Pathways that Harvest and Store Chemical Energylindblomeagles.org/ourpages/auto/2013/10/24...pathways. Chemical energy available to do work is termed free energy (G). Concept 6.1 ATP,

Pathways that Harvest and Store Chemical Energy

Chapter 6 Pathways that Harvest and Store Chemical Energy

Key Concepts

• 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism

• 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy

• 6.3 Carbohydrate Catabolism in the Absence of Oxygen Releases a Small Amount of Energy

Chapter 6 Pathways that Harvest and Store Chemical Energy

• 6.4 Catabolic and Anabolic Pathways Are Integrated

• 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy

• 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO2 to Carbohydrates

Chapter 6 Opening Question

Why does fresh air inhibit the formation of alcohol by yeast cells?

Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism

Energy is stored in chemical bonds and can be released and transformed by metabolic pathways.

Chemical energy available to do work is termed free energy (G).


Five principles governing metabolic pathways:

1. Chemical transformations occur in a series of intermediate reactions that form a metabolic pathway.

2. Each reaction is catalyzed by a specific enzyme.

3. Most metabolic pathways are similar in all organisms.


4. In eukaryotes, many metabolic pathways occur inside specific organelles.

5. Each metabolic pathway is controlled by enzymes that can be inhibited or activated.


In cells, energy-transforming reactions are often coupled:

An energy-releasing (exergonic) reaction is coupled to an energy-requiring (endergonic) reaction.


Adenosine triphosphate (ATP) is a kind of “energy currency” in cells.

Energy released by exergonic reactions is stored in the bonds of ATP.

When ATP is hydrolyzed, free energy is released to drive endergonic reactions.

Figure 6.1 The Concept of Coupling Reactions

Figure 6.2 ATP


Hydrolysis of ATP is exergonic:

ΔG is about –7.3 kcal

energyfreePADPOHATP i 2


Free energy of the bond between phosphate groups is much higher than the energy of the O—H bond that forms after hydrolysis.

text art pg 102 here(1st one, in left-hand column)


Phosphate groups are negatively charged, so energy is required to get them near enough to each other to make the covalent bonds in the ATP molecule.

ATP can be formed by substrate-level phosphorylation or oxidative phosphorylation.


Energy can also be transferred by the transfer of electrons in oxidation–reduction, or redox reactions.

• Reduction is the gain of one or more electrons.

• Oxidation is the loss of one or more electrons.


Oxidation and reduction always occur together.


Transfers of hydrogen atoms involve transfers of electrons (H = H+ + e–).

When a molecule loses a hydrogen atom, it becomes oxidized.


The more reduced a molecule is, the more energy is stored in its bonds.

Energy is transferred in a redox reaction.

Energy in the reducing agent is transferred to the reduced product.

Figure 6.3 Oxidation, Reduction, and Energy


Coenzyme NAD+ is a key electron carrier in redox reactions.

NAD+ (oxidized form)

NADH (reduced form)

Figure 6.4 A NAD+/NADH Is an Electron Carrier in Redox Reactions


Reduction of NAD+ is highly endergonic:

Oxidation of NADH is highly exergonic:

NADHeHNAD 2

OHNADOHNADH 2221

Figure 6.4 B NAD+/NADH Is an Electron Carrier in Redox Reactions


• In cells, energy is released in catabolism by oxidation and trapped by reduction of coenzymes such as NADH.

• Energy for anabolic processes is supplied by ATP.

Oxidative phosphorylation transfers energy from NADH to ATP.


Oxidative phosphorylation couples oxidation of NADH:

with production of ATP:

energyeHNADNADH 2

ATPPADPenergy i


The coupling is chemiosmosis—diffusion of protons across a membrane, which drives the synthesis of ATP.

Chemiosmosis converts potential energy of a proton gradient across a membrane into the chemical energy in ATP.

Figure 6.5 A Chemiosmosis


ATP synthase—membrane protein with two subunits:

F0 is the H+ channel; potential energy of the proton gradient drives the H+ through.

F1 has active sites for ATP synthesis.

Figure 6.5 B Chemiosmosis


Chemiosmosis can be demonstrated experimentally.

A proton gradient can be introduced artificially in chloroplasts or mitochondria in a test tube.

ATP is synthesized if ATP synthase, ADP, and inorganic phosphate are present.

Figure 6.6 An Experiment Demonstrates the Chemiosmotic Mechanism


Cellular respiration is a major catabolic pathway. Glucose is oxidized:

Photosynthesis is a major anabolic pathway. Light energy is converted to chemical energy:

tecarbohydraOenergylightOHCO 222 666

energychemicalOHCOOtecarbohydra 222 666

Figure 6.7 ATP, Reduced Coenzymes, and Metabolism

Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy

Cellular Respiration

A lot of energy is released when reduced molecules with many C—C and C—H bonds are fully oxidized to CO2 .

Oxidation occurs in a series of small steps in three pathways:

1. glycolysis

2. pyruvate oxidation

3. citric acid cycle

Figure 6.8 Energy Metabolism Occurs in Small Steps

Figure 6.9 Energy-Releasing Metabolic Pathways


Glycolysis: ten reactions.

Takes place in the cytosol.

Final products:

2 molecules of pyruvate (pyruvic acid)2 molecules of ATP 2 molecules of NADH

Figure 6.10 Glycolysis Converts Glucose into Pyruvate (Part 1)




Examples of reaction types common in metabolic pathways:Step 6: Oxidation–reductionStep 7: Substrate-level phosphorylation

text art pg 107 here


Pyruvate Oxidation:

Products: CO2 and acetate; acetate is then bound to coenzyme A (CoA)


Citric Acid Cycle: 8 reactions, operates twice for every glucose molecule that enters glycolysis.

Starts with Acetyl CoA; acetyl group is oxidized to two CO2 .

Oxaloacetate is regenerated in the last step.

Figure 6.11 The Citric Acid Cycle


Final reaction of citric acid cycle:


Electron transport/ATP Synthesis:

NADH is reoxidized to NAD+ and O2 is reduced to H2 O in a series of steps.

Respiratory chain—series of redox carrier proteins embedded in the inner mitochondrial membrane.

Electron transport—electrons from the oxidation of NADH and FADH2 pass from one carrier to the next in the chain.

Figure 6.12 Electron Transport and ATP Synthesis in Mitochondria


The oxidation reactions are exergonic; the energy is used to actively transport H+ ions out of the mitochondrial matrix, setting up a proton gradient.

ATP synthase in the membrane uses the H+

gradient to synthesize ATP by chemiosmosis.


About 32 molecules of ATP are produced for each fully oxidized glucose.

The role of O2 : most of the ATP produced is formed by oxidative phosphorylation, which is due to the reoxidation of NADH.

Concept 6.3 Carbohydrate Catabolism in the Absence of Oxygen Releases a Small Amount of Energy

Under anaerobic conditions, NADH is reoxidized by fermentation.

There are many different types of fermentation, but all operate to regenerate NAD+.

The overall yield of ATP is only two—the ATP made in glycolysis.


Lactic acid fermentation:

End product is lactic acid (lactate).

NADH is used to reduce pyruvate to lactic acid, thus regenerating NAD+.

Figure 6.13 A Fermentation


Alcoholic fermentation:

End product is ethyl alcohol (ethanol).

Pyruvate is converted to acetaldehyde, and CO2 is released. NADH is used to reduce acetaldehyde to ethanol, regenerating NAD+ for glycolysis.

Figure 6.13 B Fermentation

Concept 6.4 Catabolic and Anabolic Pathways Are Integrated

Metabolic pathways are linked.

Carbon skeletons (molecules with covalently linked carbon atoms) can enter catabolic or anabolic pathways.

Figure 6.14 Relationships among the Major Metabolic Pathways of the Cell


Catabolism:

Polysaccharides are hydrolyzed to glucose, which enter glycolysis.

Lipids break down to fatty acids and glycerol. Fatty acids can be converted to acetyl CoA.

Proteins are hydrolyzed to amino acids that can feed into glycolysis or the citric acid cycle.


Anabolism:

Many catabolic pathways can operate in reverse.

Gluconeogenesis—citric acid cycle and glycolysis intermediates can be reduced to form glucose.

Acetyl CoA can be used to form fatty acids.

Some citric acid intermediates can form nucleic acids.


Amounts of different molecules are maintained at fairly constant levels—the metabolic pools.

This is accomplished by regulation of enzymes— allosteric regulation, feedback inhibition.

Enzymes can also be regulated by altering the transcription of genes that encode the enzymes.

Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy

Photosynthesis involves two pathways:

Light reactions convert light energy into chemical energy (in ATP and the reduced electron carrier NADPH).

Carbon-fixation reactions use the ATP and NADPH, along with CO2 , to produce carbohydrates.

Figure 6.15 An Overview of Photosynthesis


Light is a form of electromagnetic radiation, which travels as a wave but also behaves as particles (photons).

Photons can be absorbed by a molecule, adding energy to the molecule—it moves to an excited state.

Figure 6.16 The Electromagnetic Spectrum


Pigments: molecules that absorb wavelengths in the visible spectrum.

Chlorophyll absorbs blue and red light; the remaining light is mostly green.

Absorption spectrum—plot of light energy absorbed against wavelength.

Action spectrum—plot of the biological activity of an organism against the wavelengths to which it is exposed

Figure 6.17 Absorption and Action Spectra


In plants, two chlorophylls absorb light energy chlorophyll a and chlorophyll b.

Accessory pigments—absorb wavelengths between red and blue and transfer some of that energy to the chlorophylls.

Figure 6.18 The Molecular Structure of Chlorophyll (Part 1)

Figure 6.18 The Molecular Structure of Chlorophyll (Part 2)


The pigments are arranged into light- harvesting complexes, or antenna systems.

A photosystem spans the thylakoid membrane in the chloroplast; it consists of antenna systems and a reaction center.


When chlorophyll (Chl) absorbs light, it enters an excited state (Chl*), then rapidly returns to ground state, releasing an excited electron.

Chl* gives the excited electron to an acceptor and becomes oxidized to Chl+.

The acceptor molecule is reduced.

acceptorChlacceptorChl *


The electron acceptor is first in an electron transport system in the thylakoid membrane.

Final electron acceptor is NADP+, which gets reduced:

ATP is produced chemiosmotically during electron transport (photophosphorylation).

NADPHeHNADP 2

Figure 6.19 Noncyclic Electron Transport Uses Two Photosystems


Two photosystems:

• Photosystem I absorbs light energy at 700 nm, passes an excited electron to NADP+, reducing it to NADPH.

• Photosystem II absorbs light energy at 680 nm, produces ATP, and oxidizes water molecules.


Photosystem II

When Chl* gives up an electron, it is unstable and grabs an electron from another molecule, H2 O, which splits the H—O—H bonds.

221

2 2*2 OHChlOHChl


Photosystem I

When Chl* gives up an electron, it grabs another electron from the end of the transport system of Photosystem II. This electron ends up reducing NADP+ to NADPH.


ATP is needed for carbon-fixation pathways.

Cyclic electron transport uses only photosystem I and produces ATP; an electron is passed from an excited chlorophyll and recycles back to the same chlorophyll.

Figure 6.20 Cyclic Electron Transport Traps Light Energy as ATP

Concept 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO2 to Carbohydrates

The Calvin cycle: CO2 fixation. It occurs in the stroma of the chloroplast.

Each reaction is catalyzed by a specific enzyme.

Figure 6.21 The Calvin Cycle


1. Fixation of CO2 :

CO2 is added to ribulose 1,5-bisphosphate (RuBP).

Ribulose bisphosphate carboxylase/oxygenase (rubisco) catalyzes the reaction.

A 6-carbon molecule results, which quickly breaks into two 3-carbon molecules: 3- phosphoglycerate (3PG).

Figure 6.22 RuBP Is the Carbon Dioxide Acceptor


2. 3PG is reduced to form glyceraldehyde 3- phosphate (G3P).


3. The CO2 acceptor, RuBP, is regenerated from G3P.

Some of the extra G3P is exported to the cytosol and is converted to hexoses (glucose and fructose).

When glucose accumulates, it is linked to form starch, a storage carbohydrate.


The C—H bonds generated by the Calvin cycle provide almost all the energy for life on Earth.

Photosynthetic organisms (autotrophs) use most of this energy to support their own growth and reproduction.

Heterotrophs cannot photosynthesize and depend on autotrophs for chemical energy.

Answer to Opening Question

Pasteur’s findings:

Catabolism of the beet sugar is a cellular process, so living yeast cells must be present.

With air (O2 ) yeasts used aerobic metabolism to fully oxidize glucose to CO2 .

Without air, yeasts used alcoholic fermentation, producing ethanol, less CO2 , and less energy (slower growth).

Figure 6.23 Products of Glucose Metabolism

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Pathways that Harvest and Store Chemical Energylindblomeagles.org/ourpages/auto/2013/10/24...pathways. Chemical energy available to do work is termed free energy (G). Concept 6.1 ATP,