How Do Animal Cells Convert Raw Nutrients Into Energy?
As we have just seen, cells require a abiding supply of energy to generate and maintain the biological order that keeps them alive. This energy is derived from the chemical bond energy in nutrient molecules, which thereby serve as fuel for cells.
Sugars are especially important fuel molecules, and they are oxidized in small steps to carbon dioxide (CO2) and water (Figure 2-69). In this department we trace the major steps in the breakdown, or catabolism, of sugars and show how they produce ATP, NADH, and other activated carrier molecules in animal cells. Nosotros concentrate on glucose breakdown, since information technology dominates free energy production in most creature cells. A very similar pathway also operates in plants, fungi, and many bacteria. Other molecules, such as fatty acids and proteins, tin also serve every bit energy sources when they are funneled through advisable enzymatic pathways.
Figure 2-69
Schematic representation of the controlled stepwise oxidation of carbohydrate in a cell, compared with ordinary burning. (A) In the jail cell, enzymes catalyze oxidation via a series of small steps in which complimentary energy is transferred in conveniently sized packets (more...)
Food Molecules Are Broken Down in Three Stages to Produce ATP
The proteins, lipids, and polysaccharides that make up most of the food we consume must be broken down into smaller molecules before our cells can utilize them—either equally a source of energy or equally building blocks for other molecules. The breakdown processes must human action on nutrient taken in from outside, but not on the macromolecules inside our ain cells. Phase 1 in the enzymatic breakdown of food molecules is therefore digestion, which occurs either in our intestine outside cells, or in a specialized organelle within cells, the lysosome. (A membrane that surrounds the lysosome keeps its digestive enzymes separated from the cytosol, as described in Chapter 13.) In either case, the big polymeric molecules in food are broken down during digestion into their monomer subunits—proteins into amino acids, polysaccharides into sugars, and fats into fatty acids and glycerol—through the action of enzymes. Afterward digestion, the small organic molecules derived from food enter the cytosol of the cell, where their gradual oxidation begins. As illustrated in Effigy 2-70, oxidation occurs in 2 farther stages of cellular catabolism: stage 2 starts in the cytosol and ends in the major energy-converting organelle, the mitochondrion; phase 3 is entirely confined to the mitochondrion.
Figure 2-70
Simplified diagram of the iii stages of cellular metabolism that lead from food to waste product products in animal cells. This series of reactions produces ATP, which is then used to drive biosynthetic reactions and other energy-requiring processes in the (more than...)
In phase 2 a chain of reactions chosen glycolysis converts each molecule of glucose into two smaller molecules of pyruvate. Sugars other than glucose are similarly converted to pyruvate subsequently their conversion to ane of the sugar intermediates in this glycolytic pathway. During pyruvate formation, two types of activated carrier molecules are produced—ATP and NADH. The pyruvate and so passes from the cytosol into mitochondria. There, each pyruvate molecule is converted into COtwo plus a ii-carbon acetyl group—which becomes fastened to coenzyme A (CoA), forming acetyl CoA, another activated carrier molecule (see Effigy 2-62). Large amounts of acetyl CoA are also produced past the stepwise breakdown and oxidation of fat acids derived from fats, which are carried in the bloodstream, imported into cells as fatty acids, and and then moved into mitochondria for acetyl CoA production.
Phase 3 of the oxidative breakdown of food molecules takes place entirely in mitochondria. The acetyl group in acetyl CoA is linked to coenzyme A through a high-energy linkage, and information technology is therefore easily transferable to other molecules. After its transfer to the four-carbon molecule oxaloacetate, the acetyl group enters a series of reactions called the citric acid cycle. Every bit nosotros discuss shortly, the acetyl group is oxidized to CO2 in these reactions, and big amounts of the electron carrier NADH are generated. Finally, the high-energy electrons from NADH are passed forth an electron-transport chain within the mitochondrial inner membrane, where the energy released by their transfer is used to bulldoze a procedure that produces ATP and consumes molecular oxygen (O2). It is in these concluding steps that most of the energy released by oxidation is harnessed to produce nigh of the cell's ATP.
Because the energy to drive ATP synthesis in mitochondria ultimately derives from the oxidative breakdown of food molecules, the phosphorylation of ADP to grade ATP that is driven by electron transport in the mitochondrion is known as oxidative phosphorylation. The fascinating events that occur within the mitochondrial inner membrane during oxidative phosphorylation are the major focus of Chapter 14.
Through the production of ATP, the energy derived from the breakup of sugars and fats is redistributed as packets of chemic free energy in a form convenient for use elsewhere in the prison cell. Roughly 109 molecules of ATP are in solution in a typical cell at any instant, and in many cells, all this ATP is turned over (that is, used up and replaced) every 1–ii minutes.
In all, nearly half of the energy that could in theory exist derived from the oxidation of glucose or fatty acids to HtwoO and CO2 is captured and used to bulldoze the energetically unfavorable reaction Pi + ADP → ATP. (By contrast, a typical combustion engine, such as a car engine, tin can convert no more than 20% of the available energy in its fuel into useful work.) The rest of the energy is released past the cell as rut, making our bodies warm.
Glycolysis Is a Central ATP-producing Pathway
The virtually important process in stage ii of the breakdown of food molecules is the degradation of glucose in the sequence of reactions known every bit glycolysis—from the Greek glukus, "sweetness," and lusis, "rupture." Glycolysis produces ATP without the interest of molecular oxygen (O2 gas). Information technology occurs in the cytosol of well-nigh cells, including many anaerobic microorganisms (those that can live without utilizing molecular oxygen). Glycolysis probably evolved early in the history of life, before the activities of photosynthetic organisms introduced oxygen into the atmosphere. During glycolysis, a glucose molecule with six carbon atoms is converted into two molecules of pyruvate, each of which contains three carbon atoms. For each molecule of glucose, two molecules of ATP are hydrolyzed to provide energy to bulldoze the early steps, just four molecules of ATP are produced in the later steps. At the terminate of glycolysis, in that location is consequently a cyberspace proceeds of two molecules of ATP for each glucose molecule broken down.
The glycolytic pathway is presented in outline in Figure 2-71, and in more than item in Panel two-viii (pp. 124–125). Glycolysis involves a sequence of x separate reactions, each producing a different carbohydrate intermediate and each catalyzed by a different enzyme. Like most enzymes, these enzymes all have names ending in ase—like isomerase and dehydrogenase—which indicate the blazon of reaction they catalyze.
Figure two-71
An outline of glycolysis. Each of the 10 steps shown is catalyzed by a different enzyme. Annotation that step four cleaves a six-carbon sugar into two three-carbon sugars, so that the number of molecules at every stage after this doubles. As indicated, step 6 (more than...)
Console 2-eight
Details of the ten Steps of Glycolysis.
Although no molecular oxygen is involved in glycolysis, oxidation occurs, in that electrons are removed by NAD+ (producing NADH) from some of the carbons derived from the glucose molecule. The stepwise nature of the process allows the free energy of oxidation to be released in pocket-size packets, so that much of information technology can be stored in activated carrier molecules rather than all of it being released as heat (see Figure ii-69). Thus, some of the energy released by oxidation drives the direct synthesis of ATP molecules from ADP and Pi, and some remains with the electrons in the high-energy electron carrier NADH.
Two molecules of NADH are formed per molecule of glucose in the course of glycolysis. In aerobic organisms (those that require molecular oxygen to live), these NADH molecules donate their electrons to the electron-transport concatenation described in Chapter 14, and the NAD+ formed from the NADH is used again for glycolysis (encounter step 6 in Console two-8, pp. 124–125).
Fermentations Permit ATP to Be Produced in the Absenteeism of Oxygen
For most fauna and plant cells, glycolysis is only a prelude to the third and final stage of the breakdown of food molecules. In these cells, the pyruvate formed at the last step of stage two is rapidly transported into the mitochondria, where it is converted into CO2 plus acetyl CoA, which is then completely oxidized to COtwo and H2O.
In contrast, for many anaerobic organisms—which practice not use molecular oxygen and tin abound and carve up without information technology—glycolysis is the principal source of the cell's ATP. This is also true for certain animate being tissues, such as skeletal muscle, that tin can continue to office when molecular oxygen is limiting. In these anaerobic weather, the pyruvate and the NADH electrons stay in the cytosol. The pyruvate is converted into products excreted from the cell—for instance, into ethanol and CO2 in the yeasts used in brewing and breadmaking, or into lactate in muscle. In this process, the NADH gives upwards its electrons and is converted back into NAD+. This regeneration of NAD+ is required to maintain the reactions of glycolysis (Effigy 2-72).
Figure 2-72
Two pathways for the anaerobic breakdown of pyruvate. (A) When inadequate oxygen is present, for instance, in a muscle cell undergoing vigorous wrinkle, the pyruvate produced by glycolysis is converted to lactate equally shown. This reaction regenerates (more...)
Anaerobic energy-yielding pathways like these are called fermentations. Studies of the commercially important fermentations carried out by yeasts inspired much of early biochemistry. Work in the nineteenth century led in 1896 to the then startling recognition that these processes could be studied outside living organisms, in jail cell extracts. This revolutionary discovery somewhen fabricated it possible to dissect out and study each of the private reactions in the fermentation procedure. The piecing together of the complete glycolytic pathway in the 1930s was a major triumph of biochemistry, and it was apace followed by the recognition of the central function of ATP in cellular processes. Thus, about of the key concepts discussed in this chapter have been understood for more than 50 years.
Glycolysis Illustrates How Enzymes Couple Oxidation to Energy Storage
We have previously used a "paddle wheel" analogy to explain how cells harvest useful energy from the oxidation of organic molecules past using enzymes to couple an energetically unfavorable reaction to an energetically favorable 1 (see Figure 2-56). Enzymes play the part of the paddle bike in our analogy, and we now render to a step in glycolysis that nosotros have previously discussed, in order to illustrate exactly how coupled reactions occur.
Two central reactions in glycolysis (steps 6 and 7) convert the three-carbon sugar intermediate glyceraldehyde 3-phosphate (an aldehyde) into 3-phosphoglycerate (a carboxylic acid). This entails the oxidation of an aldehyde group to a carboxylic acid group, which occurs in two steps. The overall reaction releases plenty free energy to convert a molecule of ADP to ATP and to transfer two electrons from the aldehyde to NAD+ to form NADH, while still releasing enough oestrus to the environs to make the overall reaction energetically favorable (ΔG° for the overall reaction is -3.0 kcal/mole).
The pathway by which this remarkable feat is achieved is outlined in Effigy 2-73. The chemical reactions are guided past two enzymes to which the sugar intermediates are tightly bound. The first enzyme (glyceraldehyde 3-phosphate dehydrogenase) forms a brusk-lived covalent bond to the aldehyde through a reactive -SH group on the enzyme, and it catalyzes the oxidation of this aldehyde while still in the attached land. The high-free energy enzyme-substrate bail created by the oxidation is and so displaced past an inorganic phosphate ion to produce a high-energy sugar-phosphate intermediate, which is thereby released from the enzyme. This intermediate and so binds to the 2nd enzyme (phosphoglycerate kinase). This enzyme catalyzes the energetically favorable transfer of the high-free energy phosphate only created to ADP, forming ATP and completing the process of oxidizing an aldehyde to a carboxylic acid (see Figure two-73).
Figure two-73
Free energy storage in steps half-dozen and 7 of glycolysis. In these steps the oxidation of an aldehyde to a carboxylic acid is coupled to the formation of ATP and NADH. (A) Pace half-dozen begins with the formation of a covalent bail between the substrate (glyceraldehyde (more...)
Nosotros accept shown this particular oxidation process in some particular because it provides a articulate example of enzyme-mediated energy storage through coupled reactions (Figure 2-74). These reactions (steps 6 and 7) are the only ones in glycolysis that create a high-energy phosphate linkage straight from inorganic phosphate. Every bit such, they account for the net yield of two ATP molecules and two NADH molecules per molecule of glucose (meet Panel 2-8, pp. 124–125).
Figure 2-74
Schematic view of the coupled reactions that grade NADH and ATP in steps 6 and 7 of glycolysis. The C-H bond oxidation energy drives the formation of both NADH and a high-energy phosphate bond. The breakage of the high-free energy bond and then drives ATP germination. (more...)
As we have just seen, ATP can be formed readily from ADP when reaction intermediates are formed with college-energy phosphate bonds than those in ATP. Phosphate bonds can exist ordered in energy by comparison the standard gratuitous-free energy change (ΔG°) for the breakage of each bond by hydrolysis. Figure 2-75 compares the high-energy phosphoanhydride bonds in ATP with other phosphate bonds, several of which are generated during glycolysis.
Figure two-75
Some phosphate bond energies. The transfer of a phosphate group from any molecule ane to any molecule ii is energetically favorable if the standard free-energy change (ΔK°) for the hydrolysis of the phosphate bond in molecule one is more negative (more...)
Sugars and Fats Are Both Degraded to Acetyl CoA in Mitochondria
Nosotros now move on to consider stage three of catabolism, a process that requires abundant molecular oxygen (O2 gas). Since the Earth is thought to have developed an atmosphere containing O2 gas between one and two billion years agone, whereas abundant life-forms are known to take existed on the World for 3.5 billion years, the utilise of Oii in the reactions that we discuss side by side is thought to be of relatively recent origin. In contrast, the mechanism used to produce ATP in Effigy two-73 does not require oxygen, and relatives of this elegant pair of coupled reactions could have arisen very early in the history of life on Earth.
In aerobic metabolism, the pyruvate produced past glycolysis is rapidly decarboxylated by a giant circuitous of three enzymes, called the pyruvate dehydrogenase complex. The products of pyruvate decarboxylation are a molecule of CO2 (a waste), a molecule of NADH, and acetyl CoA. The three-enzyme complex is located in the mitochondria of eucaryotic cells; its structure and mode of action are outlined in Figure 2-76.
Effigy 2-76
The oxidation of pyruvate to acetyl CoA and CO2. (A) The structure of the pyruvate dehydrogenase complex, which contains threescore polypeptide chains. This is an example of a large multienzyme complex in which reaction intermediates are passed directly from (more...)
The enzymes that degrade the fatty acids derived from fats too produce acetyl CoA in mitochondria. Each molecule of fatty acid (as the activated molecule fatty acyl CoA) is broken down completely by a cycle of reactions that trims two carbons at a fourth dimension from its carboxyl stop, generating i molecule of acetyl CoA for each turn of the cycle. A molecule of NADH and a molecule of FADH2 are as well produced in this process (Figure 2-77).
Figure 2-77
The oxidation of fatty acids to acetyl CoA. (A) Electron micrograph of a lipid droplet in the cytoplasm (height), and the construction of fats (lesser). Fats are triacylglycerols. The glycerol portion, to which iii fat acids are linked through ester bonds, (more than...)
Sugars and fats provide the major free energy sources for nearly non-photosynthetic organisms, including humans. However, the majority of the useful energy that tin be extracted from the oxidation of both types of foodstuffs remains stored in the acetyl CoA molecules that are produced by the two types of reactions merely described. The citric acid cycle of reactions, in which the acetyl group in acetyl CoA is oxidized to COtwo and H2O, is therefore key to the free energy metabolism of aerobic organisms. In eucaryotes these reactions all take place in mitochondria, the organelle to which pyruvate and fatty acids are directed for acetyl CoA production (Figure ii-78). We should therefore non be surprised to discover that the mitochondrion is the place where most of the ATP is produced in beast cells. In contrast, aerobic bacteria carry out all of their reactions in a single compartment, the cytosol, and information technology is here that the citric acid bicycle takes identify in these cells.
Figure 2-78
Pathways for the production of acetyl CoA from sugars and fats. The mitochondrion in eucaryotic cells is the identify where acetyl CoA is produced from both types of major nutrient molecules. It is therefore the place where near of the jail cell's oxidation reactions (more...)
The Citric Acid Cycle Generates NADH by Oxidizing Acetyl Groups to CO2
In the nineteenth century, biologists noticed that in the absenteeism of air (anaerobic conditions) cells produce lactic acid (for example, in musculus) or ethanol (for instance, in yeast), while in its presence (aerobic atmospheric condition) they consume O2 and produce CO2 and H2O. Intensive efforts to ascertain the pathways of aerobic metabolism eventually focused on the oxidation of pyruvate and led in 1937 to the discovery of the citric acid cycle, besides known every bit the tricarboxylic acid bicycle or the Krebs bicycle. The citric acid cycle accounts for about two-thirds of the total oxidation of carbon compounds in most cells, and its major end products are CO2 and high-free energy electrons in the form of NADH. The CO2 is released every bit a waste matter product, while the loftier-energy electrons from NADH are passed to a membrane-bound electron-transport concatenation, eventually combining with O2 to produce H2O. Although the citric acid cycle itself does not utilise O2, it requires Otwo in gild to proceed considering in that location is no other efficient way for the NADH to get rid of its electrons and thus regenerate the NAD+ that is needed to continue the wheel going.
The citric acid cycle, which takes place inside mitochondria in eucaryotic cells, results in the consummate oxidation of the carbon atoms of the acetyl groups in acetyl CoA, converting them into CO2. But the acetyl grouping is non oxidized directly. Instead, this grouping is transferred from acetyl CoA to a larger, 4-carbon molecule, oxaloacetate, to form the six-carbon tricarboxylic acid, citric acid, for which the subsequent wheel of reactions is named. The citric acrid molecule is then gradually oxidized, allowing the energy of this oxidation to be harnessed to produce energy-rich activated carrier molecules. The concatenation of viii reactions forms a wheel considering at the end the oxaloacetate is regenerated and enters a new plough of the cycle, as shown in outline in Figure 2-79.
Effigy 2-79
Simple overview of the citric acrid bicycle. The reaction of acetyl CoA with oxaloacetate starts the bicycle past producing citrate (citric acid). In each plow of the cycle, two molecules of COtwo are produced as waste products, plus three molecules of NADH, 1 (more...)
Nosotros have thus far discussed only i of the three types of activated carrier molecules that are produced by the citric acid cycle, the NAD+-NADH pair (encounter Figure 2-60). In add-on to three molecules of NADH, each plow of the wheel also produces one molecule of FADH 2 (reduced flavin adenine dinucleotide) from FAD and 1 molecule of the ribonucleotide GTP (guanosine triphosphate) from GDP. The structures of these two activated carrier molecules are illustrated in Figure 2-80. GTP is a close relative of ATP, and the transfer of its terminal phosphate group to ADP produces one ATP molecule in each wheel. Like NADH, FADH2 is a carrier of high-energy electrons and hydrogen. Equally we talk over shortly, the energy that is stored in the readily transferred loftier-energy electrons of NADH and FADH2 will be utilized after for ATP production through the process of oxidative phosphorylation, the only step in the oxidative catabolism of foodstuffs that directly requires gaseous oxygen (Oii) from the atmosphere.
Effigy 2-80
The structures of GTP and FADH2. (A) GTP and Gdp are shut relatives of ATP and ADP, respectively. (B) FADH2 is a carrier of hydrogens and high-energy electrons, like NADH and NADPH. It is shown hither in its oxidized form (FAD) with the hydrogen-carrying (more than...)
The complete citric acid bicycle is presented in Panel two-9 (pp. 126–127). The extra oxygen atoms required to brand CO2 from the acetyl groups inbound the citric acid bike are supplied non past molecular oxygen, merely by water. As illustrated in the panel, 3 molecules of water are divide in each cycle, and the oxygen atoms of some of them are ultimately used to make COtwo.
In add-on to pyruvate and fatty acids, some amino acids pass from the cytosol into mitochondria, where they are also converted into acetyl CoA or one of the other intermediates of the citric acid wheel. Thus, in the eucaryotic cell, the mitochondrion is the eye toward which all energy-yielding processes lead, whether they brainstorm with sugars, fats, or proteins.
The citric acid cycle also functions as a starting betoken for important biosynthetic reactions past producing vital carbon-containing intermediates, such as oxaloacetate and α-ketoglutarate. Some of these substances produced by catabolism are transferred back from the mitochondrion to the cytosol, where they serve in anabolic reactions as precursors for the synthesis of many essential molecules, such as amino acids.
Electron Send Drives the Synthesis of the Majority of the ATP in Most Cells
It is in the last step in the degradation of a food molecule that the major portion of its chemical free energy is released. In this final process the electron carriers NADH and FADH2 transfer the electrons that they accept gained when oxidizing other molecules to the electron-transport chain, which is embedded in the inner membrane of the mitochondrion. As the electrons pass along this long concatenation of specialized electron acceptor and donor molecules, they fall to successively lower energy states. The free energy that the electrons release in this procedure is used to pump H+ ions (protons) across the membrane—from the inner mitochondrial compartment to the outside (Figure two-81). A slope of H+ ions is thereby generated. This gradient serves as a source of energy, being tapped like a battery to drive a variety of energy-requiring reactions. The most prominent of these reactions is the generation of ATP by the phosphorylation of ADP.
Figure 2-81
The generation of an H+ slope across a membrane by electron-transport reactions. A high-free energy electron (derived, for instance, from the oxidation of a metabolite) is passed sequentially by carriers A, B, and C to a lower energy state. In this diagram (more...)
At the end of this series of electron transfers, the electrons are passed to molecules of oxygen gas (O2) that have diffused into the mitochondrion, which simultaneously combine with protons (H+) from the surrounding solution to produce molecules of h2o. The electrons have at present reached their everyman free energy level, and therefore all the available energy has been extracted from the food molecule being oxidized. This procedure, termed oxidative phosphorylation (Figure 2-82), also occurs in the plasma membrane of bacteria. As ane of the almost remarkable achievements of cellular evolution, information technology will be a central topic of Affiliate 14.
Figure 2-82
The final stages of oxidation of food molecules. Molecules of NADH and FADHii (FADHtwo is non shown) are produced by the citric acid cycle. These activated carriers donate high-energy electrons that are somewhen used to reduce oxygen gas to h2o. A major (more...)
In total, the consummate oxidation of a molecule of glucose to H2O and CO2 is used past the jail cell to produce almost 30 molecules of ATP. In contrast, just 2 molecules of ATP are produced per molecule of glucose by glycolysis alone.
Organisms Store Nutrient Molecules in Special Reservoirs
All organisms need to maintain a loftier ATP/ADP ratio, if biological society is to be maintained in their cells. Yet animals have just periodic admission to food, and plants need to survive overnight without sunlight, without the possibility of sugar production from photosynthesis. For this reason, both plants and animals convert sugars and fats to special forms for storage (Effigy 2-83).
Figure 2-83
The storage of sugars and fats in animal and plant cells. (A) The structures of starch and glycogen, the storage form of sugars in plants and animals, respectively. Both are storage polymers of the sugar glucose and differ just in the frequency of branch (more...)
To compensate for long periods of fasting, animals store fatty acids as fatty droplets composed of h2o-insoluble triacylglycerols, largely in specialized fat cells. And for shorter-term storage, sugar is stored as glucose subunits in the large branched polysaccharide glycogen, which is nowadays as small granules in the cytoplasm of many cells, including liver and muscle. The synthesis and deposition of glycogen are speedily regulated co-ordinate to demand. When more ATP is needed than can exist generated from the food molecules taken in from the bloodstream, cells break down glycogen in a reaction that produces glucose 1-phosphate, which enters glycolysis.
Quantitatively, fat is a far more of import storage form than glycogen, in part considering the oxidation of a gram of fat releases about twice as much energy as the oxidation of a gram of glycogen. Moreover, glycogen differs from fat in bounden a bang-up deal of water, producing a sixfold divergence in the actual mass of glycogen required to store the same amount of energy every bit fatty. An average adult human stores enough glycogen for only well-nigh a day of normal activities but enough fat to terminal for nearly a month. If our master fuel reservoir had to be carried as glycogen instead of fat, body weight would demand to exist increased by an average of about 60 pounds.
Most of our fatty is stored in adipose tissue, from which it is released into the bloodstream for other cells to utilize as needed. The need arises after a catamenia of not eating; even a normal overnight fast results in the mobilization of fat, so that in the morn most of the acetyl CoA entering the citric acid cycle is derived from fatty acids rather than from glucose. After a repast, nonetheless, nigh of the acetyl CoA entering the citric acid cycle comes from glucose derived from nutrient, and whatsoever excess glucose is used to replenish depleted glycogen stores or to synthesize fats. (While animal cells readily convert sugars to fats, they cannot convert fatty acids to sugars.)
Although plants produce NADPH and ATP by photosynthesis, this of import process occurs in a specialized organelle, called a chloroplast, which is isolated from the remainder of the constitute cell by a membrane that is impermeable to both types of activated carrier molecules. Moreover, the institute contains many other cells—such every bit those in the roots—that lack chloroplasts and therefore cannot produce their own sugars or ATP. Therefore, for well-nigh of its ATP product, the plant relies on an export of sugars from its chloroplasts to the mitochondria that are located in all cells of the institute. About of the ATP needed by the plant is synthesized in these mitochondria and exported from them to the rest of the plant jail cell, using exactly the aforementioned pathways for the oxidative breakdown of sugars that are utilized past nonphotosynthetic organisms (Effigy ii-84).
Figure 2-84
How the ATP needed for most plant jail cell metabolism is made. In plants, the chloroplasts and mitochondria interact to supply cells with metabolites and ATP.
During periods of excess photosynthetic capacity during the mean solar day, chloroplasts catechumen some of the sugars that they brand into fats and into starch, a polymer of glucose analogous to the glycogen of animals. The fats in plants are triacylglycerols, just like the fats in animals, and differ just in the types of fatty acids that predominate. Fatty and starch are both stored in the chloroplast every bit reservoirs to exist mobilized as an energy source during periods of darkness (see Figure two-83B).
The embryos inside plant seeds must live on stored sources of energy for a prolonged period, until they germinate to produce leaves that can harvest the energy in sunlight. For this reason constitute seeds often contain particularly large amounts of fats and starch—which makes them a major nutrient source for animals, including ourselves (Figure ii-85).
Effigy 2-85
Some constitute seeds that serve as important foods for humans. Corn, nuts, and peas all contain rich stores of starch and fat that provide the young establish embryo in the seed with energy and edifice blocks for biosynthesis. (Courtesy of the John Innes Foundation.) (more...)
Amino Acids and Nucleotides Are Office of the Nitrogen Cycle
In our discussion so far nosotros have concentrated mainly on saccharide metabolism. We have not yet considered the metabolism of nitrogen or sulfur. These two elements are constituents of proteins and nucleic acids, which are the two nearly important classes of macromolecules in the cell and make upward approximately ii-thirds of its dry weight. Atoms of nitrogen and sulfur pass from compound to chemical compound and between organisms and their environment in a serial of reversible cycles.
Although molecular nitrogen is abundant in the Earth's atmosphere, nitrogen is chemically unreactive equally a gas. Only a few living species are able to comprise it into organic molecules, a process called nitrogen fixation. Nitrogen fixation occurs in certain microorganisms and past some geophysical processes, such every bit lightning belch. It is essential to the biosphere as a whole, for without it life would not exist on this planet. Only a small fraction of the nitrogenous compounds in today's organisms, notwithstanding, is due to fresh products of nitrogen fixation from the atmosphere. Most organic nitrogen has been in circulation for some time, passing from one living organism to another. Thus nowadays-mean solar day nitrogen-fixing reactions can be said to perform a "topping-up" part for the total nitrogen supply.
Vertebrates receive virtually all of their nitrogen in their dietary intake of proteins and nucleic acids. In the body these macromolecules are broken downwardly to amino acids and the components of nucleotides, and the nitrogen they contain is used to produce new proteins and nucleic acids or utilized to make other molecules. About one-half of the 20 amino acids institute in proteins are essential amino acids for vertebrates (Figure 2-86), which means that they cannot be synthesized from other ingredients of the diet. The others tin be then synthesized, using a variety of raw materials, including intermediates of the citric acid cycle as described below. The essential amino acids are made by nonvertebrate organisms, usually by long and energetically expensive pathways that accept been lost in the course of vertebrate evolution.
Figure 2-86
The nine essential amino acids. These cannot be synthesized by homo cells and then must exist supplied in the nutrition.
The nucleotides needed to make RNA and DNA can be synthesized using specialized biosynthetic pathways: there are no "essential nucleotides" that must be provided in the diet. All of the nitrogens in the purine and pyrimidine bases (as well as some of the carbons) are derived from the plentiful amino acids glutamine, aspartic acid, and glycine, whereas the ribose and deoxyribose sugars are derived from glucose.
Amino acids that are not utilized in biosynthesis can be oxidized to generate metabolic energy. Most of their carbon and hydrogen atoms eventually form CO2 or H2O, whereas their nitrogen atoms are shuttled through various forms and eventually appear as urea, which is excreted. Each amino acid is processed differently, and a whole constellation of enzymatic reactions exists for their catabolism.
Many Biosynthetic Pathways Begin with Glycolysis or the Citric Acid Cycle
Catabolism produces both free energy for the jail cell and the building blocks from which many other molecules of the cell are made (see Effigy 2-36). Thus far, our discussions of glycolysis and the citric acid cycle have emphasized energy product, rather than the provision of the starting materials for biosynthesis. Simply many of the intermediates formed in these reaction pathways are too siphoned off by other enzymes that use them to produce the amino acids, nucleotides, lipids, and other small organic molecules that the prison cell needs. Some thought of the complication of this procedure can be gathered from Figure 2-87, which illustrates some of the branches from the fundamental catabolic reactions that lead to biosyntheses.
Figure 2-87
Glycolysis and the citric acid cycle provide the precursors needed to synthesize many important biological molecules. The amino acids, nucleotides, lipids, sugars, and other molecules—shown here as products—in turn serve as the precursors (more...)
The existence of and so many branching pathways in the cell requires that the choices at each co-operative be advisedly regulated, as we discuss side by side.
Metabolism Is Organized and Regulated
One gets a sense of the intricacy of a cell equally a chemical machine from the relation of glycolysis and the citric acid cycle to the other metabolic pathways sketched out in Figure ii-88. This type of nautical chart, which was used earlier in this chapter to innovate metabolism, represents only some of the enzymatic pathways in a jail cell. Information technology is obvious that our discussion of cell metabolism has dealt with only a tiny fraction of cellular chemistry.
Figure ii-88
Glycolysis and the citric acid cycle are at the center of metabolism. Some 500 metabolic reactions of a typical cell are shown schematically with the reactions of glycolysis and the citric acid cycle in red. Other reactions either atomic number 82 into these 2 (more...)
All these reactions occur in a cell that is less than 0.1 mm in diameter, and each requires a different enzyme. Every bit is clear from Figure ii-88, the same molecule can oft exist part of many different pathways. Pyruvate, for case, is a substrate for half a dozen or more unlike enzymes, each of which modifies it chemically in a different mode. Ane enzyme converts pyruvate to acetyl CoA, another to oxaloacetate; a third enzyme changes pyruvate to the amino acid alanine, a fourth to lactate, and so on. All of these dissimilar pathways compete for the same pyruvate molecule, and similar competitions for thousands of other pocket-sized molecules keep at the same fourth dimension. A better sense of this complication tin perhaps be attained from a three-dimensional metabolic map that allows the connections betwixt pathways to exist made more directly (Figure two-89).
Figure 2-89
A representation of all of the known metabolic reactions involving small molecules in a yeast prison cell. As in Effigy two-88, the reactions of glycolysis and the citric acid cycle are highlighted in red. This metabolic map is unusual in making use of three-dimensions, (more...)
The state of affairs is further complicated in a multicellular organism. Unlike cell types volition in general crave somewhat different sets of enzymes. And different tissues make singled-out contributions to the chemistry of the organism as a whole. In addition to differences in specialized products such every bit hormones or antibodies, at that place are meaning differences in the "common" metabolic pathways among various types of cells in the aforementioned organism.
Although virtually all cells contain the enzymes of glycolysis, the citric acid wheel, lipid synthesis and breakdown, and amino acid metabolism, the levels of these processes required in unlike tissues are non the same. For example, nerve cells, which are probably the most captious cells in the body, maintain almost no reserves of glycogen or fat acids and rely virtually entirely on a constant supply of glucose from the bloodstream. In contrast, liver cells supply glucose to actively contracting muscle cells and recycle the lactic acrid produced past muscle cells dorsum into glucose (Figure ii-90). All types of cells have their distinctive metabolic traits, and they cooperate extensively in the normal state, as well as in response to stress and starvation. One might think that the whole system would need to exist and so finely balanced that any minor upset, such equally a temporary change in dietary intake, would be disastrous.
Figure 2-xc
Schematic view of the metabolic cooperation between liver and muscle cells. The principal fuel of actively contracting muscle cells is glucose, much of which is supplied by liver cells. Lactic acid, the end product of anaerobic glucose breakup by glycolysis (more than...)
In fact, the metabolic residuum of a cell is amazingly stable. Whenever the remainder is perturbed, the cell reacts so as to restore the initial state. The jail cell can accommodate and continue to function during starvation or affliction. Mutations of many kinds tin harm or even eliminate particular reaction pathways, and yet—provided that certain minimum requirements are met—the cell survives. It does so because an elaborate network of control mechanisms regulates and coordinates the rates of all of its reactions. These controls remainder, ultimately, on the remarkable abilities of proteins to change their shape and their chemistry in response to changes in their immediate environs. The principles that underlie how large molecules such as proteins are built and the chemistry behind their regulation will exist our next business.
Summary
Glucose and other nutrient molecules are broken down by controlled stepwise oxidation to provide chemical free energy in the form of ATP and NADH. These are three master sets of reactions that act in series—the products of each beingness the starting textile for the side by side: glycolysis (which occurs in the cytosol), the citric acid cycle (in the mitochondrial matrix), and oxidative phosphorylation (on the inner mitochondrial membrane). The intermediate products of glycolysis and the citric acrid cycle are used both as sources of metabolic energy and to produce many of the small molecules used as the raw materials for biosynthesis. Cells store saccharide molecules as glycogen in animals and starch in plants; both plants and animals also apply fats extensively as a food store. These storage materials in turn serve as a major source of nutrient for humans, along with the proteins that contain the majority of the dry mass of the cells we eat.
Source: https://www.ncbi.nlm.nih.gov/books/NBK26882/
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