Chemical Elements, Compounds, and Molecules: Foundations of Life

Chemical Elements and Compounds

Definition and Importance

Understanding the basic building blocks of matter is essential in biology, as all living organisms are composed of chemical elements and compounds. These substances form the foundation for biological structure and function.

• Element: The smallest unit of matter that cannot be broken down by chemical means. Elements consist of protons, neutrons, and electrons.

• Compound: A substance formed by the chemical combination of two or more elements in a fixed ratio. For example, water (H2O) is a compound, not an element.

• Example: Table salt (NaCl) is a compound formed from sodium and chlorine.

The Periodic Table and Biological Elements

Classification by Biological Abundance

The periodic table organizes elements by their properties and abundance in living organisms. Elements are classified into tiers based on their biological significance.

Tier	Color	Examples	Role in Biology
1st tier (most abundant)	Green	H, C, N, O, P, S	Major components of macromolecules
2nd tier	Red	Na, Mg, K, Ca, Cl	Ions for transport and signaling
3rd tier	Blue	Fe, Zn, Cu, Mn, Co, Mo, Se, I	Cofactors for enzymes, electron transport
4th tier	Yellow	Si, F, B, Cr, V, Ni	Trace elements, specialized functions

Additional info: The color coding and tier system help students visualize which elements are most critical for life.

Chemical Bonds in Biology

Types of Chemical Bonds

Atoms combine to form molecules through chemical bonds, which determine the structure and properties of biological molecules.

• Covalent Bonds: Atoms share electrons to achieve stability. These bonds are strong and form the backbone of organic molecules.

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

• Hydrogen Bonds: Weak attractions between a hydrogen atom covalently bonded to an electronegative atom (like oxygen or nitrogen) and another electronegative atom. These are crucial for the structure of water, proteins, and DNA.

• Van der Waals Interactions: Weak attractions between non-polar molecules due to transient charge differences.

Example: The double helix structure of DNA is stabilized by hydrogen bonds between base pairs.

Water: The Solvent of Life

Properties and Biological Significance

Water is essential for all known life forms due to its unique chemical and physical properties.

• Polarity: Water is a polar molecule, allowing it to dissolve ionic and polar substances.

• High Heat Capacity: Water can absorb and retain heat, helping regulate temperature in organisms and environments.

• High Heat of Vaporization: Water requires significant energy to change from liquid to gas, enabling evaporative cooling.

• Ice Floats: Solid water (ice) is less dense than liquid water, allowing aquatic life to survive under frozen surfaces.

• Universal Solvent: Water dissolves a wide range of substances, facilitating chemical reactions and transport in cells.

Property	Biological Importance
Polarity	Dissolves salts, sugars, and other polar molecules
High heat capacity	Stabilizes temperature in organisms and environments
Ice floats	Protects aquatic life in cold climates

Additional info: Water's ability to form hydrogen bonds is the basis for its high heat capacity and solvent properties.

Carbon-Based Life

Why Carbon is the Backbone of Life

Carbon's unique bonding properties make it the central element in organic molecules.

• Four Covalent Bonds: Carbon can form up to four covalent bonds, allowing for complex and diverse molecular structures.

• Versatility: Carbon chains and rings serve as scaffolds for biological molecules.

• Functional Groups: Carbon skeletons are modified by functional groups, which confer specific chemical properties.

Common Functional Groups:

• Alcohols (-OH)

• Carbonyls (C=O)

• Carboxyls (COOH)

• Aminos (-NH2)

• Sulfhydryls (-SH)

• Phosphates (-OPO3)

• Methyls (-CH3)

Example: Amino acids contain amino and carboxyl groups attached to a carbon backbone.

Elements of Life: Scientific Applications

Radioactive Isotopes and Biological Dating

Certain elements have unstable isotopes that decay at predictable rates, which can be used to date biological materials.

• Carbon-14 Dating: Living organisms contain a constant ratio of 14C to 12C. After death, 14C decays, allowing scientists to estimate the time since death.

Equation:

$N(t) = N_0 e^{- rac{t}{ au}}$

Where $N(t)$ is the amount of 14C at time $t$, $N_0$ is the initial amount, and $\tau$ is the mean lifetime.

Lecture Review

• Life is composed primarily of the elements H, C, O, and N.

• Water is the universal medium for life, essential for biological processes.

• Carbon's versatility allows for the complexity of organic molecules.

• Understanding elements and their properties is fundamental to biology.