Concept 6.1 – Metabolism Transforms Matter and Energy

Catabolic and Anabolic Pathways

Metabolism encompasses all chemical reactions in a cell, organized into metabolic pathways. These pathways are classified as either catabolic or anabolic.

• Catabolic pathways: Break down complex molecules into simpler ones, releasing energy (e.g., cellular respiration).

• Anabolic pathways: Build complex molecules from simpler ones, consuming energy (e.g., protein synthesis).

Types of Energy and Thermodynamics

Energy exists in various forms and is governed by the laws of thermodynamics.

• Kinetic energy: Energy of motion.

• Thermal energy: A form of kinetic energy associated with the random movement of atoms or molecules.

• Potential energy: Stored energy due to position or structure.

• Chemical energy: Potential energy available for release in a chemical reaction.

• First Law of Thermodynamics: Energy cannot be created or destroyed, only transformed.

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

Entropy and Order in Living Systems

• Entropy (S): A measure of disorder or randomness. Systems tend to move toward greater entropy.

• Living organisms maintain order by increasing the entropy of their surroundings.

Example:

• During cellular respiration, glucose is broken down, releasing energy and increasing entropy.

Concept 6.2 – Free Energy Change of a Reaction

Free Energy and Spontaneity

Free energy (Gibbs free energy, G) determines whether a reaction is spontaneous.

• Free energy (G): The portion of a system's energy that can perform work at constant temperature and pressure.

• Exergonic reaction: Releases free energy; spontaneous (ΔG < 0).

• Endergonic reaction: Requires input of energy; non-spontaneous (ΔG > 0).

Equation:

• Where = change in free energy, = change in enthalpy (total energy), = temperature in Kelvin, = change in entropy.

Predicting Reaction Direction

• If is negative, the reaction is spontaneous.

• If is positive, the reaction is non-spontaneous.

Example:

• Hydrolysis of ATP is exergonic and spontaneous.

Concept 6.3 – ATP Powers Cellular Work

Structure and Function of ATP

Adenosine triphosphate (ATP) is the primary energy currency of the cell.

• Composed of adenine (a nitrogenous base), ribose (a sugar), and three phosphate groups.

• Energy is stored in the bonds between phosphate groups, especially the terminal phosphate bond.

ATP Hydrolysis and Energy Coupling

• Hydrolysis of ATP releases energy:

• Cells use ATP hydrolysis to drive energetically unfavorable (endergonic) reactions by coupling them to ATP breakdown.

Example:

• Active transport of ions across a membrane is powered by ATP hydrolysis.

Concept 6.4 – Enzymes Speed Up Metabolic Reactions

Enzyme Function and Activation Energy

Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy required.

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

• Enzymes do not change the overall free energy change () of a reaction.

Enzyme Specificity and Mechanism

• Enzymes are specific to their substrates due to the shape of their active site.

• The induced fit model describes how enzyme and substrate change shape to enhance binding.

• Enzymes can be reused for multiple reactions.

Regulation of Enzyme Activity

• Enzyme activity can be regulated by molecules that act as inhibitors or activators.

• Allosteric regulation involves binding of regulators at sites other than the active site, affecting enzyme function.

Example:

• Digestive enzymes such as amylase break down starch into sugars.