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Principles of Biology 5 contents Structure of Molecules and Compounds Complex carbon-based molecules form the basis for life on Earth. Carbon atoms. Carbon atoms bond in multiple patterns, forming structures with widely varying properties such as graphite and diamond. National Institute of General Medical Sciences/NIH. Topics Covered in this Module Chemical Bonds Organic Chemistry Major Objectives of this Module Distinguish between molecules and compounds. Explain what intermolecular forces are. Relate the importance of chemical bonds and chemical reactions to life processes. Describe the properties of carbon that allow it to form organic molecules. Summarize the Miller-Urey experiment and explain its significance. page 24 of 989 4 pages left in this module Principles of Biology 5 Structure of Molecules and Compounds We all recycle components in our trash, and our society, as a whole, is quite aware of the importance of re-using materials. Interestingly, Earth also recycles and reuses materials on the planet's surface. Carbon is an example of an element that Earth recycles via a set of chemical reactions known collectively as the carbon cycle. Carbon is an especially important element because it is the primary building block of all life on the planet. The carbon cycle moves carbon molecules between carbon dioxide gas (CO2) and a wide variety of organic compounds. Organic compounds contain carbon and hydrogen and may contain other elements as well. What exactly is carbon and why is it so versatile? The answer lies in its molecular structure. Chemical Bonds Look around. What is everything made from? All the materials on Earth are made up of the 92 naturally occurring elements. An element is a substance that cannot be reduced to simpler substances by chemical reactions. An atom is the smallest unit of matter that retains the properties of the element. Elements combine in different ways. A compound is two or more elements that combine in a fixed ratio. Compounds are held together by chemical bonds. Chemical bonds form because a full valence shell is more stable than a partly filled one. The valence shell is the outermost electron shell in an atom. Atoms gain, lose or share electrons to achieve a full valence shell. There are several types of chemical bonds that are important in biological organisms: covalent bonds, ionic bonds, hydrogen bonds, and van der Waals forces are ones we will discuss here. The bonds vary in their strength, that is, in the amount of energy (in kcal/mol or kJ/mol) required to form and break the bonds. Covalent bonds are strong, with free energies of 30 to 200 kcal/mol. Strong ionic bonds, such as those in a salt lattice like NaCl also have bond energies in the same range. At physiological temperatures, these types of bonds do not break easily. Strong bonds often co-exist with a variety of weak bonds, like hydrogen bonds, van der Waals forces, and weak electrostatic forces. These weak forces are usually intermolecular forces, holding different molecules together. At physiological temperatures, individual weak bonds are transient, with energies in the range of 1 to 7 kcal/mol. But when many weak bonds occur together, the bond energies are additive and result in a significant force that is difficult to break. These properties of weak bonds — individually weak but strong in the aggregate — are absolutely essential for life on Earth. Covalent bonds. A covalent bond forms when two non-metals share electrons. Non-metals have a valence shell that is at least half full but not completely filled. Non-metals tend to gain electrons to achieve a full valence shell. Water (H2O) contains two hydrogen atoms and an oxygen atom. Hydrogen has one electron in the first valence shell and needs one more to fill it because the first valence shell holds only two electrons. Therefore, hydrogen has a tendency to share one electron. Oxygen has six valence electrons and needs eight. Therefore, oxygen has a tendency to share two electrons. In the water molecule, each hydrogen atom shares a pair of electrons with the oxygen (Figure 1). This sharing of electrons allows all three atoms to achieve a full valence shell. Two or more atoms covalently bonded together form a molecule. contents Figure 1: Three types of diagrams depicting common biological molecules. A molecular formula indicates the number of atoms in a molecule. A structural formula shows the arrangement of atoms, and an electronsharing diagram depicts how electrons are shared in the covalent bonds. © 2014 Nature Education All rights reserved. Figure Detail Test Yourself What is the relationship between a compound and a molecule? Submit Covalent bonds come in several varieties. A single bond forms between two atoms that share one pair of electrons. Consider the element carbon. It has four valence electrons. Carbon requires four additional electrons to reach a stable configuration. It can gain these electrons, for example, by combining with four hydrogen atoms. Each hydrogen atom has one electron in its outer shell and requires one electron to reach a stable configuration. The carbon atom shares one electron with each hydrogen atom, forming four covalent single bonds. This pattern of bonding results in the methane molecule (CH4). A double bond forms between two atoms that share two pairs of electrons. Consider oxygen gas, which exists as the molecule O2 (Figure 1). Each oxygen atom needs two more electrons to achieve a full valence shell. If only a single bond were formed, each atom would gain only one electron, for a valence of seven. Instead, the two oxygen atoms form a double bond, and so share two electrons to each achieve a full valence shell. Electronegativity is the ability of an atom to attract electrons. The greater the electronegativity, the more attraction an atom has for electrons. Electronegativity increases across the periodic table. Two non-metals with similar electronegativity share electrons equally. The result is a nonpolar covalent bond. In oxygen gas (O2), the bond between the two oxygen atoms is nonpolar covalent. When two atoms covalently bonded together have different electronegativities, the electrons are more closely associated with the atom with the greater electronegativity. As a result, the more electronegative atom has a partial negative charge, and the less electronegative atom has a partial positive charge. In such cases, a polar covalent bond forms between the two atoms. Water is an example of a molecule with polar covalent bonds. The oxygen atom is more electronegative than the hydrogen atoms; therefore, it draws the electron cloud closer to its nucleus. This produces a net negative charge over the oxygen atom and a net positive charge over the hydrogen atoms. Ionic bonds. An ionic bond forms between a non-metal and a metal. A metal has a valence shell that is less than half full. Metals tend to lose electrons to empty the valence shell. In an ionic bond, one or more electrons are transferred from a metal to a non-metal. The non-metal acquires a negative charge, becoming an anion. The metal acquires a positive charge, becoming a cation. The result is an ionic compound. An ionic compound is held together by ionic bonds — the electrostatic attraction between the negatively charged anions and positively charged cations. An ionic compound is also called a salt. Is the salt used in food an ionic compound? Yes: table salt (NaCl), also called sodium chloride, is a common ionic compound. Chlorine has seven valence electrons and a high electronegativity. Sodium has one valence electron and low electronegativity. The chlorine atom strips the electron from the sodium, producing oppositely charged ions (Figure 2) that form an ionic compound. Figure 2: Ionic bonding. In sodium chloride, or table salt, an ionic bond forms when an electron is transferred from sodium to chorine. © 2014 Nature Education All rights reserved. Non-covalent attractions between molecules are called intermolecular forces. There are several different types of intermolecular forces. We will describe two of them: hydrogen bonds and van der Waals forces. Hydrogen bonds. In polar covalent molecules one end of the molecule has a partial positive charge, and the other end has a partial negative charge (Figure 3a). As a result of the uneven charge distribution, each molecule behaves like a tiny magnet. Like the opposite poles of magnets, the opposite poles of polar covalent molecules are attracted to one another. A hydrogen bond occurs when molecules that have hydrogen covalently bonded to oxygen, nitrogen or fluorine interact. For example, water is held together by hydrogen bonds that occur between the oxygen of one water molecule and a hydrogen on another (Figure 3b). As we will see in later modules, hydrogen bonds are extremely important in maintaining the double-helical structure of DNA and the structures of proteins. Figure 3: Hydrogen bonding. a) In the water molecule, the shared electrons (represented by the blue arrows) are more closely associated with the oxygen atom than with the hydrogen atoms due to oxygen's higher electronegativity. Because of the water molecule's bent shape and its atoms' unequal sharing of electrons, it has partial positive (δ+) and partial negative (δ-) poles. b) The partially positive charge of the hydrogen on one molecule is attracted to the partially negative charge of the oxygen on another, forming a hydrogen bond. © 2012 Nature Education All rights reserved. van der Waals forces. The electrons around an atom are in constant motion. Thus, the charge around an atom fluctuates with time. At any instant, the distribution of charge around an atom is not completely symmetric. This asymmetry of charge is very small but strong enough to affect the charge distribution around nearby atoms. This complementary redistribution of charge causes the neighboring atoms to attract one another, a force that is called a van der Waals interaction. Geckos benefit from van der Waals interactions. The hair-like setae on their feet can interact with a wall at a molecular level to form temporary, weak bonds. This interaction allows a gecko to walk up walls and even on vertical surfaces as smooth as glass (Figure 4). Figure 4: Gecko feet. Millions of hair-like setae on the feet of geckos allow these animals to climb smooth glass walls. Van der Waals forces adhere the setae to the surface. (Top) Volker Steger/Science Source. (Bottom) Eye of Science/Science Source. Test Yourself What is the difference between weak and strong bonds and how do each of these bonds form? Submit Chemical reactions. A chemical reaction occurs when chemical bonds are made or broken, changing a substance's composition. Many chemical reactions occur when you bake a cake. A cake requires a specific number of ingredients mixed together in the correct proportions. Reactants are the ingredients in a chemical reaction. The reactants in a cake may include flour, sugar, butter, and eggs. In a chemical reaction, the ingredients are elements or compounds. The reactants combine during the chemical reaction to form the product. In the cake analogy, the product is the cake. In a chemical reaction, the reactants and product may be a solid, a liquid or a gas. What is an example of a chemical reaction in living organisms? Plants perform photosynthesis, a multi-step chemical reaction. In essence, a plant takes carbon dioxide from the air and water from the soil. The plant then uses energy from sunlight to form a more complex molecule, glucose, using the chemical reaction shown below. Glucose provides the plant with energy to fuel cellular processes. 6CO2 + 6H2O → C6H12O6 + 6O2 Chemical reactions can occur in the forward and reverse directions. The overall chemical reaction for cellular respiration, shown below, is the reverse of the chemical reaction for photosynthesis. Respiration is a process used by both plants and animals to extract energy from the chemical bonds that hold glucose (C6H12O6) together. However, both processes are complex, multi-step reactions that involve different intermediate steps. The products of respiration, CO2 and H2O, cycle back into the environment, feeding ultimately back into the process of photosynthesis. C6H12O6 + 6O2 → 6CO2 + 6H2O When the rate of a forward reaction equals the rate of its reverse reaction, the reaction has reached chemical equilibrium. IN THIS MODULE Chemical Bonds Organic Chemistry Summary Test Your Knowledge WHY DOES THIS TOPIC MATTER? The Climate Connection How is life on Earth reacting to climate change? A Sea of Microbes Drives Global Change Do floating microbes in the ocean’s surface waters play an outsize role in global climate? PRIMARY LITERATURE How elevated carbon dioxide levels affect coral reefs Losers and winners in coral reefs acclimatized to elevated carbon dioxide concentrations. View | Download Synthetic solanoeclepin A can defeat crop pests Total synthesis of solanoeclepin A. View | Download Classic paper: The discovery of the neutron (1932) Possible existence of a neutron. View | Download Classic paper: The idea of the DNA double helix (1953) Molecular structure of nucleic acids. View | Download Man-made leaves may solve energy crisis A renewable amine for photochemical reduction of CO2. View | Download SCIENCE ON THE WEB ChemEd DL Interact with molecular models: Rotate them and look at their bond angles Be a Scientist, Meet a Scientist See if you can create organic matter with this simulation and watch a video of Stanley Miller How Small? See the difference between a coffee bean and a single atom. page 25 of 989 3 pages left in this module Principles of Biology 5 Structure of Molecules and Compounds Organic Chemistry Carbon's chemical properties make it uniquely suited as the basis for all of life. Because a carbon atom has four electrons in its outer shell, it can form connections with four other atoms. Many carbons linked together can form an endless variety of molecules. Carbon can form double as well as single bonds. This greatly increases the variety of molecules that can be formed using carbon. A great variety of carbon-based compounds, called organic compounds, are needed for life as we know it. All organic compounds contain both carbon and hydrogen. Most organic compounds in living organisms also contain other elements, including oxygen, nitrogen, phosphorus and sulfur. Organic chemistry focuses on the study of organic compounds. Organic compounds arise when carbon, hydrogen and often times other atoms form covalent bonds among themselves. An organic compound composed of only hydrogen and carbon is a hydrocarbon. The hydrocarbon skeleton can exist in a variety of arrangements, including straight, branched, and ring configurations (Figure 5). contents Figure 5: Altering carbon-carbon bonding within a hydrocarbon produces different organic compounds. The molecule pentane contains five carbons, which can be numbered from left to right (or right to left). Each carbon can form a maximum of four bonds. Changing the number of carbons in the chain, changing the branching arrangement of the carbon chain, introducing double or triple bonds, and creating ring compounds all produce new hydrocarbons with unique names. The numbers within the name of the altered molecules refer to the carbons that have attached chain branches or multiple bonds. © 2012 Nature Education All rights reserved. Figure Detail Carbon's unique bonding properties allow complex structures to form around the carbon atom. Isomers are compounds with the same number of atoms of the same elements configured in different structures (Figure 6). Receptors and enzymes can recognize particular isomers; many life processes depend on their preference for one isomer over another. Structural isomers differ in the arrangement of atoms between two molecules that have the same chemical formula. With organic structural isomers the difference is in the arrangement of atoms around the carbon chain. Because carbon chains may be straight or branched, many different arrangements are possible. In Figure 6a, septane and 3-methylhexane are examples of structural isomers. isomers differ in the spatial arrangement of atoms around the inflexible, flat covalent double bond. In Figure 6b, cis-2-butene and trans2-butene are examples of cis-trans isomers. Cis-trans Enantiomers are mirror-image isomers that differ in shape due to an asymmetric carbon atom. In Figure 6c, D-alanine and L-alanine are examples of enantiomers. Figure 6: Examples of structural isomers, cis-trans isomers and enantiomers are shown. (a) Structural isomers possess the same number and type of atoms, but the atoms are bonded together in a different arrangement. (b) Cis- and trans- isomers differ in how the atoms are arranged around a double bond. In the cis- form, two similar groups are on the same side of the bond, while in the trans- form, the similar groups are on opposite sides of the bond. (c) Enantiomers are isomers where one molecule is a mirror image of the other. © 2014 Nature Education All rights reserved. Figure Detail BIOSKILL Interpret Different Types of Molecular Models How can we describe or illustrate molecules? They have a physical form derived from the specific spatial relationships of multiple chemical elements. Knowing what those elements are is one thing, but knowing how they are arranged, and in what proportions, is the basis for creating molecular models. Figure 7: Molecular models. Explore how molecules can be modeled in different ways. © 2014 Nature Education All rights reserved. Figure Detail BIOSKILL Functional groups. Organic molecules often contain functional groups. A functional group is an arrangement of atoms that together have specific chemical properties, which they impart to the entire molecule. Functional groups are often involved in chemical reactions. There are several common functional groups in biological organisms (Figure 8). Amino Carbonyl Carboxyl Hydroxyl Phosphate Sulfhydryl Figure 8: Functional groups. Hydroxyl, carbonyl, carboxyl, amino, sulfhydryl and phosphate groups are functional groups that are common in the organic molecules found in living organisms. "R" represents the rest of the molecule bonded to the functional group. © 2014 Nature Education All rights reserved. Figure Detail Polymers. A polymer is a long molecule consisting of many similar or identical building blocks linked by covalent bonds. The repeating units that are used to build a polymer are called monomers. Examples of polymers include polysaccharides, proteins and nucleic acids (Figure 9). Figure 9: DNA is a type of nucleic acid, which is a polymer. (a) The monomers of DNA are the four deoxyribonucleotides, dATP, dGTP, dCTP, and dTTP. The monomers are shown schematically. (b) A short section of the DNA polymer is shown. The backbone, shaded in blue, is made of repeating sugars and phosphates. The unique element of each monomer, the nitrogenous bases, point away from the backbone. (c) A segment of DNA is shown as a ribbon, illustrating the double helix of DNA that forms spontaneously because the bases (A, G, C, and T) on opposite strands hydrogen bond to one another. © 2014 Nature Education All rights reserved. Polymers are essential for life. But how did they arise? To answer that question requires asking a more fundamental question: How did life arise? Scientists and non-scientists alike have long wondered how life arose. In 1953, Harold Urey and his graduate student Stanley Miller developed an experiment demonstrating that, under certain conditions, complex organic compounds could form from basic chemicals, including water (H2O), methane (CH4), ammonia (NH3) and hydrogen (H2). They used a closed system of interconnected chemistry glassware (Figure 10). In this closed system, a beaker of water simulated the ocean. The water was heated to produce water vapor, which interacted with CH4, NH3, and H2 gases in a simulated atmosphere. An electrode discharged electricity into the gases, simulating a lightning strike. The water vapor cooled and condensed into "rain" and re-entered the simulated ocean. After a period of time, Miller analyzed samples of the simulated ocean water (Figure 11). He found simple organic compounds, such as formaldehyde and hydrogen cyanide, as well as more complex organic molecules, such as amino acids. The finding of amino acids was particularly significant because amino acids are the building blocks of proteins. Figure 10: Apparatus of the Miller-Urey experiment. Stanley Miller and his graduate adviser, Harold Urey, developed this apparatus to simulate the oceanic and atmospheric conditions that may have been prevalent when life began. They first placed salt water into the oceanic bulb. Next, all air was suctioned from the apparatus through the vacuum connection and replaced with a mixture of hydrogen gas (H2), methane (CH4), and ammonia (NH3). © 2012 Nature Education All rights reserved. Figure Detail Figure 11: Can early Earth conditions produce amino acids? Applying heat to the oceanic bulb produces water vapor. The vapor enters the atmospheric bulb, completing the mixture of early atmospheric gases. After the atmospheric mixture is exposed to an electrical discharge, it enters the cooled condenser tube. There the gases condense into liquid "rain" that returns to the oceanic bulb. After running the experiment continually for one week, Miller and Urey observed the oceanic water acquire a reddish coloration. They found several amino acids in the oceanic water that were not present previously. These included aspartic acid, glycine, and both enantiomers of alanine. © 2014 Nature Education All rights reserved. Figure Detail Controversy. The gases used in the Miller-Urey experiment simulated what researchers believed at the time of the experiment were the gases in Earth's early atmosphere. Today, scientists believe that Earth's early atmosphere was quite different from that simulated in the Miller-Urey experiment. However, some scientists speculate that the Miller-Urey conditions might have occurred in certain locations, including near volcanoes or deep sea hydrothermal vents. If so, organic compounds necessary to produce life could have formed in these isolated locations. Researchers have also examined how a meteor impact in the ocean could generate the conditions necessary for life to form on the planet. Scientists performed shock compression experiments to try to recreate these conditions. The researchers determined that simple products, like carbon, water, gaseous nitrogen, and metals, could produce complex organic molecules, like carboxylic acids, amines, and hydrocarbons. Another group of researchers propose that complex organic molecules, and perhaps life itself, did not form on Earth. Instead, these molecules arrived on the planet in comets. Scientists created computer models to simulate how the starting products, like water, methane, ammonia, carbon monoxide, and carbon dioxide, would react to the shock of an impact collision. The models predicted the creation of organic molecules, including molecules with carbonnitrogen bonds, which are required for amino acid production. The origin of life on Earth remains an area of strong scientific interest. Scientists continue to find new ways to assemble the basic building blocks of life proposed in the Miller-Urey experiment. Each approach yields new explanations for how life began on Earth. Scientists debate these ideas in an effort to solve this ancient riddle. Test Yourself How did the Miller-Urey experiment energize scientists to continue the study of the development of life on young Earth? Submit IN THIS MODULE Chemical Bonds Organic Chemistry Summary Test Your Knowledge WHY DOES THIS TOPIC MATTER? The Climate Connection How is life on Earth reacting to climate change? A Sea of Microbes Drives Global Change Do floating microbes in the ocean’s surface waters play an outsize role in global climate? PRIMARY LITERATURE How elevated carbon dioxide levels affect coral reefs Losers and winners in coral reefs acclimatized to elevated carbon dioxide concentrations. View | Download Synthetic solanoeclepin A can defeat crop pests Total synthesis of solanoeclepin A. View | Download Classic paper: The discovery of the neutron (1932) Possible existence of a neutron. View | Download Classic paper: The idea of the DNA double helix (1953) Molecular structure of nucleic acids. View | Download Man-made leaves may solve energy crisis A renewable amine for photochemical reduction of CO2. View | Download SCIENCE ON THE WEB ChemEd DL Interact with molecular models: Rotate them and look at their bond angles Be a Scientist, Meet a Scientist See if you can create organic matter with this simulation and watch a video of Stanley Miller How Small? See the difference between a coffee bean and a single atom. page 26 of 989 2 pages left in this module Principles of Biology 5 Structure of Molecules and Compounds Summary Distinguish between molecules and compounds. A compound is composed of two or more different elements in a fixed ratio. A molecule is composed of two or more non-metals that share electrons in a covalent bond. An ionic compound forms when a non-metal takes one or more electrons from a metal. OBJECTIVE Explain what intermolecular forces are. Intermolecular forces are interactions between molecules. Hydrogen bonds occur between molecules in which a hydrogen is covalently bonded to oxygen, nitrogen or fluorine. The partial positive charge on the hydrogen attracts the partial negative charge on the oxygen, nitrogen or fluorine on a second molecule. Since hydrogen has low electronegativity and nitrogen, fluorine and oxygen have high electronegativity, the interaction is particularly strong. Van der Waals interactions between molecules and atoms occur due to the asymmetric distribution of electrons in their outer shells. OBJECTIVE Relate the importance of chemical bonds and chemical reactions to life processes. Chemical bonds and chemical reactions are essential for life processes. Chemical bonds form molecules such as DNA, amino acids, and proteins that are required for life structure and function. Chemical reactions are the processes by which essential materials are formed and broken down. OBJECTIVE Describe the properties of carbon that allow it to form organic molecules. Carbon atoms form the basis for all life on the planet. Carbon has four valence electrons, which means it can form four bonds. These four bonds allow complex molecules to form, including straight and branched chains and rings. Single bonds have a tetrahedral shape, and double bonds have a flat shape, so organic molecules exhibit incredible diversity. OBJECTIVE Summarize the Miller-Urey experiment and explain its significance. The Miller-Urey experiment produced complex organic compounds from inorganic compounds. The scientists' goal was to mimic early Earth conditions to understand how life first formed. The experiments showed that, under certain conditions, inorganic compounds could react to form amino acids and hydrocarbons. However, scientists now believe that Earth's early atmosphere was very different from the one simulated in the Miller-Urey experiment. Many scientists continue research to explain how life could have formed on Earth. OBJECTIVE Key Terms anion Atom that has gained an electron, resulting in a net negative charge. cation Atom that has lost an electron, resulting in a net positive charge. chemical bond The attraction between atoms resulting from the sharing of electrons or the transfer of electrons. chemical equilibrium contents Balance achieved when the forward reaction of the reactants equals the reverse reaction of the products. chemical reaction Change of a substance's composition by making or breaking chemical bonds between component atoms or molecules. compound Two or more elements that are chemically combined in a specific ratio. covalent bond Type of bond that occurs between non-metals in which electrons are shared. double bond Covalent bond involving two pairs of electrons, with two electrons from each atom. electronegativity The relative strength of the attraction between an atom and an electron. functional group A part of a molecule that is often directly involved in a chemical reaction. hydrocarbon Organic compound composed of only hydrogen and carbon. hydrogen bond Intermolecular force that occurs between molecules in which hydrogen is covalently bonded to oxygen, nitrogen or fluorine. The slight positive charge on the hydrogen of one molecule interacts with the slightly negative charge on the nitrogen, oxygen or fluorine of another molecule. intermolecular force An attraction between two molecules. ion An atom or molecule that has gained or lost electrons and thus carries a charge. ionic bond A bond in which one or more electrons are transferred from a metal to a non-metal. ionic compound A metal and a non-metal joined by an ionic bond; also defined as a salt. isomer A compound that shares the same number and type of atoms as another compound, but the arrangement of atoms is different. metal An element with a valence shell that is less than half full. Metals, found on the left two-thirds of the periodic table, tend to lose electrons. molecule Two or more non-metals joined with a covalent bond. monomer A small molecule that can be joined to many similar or identical molecules to form a long polymer. non-metal An element with a valence shell that is at least half full but not completely filled. Non-metals, found on the right third of the periodic table, tend to gain or share electrons. nonpolar covalent bond Covalent bond between atoms with similar electronegativity; also defined as the bond between atoms that share electrons equally. organic chemistry Branch of chemistry focused on compounds containing carbon and hydrogen. organic compound A molecule containing carbon and hydrogen. polar covalent bond Covalent bond between atoms with different electronegativities; also defined as the bond between atoms that share electrons unequally. polymer Long molecular chain consisting of many similar or identical building blocks (monomers) linked by covalent bonds. product The result of a chemical reaction. A reaction may have one or more products. reactant An ingredient in a chemical reaction. A reaction may have one or more reactants. salt Ionic compound; also defined as the result of a bond between a metal and a non-metal. single bond Covalent bond involving a single pair of electrons, one from each atom. valence The number of electrons in an atom's outermost electron shell. van der Waals interaction An intermolecular force that attracts molecules together due to the asymmetric distribution of electrons. IN THIS MODULE Chemical Bonds Organic Chemistry Summary Test Your Knowledge WHY DOES THIS TOPIC MATTER? The Climate Connection How is life on Earth reacting to climate change? A Sea of Microbes Drives Global Change Do floating microbes in the ocean’s surface waters play an outsize role in global climate? PRIMARY LITERATURE How elevated carbon dioxide levels affect coral reefs Losers and winners in coral reefs acclimatized to elevated carbon dioxide concentrations. View | Download Synthetic solanoeclepin A can defeat crop pests Total synthesis of solanoeclepin A. View | Download Classic paper: The discovery of the neutron (1932) Possible existence of a neutron. View | Download Classic paper: The idea of the DNA double helix (1953) Molecular structure of nucleic acids. View | Download Man-made leaves may solve energy crisis A renewable amine for photochemical reduction of CO2. View | Download SCIENCE ON THE WEB ChemEd DL Interact with molecular models: Rotate them and look at their bond angles Be a Scientist, Meet a Scientist See if you can create organic matter with this simulation and watch a video of Stanley Miller How Small? See the difference between a coffee bean and a single atom. page 27 of 989 1 pages left in this module Principles of Biology contents 5 Structure of Molecules and Compounds Test Your Knowledge 1. What type of chemical bond is found in the methane molecule? covalent single bond covalent double bond covalent triple bond ionic bond None of the answers are correct. 2. Which element is present in all organic compounds? nitrogen carbon oxygen sulfur phosphorus 3. Why is carbon capable of forming single and double bonds? It has four valence electrons. It has one valence electron. It has two valence electrons. It has three valence electrons. None of the answers are correct. 4. Which of the following is an outcome of the Miller-Urey experiment? It showed that complex organic compounds could form spontaneously from inorganic compounds. It caused biologists to view natural phenomena in terms of physical and chemical laws. It raised considerable controversy that prompted additional scientific inquiry. Scientists began to propose alternative theories to explain how organic molecules formed and how life came into existence. All answers are correct. 5. What would have to happen in order for a polar covalent bond to form? One atom would have to share electrons equally with another atom. One atom would have to donate an electron to another atom. One atom would have to share electrons unequally with another atom. One atom would have to accept an electron from another atom. One atom would have to act as a cation and another would have to act as an anion. Submit IN THIS MODULE Chemical Bonds Organic Chemistry Summary Test Your Knowledge WHY DOES THIS TOPIC MATTER? The Climate Connection How is life on Earth reacting to climate change? A Sea of Microbes Drives Global Change Do floating microbes in the ocean’s surface waters play an outsize role in global climate? PRIMARY LITERATURE How elevated carbon dioxide levels affect coral reefs Losers and winners in coral reefs acclimatized to elevated carbon dioxide concentrations. View | Download Synthetic solanoeclepin A can defeat crop pests Total synthesis of solanoeclepin A. View | Download Classic paper: The discovery of the neutron (1932) Possible existence of a neutron. View | Download Classic paper: The idea of the DNA double helix (1953) Molecular structure of nucleic acids. View | Download Man-made leaves may solve energy crisis A renewable amine for photochemical reduction of CO2. View | Download SCIENCE ON THE WEB ChemEd DL Interact with molecular models: Rotate them and look at their bond angles Be a Scientist, Meet a Scientist See if you can create organic matter with this simulation and watch a video of Stanley Miller How Small? See the difference between a coffee bean and a single atom. page 28 of 989