Module 1: Chemistry of Life, Major Macromolecules, Membranes and the Cell

Chemistry of Life

Atoms: Structure and Properties

Atoms are the fundamental units of matter, composed of subatomic particles. Understanding atomic structure is essential for studying biological molecules and their interactions.

• Protons: Positively charged particles located in the nucleus of every atom. The number of protons defines the atomic number and the element.

• Neutrons: Electrically neutral particles also found in the nucleus. They contribute to the atom's mass but not its charge.

• Electrons: Negatively charged particles that orbit the nucleus in energy levels (shells). The arrangement of electrons determines chemical reactivity.

• Atoms are stable when their outermost electron shell (valence shell) is full.

• Atoms with incomplete valence shells tend to react to achieve stability.

• Atomic number (Z): Number of protons in an atom.

• Mass number (A): Number of protons plus neutrons in an atom.

Example: Carbon has 6 protons and typically 6 neutrons, so its mass number is 12.

Elements

An element is a substance that cannot be broken down into simpler substances by chemical means. Each element consists of only one type of atom.

• Four major elements make up about 96% of the human body: Carbon (C), Hydrogen (H), Oxygen (O), and Nitrogen (N).

• Other essential elements include: Sodium (Na), Calcium (Ca), Potassium (K), Phosphorus (P), Sulfur (S), Magnesium (Mg), Chlorine (Cl).

• Trace elements (required in small amounts): Iodine (I), Iron (Fe), Copper (Cu), Zinc (Zn).

Isotopes

Isotopes are atoms of the same element (same number of protons) that have different numbers of neutrons, resulting in different mass numbers.

• Example: Carbon-12 (C-12), Carbon-13 (C-13), and Carbon-14 (C-14) are isotopes of carbon.

• Some isotopes are unstable and radioactive (radioisotopes), emitting radiation as they decay.

Molecules and Chemical Bonds

Molecules are combinations of two or more atoms held together by chemical bonds. The type of bond affects the molecule's properties and behavior in biological systems.

Ionic Bonds

• Formed when one atom donates an electron to another, resulting in oppositely charged ions (cations and anions) that attract each other.

• Example: Sodium (Na) donates an electron to chlorine (Cl), forming Na+ and Cl- ions, which combine to form NaCl (table salt).

• Ionic compounds often dissociate in water, forming electrolytes.

Covalent Bonds

• Formed when atoms share pairs of electrons, resulting in strong bonds that do not dissociate in water.

• Single, double, or triple covalent bonds can form depending on the number of shared electron pairs.

• Example: Two hydrogen atoms share electrons with one oxygen atom to form H2O (water).

Hydrogen Bonds

• Weak attractions between a hydrogen atom (covalently bonded to an electronegative atom like oxygen or nitrogen) and another electronegative atom.

• Important for the structure of water, proteins, and DNA.

• Responsible for water's unique properties, such as surface tension and cohesion.

Unit I: Chemistry of Life

Importance of Water

Water is vital for life, making up about two-thirds of total body weight. Its unique properties are essential for biological processes.

• Great solvent: Many substances dissolve in water, facilitating chemical reactions.

• Participates in chemical reactions: Water is involved in hydrolysis and dehydration synthesis.

• High heat capacity: Water absorbs and retains heat, helping maintain stable body temperature.

• Less dense as a solid: Ice floats on liquid water, insulating aquatic environments.

Chemical Structure and Polarity of Water

• Water is a polar molecule: electrons are shared unequally between oxygen and hydrogen, creating partial charges (δ- on oxygen, δ+ on hydrogens).

• This polarity allows water molecules to form hydrogen bonds with each other.

• Water forms hydration spheres around ions and polar molecules, aiding in solubility.

Hydrophilic and Hydrophobic Molecules

• Hydrophilic molecules: Mix well with water (water-soluble, polar). Example: Glucose, ions.

• Hydrophobic molecules: Do not mix with water (nonpolar, not water-soluble). Example: Fats, oils, waxes, cholesterol, triglycerides.

Cohesion and Adhesion

• Cohesion: Water molecules stick to each other due to hydrogen bonding, resulting in surface tension.

• Adhesion: Water molecules stick to other polar or charged surfaces.

Summary Table: Types of Chemical Bonds

Bond Type	How Formed	Strength	Example
Ionic	Transfer of electrons	Moderate (weak in water)	NaCl (table salt)
Covalent	Sharing of electrons	Strong	H2O (water), O2 (oxygen gas)
Hydrogen	Attraction between polar molecules	Weak (individually)	Between water molecules, in DNA

Key Equations

• Atomic number: $\text{Atomic number} = \text{Number of protons}$

• Mass number: $\text{Mass number} = \text{Number of protons} + \text{Number of neutrons}$

Additional info:

• Understanding atomic structure and chemical bonding is foundational for later topics such as macromolecules, membranes, and cellular processes.

• Water's properties are central to all biological systems, influencing everything from enzyme function to membrane structure.