Elements of Life

Major Elements in Living Organisms

Life on Earth is primarily composed of a few key elements. These elements are essential for the structure and function of biological molecules.

• Oxygen (O): Makes up about 65% of the human body by mass. Essential for cellular respiration and water.

• Carbon (C): Accounts for approximately 18%. Forms the backbone of organic molecules.

• Hydrogen (H): About 10%. Found in water and most organic compounds.

• Nitrogen (N): About 3%. Key component of amino acids and nucleic acids.

• Other elements: Calcium, phosphorus, and trace elements also play important roles.

Example: The pie chart below shows the percentage by mass of major elements in the human body.

Carbon: The Foundation of Organic Chemistry

Properties of Carbon

Carbon is the central element in organic chemistry due to its unique bonding properties.

• Organic chemistry: The study of compounds with covalently bonded carbon.

• Organic compounds: Molecules containing both carbon and hydrogen.

• Valence electrons: Carbon has 4 valence electrons, allowing it to form up to four covalent bonds.

• Bonding versatility: Carbon can form single, double, or triple covalent bonds.

Example: Methane (CH4) has four single covalent bonds; ethylene (C2H4) has a double bond between carbons.

Carbon's Bonding and Molecular Structure

The type and number of covalent bonds carbon forms with other atoms affect the length and shape of the carbon chain, influencing the properties of the molecule.

• Single bonds: Allow for free rotation and formation of long chains.

• Double and triple bonds: Restrict rotation and create planar or linear structures.

• Common bonding partners: Hydrogen, oxygen, nitrogen.

Example: Carbon can form molecules such as ethane (C2H6), ethylene (C2H4), and acetylene (C2H2).

Carbon Chains and Hydrocarbons

Formation and Diversity of Carbon Skeletons

Carbon atoms can link together to form chains, which serve as the backbone for most organic molecules.

• Hydrocarbons: Organic molecules consisting only of carbon and hydrogen.

• Skeleton variations:

  • Length

  • Branching

  • Double bond position

  • Presence of rings

• Many regions of a cell's organic molecules contain hydrocarbons.

Example: Butane (C4H10) is a straight-chain hydrocarbon; isobutane is a branched isomer.

Functional Groups in Organic Molecules

Role and Types of Functional Groups

Functional groups are chemical groups attached to the carbon skeleton that participate in chemical reactions and confer specific properties to molecules.

• Hydroxyl group (-OH): Found in alcohols.

• Carbonyl group (C=O): Found in aldehydes and ketones.

• Carboxyl group (-COOH): Found in organic acids.

• Amino group (-NH2): Found in amino acids.

• Sulfhydryl group (-SH): Found in some amino acids.

• Phosphate group (-PO4): Found in nucleic acids and ATP.

Example: The carboxyl group in acetic acid gives it acidic properties.

Molecular Diversity and Macromolecules

Macromolecules and Their Formation

Variations in carbon skeletons allow for molecular diversity. Carbon forms large molecules known as macromolecules, which are essential for life.

• Macromolecules: Large molecules made of smaller subunits.

• Four major classes:

  • Carbohydrates

  • Proteins

  • Nucleic acids

  • Lipids (not true polymers)

• Polymers: Chain-like macromolecules of similar or identical repeating units (monomers) covalently bonded together.

Example: Starch is a polymer made of glucose monomers.

Formation and Breakdown of Polymers

Polymers are formed and broken down by specific chemical reactions.

• Dehydration reaction: Bonds two monomers together by removing a molecule of water.

• Hydrolysis: Breaks the bond between monomers by adding a molecule of water.

Example: To completely hydrolyze a polymer of 30 glucose monomers, 29 water molecules are required (one less than the number of monomers).

Concept Check

Properties of Carbon Essential for Life

Carbon's ability to form four covalent bonds, create diverse structures, and bond with many elements makes it indispensable for life.

• Forms stable, complex molecules

• Allows for molecular diversity

• Central to organic chemistry

Comparison with Silicon: Silicon is directly below carbon in the periodic table and shares some chemical properties, but it is less versatile in forming stable, complex molecules necessary for life.

Additional info: The study notes have expanded on the brief points and images to provide a comprehensive overview suitable for General Biology students, including definitions, examples, and academic context.