Metabolism and Thermodynamics in Biology: Study Notes

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Metabolism and Metabolic Pathways

Definition and Overview

Metabolism refers to the sum of all chemical reactions that occur within a living organism to maintain life. These reactions are organized into metabolic pathways, which are sequences of enzymatically catalyzed chemical reactions in a cell.

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).
Example: The breakdown of glucose during cellular respiration is a catabolic pathway.

Energy and Chemical Reactions

Energy in Biological Systems

Energy is the capacity to do work or cause change. In biological systems, energy transformations are governed by the laws of thermodynamics.

Exergonic reactions: Release energy; products have less free energy than reactants.
Endergonic reactions: Require an input of energy; products have more free energy than reactants.
Example: ATP hydrolysis is an exergonic reaction; photosynthesis is endergonic.

Free Energy and Spontaneity

The free energy change () of a reaction determines whether it occurs spontaneously.

Spontaneous reactions: Occur without energy input; (exergonic).
Non-spontaneous reactions: Require energy input; (endergonic).
Formula:

= change in enthalpy (total energy)
= temperature in Kelvin
= change in entropy (disorder)

Thermodynamics in Biology

First and Second Laws of Thermodynamics

First Law (Law of Energy Conservation): Energy cannot be created or destroyed, only transformed or transferred.
Second Law: Every energy transfer increases the entropy (disorder) of the universe.
Application: Organisms must obtain energy from their environment to maintain order and carry out life processes.

ATP: The Energy Currency of the Cell

Structure and Function

ATP (Adenosine Triphosphate): Consists of adenine, ribose, and three phosphate groups.
Hydrolysis of ATP releases energy by breaking the terminal phosphate bond:

ATP regeneration: ATP is regenerated from ADP and inorganic phosphate using energy from catabolic reactions.
Importance: ATP powers cellular work by coupling exergonic and endergonic reactions.

Enzymes and Activation Energy

Role of Enzymes

Enzymes: Biological catalysts that speed up chemical reactions by lowering the activation energy barrier.
Enzymes are specific to their substrates and are not consumed in the reaction.
Active site: The region on the enzyme where the substrate binds.
Induced fit: The enzyme changes shape slightly to fit the substrate more closely.

Factors Affecting Enzyme Activity

Temperature, pH, and substrate concentration can affect enzyme activity.
Enzyme activity typically increases with temperature up to an optimum, then decreases due to denaturation.

Enzyme Regulation

Inhibition and Feedback

Competitive inhibitors: Bind to the active site, blocking substrate binding.
Non-competitive inhibitors: Bind elsewhere on the enzyme, changing its shape and reducing activity.
Feedback inhibition: The end product of a metabolic pathway inhibits an enzyme involved earlier in the pathway, regulating the pathway's activity.
Example: Isoleucine inhibits the first enzyme in its biosynthetic pathway when present in high concentrations.

Summary Table: Types of Enzyme Inhibition

Type of Inhibition	Binding Site	Effect on Enzyme	Reversibility
Competitive	Active site	Blocks substrate binding	Usually reversible
Non-competitive	Allosteric site (not active site)	Changes enzyme shape, reduces activity	Usually reversible
Feedback	Usually allosteric site	Regulates pathway by inhibiting enzyme	Reversible

Additional info:

Metabolic pathways are highly regulated to ensure efficient use of resources and energy.
Cells use compartmentalization (organelles) to separate metabolic processes.