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.

• Catabolic pathways break down molecules and release energy (e.g., cellular respiration).

• Anabolic pathways build complex molecules from simpler ones and consume energy (e.g., protein synthesis).

• Example: The breakdown of glucose in glycolysis is a metabolic pathway that produces ATP.

Energy in Biological Systems

Types and Descriptions

Energy is the capacity to do work or cause change. In biological systems, energy exists in several forms:

• Kinetic energy: Energy of motion (e.g., movement of molecules).

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

• Thermal energy: Energy associated with random movement of atoms or molecules; related to temperature.

• Chemical energy: Potential energy stored in chemical bonds of molecules (e.g., glucose, ATP).

• Example: ATP contains chemical energy that cells use for work.

Thermodynamics in Biology

First and Second Laws

• First Law of Thermodynamics: Energy cannot be created or destroyed, only transformed from one form to another. (where is change in internal energy, is heat added, is work done by the system)

• Second Law of Thermodynamics: Every energy transfer increases the entropy (disorder) of the universe. Energy transformations are not 100% efficient; some energy is lost as heat.

• Example: During cellular respiration, some energy from glucose is lost as heat.

Free Energy and Biological Reactions

Definition and Impact

Free energy (G) is the portion of a system's energy that can perform work when temperature and pressure are uniform.

• Exergonic reactions: Release free energy; spontaneous ().

• Endergonic reactions: Require input of energy; non-spontaneous ().

• Equation: (where is change in enthalpy, is temperature in Kelvin, is change in entropy)

• Free energy graphs: Exergonic reactions show a decrease in free energy; endergonic reactions show an increase.

• Example: Hydrolysis of ATP is exergonic.

ATP and Reaction Coupling

Role and Mechanism

• ATP (Adenosine Triphosphate): The primary energy currency of the cell.

• Importance: ATP couples exergonic and endergonic reactions, allowing energy released from one reaction to drive another.

• Example: ATP hydrolysis provides energy for muscle contraction.

ATP: Energy Provision

How ATP Provides Energy

• ATP releases energy when its terminal phosphate bond is broken by hydrolysis.

• The reaction:

• This energy is used to power cellular processes such as active transport and biosynthesis.

Activation Energy in Chemical Reactions

Definition and Graphs

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

• Reactions with high activation energy proceed slowly unless catalyzed.

• Free energy graphs: Show reactants, activation energy peak, and products. Enzymes lower the activation energy barrier.

• Example: Enzymes in glycolysis lower activation energy for glucose breakdown.

Enzymes and Reaction Rates

Definition and Function

• Enzyme: A biological catalyst, usually a protein, that speeds up chemical reactions without being consumed.

• Enzymes lower activation energy, increasing reaction rates.

• Mechanism: Enzymes bind substrates at the active site, facilitating the conversion to products.

• Example: Amylase catalyzes the breakdown of starch into sugars.