1.5 Molecular interactions and reactions
Organisation and Composition of the Body
Chemical Bonds
Overview of Matter Composition
Atoms: Smallest stable units of matter.
Elements: Pure substances comprising only atoms of the same type (same atomic number).
Molecules and Compounds: Composed of atoms from one or more elements, linked by chemical bonds.
Importance of Chemical Bonds
Chemical bonds are fundamental to the structure and properties of matter.
They are responsible for forming:
Proteins
Carbohydrates
Lipids
Nucleic Acids
Small organic molecules (e.g., drugs)
The interactions among these molecules create a vast range of possible complexes.
Definition of Chemical Bonds
A chemical bond is an attraction that connects atoms or ions.
Bond type depends on the nature of the atoms involved (charge, distance).
Bonds are formed to achieve stability:
Strong vs Weak
Stable vs Temporary
Bonds are continuously formed and broken in living cells.
Types of Chemical Bonds
Intramolecular Bonds
These bonds create molecules and compounds.
Aim: To achieve stability by filling outer electron shells.
Valence Electrons: Electrons in the outermost shell dictate stability.
Atoms form bonds by:
Donating valence electrons
Accepting valence electrons
Sharing valence electrons
Electron shell structure:
1st shell = 2 electrons
2nd shell = 8 electrons
3rd shell = 18 electrons
Types of Intramolecular Bonds
Covalent Bonds
Atoms share electrons to fill their electron shells to gain stability.
Example: Water (H2O).
Classification based on:
Number of shared electrons (single, double, triple).
Single covalent bond
The sharing of one electron pair
Double covalent bond
The sharing of 2 electron pairs
Triple covalent bond
The sharing of 3 electron pairs
Polarity of the bond (polar and non-polar).
Polar Covalent Bond
Electrons are distributed unequally, influenced by the atoms' electronegativity (electronic pull).
In a water molecule (H2O), the oxygen atom has 8 protons, which creates a strong attraction for the shared electrons compared to the single protons of the hydrogen atoms. This results in the electrons being pulled closer to the oxygen.
As a result, a polar molecule is formed, leading to a partial negative charge at the oxygen end and a partial positive charge at the hydrogen ends.
Non-Polar Covalent Bond
Electrons are equally shared between the atoms, resulting in no net electrical charge difference across the molecule.
The distribution of partial charges is symmetrical, leading to a balanced molecular structure.
Ionic Bonds
Involve electron transfer between atoms, forming cations and anions (e.g., Na+ and Cl- in NaCl).
Between 2 oppositely charged ions
Resulting ions are held together by electrostatic forces.
Metallic Bonds
Involve a sea of delocalized electrons among metallic atoms.
Electrons move freely throughout the metallic lattice
Electrostatic attraction between cations and free electrons brings them together
Offers high tensile strength, malleability, and conductivity.
Intermolecular Bonds
Bonds occurring between molecules, influencing physical properties.
Van der Waals Bonds
Attractive forces between a positively charged region on one molecule and a negatively charged region on a neighbouring molecule
Weak attractive forces, including:
London Dispersion Forces:
Temporary dipoles in non-polar molecules.
Electrons are constantly moving, they can cluster in a region, creating a momentary dipole
Can influence neighbouring atoms via electrostatic attraction or repulsion
Dipole-Dipole Forces:
Attraction between polar molecules.
Permanent Dipole
Hydrogen Bonds
A specific (stronger) type of dipole-dipole bond.
Forms between a hydrogen atom bonded to electronegative atoms (N, O, F).
Significantly impacts molecular shape, water properties, and biological structures (e.g., DNA, proteins).
Hydrogen bonds are responsible for many of the unique physical properties of water:
High Boiling Point: Water remains liquid at room temperature, whereas other similar molecules are gaseous due to hydrogen bonding which requires more energy to break the bonds.
High Surface Tension: At the water-air interface, water molecules are more attracted to one another than they are to air, leading to high surface tension.
Ice Floats: When water freezes, hydrogen bonds lock the water molecules in a more open and less dense structure, causing ice to float.
Chemical Reactions and Metabolism
Cells act as chemical factories involving making and breaking chemical bonds.
Essential for:
Energy production
Maintenance and repair
Growth and division
Metabolism: Total sum of all chemical reactions for maintaining homeostasis.
Types of Chemical Reactions
Catabolic Reactions (Decomposition)
Break down large molecules into smaller ones, releasing energy (exergonic).
Catabolic reactions involve the release of heat, some of which are used to maintain body temperature
Example: Nutrient breakdown.
Anabolic Reactions (Synthesis)
Construct larger molecules from smaller ones, requiring energy (endergonic).
Required to replace cellular organelles, enzymes and proteins by combining amino acids to form larger proteins and to store surplus nutrients for future uses.
Example: Protein synthesis from amino acids.
Exchange Reactions
Combine elements of synthesis and decomposition.
Bonds are both formed and broken, chemical energy is absorbed, stored and released.
Reversible Reactions
Some reactions proceed in both directions, reversible under certain conditions (e.g., heat application).
Products can be converted back to the original reactants
Summary of Bonds and Reactions
Intramolecular Bonds: Covalent, ionic, metallic (strong bonds).
Intermolecular Bonds: Van der Waals, hydrogen (weaker bonds).
Chemical Reactions: Include catabolic (energy-releasing) and anabolic (energy-consuming), and exchange reactions.