Metabolism, Free Energy, ATP, and Enzymes: Core Concepts in General Biology

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Metabolism encompasses all chemical reactions within a cell, divided into catabolic (breakdown) and anabolic (biosynthetic) pathways.

Catabolic pathways: Break down complex molecules into simpler ones, releasing energy (e.g., cellular respiration).
Anabolic pathways: Build complex molecules from simpler ones, requiring energy input (e.g., protein synthesis).
Purpose: Catabolic pathways provide energy and building blocks for anabolic pathways.
Example: Glucose breakdown in glycolysis (catabolic); amino acid assembly into proteins (anabolic).

Cells utilize various forms of energy and obey the laws of thermodynamics.

Kinetic energy: Energy of motion (e.g., movement of molecules).
Thermal energy: Energy from random movement of particles; measured as temperature.
Potential energy: Stored energy due to position or structure (e.g., chemical bonds).
Chemical energy: Potential energy available for release in chemical reactions.
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.
Example: Conversion of glucose’s chemical energy to ATP and heat during cellular respiration.

Biological systems maintain order by expending energy, but overall entropy increases.

Entropy (S): Measure of disorder or randomness.
Cells create local order (low entropy) by using energy, but release heat, increasing universal entropy.
Example: Synthesis of macromolecules from monomers requires energy input.

Free energy determines whether a reaction can occur spontaneously.

Free energy (G): Energy available to do work in a system.
Exergonic reaction: Releases free energy; spontaneous ().
Endergonic reaction: Requires input of energy; non-spontaneous ().
Spontaneous reaction: Occurs without energy input; increases entropy.
Equation: Where is change in free energy, is change in enthalpy, is temperature (Kelvin), and is change in entropy.
Example: Hydrolysis of ATP is exergonic; synthesis of glucose is endergonic.

The relationship between the free energies of reactants and products predicts reaction direction.

If products have lower free energy than reactants, the reaction is exergonic and spontaneous.
If products have higher free energy, the reaction is endergonic and requires energy input.
Example: Breakdown of hydrogen peroxide into water and oxygen is exergonic.

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

Chemical structure: Adenine base, ribose sugar, and three phosphate groups.
Energy is stored in the bonds between phosphate groups, especially the terminal phosphate.
Hydrolysis of ATP to ADP + Pi releases energy for cellular work.
Example: Muscle contraction, active transport, and biosynthesis use ATP.

Cells use ATP hydrolysis to drive reactions that would not occur spontaneously.

Energy coupling: Linking exergonic (ATP hydrolysis) and endergonic reactions.
Allows cells to perform work such as synthesis, movement, and transport.
Example: Glucose phosphorylation in glycolysis is coupled to ATP hydrolysis.

Enzymes are biological catalysts that accelerate chemical reactions by lowering activation energy.

Activation energy (Ea): Energy required to initiate a reaction.
Enzymes do not change the free energy change () of a reaction.
Enzymes increase reaction rates, allowing life to proceed at physiological temperatures.
Example: Catalase speeds up breakdown of hydrogen peroxide.

Enzymes are highly specific for their substrates and can be reused for multiple reactions.

Cells regulate enzyme activity to control metabolic pathways.

Activation energy: Lowered by enzyme action.
Enzyme regulators: Molecules that increase or decrease enzyme activity (e.g., inhibitors, activators).
Allosteric regulation: Enzyme activity modulated by binding of regulatory molecules at sites other than the active site.
Example: Feedback inhibition in metabolic pathways.