Proteins, Nucleic Acids.

Proteins

Amino acids

Proteins are among the most abundant and versatile macromolecules. They have different functions as enzymes, hormones, storage, transport, structural proteins, membrane proteins, and antibodies. Eventhough their strurctural and functional diversity is overwhelming, their properties depend as much on their final shape as on their constituent parts.

Proteins are polymers of amino acids. All amino acids have the same fundamental structure.

1.- An amino group (-NH2).- It is a weak base

2.- A carboxyl group (-COOH).- It is a weak acid

3.- A hydrogen (-H)

4.- A functional group that varies from one amino acid to the next and is generally called and -R group.

The only difference between the 20 life relevant aminoacids is that of ther lateral groups (-R). In 8 of the molecules, -R is formed by short chains or rings of C, H. These amino acids are non polar thus, hydrophobic. 7 of the amino acids have –R formed by weak acids or weak bases. Depending on the solution’s pH, they can be possitively or negatively charged.

Proteins

Amino acids link to each other in a condensation reaction. The “head” of an amino acid bonds to the “tail” of another one by the elimination of a molecule of water. This bond is called peptide bond and the molecule that forms by the union of several amino acids is called a polypeptide. The sequence of aminoacids in a polypeptide gives the molecule its particular biological function, even the slightest variation in the sequence can result in an alterated or nule protein function.

Because there are about 20 different amino acids commonly found in proteins and each protein can contain anywhere from 200 to 300 individual amino acids, the diversity of possible amino acid sequences for any given polypeptide is extremely large. For example, 400 different dipeptides are possible and 8,000 tripeptides. For peptides of 300 amino acids, the possible number of sequences is so large that there are not enough atoms in the entire universe to build one example of each possible sequence!

The final properties of a protein depend largely on its three dimensional shape. Before a polypetide can function within a cell, therefore, it must twist and fold into a unique and special properties and functions as a protein emerge.

The final shape of a protein molecule is determined by three set of conditions.

1.-The sequence of amino acids in the polypeptide.

2.-The interaction between the amino acids in the polypeptide.

3.-The interactions of the amino acids with the surrounding water.

The sequence of amino acids from one end of the molecule to the other is called primary structure. Under orders from the genetic and synthetic apparatus of the cell, aminoacids are joined together in a precise and predetermined fashion. The primary structure is only the start.

In water a polypeptide twists and turns until it takes up the most stable configuration, a shape that requires the least energy to maintain and the most to disrupt. It is termed the secondary structure of the molecule. For example, amino acids close to each other interact and form hydrogen bonds. This forces the polypeptide to into and alpha helix or a beta sheet.

Next comes the interaction with water. Many parts of the molecule are either polar and hydrophillic or nonpolar and hydrophobic. As the folded and coiled polypeptide chain interacts with the surrounding water molecules, hydrophobic –R groups are forced , as much as possible, into the interior of the emerging structure, away from the surrounding water. Hydrophilic –R in the other hand, are stabilized by the interaction with water; thus, they are found close to the surface of the protein and in contact with water. The shape taken on by the macromolecule as a result of these forces is called a tertiary structure.

For many proteins, this is the highest level of structure they achieve. At the tertiary level, the protein suddenly takes on its major property and begins to function as a vital macromolecular component of the cell. But for some, there is one more step., the highest order os structure called the quaternary structure. At this level, the functioning protein unit is a complex built by the combination of more than one polypeptide chain and /or by the addition of other substances such as metallic ions, carbohydrates, lipids, nucleic acids, or other carbon based structures.

Hemoglobin, the oxygen-carrying protein found in red blood cells is an example of a protein showing the all four levels of structure. This macromolecule, consists of four polypeptide chains. Each chain has its own amino acid sequence (primary structure), and parts of chain twists into regions of alpha helix (secondary structure). All the chains fold into unique shapes in water (tertiary structure) and the come together in a fourfold complex with four additional iron-containing structures called heme groups (quaternary structure).

Function of Proteins

Protein functions are as varied as their structures. Almost every vital chemical reaction in every living cell is mediated by its own highly specialized protein known as enzyme. These enzymes are critically important in the reactions that break down food molecules, build new cellular structures, or repair existing components. Very few, if any, chemical reactions within a living organism can take place at any appreciable rate without these proteins.

The hard shell of a turtle, the skin of a new born baby, the claws of a lion and the feathers of a hummingbird all contain a type of protein known as keratin. This protein helps build and maintain physical structures, as does collagen, an essencial component of ligaments and tendons that connect bones and muscles.

Other proteins are involved in movement. Actin and myosin are fibrous contractile proteins found in muscle cells and involved in changing the shape of individual cells.

When attacked by invading organisms, our bodies respond by producing antibodies, a type of protein designed to recognize and neutralize and invader. Messenger molecules called hormones help regulate complex processes such as immune responses, growth and metabolism. Some hormones are steroids but other are proteins.

Within the human body, there is a probable minimum of 10,000 different proteins, each with its own unique structure shape and function. Information needed to construct each of these proteins, with all amino acids in the correct sequence, is stored in the genetic and hereditary material of cells: The nucleic acids.

Nucleic Acids

The information that orders the myriad of proteins that are found in the organisms is coded by two macromolecules known as nucleic acids and is translated by them. Nucleic acids are polymers formed by long chains of monomers called nucleotides. A nucleotide is a molecule formed by three subunits: A phosphate group, a five-carbon sugar and a nitrogenous base.

The sugar subunit can be either a ribose or a deoxyribose, the latter having one oxygen atom less than the ribose. Ribose is the sugar that is present in ribonucleic acid (RNA) whereas, deoxyribose is present in deoxyribonucleic acid (DNA). There are five nitrogenous bases which are the primary skeleton of the nucleic acids. Two of them adenine (A) and guanine (G) have a two ring structure and are known as purines . The other three, cytosine (C), thymidine (T) and uracil (U) have only one ring and are known as pyrimidines.

Both nucleic acids present A,C,G but RNA presents U instead of T which is present in DNA.

Most RNA molecules found in cells consist of a single polynucleotide chain, whereas DNA molecules are almost invariably double stranded, consisting of two polynucleotide chains twisted around one another in a double helix, which resembles a spiral stair case. DNA is usually synthesized only at one specific time in the cell cycle whereas RNA is more or less produced constantly throught the cell cycle.

The sugar-phosphate backbones of both DNA strands are on the outside of the molecule, forming the handrails of the staircase, the nitrogenous bases face inward, forming the stairs. Each step in the staircase is a pair of bases, one from each strand. These bases are paired together in specific combinations. Adenine is always paired with thymine and guanine is always paired with cytosine. Using the letter abreviation A=T, G=C. These are the base pairing rules. Thus if the sequence of bases is known for one DNA strand, it is possible to deduce the sequence of the partner strand, known as the complementary strand. For example, if one of the strands has the sequence

ATGTTCAAT

The sequence for the complementary strand is

TACAAGTTA

The two strands would go together like this

ATGTTCAAT

TACAAGTTA

DNA stores information for the sequence of amino acids in polypeptides and also acts as the hereditary substance, passing along this stored information from one generation to the next. Cells utilize RNA in several ways: As a messenger molecule carrying information away from DNA molecules, as structural molecules, or as transfer molecules in protein synthesis.

For a nice review on macromolecules click here.

Session 2 (August, 21st, 2009) Lipids

Lipids

They are a general group of non water soluble organic molecules that can be dissolved in non polar organic solvents, such as chloroform, ether and benzene. Tipically, lipids are energy storage molecules in the form of fatty acids and fat and when structural, as phospholipids, glucolipids and wax.

They are constructed from the simplest possible monomer. A carbon atom is covalently bonded to two hydrogen atoms forming a -CH₂ – unit. When several of these monomers are linked together, a polymer called hydrocarbon is created. Hydrocarbons have two very important properties: 1) A lot of energy is stored in the C-C and C-H covalent bonds of the molecule and 2) They are very hydrophobic and will not dissolve in water to any great extent.

Fatty Acids.- They are not usually found in the cells as free forms. They are composed of a long chain of an even number of C atoms, between 14 and 22. They differ from each other on the number of carbons and in the fact that some of them could have double bonds in different positions in the chain. A fatty acid such as stearic acid, which has no double bonds it is said to be saturated. This is, all the posibilities of more bonds are already used. In other words, all the carbons in the chain present four bonds. In the other hand oleic acid which presents double bonds between carbon atoms in its chain, is said to be unsaturated, because its carbon atoms have the potential of bonding with more atoms.

Fat.- A fat molecule is formed by three molecules of a fatty acid linked to a glycerol molecule. Glycerol is an alcohol with three carbons with three hydroxyl groups (-OH). A fatty acid is a long hydrocarbon chain that finishes with an carboxyl group (-COOH). This chain is non polar and hydrophobic while the carboxyl group gives the molecule and acid property. As with polysaccharides, each bond between glycerol and the fatty acid is formed by the elimination of water (condensation, dehydration). These molecules are said to be neutral lipids because they do not have net positive or negative charges.

Phospholipids and Glucolipids.- As in fats, phospholipids and glucolipids are formed of fatty acid chains linked to a gylcerol molecule.

Phospholipids (phosphate lipids).- In phospholipids the third carbon atom of the glycerol is linked to a phosphate group as opposed to a fatty acid. Phosphate groups are highly hydrophilic since phospate is negatively charged while the fatty acid portion is highly hydrophobic. This kind of molecules are called amphipatic molecules. The relevance of this is that phospholipids are the main component of cell membranes.

Glucolipids (sugar lipids).- In glucolipids the third carbon atom of the glycerol is linked to a short carbohydrate chain. Depending on the particular glucolipid, this chain may contain in any position between one to fifteen monomers. The carbohydrate “head” is highly hydrophilic and the fatty acid “tails” are hydrophobic. In a water solution, glucolipids behave in the same way as phospholipids so they are also important components of cell membranes.

Waxes.- They are formed by joining together a single fatty acid molecule to another hydrocarbon molecule that has as part of its structure a hydroxyl (-OH) group at one end. The wax macromolecule, therefore, has two hydrophobic ends and is insoluble in water, which makes it an excellent waterproof material and constitutes its main role in living systems as coat that cover surfaces such as insect exoskeleton, or wax that covers feathers, skin etc.

Cholesterol and steroids.- Although not strictly polymers, steroids are usually classified along with lipids because of their strong hydrophobic character. All steroids are composed of the same basic four rings of carbon atoms. One steroid, cholesterol, acts in ways not fully undestood to change the properties of cell membranes and to regulate membrane fluidity. Other steroids, called hormones, act as messengers. They are released in tiny amounts by specialized glands and tissues and circulate in the fluids of multicellular organisms. Eventually, specific target cells recognize hormones, which become attached to or taken up by these cells and trigger changes in their metabolim or developmental fate. Sex hormones, for example, control the function of both male and female reproductive organs and ensure the correct formation of sex cells.

Carbohydrates

Carbohydrates are compounds that contain only C,H and O. These molecules are not only the fundamental energy stock of most of organims but they also play structural roles as in vegetal cell walls. They are formed by small molecules called sugars. There are three different kinds of carbohydrates depending on the number of sugars they are formed of:

Monosaccharides (Simple sugars).- They only have one sugar molecule. They have the general chemical formula (CH2O)n. This proportion gaves them the name of carbohydrates to these and all molecules derived from them. The main characteristic of monosaccharides is that they present hydroxyl groups and one ketone or aldehyde group. This, gives this sugars a high solubility and when molecules have more than 5 C atoms, they react with themselves to change their conformation dramatically. In a water solution, the aldehyde or the ketone group reacts with an hydroxyl group, then turning the molecule into a ring conformation. In glucose for example, the aldehyde group of the first C atom reacts with the hydroxyl group of the 5th C, thus producing a ring of six carbons. When this ring closes it can do it in two ways, over or under the level of the ring. When the hydroxyl group is situated under the level of the ring is called the alpha glucose while the form that is over the level of the ring is called the beta glucose. This small difference between alpha and beta forms have significant differences on the bigger molecules that are formed from glucose. Monosaccharides can be burned or oxidized. This is reaction releases a big amount of energy (673Kcal/glucose molecule). So glucose is the main energy source for many organisms. That is why glucose is the fuel for cells to live. Glucose is carried by the blood stream in superior vertebrates. Monosaccharides are ready to go energy for living systems.

Dissacharids (Transport forms of sugars).- Although glucose is the main transport sugar form in vertebrates, sugars are transported in the form of dissacharids, specially in plants. Sucrose (table sugar) is the common form in which carbohydrates are transported from the photosynthetic apparatus to the rest of plant. Sucrose is built up by one glucose molecule and one fructuose molecule. Trehalose is the common form in insects and it is composed of two binded glucose units. Lactose is formed of a glucose and a galactose.

In the synthesis reaction of dissacharids, two monosaccharids bind together releasing one molecule of water during the process. This chemical reaction is called condensation or dehydration reaction. In this way, only monosaccharides have the chemical ratio CH2O because two hydrogen atoms and one of oxygen are liberated in every bond formed.

When a dissacharid breaks up into their monosaccharides, when it is going to be used as a energy source, that water molecule is reincoporated. This reaction is called hydrolysis (separated by water) Hydrolysis liberates energy, for example in the hydrolysis of sucrose, 5.5 Kcal/mol are liberated. In the other hand, synthesis of sucrose requires an input of 5.5 Kcal/mol to bind fructose and glucose together.

Storage polysaccharids.-They are formed by long chains of monosaccharides. They are the storage forms of sugar. Starch for example, is the main energy storage in plants. Starch has two forms: Amylose and Amylopectine, both forms are a series of glucose units coupled together.

Glycogen is the main energy storage in superior animals. Glycogen has a very similar structure to that of amylopectine except for the fact that glycogen is more branched. In animals, glycogen is stored in the liver and muscular tissue. If there is an excess of glucose in the blood stream, the liver stores glycogen. When there is a need for energy (glucose) glucagon is secreted by the pancreas thus hydrolyzing glycogen into glucose, the ready to use form of energy.

Glycogen and Starch are formed only by Alpha units of Glucose.

Structural polysaccharids.- One of the main functions of molecules in the living organisms is to be constituent part of cells and tissues. The main structural molecule is cellulose. Cellulose forms the rigid part of cell walls in vegetable cells, forming fibers that wrap the cells up.

Cellulose is a polymer of glucose, such as glycogen and starch but not all organisms are capable of hydrolyze cellulose. This is a product of the kind of monomer that cellulose is composed of in comparison to glycogen. Although both polysaccharids are built up by glucose units, Cellulose has only beta glucose. This difference in the conformation of the molecules makes that in cellulose, glucose molecules stack together one over the other thus, creating fibers. This makes cellulose enzime proof so it cannot be degrated like the storage polysaccharides.

Chitin is another structural polysaccharid which is the main component of arthropod exoskeletons such as insects and crustaceans, and fungus cell walls. It is a very hard and resistant polysaccharid.

Cellulose and Chitin are formed only by Beta units of Glucose.

For more on carbohydrates please click here.

Session I ( August, 19th, 2009).

Levels of Organization

Organization of matter into higher and higher levels of complexity is found within all organisms. Simple chemicals are combined into huge molecules, molecules are combined into cells, and group of cells work together to form multicellular organisms. Each level of organization is part of the next higher one, and at each level, new properties not present in the previous one emerge.

Atoms

The atomic level is the lowest level of organization of matter in Biology. Atoms are the smallest units of matter that can exist independently, but they are not however, the smallest units of matter. Subatomic particles which construct atoms, determine the many different types of atoms and their properties. Each atom type is called and element, the building materials of all physical parts of the universe. The same atoms found in Mars, are found in living organisms on Earth. No life signs are found at the atomic level of organization.

Molecules and Compounds

At the next level of organization are combinations of elements called molecules and compounds. Held together in predictable combinations by forces called bonds, molecules are the smallest neutral particles of a substance that can exist inedpendently, whereas compounds are chemical combinations of two or more elements in definite ratios. Compounds have very different properties from those of the atoms that formed them. Associations of molecules and compounds represent an increase in complexity, but they are not necessarily a sign of life. It is only among a certain sublclass of molecules, involving the element carbon extensively, that first signs of life emerge.

Monomers

Monomers are small molecules that have the ability to join together to form long strings or polymers. Among the types of monomers found in living organisms are amino acids, saccharides and nucleotides, all composed mainly of the elements C,H,O,N,P and S.

Polymers and Macromolecules

These are very large molecules synthesized by linking together smaller molecules (monomers). Polymers characteristic of life fall into four major categories: Proteins, Lipids, Carbohydrates and Nucleic Acids. Each is composed of its own special class of monomers. Proteins for example are long strings of amino acid monomers joined together in precise sequences.

It is at this macomolecular level of organization that properties emerge in which we can clearly recognize signs of life. The presence of this macromolecules on Mars for example, would be a confirmation of presence of life. For example, some proteins have the ability to speed up or accelerate the rate of chemical reactions. Amino acids alone do not have this ability but the ability to influence these reactions emerges at the macromolecular level of organization.

The Cell

It is the lowest level of organization at which we can say that life truly exists. Suddenly, at this level, we see a whole array of properties emerge that belong to life. These properties are not those of any individual macromolecule, but instead they appear only after macromolecules are correctly assembled. For example, the components of a cell have little or no capacity to grow, reproduce, respond to stimuli, and so on. Yet, the cell, can carry various of these functions.

As a result, there are many important unifying themes of Biology to be found in the study of cells. Many basic discoveries have been made in the areas of cell and molecular biology by breaking open cells and analyzing which class of macromolecule was responsible for a particular property. The DNA for example, is now known to carry the coded genetic and hereditary instructions for building and maintaining the cell and the organism. There are, however, levels of organization beyond that of the cell. Just as the properties of cellular life are more than the sum of the various constituents, so the properties of multicellular life are more than the sum of the individual properties of all their cells.

The Macromolecules

Water

Water is the most abundant molecule in living organisms (50-90% of the fresh weight) so is not a surprise that life on the planet started in it. But, why is water so important for life? This liquid, which covers 3/4 of the earth's surface its a quite unique molecule with very unique properties that made life possible on the planet.

Molecular structure of Water

Each water molecule is formed by the covalent union between an atom of Oxygen (O-2) and two atoms of Hydrogen (H+). This is, Hydrogen contributes to the bond by sharing its only electron, and Oxygen shares its two electrons with the same number of Hydrogen atoms.

Even though water as a whole, has a neutral net charge, it is a polar molecule. Due to the electronegativity of O, electrons shared in the bond, are strongly attracted toward the O. In this way, a bipolar momentum forms, where electrons in a particular moment are closer to O giving that side of the molecule a slightly negative charge, wheareas the H+ in the opposite side of the molecule, acquiere a slightly positive charge.

When this charged regions interact with another molecule of water. The bipolar momentum on both molecules will form what is called a hydrogen bond. A charge on the oxygen atom of one water molecule attracts the oppositely charged hydrogens on another water molecule, and the two molecules are momentarily held together. This interactions are very weak and only last a small part of a second but millions of them form and break every instant. Although each individual hydrogen bond is weak, all the hydrogen bonds added together create a total force that is quite strong. The property of hydrogen bonding in water molecules, explain why water is a liquid at room temperature and why is not a solid. If water molecules were not able to form hydrogen bonds between themselves, they would be jostling around as gas molecules at room temperature. The consecuence of the hydrogen bond is the unique set of physical and chemical properties of water.

Surface tension

If you put gently a razor blade on water surface, it will float as if water was a solid. This is the result of cohesion or mutual attraction between water molecules. (cohesion is by definition the union between molecules of the same substance, adhesion is the union of molecules of different substances).

Capillarity

If you hold two glass slides together and dip the corner of them in water, the combination of cohesion and adhesion will result in water climbing up between the slides. This property is relevant to plants. Capillarity allows water molecules to fill up the small spaces between soil, being available for roots to use it.

Absorption

It is the permeation of a molecule into another material, such as water permeates through wood and it swells. Absorption pressure this is, the force that water molecules apply to the other substance can be very high. Thanks to absorption, seeds can break open their teccas while sprouting.

Specific heat

The ability of water to stabilize temperature depends on its relatively high specific heat. The specific heat of a substance is defined as the amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1º C. The specific heat of water is 1.00 cal/g ºC. Compared to other substances, water has an unusually high specific heat. For example, ethyl alcohol, the type in alcoholic beverages, has a specific heat of 0.6 cal/g ºC. Because of the high specific heat of water relative to other materials, water will change its temperature less when it absorbs or loses a given amount of heat. The reason you can burn your finger by touching the metal handle of a pot on the stove when the water in the pot is still lukewarm is that the specific heat of water is ten times greater than that of iron. In other words, it will take only 0.1 cal to raise the temperature of 1 g of iron 1ºC. Specific heat can be thought of as a measure of how well a substance resists changing its temperature when it absorbs or releases heat. Water resists changing its temperature; when it does change its temperature, it absorbs or loses a relatively large quantity of heat for each degree of change. Given that a big amount of energy its needed or lost to raise water temperature, animals that live in oceans of big lakes enjoy of a relative constant temperature. In the same way, given that there is a high water content in living organisms they maintain a constant inner body temperature. This water property is crucial because all of the relevant biochemical reactions take place only in a narrow interval of temperatures.

Vaporization heat

It is the physical change from a liquid to a gas. For a water molecule to separate of its neighbor molecules, hydrogen bonds must break. This requires an input of energy (540 cal/g). As a consequence, when water turns into vapor on a leaf or on the skin surface, the escaping molecules carry large amounts of heat with them. In this way, evaporation has a cooling effect. This is a way for organisms to unload heat excess and stabilize their temperature.

Freezing Point

It is the physical change from liquid to solid. In most liquids density raises when temperature drops. This raise in density is a product of individual molecules moving slowly so space between them become smaller. Water density, behaves in the exact same way until it reaches 4C., at this temperature water molecules get so close to each other that they can form 4 hydrogen bonds, making the strutcture very stable in the form of crystals. The space needed to form these 4 hydrogens bonds is bigger than when there are only 3 (liquid phase). The result of this is a drop of ice density which allows it to float. This is of relevance because, without this property lakes would freeze from the bottom to the top and no life could take place. Also, the solubility properties of water are affected by this property. The presence of solutes in water drops water's freezing point. Some polar fish that are cold resistant, have in their blood a protein called antifreeze protein which prevents the formation of ice crystals in the blood.

Water is a Solvent

Other molecules and ions will dissolve in water. Any liquid capable of dissolving or dispersing other substances is a solvent. Water is a very good solvent because of the two opposite charges it carries on the same molecule. Almost every substance will dissolve in water in some extent. Because of it polarized nature, however, water is a better solvent for ionized or polarized substances than it is for nonionized or polarized substances.

Sodium chloride dissolves readily in water because the positively charged sodium ion attracts the partially negative oxygen atoms of water molecules. Water molecules surround the ion, forming a shell with the negative oxygen atoms facing inward and the positive hydrogens facing outward. A similar shell forms around chloride ion, but in this case the positively charged hydrogen atoms are attracted to the negatively charged chloride ion and form a shell with the hydrogen atoms facing inward. Strong interactions between water molecules and other polar substances cause the dissolving substance, the solute, to break up and disperse through the water. The result is called a solution. Any substance capable of dissolving in water is said to be hydrophilic, meaning water loving.

Some substances, however, do not form solutions when placed in water (oil, gasoline). These molecules do not ionize (like sodium chloride) or polarize (like water), so there are no charges to attract the water molecules. Such substances are called hydrophobic, meaning water fearing.

Almost all vital chemical reactions necessary to keep cells alive take place in water. Water is the universal solvent for all life's molecules, big and small. From the moment they first formed on the primitive earth, the simple precursor molecules of life dissolved in water. From this solution the higher orders of complexity arose and the first cells were created. Without the special solvents properties of water, life would not be possible.

Water can Ionize

As we have seen before, electrons in water molecules are not equally shared between the oxygen and hydrogen atoms, giving the molecule a slightly ionic character. Occasionally, however water molecules break into two unequal pieces to give an hydronium ion (H3O+) and a hydroxyl ion (OH-). A process such as this is called ionization.

Water can ionize quite readily. In the billions and billions of water molecules in a cup of pure drinking water, there will be about 1, 000,000,000,000,000 hydronium ions and exactly the same number of hydroxyl ions. Although this seems a lot, it is only a small fraction of the total water molecules in the cup. It is important to note that for each hydronium ion there is a corresponding hydroxyl ion.

This is not always the case however. Certain substances when added to water, can change the number of hydronium ions in the water, turning the solution acidic. For example HCL ----> H+ + Cl- Solutions with higher hydronium ion concentrations than that of pure water are called acids.

Other substances dissolve in water to give hydroxyl ions. For example sodium hydroxide (NaOH), when in solution with water, gives sodium ions (Na+) and hydroxyl ions (OH-). NaOH ----> Na+ + OH-

Hydroxyl ions react with the naturally ocurring hydronium ions to give water molecules again, depleting the concentration of hydronium ions in the solution. OH- + H3O+ ------> H2O

A solution with a hydronium concentration lower than that of pure water (and consequently with a higher hydroxyl ion concentration) is said to be alkaline or basic.

Hydronium ion concentration is conveniently represented by a scale of values that goes from 1 to 14. This is the pH scale. Pure water is right in the middle of the scale with a pH value of 7 (1x10-7). As the concentration of hydronium increases, the numbers decrease, so a strong acid with a very high hydronium ion concentration has a value on this scale of 1 to 2, whereas a strong alkaline solution with a very low hydrogen ion concentration has a value of 11 to 14.

Water is more than just a solvent into which life's molecules dissolve and react. Water molecules also participate in many critical synthetic and breakdown reactions that build and destroy cells and cellular structures.

Take a look at this video on water properties.