SATII生物部分资料

SAT是Scholastic Aptitude T est的缩写，是申请几乎所有美国大学必须参加的考试。

通常，希望继续接受高等教育的高中生需要参加SAT考试，并且SAT考试得分是获取奖学金的重要标准之一。

大部分美国大学要求SAT作为录取的条件并根据SAT得分授予奖学金。

The connections between the atoms in a compound are called chemical bonds. Atoms form bonds by sharing their electrons with each other, relying on the power of electric charge to keep themselves attached. Molecules and compounds can also bond with each other. Important bonds between atoms are covalent and ionic bonds. Bonds between molecules or compounds are called dipole-dipole bonds.Covalent bondsBonds formed through the more or less equal sharing of electrons between atoms are known as covalent bonds.If the electrons in a covalent bond are shared equally, the resulting bond is called a nonpolar covalent bond. When one atom pulls the shared electrons toward itself a little more tightly than the other, the resulting covalent bond is said to be a polar bond. In a polar bond, the atom that pulls electrons toward itself gains a slight negative charge (because electrons have a negative charge). Since the other atom partially loses an electron, it gains a slight positive charge. For example, the atoms in water form polar bonds because oxygen, which has eight protons in its nucleus, has a greater pull on electrons than hydrogen, which has only one proton.Ionic BondsPolar covalent bonds involve the unequal sharing of electrons. Thisinequality is brought to an extreme in a bonding arrangement called an ionic bond. In an ionic bond, one atom pulls the shared electrons away from the other atom entirely. Ionic bonds are stronger than polar bonds.One example of ionic bonding is the reaction between sodium (Na) and chlorine (Cl) to form table salt (NaCl). The chlorine atom steals an electron from the sodium atom. Because it loses an electron, the sodium atom develops a charge of +1. The chlorine atom has a charge of –1, since it gained an electron.Dipole-Dipole BondsAs seen in polar covalent compounds, due to the unequal sharing of electrons, some molecules have a slightly positive and a slightly negative end to them, or a dipole (di-pole = two magnetic poles). These compounds can form weak bonds with one another without combining together completely to create new compounds. This type of bonding, known as dipole-dipole interaction, takes places when the positively charged end of one polar covalent compound (d+) comes in contact with the negatively charged end of another polar covalent compound (d–):Dipole-dipole interactions are much weaker than the bonds within molecules, but they play a very important role in the chemistry of life. Perhaps the most important dipole-dipole bond in biochemistry (and on the SAT II Biology) is the dipole-dipole interaction between positively charged hydrogen molecules andnegatively charged oxygen molecules. This reaction is so important, it gets its own special name: hydrogen bond. These bonds account for many of the exceptional properties of water and have important effects on the structure of proteins and DNA.The cells of all organisms, prokaryotic and eukaryotic alike, are surrounded by a thin sheet called the cell membrane. This barrier keeps cellular materials in and foreign objects out. The membrane is key to the life of the cell. By regulating what gets into and out of the cell, the membrane maintains the proper chemical composition, which is crucial to the life processes the cell carries out.Structure of the Cell MembraneThe cell membrane is made up of two sheets of special fat molecules called phospholipids, placed on top of each other.This arrangement is known as a phospholipid bilayer. Phospholipid molecules naturally arrange in bilayers because they have a unique structure. The long chains of carbon and hydrogen that form the tail of this molecule do not dissolve in water; they are said to be hydrophobic or “water fearing.” The hydrophilic phosphorous heads are attracted to water. Forming a bilayer satisfies the water preferences of both the “heads” and “tails” of phospholipids: thehydrophilic heads face the watery regions inside and outside the cell, and the hydrophobic tails face each other in a water-free junction. The bilayer forms spontaneously because this situation is so favorable.The Fluid Mosaic ModelPhospholipids form the fundamental structure of the cell membrane, but they are not the only substance found there. According to the fluid-mosaic model of the cell membrane, special proteins called membrane proteins float in the phospholipid bilayer like icebergs in a sea.The sea of phospholipid molecules and gatekeeper membrane proteins is in constant motion. The membrane’s fluidity keeps the cel l from fracturing when placed under strain.Transport Through the Cell MembraneThe most important property of the cell membrane is its selective permeability: some substances can pass through it freely, but others cannot. Small and nonpolar (hydrophobic) molecules can freely pass through the membrane, but charged ions and large molecules such as proteins and sugars are barred passage.The selective permeability of the cell membrane allows a cell to maintain its internal composition at necessary levels.Molecules that can pass freely through the membrane follow concentration gradients, moving from the higher concentration area to the region of lower concentration. These processes take no energy and are called passive transport. The molecules that cannot pass freely across the phospholipid bilayer can be carried across the membrane in various processes that require energy and are therefore called active transport.Passive TransportThere are three main types of passive transport: diffusion, facilitated diffusion, and osmosis. In fact, osmosis is simply the term given to the diffusion of water.DiffusionIn the absence of other forces, substances dissolved in water move naturally from areas where they are abundant to areas where they are scarce—a process known as diffusion. If there is a higher concentration of carbon dioxide gas dissolved in the water inside the cell than in the water outside the cell, carbon dioxide will naturally flow out from the cell until its distribution is balanced, without any energy required from the cell.Nonpolar and small polar molecules can pass through the cell membrane, so they diffuse across it in response to concentration gradients. Carbon dioxide and oxygen are two molecules that undergo this simple diffusion through the membrane.The simple diffusion of water is known as osmosis. Because water is a small polar molecule, it undergoes simple diffusion. SAT II Biology problems on osmosis can be tricky: water moves from areas where it is in high concentration to areas where it is in low concentration. Remember, however, that water is found in low concentrations in places where there are many dissolved substances, called solutes. Therefore, water moves from places where there are few dissolved substances (known as hypotonic solutions) to places where there are many dissolved substances (hypertonic solutions). An isotonic solution is one in which the concentration is the same as that found inside a cell, meaning osmotic pressure in both sides is equal.Immersing cells in unusually hypotonic or hypertonic solutions can be disastrous: water can rush into cells in hypotonic conditions, causing them to fill up so fast that they burst. T o combat this possibility, many cells that need to survive in freshwater environments possess contractile vacuoles to pump out excess water.Facilitated DiffusionCertain compounds important to the functioning of the cell, such as ions, cannot enter the cell through simple diffusion because they cannot pass through the cell membrane. As with water, these substances “want” to enter the cell if the concentration gradient demands it. For that reason, cells have developed a way for such compounds to bypass the cell membrane and flow into the cell on the basis of concentration. The cell has protein channels through the phospholipid membrane. The channels can open and close based on protein membranes. When closed, nothing can get through. When open, the protein channels allow compounds to pass through along the concentration gradient, which is diffusion.Active TransportQuite often, cells have to transport a substance across the cell membrane against the normal concentration gradient. In these cases, cells use another class of membrane proteins. Instead of relying on diffusion, these proteins actively pump compounds in the direction the cell wants them to go, a process that requires energy. Cells can turn active transport on and off as needed.Endocytosis and ExocytosisCells use yet another type of transport to move large particles through the cell membrane. In exocytosis, waste products that need to be removed from the cell are placed in vesicles that then fuse with the cell membrane, releasing their contents into the space outside the cell. Endocytosis is the opposite of exocytosis: the cell membrane engulfs a substance the cell needs to import and then pinchesoff into a vesicle that is inside the cell.There are two kinds of endocytosis: in phagocytosis the cell takes in large solid food particles that it then digests. In pinocytosis, the cell takes in drops of cellular fluid containing dissolved nutrients.Sometimes atoms give their electrons up altogether instead of sharing them in a chemical bond. This process is known as disassociation. Water, for instance, dissociates by the following formula:The hydrogen atom gives up a negatively charged electron, gaining a positive charge, and the OH compound gains a negatively charged electron, taking on a negative charge. The H+is known as a hydrogen ion and OH–ion is known as a hydroxide ion.The disassociation of water produces equal amounts of hydrogen and hydroxide ions. However, the disassociation of some compounds producessolutions with high proportions of either hydrogen or hydroxide ions. Solutions high in hydrogen ions are known as acids, while solutions high in hydroxide ions are known as bases. Both types of solution are extremely reactive—likely to form bonds—because they contain so many charged particles.The technical definition of an acid is that it is a hydrogen ion donor, or a proton donor, as hydrogen ions are consist of only a single proton. Acids put H+ions into solution. The definition of a base is a little more complicated: they are H+ion or proton acceptors, which means that they remove H+ions from solution. Some bases can directly produce OH–ions that will take H+out of solution. NaOH is an example of this type of base:A second type of base can directly take H+out of an H2O solution. Ammonia (NH3) is a common example of this sort of base:From time to time, the SAT II Biology has been known to ask whether ammonia is a base.The pH ScaleThe pH scale, which ranges from 0 to 14, measures the degree to which a solution is acidic or basic. If the proportion of hydrogen ions in a solution is the same as the proportion of hydroxide ions or equivalent, the solution has a pH of 7, which is neutral. The most acidic solutions (those with a high proportion of H+)have pHs approaching 0, while the most basic solutions (those with a high proportion of OH–or equivalent) have pHs closer to 14.Water has a pH of 7 because it has equal proportions of H+and OH–ions. In contrast, when a compound called hydrogen fluoride (HF) disassociates, it forms only hydroxide ions. HF is therefore quite acidic and has a pH well below 7. Some acids are more acidic than others because they put more H+ions into solution. Stomach fluid, for example, is more acidic than saliva.When sodium hydroxide (NaOH) disassociates, it forms only hydroxide ions, making it a base and giving it a pH above 7. Like acids, bases can be strong or weak depending on how many hydroxide ions they put in solution or how many hydrogen ions they take out of solution.BuffersSome substances resist changes in pH even when acids or bases are added to them. These substances are known as buffers. The cell contains many buffers because wide swings in pH can negatively impact the chemical reactions of cell processes.EnzymesSome chemical reactions simply happen when the two reactants come into contact. For example, you may be familiar with the bubbly “volcano” that forms when baking soda and vinegar are placed together in a glass. This reaction is spontaneous because it does not require outside energy to force it to occur.Most reactions, however, require energy. For example, the chemical reactions that produce a cake do not take place when baking soda, flour, and the other ingredients of a cake are simply left in a pan on the kitchen counter. Heat is required to break the existing chemical bonds in the ingredients so that they can undergo chemical reactions and combine with each other in new ways.In the laboratory, chemists use heat to create the activation energy needed to get nonspontaneous reactions started. Animals, however, can’t rely on internal Bunsen burners to get their chemical reactions cooking. In order to perform chemical reactions at low temperatures, the body uses special proteins called enzymes, which lower the activation energy necessary for chemical reactions to achievable levels. Enzymes lower the activation energy by interacting with the substrates, the primary molecules or compounds involved in the reaction. If you think of the activation energy needed for a chemical reaction as a mountain that the reactants have to climb, think of an enzyme as opening up a tunnel through the mountain. Less energy is required to go through the tunnel than to climb all the way up the mountain.Enzymes are not themselves altered when they help reactions along. Consequently, a single enzyme can be used repeatedly in many reactions. Becauseenzymes can be used over and over again and because they can act very quickly, a relatively small amount of enzyme is needed to facilitate reactions involving relatively large amounts of material.Each enzyme is designed to fit only the substrates in the reaction that the enzyme is meant to control. The one-to-one correspondence between enzyme and substrate is referred to as specificity. An analogy to a lock and key is useful for understanding the specificity of enzymes. Each enzyme can be thought of as a lock that can interact only with the appropriate key, or substrate. The region of the enzyme that interacts with the substrate is known as the active site.Enzymes help form bonds by holding two substrates near each other in the active site. Compounds can form bonds with each other more easily when they are adjacent than when they are floating around the cell randomly.Often, enzymes are named for their substrate. The name of the enzyme is the name of the starting material f ollowed by the “-ase.” For example, maltase is an enzyme that breaks down maltose, a common sugar. (Be careful not to confuse sugars, which end in “-ose,” with enzymes, which end in “-ase.”)Factors Affecting EnzymesLike all proteins, enzymes have a unique three-dimensional structure that changes under unusual environmental conditions. Enzymes do not function well when their structure is altered.Temperature and pHDepending on where it is normally located in the body, an enzyme will have different temperature and pH values at which its structure is most stable. As conditions deviate from this point, the enzyme’s ability to help along reactions decreases.Most enzymes work best near a pH of 7, but some enzymes operate most effectively in a particularly acidic environment, such as the stomach; a neutral environment impairs their function. Likewise, the enzymes of creatures that live at high temperatures, such as bacteria that live in hot springs, do not function properly at human body temperature.Cofactors and InhibitorsIn order to control enzyme activity more precisely, the body has developed a number of compounds that turn enzymes on or off and make them work faster or slower. Sometimes these compounds attach to the active site along with the substrate, and sometimes they bind to another site on the enzyme. Activators of enzymes are known as cofactors or coenzymes. Many vitamins are coenzymes. Molecules that prevent enzymes from functioning properly are known as inhibitors.Molecules of LifeThe elements involved in life processes can, and do, form millions of different compounds. Thankfully, these millions of compounds fall into four major groups: carbohydrates, proteins, lipids, and nucleic acids. Though all of these groups are organized around carbon, each group has its own special structure and function.CarbohydratesCarbohydrates are compounds that have carbon, hydrogen, and oxygen atoms in a ratio of about 1:2:1. If you’re stuck on an SAT II Biology question about whether a compound is a carbohydrate, just count up the atoms and see if they fit this ratio. Carbohydrates are often sugars, which provide energy for cellular processes.Like all of the biologically important classes of compounds, carbohydrates can be monomers, dimers, or polymers. The names of most carbohydrates end in “-ose”: glucose, fructose, sucrose, and maltose are some common examples.MonosaccharidesCarbohydrate monomers are known as monosaccharides. This group includes glucose, C6H12O6, which is a key substance in biochemistry. Sugars that an animal eats are converted into glucose, which is then converted into energy to fuel the animal’s activ ities by respiration (see Cell Processes).Glucose has a cousin called fructose with the same chemical formula. But these two compounds have different structures:Glucose and fructose differ in one important way: glucose has a double-bonded oxygen on the top carbon, while fructose has its double-bonded carbon on the second carbon. This difference is most apparent when the two monosaccharides are in their ring forms. Glucose generally forms a hexagonal ring (six sided), while fructose forms a pentagonal ring (five sided). Whereas fructose is the sugar most often found in fruits, glucose is most often used as the major source of energy for cellular activities.Disaccharides Disaccharides are carbohydrate dimers. These dimers are formed from two monomers by dehydration synthesis. Any two monosaccharides can form a disaccharide. For example, maltose is formeDisaccharidesDisaccharides are carbohydrate dimers. These dimers are formed from two monomers by dehydration synthesis. Any two monosaccharides can form a disaccharide. For example, maltose is formed by the dehydration synthesis of two glucose molecules. Sucrose, commontable sugar, comes from the linkage of one molecule of glucose and one of fructose.PolysaccharidesPolysaccharides can consist of as few as three and as many as several thousand monosaccharides. Depending on their structure and the monosaccharides they contain, polysaccharides can function as a means of storing excess energy or provide structural support.When cells ingest more carbohydrates than they need for fuel, they link the sugars together to form polysaccharides. The structure of these polysaccharides is different in plants and animals: in plants, polysaccharides take the form of starch, whereas in animals, they are linked in a structure called glycogen.Polysaccharides can also have structural roles in plants and animals. Cellulose, which forms the cell walls of plant cells, is a structural polysaccharide. In animals, the polysaccharide chitin forms the hard outer armor of insects, crabs, spiders, and other arthropods. Many fungi also use chitin as a structural carbohydrate.ProteinsMore than half of the organic compounds in cells are proteins, which play an important function in almost every cellular process. Proteins, for example, provide structural support to the cell in the cytoskeleton and make up many of the hormones that send messages around the body. Enzymes, which regulate chemical reactions in the cell, are also proteins.Amino AcidsProteins are made up of monomers called amino acids. The names of many, but not all, amino acids end in -ine: methionine, lysine, serine, etc. Each amino acid consists of a central carbon atom attached to a set of three designated groups: an atom of hydrogen (–H), an amino group (–NH2), and a carboxyl group (–COOH). The final group, designated (–R) in the diagram below, varies between different amino acids.It is possible to make an infinite number of amino acids by attaching different compounds to the R position of the central carbon. However, only 20 types of R groups exist in nature, so there are only 20 naturally occurring amino acids.PolypeptidesAll proteins are made of chains of some or all of these 20 amino acids. The bond formed between two amino acids by dehydration synthesis is known as a peptide bond.A particular protein has a specific sequence of amino acids, which is known as its primary structure. Every protein also winds, coils, and folds in three-dimensional space in specific and predetermined ways, taking on a unique secondary (initial winding and coiling) and tertiary structure (overall folding). In harsh conditions, such as high temperature or extreme pH, proteins can lose their normal tertiary shape and cease to function properly. When a protein unfolds in harsh conditions, it has been “denatured.”Lipids Lipids are carbon compounds that do not dissolve in water. They are distinguished from other macromolecules by characteristic hydrocarbon chainslong strings of carbon molecules with hydrogensLipidsLipids are carbon compounds that do not dissolve in water. They are distinguished from other macromolecules by characteristic hydrocarbon chains—long strings of carbon molecules with hydrogens attached. Such chains do not dissolve well in water because they are nonpolar.TriglyceridesTriglycerides consist of three long hydrocarbon chains known as fatty acids attached to each other by a molecule called glycerol.Because they include three fatty acids, fats and oils are also known as triglycerides. As you might expect by this point, glycerol and each fatty acid chain are joined to each other by dehydration synthesis.Some fats are saturated, while others are unsaturated. These terms refer to the presence or absence of double bonds in the fatty acids of fats. Saturated fats have no double bonds, whereas unsaturated fats contain one or more such bonds. In general, plant fats are unsaturated and animal fats are saturated. Saturated fats are generally solid at room temperature, while unsaturated fats are typically liquid.PhospholipidsPhospholipids, which are important components of cell membranes, consist of a glycerol molecule attached to two fatty acid chains and one phosphate group (PO4–2):Like all fats, the hydrocarbon tails of phospholipids do not dissolve in water. However, phosphate groups do dissolve in water because they are polar. The different solubilities of the two ends of phospholipid molecules allow them to form the bilayers that make up the cell membrane.SteroidsSteroids are the primary structure in hormones, substances that play important signaling roles in the body. Structurally, steroids are made up of four fused carbon rings attached to a hydrocarbon chain.The linked rings indicate that each carbon atom is attached to other carbon atoms that form multiple loops. Cholesterol, the steroid in the image above, is the central steroid from which other steroids, such as the sex hormones, are synthesized. Cholesterol is only found in animal cells.Nucleic AcidsCells use a class of compounds called nucleic acids to store and use hereditary information. Individual nucleic acid monomers, known as nucleotides, consist of three main units: a nitrogenous base (a compound made with nitrogen), a phosphate group, and a sugar:There are two main types of nucleotides, differentiated by their sugars: deoxyribonucleic acid (DNA)and ribonucleic acid (RNA). DNA nucleotides have one less oxygen than RNA nucleotides. The “deoxy” in deoxyribonucleic acid refers to the missing oxygen molecule. In terms of function, DNA molecules store genetic information for the cell, while RNA molecules carry genetic messages from the DNA in the nucleus to the cytoplasm for use in protein synthesis and other processes.Within both DNA and RNA, there are further subdivisions of nucleotides by nitrogenous bases. For DNA, there are four kinds of nitrogenous bases:1adenine (A)2guanine (G)3cytosine (C)4thymine (T)The nitrogenous base of a nucleotide provides it with its chemical identity, so the nucleotides are called by the name of their nitrogenous base. RNA also has four nitrogenous bases. Three—adenine, guanine, and cytosine—are identical to those found in DNA. The fourth, uracil, replaces thymine.DNA and RNAIn 1953, James Watson and Francis Crick published the discovery of the three-dimensional structure of DNA. Watson and Crick hypothesized that DNA nucleotides are organized into a polymer that looks like a ladder twisted into a coil. They called this structure the double helix.Two separate DNA polymers make up each side of the ladder. The sugar and phosphate molecules of the DNA form the vertical supports, while the nitrogenous bases stick out to formthe rungs. The rungs attach to each other by hydrogen bonding.The nitrogen bases attach to each other according to two simple rules: adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). The exclusivity of the attachments between nitrogen bases is known as base pairing.The rules of base pairing are frequently tested on the SA T II Biology. A test question might ask, “What is the complementary DNA strand to ‘CA T’?” Following the rules of DNA base pair ing, you can deduce that the answer is “CA T.” (“DOG” is the wrong answer, smart guy.) RNA StructureUnlike the double-stranded DNA, RNA is single stranded. It looks like a ladder cut down the middle. As you will see when we discuss protein synthesis in the chapter on Cell Processes, this structure of RNA is very important to its functions as a messenger from the DNA in the nucleus to the cytoplasm.Protein SynthesisNow that we’ve described DNA and RNA, it’s time to take a look at theprocess of protein synthesis. The synthesis of proteins takes two steps: transcription and translation. Transcription takes the information encoded in DNA and encodes it into mRNA, which heads out of the cell’s nucleus and into the cytoplasm. During translation, the mRNA works with a ribosome and tRNA to synthesize proteins.TranscriptionThe first step in transcription is the partial unwinding of the DNA molecule so that the portion of DNA that codes for the needed protein can be transcribed. Once the DNA molecule is unwound at the correct location, an enzyme called RNA polymerase helps line up nucleotides to create a complementary strand of mRNA. Since mRNA is a single-stranded molecule, only one of the two strands of DNA is used as a template for the new RNA strand.The new strand of RNA is made according to the rules of base pairing:DNA cytosine pairs with RNA guanineDNA guanine pairs with RNA cytosineDNA thymine pairs with RNA adenineDNA adenine pairs with RNA uracilFor example, the mRNA complement to the DNA sequence TTGCAC is AACGUG. The SAT II Biology frequently asks about the sequence of mRNA that will be produced from a given sequence of DNA. For these questions, don’t forget t hat RNA uses uracil in place of thymine.After transcription, the new RNA strand is released and the two unzipped DNA strands bind together again to form the double helix. Because the DNA template remains unchanged after transcription, it is possible to transcribe another identical molecule of RNA immediately after the first one is complete. A single gene on a DNA strand can produce enough RNA to make thousands of copies of the same protein in a very short time.TranslationIn translation, mRNA is sent to the cytoplasm, where it bonds with ribosomes, the sites of protein synthesis. Ribosomes have three important binding sites: one for mRNA and two for tRNA. The two tRNA sites are labeled the A site and P site.。