Energy and Metabolism Study Pack
Kibin's free study pack on Energy and Metabolism includes a 7-section study guide, 25 quiz questions, 30 flashcards, and 5 open-ended Explain review questions. Sign up free to track your progress toward mastery, plus upload your own notes and recordings to create personalized study packs organized by course.
Last updated May 28, 2026
Energy and Metabolism Study Guide
Break down the core principles of cellular metabolism — from anabolic and catabolic reactions to Gibbs free energy, ATP coupling, and how enzymes lower activation energy — to master every concept AP Biology expects you to know.
Key Takeaways
- •Metabolism encompasses all chemical reactions in a cell, divided into anabolic reactions that build complex molecules and catabolic reactions that break them down.
- •Energy exists as either potential energy (stored, positional) or kinetic energy (energy of motion), and cells constantly convert between these forms to drive biological work.
- •The laws of thermodynamics govern all energy transformations: the first law states that energy cannot be created or destroyed, and the second law states that every energy conversion increases the entropy of the universe.
- •Gibbs free energy (G) determines whether a reaction proceeds spontaneously — reactions with a negative ΔG (exergonic) release usable energy, while those with a positive ΔG (endergonic) require an energy input.
- •Activation energy is the minimum energy threshold that reactants must overcome to initiate a chemical reaction, and enzymes lower this barrier without changing the overall free energy change of the reaction.
- •ATP serves as the cell's primary energy currency, coupling exergonic catabolic reactions to endergonic anabolic reactions through phosphate group transfer.
Metabolism: The Sum of Cellular Chemical Reactions
Every living cell maintains itself through a continuous network of chemical reactions collectively called metabolism, which can be organized into two opposing categories based on whether they build or break down molecules.
Anabolism: Building Complex Molecules
- •Anabolic reactions use energy to synthesize large, complex molecules from smaller subunits — for example, linking amino acids together to form a protein or assembling glucose monomers into glycogen.
- •Because anabolic reactions increase molecular complexity and order, they require a net input of energy and are therefore endergonic.
- •These reactions are essential for growth, tissue repair, and energy storage.
Catabolism: Breaking Down Complex Molecules
- •Catabolic reactions dismantle large molecules into smaller components, releasing the chemical energy stored in molecular bonds — for example, breaking down glucose through cellular respiration.
- •The energy released by catabolism is captured in molecules like ATP and NADH, which the cell then uses to power anabolic and other energy-demanding processes.
- •Digestion, glycolysis, and the citric acid cycle are all catabolic processes.
Metabolic Pathways and Coupling
- •Individual reactions rarely operate in isolation; they are organized into metabolic pathways where the product of one reaction becomes the substrate for the next.
- •Cells couple exergonic catabolic reactions to endergonic anabolic ones so that energy released by breakdown is immediately captured and redirected, minimizing waste.
Forms of Energy Relevant to Biological Systems
Understanding how cells manage energy requires distinguishing between the two fundamental forms energy takes and recognizing how biological structures store and release each form.
Potential Energy in Biological Molecules
- •Potential energy is stored energy that depends on an object's position, configuration, or chemical state — it has the capacity to do work but is not currently doing so.
- •In biological systems, potential energy is held in chemical bonds (especially the covalent bonds of organic molecules like glucose and fatty acids), in concentration gradients across membranes, and in the positions of electrons within atoms.
- •The more complex and reduced (hydrogen-rich) an organic molecule, the greater its chemical potential energy — this is why fats store more energy per gram than carbohydrates.
Kinetic Energy and Molecular Motion
- •Kinetic energy is the energy of motion — any object that is moving, including electrons, ions, and entire molecules, possesses kinetic energy.
- •At the cellular level, kinetic energy manifests as the random thermal motion of molecules (heat), the movement of ions across a membrane down a concentration gradient, and the mechanical motion of motor proteins along cytoskeletal tracks.
- •When a cell converts potential energy (for example, by hydrolyzing ATP) into kinetic energy (for example, muscle contraction), some energy is inevitably lost as heat, consistent with the second law of thermodynamics.
Thermodynamic Laws That Govern Energy Transformations
Two fundamental principles of physics — the laws of thermodynamics — set absolute limits on how cells can acquire, transform, and use energy, and they explain why no biological system can operate with perfect efficiency.
First Law of Thermodynamics: Conservation of Energy
- •The first law states that energy cannot be created or destroyed; it can only be converted from one form to another or transferred between objects.
- •For a cell, this means the total energy input (from nutrients or sunlight) must equal the total energy output (work performed plus heat released) — cells do not generate energy from nothing.
Second Law of Thermodynamics: Entropy Always Increases
- •The second law states that every energy conversion increases the total entropy (disorder) of the universe, even if a local region temporarily becomes more ordered.
- •Entropy is a measure of disorder or the number of equivalent arrangements a system can adopt; highly structured molecules like proteins represent low-entropy states.
- •Because living cells maintain highly ordered structures, they must continuously consume energy to counteract the universal tendency toward disorder — this is why life requires a constant energy input.
- •Heat released during metabolic reactions represents energy that has become unavailable to do further cellular work, representing an increase in entropy.
Free Energy and Reaction Spontaneity
Whether a particular chemical reaction can occur spontaneously in a cell — without a continuous external energy input — is quantified by Gibbs free energy, a thermodynamic measure that accounts for both the energy and entropy changes of a reaction.
Gibbs Free Energy (G) and ΔG
- •Gibbs free energy (G) represents the portion of a system's total energy that is available to do useful work at constant temperature and pressure.
- •The change in free energy (ΔG) for a reaction equals the free energy of the products minus the free energy of the reactants: ΔG = G_products − G_reactants.
- •A negative ΔG means the products have less free energy than the reactants, so the reaction releases usable energy and can proceed spontaneously — this is called an exergonic reaction.
- •A positive ΔG means the products have more free energy than the reactants, so energy must be supplied to drive the reaction forward — this is called an endergonic reaction.
Exergonic Reactions in Metabolism
- •Cellular respiration — the oxidation of glucose to CO₂ and H₂O — is exergonic overall, with a ΔG of approximately −686 kcal/mol under standard conditions.
- •The cell captures a portion of this released free energy in the form of ATP rather than allowing it all to dissipate as heat.
Endergonic Reactions and Energy Coupling
- •Protein synthesis, active transport against a concentration gradient, and DNA replication are all endergonic processes that require energy input.
- •Cells drive endergonic reactions by coupling them to the hydrolysis of ATP, which is sufficiently exergonic (ΔG ≈ −7.3 kcal/mol under standard conditions, and more negative under physiological conditions) to push the linked reaction forward.
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About this Study Pack
Created by Kibin to help students review key concepts, prepare for exams, and study more effectively. This Study Pack was checked for accuracy and curriculum alignment using authoritative educational sources. See sources below.
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Question 1 of 25
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Which term describes the complete network of all chemical reactions occurring within a living cell?
Card 1 of 30
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Concept 1 of 5
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Anabolism vs. Catabolism
Explain the difference between anabolic and catabolic reactions in your own words. How do these two types of reactions work together to keep a cell functioning, and why does each require or release energy?
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