Concept 9.1: Catabolic pathways yield energy by oxidizing organic fuels

Question 1

Metabolic pathways that release stored energy by breaking down complex molecules are called

Answer 1

catabolic pathway

Question 2

Organic compounds possess potential energy as a result of the arrangement of electrons in the bonds between their

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atoms

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Compounds that can participate in exergonic reactions can act as

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fuels.

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is a partial degradation of sugars or other organic fuel that occurs without the use of oxygen.

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fermentation

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most efficient catabolic pathway is ____________________ in which oxygen is consumed as a reactant along with the organic fuel

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aerobic respiration

Question 6

The cells of most eukaryotic and many prokaryotic organisms can carry out

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aerobic respiration

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Some prokaryotes use substances other than oxygen as reactants in a similar process that harvests chemical energy without oxygen; this process is called

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anaerobic respiration

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includes both aerobic and anaerobic processes.

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cellular respiration

Question 9

cellular respiration is often used to refer to the

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aerobic process

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Carbohydrates, fats, and protein molecules from food can all be processed and consumed as

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fuel

Question 11

In animal diets, a major source of carbohydrates is starch, a storage polysaccharide that can be broken down into

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glucose subunits.

Question 12

This breakdown of glucose is exergonic, having a free-energy change of

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-686 kcal (2,870 kJ) per mole of glucose decomposed (∆G= -686 kcal/mol)

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Recall that a negative ∆G ( ∆G<0) indicates that the products of the chemical process store less energy than the reactants and that the reaction can happen

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spontaneously—in other words, without an input of energy.

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Catabolic pathways do not directly move

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flagella, pump solutes across membranes, polymerize monomers, or perform other cellular work.

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to keep working, the cell must regenerate its supply of

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ATP from ADP and ℗

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The relocation of electrons releases energy stored in organic molecules, and this energy ultimately is used to

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synthesize ATP.

Question 17

In many chemical reactions, there is a transfer of one or more electrons (eˉ) from

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one reactant to another.

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These electron transfers are called

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oxidation-reduction reactions, or redox reactions

Question 19

In a redox reaction, the loss of electrons from one substance is called

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oxidation

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the addition of electrons to another substance is known as

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reduction. (Note that adding electrons is called reduction; adding negatively charged electrons to an atom reduces the amount of positive charge of that atom.)

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in the generalized reaction, substance Xeˉ, the electron donor, is called the

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reducing agent; it reduces Y, which accepts the donated electron.

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Substance Y, the electron acceptor, is the

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oxidizing agent; it oxidizes Xeˉ by removing its electron.

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Because an electron transfer requires both an electron donor and an acceptor, oxidation and reduction always

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go hand in hand.

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Not all redox reactions involve the complete transfer of electrons from one substance to another; some change

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the degree of electron sharing in covalent bonds.

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a reduction-oxidation (redox) reaction always involves two events:

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one substance loses electron and is said to be oxidized, while another substance gains electrons and is said to be reduced

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sodium chloride, or table salt offers an example of

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atoms that have undergone oxidation and reduction to become ions

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when sodium and chlorine react to form sodium chloride, one electron is completely transferred from the

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sodium atom to the chlorine atom, the result is a sodium cation and a chloride anion

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in this redox reaction, the sodium atom lost an electron- it was

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oxidized

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the new sodium ion is a

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positively charged cation

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the chlorine atom gained an electron- it was

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reduced

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the new chloride ion is a

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negatively charged anion

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in redox reactions, electrons can be transferred completely from one molecule or atom to another forming an

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ionic bond, or they can simply shift positions in covalent bonds.

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the combustion of methane is a good example of this second type of reaction. The electrons in the two reactants- methane and oxygen are shown as dots at equal distance from the atoms nuclei, indicating that

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electrons are shared equally

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when electrons are shared equally the bonds are

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nonpolar

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methane combustion involves the reaction of

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two oxygen molecules with each methane molecule

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the reaction yields

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carbon dioxide and water as the products

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the atoms in the products share electrons

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unequally. meaning that the bonds are polar

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in carbon dioxide, the nucleus of the

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carbon atom holds the electrons less tightly

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carbon has been

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oxidized- it lost electrons

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in water, the nucleus of the atom holds the electrons

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more tightly

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oxygen has been

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reduced- it gained electrons

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because the atomic nuclei of the product molecules hold the electrons more tightly than they were held in the reactant molecules the product have

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lower potential energy

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such reactions tend to be

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spontaneous, or exergonic, and release free energy

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The two atoms of the oxygen molecule share their electrons

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equally

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when oxygen reacts with the hydrogen from methane, forming water, the electrons of the covalent bonds spend more time near the

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oxygen

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each oxygen atom has partially “gained” electrons, so the oxygen molecule has been

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reduced.

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The more electronegative the atom (the stronger its pull on electrons), the more energy is required to take an

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electron away from it

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An electron loses potential energy when it shifts from a less electronegative atom toward a more

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electronegative one

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learn summary equation for cellular respiration

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look under oxidation of organic fuel molecules during cellular respiration

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organic molecules that have an abundance of hydrogen are excellent fuels because their bonds are a source of ____________________whose energy may be released as these electrons ______ down an energy gradient during their transfer to oxygen.

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“hilltop” electrons, “fall”

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energy state of the electron changes as hydrogen (with its electron) is transferred to

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oxygen.

Question 52

In respiration, the oxidation of glucose transfers electrons to a lower energy state, liberating energy that becomes available for

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ATP synthesis.

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we see fuels with multiple C—H bonds oxidized into products with multiple

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C—O bonds.

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The main energy-yielding foods—carbohydrates and fats—are reservoirs of electrons associated with

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hydrogen, often in the form of C—H bonds

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Only the barrier of activation energy holds back the

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flood of electrons to a lower energy state

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glucose is broken down in a series of steps, each one catalyzed by an

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enzyme.

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At key steps, electrons are stripped from the

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glucose.

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As is often the case in oxidation reactions, each electron travels with a proton—thus, as a

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hydrogen atom.

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The hydrogen atoms are not transferred directly to oxygen, but instead are usually passed first to an electron carrier, a coenzyme called

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nicotinamide adenine dinucleotide, a derivative of the vitamin niacin

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This coenzyme is well suited as an electron carrier because it can cycle easily between its oxidized form

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,NAD+ , and its reduced form, NADH

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As an electron acceptor, NAD+ functions as an

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oxidizing agent during respiration.

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How does trap electrons from glucose and the other organic molecules in food? Enzymes called ___________________ remove a pair of hydrogen atoms (2 electrons and 2 protons) from the substrate (glucose, in the preceding example), thereby oxidizing it

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dehydrogenases

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The enzyme delivers the 2 electrons along with 1 proton to its coenzyme,

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NAD+ , forming NADH

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The other proton is released as a hydrogen ion (H+) into the

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surrounding solution

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Learn surrounding solution above figure 9.4

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By receiving 2 negatively charged electrons but only 1 positively charged proton, the nicotinamide portion of NAD+ has its charge neutralized when

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NAD+ is reduced to NADH.

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The name NADH shows the hydrogen that has been received in the reaction. NAD+ is the most versatile electron acceptor in cellular respiration and functions in several of the

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redox steps during the breakdown of glucose.

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Electrons lose very little of their potential energy when they are transferred from

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glucose to NAD+.

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Each NADH molecule formed during respiration represents

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stored energy.

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Cellular respiration also brings hydrogen and oxygen together to form water, but there are two important differences. First, in cellular respiration, the hydrogen that reacts with oxygen is derived from

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organic molecules rather than .

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Second, instead of occurring in one explosive reaction, respiration uses an electron transport chain to break the fall of electrons to oxygen into

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several energy-releasing steps

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consists of a number of molecules, mostly proteins, built into the inner membrane of the mitochondria of eukaryotic cells (and the plasma membrane of respiring prokaryotes)

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electron transport chain

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Electrons removed from glucose are shuttled by NADH to the

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“top,” higher-energy end of the chain.

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At the “bottom,” lower-energy end, O2 captures these electrons along with hydrogen nuclei (H+) , forming

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water

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Electron transfer from NADH to oxygen is an exergonic reaction with a free-energy change of

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-53 kcal/mol (-222 kJ/mol)

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Each “downhill” carrier is more electronegative than, and thus capable of

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oxidizing, its “uphill” neighbor, with oxygen at the bottom of the chain.

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during cellular respiration, most electrons travel the following “downhill” route:

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glucose → NADH → electron transport chain → oxygen

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The harvesting of energy from glucose by cellular respiration is a cumulative function of three metabolic stages.

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1. Glycolysis, 2. Pyruvate oxidation and the citric acid cycle, 3. oxidative phosphorylation

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occurs in the cytosol, begins the degradation process by breaking glucose into two molecules of a compound

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Glycolysis

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compound is called

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pyruvate

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eukaryotes, pyruvate enters the mitochondrion and is oxidized to a compound which enters the

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citric acid cycle

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compound is called what for citric acid cycle

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acetyl CoA

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organic compounds such as glucose store energy in their arrangements of

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atoms

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these molecules are broken down and their energy extracted in

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cellular respiration

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the first stage of cellular respiration occurs in the

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cytosol

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the second and third stages occur in

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mitochondria

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in cellular respiration, electrons are transferred from glucose to

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electron carriers such as NAD+ and finally oxygen

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the energy released by this transfer of electrons is used to make

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ATP

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carbon dioxide and water are given off as

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by-products

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the stages of respiration- Glycolysis is a series of steps in which a

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glucose molecule is broken down into two molecules of pyruvate

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as the chemical bond in glucose are broken electrons and hydrogen ions are picked up by

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NAD+, forming NADH

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glucose is oxidized and NAD+ is

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reduced

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a small amount of ATP is also produced in glycolysis by

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substrate-level phosphorylation

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but most of the energy released by the breakdown of glucose is carried by the

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electrons attached to NADH

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the pyruvate molecule are oxidized to acetyl CoA as they the _____________, releasing carbon dioxide

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mitochondrion

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the acetyl CoA molecules enter a series of reactions called the

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citric acid cycle

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more carbon dioxide is released as the citric acid cycle completes the

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oxidation of glucose

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most of the energy released by the oxidation of glucose is carried by

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NADH and FADH2

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the final stage of cellular respiration the NADH and FADH2 molecules produced in glycolysis and the citric acid cycle donate their electrons to the electron transport chain

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oxidative phosphorylation

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electrons move down the chain to the end, where

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oxygen exerts a strong pull on the electrons

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oxygen, electrons, and hydrogen ions combine, forming

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water

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the electron transport chain converts the chemical energy from moving electrons to a form of

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energy that can be used to drive oxidative phosphorylation

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which produces most of the ATP generated by

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cellular respiration

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in eukaryotic cells, the inner membrane of the mitochondrion is the site of electron transport and another process called

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chemiosmosis

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A smaller amount of ATP is formed directly in a few reactions of glycolysis and the citric acid cycle by a mechanism called

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substrate-level phosphorylation