Campbell Biology Chapter 5 Review: The Structure and Function of Large Biological Molecules

5.1 Building polymers from monomers

pp. 68–69

On the AP exam? Yes

This is Topic 1.3: know dehydration synthesis and hydrolysis cold, and be ready to apply them to any type of large molecule.

In the course: Topic 1.3 Introduction to Macromolecules (notes, videos and more questions)

Key points

Three of the four groups are polymers: carbohydrates, proteins and nucleic acids. Each is a long string of small repeating units called monomers, held together by covalent bonds. Lipids are the exception. They're assembled from smaller parts, but those parts don't repeat to form a chain.
Cells link monomers by dehydration synthesis. One monomer gives up an –OH and the other gives up an –H; they leave together as a water molecule, and a new covalent bond forms.
Hydrolysis runs the other way: a water molecule is split across the bond, an H capping one monomer and an OH capping the other, so the chain comes apart.
Joining n monomers into one unbranched chain takes n − 1 dehydration reactions and gives off n − 1 waters. Breaking that chain all the way down uses n − 1 waters.
Enzymes speed up both kinds of reaction. Building a polymer takes an input of energy, while hydrolysis releases a little energy.
Your digestive system runs on hydrolysis: the starch, protein and fat in a meal are cut into units small enough for your gut to absorb. Your cells then use dehydration synthesis to reassemble those units into the polymers they need.
Only a few dozen kinds of common monomers make a nearly endless variety of polymers, because the order and number of units can vary so much.

Key terms (8)

macromolecule: A very large molecule, usually a polymer. Carbohydrates like starch, proteins and nucleic acids are macromolecules.
monomer: A small molecule that can link with others like it to build a polymer, such as glucose, an amino acid or a nucleotide.
polymer: A chain built from many monomers bonded together, like beads on a string. Starch, proteins and DNA are polymers.
dehydration synthesis: A reaction that joins two monomers with a covalent bond and gives off a water molecule. Also called a dehydration or condensation reaction.
hydrolysis: A reaction that breaks the bond between monomers by adding a water molecule. It's how polymers are taken apart, as in digestion.
enzyme: A protein (or sometimes an RNA) that speeds up a specific chemical reaction without being used up.
covalent bond: A strong bond in which two atoms share electrons. Monomers in a polymer are held together by covalent bonds.
emergent property: A property that shows up only when parts are put together in a certain way, like a protein's ability to bind a molecule that none of its amino acids could bind alone.

Check yourself: 5.1 Building polymers from monomers

4 questions on 5.1 Building polymers from monomers. Pick an answer to see if you got it, and why.

Question 1 of 4

Which statement correctly describes a dehydration synthesis reaction between two monomers?

Question 2 of 4Calculator allowed

An enzyme from a soil bacterium is made of three polypeptide chains: two identical chains of 212 amino acids each and one chain of 95 amino acids. The chains are held together by weak interactions, not by peptide bonds. How many water molecules were released in forming all the peptide bonds in one molecule of this enzyme?

Question 3 of 4

Researchers supply a cell with water in which the oxygen atoms are the heavy isotope ¹⁸O. The cell then hydrolyzes a protein into free amino acids. Where would the ¹⁸O most likely be found?

Question 4 of 4Calculator allowed

Cells build proteins from 20 kinds of amino acids. How many different sequences are possible for a polypeptide exactly 4 amino acids long?

0 of 4 answered

5.2 Carbohydrates: from simple sugars to cellulose

pp. 69–74

On the AP exam? Yes

Topic 1.4 covers this: know that sugars are the monomers, that polysaccharides can be straight or branched, and how starch, glycogen and cellulose differ in job. The AP exam won't ask for sugar structures or α versus β linkages, but a class test on this chapter likely will.

In the course: Topic 1.4 Carbohydrates, Topic 1.3 Introduction to Macromolecules (notes, videos and more questions)

Key points

Most simple sugars (monosaccharides) have carbon, hydrogen and oxygen in a 1:2:1 ratio; glucose is C₆H₁₂O₆. Each has several –OH groups and one carbonyl (C=O) group: on an end carbon in an aldose (like glucose) or on an inner carbon in a ketose (like fructose). Sugars are also grouped by carbon count, such as 5-carbon ribose and 6-carbon glucose.
Sugars with the same formula can still be different molecules (isomers), like glucose, fructose and galactose. Dissolved in water, sugars with five or six carbons exist mostly as rings rather than straight chains.
A disaccharide is two monosaccharides joined by a glycosidic linkage, made through dehydration synthesis. Maltose is glucose + glucose, sucrose is glucose + fructose and lactose is glucose + galactose.
Storage polysaccharides are glucose chains that cells can hydrolyze for fuel. Plant starch comes as unbranched amylose and branched amylopectin; animal glycogen branches even more often and is kept mostly in the liver and muscles. Plants move sugar between their organs mainly as the disaccharide sucrose.
Cellulose, the main material of plant cell walls, is also a glucose chain, but its β linkages flip every other glucose. Each chain is straight and never branches, so neighboring chains line up and stick together through hydrogen bonds, forming strong microfibrils.
Humans and most other animals have no enzyme that fits the β linkages between cellulose's glucose units, so cellulose passes through the gut largely undigested as fiber. (Not every β linkage is off limits: the enzyme that splits milk sugar cuts one.) Plant eaters such as horses and rabbits rely mostly on gut microbes that make cellulose-cutting enzymes.
Chitin, the tough material in insect and crab exoskeletons and in fungal cell walls, is a β-linked chain like cellulose, but each of its sugar units carries an extra nitrogen-containing group.

Key terms (15)

carbohydrate: A sugar or a polymer of sugars. Carbohydrates are used for fuel, for storing energy and as building material.
monosaccharide: A simple sugar, the monomer of carbohydrates. Glucose, fructose, galactose and ribose are examples.
glucose: A six-carbon sugar, C₆H₁₂O₆. It's the main fuel for cellular respiration and the monomer of starch, glycogen and cellulose.
disaccharide: Two monosaccharides joined by a glycosidic linkage, such as sucrose, lactose or maltose.
glycosidic linkage: The covalent bond that joins two sugars, made by dehydration synthesis.
polysaccharide: A polymer of many sugar units, used for storage (starch, glycogen) or structure (cellulose, chitin).
starch: The storage polysaccharide of plants: α-linked glucose units, in unbranched or branched chains.
glycogen: The storage polysaccharide of animals: a highly branched chain of glucose units, kept mostly in liver and muscle cells.
cellulose: The structural polysaccharide of plant cell walls. Its β-linked glucose units form straight chains that bundle together into strong fibers.
α and β glucose: Two ring forms of glucose that differ in which way the –OH on carbon 1 points. Starch and glycogen use α; cellulose uses β.
microfibril: A rope-like bundle of parallel cellulose chains held together by hydrogen bonds. It's what makes plant cell walls strong.
chitin: A structural polysaccharide like cellulose but with a nitrogen-containing group on each unit. It makes up the hard outer skeletons of insects and crabs and the cell walls of fungi.
isomers: Molecules with the same formula but different arrangements of atoms, like glucose and fructose. Different arrangements mean different properties.
aldose and ketose: Two kinds of sugar, sorted by where the C=O (carbonyl) group sits: on the end carbon in an aldose such as glucose, or on an inner carbon in a ketose such as fructose.
amylose and amylopectin: The two forms of starch. Amylose is an unbranched chain of glucose; amylopectin has side branches every few dozen units.

Check yourself: 5.2 Carbohydrates: from simple sugars to cellulose

4 questions on 5.2 Carbohydrates: from simple sugars to cellulose. Pick an answer to see if you got it, and why.

Question 1 of 4

Volunteers drank 50 g of purified starch on one day and 50 g of purified cellulose on another, both stirred into water. Their blood glucose was measured before and 1 hour after each drink (invented data). Drink | Blood glucose before (mg/dL) | Blood glucose after 1 hour (mg/dL) Starch | 88 | 141 Cellulose | 89 | 90 Which explanation best accounts for these results, given that both polysaccharides are made of glucose?

Question 2 of 4

Starch and cellulose are both polymers of glucose. Which difference best explains why cellulose forms strong fibers in plant cell walls while starch serves as stored fuel?

Question 3 of 4

Ribose, the sugar in RNA, has a five-carbon chain with its carbonyl (C=O) group on the end carbon. Fructose has a six-carbon chain with its carbonyl group on the second carbon. How is each sugar classified?

Question 4 of 4

People who are lactose intolerant don't make enough of the enzyme that hydrolyzes lactose, the main sugar in milk. When this enzyme does act, which products does it release?

0 of 4 answered

5.3 Lipids: fats, phospholipids and steroids

pp. 74–77

On the AP exam? Yes

Topic 1.5 covers the lipid types and saturated versus unsaturated fatty acids, and Topic 2.3 builds membranes from phospholipids. Health details like trans-fat rules won't be tested.

In the course: Topic 1.5 Lipids, Topic 2.3 Plasma Membrane (notes, videos and more questions)

Key points

Lipids are grouped by how they act, not by a shared monomer: they're mostly hydrocarbon, so they're nonpolar and don't mix with water. They aren't polymers, though the AP course lists them with the other three types of large molecules.
A fat (triglyceride) is one glycerol with three fatty acids attached by ester linkages, each made by dehydration synthesis. Fat is the most compact fuel reserve, at about 9 kcal per gram versus about 4 for carbohydrate, and body fat also pads organs and holds in heat.
A saturated fatty acid has only single C–C bonds, so it holds as much hydrogen as possible and its tail is straight. Fats made mostly of saturated tails, which are common in meat and dairy, pack tightly and are solid at room temperature.
Unsaturated fatty acids contain at least one C=C double bond. Cis double bonds kink the tail, and the kinks keep molecules from packing tightly, so most plant fats are liquid oils at room temperature. The more double bonds, the lower the melting point.
Adding hydrogen to an oil (hydrogenation) straightens tails and makes it solid. Partial hydrogenation also creates trans fats, which raise heart-disease risk; the U.S. has since largely removed artificial trans fats from foods. A few unsaturated fats, such as omega-3s, are essential: your body can't make them, so they must come from food.
A phospholipid has glycerol, two fatty acid tails and a phosphate group. Its head is hydrophilic and its tails are hydrophobic, so in water phospholipids form a bilayer, the basic structure of cell membranes.
Steroids share a core of four carbon rings joined edge to edge; the side groups attached to that core make each steroid different. Cholesterol helps keep animal cell membranes at the right fluidity, and it's the starting molecule for steroid hormones such as estradiol and testosterone.

Key terms (15)

lipid: A large, mostly nonpolar molecule that doesn't mix with water. Fats, phospholipids and steroids are lipids.
hydrophobic: Water-fearing: nonpolar, so it doesn't dissolve in or mix with water. Hydrocarbon chains are hydrophobic.
fat (triglyceride): A lipid made of one glycerol and three fatty acids. Its main job is long-term energy storage.
glycerol: A small three-carbon molecule with three –OH groups. It's the backbone that fatty acids attach to in fats and phospholipids.
fatty acid: A long hydrocarbon chain with a carboxyl group at one end. Its chain is the hydrophobic tail in fats and phospholipids.
ester linkage: The covalent bond between a fatty acid's carboxyl group and one of glycerol's –OH groups, made by dehydration synthesis.
saturated fatty acid: A fatty acid with no C=C double bonds, so it holds as much hydrogen as it can. Its straight tail packs tightly.
unsaturated fatty acid: A fatty acid with one or more C=C double bonds. Cis double bonds kink the tail, which keeps oils liquid.
trans fat: An unsaturated fat with trans double bonds, mostly made when oils are partly hydrogenated. It's linked to heart disease.
phospholipid: A lipid with two fatty acid tails and a phosphate-containing head on glycerol. It's the main building block of cell membranes.
amphipathic: Having both a hydrophilic part and a hydrophobic part, like a phospholipid with its polar head and nonpolar tails.
phospholipid bilayer: A double layer of phospholipids with tails facing inward and heads facing the water on both sides. It's the basic fabric of membranes.
steroid: A lipid built on four fused carbon rings. Cholesterol and the sex hormones are steroids.
cholesterol: A steroid found in animal cell membranes. Cells use it to make other steroids, including sex hormones.
hydrogenation: Adding hydrogen to the C=C double bonds of an oil, which straightens its tails and makes it more solid. Doing it only partway produces trans fats.

Check yourself: 5.3 Lipids: fats, phospholipids and steroids

4 questions on 5.3 Lipids: fats, phospholipids and steroids. Pick an answer to see if you got it, and why.

Question 1 of 4

When a cell builds a fat (triglyceride) molecule, which parts are joined, and how?

Question 2 of 4

Question 3 of 4

Lab detergents such as SDS have one charged head group and one long hydrocarbon tail. Adding enough detergent to cells breaks up their plasma membranes, which is how biologists release what's inside the cells. Which explanation best accounts for this?

Question 4 of 4Calculator allowed

Fat releases about 9 kcal of energy per gram, and carbohydrate releases about 4 kcal per gram. A well-fed adult stores about 400 g of glycogen in the liver and muscles. About how many grams of fat would store the same amount of energy?

0 of 4 answered

5.4 Proteins: amino acids, folding and function

pp. 77–86

On the AP exam? Yes

Topic 1.7 is the core here: know how R groups shape the four levels of structure. Denaturation comes back in Topic 3.2 and sickle-cell in 6.7; you won't need individual amino acid structures, chaperonins or lab methods for finding shapes.

In the course: Topic 1.7 Proteins, Topic 3.2 Environmental Impacts on Enzyme Function, Topic 6.7 Mutations (notes, videos and more questions)

Key points

Proteins do most of the work in a cell. Most enzymes are proteins, and other proteins build fibers and frameworks, carry substances (hemoglobin carries oxygen), act as hormones and receptors, make muscles contract and defend against infection (antibodies).
Every amino acid has a central carbon bonded to an amino group, a carboxyl group, a hydrogen and an R group (side chain). The 20 common R groups can be nonpolar, polar, acidic (negative) or basic (positive), and they give each amino acid its personality.
A peptide bond forms by dehydration synthesis, linking one amino acid's carboxyl group to the next one's amino group. Every polypeptide has an N-terminus (free amino group) and a C-terminus (free carboxyl group), and it's built starting from the N-terminus.
Primary structure is the amino acid sequence, set by a gene. Secondary structure is α helices and β pleated sheets held by hydrogen bonds in the backbone. Tertiary structure is the full 3-D shape of one chain, held by R-group interactions: hydrophobic clustering, hydrogen bonds, ionic bonds and covalent disulfide bridges between cysteines. Quaternary structure is two or more chains working together, as in hemoglobin (four chains) or an antibody (two long and two short chains).
Shape decides function, because most proteins work by binding a specific molecule. One amino acid change can matter: in sickle-cell disease, a nonpolar valine replaces a charged glutamic acid in hemoglobin, and the altered molecules clump into fibers.
A protein's fold is held mostly by weak attractions, so heat, strong acid or base, a lot of salt or an oily solvent can shake it loose. The unfolded (denatured) protein stops working. Returned to normal conditions, some denatured proteins fold back up on their own, evidence that the amino acid sequence carries all the folding information.
Inside cells, helper proteins called chaperones help new chains fold (some, the chaperonins, form a sheltered chamber), and misfolded proteins are linked to diseases like Alzheimer's and Parkinson's. Scientists find protein shapes with X-ray crystallography, NMR and cryo-electron microscopy, and AI tools such as AlphaFold now predict shapes from sequence alone.

Key terms (15)

protein: A working molecule made of one or more polypeptides folded into a specific 3-D shape.
amino acid: The monomer of proteins: a central carbon with an amino group, a carboxyl group, a hydrogen and a variable R group.
R group (side chain): The part of an amino acid that differs from one to the next. It can be nonpolar, polar or charged, and it decides how that amino acid behaves.
peptide bond: The covalent bond that links one amino acid's carboxyl group to the next one's amino group, made by dehydration synthesis.
polypeptide: A chain of amino acids joined one after another by peptide bonds. A protein is one or more polypeptides, folded.
N-terminus and C-terminus: The two ends of a polypeptide: the end with a free amino group and the end with a free carboxyl group. They give the chain a direction.
primary structure: A protein's exact sequence of amino acids. A gene sets it, and it determines all the other levels.
secondary structure: Local coils and folds held by hydrogen bonds between backbone atoms, repeated along part of the chain.
α helix and β pleated sheet: The two main kinds of secondary structure: a coiled stretch of chain, and side-by-side stretches linked into a folded sheet.
tertiary structure: The overall 3-D shape of one polypeptide, held by interactions among its R groups.
disulfide bridge: A strong covalent bond between the sulfur atoms of two cysteine side chains. It helps lock a protein's shape in place.
hydrophobic interaction: The clustering of nonpolar side chains away from water, which helps fold a protein. It's driven mostly by water bonding with itself.
quaternary structure: How two or more polypeptide chains fit together to form one working protein, as in hemoglobin.
denaturation: Loss of a protein's shape, and so its function, when heat, pH, salt or chemicals break the weak bonds holding it together.
chaperonin: A protein complex that gives a newly made polypeptide a sheltered space to fold correctly. It doesn't decide the final shape.

Check yourself: 5.4 Proteins: amino acids, folding and function

4 questions on 5.4 Proteins: amino acids, folding and function. Pick an answer to see if you got it, and why.

Question 1 of 4

Two polypeptides each contain exactly 50 alanines, 30 serines, 20 lysines and 25 leucines, 125 amino acids in all, but the amino acids are joined in a different order in each. Which prediction is best supported?

Question 2 of 4

In a water-soluble enzyme, an amino acid with a nonpolar R group sits deep in the protein's core. A mutation replaces it with an amino acid whose R group is negatively charged at cell pH. What is the most likely result?

Question 3 of 4

Proteins in the lens of the eye must stay dissolved for the lens to stay clear. Suppose a mutation replaces one charged amino acid on the outer surface of a lens protein with a nonpolar one. Which sequence of events is most likely?

Question 4 of 4

Hemoglobin is made of four polypeptide chains. A gentle chemical treatment separates the four chains from one another, but each chain keeps its own folded shape, including its α helices. Which level of structure has been disrupted?

0 of 4 answered

5.5 Nucleic acids: DNA and RNA

pp. 86–89

On the AP exam? Yes

Topic 1.6 covers nucleotide parts, DNA versus RNA and the 5′/3′ direction; base pairing comes back in Topic 6.1, the DNA → RNA → protein flow in 6.3, and sequence comparisons as evidence of common ancestry in 7.6.

In the course: Topic 1.6 Nucleic Acids, Topic 6.1 DNA and RNA Structure, Topic 6.3 Transcription and RNA Processing, Topic 7.6 Evidence of Evolution (notes, videos and more questions)

Key points

Nucleic acids store and pass on hereditary information. A gene's DNA sequence is copied into messenger RNA (mRNA), which carries it to ribosomes, where a polypeptide is built: DNA → RNA → protein.
The monomer is a nucleotide: a five-carbon sugar, a phosphate group and a nitrogenous base. Without the phosphate, the sugar and base are called a nucleoside.
Bases come in two families. Pyrimidines (C, T and U) have one ring; purines (A and G) have two. DNA uses A, T, G and C, while RNA uses U in place of T. DNA's sugar, deoxyribose, lacks the –OH on the 2′ carbon that RNA's ribose has.
Phosphodiester linkages join the 3′ carbon of one sugar to the phosphate on the 5′ carbon of the next, making a sugar-phosphate backbone with the bases sticking out. Each strand has a 5′ end and a 3′ end, and new nucleotides are always added to the 3′ end.
DNA is a double helix: two antiparallel strands held together by hydrogen bonds between paired bases, A with T and G with C. Because each strand predicts the other, DNA can be copied accurately before a cell divides.
RNA is usually a single strand, but it can fold by pairing with itself (A with U, G with C), as transfer RNA does. Some RNAs act as enzymes, and others help control which genes are used.
Sequences are inherited and change slowly over generations, so the fewer differences two species show in a gene or protein, the more recently they likely shared an ancestor. Sequence comparisons are now some of the strongest evidence for evolution.

Key terms (15)

gene: A stretch of DNA that holds the instructions for making a specific RNA, and usually a protein.
DNA (deoxyribonucleic acid): The double-stranded nucleic acid that stores hereditary information, built with the sugar deoxyribose and the bases A, T, G and C.
RNA (ribonucleic acid): A usually single-stranded nucleic acid built with ribose and the bases A, U, G and C. It helps turn genes into proteins, among other jobs.
messenger RNA (mRNA): An RNA copy of a gene that carries its instructions from the DNA to the ribosomes, where a protein is made.
nucleotide: The monomer of nucleic acids: a five-carbon sugar, a phosphate group and a nitrogenous base.
nitrogenous base: The ring-shaped, nitrogen-containing part of a nucleotide: A, G, C, T or U. The order of bases carries the information.
pyrimidine: A base with a single ring: cytosine (C), thymine (T) or uracil (U).
purine: A base with two fused rings: adenine (A) or guanine (G).
deoxyribose and ribose: The five-carbon sugars in DNA and RNA. Ribose has an –OH on its 2′ carbon; deoxyribose has only an H there.
phosphodiester linkage: The link between nucleotides in a strand: a phosphate group joining the 3′ carbon of one sugar to the 5′ carbon of the next.
sugar-phosphate backbone: The repeating chain of sugars and phosphates that forms the outside of a nucleic acid strand, with the bases attached.
5′ and 3′ ends: The two different ends of a nucleic acid strand: one with a phosphate on a 5′ carbon, the other with an –OH on a 3′ carbon. Strands grow at the 3′ end.
antiparallel: Running in opposite directions. The two strands of DNA are antiparallel: one runs 5′ → 3′ and the other 3′ → 5′.
complementary base pairing: The matching of A with T (or U in RNA) and G with C through hydrogen bonds. It lets one strand serve as a template for another.
double helix: DNA's shape: two strands wound around each other, with the backbones outside and the paired bases inside.

Check yourself: 5.5 Nucleic acids: DNA and RNA

4 questions on 5.5 Nucleic acids: DNA and RNA. Pick an answer to see if you got it, and why.

Question 1 of 4

Some RNA molecules, such as transfer RNA, fold back on themselves so that one stretch of the strand pairs with another stretch of the same strand. Which RNA could fold into a hairpin in which its first four bases pair with its last four bases, with the middle four bases forming the loop?

Question 2 of 4Calculator allowed

A sample of double-stranded DNA is 28% adenine (A). What percent of its bases are guanine (G)?

Question 3 of 4

Which pair of features distinguishes an RNA nucleotide from a DNA nucleotide?

Question 4 of 4

As a DNA or RNA strand grows, where is each new nucleotide added?

0 of 4 answered