CHEM 120 W8 Exam3
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Free CHEM 120 W8 Exam3 Questions
You have a chunk of radioactive iron. This isotope has a half-life of about 2.5 years. How many grams of an 80-gram sample of this radioactive iron will remain intact after 5 years?
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80 g
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40 g
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20 g
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35 g
Explanation
A half-life is the time it takes for half of a radioactive substance to decay. After one half-life (2.5 years), 80 g becomes 40 g. After another half-life (5 years total), half of the remaining 40 g decays, leaving 20 g. Therefore, after two half-lives, only one-quarter of the original sample (20 g) remains intact.
This type of biomolecule consists of a long carbon chain with a carboxylic acid end and can be classified as saturated or unsaturated.
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Hormone
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Triglyceride
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Wax
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Fatty acid
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Protein
Explanation
A fatty acid is a long hydrocarbon chain with a carboxylic acid group (-COOH) at one end. Fatty acids can be classified as:
Saturated, when they contain only single carbon–carbon bonds (solid at room temperature), or
Unsaturated, when they contain one or more double bonds (liquid at room temperature).
Fatty acids are the building blocks of lipids, including triglycerides and phospholipids, and play key roles in energy storage and membrane structure.
What is the name for: CH₃CH₂COCH₃?
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Methyl ethyl ketone
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Butanone
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Ethyl propanoate
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Methyl propyl ether
Explanation
The compound CH₃CH₂COCH₃ contains four carbon atoms and a carbonyl group (C=O) bonded to two alkyl groups, which classifies it as a ketone. According to IUPAC naming rules, the parent chain is “butane,” and the ketone functional group changes the suffix to “-one.” Thus, the correct IUPAC name is butanone, though it is also commonly known as methyl ethyl ketone in industry.
The enzyme OREOase acts on chocolate sandwich cookies to break the cookies apart to get to the creamy center. Occasionally, a vanilla cookie also fits into the OREOase active site, but the enzyme cannot act on the vanilla cookie so the reaction halts. Eventually, the vanilla cookie leaves the active site. What is the best description of the vanilla cookie in this scenario?
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Competitive inhibitor
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Denaturing of the protein
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Cofactor
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Coenzyme
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Non-competitive inhibitor
Explanation
A competitive inhibitor is a substance that resembles the enzyme’s normal substrate and competes for binding at the active site. In this case, the vanilla cookie fits into OREOase’s active site just like the chocolate cookie (the true substrate) but does not undergo a reaction. This temporarily blocks the enzyme’s activity until the inhibitor leaves, allowing the correct substrate to bind again.
Which of the following would NOT be considered a monosaccharide?
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Glucose
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Fructose
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Sucrose
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Galactose
Explanation
Sucrose is a disaccharide, not a monosaccharide. It is formed when glucose and fructose join together through a dehydration (condensation) reaction. Disaccharides must be broken down into their monosaccharide components before they can be absorbed by the body.
Select all of the following that you would expect to be a liquid oil at room temperature:
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A fatty acid containing multiple carbon-to-carbon double bonds
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A dipeptide
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A fatty acid containing hexane
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A fatty acid classified as unsaturated
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A carbohydrate with a glycosidic bond
Explanation
A. A fatty acid containing multiple carbon-to-carbon double bonds
Fatty acids with multiple double bonds (polyunsaturated fatty acids) have kinks in their hydrocarbon chains that prevent tight packing of molecules. This results in weaker intermolecular forces, keeping them liquid at room temperature. Examples include omega-3 and omega-6 fatty acids found in vegetable oils and fish oils.
D. A fatty acid classified as unsaturated
Unsaturated fatty acids (both mono- and polyunsaturated) contain one or more double bonds, which reduce molecular rigidity and melting point. Thus, they are typically liquid oils at room temperature, unlike saturated fats, which are solid
A long chain of glucose molecules forms. Which of the following terms would best describe the resulting biomolecule?
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Monosaccharide
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Polysaccharide
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Sugar
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Disaccharide
Explanation
A polysaccharide is a complex carbohydrate composed of many glucose (monosaccharide) units bonded together through glycosidic linkages. Examples include starch, glycogen, and cellulose. These large molecules serve important biological functions such as energy storage (starch in plants, glycogen in animals) and structural support (cellulose in plant cell walls).
Which form of radiation is able to penetrate most deeply into the human body?
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Alpha decay
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Gamma ray emission
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Skin would block all of these types of radiation
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Beta decay
Explanation
Gamma rays are high-energy electromagnetic waves with no mass and no charge, allowing them to penetrate deeply through human tissue and even several centimeters of lead or concrete. Because of their penetrating power, they pose a significant internal and external radiation hazard.
The active site of an enzyme allows which of the following? Select all that apply.
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Rigid stability to the protein
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A location for allosteric inhibitors to bind
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The chemical reaction to happen
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The shape ensures only the correct molecule(s) fit
Explanation
C. The chemical reaction to happen
The active site of an enzyme is the specific region where substrates bind and the chemical reaction occurs. It provides the proper microenvironment (such as pH, charge, and orientation) to lower activation energy and facilitate the conversion of substrate(s) into product(s).
D. The shape ensures only the correct molecule(s) fit
The active site has a unique 3D shape that follows the lock-and-key or induced-fit model, allowing only specific substrates to bind. This specificity ensures that enzymes catalyze only their intended reactions.
Which is the best description of rRNA?
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It is part of the ribosome
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It holds the entire genetic code
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It carries a copy of the DNA code to the ribosome
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It carries the amino acid to the ribosome
Explanation
Ribosomal RNA (rRNA) forms the structural and functional core of ribosomes, the cellular organelles responsible for protein synthesis. It helps align messenger RNA (mRNA) and transfer RNA (tRNA) during translation and catalyzes the formation of peptide bonds between amino acids. rRNA thus plays a direct enzymatic and structural role in building proteins, making it essential for gene expression and cellular function.
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