Biology - The Chemical Basis of Life - Quick Guides

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Study GuideBiology–The Chemical Basis of Life1. Acids and BasesIn chemistry,acidsandbasesare important substances that behave in different ways when mixedwith water. Understanding how they work helps explain many chemical reactions in biology andeveryday life.1.1 What Are Acids?Acidsare chemical compounds that releasehydrogen ions (H⁺)when they are dissolved in water.This release of hydrogen ions gives acids their special properties.For example, whenhydrogen chlorideis placed in water, it breaks apart and releases hydrogenions. Once this happens, the solution becomeshydrochloric acid. The presence of free hydrogenions is what makes the solution acidic.1.2 What Are Bases?Basesbehave differently from acids. When placed in water, basesattract hydrogen ionsinstead ofreleasing them.A common example of a base issodium hydroxide (NaOH). When sodium hydroxide dissolves inwater, it attracts hydrogen ions. As a result,hydroxyl ions (OH⁻)build up in the solution. Thisaccumulation of hydroxyl ions makes the solutionbasic, also known asalkaline.1.3 Key Difference to Remember•Acidsrelease hydrogen ions (H⁺) in water•Basesremove hydrogen ions and increase hydroxyl ions (OH⁻) in waterBoth acids and bases play important roles in chemical processes, especially in biological systems.

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Study Guide2.Quiz Acids and Bases1. QuestionFill in the blank:How does an acid differ from a base?Answer Choices•An acid releases hydroxide (OH⁻)ions, whereas a base releases hydrogen ions (H⁺).•Bases attract hydrogen ions in water and acids do not.•Bases have low pH’s, whereas acids have high pH’s.Correct AnswerBases attract hydrogen ions in water and acids do not.Why This Is CorrectIn water,acids release hydrogen ions (H⁺), whilebases attract hydrogen ions or releasehydroxide ions (OH⁻). This difference explains how acids and bases behave insolution.The other options are incorrect because:•Acids donotrelease hydroxide ions.•Bases havehigh pH values, while acids havelow pH values.3. MoleculesMost of the chemical compounds studied in biology are made ofmolecules. Amoleculeis a specificgroup of atoms held together bychemical bonds. These bonds keep the atoms connected in astable structure.Acompoundis a type of molecule that contains atoms frommore than one element. However, notall molecules are compounds. For example, oxygen gas (O₂) is a molecule made of two atoms of thesame element—oxygen.Molecules can be made in different ways:•Some contain atoms of thesame element, like O₂

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Study Guide•Others contain atoms ofdifferent elements, like water (H₂O)The way atoms are arranged in a molecule determines thepropertiesof that compound, such ashow it reacts or behaves.Themolecular weightof a molecule is found by adding together the atomic weights of all the atomsin that molecule.3.1 Chemical BondsAtoms in molecules are held together by special connections calledbonds. There are different typesof bonds, and each type forms in a different way.3.2 Ionic BondsAnionic bondforms when one atomgives electronsto another atom. This transfer of electronscreates charged atoms calledions.•The atom that loses electrons becomespositively charged•The atom that gains electrons becomesnegatively chargedBecause opposite charges attract each other, these ions pull toward one another. This attractioncreates the ionic bond that holds the atoms together.3.3 Covalent BondsAcovalent bondforms when two atomsshare electronsinstead of transferring them. This sharingallows both atoms to become more stable.Here are two examples:•Inwater (H₂O), one oxygen atom shares electrons with two hydrogen atoms•Inammonia (NH₃), one nitrogen atom shares electrons with three hydrogen atomsCovalent bonds can vary depending on how many electrons are shared:•Asingle bondforms when one pair of electrons is shared•Adouble bondforms when two pairs of electrons are shared

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Study Guide3.4Key Idea to RememberMolecules are the building blocks of biological compounds. The type of atoms involved and the waythey bond—whether by transferring or sharing electrons—determine the structure and behavior ofeach molecule.4.Quiz Molecules1. QuestionFill in the blank:A molecule:Answer Choices•cannot consist of only one element•consists of elements arranged in a specific arrangement and ratio•has properties that are not related to the arrangement of the atoms

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Study GuideCorrect Answerconsists of elements arranged in a specific arrangement and ratioWhy This Is CorrectA molecule is formed whenatoms are joined together in a specific ratio and arrangement. Thisstructure determines the molecule’s properties andbehavior.The other options are incorrect because:•A moleculecanconsist of only one element (for example, O₂).•A molecule’s properties aredirectly relatedto how its atoms are arranged and bonded.2.QuestionFill in the blank:Which of the following statements about an ionic bond isFALSE?Answer Choices•It involves the sharing of electrons.•It must occur between two oppositely charged ions.•It requires that one atom gains at least one electron.Correct AnswerIt involves the sharing of electrons.Why This Is CorrectAnionic bondforms whenelectrons are transferred, not shared. One atomlosesone or moreelectrons and becomes positively charged, while the other atomgainselectrons and becomesnegatively charged. The bond forms due to the attraction between these oppositely charged ions.Sharing of electrons occurs incovalent bonds, not ionic bonds.3.QuestionFill in the blank:Which of the following statements about a covalent bond isFALSE?

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Study GuideAnswer Choices•An atom may share electrons with more than one atom.•If two pairs of electrons are shared, it is a single bond.•It is based onthe sharing of electrons between atoms.Correct AnswerIf two pairs of electrons are shared, it is a single bond.Why This Is CorrectIn acovalent bond, atoms share electrons.•Sharingone pairof electrons forms asingle bond.•Sharingtwopairsof electrons forms adouble bond.Therefore, saying that two shared pairs create a single bond is incorrect. The other statements aretrue descriptions of covalent bonding.5. Organic CompoundsThe chemical compounds found in living things are calledorganic compounds. They are called“organic” because they are connected with life and always containcarbon. The study of thesecompounds is known asorganic chemistry.Although there are many kinds of organic compounds,four major groupsare found in all livingorganisms:•Carbohydrates•Lipids•Proteins•Nucleic acidsEach group plays a special role in keeping living things alive and functioning.

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Study Guide5.1 CarbohydratesCarbohydrates are one of the mainenergy sourcesfor almost all living organisms. Somecarbohydrates also providestructure, helping form important body parts.All carbohydrates are made ofcarbon, hydrogen, and oxygen. In these molecules, the ratio ofhydrogen atoms to oxygen atoms is2:1.5.1.1Simple Carbohydrates (Sugars)Simple carbohydrates are commonly calledsugars. They come in two main types:•Monosaccharides–made of one sugar molecule•Disaccharides–made of two sugar moleculesThe most important monosaccharide isglucose. Its chemical formula isC₆H₁₂O₆. Glucose is themain fuelused by living cells.In multicellular organisms, glucose dissolves in body fluids and is transported to cells throughout thebody. Inside cells, glucose is broken down to release energy. It is the starting material forcellularrespirationand the main product ofphotosynthesis.5.1.2DisaccharidesThree important disaccharides found in living things are:•Maltose–made of two glucose molecules joined together•Sucrose–common table sugar, made of glucose and fructose•Lactose–made of glucose and galactoseWhen sucrose forms, awater molecule is released. This process is called adehydration reaction.The reverse process, where water is added to split a molecule, is calledhydrolysis.5.1.3Complex Carbohydrates (Polysaccharides)Polysaccharidesare complex carbohydrates made by linking many monosaccharides together.Important polysaccharides include:•Starch–made of hundreds or thousands of glucose units; used by plants to store energy

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Study Guide○Humans get much of their energy from starch found in rice, wheat, corn, and potatoes•Glycogen–also made of glucose, but linked differently than starch○It is the form in which glucose is stored in thehuman liver•Cellulose–a structural carbohydrate made of glucose○Found in plant cell walls, wood, cotton, and paper○Most organisms cannot break cellulose down5.2LipidsLipidsare organic molecules made of carbon, hydrogen, and oxygen. However, they containmuchmore hydrogencompared to oxygen than carbohydrates do.Lipids include:•Fats•Waxes•Steroids(many hormones are made from steroids)

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Study Guide5.2.1Structure of FatsAfat moleculeis made of:•Oneglycerolmolecule•One, two, or threefatty acidsGlycerol contains threehydroxyl (–OH)groups.A fatty acid is a long chain of carbon atoms (4–24 carbons) with acarboxyl (–COOH)group at oneend.Fatty acids attach to glycerol through a process thatremoves water, similar to dehydration synthesis.

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Study Guide5.2.2Saturated vs Unsaturated Fats•Unsaturated fatscontain one or moredouble bonds•Saturated fatscontainno double bondsFor most health situations, eatingunsaturated fatsis considered healthier than eating saturated fats.5.2.3Storage and Function of FatsFats do not dissolve in water, so they form clear oil droplets calledglobulesinside cells.•Plants store fats mainly inseeds•Animals store fats inadipose tissueFats containlarge amounts of stored energy, making them an important energy reserve. Indigestion, fats are broken down into fatty acids and glycerol by the enzymelipase.5.3ProteinsProteinsare some of the most complex organic molecules. They are made of smaller units calledamino acids, which contain carbon, hydrogen, oxygen, and nitrogen. Some amino acids also containsulfur, phosphorus, or trace elements like iron or copper.5.3.1Amino Acids and Protein StructureThere are20 different amino acids. Each amino acid has:•anamino group (–NH₂)•acarboxyl group (–COOH)•anR group, which makes each amino acid differentExamples include alanine, valine, glutamic acid, tryptophan, tyrosine, and histidine.Amino acids join together whenwater is removedin a process calleddehydration synthesis. Thebonds formed are calledpeptide bonds, and small protein chains are calledpeptides.

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