Biology Unit 1: The Chemistry of Life – Elements, Water, and Biological Molecules

Elements of Life

Atomic Structure

Atoms are the fundamental units of matter, composed of protons, neutrons, and electrons. The arrangement of these subatomic particles determines the chemical properties of each element.

• Protons: Positively charged particles found in the nucleus; the number of protons defines the atomic number.

• Neutrons: Neutral particles also located in the nucleus; they contribute to atomic mass.

• Electrons: Negatively charged particles that orbit the nucleus in energy levels (shells).

• Energy Levels: The first shell holds up to 2 electrons, the second up to 8, and so on.

• Valence Electrons: Electrons in the outermost shell; they determine chemical reactivity.

Example: Carbon has 6 protons, 6 neutrons, and 6 electrons (2 in the first shell, 4 in the second shell).

Chemically Important Elements

Although the periodic table contains many elements, only a subset is essential for life. These elements are involved in the structure and function of biological molecules.

• Major Elements: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Phosphorus (P), and Sulfur (S).

• Trace Elements: Elements required in small amounts, such as Iron (Fe), Iodine (I), and Zinc (Zn).

Example: Oxygen is crucial for cellular respiration, while iron is necessary for oxygen transport in blood.

Chemical Bonds

Atoms form chemical bonds to achieve stable electron configurations. The type of bond depends on the difference in electronegativity between atoms.

• Covalent Bonds: Atoms share electrons. Can be nonpolar (equal sharing) or polar (unequal sharing).

• Ionic Bonds: Electrons are transferred from one atom to another, creating charged ions.

• Electronegativity: The ability of an atom to attract electrons in a bond. Determines bond polarity.

Example: In water (H2O), oxygen is more electronegative than hydrogen, resulting in polar covalent bonds.

Bond Type	Electronegativity Difference	Example
Nonpolar Covalent	< 0.5	CH4 (Methane)
Polar Covalent	0.5 – 2.0	H2O (Water)
Ionic	> 2.0	NaCl (Sodium Chloride)

Water

Structure and Properties

Water is a polar molecule with unique properties essential for life. Its structure allows for hydrogen bonding, which leads to several emergent properties.

• Polarity: Oxygen is partially negative, hydrogens are partially positive.

• Hydrogen Bonds: Weak attractions between the hydrogen of one water molecule and the oxygen of another.

• Cohesion: Water molecules stick to each other.

• Adhesion: Water molecules stick to other surfaces.

• Surface Tension: Result of cohesion and adhesion; allows small objects to float.

Example: Water transport in plants relies on cohesion and adhesion to move water upward through xylem.

Thermal Properties

Water has a high specific heat, meaning it can absorb or release large amounts of heat with little temperature change.

• High Specific Heat: Moderates Earth's climate and stabilizes temperatures in organisms.

• Expansion Upon Freezing: Ice is less dense than liquid water, so bodies of water freeze from the top down.

Example: Coastal cities have milder climates due to the heat-absorbing capacity of nearby oceans.

pH and Acidity

The pH scale measures the concentration of hydrogen ions (H+) in a solution.

• Acidic Solutions: pH < 7; higher concentration of H+.

• Basic Solutions: pH > 7; higher concentration of OH-.

• Neutral Solution: pH = 7; pure water.

Formula:

$pH = -\log_{10}[H^+]$

Example: Human blood has a pH of about 7.4, which is slightly basic.

Biological Molecules

Carbon and Organic Chemistry

Carbon is the backbone of biological molecules due to its ability to form four covalent bonds, resulting in diverse structures.

• Tetrahedral Geometry: Carbon forms a three-dimensional shape with four bonds.

• Variety of Structures: Chains, rings, and branches.

Example: Methane (CH4) is a simple carbon compound with tetrahedral geometry.

Carbohydrates

Carbohydrates are energy-rich molecules composed of carbon, hydrogen, and oxygen. They exist as monomers (monosaccharides) and polymers (polysaccharides).

• Monomer: Monosaccharide (e.g., glucose, fructose).

• Polymer: Disaccharide (e.g., sucrose) and polysaccharide (e.g., starch, cellulose).

• General Formula: Cn(H2O)n

• Function: Quick energy (monomers), energy storage (polymers), and structural support (cellulose).

Type	Example	Function
Monosaccharide	Glucose	Quick energy
Disaccharide	Sucrose	Transported energy
Polysaccharide	Starch, Glycogen, Cellulose	Energy storage, structure

Lipids

Lipids are hydrophobic molecules primarily composed of carbon and hydrogen. They include fats, phospholipids, and steroids.

• Monomer: Fatty acid

• Polymer: Triglyceride, phospholipid

• Saturated Fatty Acids: All single C-C bonds; solid at room temperature.

• Unsaturated Fatty Acids: One or more C=C double bonds; liquid at room temperature.

• Function: Long-term energy storage, insulation, waterproofing, cell membrane structure (phospholipids), signaling (steroids).

Example: Phospholipids form the bilayer of cell membranes, with hydrophilic heads and hydrophobic tails.

Proteins

Proteins are polymers of amino acids, responsible for most cellular functions. Their structure determines their function.

• Monomer: Amino acid

• Polymer: Polypeptide/protein

• Levels of Structure: Primary (sequence), secondary (alpha helix, beta sheet), tertiary (3D folding), quaternary (multiple polypeptides).

• Function: Enzymes, structural support, transport, signaling, immune response.

Example: Hemoglobin is a protein that transports oxygen in blood.

Nucleic Acids

Nucleic acids store and transmit genetic information. They are polymers of nucleotides.

• Monomer: Nucleotide (composed of a sugar, phosphate group, and nitrogenous base)

• Polymer: DNA, RNA

• Function: DNA stores genetic information; RNA retrieves and uses this information for protein synthesis.

Example: DNA is double-stranded and forms a double helix; RNA is usually single-stranded.

Type	Monomer	Polymer	Function
Carbohydrate	Monosaccharide	Polysaccharide	Energy, structure
Lipid	Fatty acid	Triglyceride, phospholipid	Energy storage, membranes
Protein	Amino acid	Polypeptide	Catalysis, structure, transport
Nucleic Acid	Nucleotide	DNA, RNA	Information storage/retrieval

Additional info: These notes expand on the slide headings and brief points, providing academic context and examples for each major topic in introductory biology.