UPDATED:
09/20/01
UPDATED:
09/20/01
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WE ARE WHAT WE EAT"METABOLISM IS THE STUDY OF WHAT LIVING ORGANISMS ARE CHEMICALLY AND HOW THEY USE THE RAW MATERIALS IN THE ENVIRONMENT TO LIVE, GROW AND REPRODUCE
1. Metabolism has to do with acquiring and using# energy EFFICIENCY.
2. The most EFFICIENT energy users SURVIVE AND REPRODUCE their GENES so that any advantage they had is continued on into the FUTURE in their offspring. AS LONG AS THE ENVIRONMENT THAT SELECTED THOSE GENES STAYS THE SAME (#SURVIVAL OF THE FIT) the offspring carrying those genes have a survival advantage.
3. Metabolism follows UNBREAKABLE physical laws. As far as we know, the physical laws that function in our part of the universe, also function everywhere else in the universe. No one in a scientific laboratory as ever reported observing a "MIRACLE" where a natural law has been over turned.
4. To understand metabolism one must understand the components of metabolism and how they function together in living organisms.
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Figure 1. Basic steps of ANABOLISM and CATABOLISM. Green plants use the energy from the sun to convert carbon dioxide and water to simple sugar molecules. These sugar monomers are fastened together by #covalent bonds to form larger structures such are starch, cellulose and bacterial cell walls. Each of these steps takes energy, which is trapped within the synthesized molecule. When larger macromolecules are broken down (catabolism) they are cleaved into smaller units which are eventually oxidized and the original energy trapped within them is released. |
For purposes of convenient discussion the chemical processes in living organisms have been divided into two groups;
CATABOLISM and ANABOLISM. Catabolic processes are those that result in large macromolecules being BROKEN DOWN into their smaller component parts. These small units may either be burned for energy or they may be used as building blocks for making new macromolecules. Conversely, anabolic processes are those that use energy and simple building blocks to make NEW MACROMOLECULES for the cell. In reality all the chemical processes of a cell are always intertwined and work as a single coordinated unit, otherwise the cell is SICK.We all know that living organisms are very
complicated. If you've ever wondered just how you developed from a single fertilized egg to the thinking, feeling, active and beautiful person you are today with two hands, two eyes, two legs etc., all in the right place on your body, you have likely been amazed at how such perfection could occur. Consider just how the cells in the embryo that produced your nose, "knew" where to grow as a nose, or other cells as an eye, or a liver etc.? In some cases terrible errors occur during fetal development and brains or other organs develop outside of the body, or aren't produced at all, or appear at a point in the body where it is monstrous. Why and how do such mistakes happen and why don't they happen more often?When you were born the FIRST THING your mother asked was "
is my baby all right?", meaning normal; with five fingers, five toes etc.The simple answer to the above questions, and to life itself, is
REGULATION. That is, life is only possible if there is a high level of regulation to control every event in every living cell all the time. You may well say "that's obvious, but it still doesn't tell me how that fantastic level of regulation is achieved!", and you would be right of course. As I will describe to you below and in subsequent chapters, this regulation is achieved through the ability of the molecules of life to RECOGNIZE and IDENTIFY each other. Based on this recognition, these molecules respond in preprogrammed ways which in turn result in things coming out the way they evolved. Everything I will tell you concerning how living organisms do anything is predicated on this single UNIVERSAL principle. If you learn this principle, you will have a basic understanding of how life works. An analogy is that most people understand the basic principle behind the internal combustion engine, so cars, trucks etc. are no great mystery. Of course we don't know every detail of the inner workings of our automobiles, but most of us comprehend the basics.The cartoon below illustrates this basic principle and I will refer to it through out the following chapters and my lectures. So if you learn only one figure in this entire course, this is the
ONE!
Figure 2. The principle of molecular recognition.
A typical cell contains a number of molecules exposed to the environment and in communication with it. These molecules act as the "eyes, ears and nose" of a cell. They contain, as part of each molecule, specific portions called RECEPTORS or BINDING SITES. Other molecules in the environment contain specific components called LIGANDS. Ligands are sections or regions of a molecule that have the characteristic of binding or attaching (docking) specifically to unique receptors on the cells. Following this attachment a message is passed to the interior of each cell involved as to the situation it has found. This information, in turn, triggers the COMMAND CENTER of each cell to carry out a series of preprogrammed responses based on the data it has received. We will discuss some of these responses throughout the course. Permission to use this cartoon was granted by Sigma Chemical Co.All proteins are composed of
20 AMINO ACIDS arranged in a LINEAR chain. The 20 amino acids differ in their side chains (click here for an excellent sites on amino acids) which determines their unique chemical natures. The amino acids are bonded together with #covalent bonds called PEPTIDE bonds. They are very strong, but may be broken with enzymes called PROTEASES.Enzymes are proteins. Enzymes are the tools of life. Enzymes are designed to do the work of life. Each enzyme has ONE specific job to do. Enzymes are nanometer (10-9 meters) scale machines.
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Since most proteins within cells are enzymes, we will focus our discussion on enzymes. The chemical nature of an enzyme, thus and what it does in the cell (its function), is determined by the
SUM of the chemical characteristics of the amino acids that comprise it and by their arrangements, one amino acid to another, in the linear chain. Think of a protein as a string of the numbers 1-20 arranged linearly in groups of ~300 to 400. How many combinations of arrangements could you make this way? Further, consider if there were rigid rules that certain numbers or clusters of numbers in the chain would tend to interact so they touched each other. This would cause the linear chain of numbers to fold up in certain ways. However, if one number were to be chained it might change this interaction significantly.The various amino acids fall into several chemical types:
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Enzymes do EVERYTHING in a cell. Enzymes determine how a pathogen attacks a victim, and they determine how the host defends itself against a pathogen. Enzymes are responsible for EVERY thought, action, feeling, emotion--everything we do! EVERY ENZYME IS UNIQUE and has only ONE JOB to do. Consider a cell a living TOOL BOX. A bacterial cell contains about 1,000 different enzyme tools at any one time but may be able to make up to 4,000 different enzyme tools when required. Each of these tools has ONE PARTICULAR JOB to perform and they are limited to that one job only. My son, the amateur auto mechanic, has an expensive tool box to hold his many tools. As he repairs different cars, he must buy special tools for the different jobs (that's why your auto repair bills are so high!). A cow's enzyme-tool box contains a different set of tools for doing its "jobs" than does a human's "tool box". However, many of the tools are the same or nearly the same and all are variations on a few basic designs, modified slightly to fit cow parts rather than human parts & vice versa (e.g. metric wrenches & English wrenches have the same basic design).
Enzymes are
ORGANIC CATALYSTS. A CATALYST is anything that speeds up a chemical reaction that is occurring slowly. For example, if one mixes the elements of hydrogen and oxygen together they do not explode spontaneously at room temperature. However, in that mixture, the two elements are forming water, but at a rate that is so slow that it might take a million years for even half of the molecules to combine. But if we place a burning match in that gas mixture a violent explosion occurs as the hydrogen and oxygen rapidly combined. Also if we expose the gas mixture to certain METAL CATALYSTS a similar explosion would occur. One important characteristic of catalysts is that they are NOT USED UP or changed when catalyzing a chemical reaction, thus they are available to repeat the catalysis repeatedly. How does a catalyst work?"| ACTIVATION ENERGY |
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The explanation of what happens lies in the fact that most chemical reactions that
RELEASE ENERGY (exothermic reactions) require an INPUT of some energy to get them going. The initial input of energy is called the ACTIVATION ENERGY. Once started, these reactions will continue. The burning match above supplied the activation energy, but how can metal & protein catalysts, which do not add any heat, do the same thing? Catalysts act by BINDING the molecules they act on in a way that makes it much easier for them to do what THEY WANT TO DO ANYWAY at the energy level (the temperature) currently in the environment. In our bodies this temperature is 37oC or 98.6oF.An analogy is that of a couple that like each other, but are too shy to make the first move. But a friend (I. M. A. Catalyst) invites them to dinner & seats them beside each other. Later he arranges that one of them will have to take the other home. No one is surprised when the couple subsequently become "an item"; we even describe them as "having the right
CHEMISTRY". The couple involved didn't have the ENERGY of ACTIVATION to get together on their own, but I. M. A. Catalyst LOWERED the amount of energy by putting the couple in a situation where nature could TAKE ITS COURSE.Another analogy is the use of a starter (small amount of input energy) to start a car which will then run on its own all day.
An enzyme works in much the same way; i.e. by binding the components of a potential reaction together in a situation where the activation energy is much lower: i.e.,
LOWERING the activation energy, enzymes start the reaction going, after which it runs on its own. Enzymes lower the activation energy to the TEMPERATURE OF THE ENVIRONMENT. Before discussing the details of the exact mechanism we should learn a few more things about the typical enzyme that make it easier to understand how they work.The lock/key paradigm (#
Fig. 2) describes the most FUNDAMENTAL PRINCIPLE or MECHANISM of life. Remember that life is organization and non-life is chaos. the lock/key model explains how this organization is achieved by:|
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Briefly, the organization necessary to produce and maintain life is accomplished through
EXTREMELY SPECIFIC INTERACTIONS between pairs of molecules. In the vast majority of cases one of the components of these interactions is a PROTEIN and most of these proteins are ENZYMES. In every case you have a SITE on the protein that is SHAPED TO UNIQUELY FIT a MOLECULE which the protein is designed to interact with; this molecule is usually called the LIGAND. When the correct two molecules come together SOMETHING IS PRE-PROGRAMMED TO HAPPEN:As we go on in this course to discuss enzyme, antibodies, antibiotics, viruses, embryo development and hormone interaction
ALL OF THESE PHENOMENA involve this paradigm. To not understand this principle essentially eliminates a person from the classification of someone with a modern education; i.e., it would be similar to not knowing about Atomic Energy, the Internal Combustion Engine, the Telephones, Television, Electricity or MTV. FUNCTIONAL SHAPE its SUBSTRATE binds only at a unique location on the enzyme's surface, called the ACTIVE SITE, where the chemical reaction occurs. The analogy used to explain this specific interaction is that of a LOCK & KEY. Just as a uniquely SHAPED KEY will only fit into, and open, a MATCHING LOCK, so it is with enzymes and their substrates. The wrong key may fit into the lock, but nothing can happen because the MATCH OF SHAPES is not correct. This fit is so specific that the change in a SINGLE HYDROGEN atom in a molecule usually means that its specificity for a certain enzyme is LOST; that is, it may not even bind in the active site or even if it does the enzyme will be unable to do anything chemically to it.|
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The union between an enzyme and its substrate is called the ENZYME SUBSTRATE COMPLEX. When a substrate is bound to the active site the enzyme can best be envisioned as BENDING THE SUBSTRATE so that particular chemical bonds are WEAKENED. This has the effect of LOWERING the activation energy to the point where the heat in the environment is sufficient to supply the activation energy to initiate the reaction.
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CRITICAL
THINKING QUESTION: If a thermophile and a #psychrophile
both make an enzyme that carries out the same chemical reaction, which enzyme
would lower the activation energy the most and explain?
Many, but not all, enzymes require certain
SMALL MOLECULES in order to function correctly. These small molecules are called COFACTORS. There are two types of cofactors: organic molecules, commonly known as VITAMINS and inorganic molecules known as MINERALS. These cofactors are molecular "GOFERS", that carry (SHUTTLE) pieces of molecules from one place (SOURCE) to another (DESTINATION). Cofactors are part of the active site and as such are in intimate contact with the substrate. This puts them in position either to receive a piece removed from a substrate or to add a piece to a substrate. Cofactors carry methyl groups (CH3), electrons (e-), and protons (H+) between the donor and recipient molecules. One common cofactor NADH carries protons and electrons around a cell.|
Figure 9. Action of cofactors. In the cartoon on the left an empty cofactor sits in the active site of an enzyme. When the substrate binds at the active site, a piece of it is cut away and binds to the cofactor. The product and the filled cofactor then fall off of the enzyme. In the cartoon on the right, the loaded cofactor binds to the active site of another enzyme. Then another substrates comes along and the molecular fragment carried by the cofactor is ADDED to the substrate converting it to a PRODUCT. Both the product and the empty cofactor fall off the enzyme. The cofactor is now ready to repeat the process. |
A person lacking a required vitamin or mineral will sicken and die. However, it generally takes a long time for this to happen as most cofactors last a long time and are required only in tiny amounts. The metals cofactors are often called
TRACE ELEMENTS. For example, microbiologists frequently do not add trace metals to bacterial media as there is often enough CONTAMINATING the various media components, including the glassware, to supply the microbe's requirements. In the study of iron metabolism all media and glassware must be treated so as to remove all traces of iron, otherwise the effects of iron deficiency can not be observed.|
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Higher plants and animals are generally more susceptible to vitamin or mineral deficiencies. Humans are one of the few life forms that don't make vitamin C. Vitamin C has a short life in our bodies and we need it in fairly large amounts, so we must have a constant supply or we become ill from the disease called
SCURVY. Scurvy was a scourge of sailors throughout history. The major source of vitamin C is fresh fruits and vegetables and these were in short supply on long voyagers. It is thought that more sailors died from scurvy than from any other disease. For example, Caption Cook lost 41 of 98 men in his crew to scurvy during his Pacific cruise in 1768. By 1795 the relationship between fresh fruit and scurvy was recognized and the English Admiralty was insisting that the sailors eat citrus, hence the term LIMY for English sailors. However, Vikings also sailed the seas for months at a time but they rarely suffered from scurvy even though they didn't know about its relationship to fresh fruit; why was that? I'll give the first person who can tell me 2 extra points.
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Enzymes are relatively STABLE molecules that remain active in a cell for long periods of time. Some enzymes can survive boiling temperatures, but most are destroyed or
DENATURED by conditions that significantly differ from those found in the cells they inhabit. For example most enzymes are destroyed by temperatures above 60oC. The protein white of an egg, denatures when it is heated in boiling water for a few seconds. In many cases enzymes can be stored as dried powders, or frozen for many years. Washing detergents contain heat- and alkaline-tolerant enzymes to digest protein stains. We purchase proteolytic enzymes to tenderize our meat, to digest the gas-producing polysaccharides in beans and the lactose that makes lactase-intolerant people ill. Do you know anyone who is lactose intolerant?Enzyme inhibitors are divided into two major classes; the
COMPETITIVE and the NONCOMPETITIVE inhibitors. The competitive inhibitors are molecules that chemically mimic the true substrate close enough to fit into the active site; it is like a key that fits into a lock but doesn't work (e.g. Fig. 11). However, these mimics or ANALOGUES can not be acted upon by the enzyme, but as long as they occupy the active site they COMPETE with the natural substrates and prevent its modification. Such chemicals are used to inhibit crucial enzymes of pathogens in order to inhibit the pathogen. Examples include AZT which blocks an important enzyme of the HIV virus; sulfa drugs which prevent the synthesis of folic acid by bacteria; and penicillin which blocks a bacterial enzyme that forms a crucial bond in #peptidoglycan.Noncompetitive inhibitors fall into two subclasses. One of them, the
NONSPECIFIC inhibitors, denature or inhibit a lot of different enzymes. These include things like heavy metals (lead, mercury, cadmium, nickel), cyanide and carbon monoxide. The latter two bind iron, an important mineral required by all biological systems. For example, cyanide kills because it binds to the iron in hemoglobin and prevents it from carrying oxygen thereby causing suffocation.Consider the efficiencies of energy metabolism in the same light as you would each activity in your life. Rarely do we, as individuals, expend energy if it is
unnecessary do we? Professors rarely write lectures that they never plan to give and students hardly ever study for exams in courses they are not taking. Further, few of us expend energy far ahead of time; i.e., professors infrequently write lectures of courses they plan to give in the distant future and students don't always study for an exam until a short time before taking it. Thus it is with cells, since making an enzyme, or converting a substrate to a product is wasteful unless that enzyme or product is REQUIRED. However, as noted above, enzymes are stable in a cell once made, so how do you SHUT AN ENZYME DOWN? This is done via a process known as FEEDBACK INHIBITION. In feedback inhibition, a product of an enzyme or a series of enzymes in a PATHWAY, acts on an enzyme earlier in the pathway so as to TURN IT OFF. It does this via an ALLOSTERIC effect. The SPECIFIC NONCOMPETITIVE inhibitors are ORGANIC MOLECULES, or ALLOSTERIC INHIBITORS, that bind at specific sites, called ALLOSTERIC SITES, on enzymes and influence the active site. Even more LOGICAL is the observation that the target of most allosteric effects are those enzymes that USE ENERGY to perform a reaction or they are enzymes at a JUNCTION of two pathways. Since the allosteric effector molecules are not permanently bound to the allosteric sites an allosteric effect is REVERSIBLE. When the inhibiting product is once again required, the allosteric molecule effectively disappears (by being used up in reactions where it is needed) and the enzyme again becomes FUNCTIONAL. This process of feedback inhibition can become extremely complex as a single critical enzyme may be effected or regulated by a number of PRODUCTS in a variety of ways.|
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Allosteric inhibitors fit into Al.S.s via the same
LOCK & KEY mechanism as substrates. However, it is important to recognize that Al.S. are NOT located at the active site, but are located somewhere else on the enzyme. They influence the active site by causing the enzyme to change its FUNCTIONAL SHAPE (Fig. 11). A single enzyme may be influenced by more than one allosteric inhibitor, each binding to its own Al.S. Allosteric molecules may also cause active sites to form, that is there are POSITIVE allosteric EFFECTORS that turn on enzymes. Allosteric inhibitors are required for regulation of organized life. For example, hormones (ligand) cause their effects by binding to proteins (receptors) at allosteric sites and effecting the cells they bind to in positive or negative ways as required. When allosteric sites are damaged (e.g. by #mutation) or otherwise not functional, illnesses like cancer, birth defects etc. result.It is important to understand that the binding of allosteric molecules is
REVERSIBLE. Binding strength is the multiple of two effects, (1) the concentration of the allosteric molecule and (2) the strength of the binding points between the allosteric molecule and the allosteric sites. If the concentration of the allosteric molecule is low its chances of finding a proper binding site is also low so the enzyme is not likely to be inhibited. However, if the strength of the binding at the allosteric site is high then any allosteric molecules that do find the site will tend to stay and inhibit the enzyme for a long time. Eventually, all allosteric molecules FALL OFF their allosteric site.
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Another way of controlling PREFORMED enzymes is to
CHEMICALLY MODIFY them. This is done by another enzyme covalently adding something to the target enzyme, such as a PHOSPHATE GROUP. This modification may either "switch on" or "switch off" a given enzyme depending on the regulation required. When a REVERSAL in the enzyme's FUNCTIONAL STATE is required, the phosphate is removed by yet another enzyme, allowing it to return to its original state. A particular enzyme WITHOUT a phosphate may be inactive and may become activated by the addition of a phosphate, whereas with a different enzyme the reverse occurs.An effective analogy of the enzyme regulatory situation, is to consider how a BUSINESS IS RUN. If too many of a business's products pile up (inventory), the owners of the business slows down or halts the manufacturing process. Further, they stop buying materials used in the manufacturing to save money (ENERGY). Once sales pick up and the supply (inventory) of product has dropped to a certain level, the owners purchase new materials and begin making their product again. So it is in a cell.
One summer I earned my college tuition working on the grape harvest making boxes for the grapes. Every AM the boss would come out and tell us how many boxes they needed that day based on the estimated size of that day's grape harvest (feedback). Since we were getting paid "piece work" wages (by-the-box), we always made more than the boss requested which caused considerable yelling, cursing and threats to fire "our A......!!!" (regulation by inhibition).
An enzyme is studied by measuring its activity. An enzyme's activity is usually measured by following the
disappearance of the substrate or the appearance of the PRODUCTS of the enzyme's reaction. To obtain an enzyme for study, plant, bacterial or animal cells are DISRUPTED, to allow the CYTOPLASM TO ESCAPE. The cytoplasm, containing all the SOLUBLE ENZYMES in a cell, is collected and a SINGLE DESIRED ENZYME is separated away from several thousand unwanted substances (e.g. other enzymes, cell wall pieces, storage granules etc.). Enzyme purification use to be a complex task that sometimes took years to accomplish and then might yield only a few milligrams of purified enzyme from pounds of starting material. Improvements, in the last 30 years have greatly simplified this job, to the point where grams of purified enzyme can often be isolated in as little as two days. Some enzymes, however, are still formidable to isolate. Once isolated the PURE ENZYME is characterized as to its #pH optimum, its size, cofactor requirement, if any, etc. In today's molecular biology world an early step is to isolate the gene that directs the synthesis of the enzyme and to study the regulation of that gene in vivo (= in a living cell).|
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Biochemical processes may be conveniently viewed as an
ASSEMBLY LINE of SEQUENTIAL CHEMICAL REACTIONS (examine three or more different pathways at this site), each controlled by a unique enzyme. The biochemists have given the various series of enzyme-directed reactions names, like the TCA CYCLE, GLYCOLYSIS etc. (click the pathways to view the animated steps using the helper app Chime). for ease of discussion. To see animations of how specific chemical reactions look click here then click on "Glycolysis" followed by "Glycolysis--Step by Step". However, remember that everything in a cell is INTIMATELY INTERCONNECTED and every individual reaction must be thought of as affecting every other reaction. One proof that all life is related, is that a core of biochemical pathways run throughout life. The sugar glucose, for example, is used as a food by almost every organism on the planet and the enzymatic pathways for utilizing glucose are extremely similar. The various enzymes involved may differ in physical characteristics, but the chemical steps involved in glucose breakdown are the same. "Do you think that this continuity of life does or does not support the theory of evolution?" Click here to see a metabolic map like that on the wall in the lab. Click on the Carbohydrate Metabolism label and view that portion of the metabolic map.Although most of the core biochemical pathways are known, some specialized pathways are still unknown: such as those involved in the synthesis of some antibiotics. The major research issue today is how enzyme activity and synthesis are
REGULATED. For example, every human cell with a nucleus contains the ENTIRE COMPLEMENT OF HUMAN GENES, yet only a small portion of these genes are active at any time in any given cell; in the eye only genes involved for vision are active, in the liver, only those required for liver cell functioning are active etc. The development of a human from a single egg is a poorly understood process. How specific sub-sets of genes are turned on and off in time and space present an exciting challenge for scientists. Click here or here for excellent presentations of glycolysis. Click here to have a choice of pathways you want to see. Click here for a great site on pathways and for a colored map showing the interconnectivity of the metabolic pathways. Click here to view the complete biochemical chart. Type in "glucose 6 phosphate" and hit ENTER. Use the arrows to move around the chart.Living organisms obtain their energy in one of two ways; either directly by the process of
PHOTOSYNTHESIS or indirectly from ENERGY RICH molecules. The SUN is the source of energy for the MAJORITY OF THE LIFE forms on earth, either directly by the trapping of light using various pigmented molecules, or indirectly from organic molecules formed in photosynthesizing organisms. A few prokaryotes are able to oxidize energy-rich INORGANIC molecules like sulfur, iron, hydrogen, ammonia and carbon monoxide and to trap the released energy in the form of organic molecules.Both eukaryotes and prokaryotes contains species that can photosynthesize. Both employ variations of the
CHLOROPHYLL, a light-trapping molecule that contains MAGNESIUM, to capture the energy of the sun. Many photosynthesizing organisms use accessory pigments to absorb certain parts of the light spectrum and transfer it to chlorophyll. One can observe some of the accessory pigments in the leaves in the fall. In prokaryotes, the accessory pigments often obscure the green chlorophyll giving these organisms a range of colors, including brown, various reds, purples, yellows, blues and greens. The majority of the photosynthetic prokaryotes are OBLIGATE ANAEROBES that do not produce oxygen as a by-product of photosynthesis. However, recently a number of aerobic photosynthetic aerobes have been discovered that also appear to be able to convert light into chemical energy, but they do not produce oxygen in the process. But one important group, the CYANOBACTERIA are OXYGENIC (produce oxygen) like the green plants. There is strong evidence that the evolution of the Cyanobacteria changed the earth from an anaerobic world to an aerobic one in which creatures like ourselves could evolve. There is also good evidence that the chloroplasts of the green plants originated from cyanobacteria that became #organelles in larger cells.|
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To obtain energy from energy-rich organic molecules eukaryotes and prokaryotes
OXIDIZE them. Oxidation consists of removing the electron/proton pairs from the energy source molecules and adding these electron/proton pairs to the oxygen atom to form water, which releases energy that is CAPTURED in an organic molecule called ADENOSINE TRIPHOSPHATE or ATP. ATP is the energy BATTERY of all living organisms. In photosynthesis the light energy absorbed by the pigments is also converted into ATP. ATP contains a chain of 3 phosphate molecules; requires Chime, click on the molecule you want to see. The bonds between the last two phosphate atoms are said to be ENERGY RICH, in that the energy trapped in these bonds can be released by enzymes in a controlled manner so as to allow the construction of the energy-rich macromolecules of life. That is, ATP supplies the energy for the #ANABOLIC processes in life. ATP is a small molecule that moves around the cell easily. Also it is stable (one can buy bottles of powdered ATP and store it for years in the freezer), but not too stable. If ATP's bonds were stable covalent bonds, it would be TOO DIFFICULT to extract useful energy from it, and if it was highly unstable it wouldn't last long enough to be used in the cell. The best analogy is that of an organic BATTERY. Like flashlight or car batteries, ATP is portable and can be plugged into enzyme systems that require energy. As you now recognize, enzymes that make or use ATP have active sites into which the ATP BINDS so its energy is available as needed. In the synthesis of ATP a phosphate atom is added to ADP (adenosine diphosphate) in a process called PHOSPHORYLATION (ADP + Phosphate atom + energy = ATP).Copyright © Dr. R. E. Hurlbert, 1999.
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