How is covalent bonding critical to human anatomy and physiology

Nearly every molecule of a strong base dissociates, producing a high concentration of OH—. The relative acidity or alkalinity of a solution can be indicated by its pH.

That is, a solution with a pH of 4 is ten times more acidic than a solution with a pH of 5. The concept of pH will begin to make more sense when you study the pH scale, like that shown in Figure 2. The scale consists of a series of increments ranging from 0 to A solution with a pH of 7 is considered neutral—neither acidic nor basic. Pure water has a pH of 7. The concentration of hydrogen ions at each pH value is 10 times different than the next pH. For instance, a pH value of 4 corresponds to a proton concentration of 10—4 M, or 0.

Human urine, for example, is ten times more acidic than pure water, and HCl is 10,, times more acidic than water. The pH of human blood normally ranges from 7.

At this slightly basic pH, blood can reduce the acidity resulting from the carbon dioxide CO2 constantly being released into the bloodstream by the trillions of cells in the body.

Homeostatic mechanisms along with exhaling CO2 while breathing normally keep the pH of blood within this narrow range. This is critical, because fluctuations—either too acidic or too alkaline—can lead to life-threatening disorders.

All cells of the body depend on homeostatic regulation of acid—base balance at a pH of approximately 7. The body therefore has several mechanisms for this regulation, involving breathing, the excretion of chemicals in urine, and the internal release of chemicals collectively called buffers into body fluids.

A buffer is a solution of a weak acid and its conjugate base. A buffer can neutralize small amounts of acids or bases in body fluids.

For example, if there is even a slight decrease below 7. In contrast, if pH rises above 7. Excessive acidity of the blood and other body fluids is known as acidosis. Acidosis can also be caused by metabolic problems that reduce the level or function of buffers that act as bases, or that promote the production of acids.

For instance, with severe diarrhea, too much bicarbonate can be lost from the body, allowing acids to build up in body fluids. In people with poorly managed diabetes ineffective regulation of blood sugar , acids called ketones are produced as a form of body fuel. These can build up in the blood, causing a serious condition called diabetic ketoacidosis. Kidney failure, liver failure, heart failure, cancer, and other disorders also can prompt metabolic acidosis.

In contrast, alkalosis is a condition in which the blood and other body fluids are too alkaline basic. As with acidosis, respiratory disorders are a major cause; however, in respiratory alkalosis, carbon dioxide levels fall too low. Metabolic alkalosis often results from prolonged, severe vomiting, which causes a loss of hydrogen and chloride ions as components of HCl. Medications also can prompt alkalosis. These include diuretics that cause the body to lose potassium ions, as well as antacids when taken in excessive amounts, for instance by someone with persistent heartburn or an ulcer.

Organic compounds typically consist of groups of carbon atoms covalently bonded to hydrogen, usually oxygen, and often other elements as well. Created by living things, they are found throughout the world, in soils and seas, commercial products, and every cell of the human body. The four types most important to human structure and function are carbohydrates, lipids, proteins, and nucleotides.

Before exploring these compounds, you need to first understand the chemistry of carbon. What makes organic compounds ubiquitous is the chemistry of their carbon core. Recall that carbon atoms have four electrons in their valence shell, and that the octet rule dictates that atoms tend to react in such a way as to complete their valence shell with eight electrons. Carbon atoms do not complete their valence shells by donating or accepting four electrons.

Instead, they readily share electrons via covalent bonds. Commonly, carbon atoms share with other carbon atoms, often forming a long carbon chain referred to as a carbon skeleton. When they do share, however, they do not share all their electrons exclusively with each other. Rather, carbon atoms tend to share electrons with a variety of other elements, one of which is always hydrogen.

Carbon and hydrogen groupings are called hydrocarbons. If you study the figures of organic compounds in the remainder of this chapter, you will see several with chains of hydrocarbons in one region of the compound.

Moreover, the atoms to which carbon atoms bond may also be part of a functional group. A functional group is a group of atoms linked by strong covalent bonds and tending to function in chemical reactions as a single unit.

Five functional groups are important in human physiology; these are the hydroxyl, carboxyl, amino, methyl and phosphate groups Table 2. There are many examples of monomers and polymers among the organic compounds. Monomers form polymers by engaging in dehydration synthesis see Figure 2. As was noted earlier, this reaction results in the release of a molecule of water.

Each monomer contributes: One gives up a hydrogen atom and the other gives up a hydroxyl group. The bonds between their monomers are broken, via the donation of a molecule of water, which contributes a hydrogen atom to one monomer and a hydroxyl group to the other. A carbohydrate is a molecule composed of carbon, hydrogen, and oxygen; in most carbohydrates, hydrogen and oxygen are found in the same two-to-one relative proportions they have in water.

Monosaccharides are the monomers of carbohydrates. Polysaccharides are the polymers, and can consist of hundreds to thousands of monomers. A monosaccharide is a monomer of carbohydrates. Five monosaccharides are important in the body.

Three of these are the hexose sugars, so called because they each contain six atoms of carbon. These are glucose, fructose, and galactose, shown in Figure 2. The remaining monosaccharides are the two pentose sugars, each of which contains five atoms of carbon.

They are ribose and deoxyribose, shown in Figure 2. A disaccharide is a pair of monosaccharides. Three disaccharides shown in Figure 2. These are sucrose, commonly referred to as table sugar; lactose, or milk sugar; and maltose, or malt sugar.

As you can tell from their common names, you consume these in your diet; however, your body cannot use them directly. Instead, in the digestive tract, they are split into their component monosaccharides via hydrolysis. Watch this video to observe the formation of a disaccharide.

What happens when water encounters a glycosidic bond? Polysaccharides can contain a few to a thousand or more monosaccharides. Three are important to the body Figure 2.

The body obtains carbohydrates from plant-based foods. Grains, fruits, and legumes and other vegetables provide most of the carbohydrate in the human diet, although lactose is found in dairy products. Although most body cells can break down other organic compounds for fuel, all body cells can use glucose. Moreover, nerve cells neurons in the brain, spinal cord, and through the peripheral nervous system, as well as red blood cells, can use only glucose for fuel.

In the breakdown of glucose for energy, molecules of adenosine triphosphate, better known as ATP, are produced. Adenosine triphosphate ATP is composed of a ribose sugar, an adenine base, and three phosphate groups.

ATP releases free energy when its phosphate bonds are broken, and thus supplies ready energy to the cell. More ATP is produced in the presence of oxygen O2 than in pathways that do not use oxygen. The overall reaction for the conversion of the energy in glucose to energy stored in ATP can be written:. For instance, some carbohydrate molecules bind with proteins to produce glycoproteins, and others combine with lipids to produce glycolipids, both of which are found in the membrane that encloses the contents of body cells.

A lipid is one of a highly diverse group of compounds made up mostly of hydrocarbons. The few oxygen atoms they contain are often at the periphery of the molecule. Their nonpolar hydrocarbons make all lipids hydrophobic. In water, lipids do not form a true solution, but they may form an emulsion, which is the term for a mixture of solutions that do not mix well. A triglyceride is one of the most common dietary lipid groups, and the type found most abundantly in body tissues.

This compound, which is commonly referred to as a fat, is formed from the synthesis of two types of molecules Figure 2. Notice that glycerol gives up a hydrogen atom, and the carboxyl groups on the fatty acids each give up a hydroxyl group. Triglycerides form via dehydration synthesis. Glycerol gives up hydrogen atoms from its hydroxyl groups at each bond, and the carboxyl group on each fatty acid chain gives up a hydroxyl group.

A total of three water molecules are thereby released. Fatty acid chains that have no double carbon bonds anywhere along their length and therefore contain the maximum number of hydrogen atoms are called saturated fatty acids.

These straight, rigid chains pack tightly together and are solid or semi-solid at room temperature Figure 2. Butter and lard are examples, as is the fat found on a steak or in your own body.

In contrast, fatty acids with one double carbon bond are kinked at that bond Figure 2. These monounsaturated fatty acids are therefore unable to pack together tightly, and are liquid at room temperature.

Polyunsaturated fatty acids contain two or more double carbon bonds, and are also liquid at room temperature. Plant oils such as olive oil typically contain both mono- and polyunsaturated fatty acids. Whereas a diet high in saturated fatty acids increases the risk of heart disease, a diet high in unsaturated fatty acids is thought to reduce the risk. This is especially true for the omega-3 unsaturated fatty acids found in cold-water fish such as salmon.

These fatty acids have their first double carbon bond at the third hydrocarbon from the methyl group referred to as the omega end of the molecule.

Finally, trans fatty acids found in some processed foods, including some stick and tub margarines, are thought to be even more harmful to the heart and blood vessels than saturated fatty acids. Trans fats are created from unsaturated fatty acids such as corn oil when chemically treated to produce partially hydrogenated fats. As a group, triglycerides are a major fuel source for the body. When you are resting or asleep, a majority of the energy used to keep you alive is derived from triglycerides stored in your fat adipose tissues.

Triglycerides also fuel long, slow physical activity such as gardening or hiking, and contribute a modest percentage of energy for vigorous physical activity. Dietary fat also assists the absorption and transport of the nonpolar fat-soluble vitamins A, D, E, and K. Fatty acids are also components of glycolipids, which are sugar-fat compounds found in the cell membrane. Lipoproteins are compounds in which the hydrophobic triglycerides are packaged in protein envelopes for transport in body fluids.

As its name suggests, a phospholipid is a bond between the glycerol component of a lipid and a phosphorous molecule. In fact, phospholipids are similar in structure to triglycerides. However, instead of having three fatty acids, a phospholipid is generated from a diglyceride, a glycerol with just two fatty acid chains Figure 2.

Recall that triglycerides are nonpolar and hydrophobic. This still holds for the fatty acid portion of a phospholipid compound. However, the head of a phospholipid contains charges on the phosphate groups, as well as on the nitrogen atom. These charges make the phospholipid head hydrophilic. Therefore, phospholipids are said to have hydrophobic tails, containing the neutral fatty acids, and hydrophilic heads, containing the charged phosphate groups and nitrogen atom.

Shown here is cholesterol. Prostaglandin E2 PGE2 includes hydroxyl and carboxyl groups. A steroid compound referred to as a sterol has as its foundation a set of four hydrocarbon rings bonded to a variety of other atoms and molecules see Figure 2. Although both plants and animals synthesize sterols, the type that makes the most important contribution to human structure and function is cholesterol, which is synthesized by the liver in humans and animals and is also present in most animal-based foods.

Cholesterol is an important component of bile acids, compounds that help emulsify dietary fats. In fact, the word root chole- refers to bile. Cholesterol is also a building block of many hormones, signaling molecules that the body releases to regulate processes at distant sites. Finally, like phospholipids, cholesterol molecules are found in the cell membrane, where their hydrophobic and hydrophilic regions help regulate the flow of substances into and out of the cell.

Like a hormone, a prostaglandin is one of a group of signaling molecules, but prostaglandins are derived from unsaturated fatty acids see Figure 2. One reason that the omega-3 fatty acids found in fish are beneficial is that they stimulate the production of certain prostaglandins that help regulate aspects of blood pressure and inflammation, and thereby reduce the risk for heart disease.

Prostaglandins also sensitize nerves to pain. One class of pain-relieving medications called nonsteroidal anti-inflammatory drugs NSAIDs works by reducing the effects of prostaglandins. You might associate proteins with muscle tissue, but in fact, proteins are critical components of all tissues and organs. A protein is an organic molecule composed of amino acids linked by peptide bonds. Proteins include the keratin in the epidermis of skin that protects underlying tissues, the collagen found in the dermis of skin, in bones, and in the meninges that cover the brain and spinal cord.

While carbohydrates and lipids are composed of hydrocarbons and oxygen, all proteins also contain nitrogen N , and many contain sulfur S , in addition to carbon, hydrogen, and oxygen. Proteins are polymers made up of nitrogen-containing monomers called amino acids. An amino acid is a molecule composed of an amino group and a carboxyl group, together with a variable side chain.

Just 20 different amino acids contribute to nearly all of the thousands of different proteins important in human structure and function. Body proteins contain a unique combination of a few dozen to a few hundred of these 20 amino acid monomers. All 20 of these amino acids share a similar structure Figure 2. All consist of a central carbon atom to which the following are bonded:.

For this reason, they make excellent buffers, helping the body regulate acid—base balance. What distinguishes the 20 amino acids from one another is their variable group, which is referred to as a side chain or an R-group.

This group can vary in size and can be polar or nonpolar, giving each amino acid its unique characteristics. For example, the side chains of two amino acids—cysteine and methionine—contain sulfur.

Sulfur does not readily participate in hydrogen bonds, whereas all other amino acids do. This variation influences the way that proteins containing cysteine and methionine are assembled. Amino acids join via dehydration synthesis to form protein polymers Figure 2.

The unique bond holding amino acids together is called a peptide bond. A peptide bond is a covalent bond between two amino acids that forms by dehydration synthesis. A peptide, in fact, is a very short chain of amino acids. Strands containing fewer than about amino acids are generally referred to as polypeptides rather than proteins.

The bonds between the amino acids are peptide bonds. The body is able to synthesize most of the amino acids from components of other molecules; however, nine cannot be synthesized and have to be consumed in the diet. These are known as the essential amino acids. Free amino acids available for protein construction are said to reside in the amino acid pool within cells. Structures within cells use these amino acids when assembling proteins. If a particular essential amino acid is not available in sufficient quantities in the amino acid pool, however, synthesis of proteins containing it can slow or even cease.

The sequence is called the primary structure of the protein. The example shown here is hemoglobin, a protein in red blood cells which transports oxygen to body tissues. Although some polypeptides exist as linear chains, most are twisted or folded into more complex secondary structures that form when bonding occurs between amino acids with different properties at different regions of the polypeptide. The most common secondary structure is a spiral called an alpha-helix.

If you were to take a length of string and simply twist it into a spiral, it would not hold the shape. Similarly, a strand of amino acids could not maintain a stable spiral shape without the help of hydrogen bonds, which create bridges between different regions of the same strand see Figure 2. Less commonly, a polypeptide chain can form a beta-pleated sheet, in which hydrogen bonds form bridges between different regions of a single polypeptide that has folded back upon itself, or between two or more adjacent polypeptide chains.

In this configuration, amino acids that had been very distant in the primary chain can be brought quite close via hydrogen bonds or, in proteins containing cysteine, via disulfide bonds. A disulfide bond is a covalent bond between sulfur atoms in a polypeptide. Often, two or more separate polypeptides bond to form an even larger protein with a quaternary structure see Figure 2. The polypeptide subunits forming a quaternary structure can be identical or different.

For instance, hemoglobin, the protein found in red blood cells is composed of four tertiary polypeptides, two of which are called alpha chains and two of which are called beta chains. When they are exposed to extreme heat, acids, bases, and certain other substances, proteins will denature. Denaturation is a change in the structure of a molecule through physical or chemical means.

Denatured proteins lose their functional shape and are no longer able to carry out their jobs. An everyday example of protein denaturation is the curdling of milk when acidic lemon juice is added. The contribution of the shape of a protein to its function can hardly be exaggerated. For example, the long, slender shape of protein strands that make up muscle tissue is essential to their ability to contract shorten and relax lengthen.

As another example, bones contain long threads of a protein called collagen that acts as scaffolding upon which bone minerals are deposited.

These elongated proteins, called fibrous proteins, are strong and durable and typically hydrophobic. In contrast, globular proteins are globes or spheres that tend to be highly reactive and are hydrophilic. The hemoglobin proteins packed into red blood cells are an example see Figure 2. Enzymes, introduced earlier as protein catalysts, are examples of this. The next section takes a closer look at the action of enzymes. If you were trying to type a paper, and every time you hit a key on your laptop there was a delay of six or seven minutes before you got a response, you would probably get a new laptop.

In a similar way, without enzymes to catalyze chemical reactions, the human body would be nonfunctional. It functions only because enzymes function. Enzymatic reactions—chemical reactions catalyzed by enzymes—begin when substrates bind to the enzyme. A substrate is a reactant in an enzymatic reaction.

This occurs on regions of the enzyme known as active sites Figure 2. Any given enzyme catalyzes just one type of chemical reaction.

This characteristic, called specificity, is due to the fact that a substrate with a particular shape and electrical charge can bind only to an active site corresponding to that substrate. Due to this jigsaw puzzle-like match between an enzyme and its substrates, enzymes are known for their specificity.

In fact, as an enzyme binds to its substrate s , the enzyme structure changes slightly to find the best fit between the transition state a structural intermediate between the substrate and product and the active site, just as a rubber glove molds to a hand inserted into it. This active-site modification in the presence of substrate, along with the simultaneous formation of the transition state, is called induced fit. Which of the following is a molecule, but not a compound? A molecule of ammonia contains one atom of nitrogen and three atoms of hydrogen.

Explain why CH 4 is one of the most common molecules found in nature. Are the bonds between the atoms ionic or covalent? A carbon atom has four electrons in its valence shell. According to the octet rule, it will readily participate in chemical reactions that result in its valence shell having eight electrons.

Hydrogen, with one electron, will complete its valence shell with two. Electron sharing between an atom of carbon and four atoms of hydrogen meets the requirements of all atoms. The bonds are covalent because the electrons are shared: although hydrogen often participates in ionic bonds, carbon does not because it is highly unlikely to donate or accept four electrons.

In a hurry one day, you merely rinse your lunch dishes with water. As you are drying your salad bowl, you notice that it still has an oily film. Why was the water alone not effective in cleaning the bowl? Water is a polar molecule. It has a region of weakly positive charge and a region of weakly negative charge. These regions are attracted to ions as well as to other polar molecules.

Oils are nonpolar, and are repelled by water. Identical atoms have identical electronegativity and cannot form ionic bonds. Oxygen, for example, has six electrons in its valence shell. Neither donating nor accepting the valence shell electrons of the other will result in the oxygen atoms completing their valence shells. Two atoms of the same element always form covalent bonds. Skip to content The Chemical Level of Organization. Learning Objectives By the end of this section, you will be able to: Explain the relationship between molecules and compounds Distinguish between ions, cations, and anions Identify the key difference between ionic and covalent bonds Distinguish between nonpolar and polar covalent bonds Explain how water molecules link via hydrogen bonds.

Ions and Ionic Bonds Recall that an atom typically has the same number of positively charged protons and negatively charged electrons. Ionic Bonding. Nonpolar Covalent Bonds [link] shows several common types of covalent bonds. Covalent Bonding.

Polar Covalent Bonds in a Water Molecule. Hydrogen Bonds A hydrogen bond is formed when a weakly positive hydrogen atom already bonded to one electronegative atom for example, the oxygen in the water molecule is attracted to another electronegative atom from another molecule. Hydrogen Bonds between Water Molecules. Notice that the bonds occur between the weakly positive charge on the hydrogen atoms and the weakly negative charge on the oxygen atoms.

Hydrogen bonds are relatively weak, and therefore are indicated with a dotted rather than a solid line. These are ionic bonds, covalent bonds, and hydrogen bonds. Recall that an atom typically has the same number of positively charged protons and negatively charged electrons.

As long as this situation remains, the atom is electrically neutral. But when an atom participates in a chemical reaction that results in the donation or acceptance of one or more electrons, the atom will then become positively or negatively charged.

This happens frequently for most atoms in order to have a full valence shell, as described previously. This can happen either by gaining electrons to fill a shell that is more than half-full, or by giving away electrons to empty a shell than is less than half-full, thereby leaving the next smaller electron shell as the new, full, valence shell. An atom that has an electrical charge—whether positive or negative—is an ion. What happens to the charged electroscope when a conductor is moved between its plastic sheets, and why?

Potassium K , for instance, is an important element in all body cells. Its atomic number is It has just one electron in its valence shell.

This characteristic makes potassium highly likely to participate in chemical reactions in which it donates one electron. It is easier for potassium to donate one electron than to gain seven electrons. In other words, the resulting potassium ion will be slightly positive. A positively charged ion is known as a cation. Now consider fluorine F , a component of bones and teeth. Its atomic number is nine, and it has seven electrons in its valence shell.

Thus, it is highly likely to bond with other atoms in such a way that fluorine accepts one electron it is easier for fluorine to gain one electron than to donate seven electrons. When it does, its electrons will outnumber its protons by one, and it will have an overall negative charge. The ionized form of fluorine is called fluoride, and is written as F —.

A negatively charged ion is known as an anion. Atoms that have more than one electron to donate or accept will end up with stronger positive or negative charges. Synthesis reactions bond reactants together, a process that requires energy, whereas decomposition reactions break the bonds within a reactant and thereby release energy. In exchange reactions, bonds are both broken and formed, and energy is exchanged.

The rate at which chemical reactions occur is influenced by several properties of the reactants: temperature, concentration and pressure, and the presence or absence of a catalyst.

An enzyme is a catalytic protein that speeds up chemical reactions in the human body. Inorganic compounds essential to human functioning include water, salts, acids, and bases. These compounds are inorganic; that is, they do not contain both hydrogen and carbon. Water is a lubricant and cushion, a heat sink, a component of liquid mixtures, a byproduct of dehydration synthesis reactions, and a reactant in hydrolysis reactions.

A solution with pH 7 is neutral. Solutions with pH below 7 are acids, and solutions with pH above 7 are bases. A change in a single digit on the pH scale e. In a healthy adult, the pH of blood ranges from 7. Homeostatic control mechanisms important for keeping blood in a healthy pH range include chemicals called buffers, weak acids and weak bases released when the pH of blood or other body fluids fluctuates in either direction outside of this normal range.

Organic compounds essential to human functioning include carbohydrates, lipids, proteins, and nucleotides. These compounds are said to be organic because they contain both carbon and hydrogen. Carbon atoms in organic compounds readily share electrons with hydrogen and other atoms, usually oxygen, and sometimes nitrogen.