O
C
O
H
O
O
H
H
H
C
+
H
H
Figure 9.3
Methane
(reducing
agent)
Oxygen
(oxidizing
agent)
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Carbon dioxide
Water
Oxidation of Organic Fuel Molecules During
Cellular Respiration
• During cellular respiration
– Glucose is oxidized in a series of steps and
oxygen is reduced
becomes oxidized
C6H12O6 + 6O2
6CO2 + 6H2O + Energy
becomes reduced
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• Electrons from organic compounds
– Are usually first transferred to NAD+, a
coenzyme
–
+
NAD+
O
NH2
H
C
O OCH O
2
O
O
P
–
O OH
P
O
HO
– CH HO
2
NH2
N
N
H
O
H
HO
2
N+ Nicotinamide
(oxidized form)
H
OH
N
H
OH
2e +2H –
2 e + NADH
H+
Dehydrogen
Reduction
of H HO
ase
2[H]
NH
+(from
+
+
C
NAD
Oxidation of
N Nicotinamide
food)
NADH
(reduced form)
N
H
Figure 9.4
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• NADH, the reduced form of NAD+
– Passes the electrons to the electron transport
chain
• At the end of the chain
– Electrons are passed to oxygen, forming water
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• There are three main processes in this
metabolic enterprise
Electron shuttles
span membrane
CYTOSOL
MITOCHONDRION
2 NADH
or
2 FADH2
2 NADH
2 NADH
Glycolysis
Glucose
2
Pyruvate
6 NADH
Citric
acid
cycle
2
Acetyl
CoA
+ 2 ATP
by substrate-level
phosphorylation
Maximum per glucose:
+ 2 ATP
2 FADH2
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
+ about 32 or 34 ATP
by substrate-level by oxidative phosphorylation, depending
on which shuttle transports electrons
phosphorylation
from NADH in cytosol
About
36 or 38 ATP
Figure 9.16
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Excitation of Chlorophyll by Light
• When a pigment absorbs light
– It goes from a ground state to an excited state,
which is unstable
e–
Excited
state
Heat
Photon
(fluorescence)
Photon
Chlorophyll
molecule
Figure 10.11 A
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Ground
state
• Produces NADPH, ATP, and oxygen
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Primary
acceptor
Fd
2
2 H+
+
O2
Pq
e
H2O
e
NADP+
NADP+
+ 2 H+
reductase
3
NADPH
PC
e–
5
+ H+
P700
P680
Light
6
ATP
Figure 10.13
8
e–
Cytochrome
complex
e–
Light
1
7
4
Photosystem II
(PS II)
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Photosystem-I
(PS I)
A Comparison of Chemiosmosis in Chloroplasts
and Mitochondria
• Chloroplasts and mitochondria
– Generate ATP by the same basic mechanism:
chemiosmosis
– But use different sources of energy to
accomplish this
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• Concept 10.3: The Calvin cycle uses ATP and
NADPH to convert CO2 to sugar
• The Calvin cycle
– Is similar to the citric acid cycle
– Occurs in the stroma
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• The Calvin cycle
Light
H2 O
CO2
Input
3 (Entering one
CO2 at a time)
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTION
ATP
Phase 1: Carbon fixation
NADPH
O2
Rubisco
[CH2O] (sugar)
3 P
3 P
P
Short-lived
intermediate
P
Ribulose bisphosphate
(RuBP)
P
6
3-Phosphoglycerate
6
ATP
6 ADP
CALVIN
CYCLE
3 ADP
3
ATP
6 P
P
1,3-Bisphoglycerate
6 NADPH
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
6 NADPH+
6 P
P
5
(G3P)
6
P
Glyceraldehyde-3-phosphate
(G3P)
P
1
Figure 10.18
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G3P
(a sugar)
Output
Glucose and
other organic
compounds
Phase 2:
Reduction
• Overview: Ecosystems, Energy, and Matter
• An ecosystem consists of all the organisms
living in a community
– As well as all the abiotic factors with which
they interact
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• Concept 54.1: Ecosystem ecology emphasizes
energy flow and chemical cycling
• Ecosystem ecologists view ecosystems
– As transformers of energy and processors of
matter
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Ecosystems and Physical Laws
• The laws of physics and chemistry apply to
ecosystems
– Particularly in regard to the flow of energy
• Energy is conserved
– But degraded to heat during ecosystem
processes
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• Energy flows through an ecosystem
– Entering as light and exiting as heat
Tertiary
consumers
Microorganisms
and other
detritivores
Detritus
Secondary
consumers
Primary consumers
Primary producers
Heat
Key
Chemical cycling
Energy flow
Figure 54.2
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Sun
• Concept 54.2: Physical and chemical factors
limit primary production in ecosystems
• Primary production in an ecosystem
– Is the amount of light energy converted to
chemical energy by autotrophs during a given
time period
• The extent of photosynthetic production
– Sets the spending limit for the energy budget
of the entire ecosystem
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Gross and Net Primary Production
• Total primary production in an ecosystem
– Is known as that ecosystem’s gross primary
production (GPP)
• Not all of this production
– Is stored as organic material in the growing plants
• Net primary production (NPP)
– Is equal to GPP minus the energy used by the primary
producers for respiration
• Only NPP
– Is available to consumers
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Pyramids of Production
• This loss of energy with each transfer in a food chain
– Can be represented by a pyramid of net production
Tertiary
consumers
Secondary
consumers
Primary
consumers
Primary
producers
Figure 54.11
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10 J
100 J
1,000 J
10,000 J
1,000,000 J of sunlight
Pyramids of Numbers
• A pyramid of numbers
– Represents the number of individual
organisms in each trophic level
Trophic level
Tertiary consumers
Number of
individual organisms
3
Secondary consumers
354,904
Primary consumers
708,624
Primary producers
Figure 54.13
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5,842,424
• Worldwide agriculture could successfully feed
many more people
– If humans all fed more efficiently, eating only
plant material
Trophic level
Secondary
consumers
Primary
consumers
Primary
producers
Figure 54.14
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• In biological magnification
– Toxins concentrate at higher trophic levels
because at these levels biomass tends to be lower
Concentration of PCBs
Herring
gull eggs
124 ppm
Figure 54.23
Lake trout
4.83 ppm
Smelt
1.04 ppm
Zooplankton
0.123 ppm
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Phytoplankton
0.025 ppm