• The most important molecule for capturing and transferring free energy in biological systems is adenosine triphosphate or ATP

8.1 - Oxidation of Glucose and Fatty Acids to CO2

• Glycolysis, the initial stage of glucose metabolism, takes place in the cytosol and does not involve molecular O2

• It produces a small amount of ATP and the three-carbon compound pyruvate

8.2 - Electron Transport and Generation of the Proton-Motive Force

• Most of the free energy released during the oxidation of glucose to CO2 is retained in the reduced coenzymes NADH and FADH2 generated during glycolysis and the citric acid cycle

• The free energy released during oxidation of a single NADH or FADH2 molecule by O2 is sufficient to drive the synthesis of several molecules of ATP from ADP and Pi, a reaction with a △Go, of 7.3 kcal/mol

• The synthesis of ATP from ADP and Pi, driven by the transfer of electrons from NADH or FADH2 to O2, is the major source of ATP in aerobic nonphotosynthetic cells

• Much evidence shows that in mitochondria and bacteria this process, called oxidative phosphorylation, depends on the generation of a proton-motive force across the inner membrane, with electron transport, proton pumping, and ATP formation occurring simultaneously

The Proton-Motive Force in Mitochondria Is Due Largely to a Voltage Gradient Across the Inner Membrane

• The proton-motive force (pmf) is the sum of a transmembrane proton concentration (pH) gradient and electric potential or voltage gradient

• The relative contribution of the two components to the total pmf depends on the permeability of the membrane to ions other than H+

• A significant voltage gradient can develop only if the membrane is poorly permeable to other cations and to anions, as is the inner mitochondrial membrane

• Researchers can measure the inside pH by trapping fluorescent pH-sensitive dyes inside vesicles formed from the inner mitochondrial membrane

• They also can determine the electric potential by adding radioactive 42K+ ions and a trace amount of valinomycin to a suspension of respiring mitochondria

• Although the inner membrane is normally impermeable to K+, valinomycin is an ionophore, a small lipid-soluble molecule that selectively binds a specific ion (in this case, K ) in its hydrophilic interior and carries it across otherwise impermeable membranes

Electron Transport in Mitochondria Is Coupled to Proton Translocation

• The coupling between electron transport from NADH (or FADH2) to O2 and proton transport across the inner mitochondrial membrane, which generates the proton-motive force, also can be demonstrated experimentally with isolated mitochondria

• As soon as O2 is added to a suspension of mitochondria, the medium outside the mitochondria becomes acidic

• During electron transport from NADH to O2, protons translocate from the matrix to the intermembrane space; since the outer membrane is freely permeable to protons, the pH of the outside medium is lowered briefly

Electrons Flow from FADH2 and NADH to O2 Through a Series of Four Multiprotein Complexes

• As electrons move from NADH to O2, their potential declines by 1.14 V, which corresponds to 26.2 kcal/mol of electrons transferred, or ≈ 53 kcal/mol for a pair of electrons

• Each of the four large multiprotein complexes in the respiratory chain spans the inner mitochondrial membrane and contains several prosthetic groups that participate in moving electrons

• These small nonpeptide organic molecules or metal ions are tightly and specifically associated with the multiprotein complexes

• Several types of heme, an iron-containing prosthetic group similar to that in hemoglobin and myoglobin, are tightly bound or covalently linked to mitochondrial proteins, forming the cytochromes

• The various cytochromes have slightly different heme groups and axial ligands, which generate different environments for the Fe ion

• Iron-sulfur clusters are nonheme, iron-containing prosthetic groups consisting of Fe atoms bonded both to inorganic S atoms and to S atoms on cysteine residues on a protein

• Coenzyme Q (CoQ), also called ubiquinone, is the only electron carrier in the respiratory chain that is not a protein-bound prosthetic group

• It is a carrier of hydrogen atoms, that is, protons plus electrons

• CoQ accepts electrons released from the NADH-CoQ reductase complex (I) and the succinate-CoQ reductase complex (II) and donates them to the CoQH2–cytochrome c reductase complex (III)

• Importantly, reduction and oxidation of CoQ are coupled to the pumping of protons

• NADH-CoQ Reductase (Complex I): Electrons are carried from NADH to CoQ by the NADH-CoQ reductase complex

  • NAD is exclusively a two-electron carrier: it accepts or releases a pair of electrons at a time

• Succinate-CoQ Reductase (Complex II): Succinate dehydrogenase, the enzyme that oxidizes a molecule of succinate to fumarate in the citric acid cycle, is an integral component of the succinate-CoQ reductase complex

  • The two electrons released in the conversion of succinate to fumarate are transferred first to FAD, then to an iron-sulfur cluster, and finally to CoQ

• CoQH2–Cytochrome c Reductase (Complex III): A CoQH2 generated either by complex I or complex II donates two electrons to the CoQH2–cytochrome c reductase complex, regenerating oxidized CoQ

  • Concomitantly it releases two protons picked up on the cytosolic face into the intermembrane space, generating part of the proton-motive force

• Cytochrome c Oxidase (Complex IV): Cytochrome c, after being reduced by the CoQH2–cytochrome c reductase complex, transports electrons, one at a time, to the cytochrome c oxidase complex

• CoQ and Cytochrome c as Mobile Electron Shuttles: The four electron-transport complexes just described are laterally mobile in the inner mitochondrial membrane; moreover, they are present in unequal amounts and do not form stable contacts with one another

CoQ and Three Electron-Transport Complexes Pump Protons Out of the Mitochondrial Matrix

• The multiprotein complexes responsible for proton pumping coupled to electron transport have been identified by selectively extracting mitochondrial membranes with detergents, isolating each of the complexes in near purity

  • Then preparing artificial phospholipid vesicles (liposomes) containing each complex

• Current evidence suggests that a total of 10 protons are transported from the matrix space across the inner mitochondrial membrane for every electron pair that is transferred from NADH to O2

• After cytochrome c is reduced by the QH2–cytochrome c reductase complex, it is reoxidized by the cytochrome c oxidase complex, which transfers electrons to oxygen

• The cyclic oxidation and reduction of the iron and copper in the oxygen reduction center of cytochrome c oxidase, together with the uptake of four protons from the matrix space, are coupled to the transfer of the four electrons to oxygen and the formation of water

The Q Cycle Increases the Number of Protons Translocated as Electrons Flow Through the CoQH2–Cytochrome c Reductase Complex

• CoQH2 is generated both by the NADH-CoQ reductase and succinate-CoQ reductase complexes and, as we shall see, by the CoQH2–cytochrome c reductase complex itself

• In all cases, a molecule from the pool of reduced CoQH2 in the membrane binds to the Qo site on the intermembrane

• In the Q cycle, two molecules of CoQH2 are oxidized to CoQ at the Qo site and release a total of four protons into the intermembrane space, but one molecule of CoQH2 is regenerated from CoQ at the Qi site

8.3 - Harnessing the Proton-Motive Force for Energy-Requiring Process

Bacterial Plasma-Membrane Proteins Catalyze Electron Transport and Coupled ATP Synthesis

• Although bacteria lack any internal membranes, aerobic bacteria nonetheless carry out oxidative phosphorylation by the same processes that occur in eukaryotic mitochondria

• Enzymes that catalyze the reactions of both the glycolytic pathway and the citric acid cycle are present in the cytosol of bacteria

  • Enzymes that oxidize NADH to NAD and transfer the electrons to the ultimate acceptor O2 are localized to the bacterial plasma membrane

ATP Synthase Comprises Two Multiprotein Complexes Termed F0 and F1

• The F0F1 complex, or ATP synthase, has two principal components, F0 and F1, both of which are multimeric proteins

• The F0 component contains three types of integral membrane proteins, designated a, b, and c

• In bacteria and in yeast mitochondria the most common subunit composition is a1b2c10, but F0 complexes in animal mitochondria have 12 c subunits and those in chloroplasts have 14

• In all cases, the c subunits form a donut-shaped ring in the plane of the membrane

• The a and two b subunits are rigidly linked to one another but not to the ring of c subunits

Rotation of the F1 y Subunit, Driven by Proton Movement Through F0, Powers ATP Synthesis

• Each of the three subunits in the complete F0F1 complex can bind ADP and Pi and catalyze ATP synthesis

• But, the coupling between proton flow and ATP synthesis must be indirect, since the nucleotide-binding sites on the subunits of F1, where ATP synthesis occurs, are 9–10 nm from the surface of the mitochondrial membrane

• The most widely accepted model for ATP synthesis by the F0F1 complex—the binding-change mechanism—posits just such an indirect coupling

• Later x-ray crystallographic analysis of the (oxB)3 hexamer yielded a striking conclusion: although the three subunits are identical in sequence and overall structure, the ADP/ATP-binding sites have different conformations in each subunit

• The most reasonable conclusion was that the three subunits cycle between three conformational states, with different nucleotide-binding sites, in an energy-dependent reaction

• Proton Movement Through F0 and Rotation of the c Ring: Each copy of subunit c contains two membrane-spanning ox helices that form a hairpin

• An aspartate residue, Asp61, in the center of one of these helices is thought to participate in proton movement

ATP-ADP Exchange Across the Inner Mitochondrial Membrane Is Powered by the Proton-Motive Force

• The proton-motive force across the inner mitochondrial membrane also powers the exchange of ATP formed by oxidative phosphorylation inside the mitochondrion for ADP and Pi in the cytosol

• The phosphate transporter catalyzes the import of one HPO4 2 coupled to the export of one OH

• Likewise, the ATP/ADP antiporter allows one molecule of ADP to enter only if one molecule of ATP exits simultaneously

Rate of Mitochondrial Oxidation Normally Depends on ADP Levels

• If intact isolated mitochondria are provided with NADH (or FADH2), O2, and Pi, but not with ADP, the oxidation of NADH and the reduction of O2 rapidly cease, as the amount of endogenous ADP is depleted by ATP formation

• If ADP then is added, the oxidation of NADH is rapidly restored

• Certain poisons, called uncouplers, render the inner mitochondrial membrane permeable to protons

Brown-Fat Mitochondria Contain an Uncoupler of Oxidative Phosphorylation

• Brown-fat tissue, whose color is due to the presence of abundant mitochondria, is specialized for the generation of heat

• In contrast, white-fat tissue is specialized for the storage of fat and contains relatively few mitochondria

• The inner membrane of brown-fat mitochondria contains thermogenin, a protein that functions as a natural uncoupler of oxidative phosphorylation

• Environmental conditions regulate the amount of thermogenin in brown-fat mitochondria

• Adult humans have little brown fat, but human infants have a great deal

8.4 - Photosynthetic Stages and Light-Absorbing Pigments

Thylakoid Membranes Are the Sites of Photosynthesis in Plants

• Chloroplasts are bounded by two membranes, which do not contain chlorophyll and do not participate directly in photosynthesis

• As in mitochondria, the outer membrane of chloroplasts contains porins and thus is permeable to metabolites of small molecular weight

• The inner membrane forms a permeability barrier that contains transport proteins for regulating the movement of metabolites into and out of the organelle

Three of the Four Stages in Photosynthesis Occur Only During Illumination

• The photosynthetic process in plants can be divided into four stages, each localized to a defined area of the chloroplast,

  • (1) absorption of light

    • Absorption of Light The initial step in photosynthesis is the absorption of light by chlorophylls attached to proteins in the thylakoid membranes

    • Like the heme component of cytochromes, chlorophylls consist of a porphyrin ring attached to a long hydrocarbon side chain

  • (2) electron transport leading to the formation of O2 from H2O, reduction of NADP to NADPH, and generation of a proton-motive force

    • Electron Transport and Generation of a Proton-Motive Force: Electrons move from the quinone primary electron acceptor through a series of electron carriers until they reach the ultimate electron acceptor, usually the oxidized form of nicotinamide adenine dinucleotide phosphate (NADP), reducing it to NADPH

  • (3) synthesis of ATP

    • Protons move down their concentration gradient from the thylakoid lumen to the stroma through the F0F1 complex (ATP synthase), which couples proton movement to the synthesis of ATP from ADP and Pi

  • (4) conversion of CO2 into carbohydrates, commonly referred to as carbon fixation

    • Generated by the second and third stages of photosynthesis provide the energy and the electrons to drive the synthesis of polymers of six-carbon sugars from CO2 and H2O

Each Photon of Light Has a Defined Amount of Energy

• Quantum mechanics established that light, a form of electromagnetic radiation, has properties of both waves and particles

• When light interacts with matter, it behaves as discrete packets of energy (quanta) called photons

Photosystems Comprise a Reaction Center and Associated Light-Harvesting Complexes

• The absorption of light energy and its conversion into chemical energy occurs in multiprotein complexes called photosystems

• Found in all photosynthetic organisms, both eukaryotic and prokaryotic, photosystems consist of two closely linked components: a reaction center, where the primary events of photosynthesis occur, and an antenna complex consisting of numerous protein complexes, termed light-harvesting complexes (LHCs), which capture light energy and transmit it to the reaction center

Photoelectron Transport from Energized Reaction-Center Chlorophyll a Produces a Charge Separation

• The absorption of a photon of light of wavelength ≈680 nm by chlorophyll increases its energy by 42 kcal/mol (the first excited state)

• Such an energized chlorophyll a molecule in a plant reaction center rapidly donates an electron to an intermediate acceptor, and the electron is rapidly passed on to the primary electron acceptor, quinone Q, on the stromal surface of the thylakoid membrane

• This light-driven electron transfer, called photoelectron transport, depends on the unique environment of both the chlorophylls and the acceptor within the reaction center

Light-Harvesting Complexes Increase the Efficiency of Photosynthesis

• Although chlorophyll molecules within a reaction center are capable of directly absorbing light and initiating photosynthesis, they most commonly are energized indirectly by energy transferred from light-harvesting complexes (LHCs) in an associated antenna

• Even at the maximum light intensity encountered by photosynthetic organisms (tropical noontime sunlight), each reaction-center chlorophyll a molecule absorbs only about one photon per second

  • Which is not enough to support photosynthesis sufficient for the needs of the plant

8.5 - Molecular Analysis of Photosynthesis

The Single Photosystem of Purple Bacteria Generates a Proton-Motive Force but No O2

• The three-dimensional structures of the photosynthetic reaction centers from two purple bacteria have been determined, permitting scientists to trace the detailed paths of electrons during and after the absorption of light

• Similar proteins and pigments compose photosystem II of plants as well, and the conclusions drawn from studies on this simple photosystem have proven applicable to plant systems

• Initial Charge Separation: The mechanism of charge separation in the photosystem of purple bacteria is identical to that in plants outlined earlier; that is, energy from absorbed light is used to strip an electron from a reaction-center bacteriochlorophyll a molecule and transfer it

  • Via several different pigments, to the primary electron acceptor QB, which is loosely bound to a site on the cytosolic membrane face

• Subsequent Electron Flow and Coupled Proton Movement: After the primary electron acceptor, QB, in the bacterial reaction center accepts one electron, forming QB·, it accepts a second electron from the same reaction-center chlorophyll following its absorption of a second photon

• The quinone then binds two protons from the cytosol, forming the reduced quinone (QH2), which is released from the reaction center

Chloroplasts Contain Two Functionally and Spatially Distinct Photosystems

• The rate of plant photosynthesis generated by light of wavelength 700 nm can be greatly enhanced by adding light of a shorter wavelength

• A combination of light at, say, 600 and 700 nm supports a greater rate of photosynthesis than the sum of the rates for the two separate wavelengths

Linear Electron Flow Through Both Plant Photosystems, PSII, and PSI Generates a Proton-Motive Force, O2, and NADPH

• Linear electron flow in chloroplasts involves PSII and PSI in an obligate series in which electrons are transferred from H2O to NADP

• The process begins with the absorption of a photon by PSII, causing an electron to move from a P680 chlorophyll a to an acceptor plastoquinone (QB) on the stromal surface

An Oxygen-Evolving Complex Is Located on the Luminal Surface of the PSII Reaction Center

• Somewhat surprisingly, the structure of the PSII reaction center, which removes electrons from H2O to form O2, resembles that of the reaction center of photosynthetic purple bacteria, which does not form O2

• The photochemically oxidized reaction-center chlorophyll of PSII, P680, is the strongest biological oxidant known

• The reduction potential of P680 is more positive than that of water, and thus it can oxidize water to generate O2 and H ions

• Manganese is known to exist in multiple oxidation states with from two to five positive charges

• Subsequent spectroscopic studies indeed showed that the bound Mn ions in the oxygen-evolving complex cycle through five different oxidation states, S0–S4

Cyclic Electron Flow Through PSI Generates a Proton-Motive Force but No NADPH or O2

• Reduced ferredoxin can donate two electrons to a quinone (Q) bound to a site on the stromal surface of PSI; the quinone then picks up two protons from the stroma, forming QH2

• This QH2 then diffuses through the thylakoid membrane to the Qo binding site on the luminal surface of the cytochrome bf complex

Relative Activity of Photosystems I and II Is Regulated

• In order for PSII and PSI to act in sequence during linear electron flow, the amount of light energy delivered to the two reaction centers must be controlled so that each center activates the same number of electrons

• If the two photosystems are not equally excited, then cyclic electron flow occurs in PSI, and PSII becomes less active

8.6 - CO2 Metabolism During Photosynthesis

• Chloroplasts perform many metabolic reactions in green leaves

• In addition to CO2 fixation, the synthesis of almost all amino acids, all fatty acids and carotenes, all pyrimidines, and probably all purines occurs in chloroplasts

CO2 Fixation Occurs in the Chloroplast Stroma

• The reaction that actually fixes CO2 into carbohydrates is catalyzed by ribulose 1,5-bisphosphate carboxylase (often called rubisco), which is located in the stromal space of the chloroplast

• This enzyme adds CO2 to the five-carbon sugar ribulose 1,5-bisphosphate to form two molecules of 3-phosphoglycerate

• When photosynthetic algae are exposed to a brief pulse of 14C-labeled CO2 and the cells are then quickly disrupted, 3-phosphoglycerate is radiolabeled most rapidly, and all the radioactivity is found in the carboxyl group

Synthesis of Sucrose Incorporating Fixed CO2 Is Completed in the Cytosol

• After its formation in the chloroplast stroma, glyceraldehyde 3-phosphate is transported to the cytosol in exchange for phosphate

• The transport protein in the chloroplast membrane that brings fixed CO2 (as glyceraldehyde 3-phosphate) into the cytosol when the cell is exporting sucrose vigorously is a strict antiporter

• No fixed CO2 leaves the chloroplast unless phosphate is fed into it.

Light and Rubisco Activase Stimulate CO2 Fixation

• The Calvin cycle enzymes that catalyze CO2 fixation are rapidly inactivated in the dark, thereby conserving ATP that is generated in the dark for other synthetic reactions, such as lipid and amino acid biosynthesis

• A stromal protein called thioredoxin (Tx) also plays a role in controlling some Calvin cycle enzymes

Photorespiration, Which Competes with Photosynthesis, Is Reduced in Plants That Fix CO2 by the C4 Pathway

• Photosynthesis is always accompanied by photorespiration--- a process that takes place in light, consumes O2, and converts ribulose 1,5-bisphosphate in part to CO2

• Photorespiration is wasteful to the energy economy of the plant: it consumes ATP and O2, and it generates CO2

• In a hot, dry environment, plants must keep the gas-exchange pores (stomata) in their leaves closed much of the time to prevent excessive loss of moisture

• This causes the CO2 level inside the leaf to fall below the Km of rubisco for CO2

• Because of the transport of CO2 from mesophyll cells, the CO2 concentration in the bundle sheath cells of C4 plants is much higher than it is in the normal atmosphere

• In contrast, the high O2 concentration in the atmosphere favors photorespiration in the mesophyll cells of C3 plants

Sucrose Is Transported from Leaves Through the Phloem to All Plant Tissues

• Of the two carbohydrate products of photosynthesis, starch remains in the mesophyll cells of C3 plants and the bundle sheath cells in C4 plants

  • In these cells, starch is subjected to glycolysis, mainly in the dark, forming ATP, NADH, and small molecules that are used as building blocks for the synthesis of amino acids, lipids, and other cellular constituents

• Glucose metabolism is controlled differently in various mammalian tissues to meet the metabolic needs of the organism as a whole

• The three glycolytic enzymes that are regulated by allosteric molecules catalyze reactions with large negative △Go, values—reactions that are essentially irreversible under ordinary conditions

• Fructose 6-phosphate accelerates the formation of fructose 2,6-bisphosphate, which, in turn, activates phosphofructokinase-1

• This metabolite is formed from fructose 6-phosphate by phosphofructokinase- 2, an enzyme different from phosphofructokinase-1

• Another important allosteric activator of phosphofructokinase-1 is fructose 2,6-bisphosphate

• Phosphofructokinase-1 is allosterically inhibited by ATP and allosterically activated by AMP

• Three allosterically controlled glycolytic enzymes play a key role in regulating the entire glycolytic pathway

• The primary function of the oxidation of glucose to CO2 via the glycolytic pathway, the pyruvate dehydrogenase reaction, and the citric acid cycle is to produce NADH and FADH2, whose oxidation in mitochondria generates ATP

• All enzyme-catalyzed reactions and metabolic pathways are regulated by cells so as to produce the needed amounts of metabolites but not an excess

The Rate of Glucose Oxidation Is Adjusted to Meet the Cell’s Need for ATP

• The reaction pathway by which fatty acids are degraded to acetyl CoA in peroxisomes is similar to that used in liver mitochondria

• Indeed, very-long-chain fatty acids containing more than about 20 CH2 groups are degraded only in peroxisomes; in mammalian cells, mid-length fatty acids containing 10–20 CH2 groups can be degraded in both peroxisomes and mitochondria

• The peroxisome is now recognized as the principal organelle in which fatty acids are oxidized in most cell types

• Mitochondrial oxidation of fatty acids is the major source of ATP in mammalian liver cells, and biochemists at one time believed this was true in all cell types

Peroxisomal Oxidation of Fatty Acids Generates No ATP

• Each molecule of a fatty acyl CoA in the mitochondrion is oxidized in a cyclical sequence of four reactions in which all the carbon atoms are converted to acetyl CoA with the generation of NADH and FADH2

• Then the fatty acyl group is transferred to carnitine and moved across the inner mitochondrial membrane by an acylcarnitine transporter protein

• Subsequent hydrolysis of PPi to two molecules of phosphate (Pi) drives this reaction to completion

• They are released into the blood are taken up and oxidized by most other cells, constituting the major energy source for many tissues, particularly heart muscle

• Fatty acids are stored as triacylglycerols, primarily as droplets in adipose (fat-storing) cells

Mitochondrial Oxidation of Fatty Acids Is Coupled to ATP Formation

• As with NADH generated in the mitochondrial matrix, electrons from cytosolic NADH are ultimately transferred to O2 via the respiratory chain, concomitant with the generation of a proton-motive force

• For aerobic oxidation to continue, the NADH produced during glycolysis in the cytosol must be oxidized to NAD

• Transporters in the Inner Mitochondrial Membrane Allow the Uptake of Electrons from Cytosolic NADH

• Synthesis of most of the ATP generated in aerobic oxidation is coupled to the reoxidation of NADH and FADH2 by O2 in a stepwise process involving the respiratory chain, also called the electron transport chain

• Since glycolysis of one glucose molecule generates two acetyl CoA molecules, the reactions in the glycolytic pathway and citric acid cycle produce six CO2 molecules, ten NADH molecules, and two FADH2 molecules per glucose molecule

• Succinate dehydrogenase and -ketoglutarate dehydrogenase are integral proteins in the inner membrane, with their active sites facing the matrix

• These include CoA, acetyl CoA, succinyl CoA, NAD, and NADH, as well as six of the eight cycle enzymes

• Most enzymes and small molecules involved in the citric acid cycle are soluble in an aqueous solution and are localized to the mitochondrial matrix

• Acetyl CoA plays a central role in the oxidation of fatty acids and many amino acids.

  • It is an intermediate in numerous biosynthetic reactions, such as the transfer of an acetyl group to lysine residues in histone proteins and to the N-termini of many mammalian proteins

• This reaction, catalyzed by pyruvate dehydrogenase, is highly exergonic (G 8.0 kcal/mol) and essentially irreversible

• Immediately after pyruvate is transported from the cytosol across the mitochondrial membranes to the matrix, it reacts with coenzyme A, forming CO2 and the intermediate acetyl CoA

Acetyl CoA Derived from Pyruvate Is Oxidized to Yield CO2 and Reduced Coenzymes in Mitochondria

1. Harnessing of the energy stored in the electrochemical proton gradient for ATP synthesis by the F0F1 complex in the inner membrane

2. Electron transfer from NADH and FADH2 to O2, regenerating the oxidized electron carriers NAD and FAD

   1. These reactions occur in the inner membrane and are coupled to the generation of a proton-motive force across it

3. Oxidation of pyruvate and fatty acids to CO2 coupled to the reduction of NAD to NADH and of flavin adenine dinucleotide (FAD), another oxidized electron carrier, to its reduced form, FADH2

   1. These electron carriers are often referred to as coenzymes. NAD+, NADH, FAD, and FADH2 are diffusible and not permanently bound to proteins

   2. Most of the reactions occur in the matrix; two are catalyzed by inner-membrane enzymes that face the matrix

• These processes involve many steps but can be subdivided into three groups of reactions, each of which occurs in a discrete membrane or space in the mitochondrion

• The mitochondrial inner membrane, cristae, and matrix are the sites of most reactions involving the oxidation of pyruvate and fatty acids to CO2 and H2O and the coupled synthesis of ATP from ADP and Pi

• Various transport proteins located in the inner membrane and cristae allow otherwise impermeable molecules, such as ADP and Pi, to pass from the cytosol to the matrix, and other molecules, such as ATP, to move from the matrix into the cytosol

• Although the flow of metabolites across the outer membrane may limit their rate of mitochondrial oxidation

  • The inner membrane and cristae are the major permeability barriers between the cytosol and the mitochondrial matrix

• Ions and most small molecules (up to about 5000 Da) can readily pass through these channel proteins

• Most eukaryotic cells contain many mitochondria, which collectively can occupy as much as 25 percent of the volume of the cytoplasm

• Mitochondria are among the larger organelles in the cell, each one being about the size of an E. coli bacterium

Mitochondria Possess Two Structurally and Functionally Distinct Membranes

• In the presence of oxygen, however, pyruvate formed in glycolysis is transported into mitochondria

  • Where it is oxidized by O2 to CO2 in a series of oxidation reactions collectively termed cellular respiration

• In the absence of oxygen, facultative anaerobes convert glucose to one or more two- or three-carbon compounds, which are generally released into the surrounding medium

• A few eukaryotes are facultative anaerobes: they grow in either the presence or the absence of oxygen

• Most eukaryotes can generate some ATP by anaerobic metabolism

• Many eukaryotes are obligate aerobes: they grow only in the presence of oxygen and metabolize glucose (or related sugars) completely to CO2, with the concomitant production of a large amount of ATP

Anaerobic Metabolism of Each Glucose Molecule Yields Only Two ATP Molecules

• Four molecules of ATP are formed from ADP during glycolysis via substrate-level phosphorylation, which is catalyzed by enzymes in the cytosol

• A set of 10 water-soluble cytosolic enzymes catalyze the reactions constituting the glycolytic pathway

  • In which one molecule of glucose is converted to two molecules of pyruvate