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Are you feeling adventurous? Then spread your wings and travel km southeast of Taiwan, where you'll discover a part of the country unlike any other. Known as Lanyu* or Orchid Island, this tiny drop of earth in the Pacific* is home to the Tao*, Taiwan's only ocean-dependent indigenous* tribe. The Tao people are the people of the "alibangbang*," or flying fish, one of Mother Nature's greatest gifts. The annual flying fish season is at the heart of Tao culture and comes with many traditions and taboos. Spanning around eight months of the year, the flying fish season involves multiple customs and rituals. A special religious ceremony, usually held in February, marks the beginning of this important period and ensures a prosperous and plentiful season. Tao boat captains head to the beach with elaborate costumes on. There, they pray and make sacrifices to appeal to the flying fish spirits to bless their tribe, and to the gods for good fortune and courage on the dangerous seas. Although local fishers can catch flying fish between March and June, they are restricted to capturing only those found in shallow waters. During this period, netting* any deep-sea or coral reef fish is not permitted. If the fishers didn't follow this rule, various species would not have the time needed to recover their numbers, and harmony with nature would be lost. At the end of June, the annual sea harvest concludes with another ceremony to express gratitude and respect for nature's endless* cycles. After the ceremony, flying fish can no longer be caught. Therefore, from July onward*, only dried and stored alibangbang can be eaten. Furthermore, after the Mid-Autumn Festival*, even these dried fish must no longer be consumed. This custom seems to have been engraved in the locals’ hearts* and is believed to prevent ill luck and guarantee abundance in seasons to come. Just as the locals are concluding their scaly* harvest for the year, scores of tourists begin arriving on Lanyu for the summer season. To make sure you don't feel like a fish out of water, here are some basic guidelines to help you enjoy your stay while respecting local traditions. First, it is considered a taboo to touch or photograph the Tao fishing boats without permission since it would bring bad luck. Also, you should never enter locals’ private gardens uninvited* to view or sample drying fish. Additionally, avoid asking about future fishing trips, as locals believe that the gods may be angered and that the harvest may be spoiled as a consequence. Finally, as a sign of respect for the local community and environment, don't snorkel* or dive in the island's traditional fishing waters during the flying fish season. Such activities can disturb the local ecosystem. The customs and taboos mentioned above reflect the importance of preserving and cherishing natural resources and local culture. The ancient wisdom of Lanyu’s Tao inhabitants thus appears to give us much to learn about living in harmony with this planet. Everyone is, after all, in the same boat under nature's wing.

Generate all of these 25 questions Part A: Each correct answer is worth 5. 1. The regular pentagon shown has a side length of 2 cm. The perimeter of the pentagon is (A) 2 cm (B) 4 cm (C) 6 cm (D) 8 cm (E) 10 cm 2 cm 2. The faces of a cube are labelled with 1, 2, 3, 4, 5, and 6 dots. Three of the faces are shown. What is the total number of dots on the other three faces? (A) 6 (B) 8 (C) 10 (D) 12 (E) 15 3. The equation that best represents \a number increased by _ve equals 15" is (A) n 􀀀 5 = 15 (B) n _ 5 = 15 (C) n + 5 = 15 (D) n + 15 = 5 (E) n _ 5 = 15 4. The line graph shows the number of bobbleheads sold at a store each year. The sale of bobbleheads increased the most between (A) 2016 and 2017 (B) 2017 and 2018 (C) 2018 and 2019 (D) 2019 and 2020 (E) 2020 and 2021 Number of 2016 2017 2018 2019 2020 Year Sale of Bobbleheads 2021 Bobbleheads 20 40 60 80 5. Starting at 72, Aryana counts down by 11s: 72; 61; 50; : : : . What is the last number greater than 0 that Aryana will count? (A) 4 (B) 5 (C) 6 (D) 7 (E) 8 6. In the diagram, \ABC = 90_. The value of x is (A) 68 (B) 23 (C) 56 (D) 28 (E) 26 Day of the Week 44° x° A B C x° 7. Which of the following values is closest to zero? (A) 􀀀1 (B) 5 4 (C) 12 (D) 􀀀4 5 (E) 0:9 Grade 8 8. A jar contains 267 quarters. One quarter is worth $0.25. How many quarters must be added to the jar so that the total value of the quarters is $100.00? (A) 33 (B) 53 (C) 103 (D) 133 (E) 153 9. A package of 8 greeting cards comes with 10 envelopes. Kirra has 7 cards but no envelopes. What is the smallest number of packages that Kirra needs to buy to have more envelopes than cards? (A) 3 (B) 4 (C) 5 (D) 6 (E) 7 10. For the points in the diagram, which statement is true? (A) e > c (B) b < d (C) f > b (D) a < e (E) a > c y x (e, f ) (a, b) (c, d ) Part B: Each correct answer is worth 6. 11. The 26 letters of the English alphabet are listed in an in_nite, repeating loop: ABCDEFGHIJKLMNOPQRSTUVWXY ZABC : : : What is the 258th letter in this sequence? (A) V (B) W (C) X (D) Y (E) Z 12. A public holiday is always celebrated on the third Wednesday of a certain month. In that month, the holiday cannot occur on which of the following days? (A) 16th (B) 22nd (C) 18th (D) 19th (E) 21st 13. A circular spinner is divided into three sections. An arrow is attached to the centre of the spinner. The arrow is spun once. The probability that the arrow stops on the largest section is 50%. The probability it stops on the next largest section is 1 in 3. The probability it stops on the smallest section is (A) 1 4 (B) 2 5 (C) 1 6 (D) 2 7 (E) 3 10 14. A positive number is divisible by both 3 and 4. The tens digit is greater than the ones digit. How many positive two-digit numbers have this property? (A) 4 (B) 5 (C) 6 (D) 7 (E) 8 15. A rectangular pool measures 20 m by 8 m. There is a 1 m wide walkway around the outside of the pool, as shown by the shaded region. The area of the walkway is (A) 56 m2 (B) 60 m2 (C) 29 m2 (D) 52 m2 (E) 50 m2 20 m 8 m 1 m Grade 8 16. The results of asking 50 students if they participate in music or sports are shown in the Venn diagram. What percentage of the 50 students do not participate in music and do not participate in sports? (A) 0% (B) 80% (C) 20% (D) 70% (E) 40% Music Sports 15 5 20 17. There are 2 3 as many golf balls in Bin F as in Bin G. If there are a total of 150 golf balls, how many fewer golf balls are in Bin F than in Bin G? (A) 15 (B) 30 (C) 50 (D) 60 (E) 90 18. In the sequence shown, Figure 1 is formed using 7 squares. Each _gure after Figure 1 has 5 more squares than the previous _gure. What _gure has 2022 squares? (A) Figure 400 (B) Figure 402 (C) Figure 404 (D) Figure 406 (E) Figure 408 Figure 1 Figure 2 Figure 3 19. Mateo's 300 km trip from Edmonton to Calgary passed through Red Deer. Mateo started in Edmonton at 7 a.m. and drove until stopping for a 40 minute break in Red Deer. Mateo arrived in Calgary at 11 a.m. Not including the break, what was his average speed for the trip? (A) 83 km/h (B) 94 km/h (C) 90 km/h (D) 95 km/h (E) 64 km/h 20. Equilateral triangle ABC has sides of length 4. The midpoint of BC is D, and the midpoint of AD is E. The value of EC2 is (A) 7 (B) 6 (C) 6:25 (D) 8 (E) 10 Part C: Each correct answer is worth 8. 21. The positive factors of 6 are 1, 2, 3, and 6. There are two perfect squares less than 100 that have exactly _ve positive factors. What is the sum of these two perfect squares? (A) 177 (B) 80 (C) 145 (D) 52 (E) 97 22. In the list p; q; r; s; t; u; v, each letter represents a positive integer. The sum of the values of each group of three consecutive letters in the list is 35. If q + u = 15, then p + q + r + s + t + u + v is (A) 85 (B) 70 (C) 80 (D) 90 (E) 75 Grade 8 23. The net shown is folded to form a cube. An ant walks from face to face on the cube, visiting each face exactly once. For example, ABCFED and ABCEFD are two possible orders of faces the ant visits. If the ant starts at A, how many possible orders are there? (A) 24 (B) 48 (C) 32 (D) 30 (E) 40 A D B C E F 24. The number 385 is an example of a three-digit number for which one of the digits is the sum of the other two digits. How many numbers between 100 and 999 have this property? (A) 144 (B) 126 (C) 108 (D) 234 (E) 64 25. Student A, Student B, and Student C have been hired to help scientists develop a new avour of juice. There are 4200 samples to test. Each sample either contains blueberry or does not. Each student is asked to taste each sample and report whether or not they think it contains blueberry. Student A reports correctly on exactly 90% of the samples containing blueberry and reports correctly on exactly 88% of the samples that do not contain blueberry. The results for all three students are shown below. Student A Student B Student C Percentage correct on samples 90% 98% (2m)% containing blueberry Percentage correct on samples 88% 86% (4m)% not containing blueberry Student B reports 315 more samples as containing blueberry than Student A. For some positive integers m, the total number of samples that the three students report as containing blueberry is equal to a multiple of 5 between 8000 and 9000. The sum of all such values of m is (A) 45 (B) 36 (C) 24 (D) 27 (E) 29

Broken windows are covered. Floorboards are patched and doors screwed back on. The road that was ruined by German tanks is shovelled and raked smooth. Boot-shaped bruises turn yellow then fade and disappear. Flowers grow and spread across the ugly German footprints stomped into garden beds. The village looks pretty once more. School stops for the summer and everyone is put to work on the kolkhoz, the village farm. Women and big boys begin harvesting the barley crops in the outer fields. The biggest girls milk the cows, morning and night, and keep the barns clean. Old Nikolay mends ploughs, horse harnesses, pitchforks and scythes in his workshop. Anna Pushinka teaches Yelena and her friends how to get the honey from the beehives that are scattered through the orchards. I am in charge of collecting eggs. My friends Olga and Nina help. Olga and Nina are five, a year younger than me. They are twins and look exactly alike, except Nina’s nose is a little bit crooked from when she fell out of bed and squashed it sideways on the floor. The hens, ducks and geese wander free in the summer, so collecting eggs is like a treasure hunt and takes hours. Catching the hens for their daily hugs takes even longer, but I think it’s important because hugs make everyone happy and happy hens lay bigger eggs. Olga says I’m the best hen-hugger in all of Russia. Nina says I’ll be the best cow-hugger, too, when my arms grow longer. But good hugs have nothing to do with the size of your arms. It’s all to do with the size of your heart. When we are done with the hens, Olga, Nina and I can spend the rest of the day doing whatever we like. We climb the apricot trees, chase squirrels, lie in the meadow marvelling at how hot Ushanka’s black fur becomes in the sunshine, make daisy chains and race little boats of bark in the stream. I teach Olga and Nina the alphabet and we use charcoal to write our letters and our names all over the village – on doors and walls and the freshly cut ends of firewood. In between, I practise my knots. In case the German princemonsters return. I slip into Old Nikolay’s workshop and tie knots in the harnesses hanging on the walls. I wander into gardens where the washing is hung out to dry and tie knots in the laces on pants and smocks. I creep up behind Anna Pushinka and tie knots in her apron strings. I find baling twine in the hay shed and tie my own ankles together. I do such a good job of these last knots that I can’t get them undone. I have to jump all the way to Olga and Nina’s house and ask them to cut me free with their mama’s knife. At the end of each day, Ushanka and I run out into the distant barley fields to meet Mama. This is my favourite part of the day, because Mama always shouts, ‘Little Rabbit!’ and smothers my head with kisses. And as we walk home, we sing. Everyone – women, big boys and me. I love to sing. Almost as much as I love to be kissed by Mama. Sometimes one of the boys, Mikhail, has his balalaika with him. He takes the instrument out from beneath the sheaves of barley piled high on the wagon and plays music. We sing about forests and orchards and people who find their true love. As we walk home, arm in arm, my heart fills with happiness and my belly swells with pride that I am allowed to sing along with the big boys. And I can almost forget about the German prince-monsters and their lies about Russia and their big ugly boots. Almost. But today, when Mikhail reaches for his balalaika, I see other things hiding beneath the barley sheaves. Three of the mamas rush forward and cover them up, but it’s too late. I know they are there. I’ve already seen them. Rifles. Lots of rifles. Mikhail hugs his balalaika to his chest and blushes. ‘So play!’ cries Mama, her voice oddly loud and high. ‘Let’s play Sasha’s favourite song, “The Little Birch Tree”.’ So Mikhail plays and everyone sings about the lovely birch tree with its curly leaves and the branches that will be turned into silver flutes. They sing too quickly, too loudly, and as they sing and walk, they cast nervous sideways glances at me. ‘It’s alright,’ I say, when the song comes to an end. ‘I didn’t see the rifles.’ Mama nods and smiles, and I know it was the right thing to say. But I did see the rifles. And I think about Yelena wanting to get lots of guns and dynamite for the Partisans so they can shoot the Germans and blow them into thousands of tiny pieces, and Mama looking as though she agreed, and I know this is what the mamas and the big boys are doing. As well as harvesting, they are helping the Partisans. Three days later, I wake before dawn and I am all alone. Yelena is always here beside me when I wake. But not this morning. I climb down from our bed above the stove. Mama is filling a cloth sack with bread. She ties it closed with a piece of string and hands it to Yelena. ‘Stay out of sight,’ says Mama. ‘And don’t return until after dark.’ ‘Where’s she going?’ I ask. ‘Nowhere,’ snaps Mama. ‘Then why does she need all that bread?’ I ask. ‘There’s nothing left for us.’ Mama baked four loaves last night and she has stuffed them all into the sack. Yelena opens her mouth, but before she can speak, Mama shoves her out the door and sends her on the way to nowhere. Mama turns and stares at me, her blue, blue cornflower eyes wide with worry. ‘I know,’ I say, flopping down on the bench. ‘I didn’t see any bread.’ Mama sits beside me and takes my hand. ‘And . . .?’ she prods, obviously waiting for more. I puzzle for a while, then say, ‘And I don’t have a sister called Yelena.’ Mama laughs, softly and with a little bit of sadness around the edges. ‘Sweet Little Rabbit! You do have a sister called Yelena.’ ‘I do?’ I ask, now confused. ‘I haven’t seen the rifles or the bread, but I have seen Yelena?’ ‘Yes.’ Mama smiles and the magic makes me smile, too. And I am glad that Yelena is real because I love her very much. ‘Yelena is real,’ Mama explains, ‘but she does not carry sacks of bread into the forest for the Partisans.’ ‘Of course not!’ I shout, slapping my forehead. ‘Because there is no bread!’ Mama laughs loudly now, with not a hint of sadness. She hugs me, pressing me against her warm, loving heart, covering my head with kisses. ‘Clever Little Rabbit,’ she murmurs, and then, in barely a whisper, ‘Your papa would be so proud.’ When I wake the next morning, Yelena is sleeping beside me, her mouth open, her braided hair unravelling. Mama is serving kasha to a strange woman seated at our table. I crawl down from above the stove and slide along the bench beside her. I stare at her pants, her tunic, the rope she is using as a belt and her big boots. She’s dressed like a man! And there’s a rifle leaning against the wall near the door. ‘Hello,’ I say. ‘I’m Sasha.’ The woman doesn’t reply. She just shovels down her kasha. I line my four wooden bears along the table in front of her bowl and say, ‘These are my bears: Big Bear, Medium Bear, Little Bear and Even Littler Bear.’ ‘Hello, Sasha. Hello, bears.’ She smiles but she doesn’t tell me her name. ‘Why are you dressed like a man?’ I ask, tugging at the sleeve of her tunic. ‘Because men’s clothes make it easier to run and climb and crawl and shoot,’ she says. ‘You’re a Partisan!’ I gasp. ‘But she’s not real,’ says Mama, placing a bowl of kasha before me. ‘Is the kasha real?’ I ask. Mama laughs. ‘Yes, Little Rabbit.’ I’m glad the food is real, because I’m hungry. But I’m disappointed that the woman is not real. I was going to ask if I could use her rope-belt to tie her ankles together. For practice. But if she’s not real, then the rope and her ankles aren’t either. The woman finishes her kasha, hangs her rifle over her shoulder, kisses Mama on the cheek then slips out the door. I run to the window to watch her leave, but by the time I get there, she’s gone. Vanished. ‘Because she’s not real,’ I whisper. A week later, Mama and I are working in the garden. We sing as we weed between the flowers and pluck caterpillars from the vegetables. Anna Pushinka is picking strawberries in her garden and wanders over. ‘Taste these,’ she says, holding out the basket. Mama reaches in and takes out a fat strawberry and a tiny piece of folded paper. The strawberry goes into her mouth, the paper into her pocket. ‘What’s on the paper?’ I ask. ‘Paper?’ Anna Pushinka replies with a wave of her hand. ‘Goodness, Sasha! Who has money for paper? These are lean times. We must choose between paper for writing and noodles for our soup. And I always choose noodles.’ She chuckles and I know the paper is yet another thing that is not real. That night, Mama slips the paper to Yelena, but she drops it on the floor. I pick it up for her, and I see that there are tiny words and numbers written all over it. I wish I could read better. I’m desperate to know what it says. Or rather, what it doesn’t say, because it’s not real. Later, when Mama has tucked us into our bed above the stove and Ushanka has wrapped herself around the top of my head, I ask Yelena, ‘What’s on the paper?’ ‘What paper?’ says Yelena. ‘The paper that isn’t real,’ I reply. Yelena stares at me, nibbling her lip, then whispers, ‘A message for the Partisans. Stuff about where the Germans have their headquarters and when their trains are travelling and where they store their ammunition.’ ‘Why?’ ‘So the Partisans can blow them up.’ Yelena grabs my arm. ‘But don’t tell anyone. It’s a secret.’ ‘What’s a secret?’ I ask. ‘The message.’ ‘What message?’ I say, my eyes wide. Yelena laughs. ‘Good boy, Sasha.’ My belly swells with pride. I know how to play this game. ‘How are your knots coming along?’ asks Yelena. ‘Good! Yesterday, I crept into the dairy and tied knots in the apron strings of all the girls who were milking and only one of them noticed. Today, I tied Olga’s ankles together with Mama’s embroidery thread and just now, while you were taking a bath, I tied the sleeves of your blouse together in an enormous knot.’ Yelena rolls her eyes, then says, ‘I’ll see if I can find you some rope for practising.’ ‘Practising what?’ I ask. ‘Your knots,’ she says. ‘What knots?’ Yelena, my big sister who is twelve and always serious t

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CARBOHYDRATES Carbohydrates are organic compounds composed of carbon, hydrogen, and oxygen in a ratio of about one carbon atom to two hydrogen atoms to one oxygen atom. The number of carbon atoms in a carbohydrate varies. Some carbohydrates serve as a source of energy. Other carbohydrates are used as structural materials. Carbohydrates can exist as monosaccharides, disaccharides, or polysaccharides. Monosaccharides A monomer of a carbohydrate is called a monosaccharide (MAHN-oh-SAK-uh-RIED). A monosaccharide—or simple sugar— contains carbon, hydrogen, and oxygen in a ratio of 1:2:1. The gen- eral formula for a monosaccharide is written as (CH2O)n, where n is any whole number from 3 to 8. For example, a six-carbon mono- saccharide, (CH2O)6, would have the formula C6H12O6. The most common monosaccharides are glucose, fructose, and galactose, as shown in Figure 3-6. Glucose is a main source of energy for cells. Fructose is found in fruits and is the sweetest of the monosaccharides. Galactose is found in milk. Notice in Figure 3-6 that glucose, fructose, and galactose have the same molecular formula, C6H12O6, but differing structures. The different structures determine the slightly different properties of the three compounds. Compounds like these sugars, with a single chemical formula but different structural forms, are called isomers (IE-soh-muhrz). SECTION 2 OBJECTIVES ● Distinguish between monosaccharides, disaccharides, and polysaccharides. ● Explain the relationship between amino acids and protein structure. ● Describe the induced fit model of enzyme action. ● Compare the structure and function of each of the different types of lipids. ● Compare the nucleic acids DNA and RNA. VOCABULARY carbohydrate monosaccharide disaccharide polysaccharide protein amino acid peptide bond polypeptide enzyme substrate active site lipid fatty acid phospholipid wax steroid nucleic acid deoxyribonucleic acid (DNA) ribonucleic acid (RNA) nucleotide C HO H C H OH C OH H C CH2OH H C H OH O Glucose C OH C O H OH C OH H CH2OH C H CH2OH Fructose C H HO C OH H C OH H C CH2OH H C H OH O Galactose Glucose, fructose, and galactose have the same chemical formula, but their structural differences result in different properties among the three compounds. FIGURE 3-6 Copyright © by Holt, Rinehart and Winston. All rights reserved. 56 CHAPTER 3 Disaccharides and Polysaccharides In living things, two monosaccharides can combine in a condensa- tion reaction to form a double sugar, or disaccharide (die-SAK-e-RIED). For example in Figure 3-4, the monosaccharides fructose and glu- cose can combine to form the disaccharide sucrose. A polysaccharide is a complex molecule composed of three or more monosaccharides. Animals store glucose in the form of the polysaccharide glycogen. Glycogen consists of hundreds of glucose molecules strung together in a highly branched chain. Much of the glucose that comes from food is ultimately stored in your liver and muscles as glycogen and is ready to be used for quick energy. Plants store glucose molecules in the form of the polysaccha- ride starch. Starch molecules have two basic forms—highly branched chains that are similar to glycogen and long, coiled, unbranched chains. Plants also make a large polysaccharide called cellulose. Cellulose, which gives strength and rigidity to plant cells, makes up about 50 percent of wood. In a single cellu- lose molecule, thousands of glucose monomers are linked in long, straight chains. These chains tend to form hydrogen bonds with each other. The resulting structure is strong and can be broken down by hydrolysis only under certain conditions. PROTEINS Proteins are organic compounds composed mainly of carbon, hydrogen, oxygen, and nitrogen. Like most of the other biological macromolecules, proteins are formed from the linkage of monomers called amino acids. Hair and horns, as shown in Figure 3-7a, are made mostly of proteins, as are skin, muscles and many biological catalysts (enzymes). Amino Acids There are 20 different amino acids, and all share a basic structure. As Figure 3-7b shows, each amino acid contains a central carbon atom covalently bonded to four other atoms or functional groups. A single hydrogen atom, highlighted in blue in the illustration, bonds at one site. A carboxyl group, —COOH, highlighted in green, bonds at a second site. An amino group, —NH2, highlighted in yel- low, bonds at a third site. A side chain called the R group, high- lighted in red, bonds at the fourth site. The main difference among the different amino acids is in their R groups. The R group can be complex or it can be simple, such as the CH3 group shown in the amino acid alanine in Figure 3-7b. The differences among the amino acid R groups gives different proteins very different shapes. The different shapes allow pro- teins to carry out many different activities in living things. Amino acids are commonly shown in a simplified way such as balls, as shown in Figure 3-7c. (a) Many structures, such as hair and horns are made of proteins. (b) Proteins are made up of amino acids. Amino acids differ only in the type of R group (shown in red) they carry. Polar R groups can dissolve in water, but nonpolar R groups cannot. (c) Amino acids have complex structures, so, in this and other textbooks, they are often simplified into balls. FIGURE 3-7 (b) Alanine (an amino acid) (c) Simplified version of amino acid CH3 H N OH C C H O H (a) Copyright © by Holt, Rinehart and Winston. All rights reserved. BIOCHEMISTRY 57 H H N C C OH H O H CH3 H2O Glycine Alanine H N OH C C H O H H H N C C H O H CH3 N OH C C H O H (a) (b) (a) The peptide bond (shaded blue) that binds amino acids together to form a polypeptide results from a condensation reaction that produces water. (b) Poly- peptides are commonly shown as a string of balls in this textbook and elsewhere. Each ball represents an amino acid. FIGURE 3-8 Substrate Products Enzyme 1 2 3 In the induced fit model of enzyme action, the enzyme can attach only to a substrate (reactant) with a specific shape. The enzyme then changes and reduces the activation energy of the reaction so reactants can become products. The enzyme is unchanged and is available to be used again. 3 2 1 FIGURE 3-9 Dipeptides and Polypeptides Figure 3-8a shows how two amino acids bond to form a dipeptide (die-PEP-TIED). In this condensation reaction, the two amino acids form a covalent bond, called a peptide bond (shaded in blue in Figure 3-8a) and release a water molecule. Amino acids often form very long chains called polypeptides (PAHL-i-PEP-TIEDZ). Proteins are composed of one or more polypep- tides. Some proteins are very large molecules, containing hun- dreds of amino acids. Often, these long proteins are bent and folded upon themselves as a result of interactions—such as hydrogen bonding—between individual amino acids. Protein shape can also be influenced by conditions such as temperature and the type of solvent in which a protein is dissolved. For exam- ple, cooking an egg changes the shape of proteins in the egg white. The firm, opaque result is very different from the initial clear, runny material. Enzymes Enzymes—RNA or protein molecules that act as biological catalysts—are essential for the functioning of any cell. Many enzymes are proteins. Figure 3-9 shows an induced fit model of enzyme action. Enzyme reactions depend on a physical fit between the enzyme molecule and its specific substrate, the reactant being catalyzed. Notice that the enzyme has folds, or an active site, with a shape that allows the substrate to fit into the active site. An enzyme acts only on a specific substrate because only that substrate fits into its active site. The linkage of the enzyme and substrate causes a slight change in the enzyme’s shape. The change in the enzyme’s shape weakens some chemical bonds in the substrate, which is one way that enzymes reduce activation energy, the energy needed to start the reaction. After the reaction, the enzyme releases the products. Like any catalyst, the enzyme itself is unchanged, so it can be used many times. An enzyme may not work if its environment is changed. For example, change in temperature or pH can cause a change in the shape of the enzyme or the substrate. If such a change happens, the reaction that the enzyme would have catalyzed cannot occur.

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