Chemistry of Life
Atoms & Bonds
All material things are made of atoms, including us. Even though atoms are themselves composed of subatomic particles, atoms are the smallest, stable units of matter. The atom is made of three basic subatomic parts: the proton, the neutron, and the electron. The proton is positively charged and relatively large; it weighs one Dalton. The neutron has no charge (i.e., neutral) and also weighs one Dalton. The electron is negative and is very, very small. The electron weighs 1/1836 of a Dalton. In other words, both the proton and the neutron are 1,836 times heavier than an electron. If an electron weighs the same as a book, then a proton would weigh about as much as a car. Placing a book inside your car has virtually no impact on the weight of your vehicle. Likewise, electrons contribute very little to the mass of an atom, and so they are ignored when discussing the weight of atoms.
The protons and neutrons of an atom are located in the center in an area called the nucleus. The electrons swirl around the nucleus at very high speeds and rather far away. If the nucleus were the size of a golf ball, the nearest electron would be over a mile away. Atoms are usually depicted visually with the planetary model, which shows the electrons orbiting the nucleus. Another common depiction is the orbital model which shows the electrons in a "cloud" with no distinct location.
The number of protons in an atom is called the atomic number and determines what type of element the atom is. The element hydrogen is the smallest atom; it has just one proton. Atoms of helium possess two protons; lithium has three. There are 92 naturally-occurring elements, but heavier man-made elements also exist. The atomic mass of an element is determined by the number of protons plus the number of neutrons. For all neutral atoms, the number of electrons will equal the number of protons, but remember that electrons are so small and light they can be ignored when calculating atomic mass.
The Periodic Table of the Elements
As stated, electrons swirl around the nucleus of the atom at some distance. Like planets orbiting the sun, electrons occupy different energy levels (called shells). There are rules governing how many electrons can occupy a shell. The first shell can hold only two electrons. The second and third shells can hold up to eight electrons each. There can be more than three shells, but things get really complicated and, for our purposes, the first three is all we need to consider.
Looking at the Periodic Table, each row represents the number of shells. Thus, the atoms in the first three rows possess one, two, and three shells, respectively. Moving across the Periodic Table, each column represents the number of atoms in the outermost shell. Again, we are only considering the first three rows, so we will ignore columns 3-12. Row one only has two columns because the first shell can only possess a maximum of two electrons. Rows two and three possess eight columns because shells two and three can possess up to eight electrons. The atoms in column one have one electron in the outermost shell, those in column two possess two electrons in the outermost shell, and so on. Skipping rows 3-12, the atoms in column 13 have three electrons in the outermost shell, those in column 14 have four electrons in the outermost shell, and so on.
Atoms are most stable when the outer shell is full of electrons. If the outer shell is not full, an atom will seek to fill it by interacting with other atoms - sharing electrons or donating or receiving electrons. The number of electrons in the outermost shell will largely determine the chemical properties of the element. An atom needing just one electron to fill its outer shell will behave differently than an atom that needs four electrons to fill its outer shell. On the Periodic Table, notice that all the atoms in a given column have the same number of electrons in the outer shell (and all the atoms on a given row have the same number of shells). Because multiple elements possess the same number of electrons in their outer shells, they will behave almost the same chemically. If you line up the elements with increasing number of electrons, you find they have a repeating pattern of chemical properties. This phenomenon is what led to the discovery of the Periodic Table of the Elements.
The atoms in the column all the way to the right of the Periodic Table have full outer shells. Consequently, they are very 'happy' and very stable. They do not react with other atoms and called the inert elements (or noble gases because they are all gases and, like nobility, too snobbish to interact with others).
The atoms not in the last column are reactive because they have less-than-full outer shells (and all atoms are 'happiest' when their outer shells are full). These atoms will share electrons or donate/receive electrons to fill their outer shells.
The Noble Gases
Reactive elements (e.g., Fluorine & Chlorine) have less-than-full outer shells
Atoms combine (and recombine) through chemical reactions to form molecules and compounds. They are joined together by chemical bonds. There are three types of chemical bonds:
1. Covalent bond - occurs when atoms share outer-shell electrons
2. Ionic bond - occurs when one atom donates, and another receives, one or more electrons
3. Hydrogen bond - occurs when molecules are electrically charged and attracted to each other
Sodium gives Chlorine an electron to fill their outer shells
Ionic bond between sodium and chloride
In the example shown below using water, we see that oxygen has two shells and hydrogen has one shell. In oxygen, the first shell holds two electrons and is full. The first shell in hydrogen (which is also the outer shell for that atom) has only one electron. The outer shell "wants" to have two electrons to be full so hydrogen will interact with other atoms (in this case, oxygen) to fill its outer shell.
Oxygen also has a second shell. The second shell (which is also the outer shell for oxygen) contains six electrons but 'wants' to have eight to be full. Oxygen will interact with other atoms (in this case, two hydrogen atoms) to fill its outer shell. One oxygen will interact with two hydrogen atoms to form water (H2O). Each hydrogen shares its electron with oxygen to fill oxygen's outer shell. Likewise, oxygen shares one electron with each hydrogen to fill their outer shells. Everyone is full, everyone is happy! The water molecule is very stable.
Covalent bonds between oxygen and hydrogens
There is a fourth type of bond known as the hydrophobic bond. It is not a traditional form of bonding, but occurs whenever hydrophobic (water-hating) molecules find themselves in water. They will group closely together and separate themselves from the water. This form of bonding is on display in a lava lamp and is the force behind the saying: "water and oil don't mix." In living organisms, hydrophobic interactions are critical for protein formation and cell membrane integrity.
Molecule vs Compound
What is the difference between a molecule and a compound? Molecules are chemical structures consisting of atoms held together by covalent bonds. Oxygen gas (O2) and water (H2O) are examples. A compound is a chemical substance composed of two or more elements regardless of the type of bond. Sodium Chloride (NaCl) and water (H2O) are examples. Notice that water is both a molecule and a compound.
Covalent bonding occurs when atoms share outer-shell electrons to become a molecule. There are three types of covalent bonds: single, double, and triple. In single bonds, the two atoms share one pair of electrons; in double bonds, the two atoms share two pairs of electrons; and in triple bonds, the two atoms share three pairs of electrons. Double bonds are stronger than single bonds, and triple bonds are stronger than double bonds.
Covalent bonds may also be polar or non-polar. In non-polar covalent bonds, the electrons are shared equally by the two atoms. In polar covalent bonds, the electron pair is not shared equally between the two atoms. In other words, the electrons spend more time with one atom than the other. Consequently, because electrons are negatively charged, the atom with the electrons more often is slightly more negative than the atom with the electrons less often. The molecule as a whole has a positive end and negative end (i.e., it's polarized).
Ionic bonds are formed when one atom gives up an electron to another atom. The atom that gives up and electron becomes positively charged and the atom that receives the electron becomes negatively charged. A positively-charged atom is called a cation; a negatively-charged atom is called an anion. Because opposite charges attract, the cation stably associates with the anion in the form of an ionic bond. Although the association is stable, ionic bonds are considerably weaker than covalent bonds.
Because of hydrogen's low electronegativity, it is often slightly positive when bonded to another atom. In water, the two hydrogens are slightly positive (delta +) and the oxygen is slightly negative (delta -). Because the water molecule is non-linear (the two hydrogens are on one side of the oxygen), the molecule as a whole is polarized. When groups of water molecules are present, as in a glass of water or inside a cell, the positive side of one water molecule will interact weakly with the negative side of another water molecule. This weak inter-molecular interaction is called hydrogen bonding (as opposed to the intra-molecular interaction found in ionic bonds). Although the interaction is very weak, there are usually trillions and trillions of them. Collectively, the force can be quite strong. This is like Velcro; each Velcro connection is weak, but millions of connections make Velcro tape a strong tie down.
Water has a positive and negative pole.
Hydrogen bonds in water
Chemical Reactions & Metabolism
A chemical reaction occurs whenever two reactants combine (or recombine) to generate one or more products. At any given moment, there may be more than 2,000 chemical reactions occurring within a single cell. The sum total of all the chemical reactions in the body is called metabolism. Catabolic reactions (or catabolism) are reactions where large molecules are broken down releasing energy in the process. Anabolic reactions (or anabolism) occurs when small molecules are pieced together to make larger molecules. This general requires the input of energy (which was obtained from catabolic reactions). Metabolic reactions are not 100% efficient, resulting in the release of some energy as heat.
Anabolic reactions are synthetic reactions. Catabolic reactions are decomposition reactions. Exchange reactions reshuffle atoms without a net synthesis or decomposition.
Metabolites are small molecules synthesized or broken down inside the cell. Nutrients are essential compounds obtained from the environment.
Metabolites and nutrients can be classified as organic or inorganic. Organic compounds are made primarily from carbon and hydrogen; inorganic compounds are not.
A solution is a uniform mixture of two or more substances. In a solution, the solvent is the medium in which a solute is dispersed. Water is the solvent in aqueous solutions. In salt water, salt (usually sodium chloride) is the solute. All cells live in aqueous solutions and maintain an internal aqueous environment (the cytosol). Water is the most abundant and most important inorganic molecule on Earth. Water is a major constituent of our bodies (45-65% by weight). While not everything will dissolve in water, so many things will dissolve in water that water is called the universal solvent.
When a solute is placed into a solvent, it will disperse to form a uniform solution by diffusion. Diffusion is driven by heat energy randomly bouncing molecules against each other (a process called Brownian motion).
Because water molecules are polar, water molecules surround ions placed in aqueous solutions. The hydration spheres can surround both cations and anions. Pure water does not conduct electricity, but when ions are placed in the water it conducts electricity very well. Consequently, Ions in water are called electrolytes. Some of the most important electrolytes in the body are sodium, potassium, calcium, magnesium, chloride, and phosphate.
Acids are molecules that dissociate in water releasing hydrogen ions (H+), or protons. Strong acids dissociate completely (100% of the molecules release H+) and weak acids dissociate only partly (for example, only 10% of the acid molecules release a H+). Hydrochloric acid (HCl) is a strong acid and deoxyribonucleic acid (DNA) is a weak acid. The pH scale is a measure of acidity. Water has a neutral pH (pH 7). Solutions with a pH lower than 7 are acidic; solutions with a pH higher than 7 are basic. The pH scale is logarithmic, so a solution of pH 6 has 10 times more H+ than a solution of pH 7. Buffers are chemicals that can release or absorb H+ and keep the pH from changing, even when acids or bases are added to the solution.
When an acid and a base are mixed, they produce a salt. For example mixing HCl with NaOH will produce water (H2O) and NaCl, a salt.
Usually, only a certain number of neutrons in the nucleus will allow an atom to be stable. For some elements, however, the number of acceptable neutrons can be variable. Elements with a different number of neutrons than 'standard' are called isotopes. For example, we learned already that a hydrogen atom has only one proton. Neutral hydrogen atoms will also have one electron. Most neutral hydrogen atoms have no neutrons, but some hydrogen atoms have one neutron and some even have two neutrons. The hydrogen isotopes with one neutron are called deuterium and those with two neutrons are called tritium. Although isotopes are stable enough to allow them to even exist, they are unstable in that they eventually decay to the most stable form. Both tritium and deuterium eventually decay to hydrogen. Sometimes the element is changed by this radioactive decay process. For example, phosphorus-32 contains 15 protons and 17 neutrons. When phosphorus-32 decays it becomes sulfur-32 (16 protons and 16 neutrons). For many years, medieval alchemist tried to convert cheap elements into costly gold. Using nuclear conversion we can convert lead into gold, and even mercury into gold, but the process is far more expensive than the gold obtained.
How many is a dozen? How many is a few? How many is a mole?
Just as a dozen is 12 and a few is 3, a mole is simply a word that represents a specific number, in this case 6.22 x 10^23. Yes, it's a really big number, but it's just a number. Where does this number come from?
As it turns out, 6.22 x 10^23 just happens to be the number of atoms whose combined weight (in grams) equals the atomic mass of the element being weighed. For example, the atomic mass of carbon is 12 Daltons. One mole (6.22 x 10^23) of carbon atoms weighs 12 grams! The atomic mass of sodium is 23 Daltons. One mole of sodium atoms weighs 23 grams. The atomic mass of hydrogen is 1 Dalton. One mole of hydrogen weighs 1 gram. So, if you go to a scale and weigh out one gram of hydrogen, you will have 6.22 x 10^23 hydrogen atoms on your scale. If you weigh out 12 grams of carbon, you will have 6.22 x 10^23 carbon atoms on your scale. One mole of carbon.
Concentrations are often measured in molarity. A one molar (1 M) solution contains one mole of solute per liter of solution. For example, 1 M NaCl contains one mole of NaCl per liter of solution. If the solution is 0.5 M, it contains 0.5 mole of NaCl per liter of solution. A millimole is 1/1000 of a mole and solutions of biological concentrations are often in millimolar. A 0.15 M solution of NaCl is also 150 mM (millimolar), which is the concentration of NaCl in your blood. Sometimes, solutions are reported in micromolar(uM); a micromole (umol) is one millionth of a mole.
Basic Energy Concepts
Energy = the capacity to perform work. If you have no energy, you can't get anything done!
Work = the ability to move. That's it! If you are not moving, you are not doing any work. Movement can also come in the form of changing shape.
Kinetic energy = the released energy of motion. When something moves, it releases kinetic energy.
Potential energy = the stored energy that results from position or shape. A cell phone on the very edge of a table has more potential energy than a cell phone in the middle of the table. If the cell phone falls off the table, its potential energy is converted into kinetic energy. If enough potential energy is converted into enough kinetic energy, it could result in pieces of your cell phone flying off in different directions after impacting the floor!
Catalyst = an agent that speeds up a chemical reaction without being consumed in the reaction. Enzymes are biological catalysts. Enzymes can speed up chemical reactions up to a million-fold. That is, if a reaction takes one million seconds (about 12 days) without the enzyme, it takes only 1 second with the enzyme. Most cellular reactions are far too slow to sustain life without enzymes.
"The only distinguishing characteristic of organic chemicals is that they all contain the element carbon."
~ John McMurry, Organic Chemistry, 2nd ed.
Organic compounds are tremendously varied. The main reason there can be so many different carbon compounds is because only the element carbon can bond to itself to form chains. The smallest organic compound has only one carbon (and four hydrogens): CH4, or methane. DNA molecules are the largest organic molecules, possessing millions of carbon atoms in a single molecule.
Most organic compounds possess "functional groups" with certain unique chemical characteristics. There are many functional groups, but some common ones are the carboxyl group (R-COOH), the amino group (R-NH2), the hydroxyl group (R-OH), and the phosphate group (R-PO4).
Somewhat amazingly, all of the varied organic compounds in the body - and there are thousands - can be classified into just four classes:
Carbohydrates - compounds that contain carbon and hydrogen and oxygen in a 1:2:1 ratio (CH2O). In other words, a hydrated carbon. The chemical formula for glucose, for example, is C6H12O6.
Lipids - compounds that contain carbon and hydrogen and oxygen not in a 1:2:1 ratio. Usually, lipids possess carbon and hydrogen in a 1:2 ratio, but only one or two oxygen atoms per molecule.
Proteins - organic compounds composed of amino acids. There are 20 naturally occurring amino acids and they all contain a carboxylic acid group (-COOH) and an amino group (-NH2).
Nucleic acids - compounds that store and transmit information. DNA, for example, stores genetic information that is transmitted from one generation to the next during reproduction.
Carbohydrates, also known as sugars or carbs, are the most important energy source for metabolism. Glucose, or blood sugar, is the main carbohydrate used by our cells for energy. Glucose is broken down inside cells by a process called glycolysis ("sugar splitting"). The energy released by glycolysis is store in ATP (a nucleotide). ATP is analogous to money; almost everything a cell does requires ATP, just as almost everything we do in life requires money.
Plants are responsible for extracting carbon from the air (as carbon dioxide) and converting that carbon to sugar in a process called carbon fixation. This requires the energy of the sun in photosynthesis. Animals in turn eat plants and convert the sugar back into carbon dioxide.
Glucose is an example of a simple sugar. Simple sugars can be linked together to make complex sugars. Long chains of simple sugars are called polysaccharides. Glycogen is a polysaccharide and is a storage form of sugar in our bodes. Glucose only lasts a matter of minutes in our bodies because it is constantly consumed by our cells. Glycogen lasts for hours or days. Starch is a plant polysaccharide also made of glucose.
Glucose is consumed in minutes and glycogen lasts only days. If we need to store energy for the long haul, we turn to lipids. Lipids can last for months or years in our bodies as fat tissue. Lipids are very energy dense, meaning that a lot of energy can be stored in a relatively small space. Lipids are much more energy dense than glycogen. However, lipids are hydrophobic and this makes them hard to process since the enzymes that act on them are water-soluble (hydrophilic).
Lipids are not only used for energy storage. In addition to the lipids used for energy (fatty acids and glycerides), lipids are also the main component of cell membranes (phospholipids) and are sometimes used as hormones (eicosanoids and steroids).
Fatty acids are chains usually 10-20 carbon atoms long. One end of the chain terminates with a carboxyl group. If the fatty acid contains only single bond between carbon atoms in the chain, it is saturated with hydrogen atoms and is said to be a saturated fatty acid. If you remove one pair of hydrogens and create a double bond between two carbons in the chain, you have an unsaturated fatty acid. Polyunsaturated fatty acids possess multiple double bonds.
Glycerides are formed when one or more fatty acid is attached to a molecule of glycerol. One fatty acid attachment creates a monoglyceride; two fatty acids attached creates a diglyceride; three fatty acids attached creates a triglyceride. Body fat is composed of triglycerides store in adipocytes (fat cells).
Phospholipids (PLs) and glycolipids (GLs) are structural components of membranes. These molecules possess a hydrophilic "head" and hydrophobic fatty acid "tails." When placed in water, the molecules will spontaneously form spherical micelles or bilayers. The cell membrane is a bilayer created by PLs and GLs.
Steroids are lipids with a particular 4-ring structure. The steroid cholesterol is an important component of cell membranes; it regulates membrane fluidity. The steroid hormones estrogen and testosterone produce secondary sexual characteristics in women and men. Estrogen produces higher voices, more body fat, and developed breasts in women; testosterone produces lower voices, more body hair, and bigger muscles in men. Interestingly, these hormones almost identical structurally.
Proteins are the work horses of biological molecules. They do most of what needs to be done to stay alive. Structural proteins support the shapes of cells, tissues, and organs throughout the body. Contractile proteins enable muscle activity and body movement. Transport proteins move substances from one place in a cell to another, or through the blood from one organ to another. Buffering proteins help stabilize pH in body tissues. Enzymes are proteins that catalyze chemical reactions. Antibodies are proteins that protect us pathogens. There are roughly 50,000 unique proteins and most of them exist in the millions within each cell.
Considering how much proteins vary in function, it's not surprising that they also vary in shape and size, yet all proteins are made from the 20 naturally-occurring amino acids. Theses amino acids are strung together into long chains often hundreds of amino acids in length. These chains then fold into unique and complex three-dimensional structures suitable for the protein's function. The function of a protein is determined by the structure of that protein. That is, the structure of the protein allows it to perform a particular function. This is true for just about everything. The tires on your car are circular because only circular tires will allow the car to roll efficiently.
The covalent bond that links two amino acids together is called a peptide bond. A chain of amino acids is called a polypeptide. Polypeptides longer than 200 amino acids are called proteins.
The primary structure of a protein is simply the order of amino acids in the chain. Sections of the chain usually fold into one of two secondary structures: the alpha helix (shaped like a rod) and the beta sheet (shaped like a wall). The walls and rods fold up into the tertiary structure, which for many proteins is the final structure. Sometimes multiple polypeptide chains (each folded into tertiary structures) combine to form the final protein. In that case, the final protein has a quaternary structure.
As mentioned already, the shape of a protein determines its function. Although proteins are durable to physical stresses, they are fragile to some other forces. Small changes in pH, for example, can unravel (i.e., denature) a protein. High temperatures can also cause a protein to unravel.
Accidental changes in the DNA that codes for the structure of a protein can also alter the protein's shape, usually destroying the protein's function. CFTR is a protein 1480 amino acids long, yet changing just one amino acid can destroy its function and cause cystic fibrosis, a lethal disease.
Nucleic acids store and process information at the molecular level. DNA is designed for long-term storage of genetic information, i.e., instructions for making proteins and directing their use. RNA is a working copy of DNA used during protein production; they are short-lived and disposable. The DNA is kept in the nucleus - a vault for protecting this most precious molecule from damage (i.e., mutation). After being copied from DNA, the RNA is transported into the cytoplasm where it is used to make proteins. After the proteins are made, the RNA is discarded.
Proteins are chains of amino acids, of which there are twenty. Nucleic acids are chains of nucleotides, of which there are four: adenine (A), thymine (T), guanine (G), and cytosine (C). In a DNA double-helix, A binds only with T and G binds only with C. The sequence of nucleotides along a DNA strand encodes the instructions for making proteins. The genetic language consists of 3-letter words called codons and the different codons code for specific amino acids. For example, ATT codes for the amino acid isoleucine and GAA codes for the amino acid glutamate (so does the codon GAG; see Figure 2.4). Thus, a sequence of DNA can be translated into a sequence of amino acids that fold into a protein. For example, the DNA sequence ATGGGGCAATTTGTTGCGAATAAT... translates to the protein sequence Methionine-Glycine-Glutamine-Phenylalaine-Alanine-Asparagine-Asparagine.