Although life-forms exist in many diverse forms, they all have one thing in common: they are all made up of matter. Matter is made up of elements. Elements, by definition, are substances that cannot be broken down into simpler substances by chemical means.
Although there are 92 natural elements, 96% of the mass of all living things is made up of just four of them: oxygen (O), carbon (C), hydrogen (H), and nitrogen (N). Other elements such as calcium (Ca), phosphorus (P), potassium (K), sulfur (S), sodium (Na), Chlorine (Cl), and magnesium (Mg) are also present, but in smaller quantities. These elements make up most of the remaining four percent of a living thing’s weight. Some elements are known as trace elements because they are required by an organism only in very small quantities. Trace elements include iron (Fe), iodine (I), and copper (Cu).
The smallest unit of an element that retains its characteristic properties is an atom. Atoms are the building blocks of the physical world.
Within atoms, there are even smaller subatomic particles called protons, neutrons, and electrons. Let’s take a look at a typical atom.
Protons and neutrons are particles that are packed together in the core of an atom called the nucleus. You’ll notice that protons are positively charged (+) particles, whereas neutrons are uncharged particles.
Electrons, on the other hand, are negatively charged (–) particles that spin around the nucleus. Electrons are pretty small compared to protons and neutrons. In fact, for our purposes, electrons are considered massless. Most atoms have the same number of protons and electrons, making them electrically neutral. Some atoms have the same number of protons but differ in the number of neutrons in the nucleus. These atoms are called isotopes.
When two or more individual elements are combined in a fixed ratio, they form a chemical compound. You’ll sometimes find that a compound has different properties from those of its elements. For instance, hydrogen and oxygen exist in nature as gases. Yet when they combine to make water, they often pass into a liquid state. When hydrogen atoms get together with oxygen atoms to form water, we’ve got a chemical reaction.
2H2 (g) + O2 (g) → 2H2O (l)
The atoms of a compound are held together by chemical bonds, which may be ionic bonds, covalent bonds, or hydrogen bonds.
An ionic bond is formed between two atoms when one or more electrons are transferred from one atom to the other. In order for this to occur, first, one atom loses electrons and becomes positively charged, and the other atom gains electrons and becomes negatively charged. The charged forms of the atoms are called ions. An ionic bond results from the attraction between the two oppositely charged ions. For example, when Na reacts with Cl, the charged ions Na+ and Cl– are formed.
A covalent bond is formed when electrons are shared between atoms. If the electrons are shared equally between the atoms, the bond is called nonpolar covalent. If the electrons are shared unequally, the bond is called polar covalent. When one pair of electrons is shared between two atoms, the result is a single covalent bond. When two pairs of electrons are shared, the result is a double covalent bond. When three pairs of electrons are shared, the result is a triple covalent bond.
One of the most important substances in nature is water. Water is considered a unique molecule because it plays an important role in chemical reactions.
Let’s take a look at one of the properties of water. Water has two hydrogen atoms joined to an oxygen atom.
In water molecules, the electrons are not shared equally in the bonds between hydrogen and oxygen. This means that the hydrogen atoms have a partial positive charge and the oxygen atom has a partial negative charge. Molecules that have partially positive and partially negative charges are said to be polar. Water is therefore a polar molecule. The positively charged elements of the water molecules strongly attract the negatively charged ends of other polar compounds (including water). Likewise, the negatively charged ends strongly attract the positively charged ends of polar compounds. These forces are most readily apparent in the tendency of water molecules to stick together, as in the formation of water beads or raindrops.
These types of intermolecular attractions are called hydrogen bonds. Hydrogen bonds are weak chemical bonds that form when a hydrogen atom that is covalently bonded to one electronegative atom is also attracted to another electronegative atom. Water molecules are held together by hydrogen bonds.
Although hydrogen bonds are individually weak, they are strong when present in large numbers. Because water reacts well with other polar substances, it makes a great solvent; it can dissolve many kinds of substances. The hydrogen bonds that hold water molecules together contribute to a number of special properties, including cohesion, adhesion, surface tension, high heat capacity, and expansion on freezing.
As mentioned above, water molecules have a strong tendency to stick together. That is, water exhibits cohesive forces. These forces are extremely important to life. For instance, during transpiration, water molecules evaporate from a leaf, “pulling” on neighboring water molecules. These, in turn, draw up the molecules immediately behind them, and so on, all the way down the plant vessels. The resulting chain of water molecules enables water to move up the stem.
Water molecules also like to stick to other substances—that is, they’re adhesive. Have you ever tried to separate two glass slides stuck together by a film of water? They’re difficult to separate because of the water sticking to the glass surfaces.
These two forces taken together—cohesion and adhesion—account for the ability of water to rise up the roots, trunks, and branches of trees. Since this phenomenon occurs in thin vessels, it’s called capillary action.
The cohesion of water molecules contributes to another property of water, its surface tension. Like a taut trampoline, the surface of water has a tension to it. The water molecules are stuck together and light things like leaves and water striders can sit atop the surface without sinking.
Another remarkable property of water is its high heat capacity. What’s heat capacity? Your textbook will give you a definition something like this: “heat capacity is the quantity of heat required to change the temperature of a substance by 1 degree.” What does that mean? In plain English, heat capacity refers to the ability of a substance to resist temperature changes. For example, when you heat an iron kettle, it gets hot pretty quickly. Why? Because it has a low specific heat. It doesn’t take much heat to increase the temperature of the kettle. Water, on the other hand, has a high heat capacity. You have to add a lot of heat to get an increase in temperature. The boiling point of water is pretty darn high. Liquid water is essential for cellular structures, and it is important that it doesn’t boil away if someone goes out on a warm day. Water’s ability to resist temperature changes is one of the things that helps keep the temperature in our oceans fairly stable. It’s also why organisms that are mainly made up of water, like us, are able to keep a constant body temperature.
One of the most important properties of water is yet another result of hydrogen bonding. When four water molecules are bound in a solid lattice of ice, the hydrogen bonds actually cause solid water to expand on freezing. In most materials, when the molecules lose kinetic energy and cool from liquid to solid, the molecules get more dense, moving closer together. However, in liquid water, the molecules are slightly more dense than in solid water. Because water expands on freezing, becoming slightly less dense than liquid water, ice can crack lead pipes in the winter, or a soda can will pop if it is left in your cold car. The important consequence of this property of water is that ice floats on the top of lakes or streams, allowing animal life to live underneath the ice. If ice was denser than water, it would sink to the bottom, freezing the body of water solid. All aquatic life would be unable to survive.
So let’s review the unique properties of water:
Water is polar and can dissolve other polar substances.
Water has cohesive and adhesive properties.
Water has a high heat capacity.
Water expands when it freezes (or, ice floats).
We just said that water is important because most reactions occur in watery solutions. Well, there’s one more thing to remember: reactions are also influenced by whether the solution in which they occur is acidic, basic, or neutral.
What makes a solution acidic or basic? A solution is acidic if it contains a lot of hydrogen ions (H+). That is, if you dissolve an acid in water, it will release a lot of hydrogen ions. When you think about acids, you usually think of substances with a sour taste, like lemons. For example, if you squeeze a little lemon juice into a glass of water, the solution will become acidic. That’s because lemons contain citric acid, which releases a lot of H+ into the solution.
Bases, on the other hand, do not release hydrogen ions when added to water. They release a lot of hydroxide ions (OH–). These solutions are said to be alkaline (the fancy name for a basic solution). Bases usually have a slippery consistency. Common soap, for example, is composed largely of bases.
The acidity or alkalinity of a solution can be measured with a pH scale. The pH scale is numbered from 1 to 14. The midpoint, 7, is considered neutral pH. The concentration of hydrogen ions in a solution will indicate whether it is acidic, basic, or neutral. If a solution contains a lot of hydrogen ions, then it will be acidic and have a low pH. Here’s the trend:
An increase in H+ ions causes a decrease in the pH.
pH = –log [H+]
One more thing to remember: the pH scale is not a linear scale—it’s logarithmic. That is, a change of one pH number actually represents a tenfold change in hydrogen ion concentration. For example, a pH of 3 is actually ten times more acidic than a pH of 4. This is also true in the reverse direction: a pH of 4 represents a tenfold decrease in acidity compared to a pH of 3.
Therefore, as the concentration of H+ ions increases by a factor of 10, the pH becomes one number smaller. For example, stomach acid has a pH of 2, and if we use the equation, we discover that the concentration of H+ ions in stomach acid is 10–2 M. This is pretty high when you consider the other extreme is lye, which has a pH of 14, a concentration of H+ ions around 10–14 M! Use your calculator to double-check that these numbers are correct.
You’ll notice from the scale below that stronger acids have lower pHs. If a solution has a low concentration of hydrogen ions, it will have a high pH.
The equation for pH is listed on the AP Biology Equations and Formulas sheet. However, you will not be expected to perform calculations using this equation. Instead, you should understand how the equation works and when pH calculations are useful.
Now that we’ve discussed chemical compounds in general, let’s talk about a special group of compounds. Most of the chemical compounds in living organisms contain a skeleton of carbon atoms surrounded by hydrogen atoms and often other elements. These molecules are known as organic compounds. By contrast, molecules that do not contain carbon atoms are called inorganic compounds. For example, salt (NaCl) is an inorganic compound.
Carbon is important for life because it is a versatile atom, meaning that it has the ability to bind not only with other carbons but also with a number of other elements including nitrogen, oxygen, and hydrogen. The resulting molecules are key in carrying out the activities necessary for life.
To recap:
Organic compounds contain carbon atoms AND hydrogen atoms (and sometimes other things too).
Inorganic compounds don’t contain both carbon atoms and hydrogen atoms.
Now let’s focus on the following four classes of organic compounds central to life on Earth:
carbohydrates
proteins
lipids
nucleic acids
Most macromolecules are chains of building blocks, called polymers. The individual building blocks of a polymer are called monomers.
Organic compounds that contain carbon, hydrogen, and oxygen are called carbohydrates. They usually contain these three elements in a ratio of appoximately 1:2:1, respectively. We can represent the proportion of elements within carbohydrate molecules by the formula CnH2nOn, which can be simplified as (CH2O)n.
Most carbohydrates are categorized as either monosaccharides, disaccharides, or polysaccharides. The term saccharides is a fancy word for “sugar.” The prefixes mono-, di-, and poly- refer to the number of sugars in the molecule. Mono- means “one,” di- means “two,” and poly- means “many.” A monosaccharide is therefore a carbohydrate made up of a single saccharide.
Monosaccharides, the simplest sugars, serve as an energy source for cells. The two most common sugars are (1) glucose and (2) fructose. Galactose, ribose, and deoxyribose are also monosaccharides.
Both glucose and fructose are six-carbon sugars with the chemical formula C6H12O6. Glucose, the most abundant monosaccharide, is the most popular sugar around. Glucose is an important part of the food we eat and it is the favorite food made by plants during photosynthesis. Living organisms break down glucose in order to provide cells with energy. Almost all living organisms can perform this ancient biochemical process (which is called cell respiration; you’ll learn more about this process in Chapter 6). In fact, the evolution of glycolysis, photosynthesis, and oxidative metabolism is thought to have occurred about 3 billion years ago! Fructose, the other monosaccharide you need to know for the test, is a common sugar in fruits.
Glucose and fructose can be depicted as either “straight” or “rings.” Both of them are pretty easy to spot; just look for the carbon molecules with a lot of OHs and Hs attached to them.
Here are the two different forms.
What happens when two monosaccharides are brought together? The hydrogen (–H) from one sugar molecule combines with the hydroxyl group (–OH) of another sugar molecule. What do H and OH add up to? Water (H2O)!
The process illustrated above is called dehydration synthesis, or condensation, because during this process, a water molecule is lost. When two monosaccharides are joined, the bond is called a glycosidic linkage, and the resulting sugar is called a disaccharide. The disaccharide formed from two glucose molecules is maltose. Other common disaccharides include sucrose (table sugar) and lactose (found in dairy products).
Now, what if you want to break up the disaccharide and form two monosaccharides again? Just add water. That’s called hydrolysis, which means “water” (hydro-) and “breaking” (-lysis). The water breaks the bond between the two monomers. Like condensation, hydrolysis is a common reaction in biology.
Polysaccharides are made up of many repeated units of monosaccharides. Polysaccharides can consist of branched or unbranched chains of monosaccharides. The most common polysaccharides you’ll need to know for the test are starch, cellulose, and glycogen. Glycogen and starch are sugar storage molecules. Glycogen stores sugar in animals and starch stores sugar in plants. Cellulose, on the other hand, is made up of β-glucose and is a major part of the cell walls in plants. Its function is to lend structural support. Chitin, a polymer of β-glucose molecules, serves as a structural molecule in the walls of fungus and in the exoskeletons of arthropods.
Proteins perform most of the work in your cells, and are important for structure, function, and regulation of your tissues and organs. There are thousands of different types of proteins. Amino acids are organic molecules that serve as the building blocks of proteins. They contain carbon, hydrogen, oxygen, and nitrogen atoms. There are 20 different amino acids commonly found in proteins. Fortunately, you don’t have to memorize all 20 amino acids. However, you do have to remember that every amino acid has four important parts around a central carbon: an amino group (–NH2), a carboxyl group (–COOH), a hydrogen, and an R-group.
Here’s a typical amino acid.
Amino acids differ only in the R-group, which is also called the side chain. The R-group associated with an amino acid could be as simple as a hydrogen atom (as in the amino acid glycine) or as complex as a charged carbon skeleton (as in the amino acid arginine).
When it comes to spotting an amino acid, simply keep an eye out for the amino group (NH2); then look for the carboxyl molecule (COOH). The side chains for each amino acid vary greatly. You don’t have to memorize the structures of the side chains for individual amino acids, but you should know that they can vary in composition, polarity, charge, and shape depending on the side chain that they have.
How do the side chains of the amino acids differ?
Composition of elements (C, H, O, N, and S)
Polarity (polar, nonpolar)
Charge (neutral, positive, negative)
Shape (long chain, short chain, ring-shape)
Side chain polarity affects whether an animo acid is more hydrophobic or more hydrophilic.
You can see all 20 amino acids on the next page. We’ve put them in some common categories: nonpolar, polar uncharged, and polar charged. In addition to the three-letter abbreviations we’ve listed, each amino acid is also given a one-letter name. For example, histidine can be referred to as His or just H. Don’t worry about memorizing these abbreviations for the test.
Check out the bottom of the chart on the next page: at physiological pH (which is about 7.4), two of the amino acids (glutamic acid and aspartic acid) donate a proton, making them negatively charged. In contrast, lysine and arginine accept protons at physiological pH, so they are usually positively charged.
Finally, notice that only two amino acids contain the atom sulfur: methionine and cysteine.
When two amino acids join, they form a dipeptide. The carboxyl group of one amino acid combines with the amino group of another amino acid. Here’s an example:
This is the same dehydration synthesis process we saw earlier. Why? Because a water molecule is removed to form a bond. By the way, the bond between two amino acids has a special name—a peptide bond. If a group of amino acids is joined together in a “string,” the resulting organic compound is called a polypeptide. Once a polypeptide chain twists and folds on itself, it forms a three-dimensional structure called a protein.
The polypeptide has to go through several changes before it can officially be called a protein. Proteins can have four levels of structure. The linear sequence of the amino acids is called the primary structure of a protein. Now the polypeptide begins to twist, forming either a coil (known as an alpha helix) or zigzagging pattern (known as beta-pleated sheets). These are examples of proteins’ secondary structures.
A polypeptide folds and twists because the different R-groups of the amino acids are interacting with each other. Remember, each R-group is unique and has a particular size, shape, charge, and so on. that allows it to react to things around it. Depending on which amino acids are in a protein and the order that they are in, the protein can twist and fold in very different ways. This is why proteins can form so many different shapes.
The secondary structure is formed by amino acids that interact with other amino acids close by in the primary structure. However, after the secondary structure reshapes the polypeptide, amino acids that were far away in the primary structure arrangement can now also interact with each other. This is called the tertiary structure. Because most proteins are found in an aqueous environment, hydrophilic amino acids and regions of the peptide chain are often located on the exterior of the protein. Hydrophobic amino acids and regions are usually found on the interior of proteins. Sometimes, two cysteine amino acids can react with each other to form a covalent disulfide bond that stabilizes the tertiary structure. Tertiary structure often minimizes the free energy of the molecule and locks it into a stable 3D shape.
Lastly, several different polypeptide chains sometimes interact with each other to form a quaternary structure. The different polypeptide chains that come together are often called subunits of the final whole protein.
Only proteins that have folded correctly into their specific three-dimensional structure can perform their function properly. Mistakes in the amino acid chain can create oddly shaped proteins that are nonfunctional. One more thing: in some cases, the folding of proteins involves other proteins known as chaperone proteins (chaperonins). They help the protein fold properly and make the process more efficient.
Like carbohydrates, lipids consist of carbon, hydrogen, and oxygen atoms, but not in the 1:2:1 ratio typical of carbohydrates. The most common examples of lipids are triglycerides, phospholipids, and steroids. Lipids are important because they function as structural components of cell membranes, sources of insulation, signaling molecules, and a means of energy storage.
Our bodies store fat in tissue called adipose, which is made of cells called adipocytes; these cells are filled with lipids called triglycerides. Each triglyceride is made of a glycerol molecule (sometimes called the glycerol backbone) with three fatty acid chains attached to it. A fatty acid chain is mostly a long chain of carbons in which each carbon is covered in hydrogen. One end of the chain has a carboxyl group (just like we saw in an amino acid).
Let’s take a look.
To make a triglyceride, each of the carboxyl groups (–COOH) of the three fatty acids must react with one of the three hydroxyl groups (–OH) of the glycerol molecule. This happens by the removal of a water molecule. So, the creation of a fat requires the removal of three molecules of water. Once again, what have we got? You probably already guessed it—dehydration synthesis! The linkage now formed between the glycerol molecule and the fatty acids is called an ester linkage. A fatty acid can be saturated with hydrogens along its long carbon chain or it can have a few gaps where double bonds exist instead of a hydrogen. If there is a double bond in the chain, then it is an unsaturated fatty acid. A polyunsaturated fatty acid has many double bonds within the fatty acid.
Another special class of lipids is known as phospholipids. Phospholipids contain two fatty acid “tails” and one negatively charged phosphate “head.” Take a look at a typical phospholipid.
Phospholipids are extremely important, mainly because of some unique properties they possess, particularly with regard to water.
The two fatty acid tails are hydrophobic (“water-hating”). In other words, just like oil and vinegar, fatty acids and water don’t mix. The reason for this is that fatty acid tails are nonpolar, and nonpolar substances don’t mix well with polar ones, such as water.
On the other hand, the phosphate “head” of the lipid is hydrophilic (“water-loving”), meaning that it does mix well with water. Why? It carries a negative charge, and this charge draws it to the positively charged end of a water molecule. Since a phospholipid has both a hydrophilic region and a hydrophobic region, it is an amphipathic molecule. One side of a phospholipid loves to hang out with water, and the other side hates to.
Thus, the two fatty acid chains orient themselves away from water, while the phosphate portion orients itself toward the water. Keep these properties in mind. We’ll see later how this orientation of phospholipids in water relates to the structure and function of cell membranes.
Cholesterol is another important type of lipid. Cholesterol is a four-ringed molecule that is found here and there in membranes. It generally increases membrane fluidity, except at very high temperatures when it helps to hold things together instead. Cholesterol is also important for making certain types of hormones and for making vitamin D. Here are some examples:
The fourth class of organic compounds is the nucleic acids. Like proteins, nucleic acids contain carbon, hydrogen, oxygen, and nitrogen, but nucleic acids also contain phosphorus. Nucleic acids are molecules that are made up of simple units called nucleotides. For the AP Biology Exam, you’ll need to know about two kinds of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
DNA is important because it contains the hereditary “blueprints” of all life. RNA is important because it’s essential for protein synthesis. We’ll discuss DNA and RNA in greater detail when we discuss heredity (see Chapter 8). The following table summarizes the important macromolecules discussed here.
Important Biological Macromolecules
elements
oxygen
carbon
hydrogen
nitrogen
trace elements
atom
protons
neutrons
electrons
nucleus
isotopes
radiometric dating
compound
chemical reaction
chemical bond
ionic bond
covalent bond
hydrogen bond ions
nonpolar covalent
polar covalent
polar molecule
cohesion
adhesion
surface tension
heat capacity
expansion on freezing
adhesive
capillary action
acidic
basic
neutral
alkaline
pH scale
organic compounds
inorganic compounds
polymer
monomer
carbohydrates
monosaccharides
disaccharides
polysaccharides
glucose
fructose
dehydration synthesis (condensation)
glycosidic linkage
hydrolysis
starch
cellulose
glycogen
amino acids
amino group
carboxyl group
R-group
side chain
dipeptide
peptide bond
polypeptide
protein
primary structure
secondary structure
tertiary structure
quaternary structure
chaperone proteins (chaperonins)
lipids
triglycerides
phospholipids
steroids
glycerol
ester linkage
saturated
unsaturated
polyunsaturated
hydrophobic
hydrophilic
amphipathic molecule
cholesterol
nucleic acids
nucleotides
deoxyribonucleic acid (DNA)
ribonucleic acid (RNA)
Compounds are composed of two or more types of elements in a fixed ratio.
Molecules are held together by ionic bonds and covalent bonds.
Hydrogen bonding is essential for the following emergent properties of water:
cohesion
adhesion
surface tension
high heat capacity
expansion on freezing
polar structure
ability to dissolve other polar (hydrophilic) substances but inability to dissolve nonpolar (hydrophobic) substances
pH is important for biological reactions. Solutions may be acidic, basic or alkaline, or neutral.
Organic molecules contain carbon atoms. Many molecules in biology are composed of several monomers bound together into polymers. The creation of these polymers occurs through dehydration synthesis reactions. The breakdown of these polymers occurs through hydrolysis reactions.
Important organic macromolecules in biology include:
Carbohydrates: monosaccharides link by glyosidic bonds to form di- and polysaccharides. Examples are glucose, fructose, glycogen, cellulose, and starch.
Proteins: 20 different amino acids, each with a different R-group, link together via peptide bonds to form polypeptides. The chain further folds up into special shapes to make the final protein structure. The structure can be divided into primary (amino acid sequence), secondary (alpha helices and beta-pleated sheets), tertiary (disulfide bonds), and quaternary (multiple polypeptides).
Lipids: chains called fatty acids (either saturated or unsaturated) combine on a glycerol backbone to form phospholipids and triglycerides. Cholesterol is another type of lipid. Lipids are hydrophobic.
Nucleic acids: chains of either DNA or RNA nucleotides. They carry the genetic recipes in the body.
Answers and explanations can be found in Chapter 15.
1. Water is a critical component of life due to its unique structural and chemical properties. Which of the following does NOT describe a way that the exceptional characteristics of water are used in nature to sustain life?
(A) The high heat capacity of water prevents lakes and streams from rapidly changing temperature and freezing completely solid in the winter.
(B) The high surface tension and cohesiveness of water facilitates capillary action in plants.
(C) The low polarity of water prevents dissolution of cells and compounds.
(D) The high intermolecular forces of water, such as hydrogen bonding, result in a boiling point which exceeds the tolerance of most life on the planet.
2. The intracellular pH of human cells is approximately 7.4. Yet, the pH within the lumen (inside) of the human stomach averages 1.5. Which of the following accurately describes the difference between the acidity of the cellular and gastric pH?
(A) Gastric juices contain approximately 6 times more H+ ions than the intracellular cytoplasm of cells and are more acidic.
(B) Gastric juices contain approximately 1,000,000-fold more H+ ions than the intracellular cytoplasm of cells and are more acidic.
(C) The intracellular cytoplasm of cells contain approximately 6 times more H+ ions than gastric juices and is more acidic.
(D) The intracellular cytoplasm of cells contains approximately 1,000,000-fold more H+ ions than gastric juices and is more acidic.
3. Amino acids are the basic molecular units which compose proteins. All life on the planet forms proteins by forming chains of amino acids. Which labeled component of the amino acid structure of phenylalanine shown below will vary from amino acid to amino acid?
Questions 4–6 refer to the following paragraph and diagram.
In 1953, Stanley Miller and Harold Urey performed an experiment at the University of Chicago to test the hypothesis that the conditions of the early Earth would have favored the formation of larger, more complex organic molecules from basic precursors. The experiment, as shown below, consisted of sealing basic organic chemicals (representing the atmosphere of the primitive Earth) in a flask, which was exposed to electric sparks (to simulate lightning) and water vapor.
After one day of exposure, the mixture in the flask had turned pink in color, and later analysis showed that at least 10% of the carbon had been transformed into simple and complex organic compounds including at least 11 different amino acids and some basic sugars. No nucleic acids were detected in the mixture.
4. Which of the following contradicts the hypothesis of the experiment that life may have arisen from the formation of complex molecules in the conditions of the primitive Earth?
(A) Complex carbon-based compounds were generated after only one day of exposure to simulated primitive Earth conditions.
(B) Nucleic acid compounds such as DNA and RNA were not detected in the mixture during the experiment.
(C) Over half of known amino acids involved in life were detected in the mixture during the experiment.
(D) Basic sugar molecules were generated and detected in the mixture during the experiment.
5. Some amino acids, such as cysteine (shown below) and methionine, could not be formed in this experiment. Which of the following best explains why these molecules could not be detected?
(A) The chemical reactions necessary to create amino acids such as cysteine and methionine require more energy than the simulated lightning provided in the experiment.
(B) The chemical reactions necessary to create amino acids such as cysteine and methionine require enzymes for catalysis to occur, which were not included in the experiment.
(C) Sulfur-based compounds were not included in the experiment.
(D) Nitrogen-based compounds were not included in the experiment.
6. A scientist believes that the Miller-Urey experiment failed to yield the remaining amino acids and the nucleic acids because of the absence of critical chemical substrates that would have existed on the primordial Earth due to volcanism. Which of the following basic compounds, which are associated with volcanism, would NOT need to be added in a follow-up Miller-Urey experiment?
(A) H2S (gas)
(B) SiO2 (silica)
(C) SO2
(D) H3PO4 (phosphoric acid)
7. Catabolism refers to breaking down complex macromolecules into their basic components. Many biological processes use hydrolysis for catabolism. Hydrolysis of proteins could directly result in
(A) free water
(B) adenine
(C) cholesterol
(D) dipeptides
8. Which of the following contain both hydrophilic and hydrophobic properties and are often found in cell plasma membranes?
(A) Nucleotides
(B) Phospholipids
(C) Water
(D) Amino acids
9. Maltotriose is a trisaccharide composed of three glucose molecules linked through α-1,4 glycosidic linkages formed via dehydration synthesis. What would the formula be for maltotriose?
(A) C18H36O18
(B) C18H10O15
(C) C18H32O16
(D) C3H6O3
10. A radioactive isotope of hydrogen, 3H, is called tritium. Tritium differs from the more common form of hydrogen because
(A) it contains two neutrons and one proton in its nucleus
(B) it contains one neutron and two protons in its nucleus
(C) it differs by its atomic number
(D) it is radioactive and therefore gives off one electron
Respond to the following questions:
Which topics from this chapter do you feel you have mastered?
Which content topics from this chapter do you feel you need to study more before you can answer multiple-choice questions correctly?
Which content topics from this chapter do you feel you need to study more before you can effectively compose a free response?
Was there any content that you need to ask your teacher or another person about?