Waxes Definition

Waxes Definition - Waxes consist of long-chain alcohols bonded to long-chain fatty acids and are used for protective coverings in living things.

Through the process of dehydration synthesis, long-chain alcohols sometimes combine with long-chain fatty acids to form waxes. Waxes differ from triglycerides in that waxes contain alcohols with more than three carbons. Waxes are either solid or oily at room temperature, depending on whether they are saturated or unsaturated, and they serve to cover and protect portions of plants and animals. For example, as you walk through a park, run your finger lightly over the surface of one of the green leaves growing on the trees there. The slightly waxy feel of the surface is caused by a substance called cutin.

Solid and oily waxes also form protective coverings for insects, skin, fur, and feathers. The mink oil that people use to waterproof their boots is made from a natural wax. If you have ever seen a duck preening its feathers as it floats along in the water, you have actually seen it spreading an oily wax over the feathers in order to waterproof them. Without lipids, a duck would sink because of the weight of the water soaked up by its feathers.

Triglycerides or Fats

Triglycerides, or Fats - Triglycerides consist of a glycerol (3-carbon alcohol) and three fatty acids. In saturated fats, all the carbon molecules in the carbon skeleton of the fatty acid are bonded with hydrogen atoms. In unsaturated fats, some carbon atoms in the carbon skeleton are double bonded to each other rather than to hydrogen. Triglycerides function in energy storage.

Ilustration

Fats are composed only of atoms of carbon, hydrogen, and oxygen (C, H, and O), which are arranged into two subunits: a glycerol unit (which is a three-carbon alcohol) and a fatty acid unit. Glycerol is composed of three carbons, to which three alcohol (OH) groups are attached. Fatty acids consist of an unbranched carbon skeleton to which hydrogen and a carboxyl group (COOH), an organic acid functional group, is attached.

Fats are formed by the dehydration synthesis of fatty acid chains to a glycerol molecule. As many as three fatty acid chains can bond to glycerol. When three fatty acids bind with glycerol, the fat is called a triglyceride, since tri- means three. Since the type of fatty acids attached to the glycerol can differ from one triglyceride to another, there are many types of this polymer. In Figure 3.7, stearic, oleic, and palmitic acids form the fatty acid tails.

Fats contain fewer oxygen atoms than other macromolecules and have more hydrogen bonds. For this reason these compounds hold more energy than other molecules. (The reason for this will be clear when we discuss oxidation and reduction in Chapter 6.) More oxygen is required to break down fat molecules during energy-releasing activities in cells. Since fat molecules weigh less than carbohydrate molecules, fats are ideal for the efficient storage of energy.

Having fewer oxygen atoms, fats lack the electropositive regions that would attract the hydrogen atoms (electronegative regions) in water molecules. In other words, fat molecules are nonpolar and hydrophobic they do not dissolve in water, only in other nonpolar substances like ether or methane. For instance, did you know that the wax you apply to a kitchen floor is a fat? That's why you cannot remove it by simply washing the floor with water water molecules are polar. You have to use wax remover (which is nonpolar) to remove wax, since fats are hydrophobic. In general, we can say that like dissolves like. That is, polar substances dissolve polar substances, and nonpolar dissolve nonpolar. That's why wax, a fat and therefore nonpolar, will dissolve only in a nonpolar substance.

As we mentioned in Chapter 1, living things are constantly changing their chemical makeup in order to remain the same, to maintain homeostasis. Fats are constantly being turned over, or converted. They are hydrolyzed to fatty acids and glycerol, and when completely broken down they yield as much as 9 Calories per gram, almost twice as much energy as a gram of carbohydrate, which yields 3.5 to 4 Calories per gram. (A Calorie is the amount of heat energy required to raise the temperature of 1000 grams of water 1 degree Celsius; see page 125.) This high-energy yield is due to the fact that fats contain a higher proportion of carbon-hydrogen bonds, or CH, than any other class of molecules in living things.

Triglycerides contain large amounts of chemical energy and are stored in seeds or in the fatty tissue of animals. Triglycerides are also used for thermal insulation and protection in animals such as the arctic seal.

The fatty acid units in a triglyceride can be saturated or unsaturated. In a saturated fat, the carbon

skeleton of the fatty acid is saturated with hydrogens in other words, there are hydrogen atoms covalently bonded to all possible locations on the carbon atoms . Animal fats, such as butter, lard, and bacon fat, are saturated fats. Since saturated fats usually have fairly high melting temperatures, they are solid at room temperature.

In an unsaturated fat, on the other hand, some carbon atoms form double bonds with other carbon atoms , so that the molecule contains fewer hydrogen atoms than a saturated fat. The formation of these double bonds produces kinks in the fatty acid chains, preventing a tight fit between the fatty acid molecules (Figure 3.8c). The "looseness" of the molecule's structure affects its consistency. For example, plant fats (called oils) are usually unsaturated. This type of fat generally has a low melting point, which is why vegetable oils are usually liquid at room temperature.

When individuals, be they human or house pets, take in more energy in the form of food than they require, the excess energy is stored in fat, and excess fat can lead to obesity. Obesity is the most common chronic medical problem in the United States today. When individuals diet, they are living off their stored fat reserves. If they Use up these fat reserves faster than they replace them, they lose weight. As some of you may know, losing excess fat is not all that easy. In order to lose one pound of body weight, a human must decrease caloric consumption by 3500 Calories or exercise sufficiently to use up 3500 Calories. (Source: Avila, Vernon L. Biology : Investigating Life On Earth Jones and Bartlett/Bookmark Series in Biology Page 61-64)

Lipids Meaning

Lipids Meaning - Lipids are a diverse group of molecules that consist mainly of carbon, hydrogen, and oxygen and are nonpolar. Lipids function in energy storage and structural support.

Lipids Meaning

Lipids are a very diverse group of organic compounds. They consist of triglycerides, phospholipids, steroids, terpenes, and glycolipids. Because they are so diverse, you might wonder why they are all grouped together. They are grouped together because they are all nonpolar molecules (hydrophobic) and do not dissolve in water. Most lipids usually have an oily or waxy consistency, and they function primarily as a means of energy storage and as structural components for living cells. They also play a role in insulation, in padding (for protection), and as an insect attractant in flowers.

Approximately 15 to 25 percent of the human body is made of lipids, the majority of which are stored in fat cells. The two genders tend to distribute their fat cells unevenly. Women tend to store fat in the breasts, hips, and thighs, resulting in their characteristic shape. Men tend to store fat around their waist, hence their characteristic shape.

Polysaccharides: Definition, Structure & Examples

Polysaccharides: Definition, Structure & Examples - Polysaccharides are carbohydrates containing more than 10 monosaccharides; they function in energy storage and structural support in plants and animals. The way that monomer units are linked together determines the polysaccharide's properties.

Polysaccharides structure

When dehydration synthesis covalently bonds more than 10 monosaccharides together, the result is a macromolecule, sometimes called a polymer. Macromolecules containing monosaccharides are called polysaccharides, meaning many sugars. A polysaccharide may have a branched shape, or it may be arranged like a long linear chain. The chemical properties of polysaccharides are due to the different types of linkages that can occur between the monomer units.

Storage Polysaccharides

Both glycogen (animal starch stored in the liver and muscles of higher animals) and plant starch consist of several hundred glucose units. The two types of polysaccharides react differently because of the way their long chains are arranged. In glycogen, the glucose units are aligned in such a way that they form highly branched forms. In plant starch, the glucose units are aligned to form a twisted, coiled structure. The properties of polysaccharides are due to different types of linkage alignments that can occur between monomer units.

Polysaccharides serve as a means of storing carbohydrates. However, before their elements can be transported through living systems and used for energy, they must undergo hydrolysis and be broken down into monosaccharides or disaccharides. Many times large carbohydrates are formed of only one type of monosaccharide, but some are composed of two, three, or four different types.

Structural Polysaccharides.

Polysaccharides not only function in energy relationships in cells. They are also very important structural components of living things. For example, cellulose, which is made up of glucose monomers, is a fibrous, water-insoluble substance that functions as the main structural material in plant cell walls. In fact, cellulose may well be the most abundant organic chemical in the world. Humans benefit from this abundance, for we use this molecule for such things as lumber, rope fiber, and paper.

Chitin is another polysaccharide, one that is called a modified carbohydrate because it contains nitrogen atoms in its monomer units. Chitin, which is secreted by the outer tissue layer of the animal group that includes lobsters and insects, is the main structural component of the animals' outer skeletons (or exoskeleton). Until recently chitin was thought to have little food value for humans, since humans do not have enzymes to break down this polysaccharide to simpler units. However, biologists now know that some bacteria contain enzymes that can break down chitin into sugars so that the material could be used as a dietary energy source. As a result, some molecular biologists are trying to determine the nature of the gene that controls the production of the enzyme so that we can produce it in large amounts. With this enzyme we can transform the chitin previously believed to be of no use to humans and break it down into sugar.

(Source: Avila, Vernon L. Biology : Investigating Life On Earth Jones and Bartlett/Bookmark Series in Biology Page 59-60)

Disaccharides: Definition, Structure & Examples

Disaccharides: Definition, Structure & Examples - Disaccharides are formed when two monosaccharide units covalently bond together through dehydration synthesis. A common example is sucrose, or table sugar.

When the process of dehydration synthesis covalently bonds two monosaccharides together, the result is a disaccharide. (Remember that saccharide means sugar. The prefix di- means two.) We are all aware of many disaccharides. Common table sugar, sucrose, is a disaccharide, composed of a molecule of glucose bonded to a molecule of fructose. Lactose, or milk sugar, is a disaccharide composed of glucose combined with the monosaccharide galactose. Oligosaccharides consist of two to ten monosaccharide units bonded together, and some types can cause very unique problems, such as the excessive gas and stomach distress you get when you eat beans. Beans contain large amounts of oligosaccharides.

Some adults lose the ability to digest the milk sugar lactose, because their bodies produce insufficient amounts of the enzyme lactase. Lactase is responsible for the hydrolysis of lactose to glucose and galactose. Individuals with this condition suffer from gas, diarrhea, and cramps after the ingestion of lactose. The condition is more prominent in blacks than in members of other races, compounding the difficulty of relief efforts in African countries suffering from famine due to years of drought. A major food supplement shipped to famine-stricken areas is dried milk, because it is light and easier to transport than other protein sources. However, because of enzyme deficiencies, the recipients of this aid do not always benefit as much as their benefactors intend. (Source: Avila, Vernon L. Biology : Investigating Life On Earth Jones and Bartlett/Bookmark Series in Biology Page 59)

Nanotechnology: Definition & Products

Nanotechnology: Definition & Products - Can we see the birth of molecules? What really happens to atoms during chemical reactions? Can we move atoms? Can we actually visualize biological processes at the molecular level? The answer to all of these questions is yes. Today, because of the advent of new instrumentation like the family of scanning-tunneling microscopes, scientists can now pick up and move atoms, and, with the use of molecular cameras, they can take photographs as fast as one quadrillionth of a second (a femtosecond). Scientists can image the step -by-step changes that occur in atoms and molecules as chemical reactions progress.

Nanotechnology product

As was discussed in the Human Endeavors box in Chapter 2, scanning-tunneling microscopes ''feel" the surface of the molecular or atomic specimens to form the image. A tiny platinum probe is given a small electrical charge and the specimen is scanned, atomic diameter by atomic diameter, across its surface. As the probe moves, electrons respond to the voltage differential between the probe and the surface of the specimen by tunneling across the gap, which produces a very small current. The variations in the currentwhich are due to the miniscule differences in the distance between the probe and the objectare detected. The computer produces the image from these distances.

In 1989, Donald Eigler and Erhard Schweitzer of IBM in San Jose, California, lowered the temperature of a plate of nickel to about absolute zero (-456° F), which minimized atomic vibrations. Then, by spraying atoms of xenon gas over the plate and by increasing the electrical charge on the probe of the scanning-tunneling microscope, they were able to drag the xenon atoms across the nickel to spell out "IBM" in letters only five atoms tall, the entire logo only about 660 billionths of an inch long (Figure A). This example of the manipulation of materials, be they inorganic or organic, atom by atom, is what is meant by nanoengineering. The new nanotechnology combines the technological capabilities of the scanning-tunneling microscope with laser cameras that can detect changes that occur in quadrillionths of a second, and you can image what happens in atoms and molecules as they react. The synchroton ray camera can do just that. When you pulse an atom with a specific frequency of energy, the atom will absorb that energy and the electrons are moved to a higher energy state. When the electrons return to their original ground state, energy of a specific frequency is emitted. Using this principle, scientists can pulse molecule A and molecule B and detect how their frequencies change to form molecule C. But how can you do that and how can you record the snapshots or "frames" of this continuous event, since it only occurs in quadrillionths of a second? Figure B illustrates how this camera works.

So today we can see the advent of a new technology even more exciting or as exciting as genetic engineering. Not only can we now manipulate genetic variation, but we can actually manipulate atoms and molecules. With this technology our advances in science, including biology, will truly expand. Just think, today not only can we image atoms, but we can manipulate them as well and detect the actual events in the birth of new molecules as atoms and molecules react.

Source: Zewail, Ahmed H., "The Birth of Molecules," Scientific American, 263:6, Dec. 1990, pp. 7682. - See more at: http://www.biologysmart.com/2015/07/the-functions-of-carbohydrates-in-body.html#sthash.RXSQcqFS.dpuf

Monosaccharides: Definition, Structure & Examples

Monosaccharides: Definition, Structure & Examples - The simplest carbohydrate unit is called a monosaccharide. In some cases, monosaccharides (as well as other molecules) have identical empirical formulas, but their structural formulas differ, giving each variety unique chemical properties. Such molecules are called isomers.

The simplest units of carbohydrates (which are the building blocks of all complex carbohydrates) are monomers known as simple sugars, or monosaccharides. (The term mono-means one, and the term -saccharide means sugar unit.) Typically, simple sugars consist of three to seven carbon atoms. Of special importance to biologists are two types of monosaccharides: the five-carbon (or pentose) molecules, such as those that are part of DNA and RNA; and the six-carbon (hexose) molecules, such as glucose and fructose. The common ending -ose indicates a sugar.

Mnosaccharides Structure

Glucose and fructose are examples of what are called isomers olecules that have identical empirical formulas but have different properties. We touched briefly on this topic in Chapter 2. The empirical formula for both glucose and fructose is C6H12O6, indicating that each consists of 6 atoms of carbon, 12 atoms of hydrogen, and 6 atoms of oxygen.

In an instance like this, in order to see the differences between the two kinds of molecules, a structural formula is more useful than a general formula. (A structural formula shows the position of the atoms within a molecule.) This type of formula illustrates the significant nature of each isomer by showing variations in molecular arrangement. The differences in the bonds between the carbons change the properties of the chemicals. For example, because of the molecular structure of fructose, it stimulates our taste buds more than glucose, so it tastes sweeter than glucose.

One comment about the notation you will see in a structural formula of carbon molecules. Some monosaccharides, especially those having five or six carbon atoms, form a ring when placed in water, as in the watery interior of living cells. In a structural formula, as a matter of convention, the locations of the carbon atoms are numbered for reference, and the numbers serve as an indication of the placement of the carbons at the intersection of the links in the ring. (Source: Avila, Vernon L. Biology : Investigating Life On Earth Jones and Bartlett/Bookmark Series in Biology Page 55)

The Functions of Carbohydrates in the Body

The Functions of Carbohydrates in the Body - Carbohydrates function as the primary cellular energy source. They have an empirical formula of [C(H2O)]n/sub>. The main types of carbohydrates are monosaccharides, disaccharides, and polysaccharides.

Function of Carbohydrates

Carbohydrates are not just those things that you try to avoid when you are on a diet. All living things (plants, animals, fungi, and microorganisms) contain and utilize carbohydrates for their primary) source of energy. In addition, carbohydrates also serve a structural function in all living things. For example, plants are supported structurally by a carbohydrate called cellulose, which exists in their cell walls. Both plants and animals store reserves of excess carbohydrates, as either starch (in plants) or glycogen (animal starch). Carbohydrates fall into three major categories: monosaccharides, disaccharides, and polysaccharides.

An empirical formula is one that indicates the type and number of atoms within a molecule. When we compare the empirical formulas of most carbohydrates, we find carbon, hydrogen, and oxygen. Carbohydrates are literally watered (or hydrated) carbons C(H2O) as the name carbohydrate suggests, and their generalized empirical formula is [C(H2O)]n. The n designates the number of carbohydrate units in the molecule. The category to which a specific carbohydrate belongs depends on the number of units it possesses. (Source: Avila, Vernon L. Biology : Investigating Life On Earth Jones and Bartlett/Bookmark Series in Biology Page 55)

Chemical Reactions in Biological Terms

Chemical Reactions in Biological Terms - During hydrolysis macromolecules are cleaved into their smaller subunits by adding water. Dehydration synthesis is the linking of smaller subunits by removing water to form larger molecules.

Chemical

Let us see how some of these types of reactions affect my meal. Earlier I said that my meal actually consisted of large molecules (often called macromolecules or polymers) of proteins, carbohydrates, and lipids. To change those macromolecules into the types of proteins, carbohydrates, and lipids that my body can use, a set of paired reactions occurs in my cells: hydrolysis and dehydration synthesis (Figure 3.2). Both of these reactions are mediated by enzymes.

The first type of reaction is a degradation reaction called hydrolysis, a term that means water

 splitting. In hydrolysis, water molecules are enzymatically added to macromolecules, splitting them into their component subunits. Such small molecules are called monomers.

The second type of reaction is a synthesis reaction called dehydration synthesis. This step recycles the monomers by removing the water used in hydrolysis. The monomers are then joined to form polymers, such as the proteins, carbohydrates, and lipids that my body needs. Hence the name of the process, which means synthesis (or putting together) by dehydration (the removal of water).

So I benefit from my breakfast because of a pair of reactions that are the reverse of each other. One reaction adds water to a macromolecule to degrade it into monomers, and the other process removes water from monomers to form macromolecules. At present I am hydrolyzing, or breaking down, my meal into its component parts, its monomers. Once these simple molecules get into my cells, they will be utilized as sources of energy or as the raw materials used to synthesize the macromolecules that make up me.

So through these two paired reactions, monomers are synthesized into macromolecules that are of biological importance. What are the properties of these biological compounds, and what are their functions? Let us discuss them.

There are four groups of biological compounds that are important in living things: carbohydrates, lipids, proteins, and nucleic acids. Each group is divided into smaller subgroups that play an important role in the chemical activities of living things. (Source: Avila, Vernon L. Biology : Investigating Life On Earth Jones and Bartlett/Bookmark Series in Biology Page 53-54)

Displacement Reactions

Displacement Reactions  - In displacement reactions, there is an exchange of atoms (or groups of atoms) between molecules. There are two types of displacement reactions, single and double displacement. In a single displacement reaction, such as A + BC « AC + B, one element shifts position. In living systems, for example, hemoglobin (an iron-containing molecule) can combine with CO2 from the cells to form carbamino hemoglobin. When this carbamino hemoglobin is transported by the blood to the lungs, the carbon dioxide is displaced by O2. This single displacement reaction can be represented as follows:

In a double displacement, as in AB + CD « AC + BD, two elements shift position. A double displacement reaction occurs when silver nitrate reacts with hydrochloric acid to form silver chloride and nitric acid.

All four types of reactions take place during the various life processes in living things, such as the utilization of my breakfast. Often the reactions occur in pairs something is taken apart (degraded) in order to be put together (synthesized) in another arrangement. Such an arrangement is called a paired reaction. It is rather like cutting apart several types of board in order to build a bookcase. You have the same wood when you have finished that you had when you started, but the structure and arrangement are different. Paired reactions are an essential characteristic of chemical reactions in living things. In other words, if you put things together, you can usually break them apart. In fact, most chemical reactions are reversible. Chemists use a pair of arrows to indicate a reversible reaction:

Collision Theory

Collision Theory - The collision theory states that chemical reactions occur when molecules collide. The energy required to start a reaction is called the minimum energy of activation. There are four general types of chemical reactions: rearrangement, synthesis, degradation, and displacement.

Collision Theory

Today is a very exciting time in chemistry, since scientists can actually move atoms and also image the progress of chemical reactions. But, what are chemical reactions?

Simply stated, chemical reactions involve breaking and reforming chemical bonds. In order to do this, energy is required, and the amount of energy available in the system determines whether or not a reaction will occur. For example, if you assume that all atoms, ions, and molecules are constantly moving, then a chemical reaction can occur when they collide with one another. This is what happens when two atoms of hydrogen unite with a molecule of oxygen to form water (H2O). Chemists call this explanation the collision theory.

For example, when you played with your old chemistry set you learned that if you simply mixed two chemicals together, it usually took a long time for a chemical reaction to occur. However, if you heated the mixture (which is one way to put energy into a system), the reaction proceeded much more quickly. Basically, according to the collision theory, the addition of heat increased the speed at which the atoms and molecules were moving, and thus increased the likelihood that they would collide and react. (There are other ways to add energy to a material by shaking or increasing pressure, for instance but heat is the one that is most common, especially in biological processes.)

Adding just a little heat will not necessarily get results. Before a reaction can occur, a certain minimum amount of energy must be added, which chemists call the minimum energy of activation. What is more, the level of the minimum energy of activation depends on the substances involved. For instance, if you light a match and toss it into a small pan of gasoline, you provide enough energy (in the form of heat) to combine the gasoline and oxygen to form carbon dioxide, water, and a tremendous amount of liberated energy (in other words, the explosion will be something to see). However, if you fill the pan with another liquid, such as plain water, the lighted match will have no effect. (Source: Avila, Vernon L. Biology : Investigating Life On Earth Jones and Bartlett/Bookmark Series in Biology Page 52)

Chemical Reactions and the Molecules of Life

Chemical Reactions and the Molecules of Life - I just finished eating breakfast, which today consisted of a glass of low-fat milk and French toast covered with peanut butter and syrup. (I happen to like peanut butter on my pancakes and French toast you should try it.) How do we as living things break this meal down into its constituent parts and reconstitute or assimilate the chemical components into ourselves?

Chemical Reaction (source: http://www2.estrellamountain.edu/faculty/farabee/BIOBK/atweights.gif)

Although my breakfast consisted of the things I just mentioned, to the biological system that is my body, my meal consisted of carbohydrates, lipids, and proteins, the large molecular constructions that are found in all foods. It also contained water, minerals, and nucleic acids. The cells of my body and of other living things do not distinguish between a meal of insects or of steak. Protein is protein, and it will be broken down into the amino acids that make up that protein. The amino acids will eventually enter my cells and be reunited into new amino acid sequences that form the protein in my cells. 

How does nature degrade, or break down, large molecules into their component subunits? And how does nature take these component subunits and reunite them to form other complex molecules?

To begin with, all these activities are chemical reactions; so in order to answer our questions, we must first understand how chemical reactions take place.

Cohesiveness and Tensile Strength

Cohesiveness and Tensile Strength  - You have all heard of adhesion (the holding together of unlike substances). For example, the adhesion of adhesive tape holds you and the tape together. There is another way of "holding together," called cohesion, or the holding together of like substances. All of you have seen evidence of the cohesiveness of water molecules at one time or another. As you try to sleep, and finally locate the dripping faucet that has been keeping you awake, you notice that for a few moments before it falls, the trickle of water clings to the faucet as it forms a drop. Others have seen water striders run across the surface of a pond. All these phenomena are due to the surface tension of water. The water moisture in our lungs also exerts a surface tension that we must counteract to avoid the collapse of the air sacs in our lungs. Surface tension is a result of the hydrogen bonds that have formed because of the electronegative and electropositive qualities of water molecules.

Cohesiveness and Tensile strenght (www.gordonengland.co.uk/img/img00003.jpg)

Because of these cohesive properties, water also has a remarkable tensile strength in other words it resists being pulled apart. As a matter of fact, under certain conditions the tensile strength of

water exceeds that of steel wire. The cohesiveness and tensile strength of water help to explain how water can rise hundreds of feet from roots to the needles of a giant redwood tree.

What is ionization? Definition and Meaning

What is ionization? definition and meaning - With a predictable frequency, water itself can ionize into a hydrogen ion (H+) and hydroxide ion (OH-),thus providing the ions required in many fundamental reactions essential to the continuation of life. In addition, other molecules containing H+ and OH- groups dissociate in water. A substance that releases hydrogen ions (H+) is called an acid. Substances that release hydroxide ions (OH-) in water are called bases. (This is not a complete definition of an acid or base, but it will serve our purposes.) 

Ionization Energy

We use a scale called pH, which runs from 0 to 14, to indicate the relationship between these two ions in solutions. In pure water, the numbers of hydrogen ions (H+) and hydroxide ions (OH-) are equal; hence the water is said to have a neutral pH, or a pH of 7. When the number of H+ exceeds the number of OH-, the solution is said to be acidic, and has a pH of less than 7. Conversely, when the number of OH- exceeds the number of H+, the solution is said to be basic or alkaline; it has a pH between 7 and 14 (Figure 2.15). The scale is based on powers of 10, so that the difference of just one indicates a change ten times as great. For example, stomach acid has an acidity level around 2. Apples have a pH of about 5. That means that stomach acid is 1000 times as acidic as apples.

Chemical reactions of living organisms usually occur at a pH range of 6.9 to 7.5, a range that is called "neutral." (There are exceptions, such as the highly acidic environment inside your stomach.) However, many of the chemical reactions that occur in aqueous solutions either release or utilize hydrogen, which affects the pH. How does the cell prevent pH shifts away from neutrality? The maintenance of the internal pH of all cells is primarily due to buffers, chemical substances that play one of two roles. When there are too many hydrogen ions (when the solution is acidic), buffers combine with excess hydrogen ions to bring the solution to a neutral state. When there are too many hydroxide ions (when the solution is basic), buffers combine with hydroxide ions to bring the solution to a neutral state. Hence neutrality is maintained.

Carbonic acid is one of the major buffering substances of blood. Carbonic acid when present in water dissociates into bicarbonate and hydrogen ions. When chemical reactions in the body cause a high concentration of hydrogen ions in the blood, they combine with the bicarbonate to form carbonic acid, thus removing the hydrogen ions from the blood. When there is an excess of hydroxide ions, they combine with the hydrogen ions to form water. (Source: Avila, Vernon L. Biology : Investigating Life On Earth Jones and Bartlett/Bookmark Series in Biology Page 46-48)

A Universal Solvent

A Universal Solvent - A Solvent is any substance in which another substance (called a solute) can dissolve. Water's polar nature makes it an excellent solvent. As we discussed earlier, because there is an unequal distribution of electrons between the atoms of the molecule, the atom of oxygen behaves as if it had a negative charge (or as if it were electronegative). The hydrogen regions behave as if they had positive charges (or as if they were electropositive). When a polar molecule is placed with other polar substances, there is an electrical interaction and the molecules are dispersed spread out. In other words, they dissolve.

A Universal Solvent

		Polar molecules that form weak hydrogen bonds with water molecules and thus dissolve are said to be hydrophilic (water-loving). Some molecules are not polar, however, so they do not dissolve in water. Instead they are usually insoluble in water, so they are called hydrophobic, or water-fearing. For example, if you mix oil and water, the oil (a liquid fat) is nonpolar, so it remains in the form of droplets in the water (Figure 2.14). The oil and water separate. That is why you need to remix your salad dressing every so often. As we will see when we discuss cell membranes, the hydrophobic and hydrophilic properties of molecules are very important in the movement of molecules through membranes.

Water, the Crucial Molecule of Life

Water, the Crucial Molecule of Life - Water is essential to life. Because of its polar structure, water has several important properties: it is a universal solvent and a temperature stabilizer, it can dissociate ionic compounds, and it has great tensile strength and cohesiveness.

The hydrogen bonding that occurs in water has profound biological consequences. Some astronomers refer to our planet as ''the water planet," for in many ways the existence of water makes earth unique in our solar system. In some ways, all living things on earth are "water beings," because about 50 to 90 percent of the bodies of living things consists of water. What is more, it is the special structure of water, with its polarity and its ability to form weak hydrogen bonds between one water molecule and another, that made it possible for life to arise. Because of hydrogen bonding, water is, among other things, a universal solvent and a remarkable temperature stabilizer, and it has great cohesiveness and tensile strength. (Source: Avila, Vernon L. Biology : Investigating Life On Earth Jones and Bartlett/Bookmark Series in Biology Page 46)

Hydrogen Bonds

Hydrogen Bonds - Hydrogen bonds are weak bonds between polar molecules. An especially important hydrogen bond occurs between the oxygen of one water molecule and the hydrogen of another.

Ionic and covalent bonds form between the atoms of a molecule. Hydrogen bonds, however, form between molecules, not between atoms, and are made possible because of the polarity of certain molecules. To illustrate hydrogen bonds, we will look at the water molecule again, but remember that other polar molecules will act in a similar manner.

How does polarity affect the way a water molecule reacts with other molecules? The hydrogen atoms at each side have a slightly positive charge, so they will form attractions with atoms in other molecules having negative charges, such as oxygen, nitrogen, or fluorine. The oxygen atom in the center has a slightly negative charge, so it will form attractions with atoms or molecules having positive charges. This weak electrostatic attraction is called the hydrogen bond. For example, in water a hydrogen atom in one molecule (the relatively positive end) is attracted to the oxygen atom (the relatively negative portion) in another molecule, forming a hydrogen bond.

Hydrogen bonds are very important in biology, in part because they are so easy to break. For example, hydrogen bonds are responsible for holding certain portions of the DNA molecule together, and their weak hold allows the molecule to separate very easily in order to replicate itself. Hydrogen bonds also play a role in the activities of enzymes in chemical reactions. (Source: Avila, Vernon L. Biology : Investigating Life On Earth Jones and Bartlett/Bookmark Series in Biology Page 45-46)

Polar Covalent Bonds

Polar Covalent Bonds - In some molecules, atoms share electrons unequally, developing areas that have slightly positive and negative charges, which form a polar covalent bond. When atoms form covalent bonds, they often share the electrons equally, so that the charges are equally distributed. However, in some special cases, the shared electrons are drawn to one section of the molecule because the electrostatic pull of that section is stronger. In these cases, the bond is called a polar covalent bond.

Polar Covalent Bonds

For instance, think about the structure of a water molecule. Most of us already know that water is H2O two hydrogen atoms and one oxygen molecule. And we know that hydrogen has an atomic number of 1. Oxygen has an atomic number of 8, which means that it has 2 electrons in its first energy level and 6 electrons in its second energy level. In a water molecule, oxygen and two hydrogen atoms form covalent bonds in order to fill oxygen's second energy level and hydrogen's first energy level.

However, even though the three atoms share electrons in order to fill their energy levels, the oxygen nucleus, with its eight positively charged protons, has a stronger attraction for electrons than the hydrogen, with only one proton. As a result, the majority of the electrons are near the oxygen portion of the molecule, which gives that region a slight negative charge. (Remember, electrons have a negative charge.) The two hydrogens, on the other hand, have fewer electrons near them, so they have a slight positive charge. As you can see in Figure 2.11, the water molecule assumes a shape having four corners, with two positively charged regions and two negatively charged regions. In other words, there is an unequal distribution of charges in the molecule.

Because given portions of the molecule have differing electrostatic charges, the molecule is said to be polar, and the bonds are said to be polar covalent bonds. (The use of the word polar here is meant in the same sense as the term poles when you are dealing with magnets.) The polarity of the water molecule leads us to the discussion of the next type of bond that is important in biological systems, the hydrogen bond. (Source: Avila, Vernon L. Biology : Investigating Life On Earth Jones and Bartlett/Bookmark Series in Biology Page 45)

Carbon's Valence Electrons

Carbon's Valence Electrons - Carbon's uniqueness and versatility are due to its electronic configuration that is, the arrangement of electrons in its outermost energy level. Remember that we said earlier that carbon has six protons and six electrons. The first energy level contains two electrons, so how many electrons does the second energy level have? Correct, it has four. Because of this fact, carbon can either gain or lose four electrons in order to satisfy its electron requirements, and that fact makes carbon a very versatile atom.

Carbon's Valence Electrons

For example, as we saw earlier, carbon can combine with four hydrogen atoms to form methane (swamp gas), a type of compound called a hydrocarbon, because it is made up of hydrogen and carbon. (There are numerous other hydrocarbon molecules besides methane.) Or carbon can join with two oxygen atoms to form carbon dioxide (O=C=O). Carbon bonds very readily with other elements as well, such as nitrogen, sulfur, chlorine, and phosphorus. It can also combine with one or more other carbon atoms, by forming a single (CC), double (C=C), or triple (C=C) bond. You will see part of the importance of this fact when we discuss carbon chains in saturated and unsaturated bonds in the next chapter.

Now if carbon participated in an ionization process, it would have to either lose or gain four electrons. That would make the atom's electrical balance very uneven, since it would have either four more positive than negative charges, or vice versa. Therefore, carbon usually forms covalent bonds it shares four electrons. Because of this ability to share four electrons, carbon is the most versatile of all the atoms. It can combine convalently very readily with many other atoms.

Carbon also bonds with functional groups. A functional group is a group of atoms that provides a distinctive chemical property to any molecule of which the group is a part. For example, OH is what is known as a hydroxyl group, which is sometimes called an alcohol group. A hydroxyl group imparts its properties to all alcohols. When a hydrogen atom of methane (CH4) is replaced by a hydroxyl, it becomes methanol (CH3OH), or wood alcohol, which is poisonous. When ethane (C2H6 has a hydrogen atom replaced by a hydroxyl, it becomes ethyl alcohol (C2H5OH), the grain alcohol contained in vodka. We will take a closer look at functional groups in Chapter 3.

Thinking of Carbon-Based Molecules in Three Dimensions. To picture a carbon-based molecule accurately, you have to realize that the structural formula of molecules actually represents a three-dimensional object (Figure 2.10), even though, for convenience, they are usually drawn in only two dimensions. As we'll see in the next chapter not only can carbon combine with a variety of other atoms and with functional groups, but the combinations come in a variety of shapes. In some molecules, carbon atoms form a basic backbonelike arrangement for an array of atoms or functional groups that branch out to one side or another.

The shape of a carbon-based molecule is important because, as you will learn in the next chapter, some molecules (called isomers) have the same number of atoms, but because their shapes are different, they exist as different molecules. As a result, they react with other molecules in different ways, and they serve different purposes.

So the importance of the chemistry of carbon is its ability to combine in several ways with itself, other atoms, and molecules. As a matter of fact, because of their significance in living things, carbon-containing molecules are called organic (life-formrelated), as opposed to inorganic (non-carbon, nonlife molecules). Life is truly carbon-based. (Source: Avila, Vernon L. Biology : Investigating Life On Earth Jones and Bartlett/Bookmark Series in Biology Page 43-44)

The Carbon Base of Life Forms

The Carbon Base of Life Forms - A carbon atom contains four valence electrons, which can bond with up to four other atoms. Carbon atoms can join in chains or rings of various shapes. To a large extent, the shape of a molecule determines its chemical properties.

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Substances that contain carbon are called organic, because all carbon-based substances had their origin in living things. The life forms that have evolved on this planet are said to be carbon-based in other words, the element carbon has been the primary building block of living things on earth. But with 92 naturally occurring elements, why is life on this earth based on carbon? What is so unique about this atom? To understand that you must look carefully at carbon's structure. (Source: Avila, Vernon L. Biology : Investigating Life On Earth Jones and Bartlett/Bookmark Series in Biology)