Which Of The Following Describe The Ways Photosynthetic Algae And Animals Obtain And Use Energy.
Ane holding of living things above all makes them seem most miraculously different from nonliving matter: they create and maintain society, in a universe that is tending always to greater disorder (Effigy 2-33). To create this club, the cells in a living organism must perform a never-ending stream of chemical reactions. In some of these reactions, minor organic molecules—amino acids, sugars, nucleotides, and lipids—are existence taken apart or modified to supply the many other small-scale molecules that the jail cell requires. In other reactions, these small molecules are being used to construct an enormously diverse range of proteins, nucleic acids, and other macromolecules that endow living systems with all of their near distinctive properties. Each cell can be viewed as a tiny chemical factory, performing many millions of reactions every second.
Figure 2-33
Order in biological structures. Well-defined, ornate, and beautiful spatial patterns can exist plant at every level of organization in living organisms. In order of increasing size: (A) poly peptide molecules in the glaze of a virus; (B) the regular array of microtubules (more...)
Cell Metabolism Is Organized past Enzymes
The chemic reactions that a jail cell carries out would normally occur only at temperatures that are much college than those existing inside cells. For this reason, each reaction requires a specific boost in chemical reactivity. This requirement is crucial, because it allows each reaction to be controlled past the cell. The control is exerted through the specialized proteins called enzymes, each of which accelerates, or catalyzes, merely one of the many possible kinds of reactions that a particular molecule might undergo. Enzyme-catalyzed reactions are usually continued in series, so that the product of 1 reaction becomes the starting textile, or substrate, for the side by side (Figure 2-34). These long linear reaction pathways are in turn linked to i another, forming a maze of interconnected reactions that enable the cell to survive, abound, and reproduce (Figure two-35).
Effigy 2-34
How a set of enzyme-catalyzed reactions generates a metabolic pathway. Each enzyme catalyzes a particular chemic reaction, leaving the enzyme unchanged. In this example, a set of enzymes acting in series converts molecule A to molecule F, forming a (more than...)
Figure 2-35
Some of the metabolic pathways and their interconnections in a typical prison cell. About 500 common metabolic reactions are shown diagrammatically, with each molecule in a metabolic pathway represented past a filled circle, every bit in the xanthous box in Figure 2-34. (more...)
Two opposing streams of chemical reactions occur in cells: (1) the catabolic pathways pause down foodstuffs into smaller molecules, thereby generating both a useful grade of energy for the prison cell and some of the small molecules that the cell needs every bit building blocks, and (ii) the anabolic, or biosynthetic, pathways use the energy harnessed by catabolism to bulldoze the synthesis of the many other molecules that grade the cell. Together these ii sets of reactions constitute the metabolism of the jail cell (Effigy 2-36).
Figure two-36
Schematic representation of the relationship between catabolic and anabolic pathways in metabolism. Every bit suggested here, since a major portion of the energy stored in the chemical bonds of food molecules is dissipated equally heat, the mass of nutrient required (more...)
Many of the details of cell metabolism form the traditional subject of biochemistry and need not concern us here. Simply the general principles by which cells obtain energy from their environment and apply it to create gild are cardinal to cell biology. We begin with a discussion of why a constant input of energy is needed to sustain living organisms.
Biological Order Is Made Possible by the Release of Oestrus Energy from Cells
The universal tendency of things to become disordered is expressed in a central law of physics—the second law of thermodynamics—which states that in the universe, or in any isolated system (a collection of matter that is completely isolated from the rest of the universe), the degree of disorder tin can only increment. This law has such profound implications for all living things that it is worth restating in several ways.
For example, we can present the second police in terms of probability and country that systems volition change spontaneously toward those arrangements that have the greatest probability. If we consider, for case, a box of 100 coins all lying heads up, a serial of accidents that disturbs the box will tend to motion the arrangement toward a mixture of 50 heads and l tails. The reason is elementary: in that location is a huge number of possible arrangements of the individual coins in the mixture that can achieve the fifty-50 result, but only i possible arrangement that keeps all of the coins oriented heads up. Considering the l-50 mixture is therefore the most probable, we say that it is more "disordered." For the aforementioned reason, information technology is a common experience that one'south living infinite will become increasingly disordered without intentional effort: the movement toward disorder is a spontaneous process, requiring a periodic attempt to opposite it (Effigy two-37).
Figure ii-37
An everyday illustration of the spontaneous drive toward disorder. Reversing this trend toward disorder requires an intentional effort and an input of energy: it is not spontaneous. In fact, from the second law of thermodynamics, we can be certain (more than...)
The amount of disorder in a system tin be quantified. The quantity that we apply to mensurate this disorder is called the entropy of the system: the greater the disorder, the greater the entropy. Thus, a third fashion to limited the 2d police force of thermodynamics is to say that systems will change spontaneously toward arrangements with greater entropy.
Living cells—by surviving, growing, and forming complex organisms—are generating order and thus might appear to defy the second police of thermodynamics. How is this possible? The answer is that a cell is not an isolated system: information technology takes in energy from its environs in the form of food, or as photons from the sun (or fifty-fifty, as in some chemosynthetic bacteria, from inorganic molecules alone), and it and then uses this energy to generate order within itself. In the form of the chemical reactions that generate society, part of the energy that the cell uses is converted into heat. The heat is discharged into the cell's environment and disorders it, so that the total entropy—that of the cell plus its surroundings—increases, as demanded by the laws of physics.
To empathize the principles governing these energy conversions, think of a cell as sitting in a sea of matter representing the rest of the universe. As the prison cell lives and grows, it creates internal order. But it releases estrus energy as it synthesizes molecules and assembles them into cell structures. Heat is energy in its most disordered grade—the random jostling of molecules. When the cell releases heat to the bounding main, information technology increases the intensity of molecular motions there (thermal motion)—thereby increasing the randomness, or disorder, of the ocean. The 2nd police of thermodynamics is satisfied because the increase in the amount of gild within the cell is more than than compensated by a greater subtract in order (increase in entropy) in the surrounding sea of matter (Figure ii-38).
Figure 2-38
A unproblematic thermodynamic analysis of a living prison cell. In the diagram on the left the molecules of both the jail cell and the residue of the universe (the ocean of matter) are depicted in a relatively disordered state. In the diagram on the right the cell has taken (more...)
Where does the estrus that the jail cell releases come from? Here we encounter some other important law of thermodynamics. The first law of thermodynamics states that energy tin exist converted from ane form to another, but that it cannot be created or destroyed. Some forms of free energy are illustrated in Effigy 2-39. The amount of free energy in different forms volition change as a result of the chemic reactions inside the jail cell, merely the first constabulary tells us that the full corporeality of energy must always exist the aforementioned. For example, an fauna cell takes in foodstuffs and converts some of the free energy nowadays in the chemic bonds betwixt the atoms of these food molecules (chemical bond energy) into the random thermal motion of molecules (heat energy). This conversion of chemical energy into heat energy is essential if the reactions inside the jail cell are to cause the universe as a whole to become more disordered—as required by the second police.
Effigy 2-39
Some interconversions between dissimilar forms of free energy. All free energy forms are, in principle, interconvertible. In all these processes the total amount of energy is conserved; thus, for example, from the superlative and weight of the brick in the first instance, (more than...)
The prison cell cannot derive any benefit from the estrus energy information technology releases unless the rut-generating reactions within the cell are directly linked to the processes that generate molecular order. Information technology is the tight coupling of heat production to an increase in order that distinguishes the metabolism of a cell from the wasteful burning of fuel in a burn. Later in this chapter, nosotros shall illustrate how this coupling occurs. For the moment, it is sufficient to recognize that a straight linkage of the "called-for" of food molecules to the generation of biological order is required if cells are to be able to create and maintain an island of order in a universe tending toward chaos.
Photosynthetic Organisms Use Sunlight to Synthesize Organic Molecules
All animals live on energy stored in the chemical bonds of organic molecules made past other organisms, which they take in as food. The molecules in food as well provide the atoms that animals demand to construct new living matter. Some animals obtain their food by eating other animals. But at the lesser of the animal food chain are animals that eat plants. The plants, in turn, trap energy directly from sunlight. Equally a event, all of the energy used by animal cells is derived ultimately from the sun.
Solar free energy enters the living world through photosynthesis in plants and photosynthetic leaner. Photosynthesis allows the electromagnetic energy in sunlight to be converted into chemic bond energy in the cell. Plants are able to obtain all the atoms they need from inorganic sources: carbon from atmospheric carbon dioxide, hydrogen and oxygen from water, nitrogen from ammonia and nitrates in the soil, and other elements needed in smaller amounts from inorganic salts in the soil. They apply the energy they derive from sunlight to build these atoms into sugars, amino acids, nucleotides, and fat acids. These pocket-sized molecules in turn are converted into the proteins, nucleic acids, polysaccharides, and lipids that form the plant. All of these substances serve as nutrient molecules for animals, if the plants are later eaten.
The reactions of photosynthesis accept place in two stages (Figure two-40). In the first stage, energy from sunlight is captured and transiently stored as chemical bail energy in specialized small molecules that act as carriers of energy and reactive chemical groups. (We discuss these activated carrier molecules later.) Molecular oxygen (Otwo gas) derived from the splitting of h2o by low-cal is released equally a waste material product of this starting time stage.
Figure 2-xl
Photosynthesis. The two stages of photosynthesis. The energy carriers created in the commencement stage are 2 molecules that we discuss shortly—ATP and NADPH.
In the second stage, the molecules that serve as free energy carriers are used to help bulldoze a carbon fixation process in which sugars are manufactured from carbon dioxide gas (CO2) and water (H2O), thereby providing a useful source of stored chemical bond free energy and materials—both for the plant itself and for whatever animals that eat it. We describe the elegant mechanisms that underlie these two stages of photosynthesis in Chapter xiv.
The net outcome of the unabridged process of photosynthesis, so far as the green plant is concerned, can exist summarized just in the equation
The sugars produced are then used both equally a source of chemical bail energy and equally a source of materials to make the many other small-scale and large organic molecules that are essential to the plant cell.
Cells Obtain Free energy past the Oxidation of Organic Molecules
All brute and plant cells are powered by energy stored in the chemical bonds of organic molecules, whether these be sugars that a plant has photosynthesized as food for itself or the mixture of large and small molecules that an brute has eaten. In order to use this energy to live, grow, and reproduce, organisms must extract it in a usable form. In both plants and animals, free energy is extracted from food molecules by a procedure of gradual oxidation, or controlled burning.
The Earth's atmosphere contains a great bargain of oxygen, and in the presence of oxygen the nigh energetically stable form of carbon is as CO2 and that of hydrogen is every bit HiiO. A cell is therefore able to obtain energy from sugars or other organic molecules past assuasive their carbon and hydrogen atoms to combine with oxygen to produce CO2 and HiiO, respectively—a process called respiration.
Photosynthesis and respiration are complementary processes (Figure 2-41). This means that the transactions between plants and animals are not all i style. Plants, animals, and microorganisms have existed together on this planet for and then long that many of them have become an essential part of the others' environments. The oxygen released by photosynthesis is consumed in the combustion of organic molecules by nigh all organisms. And some of the CO2 molecules that are fixed today into organic molecules past photosynthesis in a green leaf were yesterday released into the temper by the respiration of an animal—or by that of a fungus or bacterium decomposing expressionless organic matter. We therefore see that carbon utilization forms a huge cycle that involves the biosphere (all of the living organisms on World) every bit a whole, crossing boundaries between individual organisms (Figure 2-42). Similarly, atoms of nitrogen, phosphorus, and sulfur move between the living and nonliving worlds in cycles that involve plants, animals, fungi, and bacteria.
Figure 2-41
Photosynthesis and respiration as complementary processes in the living world. Photosynthesis uses the energy of sunlight to produce sugars and other organic molecules. These molecules in turn serve as food for other organisms. Many of these organisms (more...)
Figure 2-42
The carbon cycle. Individual carbon atoms are incorporated into organic molecules of the living world by the photosynthetic activity of plants, leaner, and marine algae. They pass to animals, microorganisms, and organic material in soil and oceans in (more...)
Oxidation and Reduction Involve Electron Transfers
The jail cell does not oxidize organic molecules in 1 step, as occurs when organic cloth is burned in a fire. Through the use of enzyme catalysts, metabolism takes the molecules through a large number of reactions that only rarely involve the direct addition of oxygen. Before nosotros consider some of these reactions and the purpose behind them, we need to discuss what is meant by the process of oxidation.
Oxidation, in the sense used to a higher place, does non hateful only the addition of oxygen atoms; rather, information technology applies more more often than not to any reaction in which electrons are transferred from one atom to another. Oxidation in this sense refers to the removal of electrons, and reduction—the converse of oxidation—means the addition of electrons. Thus, Iron2+ is oxidized if it loses an electron to become Feiii+, and a chlorine atom is reduced if it gains an electron to become Cl-. Since the number of electrons is conserved (no loss or gain) in a chemic reaction, oxidation and reduction always occur simultaneously: that is, if one molecule gains an electron in a reaction (reduction), a 2d molecule loses the electron (oxidation). When a sugar molecule is oxidized to CO2 and H2O, for example, the Oii molecules involved in forming HtwoO gain electrons and thus are said to have been reduced.
The terms "oxidation" and "reduction" utilise even when at that place is only a partial shift of electrons between atoms linked by a covalent bond (Effigy 2-43). When a carbon atom becomes covalently bonded to an atom with a stiff affinity for electrons, such as oxygen, chlorine, or sulfur, for example, it gives upward more its equal share of electrons and forms a polar covalent bail: the positive charge of the carbon nucleus is at present somewhat greater than the negative charge of its electrons, and the atom therefore acquires a partial positive charge and is said to be oxidized. Conversely, a carbon atom in a C-H linkage has slightly more than its share of electrons, and so it is said to be reduced (see Figure 2-43).
Effigy ii-43
Oxidation and reduction. (A) When two atoms course a polar covalent bond (come across p. 54), the atom catastrophe upwardly with a greater share of electrons is said to exist reduced, while the other atom acquires a lesser share of electrons and is said to be oxidized. The reduced (more than...)
When a molecule in a cell picks up an electron (e-), information technology often picks up a proton (H+) at the same time (protons being freely available in water). The net effect in this example is to add a hydrogen atom to the molecule
Fifty-fifty though a proton plus an electron is involved (instead of just an electron), such hydrogenation reactions are reductions, and the reverse, dehydrogenation reactions, are oxidations. It is specially easy to tell whether an organic molecule is being oxidized or reduced: reduction is occurring if its number of C-H bonds increases, whereas oxidation is occurring if its number of C-H bonds decreases (encounter Figure 2-43B).
Cells utilize enzymes to catalyze the oxidation of organic molecules in modest steps, through a sequence of reactions that allows useful energy to be harvested. Nosotros now need to explicate how enzymes piece of work and some of the constraints under which they operate.
Enzymes Lower the Barriers That Cake Chemic Reactions
Consider the reaction
The newspaper burns readily, releasing to the atmosphere both free energy as heat and h2o and carbon dioxide as gases, simply the smoke and ashes never spontaneously retrieve these entities from the heated temper and reconstitute themselves into paper. When the newspaper burns, its chemical energy is dissipated every bit heat—not lost from the universe, since free energy tin can never be created or destroyed, merely irretrievably dispersed in the chaotic random thermal motions of molecules. At the same fourth dimension, the atoms and molecules of the newspaper go dispersed and disordered. In the language of thermodynamics, there has been a loss of free free energy, that is, of energy that tin can exist harnessed to practice work or drive chemical reactions. This loss reflects a loss of orderliness in the way the energy and molecules were stored in the paper. Nosotros shall hash out free energy in more detail presently, but the general principle is clear enough intuitively: chemical reactions proceed only in the management that leads to a loss of free energy; in other words, the spontaneous direction for whatsoever reaction is the direction that goes "downhill." A "downhill" reaction in this sense is oft said to exist energetically favorable.
Although the most energetically favorable form of carbon nether ordinary conditions is equally CO2, and that of hydrogen is as HiiO, a living organism does not disappear in a puff of smoke, and the book in your hands does non burst into flames. This is because the molecules both in the living organism and in the book are in a relatively stable state, and they cannot be inverse to a state of lower energy without an input of energy: in other words, a molecule requires activation energy—a kick over an free energy barrier—before information technology can undergo a chemical reaction that leaves it in a more stable state (Figure 2-44). In the case of a burning book, the activation energy is provided by the rut of a lighted match. For the molecules in the watery solution inside a cell, the kicking is delivered by an unusually energetic random collision with surrounding molecules—collisions that go more violent every bit the temperature is raised.
Effigy 2-44
The important principle of activation free energy. Compound X is in a stable state, and energy is required to convert it to compound Y, even though Y is at a lower overall energy level than X. This conversion will not take place, therefore, unless compound (more...)
In a living cell, the kick over the energy barrier is profoundly aided by a specialized class of proteins—the enzymes. Each enzyme binds tightly to one or two molecules, called substrates, and holds them in a way that greatly reduces the activation energy of a particular chemical reaction that the bound substrates tin undergo. A substance that can lower the activation energy of a reaction is termed a goad; catalysts increase the rate of chemical reactions considering they allow a much larger proportion of the random collisions with surrounding molecules to kick the substrates over the energy barrier, as illustrated in Figure ii-45. Enzymes are amidst the near constructive catalysts known, speeding upward reactions by a gene of as much as 10fourteen, and they thereby allow reactions that would not otherwise occur to go along apace at normal temperatures.
Effigy 2-45
Lowering the activation energy greatly increases the probability of reaction. A population of identical substrate molecules will have a range of energies that is distributed as shown on the graph at whatever 1 instant. The varying energies come from collisions (more...)
Enzymes are also highly selective. Each enzyme unremarkably catalyzes only one particular reaction: in other words, it selectively lowers the activation energy of only one of the several possible chemical reactions that its bound substrate molecules could undergo. In this way, enzymes direct each of the many different molecules in a cell forth specific reaction pathways (Figure two-46).
Figure 2-46
Floating brawl analogies for enzyme catalysis. (A) A barrier dam is lowered to correspond enzyme catalysis. The dark-green ball represents a potential enzyme substrate (chemical compound X) that is billowy up and down in free energy level due to abiding encounters with (more...)
The success of living organisms is attributable to a jail cell'southward power to make enzymes of many types, each with precisely specified properties. Each enzyme has a unique shape containing an active site, a pocket or groove in the enzyme into which only particular substrates will fit (Figure 2-47). Like all other catalysts, enzyme molecules themselves remain unchanged subsequently participating in a reaction and therefore tin function over and over again. In Chapter three, we discuss further how enzymes piece of work, subsequently we have looked in detail at the molecular construction of proteins.
Effigy ii-47
How enzymes piece of work. Each enzyme has an active site to which one or 2 substrate molecules bind, forming an enzyme-substrate circuitous. A reaction occurs at the active site, producing an enzyme-product circuitous. The product is and then released, assuasive the enzyme (more...)
How Enzymes Detect Their Substrates: The Importance of Rapid Improvidence
A typical enzyme will catalyze the reaction of nigh a chiliad substrate molecules every second. This means that it must be able to demark a new substrate molecule in a fraction of a millisecond. Simply both enzymes and their substrates are present in relatively pocket-size numbers in a cell. How do they find each other and then fast? Rapid binding is possible because the motions caused by estrus energy are enormously fast at the molecular level. These molecular motions can be classified broadly into three kinds: (1) the movement of a molecule from one place to another (translational move), (ii) the rapid back-and-along movement of covalently linked atoms with respect to one another (vibrations), and (3) rotations. All of these motions are important in bringing the surfaces of interacting molecules together.
These rates of molecular motions can be measured by a diversity of spectroscopic techniques. These bespeak that a large globular protein is constantly tumbling, rotating about its centrality most a million times per second. Molecules are too in constant translational motion, which causes them to explore the infinite inside the cell very efficiently by wandering through it—a process called diffusion. In this way, every molecule in a cell collides with a huge number of other molecules each second. Every bit the molecules in a liquid collide and bounce off one another, an individual molecule moves commencement ane style so another, its path constituting a random walk (Figure 2-48). In such a walk, the boilerplate distance that each molecule travels (as the crow flies) from its starting indicate is proportional to the square root of the time involved: that is, if information technology takes a molecule 1 second on average to travel 1 μm, information technology takes four seconds to travel 2 μm, 100 seconds to travel x μm, and and then on.
Figure 2-48
A random walk. Molecules in solution move in a random fashion due to the continual buffeting they receive in collisions with other molecules. This move allows small molecules to diffuse speedily from one part of the prison cell to another, equally described in (more...)
The inside of a cell is very crowded (Figure 2-49). Notwithstanding, experiments in which fluorescent dyes and other labeled molecules are injected into cells show that minor organic molecules lengthened through the watery gel of the cytosol almost as chop-chop every bit they practise through water. A pocket-sized organic molecule, for example, takes only nearly one-5th of a second on average to diffuse a distance of 10 μm. Improvidence is therefore an efficient style for modest molecules to move the limited distances in the cell (a typical animal cell is 15 μm in bore).
Effigy 2-49
The construction of the cytoplasm. The drawing is approximately to calibration and emphasizes the crowding in the cytoplasm. Just the macromolecules are shown: RNAs are shown in blue, ribosomes in green, and proteins in cherry-red. Enzymes and other macromolecules diffuse (more than...)
Since enzymes move more slowly than substrates in cells, nosotros can recollect of them as sitting still. The rate of encounter of each enzyme molecule with its substrate will depend on the concentration of the substrate molecule. For example, some abundant substrates are present at a concentration of 0.5 mM. Since pure water is 55 M, in that location is merely about one such substrate molecule in the cell for every 105 h2o molecules. All the same, the active site on an enzyme molecule that binds this substrate will be bombarded by about 500,000 random collisions with the substrate molecule per second. (For a substrate concentration tenfold lower, the number of collisions drops to 50,000 per second, and so on.) A random see between the surface of an enzyme and the matching surface of its substrate molecule oftentimes leads immediately to the formation of an enzyme-substrate complex that is ready to react. A reaction in which a covalent bond is broken or formed can now occur extremely rapidly. When one appreciates how quickly molecules movement and react, the observed rates of enzymatic catalysis do not seem so amazing.
Once an enzyme and substrate have collided and snuggled together properly at the active site, they grade multiple weak bonds with each other that persist until random thermal motion causes the molecules to dissociate once more. In general, the stronger the binding of the enzyme and substrate, the slower their rate of dissociation. All the same, when two colliding molecules have poorly matching surfaces, few noncovalent bonds are formed and their total free energy is negligible compared with that of thermal motion. In this case the two molecules dissociate as chop-chop as they come up together. This is what prevents incorrect and unwanted associations from forming between mismatched molecules, such equally between an enzyme and the wrong substrate.
The Free-Free energy Change for a Reaction Determines Whether It Tin Occur
We must now digress briefly to introduce some primal chemistry. Cells are chemical systems that must obey all chemic and concrete laws. Although enzymes speed up reactions, they cannot by themselves force energetically unfavorable reactions to occur. In terms of a h2o analogy, enzymes by themselves cannot make water run uphill. Cells, however, must do simply that in lodge to grow and divide: they must build highly ordered and energy-rich molecules from pocket-size and simple ones. We shall come across that this is done through enzymes that directly couple energetically favorable reactions, which release energy and produce heat, to energetically unfavorable reactions, which produce biological order.
Before examining how such coupling is achieved, we must consider more than carefully the term "energetically favorable." According to the second law of thermodynamics, a chemic reaction can go along spontaneously only if it results in a internet increase in the disorder of the universe (see Figure 2-38). The criterion for an increment in disorder of the universe tin can be expressed most conveniently in terms of a quantity called the free free energy, Thousand , of a organisation. The value of Grand is of interest only when a organization undergoes a alter, and the modify in G, denoted Δ G (delta G), is disquisitional. Suppose that the system existence considered is a collection of molecules. As explained in Panel 2-7 (pp. 122–123), complimentary energy has been divers such that ΔGrand directly measures the amount of disorder created in the universe when a reaction takes place that involves these molecules. Energetically favorable reactions, by definition, are those that subtract free energy, or, in other words, accept a negative ΔK and disorder the universe (Figure 2-50).
Panel 2-vii
Free Energy and Biological Reactions.
Figure ii-50
The distinction between energetically favorable and energetically unfavorable reactions.
A familiar example of an energetically favorable reaction on a macroscopic scale is the "reaction" past which a compressed jump relaxes to an expanded state, releasing its stored elastic energy as heat to its environment; an example on a microscopic scale is the dissolving of salt in water. Conversely, energetically unfavorable reactions, with a positive ΔG—such as those in which two amino acids are joined together to form a peptide bond—by themselves create club in the universe. Therefore, these reactions can take place merely if they are coupled to a 2d reaction with a negative ΔG so large that the ΔG of the entire process is negative (Effigy 2-51).
Effigy two-51
How reaction coupling is used to bulldoze energetically unfavorable reactions.
The Concentration of Reactants Influences ΔG
Every bit we have simply described, a reaction A ⇌ B will become in the direction A → B when the associated free-free energy change, ΔG, is negative, just as a tensed leap left to itself will relax and lose its stored energy to its environment as heat. For a chemical reaction, withal, ΔG depends non only on the free energy stored in each individual molecule, but also on the concentrations of the molecules in the reaction mixture. Retrieve that ΔG reflects the caste to which a reaction creates a more disordered—in other words, a more than likely—state of the universe. Recalling our coin analogy, it is very probable that a coin will flip from a head to a tail orientation if a jiggling box contains 90 heads and 10 tails, only this is a less probable event if the box contains 10 heads and 90 tails. For exactly the same reason, for a reversible reaction A ⇌ B, a large excess of A over B will tend to drive the reaction in the direction A → B; that is, at that place will be a tendency for there to be more molecules making the transition A → B than there are molecules making the transition B → A. Therefore, the ΔYard becomes more than negative for the transition A → B (and more positive for the transition B → A) as the ratio of A to B increases.
How much of a concentration difference is needed to compensate for a given subtract in chemical bond energy (and accompanying heat release)? The reply is not intuitively obvious, but it tin can be determined from a thermodynamic analysis that makes information technology possible to separate the concentration-dependent and the concentration-independent parts of the free-energy modify. The ΔThousand for a given reaction can thereby be written as the sum of ii parts: the first, called the standard free-energy modify, Δ G° , depends on the intrinsic characters of the reacting molecules; the second depends on their concentrations. For the simple reaction A → B at 37°C,
where ΔG is in kilocalories per mole, [A] and [B] announce the concentrations of A and B, ln is the natural logarithm, and 0.616 is RT—the product of the gas abiding, R, and the abolute temperature, T.
Note that ΔG equals the value of ΔG° when the molar concentrations of A and B are equal (ln 1 = 0). As expected, ΔG becomes more negative equally the ratio of B to A decreases (the ln of a number < ane is negative).
Chemic equilibrium is reached when the concentration effect just balances the push given to the reaction by ΔG°, so that at that place is no net modify of free energy to drive the reaction in either direction (Figure ii-52). Here ΔM = 0, then the concentrations of A and B are such that
Figure 2-52
Chemical equilibrium. When a reaction reaches equilibrium, the forward and astern flux of reacting molecules are equal and reverse.
which means that there is chemical equilibrium at 37°C when
Table 2-5 shows how the equilibrium ratio of A to B (expressed as an equilibrium constant, Thou ) depends on the value of ΔYard°.
Table ii-5
Relationship Between the Standard Free- Energy Modify, ΔG°, and Equilibrium Constant.
Information technology is important to recognize that when an enzyme (or any catalyst) lowers the activation energy for the reaction A → B, information technology also lowers the activation energy for the reaction B → A by exactly the same corporeality (see Figure 2-44). The forward and astern reactions will therefore be accelerated past the same gene by an enzyme, and the equilibrium bespeak for the reaction (and ΔGrand°) remains unchanged (Figure two-53).
Figure 2-53
Enzymes cannot change the equilibrium bespeak for reactions. Enzymes, like all catalysts, speed up the forward and backward rates of a reaction by the same gene. Therefore, for both the catalyzed and the uncatalyzed reactions shown here, the number of (more...)
For Sequential Reactions, ΔG° Values Are Condiment
The course of almost reactions tin can be predicted quantitatively. A large torso of thermodynamic data has been nerveless that makes it possible to summate the standard alter in gratuitous energy, ΔG°, for most of the of import metabolic reactions of the cell. The overall complimentary-free energy change for a metabolic pathway is then just the sum of the gratuitous-energy changes in each of its component steps. Consider, for case, two sequential reactions
where the ΔG° values are +5 and -thirteen kcal/mole, respectively. (Remember that a mole is 6 × ten23 molecules of a substance.) If these 2 reactions occur sequentially, the ΔG° for the coupled reaction volition be -eight kcal/mole. Thus, the unfavorable reaction X → Y, which will not occur spontaneously, can be driven past the favorable reaction Y → Z, provided that the second reaction follows the first.
Cells can therefore cause the energetically unfavorable transition, X → Y, to occur if an enzyme catalyzing the X → Y reaction is supplemented by a second enzyme that catalyzes the energetically favorable reaction, Y → Z. In consequence, the reaction Y → Z will so deed as a "siphon" to drive the conversion of all of molecule Ten to molecule Y, and thence to molecule Z (Effigy 2-54). For example, several of the reactions in the long pathway that converts sugars into CO2 and HiiO would be energetically unfavorable if considered on their own. But the pathway withal proceeds rapidly to completion because the total ΔG° for the series of sequential reactions has a large negative value.
Figure two-54
How an energetically unfavorable reaction can exist driven by a second, following reaction. (A) At equilibrium, there are twice as many Ten molecules as Y molecules, considering X is of lower free energy than Y. (B) At equilibrium, there are 25 times more Z molecules (more than...)
But forming a sequential pathway is not acceptable for many purposes. Often the desired pathway is simply X → Y, without further conversion of Y to some other production. Fortunately, there are other more general ways of using enzymes to couple reactions together. How these work is the topic nosotros discuss next.
Activated Carrier Molecules are Essential for Biosynthesis
The energy released by the oxidation of food molecules must be stored temporarily before it can exist channeled into the construction of other small organic molecules and of the larger and more complex molecules needed by the cell. In most cases, the free energy is stored as chemical bail energy in a pocket-size set of activated "carrier molecules," which incorporate one or more energy-rich covalent bonds. These molecules lengthened quickly throughout the cell and thereby carry their bond energy from sites of energy generation to the sites where energy is used for biosynthesis and other needed cell activities (Figure 2-55).
Figure ii-55
Energy transfer and the part of activated carriers in metabolism. By serving every bit energy shuttles, activated carrier molecules perform their function equally go-betweens that link the breakdown of food molecules and the release of energy (catabolism) to the (more than...)
The activated carriers shop energy in an hands exchangeable grade, either as a readily transferable chemical group or equally high-energy electrons, and they can serve a dual role equally a source of both energy and chemical groups in biosynthetic reactions. For historical reasons, these molecules are as well sometimes referred to as coenzymes. The most important of the activated carrier molecules are ATP and 2 molecules that are closely related to each other, NADH and NADPH—every bit we discuss in detail shortly. We shall come across that cells utilise activated carrier molecules similar money to pay for reactions that otherwise could not take place.
The Formation of an Activated Carrier Is Coupled to an Energetically Favorable Reaction
When a fuel molecule such as glucose is oxidized in a cell, enzyme-catalyzed reactions ensure that a big role of the free energy that is released past oxidation is captured in a chemically useful form, rather than being released wastefully as heat. This is achieved by means of a coupled reaction, in which an energetically favorable reaction is used to bulldoze an energetically unfavorable one that produces an activated carrier molecule or some other useful energy shop. Coupling mechanisms require enzymes and are cardinal to all the energy trans-actions of the jail cell.
The nature of a coupled reaction is illustrated by a mechanical analogy in Figure 2-56, in which an energetically favorable chemic reaction is represented past rocks falling from a cliff. The energy of falling rocks would normally be entirely wasted in the form of estrus generated by friction when the rocks hit the footing (come across the falling brick diagram in Figure 2-39). By careful design, however, part of this energy could be used instead to drive a paddle wheel that lifts a bucket of water (Figure 2-56B). Because the rocks can now achieve the ground only after moving the paddle wheel, we say that the energetically favorable reaction of rock falling has been direct coupled to the energetically unfavorable reaction of lifting the bucket of water. Note that considering role of the energy is used to do work in (B), the rocks striking the footing with less velocity than in (A), and correspondingly less energy is wasted as heat.
Figure ii-56
A mechanical model illustrating the principle of coupled chemical reactions. The spontaneous reaction shown in (A) could serve as an analogy for the direct oxidation of glucose to COii and H2O, which produces heat only. In (B) the aforementioned reaction is coupled (more...)
Exactly analogous processes occur in cells, where enzymes play the role of the paddle cycle in our analogy. By mechanisms that will be discussed afterwards in this chapter, they couple an energetically favorable reaction, such as the oxidation of foodstuffs, to an energetically unfavorable reaction, such as the generation of an activated carrier molecule. As a upshot, the amount of heat released by the oxidation reaction is reduced by exactly the corporeality of energy that is stored in the energy-rich covalent bonds of the activated carrier molecule. The activated carrier molecule in turn picks up a packet of free energy of a size sufficient to power a chemical reaction elsewhere in the cell.
ATP Is the Most Widely Used Activated Carrier Molecule
The most important and versatile of the activated carriers in cells is ATP (adenosine triphosphate). Only as the free energy stored in the raised saucepan of water in Figure 2-56B can be used to drive a wide diversity of hydraulic machines, ATP serves as a convenient and versatile shop, or currency, of energy to drive a variety of chemical reactions in cells. ATP is synthesized in an energetically unfavorable phosphorylation reaction in which a phosphate grouping is added to ADP (adenosine diphosphate). When required, ATP gives upwardly its energy package through its energetically favorable hydrolysis to ADP and inorganic phosphate (Figure 2-57). The regenerated ADP is so available to be used for another circular of the phosphorylation reaction that forms ATP.
Figure 2-57
The hydrolysis of ATP to ADP and inorganic phosphate. The two outermost phosphates in ATP are held to the rest of the molecule by high-energy phosphoanhydride bonds and are readily transferred. As indicated, water tin be added to ATP to class ADP and inorganic (more...)
The energetically favorable reaction of ATP hydrolysis is coupled to many otherwise unfavorable reactions through which other molecules are synthesized. Nosotros shall encounter several of these reactions subsequently in this chapter. Many of them involve the transfer of the last phosphate in ATP to another molecule, as illustrated past the phosphorylation reaction in Figure ii-58.
Figure 2-58
An instance of a phosphate transfer reaction. Because an free energy-rich phosphoanhydride bond in ATP is converted to a phosphoester bail, this reaction is energetically favorable, having a large negative ΔG. Reactions of this type are involved in (more...)
ATP is the most abundant active carrier in cells. Equally one example, it is used to supply energy for many of the pumps that transport substances into and out of the cell (discussed in Chapter 11). It too powers the molecular motors that enable muscle cells to contract and nerve cells to transport materials from one finish of their long axons to another (discussed in Chapter sixteen).
Energy Stored in ATP Is Often Harnessed to Join 2 Molecules Together
We have previously discussed one style in which an energetically favorable reaction can be coupled to an energetically unfavorable reaction, 10 → Y, and so as to enable it to occur. In that scheme a second enzyme catalyzes the energetically favorable reaction Y → Z, pulling all of the 10 to Y in the process (see Figure ii-54). But when the required product is Y and not Z, this mechanism is non useful.
A frequent type of reaction that is needed for biosynthesis is ane in which two molecules, A and B, are joined together to produce A-B in the energetically unfavorable condensation reaction
There is an indirect pathway that allows A-H and B-OH to form A-B, in which a coupling to ATP hydrolysis makes the reaction get. Here energy from ATP hydrolysis is first used to convert B-OH to a higher-energy intermediate compound, which then reacts direct with A-H to requite A-B. The simplest possible mechanism involves the transfer of a phosphate from ATP to B-OH to make B-OPO3, in which case the reaction pathway contains only 2 steps:
The condensation reaction, which by itself is energetically unfavorable, is forced to occur by being direct coupled to ATP hydrolysis in an enzyme-catalyzed reaction pathway (Figure 2-59A).
Figure ii-59
An example of an energetically unfavorable biosynthetic reaction driven by ATP hydrolysis. (A) Schematic illustration of the formation of A-B in the condensation reaction described in the text. (B) The biosynthesis of the common amino acrid glutamine. (more...)
A biosynthetic reaction of exactly this type is employed to synthesize the amino acid glutamine, as illustrated in Figure 2-59B. We will run across presently that very similar (but more complex) mechanisms are too used to produce nearly all of the large molecules of the cell.
NADH and NADPH Are Important Electron Carriers
Other important activated carrier molecules participate in oxidation-reduction reactions and are commonly part of coupled reactions in cells. These activated carriers are specialized to acquit high-energy electrons and hydrogen atoms. The most important of these electron carriers are NAD + (nicotinamide adenine dinucleotide) and the closely related molecule NADP + (nicotinamide adenine dinucleotide phosphate). Later, we examine some of the reactions in which they participate. NAD+ and NADP+ each pick up a "packet of free energy" corresponding to two loftier-energy electrons plus a proton (H+)—being converted to NADH (reduced nicotinamide adenine dinucleotide) and NADPH (reduced nicotinamide adenine dinucleotide phosphate), respectively. These molecules can therefore too be regarded as carriers of hydride ions (the H+ plus two electrons, or H-).
Similar ATP, NADPH is an activated carrier that participates in many important biosynthetic reactions that would otherwise be energetically unfavorable. The NADPH is produced according to the general scheme shown in Figure 2-60A. During a special prepare of energy-yielding catabolic reactions, a hydrogen atom plus two electrons are removed from the substrate molecule and added to the nicotinamide band of NADP+ to form NADPH. This is a typical oxidation-reduction reaction; the substrate is oxidized and NADP+ is reduced. The structures of NADP+ and NADPH are shown in Figure ii-60B.
Effigy ii-lx
NADPH, an important carrier of electrons. (A) NADPH is produced in reactions of the general blazon shown on the left, in which ii hydrogen atoms are removed from a substrate. The oxidized form of the carrier molecule, NADP+, receives 1 hydrogen atom (more...)
The hydride ion carried by NADPH is given up readily in a subsequent oxidation-reduction reaction, considering the ring can achieve a more stable arrangement of electrons without it. In this subsequent reaction, which regenerates NADP+, it is the NADPH that becomes oxidized and the substrate that becomes reduced. The NADPH is an effective donor of its hydride ion to other molecules for the same reason that ATP readily transfers a phosphate: in both cases the transfer is accompanied by a large negative free-free energy modify. One example of the use of NADPH in biosynthesis is shown in Figure 2-61.
Effigy ii-61
The concluding stage in one of the biosynthetic routes leading to cholesterol. Every bit in many other biosynthetic reactions, the reduction of the C=C bond is achieved by the transfer of a hydride ion from the carrier molecule NADPH, plus a proton (H+) from the (more...)
The divergence of a single phosphate grouping has no result on the electron-transfer properties of NADPH compared with NADH, but information technology is crucial for their distinctive roles. The actress phosphate group on NADPH is far from the region involved in electron transfer (come across Figure 2-60B) and is of no importance to the transfer reaction. Information technology does, notwithstanding, give a molecule of NADPH a slightly different shape from that of NADH, and and so NADPH and NADH demark equally substrates to dissimilar sets of enzymes. Thus the two types of carriers are used to transfer electrons (or hydride ions) betwixt different sets of molecules.
Why should there be this partition of labor? The reply lies in the demand to regulate ii sets of electron-transfer reactions independently. NADPH operates chiefly with enzymes that catalyze anabolic reactions, supplying the high-energy electrons needed to synthesize energy-rich biological molecules. NADH, by contrast, has a special role as an intermediate in the catabolic system of reactions that generate ATP through the oxidation of food molecules, as we will discuss presently. The genesis of NADH from NAD+ and that of NADPH from NADP+ occur by unlike pathways and are independently regulated, so that the cell can independently adjust the supply of electrons for these two contrasting purposes. Inside the jail cell the ratio of NAD+ to NADH is kept high, whereas the ratio of NADP+ to NADPH is kept depression. This provides plenty of NAD+ to human activity as an oxidizing agent and plenty of NADPH to human activity equally a reducing amanuensis—as required for their special roles in catabolism and anabolism, respectively.
There Are Many Other Activated Carrier Molecules in Cells
Other activated carriers too pick up and conduct a chemic group in an hands transferred, high-free energy linkage (Tabular array two-six). For case, coenzyme A carries an acetyl group in a readily transferable linkage, and in this activated course is known as acetyl CoA (acetyl coenzyme A). The structure of acetyl CoA is illustrated in Figure 2-62; it is used to add together ii carbon units in the biosynthesis of larger molecules.
Table 2-six
Some Activated Carrier Molecules Widely Used in Metabolism.
Effigy ii-62
The structure of the important activated carrier molecule acetyl CoA. A space-filling model is shown above the construction. The sulfur atom (yellow) forms a thioester bond to acetate. Considering this is a high-energy linkage, releasing a large amount of free (more than...)
In acetyl CoA and the other carrier molecules in Table 2-6, the transferable group makes upwards only a small-scale function of the molecule. The rest consists of a large organic portion that serves as a user-friendly "handle," facilitating the recognition of the carrier molecule by specific enzymes. As with acetyl CoA, this handle portion very often contains a nucleotide, a curious fact that may be a relic from an early phase of development. It is currently thought that the principal catalysts for early life-forms—earlier DNA or proteins—were RNA molecules (or their shut relatives), as described in Affiliate 6. It is tempting to speculate that many of the carrier molecules that we discover today originated in this earlier RNA world, where their nucleotide portions could have been useful for bounden them to RNA enzymes.
Examples of the type of transfer reactions catalyzed by the activated carrier molecules ATP (transfer of phosphate) and NADPH (transfer of electrons and hydrogen) accept been presented in Figures ii-58 and 2-61, respectively. The reactions of other activated carrier molecules involve the transfers of methyl, carboxyl, or glucose group, for the purpose of biosynthesis. The activated carriers required are normally generated in reactions that are coupled to ATP hydrolysis, every bit in the case in Figure two-63. Therefore, the energy that enables their groups to exist used for biosynthesis ultimately comes from the catabolic reactions that generate ATP. Similar processes occur in the synthesis of the very large molecules of the jail cell—the nucleic acids, proteins, and polysaccharides—that we discuss next.
Figure ii-63
A carboxyl group transfer reaction using an activated carrier molecule. Carboxylated biotin is used by the enzyme pyruvate carboxylase to transfer a carboxyl grouping in the production of oxaloacetate, a molecule needed for the citric acid wheel. The acceptor (more...)
The Synthesis of Biological Polymers Requires an Energy Input
Equally discussed previously, the macromolecules of the cell found the vast bulk of its dry mass—that is, of the mass not due to water (run across Figure 2-29). These molecules are made from subunits (or monomers) that are linked together in a condensation reaction, in which the constituents of a water molecule (OH plus H) are removed from the two reactants. Consequently, the reverse reaction—the breakdown of all three types of polymers—occurs by the enzyme-catalyzed addition of water (hydrolysis). This hydrolysis reaction is energetically favorable, whereas the biosynthetic reactions require an energy input and are more complex (Figure 2-64).
Figure two-64
Condensation and hydrolysis as opposite reactions. The macromolecules of the cell are polymers that are formed from subunits (or monomers) by a condensation reaction and are cleaved down by hydrolysis. The condensation reactions are all energetically unfavorable. (more than...)
The nucleic acids (Dna and RNA), proteins, and polysaccharides are all polymers that are produced by the repeated addition of a subunit (too called a monomer) onto i end of a growing chain. The synthesis reactions for these three types of macromolecules are outlined in Figure 2-65. Every bit indicated, the condensation footstep in each case depends on energy from nucleoside triphosphate hydrolysis. And even so, except for the nucleic acids, there are no phosphate groups left in the last product molecules. How are the reactions that release the energy of ATP hydrolysis coupled to polymer synthesis?
Figure 2-65
The synthesis of polysaccharides, proteins, and nucleic acids. Synthesis of each kind of biological polymer involves the loss of water in a condensation reaction. Not shown is the consumption of high-energy nucleoside triphosphates that is required to (more...)
For each blazon of macromolecule, an enzyme-catalyzed pathway exists which resembles that discussed previously for the synthesis of the amino acrid glutamine (come across Figure 2-59). The principle is exactly the aforementioned, in that the OH grouping that volition be removed in the condensation reaction is first activated past condign involved in a loftier-energy linkage to a 2d molecule. Still, the actual mechanisms used to link ATP hydrolysis to the synthesis of proteins and polysaccharides are more complex than that used for glutamine synthesis, since a series of loftier-energy intermediates is required to generate the final high-energy bond that is broken during the condensation step (discussed in Chapter vi for protein synthesis).
There are limits to what each activated carrier can do in driving biosynthesis. The ΔK for the hydrolysis of ATP to ADP and inorganic phosphate (Pi) depends on the concentrations of all of the reactants, just under the usual conditions in a jail cell it is between -11 and -13 kcal/mole. In principle, this hydrolysis reaction can exist used to drive an unfavorable reaction with a ΔG of, possibly, +x kcal/mole, provided that a suitable reaction path is available. For some biosynthetic reactions, however, fifty-fifty -13 kcal/mole may not be enough. In these cases the path of ATP hydrolysis can exist contradistinct so that information technology initially produces AMP and pyrophosphate (PPi), which is itself and then hydrolyzed in a subsequent stride (Effigy two-66). The whole process makes available a full gratis-energy change of nearly -26 kcal/mole. An important biosynthetic reaction that is driven in this mode is nucleic acrid (polynucleotide) synthesis, as illustrated in Figure 2-67.
Figure 2-66
An alternative route for the hydrolysis of ATP, in which pyrophosphate is first formed and so hydrolyzed. This route releases about twice as much energy as the reaction shown before in Figure 2-57. (A) In the two successive hydrolysis reactions, (more...)
Figure 2-67
Synthesis of a polynucleotide, RNA or DNA, is a multistep process driven by ATP hydrolysis. In the first footstep, a nucleoside monophosphate is activated by the sequential transfer of the final phosphate groups from two ATP molecules. The high-energy (more...)
It is interesting to note that the polymerization reactions that produce macromolecules can be oriented in one of two means, giving ascent to either the head polymerization or the tail polymerization of monomers. In head polymerization the reactive bond required for the condensation reaction is carried on the end of the growing polymer, and information technology must therefore be regenerated each time that a monomer is added. In this case, each monomer brings with information technology the reactive bond that volition exist used in adding the next monomer in the serial. In tail polymerization the reactive bond carried by each monomer is instead used immediately for its own improver (Figure 2-68).
Figure 2-68
The orientation of the active intermediates in biological polymerization reactions. The head growth of polymers is compared with its alternative tail growth. As indicated, these two mechanisms are used to produce dissimilar biological macromolecules.
We shall see in afterwards chapters that both these types of polymerization are used. The synthesis of polynucleotides and some simple polysaccharides occurs by tail polymerization, for instance, whereas the synthesis of proteins occurs by a head polymerization process.
Summary
Living cells are highly ordered and need to create lodge inside themselves in order to survive and abound. This is thermodynamically possible simply because of a continual input of energy, part of which must be released from the cells to their environment equally heat. The energy comes ultimately from the electromagnetic radiation of the sun, which drives the formation of organic molecules in photosynthetic organisms such as light-green plants. Animals obtain their energy by eating these organic molecules and oxidizing them in a series of enzyme-catalyzed reactions that are coupled to the formation of ATP—a common currency of energy in all cells.
To make possible the continual generation of club in cells, the energetically favorable hydrolysis of ATP is coupled to energetically unfavorable reactions. In the biosynthesis of macromolecules, this is achieved by the transfer of phosphate groups to form reactive phosphorylated intermediates. Because the energetically unfavorable reaction at present becomes energetically favorable, ATP hydrolysis is said to drive the reaction. Polymeric molecules such as proteins, nucleic acids, and polysaccharides are assembled from small activated precursor molecules past repetitive condensation reactions that are driven in this fashion. Other reactive molecules, chosen either active carriers or coenzymes, transfer other chemical groups in the course of biosynthesis: NADPH transfers hydrogen equally a proton plus 2 electrons (a hydride ion), for instance, whereas acetyl CoA transfers an acetyl group.
Source: https://www.ncbi.nlm.nih.gov/books/NBK26838/
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