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Biology - The Chemical Basis of Life

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    Study Guide Biology – The Chemical Basis of Life 1. Acids and Bases In chemistry, acids and bases are important substances that behave in different ways when mixed with water. Understanding how they work helps explain many chemical reactions in biology and everyday life. 1.1 What Are Acids? Acids are chemical compounds that release hydrogen ions (H ⁺ ) when they are dissolved in water. This release of hydrogen ions gives acids their special properties. For example, when hydrogen chloride is placed in water, it breaks apart and releases hydrogen ions. Once this happens, the solution becomes hydrochloric acid . The presence of free hydrogen ions is what makes the solution acidic. 1.2 What Are Bases? Bases behave differently from acids. When placed in water, bases attract hydrogen ions instead of releasing them. A common example of a base is sodium hydroxide (NaOH) . When sodium hydroxide dissolves in water, it attracts hydrogen ions. As a result, hydroxyl ions (OH ⁻ ) build up in the solution. This accumulation of hydroxyl ions makes the solution basic , also known as alkaline . 1.3 Key Difference to Remember • Acids release hydrogen ions (H ⁺ ) in water • Bases remove hydrogen ions and increase hydroxyl ions (OH ⁻ ) in water Both acids and bases play important roles in chemical processes, especially in biological systems.

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    Study Guide 2. Quiz Acids and Bases 1. Question Fill 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 Answer Bases attract hydrogen ions in water and acids do not. Why This Is Correct In water, acids release hydrogen ions (H ⁺ ) , while bases attract hydrogen ions or release hydroxide ions (OH ⁻ ) . This difference explains how acids and bases behave in solution. The other options are incorrect because: • Acids do not release hydroxide ions. • Bases have high pH values , while acids have low pH values . 3. Molecules Most of the chemical compounds studied in biology are made of molecules . A molecule is a specific group of atoms held together by chemical bonds . These bonds keep the atoms connected in a stable structure. A compound is a type of molecule that contains atoms from more than one element . However, not all molecules are compounds. For example, oxygen gas (O ₂ ) is a molecule made of two atoms of the same element — oxygen. Molecules can be made in different ways: • Some contain atoms of the same element , like O ₂

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    Study Guide • Others contain atoms of different elements , like water (H ₂ O) The way atoms are arranged in a molecule determines the properties of that compound, such as how it reacts or behaves. The molecular weight of a molecule is found by adding together the atomic weights of all the atoms in that molecule. 3.1 Chemical Bonds Atoms in molecules are held together by special connections called bonds . There are different types of bonds, and each type forms in a different way. 3.2 Ionic Bonds An ionic bond forms when one atom gives electrons to another atom. This transfer of electrons creates charged atoms called ions . • The atom that loses electrons becomes positively charged • The atom that gains electrons becomes negatively charged Because opposite charges attract each other, these ions pull toward one another. This attraction creates the ionic bond that holds the atoms together. 3.3 Covalent Bonds A covalent bond forms when two atoms share electrons instead of transferring them. This sharing allows both atoms to become more stable. Here are two examples: • In water (H ₂ O) , one oxygen atom shares electrons with two hydrogen atoms • In ammonia (NH ₃ ) , one nitrogen atom shares electrons with three hydrogen atoms Covalent bonds can vary depending on how many electrons are shared: • A single bond forms when one pair of electrons is shared • A double bond forms when two pairs of electrons are shared

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    Study Guide 3.4 Key Idea to Remember Molecules are the building blocks of biological compounds. The type of atoms involved and the way they bond — whether by transferring or sharing electrons — determine the structure and behavior of each molecule. 4. Quiz Molecules 1. Question Fill 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 Guide Correct Answer consists of elements arranged in a specific arrangement and ratio Why This Is Correct A molecule is formed when atoms are joined together in a specific ratio and arrangement . This structure determines the molecule’s properties and behavior. The other options are incorrect because: • A molecule can consist of only one element (for example, O ₂ ). • A molecule’s properties are directly related to how its atoms are arranged and bonded. 2. Question Fill in the blank: Which of the following statements about an ionic bond is FALSE ? 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 Answer It involves the sharing of electrons. Why This Is Correct An ionic bond forms when electrons are transferred , not shared. One atom loses one or more electrons and becomes positively charged, while the other atom gains electrons and becomes negatively charged. The bond forms due to the attraction between these oppositely charged ions. Sharing of electrons occurs in covalent bonds , not ionic bonds. 3. Question Fill in the blank: Which of the following statements about a covalent bond is FALSE ?

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    Study Guide Answer 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 on the sharing of electrons between atoms. Correct Answer If two pairs of electrons are shared, it is a single bond. Why This Is Correct In a covalent bond , atoms share electrons. • Sharing one pair of electrons forms a single bond . • Sharing two pairs of electrons forms a double bond . Therefore, saying that two shared pairs create a single bond is incorrect. The other statements are true descriptions of covalent bonding. 5. Organic Compounds The chemical compounds found in living things are called organic compounds . They are called “organic” because they are connected with life and always contain carbon . The study of these compounds is known as organic chemistry . Although there are many kinds of organic compounds, four major groups are found in all living organisms: • Carbohydrates • Lipids • Proteins • Nucleic acids Each group plays a special role in keeping living things alive and functioning.

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    Study Guide 5.1 Carbohydrates Carbohydrates are one of the main energy sources for almost all living organisms. Some carbohydrates also provide structure , helping form important body parts. All carbohydrates are made of carbon, hydrogen, and oxygen . In these molecules, the ratio of hydrogen atoms to oxygen atoms is 2:1 . 5.1.1 Simple Carbohydrates (Sugars) Simple carbohydrates are commonly called sugars . They come in two main types: • Monosaccharides – made of one sugar molecule • Disaccharides – made of two sugar molecules The most important monosaccharide is glucose . Its chemical formula is C ₆ H ₁₂ O ₆ . Glucose is the main fuel used by living cells. In multicellular organisms, glucose dissolves in body fluids and is transported to cells throughout the body. Inside cells, glucose is broken down to release energy. It is the starting material for cellular respiration and the main product of photosynthesis . 5.1.2 Disaccharides Three 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 galactose When sucrose forms, a water molecule is released . This process is called a dehydration reaction . The reverse process, where water is added to split a molecule, is called hydrolysis . 5.1.3 Complex Carbohydrates (Polysaccharides) Polysaccharides are 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 the human liver • Cellulose – a structural carbohydrate made of glucose ○ Found in plant cell walls, wood, cotton, and paper ○ Most organisms cannot break cellulose down 5. 2 Lipids Lipids are organic molecules made of carbon, hydrogen, and oxygen. However, they contain much more hydrogen compared to oxygen than carbohydrates do. Lipids include: • Fats • Waxes • Steroids (many hormones are made from steroids)

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    Study Guide 5. 2.1 Structure of Fats A fat molecule is made of: • One glycerol molecule • One, two, or three fatty acids Glycerol contains three hydroxyl ( – OH) groups. A fatty acid is a long chain of carbon atoms (4 – 24 carbons) with a carboxyl ( – COOH) group at one end. Fatty acids attach to glycerol through a process that removes water , similar to dehydration synthesis.

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    Study Guide 5. 2.2 Saturated vs Unsaturated Fats • Unsaturated fats contain one or more double bonds • Saturated fats contain no double bonds For most health situations, eating unsaturated fats is considered healthier than eating saturated fats. 5. 2.3 Storage and Function of Fats Fats do not dissolve in water, so they form clear oil droplets called globules inside cells. • Plants store fats mainly in seeds • Animals store fats in adipose tissue Fats contain large amounts of stored energy , making them an important energy reserve. In digestion, fats are broken down into fatty acids and glycerol by the enzyme lipase . 5. 3 Proteins Proteins are some of the most complex organic molecules. They are made of smaller units called amino acids , which contain carbon, hydrogen, oxygen, and nitrogen. Some amino acids also contain sulfur, phosphorus, or trace elements like iron or copper. 5. 3.1 Amino Acids and Protein Structure There are 20 different amino acids . Each amino acid has: • an amino group ( – NH ₂ ) • a carboxyl group ( – COOH) • an R group , which makes each amino acid different Examples include alanine, valine, glutamic acid, tryptophan, tyrosine, and histidine. Amino acids join together when water is removed in a process called dehydration synthesis . The bonds formed are called peptide bonds , and small protein chains are called peptides .

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