Unit 3: Free Energy & Metabolism – Cellular Energetics Study Notes

Unit 3: Free Energy & Metabolism

Introduction to Metabolism and Thermodynamics

Metabolism encompasses all chemical reactions occurring within an organism, organized into complex networks called metabolic pathways. These pathways are essential for maintaining life, enabling cells to extract and use energy efficiently.

• Metabolism: The sum total of an organism’s chemical reactions.

• Cellular metabolism involves thousands of reactions, organized into metabolic pathways.

• Each step in a pathway is catalyzed by a specific enzyme.

Types of Metabolic Pathways

• Catabolic pathways (catabolism): Break down complex molecules into simpler compounds, releasing energy. Example: Cellular respiration (glucose → CO2 + H2O).

• Anabolic pathways (anabolism): Build complex molecules from simpler ones, consuming energy. Example: Protein synthesis (amino acids → polypeptide).

Forms of Energy

• Kinetic energy: Energy of motion (e.g., thermal energy, light).

• Potential energy: Stored energy due to position or structure (e.g., chemical energy in bonds).

Thermodynamics

• Thermodynamics: The study of energy transformations.

• First Law of Thermodynamics: Energy can be transferred and transformed, but not created or destroyed.

• Second Law of Thermodynamics: Every energy transfer increases the entropy (disorder) of the universe.

Entropy in Biological Systems

• Entropy (S): A measure of disorder or randomness.

• Organisms decrease internal entropy by creating order (e.g., building proteins), but increase the entropy of their surroundings (e.g., releasing heat and waste).

Free Energy

Free energy (G) is the portion of a system’s energy that can perform work at constant temperature and pressure. The change in free energy (ΔG) determines whether a process is spontaneous.

• ΔG = Gfinal – Ginitial

• If ΔG < 0: The process is spontaneous (exergonic), releases free energy, and increases system stability.

• If ΔG > 0: The process is non-spontaneous (endergonic), requires input of energy.

Exergonic vs. Endergonic Reactions

• Exergonic reactions: Release free energy (ΔG < 0), occur spontaneously. Example: Cellular respiration: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$ $\Delta G = -686$ kcal/mol

• Endergonic reactions: Absorb free energy (ΔG > 0), are non-spontaneous. Example: Photosynthesis: $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$ $\Delta G = +686$ kcal/mol

Activation Energy and Enzymes

• Activation energy (Ea): The initial energy needed to start a chemical reaction.

• Enzymes: Biological catalysts that lower activation energy, speeding up reactions without being consumed.

• Enzymes do not affect ΔG of the reaction.

ATP Coupling

ATP (adenosine triphosphate) is the primary energy currency of the cell. Its hydrolysis releases energy that can be used to drive endergonic reactions through a process called ATP coupling.

• ATP Structure: Three phosphate groups, ribose, and adenine.

• Hydrolysis of ATP: $ATP + H_2O \rightarrow ADP + P_i$ $\Delta G = -7.3$ kcal/mol (exergonic)

• ATP Coupling: The energy released from ATP hydrolysis is used to power endergonic cellular processes (e.g., active transport, biosynthesis, mechanical work).

• ATP is regenerated from ADP by phosphorylation, an endergonic process powered by catabolic pathways (e.g., cellular respiration).

How ATP Performs Work

• ATP hydrolysis is coupled to cellular work by transferring a phosphate group to another molecule (phosphorylation).

• Three main types of work: chemical (driving endergonic reactions), transport (pumping substances across membranes), and mechanical (muscle contraction, movement of cilia/flagella).

ATP Cycle

• ATP is continuously regenerated by adding a phosphate group to ADP.

• Energy for this process comes from exergonic breakdown of organic molecules in catabolic pathways.

Metabolic Rate & Thermoregulation

Metabolic rate refers to the amount of energy an organism uses in a given time period. Thermoregulation is the process by which organisms maintain their internal temperature within certain boundaries, even when the surrounding temperature is different.

• Endotherms (e.g., mammals, birds) regulate body temperature internally, often at a higher metabolic cost.

• Ectotherms (e.g., reptiles, fish) rely on external sources for body heat, generally with lower metabolic rates.

Self-Assessment Questions (Selected Examples)

• What is the difference between catabolic and anabolic reactions? Catabolic reactions break down molecules and release energy; anabolic reactions build molecules and require energy.

• What type of metabolic pathway provides energy while running a mile? Catabolic pathways (e.g., cellular respiration).

• What type of pathway is involved in photosynthesis? Anabolic pathway (building glucose from CO2 and H2O).

• Identify as kinetic or potential energy:

  Item	Type
  Stored energy	Potential
  Movement of matter	Kinetic
  Chemical energy	Potential
  Thermal energy	Kinetic
  ATP	Potential
  Muscle movement	Kinetic
  Water held behind a dam	Potential
  Glucose	Potential
  Light	Kinetic

• Explain how a bear eating a fish demonstrates the first law of thermodynamics: The chemical energy in the fish is transferred to the bear for movement, growth, and heat, but the total energy is conserved.

• What is entropy? Does the universe increase or decrease in entropy over time? Entropy is a measure of disorder; the universe increases in entropy over time.

• Explain how a bear running demonstrates the second law of thermodynamics: As the bear runs, chemical energy is converted to kinetic energy and heat, increasing the disorder (entropy) of the surroundings.

• Why do living systems require a constant input of energy? To maintain order and support cellular processes; without energy input, systems move toward disorder and death.

Summary Table: Exergonic vs. Endergonic Reactions

Type	ΔG	Spontaneity	Energy Flow	Example
Exergonic	Negative (<0)	Spontaneous	Energy released	Cellular respiration
Endergonic	Positive (>0)	Non-spontaneous	Energy required	Photosynthesis

Additional info: These notes are based on AP/college-level biology content and are suitable for exam preparation on cellular energetics, metabolism, and thermodynamics.