biological energy transfer | principles of biology from ...14) 24-biological energ… · 24...

contentsPrinciples of Biology

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24 Biological Energy Transfer

Cellular respiration involves the stepwise transfer of energy and electrons.

Large-scale municipal composting.The steam emitted from the compost as it is being turned is evidence of trillions of microorganismsperforming cellular respiration, breaking down molecules in the compost and generating energy, somein the form of heat.Nancy J. Pierce/Science Source.

Topics Covered in this Module How Do Organisms Obtain Energy?Redox ReactionsAn Outline of the Stages in Cellular Respiration

Major Objectives of this Module Describe the relationships among photosynthesis, respiration, producers, and consumers.Explain how reduction-oxidation (redox) reactions work.Describe how aerobic cellular respiration breaks down fuel molecules and releases energy for cellular work.



How Do Organisms Obtain Energy?No matter how wealthy you become in life, you will always work for a living.That is, your cells will always be working to keep you alive. Fromsynthesizing proteins to producing gametes to chasing down prey, living cellsare constantly at work, and all that work requires energy. Where does thatenergy come from? Ultimately it comes from our Sun as light energy.

Energy flows through all living systems. Plants, algae, and photosyntheticbacteria use energy from sunlight to generate sugar molecules through theprocess of photosynthesis. Such organisms, known as photoautotrophicproducers, convert the radiant energy of sunlight into chemical energy thatthey store in sugars and other organic compounds. Other organisms, termedheterotrophic consumers, must acquire the chemical energy they need byingesting or absorbing organic molecules from other organisms.

In both autotrophs and heterotrophs, cellular respiration is the process thatreleases energy through the breakdown of these food sources. This energyis then used to fuel cellular processes. A common misconception is thatproducers, such as plants, only conduct photosynthesis and need notperform cellular respiration. However, producers have to conduct respirationto break down the organic molecules they have produced (Figure 1), just asconsumers do in order to release chemical energy from the food theyconsume.

Carbohydrates, fats, and proteins can all be metabolized via cellularrespiration. Each of these large molecules is broken down into its smallercomponents, namely, sugars, smaller fats, and amino acids. These smallermolecules are fed into the cellular respiration process, where chemicalenergy is generated. Fats are particularly generous energy producers. Agram of fat yields more than twice as much ATP as a gram of carbohydrate.A gram of fat has 9 kcal or 38 kJ. (Nutritional calories, which are the valuesreported on food packages, are actually kilocalories or kcal.) Carbohydratesand protein each have 4 kcal per gram.

Figure 1: Overview of photosynthesis and aerobic respiration.

© 2013 Nature Education All rights reserved.

Photosynthetic organisms — plants, algae, and some bacteria — use lightenergy to generate organic molecules from carbon dioxide and water. Allorganisms, including photosynthetic organisms, perform some form ofcellular respiration to break down organic molecules. In aerobicrespiration, molecular oxygen is consumed and water and carbon dioxideare produced. Some of the energy released from the organic molecules istemporarily stored in ATP. The exergonic hydrolysis of ATP can then becoupled to endergonic reactions; some energy is always released as heat.

Energy flows into an ecosystem from the Sun and moves from producers toconsumers through a food web. We can follow the energy flow through anecosystem a bit more closely using an energy pyramid diagram, whichshows how much energy is present at different levels within a food web(Figure 2). Photosynthesizers (the plants in Figure 2) capture only a smallfraction of the energy from the solar radiation that strikes the Earth (usuallyless than 1%), which they convert into stored chemical energy. Onceproducers convert radiant energy into chemical energy, this energy becomesavailable to consumers. At the next level, primary consumers are herbivores,which can range in size from tiny worms and insects to massive elephantsand bison. Although each species converts consumed plant energy intouseful energy at a particular efficiency, 10% efficiency of transfer is a typicalrule of thumb. Likewise, only a small proportion of the energy tied up in

Figure 2: An idealized pyramid of energy flow from the Sun into ahypothetical terrestrial food web.


Typically, less than 1% of the solar radiation that is available to primaryproducers is converted into stored chemical energy throughphotosynthesis. After producers store chemical energy (given as 100%because it can vary among ecosystems), a rule of thumb is thatconsumers can convert ~10% of the energy from lower trophic levels totheir own stored energy (with the rest being lost to metabolism, waste, andother processes). Thus, primary consumers (herbivores) contain ~10% ofthe energy that was present in the producers they consumed; secondaryconsumers (predators) have 1% of the energy that was present in theconsumed producers; and tertiary consumers have 0.1% of the energythat was present in the consumed producers. Food webs generally end attertiary consumers because of the exponential loss of energy at eachhigher level.

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herbivores makes it up to the level of secondary consumers, which typicallyfeed on herbivores but are themselves often prey to larger, tertiary, or toplevel, consumers. In all, only the tiniest fraction of energy that enters a givenecosystem through photosynthesis at the base makes it up to these topconsumers (Figure 2).

Test Yourself

Calculate how many kilocalories of energy a tertiary consumer receives from a producer thattakes in 120 kilocalories. Assume for your calculations that 10% of the energy at each level ofthe food chain is passed on.

Catabolic processes break down fuel and release energy.So just how does the oatmeal you ate for breakfast become the energy youuse to do work after class? Organic molecules in food, such ascarbohydrates, fats and proteins, contain potential energy in the chemicalbonds between their atoms. Catabolic processes in the cell decomposelarge organic molecules into simpler products, releasing the energy held intheir chemical bonds. Most of this energy is transferred to and stored in the

Figure 3: Redox reaction ofmagnesium and oxygen.A reactant that donates electrons isoxidized, which results in a positivecharge. The reactant that accepts the

chemical bonds of adenosine triphosphate (ATP), the molecule that providesthe energy to drive the majority of metabolic processes in living organisms.

The 6-carbon sugar glucose (C6H12O6) is the most important fuel for cellularrespiration. The complete breakdown of glucose during aerobic cellularrespiration generates a large amount of ATP in a process that requiresoxygen to function. In some organisms, organic molecules can becatabolized without using oxygen. One such anaerobic process isfermentation, in which sugars are partially broken down to produce ATP.Other organisms use anaerobic respiration, in which sulfate or nitrate servethe role that oxygen does in cellular respiration.

The overall catabolism of glucose in aerobic cellular respiration can berepresented by the following chemical equation:

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O

∆G = -686 kcal/mol (-2870 kJ/mol)

The negative free energy change indicates that the reaction releases energy.However, if cellular respiration took place in a single step and all of theenergy was released at once, it would be a massive release of heat thatwould likely destroy the cell. To avoid self-destructing and to capture thisenergy, the cell breaks down glucose over many steps. In this manner, therelease of energy is controlled — more is captured and converted to ATP,and less is lost as heat.

Redox ReactionsDuring the cellular respiration reactions, what happens that enables theenergy of bond breaking to be captured and converted into the formation ofATP? Simply put, a series of chemical reactions moves electrons betweenmolecules. The transfer of electrons releases stored energy that can be usedto attach inorganic phosphate to ADP to form ATP. These electron-transferring chemical reactions are called reduction-oxidation reactions, orredox reactions for short. In a redox reaction, two simultaneous reactionsare occurring. One reactant donates electrons to another reactant. Theelectron donor becomes oxidized as it loses electrons. The other reactant,the electron acceptor, becomes reduced as it gains the electrons.

Redox reactions are very common in chemistry. For example, one type ofredox reaction occurs when elemental magnesium (Mg) and molecularoxygen (O2) combine to form magnesium oxide (MgO). In this reaction, twomagnesium atoms donate four electrons to the oxygen molecule (O2). As aresult, the magnesium atoms are oxidized into Mg2+ ions, each with a 2+charge. On the other hand, the oxygen molecule is reduced upon acceptingthe four electrons, resulting in two O2- ions. The oxidized Mg2+ ions andreduced O2- ions combine to form magnesium oxide (Figure 3).

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electrons is reduced, which results in anegative charge.

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Figure 4: Periodic table with electronegativities.

© 2011 Nature Education All rights reserved. Figure Detail

This periodic table includes the electronegativity of most of the elements.Notice that oxygen and fluorine have especially high electronegativities.The electronegativities of the noble gases are zero and are not shown.

Test Yourself

Determine which atom is oxidized and which is reduced in the following reaction: 2 Al + 3 Br2

→ Al2Br6

Electronegativity and redox reactions.What drives electron movement during redox reactions? Electrons tend to befound at different distances from nuclei in different atoms. Becausenegatively charged electrons are attracted to positively charged nuclei,separating electrons from nuclei requires an input of energy. As a result, thepotential energy of the electron increases with distance from the proton.Likewise, allowing an electron to move closer to a nucleus releases some ofits potential energy, which can be used to drive other chemical reactions.When bonds between atoms change during reactions, electrons can eithergain or release energy depending on how their position changes relative tonuclei.

Electronegativity is a measure of the tendency of an atom to attractelectrons. Because redox reactions move electrons closer to some atomsand farther away from others, differences in electronegativity allow us topredict the position of electrons in bonds and therefore the electrons' energystate. The periodic table shows electronegativity values for the elements(Figure 4).

Many redox reactions in the cell involve organic molecules such ascarbohydrates. In these molecules, carbon and hydrogen atoms tend tobecome oxidized, while oxygen atoms tend to become reduced. Why? In abond between atoms with a large difference in electronegativity, such as ahydroxyl group formed between oxygen and hydrogen, electrons tend tospend more time closer to the nucleus of the more electronegative oxygen

Figure 5: Electron position in different bonds.

© 2011 Nature Education All rights reserved. Figure Detail

In C–O and H–O bonds, the oxygen atom attracts electrons closer to itselfbecause of its high electronegativity.

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atom (Figure 5).

Now compare the electronegativity values of oxygen, carbon and hydrogen.Because oxygen has a higher electronegativity (3.5) than either carbon (2.5)or hydrogen (2.1), we predict that electrons will tend to be closer to theoxygen atom in either an O–C bond or an O–H bond. Thus, reactions thatbreak bonds between hydrogen and carbon and create bonds with oxygenrelease energy because electrons are being allowed to move to a position oflower potential energy — closer to the more electronegative oxygen atom.The released energy can then be used to drive other chemical reactions inthe cell.

Test Yourself

Compare the electronegativity values of oxygen, carbon and sulfur. A chemical reactionbreaks a C–S bond in an organic molecule and forms a C–O bond in its place. What is theenergetic outcome of this reaction? Explain.

Electrons and energy can be transferred through mobile carriers.Now let's apply the principles of redox reactions to cellular respiration. Recallthe overall chemical equation for the catabolism of glucose during aerobiccellular respiration:

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O

∆G = -686 kcal/mol (-2870 kJ/mol)

During cellular respiration, energy is released through a series of redoxreactions, instead of all at once as in direct combustion. As electrons movefrom higher energy states (i.e., farther from protons in a nucleus) to lowerenergy states (i.e., closer to protons in a nucleus), energy is released.Through a series of steps in aerobic respiration, glucose is oxidized intocarbon dioxide, and oxygen is reduced into water. Because electrons are nottransferred directly from glucose to molecular oxygen in aerobic respiration,electron carriers are required during the intermediate steps.

Nicotinamide adenine dinucleotide (NAD+) is the most versatile electroncarrier in aerobic cellular respiration (Figure 6). In an early step of cellularrespiration, enzymes remove two hydrogen atoms — equivalent to twoprotons and two electrons — from an intermediate of glucose breakdown.One of the hydrogen atoms is released as a H+, while the remaininghydrogen atom and the two electrons are transferred to NAD+, reducing it toNADH. Additional molecules of NADH are formed in later steps. The NADHmolecules carry both electrons and energy to the final steps of cellularrespiration (Figure 7). In aerobic respiration, molecular oxygen ultimatelyaccepts the electrons, and some of the energy can be used to synthesizeATP.

Figure 6: NAD+ moves electrons.


Nicotinamide adenine dinucleotide (NAD+) has two nucleotides. One ofthem has an adenine base, and the other has a nicotinamide group. Thetwo nucleotides are connected by a diphosphate bridge. In simple terms,nicotinamide adenine dinucleotide can exist in its oxidized form (NAD+) orin its reduced form (NADH). By alternating between these forms,NAD+/NADH functions as an electron and energy carrier within the cell.

Figure 7: NAD+/NADH functions as an energy and electron carrier incellular respiration.


NAD+ switches back and forth between its oxidized form and its reducedform, NADH, as it carries electrons and energy between phases of cellularrespiration.

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Test Yourself

Flavin adenine dinucleotide (FAD) is another electron acceptor used in cellular respiration. Thereduced form of FAD is FADH2. What does FAD accept in the redox reaction that transforms itto FADH2? How does this differ from NAD+/NADH?

How Do Organisms Obtain Energy?

Redox Reactions

An Outline of the Stages in CellularRespiration

Summary

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Figure 8: The major features of aerobic cellular respiration.


Glucose, which originates from photosynthesis or consumption of otherorganisms, enters glycolysis. Through a series of coupled reactions,glucose is converted into smaller organic molecules. A small amount ofATP is formed during glycolysis. The final product of glycolysis enters thecitric acid cycle, where a number of redox reactions move electrons to theelectron carriers NADH and FADH2. Some CO2 is generated as well. Theelectron carriers feed their electrons to molecules in the oxidativephosphorylation process, which eventually transfers them to the terminalelectron acceptor, O2. In the process of moving electrons, a large amountof ATP is synthesized from ADP.

An Outline of the Stages in Cellular RespirationThe major stages of aerobic cellular respiration are glycolysis, the citric acidcycle and oxidative phosphorylation (Figure 8).

Cellular respiration is composed of a series of reactions, many of them redoxreactions, that release energy in steps as organic molecules (such asglucose) are broken down. Glucose enters cellular respiration in glycolysis,where it is broken into smaller molecules that feed into the next stage, thecitric acid cycle. Glycolysis itself does not require O2 to function. Indeed, inthe absence of O2, the end product of glycolysis can be converted to otherorganic molecules in a process known as fermentation, which is used bymany microorganisms and by athletes during strenuous prolonged exercise.Small amounts of ATP are produced during glycolysis, but it and the citricacid cycle mainly function to transfer electrons to NAD+ and flavin adeninedinucleotide (FAD) as part of several separate redox reactions. Thereduced forms of these electron carriers, NADH and FADH2, give up theirelectrons to oxidative phosphorylation and ultimately to O2, producinglarge amounts of ATP in the process.



OBJECTIVE Describe the relationships among photosynthesis,respiration, producers, and consumers.

Photosynthetic producers convert light energy from the Sun into chemicalenergy to produce their own organic molecules. Producers then use cellularrespiration to release energy from the chemical bonds in the organicmolecules they have produced. Heterotrophic consumers acquire or absorborganic molecules from other organisms and use cellular respiration torelease energy from the chemical bonds in those food molecules.

OBJECTIVE Explain how reduction-oxidation (redox) reactions work.One reactant donates electrons to the other reactant. The electron donorbecomes oxidized as it loses electrons. The electron acceptor becomesreduced as it gains electrons. Electrons move from donor to acceptor basedon the electronegativity of each molecule in the redox pair. Theelectronegativity of O is stronger than almost every other atom, meaning thatO attracts electrons strongly, holding them in orbit relatively close to theatomic nuclei. When electrons are transferred from the breaking of a carbon-carbon or a carbon-hydrogen bond to a bond containing oxygen, theelectrons release energy because they are in a state of lower potentialenergy.

OBJECTIVE Describe how aerobic cellular respiration breaks down fuelmolecules and releases energy for cellular work.

Cellular respiration releases energy through a series of redox reactions thatrelease energy in steps and capture it in ATP. These reactions moveelectrons from organic molecules (such as glucose) to O2. The major parts ofcellular respiration are glycolysis, the citric acid cycle and oxidativephosphorylation. Small amounts of ATP are produced during glycolysis, but itand the citric acid cycle mainly function to transfer electrons to NAD+ andFAD. The reduced forms of these electron carriers, NADH and FADH2, giveup their electrons to oxidative phosphorylation, where large amounts of ATPare produced.

aerobic respirationRespiration pathway requiring oxygen.

anaerobic respirationCell respiration in which oxygen is not the final electron acceptor.

catabolic processChemical reaction that breaks down complex molecules and releases energy.

cellular respirationProcess by which energy is released from the breakdown of organic molecules.

citric acid cycleSecond stage of aerobic respiration; complete oxidation of glucose to CO2generates NADH and FADH2 for use in oxidative phosphorylation.

electronegativityA measure of the tendency of an atom to attract electrons.

flavin adenine dinucleotide (FAD)Molecule that functions as an energy and electron carrier in the cell. Adinucleotide with an attached flavin group. Occurs in both oxidized (FAD) and

Summary

Key Terms

reduced (FADH2) forms.

food webA set of interrelated food chains in an ecosystem.

glycolysisThe first stage of cellular respiration, in which carbohydrates (such as glucose) arebroken down into smaller molecules that are either converted to end-stage organicmolecules by fermentation or fed into the citric acid cycle.

heterotrophic consumerAn organism that feeds on producers or other consumers; cannot make its ownorganic molecules.

nicotinamide adenine dinucleotide (NAD+)One of the electron carriers in cellular respiration.

oxidative phosphorylationControlled electron flow is used to synthesize ATP. O2 is the terminal electronacceptor and is reduced to H2O.

photoautotrophic producerAn organism that carries out photosynthesis and uses the resulting organicmolecules as fuel for its cellular respiration.

photosynthesisUse of light energy to convert carbon dioxide and water into more complex organicmolecules.

redox reactionsIn the reaction, one participant is oxidized, the other reduced.


Redox Reactions


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PRIMARY LITERATURE









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1.

FADADPNAD+

hydrogenoxygen

What is the ultimate electron acceptor in aerobic cellular respiration?

2.

phototrophs.heterotrophs.producers.phototrophs, heterotrophs and producers.both producers and phototrophs.

Complete the following sentence: Cellular respiration is carried out by...

3.

respirationphosphorylationreductionoxidationfermentation

Which kind of reaction converts NAD+ to NADH?

4.

carbohydratesfatsproteinsnucleic acidsAll of these molecules contain an equal amount of energy per gram.

Which class of food molecules contains the most energy per gram?

5.

hydrogencarbonnitrogenoxygensulfur

Which atom below is the most electronegative?

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Redox Reactions


Summary

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