Energy and Energy transfer

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

ENERGY AND ENERGY TRANSFER

An Introduction to Metabolism


Overview• Metabolism

• Exothermic/Endothermic reactions

• ATP

• Energy pyramids and ecosystems


• Overview: The Energy of Life

• The living cell

– Is a miniature factory where thousands of reactions occur

– Converts energy in many ways


• Metabolism

– Is the totality of an organism’s chemical reactions

– Arises from interactions between molecules


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 Enzyme 2 Enzyme 3A B C D

Reaction 1 Reaction 2 Reaction 3Starting

moleculeProduct


• Catabolic pathways

– Break down complex molecules into simpler compounds

– Release energy

• Anabolic pathways

– Build complicated molecules from simpler ones

– Consume energy


• Thermodynamics

– Is the study of energy transformations

An organism’s metabolism transforms matter and energy, subject to the laws of thermodynamics


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


• Concept 8.2: The free-energy change of a reaction tells us whether the reaction occurs spontaneously


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


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


Exergonic and Endergonic Reactions in Metabolism

• An exergonic reaction

– Proceeds with a net release of free energy and is spontaneous

Figure 8.6

Reactants

ProductsEnergy

Progress of the reaction

Amount ofenergyreleased (∆G <0)

Free

ene

rgy

(a) Exergonic reaction: energy released


• An endergonic reaction

– Is one that absorbs free energy from its surroundings and is nonspontaneous

Figure 8.6

Energy

Products

Amount ofenergyreleased (∆G>0)

Reactants


Free

ene

rgy

(b) Endergonic reaction: energy required


Equilibrium and Metabolism• Reactions in a closed system

– Eventually reach equilibrium

Figure 8.7 A

(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.

∆G < 0 ∆G = 0


• Cells in our body

– Experience a constant flow of materials in and out, preventing metabolic pathways from reaching equilibrium

Figure 8.7

(b) An open hydroelectric system. Flowing water keeps driving the generator because intake and outflow of water keep the system from reaching equlibrium.

∆G < 0


• An analogy for cellular respiration

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.

∆G < 0∆G < 0

∆G < 0


• 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


The Structure and Hydrolysis of ATP• ATP (adenosine triphosphate)

– Is the cell’s energy shuttle

– Provides energy for cellular functions

Figure 8.8

O O O O CH2

H

OH OH

H

N

H H

ON C

HC

N CC

N

NH2Adenine

RibosePhosphate groups

O

O O

O

O

O

-- - -

CH


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


• Energy is released from ATP

– When the terminal phosphate bond is broken

Figure 8.9

P

Adenosine triphosphate (ATP)

H2O

+ Energy

Inorganic phosphate Adenosine diphosphate (ADP)

PP

P PP i


• The three types of cellular work– Are powered by the hydrolysis of ATP

(c) Chemical work: ATP phosphorylates key reactants

P

Membraneprotein

Motor protein

P i

Protein moved(a) Mechanical work: ATP phosphorylates motor proteins

ATP

(b) Transport work: ATP phosphorylates transport proteinsSolute

P P i

transportedSolute

Glu GluNH3

NH2P i

P i

+ +

Reactants: Glutamic acid and ammonia

Product (glutamine)made

ADP+

P

Figure 8.11


The Regeneration of ATP• Catabolic pathways

– Drive the regeneration of ATP from ADP and phosphate

ATP synthesis from ADP + P i requires energy

ATP

ADP + P i

Energy for cellular work(endergonic, energy-consuming processes)

Energy from catabolism(exergonic, energy yieldingprocesses)

ATP hydrolysis to ADP + P i yields energy

Figure 8.12


• 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


• An enzyme

– Is a catalytic protein


• 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


• The effect of enzymes on reaction rate


Products

Course of reaction without enzyme

Reactants

Course of reaction with enzyme

EA

withoutenzyme

EA with enzymeis lower

∆G is unaffected by enzymeFr

ee e

nerg

y

Figure 8.15


• The active site can lower an EA barrier by

– Orienting substrates correctly

– Straining substrate bonds

– Providing a favorable microenvironment

– Covalently bonding to the substrate


• Concept 8.5: Regulation of enzyme activity helps control metabolism

• A cell’s metabolic pathways

– Must be tightly regulated


• Energy

– Flows into an ecosystem as sunlight and leaves as heat

Light energy

ECOSYSTEM

CO2 + H2O

Photosynthesisin chloroplasts

Cellular respiration

in mitochondria

Organicmolecules+ O2

ATP

powers most cellular work

HeatenergyFigure 9.2


• Some redox reactions

– Do not completely exchange electrons

– Change the degree of electron sharing in covalent bonds

CH4

H

H

HH C O O O O OC

H H

Methane(reducingagent)

Oxygen(oxidizingagent)

Carbon dioxide Water

+ 2O2 CO2 + Energy + 2 H2O

becomes oxidized

becomes reduced

Reactants Products

Figure 9.3


Oxidation of Organic Fuel Molecules During Cellular Respiration

• During cellular respiration

– Glucose is oxidized in a series of steps and oxygen is reduced

C6H12O6 + 6O2 6CO2 + 6H2O + Energy

becomes oxidized

becomes reduced


• Electrons from organic compounds

– Are usually first transferred to NAD+, a coenzyme

NAD+H

O

OO O

–OO O

–O

O

O

P

P

CH2

CH2

HO OHH

HHO OH

HO

H

H

N+

C NH2

HN

H

NH2

N

N

Nicotinamide(oxidized form)

NH2+ 2[H](from food)

DehydrogenaseReduction of

NAD+Oxidation of NADH

2 e– + 2 H+2 e– + H+

NADHOH H

NC +

Nicotinamide(reduced form)

N

Figure 9.4


• 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


• There are three main processes in this metabolic enterprise

Electron shuttlesspan membrane

CYTOSOL 2 NADH

2 FADH2

2 NADH 6 NADH 2 FADH22 NADH

Glycolysis

Glucose2

Pyruvate

2AcetylCoA

Citricacidcycle

Oxidativephosphorylation:electron transport

andchemiosmosis

MITOCHONDRION

by substrate-levelphosphorylation

by substrate-levelphosphorylation

by oxidative phosphorylation, dependingon which shuttle transports electronsfrom NADH in cytosol

Maximum per glucose:About

36 or 38 ATP

+ 2 ATP + 2 ATP + about 32 or 34 ATP

or

Figure 9.16


Excitation of Chlorophyll by Light• When a pigment absorbs light

– It goes from a ground state to an excited state, which is unstable

Excitedstate

Ene

rgy

of e

lect

ion

Heat

Photon(fluorescence)

Chlorophyllmolecule

GroundstatePhoton

e–

Figure 10.11 A


• Produces NADPH, ATP, and oxygen

Figure 10.13 Photosystem II(PS II)

Photosystem-I(PS I)

ATP

NADPH

NADP+

ADP

CALVINCYCLE

CO2H2O

O2 [CH2O] (sugar)

LIGHTREACTIONS

Light

Primaryacceptor

Pq

Cytochromecomplex

PC

e

P680

e–

e–

O2

+

H2O2 H+

Light

ATP

Primaryacceptor

Fd

ee–

NADP+

reductase

ElectronTransportchain

Electron transport chain

P700

Light

NADPH

NADP+

+ 2 H+

+ H+

1

5

7

2

3

4

6

8


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


• 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


• The Calvin cycle

(G3P)

Input(Entering one

at a time)CO2

3

Rubisco

Short-livedintermediate

3 P P

3 P PRibulose bisphosphate

(RuBP)

P

3-Phosphoglycerate

P6 P

6

1,3-Bisphoglycerate6 NADPH

6 NADPH+

6 P

P6

Glyceraldehyde-3-phosphate(G3P)

6 ATP

3 ATP

3 ADP CALVINCYCLE

P5

P1G3P

(a sugar)Output

LightH2O CO2

LIGHTREACTION

ATP

NADPH

NADP+

ADP

[CH2O] (sugar)

CALVINCYCLE

Figure 10.18

O2

6 ADP

Glucose andother organiccompounds

Phase 1: Carbon fixation

Phase 2:Reduction

Phase 3:Regeneration ofthe CO2 acceptor(RuBP)


• 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


• Concept 54.1: Ecosystem ecology emphasizes energy flow and chemical cycling

• Ecosystem ecologists view ecosystems

– As transformers of energy and processors of matter


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


• Energy flows through an ecosystem

– Entering as light and exiting as heat

Figure 54.2

Microorganismsand other

detritivores

Detritus

Primary producers

Primary consumers

Secondaryconsumers

Tertiary consumers

Heat

Sun

Key

Chemical cyclingEnergy flow


• 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


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


Pyramids of Production• This loss of energy with each transfer in a food chain

– Can be represented by a pyramid of net production

Figure 54.11

Tertiaryconsumers

Secondaryconsumers

Primaryconsumers

Primaryproducers

1,000,000 J of sunlight

10 J

100 J

1,000 J

10,000 J


Pyramids of Numbers• A pyramid of numbers

– Represents the number of individual organisms in each trophic level

Figure 54.13

Trophic level Number of individual organisms

Primary producers

Tertiary consumers

Secondary consumers

Primary consumers

3

354,904

708,624

5,842,424


• Worldwide agriculture could successfully feed many more people

– If humans all fed more efficiently, eating only plant material

Figure 54.14

Trophic level

Secondaryconsumers

Primaryconsumers

Primaryproducers


• In biological magnification– Toxins concentrate at higher trophic levels

because at these levels biomass tends to be lower

Figure 54.23

Con

cent

ratio

n of

PC

Bs

Herringgull eggs124 ppm

Zooplankton 0.123 ppm

Phytoplankton 0.025 ppm

Lake trout 4.83 ppm

Smelt 1.04 ppm

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