CHAPTER 2  : The Chemical Basis of Life

Matter, Mass and Weight

Here’s a tricky question, so think carefully.
What weights more, a kilogram of feather or a kilogram of rock?
If you said neither and they both weight the same, then you are not easy fooled. Even though these two types of matter weight the same they are quite different from each other. Let see how. Mass is a physical characteristic of a matter. Mass is the amount of matter in an object. Different objects have different amounts of mass. Weight is another physical property.  Weight and Mass are not the same thing.  Weight is a measure of mass while the mass of an object can stay constant, its weight can change. Weight is the force of an object due to the gravitational pull of the planet.

For example the weight  of a person on Earth is much greater than the same person on the moon because the gravitational pull on the moon is much less. even though the person’s weight changes , he’s mass stays the same.The weight of an object is mostly commonly measured on scale or balance. The metric system uses kilograms (kg) and grams (g) to measure weight.

In our example of the feather and rock, they have the same weight of about  1kg.

So, what makes them different?  While they have the same weight , they have different volumes

Elements and Atoms

Did you ever wonder what things are made of? 

Everything on Earth and in space is made up of atoms.

They are the basic building blocks of Matter. Cars, flowers, toys your computer even you are made of atoms. What are atoms? Atoms are particles that are so tiny, we cant see them. If we can see them, how do we know that atoms exist? Today, we have machines that magnify things and let us see things, even the tiniest atoms. An object, after being magnified a thousand times, we can actually see individual atoms. Atoms are not all the same, but when all the atoms in a substance are exactly the same, the substance is called an element. So far scientist have discovered over hundred different elements.

Atomic Structure

Although no one has yet found a way of getting a close up view of an atom. Different experiments have made it possible to build up a picture or model of what atoms consists of. In the center of an atom is a nucleus, this is made up of even smaller or subatomic particles called protons and neutrons. The protons each carry a positive electrical change. The neutrons are as heavy as the protons but carry no charge. This means that over all the nucleus is positively charge and orbiting around it a negatively charge particles called electrons. The size of the negative charge carried by an electron is equal to the positive charge carried by a proton. Now the electrons are moving at an incredible speed. So what’ s stops shooting them of at any different direction? If we could pulled an atom apart into its component, we’d see that a neutral atom has the same number of electron as protons and it’s the force of attraction between positive and negative charges that prevents the electron from escaping. Each types of atom is identified by the number of protons and electron it has. An hydrogen atom has just one proton and one electron, an oxygen atom has eight protons and eight electron. 

Atomic Number and Mass Number

What is an atom's atomic number?

The number of protons in the nucleus of an atom determines an element's atomic number. In other words, each element has a unique number that identifies how many protons are in one atom of that element. For example, all hydrogen atoms, and only hydrogen atoms, contain one proton and have an atomic number of 1. All carbon atoms, and only carbon atoms, contain six protons and have an atomic number of 6. Oxygen atoms contain 8 protons and have an atomic number of 8. The atomic number of an element never changes, meaning that the number of protons in the nucleus of every atom in an element is always the same.

What is an atom's mass number?

All atoms have a mass number which is derived as follows.

Electons and Chemical Bonding

Chemical Bonding

Chemical compounds are formed by the joining of two or more atoms. A stable compound occurs when the total energy of the combination has lower energy than the separated atoms. The bound state implies a net attractive force between the atoms ... a chemical bond. The two extreme cases of chemical bonds are:

Covalent bond: bond in which one or more pairs of electrons are shared by two atoms.

Ionic bond: bond in which one or more electrons from one atom are removed and attached to another atom, resulting in positive and negative ions which attract each other.

Other types of bonds include metallic bonds and hydrogen bonding. The attractive forces between molecules in a liquid can be characterized as van der Waals bonds.

Covalent Bonds

Covalent chemical bonds involve the sharing of a pair of valence electrons by two atoms, in contrast to the transfer of electrons in ionic bonds. Such bonds lead to stable molecules if they share electrons in such a way as to create a noble gas configuration for each atom.

Hydrogen gas forms the simplest covalent bond in the diatomic hydrogen molecule. The halogens such as chlorine also exist as diatomic gases by forming covalent bonds. The nitrogen and oxygen which makes up the bulk of the atmosphere also exhibits covalent bonding in forming diatomic molecules.

Polar Covalent Bonds

Covalent bonds in which the sharing of the electron pair is unequal, with the electrons spending more time around the more nonmetallic atom, are called polar covalent bonds. In such a bond there is a charge separation with one atom being slightly more positive and the other more negative, i.e., the bond will produce a dipole moment. The ability of an atom to attract electrons in the presense of another atom is a measurable property called electronegativity.

Ionic Bonds

In chemical bonds, atoms can either transfer or share their valence electrons. In the extreme case where one or more atoms lose electrons and other atoms gain them in order to produce a noble gas electron configuration, the bond is called an ionic bond.

Typical of ionic bonds are those in the alkali halides such as sodium chloride, NaCl.

Metallic Bonds

The properties of metals suggest that their atoms possess strong bonds, yet the ease of conduction of heat and electricity suggest that electrons can move freely in all directions in a metal. The general observations give rise to a picture of "positive ions in a sea of electrons" to describe metallic bonding.

Hydrogen Bonding

Hydrogen bonding differs from other uses of the word "bond" since it is a force of attraction between a hydrogen atom in one molecule and a small atom of high electronegativity in another molecule. That is, it is an intermolecular force, not an intramolecular force as in the common use of the word bond.

When hydrogen atoms are joined in a polar covalent bondwith a small atom of high electronegativity such as O, F or N, the partial positive charge on the hydrogen is highly concentrated because of its small size. If the hydrogen is close to another oxygen, fluorine or nitrogen in another molecule, then there is a force of attraction termed a dipole-dipole interaction. This attraction or "hydrogen bond" can have about 5% to 10% of the strength of a covalent bond.

Hydrogen bonding has a very important effect on the properties of water and ice. Hydrogen bonding is also very important in proteins and nucleic acids and therefore in life processes. The "unzipping" of DNA is a breaking of hydrogen bonds which help hold the two strands of the double helix together.

Molecules and Compounds

What is a molecule?


molecule - A substance formed by the chemical combination (chemical bonding, usually covalent or ionic bonding) of two or more atoms (of the same or different elements); the smallest particle of a chemical compound that retains the chemical properties of the compound.  [A stable molecule occurs when the total energy of the combination has a lower energy state than the separated atoms.]


What is a compound?

compound - A substance formed by the chemical combination (chemical bonding, usually covalent or ionic bonding) of two or more atoms of different elements.  [A stable compound occurs when the total energy of the combination has a lower energy state than the separated atoms.]

                           All the examples above are molecules, but only those in the second row are also compounds.

Disassociation

Dissociation in chemistry is a general process in which ionic compounds (complexes, or salts) separate or split into smaller particles, ions, or radicals, usually in a reversible manner. For instance, when a Brønsted-Lowry acid is put in water, a covalent bond between an electronegative atom and a hydrogen atom is broken by heterolytic fission, which gives a proton and a negative ion. Dissociation is the opposite of association and recombination. The process is frequently confused with ionization.

Reversible, Irreversible Reactions and Equilibrium


It is a common observation that most of the reactions when carried out in closed vessels do not go to completion, under a given set of conditions of temperature and pressure. Infact in all such cases, in the initial state, only the reactants are present but as the reaction proceeds, the concentration of reactants decreases and that of products increases. Finally a stage is reached when no further change in concentration of the reactants and products is observed.
This state at which the concentration of reactants and products do not change with time is called a state of chemical equilibrium.
The amount of reactants unused depend on the experimental conditions such as concentration, temperature, pressure and the nature of the reaction.
If a mixture of gaseous hydrogen and iodine vapours is made to react at 717k in a closed vessel for about 2 - 3 hours, gaseous hydrogen iodide is produced according to the following equation :

But along with gaseous hydrogen iodide, there will be some amount of unreacted gaseous hydrogen and gaseous iodine left.

On the other hand if gaseous hydrogen iodide is kept at 717K in a closed vessel for about 2 - 3 hours it decomposes to give gaseous hydrogen and gaseous iodine.

In this case also some amount of gaseous hydrogen iodide will be left unreacted.

This means that the products of certain reactions can be converted back to the reactants. These types of reactions are called reversible reactions.

Thus, in reversible reactions the products can react with one another under suitable conditions to give back the reactants.

In other words, in reversible reactions the reaction takes place in both the forward and backward directions. The reversible reaction may be expressed as:

These reversible reactions never go to completion if performed in a closed container. For a reversible chemical reaction, an equilibrium state is attained when the rate at which a chemical reaction is proceeding in forward direction equals the rate at which the reverse reaction is proceeding.

At equilibrium,

Rate of forward reaction = Rate of reverse reaction

Consider the reversible reaction

When this reaction is performed at high pressure and temperature in a close container, at equilibrium,

Rate of formation of ammonia = Rate of decomposition of ammonia

Now, the question arises whether all the ammonia molecules are remaining intact and not decomposing? Are all the molecules of nitrogen and hydrogen becoming inactive and not combining?

If this is the case, we would say a static equilibrium is attained.

To understand the concept of static equilibrium, let us consider two children sitting on a see-saw. At balance point (i.e., the equilibrium position) no movement of children on the see-saw occurs.

Static Equilibrium

In the case of reversible reaction, however a static equilibrium is not being established.

In the case of ammonia, using deuterium, D (an isotope of hydrogen) it has been proved that even at equilibrium, decomposition of ammonia into hydrogen and nitrogen and combination of hydrogen and nitrogen into ammonia continues. This equilibrium is dynamic in nature and is therefore called dynamic equilibrium.

A dynamic steady state can be compared with the equilibrium of water in a reservoir, which is being simultaneously filled and discharged. If the rate of water flowing in is equal to the rate of water flowing out, the quantity of water in the reservoir will remain unchanged like the quantities of substances in a state of chemical equilibrium.

Dynamic Equilibrium

Rate of water entering = Rate of water leaving

Hence the level of water is constant

Similarly, some other reversible reactions are:

On the other hand, the chemical reaction in which the products formed do not combine to give the reactants are known as irreversible reactions.

For e.g., potassium chlorate decomposes on heating to form potassium chloride and oxygen.

However the products cannot combine to form potassium chlorate. In case of irreversible physical and chemical processes, the change occurs only in one direction and the processes go to completion. However, the reversible processes do not go to completion and appear to stop (attain state of chemical equilibrium) even though some starting materials are remaining.

Some examples of irreversible reactions are:

Chemical Reactions

Let's start with the idea of a reaction. In chemistry, a reaction happens when two or more molecules interact and something happens. That's it. What molecules are they? How do they interact? What happens? Those are all the possibilities in reactions. The possibilities are infinite. There are a few key points you should know about chemical reactions. 

Key Points

A chemical change must occur. You start with one compound and turn it into another. That's an example of a chemical change. A steel garbage can rusting is a chemical reaction. That rusting happens because the iron (Fe) in the metal combines with oxygen (O2) in the atmosphere. When a refrigerator or air conditioner cools the air, there is no reaction. That change in temperature is a physical change. Nevertheless, a chemical reaction can happen inside of the air conditioner. 

 A reaction could include ions, molecules, or pure atoms. We said molecules in the previous paragraph, but a reaction can happen with anything, just as long as a chemical change occurs (not a physical one). If you put pure hydrogen gas (H2) and pure oxygen gas in a room, they can be involved in a reaction. The slow rate of reaction will have the atoms bonding to form water very slowly. If you were to add a spark, those gases would create a reaction that would result in a huge explosion. Chemists would call that spark a catalyst. 

 Single reactions often happen as part of a larger series of reactions. Take something as simple as moving your arm. The contraction of that muscle requires sugars for energy. Those sugars need to be metabolized. You'll find that proteinsneed to move in a certain way to make the muscle contract. A whole series (hundreds actually) of different reactions are needed to make that simple movement happen. 

 RATES OF REACTIONS

The rate of a reaction is the speed at which a reaction happens. If a reaction has a low rate, that means the molecules combine at a slower speed than a reaction with a high rate. Some reactions take hundreds, maybe even thousands of years while other can happen in less than one second. The rate of reaction depends on the type of molecules that are combining.

There is another big idea for rates of reaction called collision theory. The collision theory says that the more collisions in a system, the more likely combinations of molecules will happen. If there are a higher number of collisions in a system, more combinations of molecules will occur. The reaction will go faster, and the rate of that reaction will be higher. 

Reactions happen, no matter what. Chemicals are always combining or breaking down. The reactions happen over and over but not always at the same speed. A few things affect the overall speed of the reaction and the number of collisions that can occur. 

Concentration: If there is more of a substance in a system, there is a greater chance that molecules will collide and speed up the rate of the reaction. If there is less of something, there will be fewer collisions and the reaction will probably happen at a slower speed. 

Temperature: When you raise the temperature of a system, the molecules bounce around a lot more (because they have more energy). When they bounce around more, they are more likely to collide. That fact means they are also more likely to combine. When you lower the temperature, the molecules are slower and collide less. That temperature drop lowers the rate of the reaction.

Pressure: Pressure affects the rate of reaction, especially when you look at gases. When you increase the pressure, the molecules have less space in which they can move. That greater concentration of molecules increases the number of collisions. When you decrease the pressure, molecules don't hit each other as often. The lower pressure decreases the rate of reaction.

For thousands of years people have known that vinegar, lemon juice and many other foods taste sour. However, it was not until a few hundred years ago that it was discovered why these things taste sour - because they are all acids. The term acid, in fact, comes from the Latin term acere, which means "sour". While there are many slightly different definitions of acids and bases, in this lesson we will introduce the fundamentals of acid/base chemistry.

In the seventeenth century, the Irish writer and amateur chemist Robert Boyle first labeled substances as either acids or bases (he called bases alkalies) according to the following characteristics:

Acids taste sour, are corrosive to metals, change litmus (a dye extracted from lichens) red, and become less acidic when mixed with bases.

Bases feel slippery, change litmus blue, and become less basic when mixed with acids.

PH

The pH scale measures how acidic an object is. Objects that are not very acidic are called basic. The scale has values ranging from zero (the most acidic) to 14 (the most basic). As you can see from the pH scale above, pure water has a pH value of 7. This value is considered neutral—neither acidic or basic. Normal, clean rain has a pH value of between 5.0 and 5.5, which is slightly acidic. However, when rain combines with sulfur dioxide or nitrogen oxides—produced from power plants and automobiles—the rain becomes much more acidic. Typical acid rain has a pH value of 4.0. A decrease in pH values from 5.0 to 4.0 means that the acidity is 10 times greater.

How pH is Measured

There are many high-tech devices that are used to measure pH in laboratories. One easy way that you can measure pH is with a strip of litmus paper. When you touch a strip of litmus paper to something, the paper changes color depending on whether the substance is acidic or basic. If the paper turns red, the substance is acidic, and if it turns blue, the substance is basic.

Salt

Salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations (positively charged ions) and anions (negative ions) so that the product is electrically neutral (without a net charge). These component ions can be inorganic such as chloride (Cl−), as well as organic such as acetate (CH3COO−) and monatomic ions such as fluoride (F−), as well as polyatomic ions such as sulfate (SO42−).

There are several varieties of salts. Salts that hydrolyze to produce hydroxide ions when dissolved in water are basic saltsand salts that hydrolyze to produce hydronium ions in water are acid salts. Neutral salts are those that are neither acid nor basic salts. Zwitterions contain an anionic center and a cationic center in the same molecule but are not considered to be salts. Examples include amino acids, many metabolites, peptides, and proteins.

Molten salts and solutions containing dissolved salts (e.g., sodium chloride in water) are called electrolytes, as they are able to conduct electricity. As observed in the cytoplasm of cells, in blood, urine, plant saps and mineral waters, mixtures of many different ions in solution usually do not form defined salts after evaporation of the water. Therefore, their salt content is given for the respective ions.

The blue salt copper(II) sulfate in the form of the mineralchalcanthite

What Is a Buffer?

A buffer is an aqueous solution that has a highly stable pH. If you add acid or base to a buffered solution, its pH will not change significantly. Similarly, adding water to a buffer or allowing water to evaporate will not change the pH of a buffer.

How Do You Make a Buffer?

A buffer is made by mixing a large volume of a weak acid or weak base together with its conjugate. A weak acid and its conjugate base can remain in solution without neutralizing each other. The same is true for a weak base and its conjugate acid.

How Do Buffers Work?

When hydrogen ions are added to a buffer, they will be neutralized by the base in the buffer. Hydroxide ions will be neutralized by the acid. These neutralization reactions will not have much effect on the overall pH of the buffer solution.

When you select an acid for a buffer solution, try to choose an acid that has a pKa close to your desired pH. This will give your buffer nearly equivalent amounts of acid and conjugate base so it will be able to neutralize as much H+ and OH- as possible.

Inorganic Chemistry 

Inorganic chemistry is the branch of chemistry concerned with the properties and behavior of inorganic compounds. This field covers allchemical compounds except the myriad organic compounds (carbon based compounds, usually containing C-H bonds), which are the subjects of organic chemistry. The distinction between the two disciplines is far from absolute, and there is much overlap, most importantly in the sub-discipline of organometallic chemistry.

Oxygen and Carbon Dioxide 

Carbon dioxide is an inorganic chemical compound with a wide range of commercial uses, from the production of lasers to the carbonation of soft drinks. This compound exists naturally from the Earth's environment, and it is produced in a variety of ways; commercial carbon dioxide is usually derived from the byproducts of industrial processes. This humble gas has become a topic of interest for humans because it is classified among the greenhouse gases, gases which impact the Earth's environment when they reach high concentrations in the atmosphere.

The chemical formula for carbon dioxide is CO2, and it takes the form of two oxygen molecules covalently bonded to a single carbon molecule. This compound is produced through decomposition of organic materials as well as through respiration and combustion. Amounts of carbon dioxide in the environment prior to the advent of the 20th century were kept stable by plants, which were capable of absorbing carbon dioxide as it was produced for use in photosynthesis.

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greekroots ὀξύς (oxys) ("acid", literally "sharp", referring to the sour taste of acids) and -γενής (-genēs) ("producer", literally "begetter"), because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a very pale blue, odorless, tasteless diatomic gas with the formula O2.

Oxygen is my favorite element because as you can see without it, we would die. People say that they are starving when they haven't eaten for two hours, and that they are thirsty almost five minutes after they had a glass of water. Well, the fact is, you can live weeks without food, days without water, but you can only live five minutes without a breath of oxygen. When you are asleep many people think you don't get much oxygen. They are WRONG! As a matter-of-fact, a resting person consumes five gallons of oxygen every hour he sleeps. So, if you slept eight hours you have just consumed 40 gallons of oxygen. Here is something even more strange.  People think the air is 100% oxygen. That is also wrong.  Air is filled with Carbon Dioxide, Argon, Neon, Radon, Helium, Krypton, Xenon, Hydrogen, Methane, Nitrous Oxide, Ozone, 4% Water Vapor, Dust, Smoke, Salt, Carbon Monoxide, and Microorganisms. That is a lot of substances in the air we breathe.

Carbon dioxide is an inorganic chemical compound with a wide range of commercial uses, from the production of lasers to the carbonation of soft drinks. This compound exists naturally from the Earth's environment, and it is produced in a variety of ways; commercial carbon dioxide is usually derived from the byproducts of industrial processes. This humble gas has become a topic of interest for humans because it is classified among the greenhouse gases, gases which impact the Earth's environment when they reach high concentrations in the atmosphere.

The chemical formula for carbon dioxide is CO2, and it takes the form of two oxygen molecules covalently bonded to a single carbon molecule. This compound is produced through decomposition of organic materials as well as through respiration and combustion. Amounts of carbon dioxide in the environment prior to the advent of the 20th century were kept stable by plants, which were capable of absorbing carbon dioxide as it was produced for use in photosynthesis.

Water covers 70.9% of the Earth's surface,[4] and is vital for all known forms of life. On Earth, 96.5% of the planet's water is found in oceans, 1.7% in groundwater, 1.7% in glaciers and the ice caps of Antarctica and Greenland, a small fraction in other large water bodies, and 0.001% in the air as vapor, clouds (formed of solid and liquid water particles suspended in air), andprecipitation. Only 2.5% of the Earth's water is freshwater, and 98.8% of that water is in ice and groundwater. Less than 0.3% of all freshwater is in rivers, lakes, and the atmosphere, and an even smaller amount of the Earth's freshwater (0.003%) is contained within biological bodies and manufactured products.

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atomsconnected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, andgaseous state (water vapor or steam). Water also exists in a liquid crystal state near hydrophilic surfaces.Under nomenclature used to name chemical compounds, Dihydrogen monoxide is the scientific name for water, though it is almost never used.

Carbohydrates are one of the four major classes of organic compounds in living cells. They are produced during photosynthesis and are the main sources of energy for plants and animals. The term carbohydrate is used when referring to a saccharide or sugar and its derivatives. Carbohydrates can be simple sugars or monosaccharides, double sugars or disaccharides, composed of a few sugars or oligosaccharides, or composed of many sugars or polysaccharides.

Carbohydrates: Monosaccharides

A monosaccharide or simple sugar has a formula that is some multiple of CH2O. For instance, glucose (the most common monosaccharide) has a formula of C6H12O6. Glucose is typical of the structure of monosaccharides. Hydroxyl groups (-OH) are attached to all carbons except one. The carbon without an attached hydroxyl group is double-bonded to an oxygen to form what is known as a carbonyl group. The location of this group determines whether or not a sugar is known as a ketone or an aldehyde sugar. If the group is not terminal then the sugar is known as a ketone. If the group is at the end, it is known as an aldehyde. Glucose is an important energy source in living organisms. During cellular respiration, the breakdown of glucose occurs in order to release its stored energy.

Carbohydrates: Disaccharides

Two monosaccharides joined together by a glycosidic linkage is called a double sugar or disaccharide. The most common disaccharide is sucrose. It is composed of glucose and fructose. Sucrose is commonly used by plants to transport sugar from one part of the plant to another. Disaccharides are also oligosaccharides. An oligosaccharide consists of a small number of monosaccaharide units (from about two to ten) joined together. Oligosaccharides are found in cell membranes and assist other membrane structures called glycolipids in cell recognition.

Carbohydrates: Polysaccharides

Polysaccharides can be composed of hundreds to thousands of monosaccharides combined together. These monosaccharides are joined together through dehydration synthesis. Some examples of polysaccharides include starch, cellulose and glycogen. Polysaccharides have several functions including structural support and storage.

Lipids
Lipids are very diverse in both their respective structures and functions. These diverse compounds that make up the lipid family are so grouped because they are insoluble in water. They are however soluble in other organic solvents such as ether, acetone and other lipids. Major lipid groups include fats, phospholipids, steroids and waxes.

Lipids: Fats
Fats are composed of three fatty acids and glycerol. These triglycerides can be solid or liquid at room temperature. Those that are solid are classified as fats, while those that are liquid are known as oils. Fatty acids consist of a long chain of carbons with a carboxyl group at one end. Depending on their structure, fatty acids can be saturated or unsaturated. While fats have been denigrated to the point that many believe that fat should be eliminated from the diet, fat serves many useful purposes. Fats store energy, help to insulate the body and cushion and protect organs.

Lipids: Phospholipids
A phospholipid is composed of two fatty acids, a glycerol unit, a phosphate group and a polar molecule. The phosphate group and polar head region of the molecule is hydrophillic (attracted to water), while the fatty acid tail is hydrophobic (repelled by water). When placed in water, phospholipids will orient themselves into a bilayer in which the nonpolar tail region faces the inner area of the bilayer. The polar head region faces outward and interacts with the water. Phospholipids are a major component of cell membranes which enclose the cytoplasm and other contents of a cell.

Lipids: Steroids and Waxes
Steroids have a carbon backbone that consists of four fused ring-like structures. Steroids include cholesterol, sex hormones (progesterone, estrogen and testosterone) and cortisone. Waxes are comprised of an ester of a long-chain alcohol and a fatty acid. Many plants have leaves and fruits with wax coatings to help prevent water loss. Some animals also have wax-coated fur or feathers to repel water. Unlike most waxes, ear wax is composed of phospholipids and esters of cholesterol.

Proteins

One or more polypeptide chains twisted into a 3D shape forms a protein. The unique shape of the protein determines its function. For instance, structural proteins such as collagen and keratin are fibrous and stringy. Globular proteins like hemoglobin, on the other hand, are folded and compact.

Protein Synthesis

Proteins are synthesized in the body through a process called translation. Translation occurs in the cytoplasm and involves the translation of genetic codes that are assembled during DNA transcription into proteins. Cell structures called ribosomes help translate these genetic codes into polypeptide chains that undergo several modifications before becoming fully functioning proteins.

Enzymes

At any given moment, all of the work being done inside any cell is being done by enzymes. If you understand enzymes, you understand cells. A bacterium like E. coli has about 1,000 different types of enzymes floating around in the cytoplasm at any given time.

Enzymes have extremely interesting properties that make them little chemical-reaction machines. The purpose of an enzyme in a cell is to allow the cell to carry out chemical reactions very quickly. These reactions allow the cell to build things or take things apart as needed. This is how a cell grows and reproduces. At the most basic level, a cell is really a little bag full of chemical reactions that are made possible by enzymes!

Enzymes are made from amino acids, and they are proteins. When an enzyme is formed, it is made by stringing together between 100 and 1,000 amino acids in a very specific and unique order. The chain of amino acids then folds into a unique shape. That shape allows the enzyme to carry out specific chemical reactions -- an enzyme acts as a very efficient catalyst for a specific chemical reaction. The enzyme speeds that reaction up tremendously.

For example, the sugar maltose is made from two glucose molecules bonded together. The enzyme maltase is shaped in such a way that it can break the bond and free the two glucose pieces. The only thing maltase can do is break maltose molecules, but it can do that very rapidly and efficiently. Other types of enzymes can put atoms and molecules together. Breaking molecules apart and putting molecules together is what enzymes do, and there is a specific enzyme for each chemical reaction needed to make the cell work properly.

You can see in the diagram above the basic action of an enzyme. A maltose molecule floats near and is captured at a specific site on the maltase enzyme. The active site on the enzyme breaks the bond, and then the two glucose molecules float away.

You may have heard of people who are lactose intolerant, or you may suffer from this problem yourself. The problem arises because the sugar in milk -- lactose -- does not get broken into its glucose components. Therefore, it cannot be digested. The intestinal cells of lactose-intolerant people do not produce lactase, the enzyme needed to break down lactose. This problem shows how the lack of just one enzyme in the human body can lead to problems. A person who is lactose intolerant can swallow a drop of lactase prior to drinking milk and the problem is solved. Many enzyme deficiencies are not nearly so easy to fix.

Inside a bacterium there are about 1,000 types of enzymes (lactase being one of them). All of the enzymes float freely in the cytoplasm waiting for the chemical they recognize to float by. There are hundreds or millions of copies of each different type of enzyme, depending on how important a reaction is to a cell and how often the reaction is needed. These enzymes do everything from breaking glucose down for energy to building cell walls, constructing new enzymes and allowing the cell to reproduce. Enzymes do all of the work inside cells.

RNA stands for ribonucleic acid. RNA has many functions in the cell and comes in many forms - including messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). RNA is particularly important in the manufacture of proteins.

RNA is made up of a long strand of nucleic acids similar, but not identical, to DNA. For example, where DNA is normally stored as two, paired, complementary strands that are twisted into a double helix, RNA is usually found as a single strand - which may be folded into a variety of different shapes that can affect its function. In addition, RNA uses a slightly different group of nucleic acids in its strands than does DNA. Both types of nucleic acid polymer use guanine, cytosine, and adenine as part of their code, but DNA pairs adenine with thymine while RNA pairs it with uracil.

Although most organisms store their genomes in the form of DNA, some small viruses known asretroviruses have RNA genomes instead. RNA viruses need to be reverse transcribed to DNA before the instructions they contain can be processed by the cells they infect. This process encourages mutation, which may benefit the virus by allowing faster evolution.

 

                                                                       DNA

                                                               TRANSLATION AND TRANSCRIPTION

           

DNA, which stands for deoxyribonucleic acid, is the information storage system of the body. Made of strands of linked nucleic acids, DNA spells out the recipes for all the components of a cell. These recipes are known as genes.

In humans and other complex organisms, DNA is found in the form of chromosomes in the nucleus of cells. Each chromosome is made up of paired, complementary strands of DNA that are twisted into a double helix (picture a twisted step ladder).

When a cell divides, one strand of each chromosome goes into each of the new cells created from the split. These cells can then generate new matching strands to complete the pairs on their own.

Although most organisms store their genetic codes as DNA, a small number of viruses, known as retroviruses, use RNA as their information storage system instead. An RNA genome may give several advantages to a retrovirus, such as smaller sizes and faster mutation rates than are possible with DNA genomes.

                                                                 DNA REPLICATION PROCESS