AP Biology: Cellular Energetics — Practice Questions & Answers
How cells obtain, transform, and spend energy: enzymes lower activation energy and are tuned by their environment, ATP couples reactions, photosynthesis captures light to build sugars, and cellular respiration burns those sugars to regenerate ATP.
1009 practice questions available for this unit — here are 10 with full answers and explanations.
Enzymes increase the rate of a chemical reaction primarily by which mechanism?
Increasing the activation energy of the reaction
Lowering the activation energy of the reaction
Adding free energy to the reactants
Changing the equilibrium constant of the reaction
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WhyEnzymes are biological catalysts that lower the activation energy required for a reaction to proceed. They do not change the overall free energy difference between reactants and products, nor do they make endergonic reactions spontaneous.
2Multiple choice · Easy
The specific region of an enzyme where the substrate binds and catalysis occurs is called the:
Allosteric site
Active site
Binding pocket of the cofactor
Quaternary domain
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WhyThe active site is the region of an enzyme, formed by a specific arrangement of amino acids, where the substrate binds and the chemical reaction is catalyzed.
3Multiple choice · Easy
Which molecule serves as the primary immediate energy currency of the cell?
Glucose
NADPH
ATP
Pyruvate
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WhyATP (adenosine triphosphate) is the primary energy currency. Energy released when ATP is hydrolyzed to ADP and inorganic phosphate is used to drive endergonic cellular processes.
4Multiple choice · Easy
In eukaryotic cells, the Calvin cycle takes place in which location?
Thylakoid membrane
Stroma of the chloroplast
Mitochondrial matrix
Cytosol
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WhyThe light-independent reactions (Calvin cycle) occur in the stroma of the chloroplast, while the light-dependent reactions occur in the thylakoid membranes.
5Multiple choice · Easy
During which stage of cellular respiration is glucose split into two molecules of pyruvate?
Glycolysis
The Krebs cycle
Oxidative phosphorylation
Pyruvate oxidation
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WhyGlycolysis occurs in the cytosol and breaks one six-carbon glucose molecule into two three-carbon pyruvate molecules, producing a net of 2 ATP and 2 NADH.
6Multiple choice · Medium
A reaction has a positive change in free energy (delta G > 0). Which statement best describes this reaction?
It is exergonic and releases free energy
It is endergonic and requires an input of free energy
It will proceed spontaneously without any energy input
It produces ATP directly
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WhyA positive delta G indicates an endergonic reaction that is nonspontaneous and requires an input of free energy, typically supplied by coupling to an exergonic reaction such as ATP hydrolysis.
7Multiple choice · Medium
According to the induced fit model of enzyme action, what happens when a substrate binds to an enzyme?
The substrate permanently changes the enzyme's primary structure
The active site changes shape to bind the substrate more snugly
The enzyme is consumed and must be resynthesized
The active site remains completely rigid and unchanged
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WhyIn the induced fit model, the active site changes shape slightly upon substrate binding, allowing a tighter fit that helps position the substrate and strain bonds, promoting catalysis. This contrasts with the older lock-and-key view of a rigid active site.
8Multiple choice · Medium
A competitive inhibitor reduces enzyme activity by:
Binding to the active site and blocking substrate access
Binding to an allosteric site and changing the active site shape
Permanently denaturing the enzyme
Lowering the temperature of the reaction
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WhyA competitive inhibitor binds to the active site, competing with the substrate. Its effect can be overcome by increasing substrate concentration, which outcompetes the inhibitor for the active site.
9Multiple choice · Medium
Which of the following correctly pairs a stage of aerobic respiration with its location in a eukaryotic cell?
Glycolysis - mitochondrial matrix
Krebs cycle - mitochondrial matrix
Electron transport chain - cytosol
Oxidative phosphorylation - chloroplast stroma
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WhyThe Krebs (citric acid) cycle takes place in the mitochondrial matrix. Glycolysis occurs in the cytosol, and the electron transport chain is embedded in the inner mitochondrial membrane.
10Multiple choice · Medium
In the light-dependent reactions of photosynthesis, what is the original source of the electrons that replace those lost by chlorophyll in Photosystem II?
Carbon dioxide
NADPH
Water
ATP
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WhyWater is split (photolysis) in Photosystem II, releasing electrons that replace those excited and passed down the electron transport chain. This splitting also releases O2 as a byproduct and protons into the thylakoid lumen.
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Cellular energetics: The study of how cells capture, store, transform, and release energy through chemical reactions such as photosynthesis and cellular respiration.
Metabolism: The complete set of chemical reactions occurring within a cell or organism, encompassing both energy-releasing breakdown reactions and energy-requiring building reactions.
Anabolism: The set of metabolic pathways that build larger, more complex molecules from simpler ones, consuming energy in the process.
Catabolism: The set of metabolic pathways that break down complex molecules into simpler ones, releasing stored energy that the cell can use.
Metabolic pathway: An ordered series of enzyme-catalyzed reactions in which the product of one step serves as the substrate for the next.
Bioenergetics: The branch of biology concerned with energy flow and transformation through living systems.
Energy: The capacity to do work or to cause change, existing in forms such as chemical, kinetic, and potential energy.
Potential energy: Stored energy that an object or molecule possesses because of its position or chemical structure, available to be released later.