Chemical Composition of Cells: Atoms, Bonds, and Organic Molecules

Chemical Composition of Cells

Introduction

The chemical composition of cells underpins all biological processes. Understanding the types of atoms, their properties, and the bonds they form is essential for studying cell structure and function in General Biology.

Atoms and Elements in Biology

Definitions and Key Concepts

• Matter: Anything that has mass and occupies space. Matter is composed of atoms and subatomic particles.

• Atom: The smallest unit of matter that retains the properties of an element. Atoms consist of protons (positive charge), neutrons (neutral), and electrons (negative charge).

• Element: A substance consisting of a single type of atom that cannot be broken down by chemical reactions. Examples: carbon, hydrogen, oxygen, nitrogen.

• Molecule: Two or more atoms chemically bonded together. Examples: H2O, CO2.

• Compound: A molecule consisting of two or more different elements in a fixed ratio. Example: NaCl (sodium chloride).

Subatomic Structure

• Protons: Positive charge, located in the nucleus, mass ≈ 1 Dalton (Da).

• Neutrons: No charge, located in the nucleus, mass ≈ 1 Da.

• Electrons: Negative charge, orbit the nucleus, negligible mass.

• Atomic number: Number of protons in the nucleus, defines the element.

• Valence electrons: Electrons in the outermost shell, determine chemical reactivity.

Major Elements in Biological Molecules

Abundance and Importance

Biological molecules are primarily composed of four elements: carbon (C), hydrogen (H), oxygen (O), and nitrogen (N). These elements make up approximately 96% of the mass of living organisms.

• Carbon: Forms the backbone of organic molecules due to its ability to make four covalent bonds.

• Hydrogen: Found in water and organic compounds.

• Oxygen: Essential for cellular respiration and water.

• Nitrogen: Key component of amino acids and nucleic acids.

Table: Elements in the Human Body

Element	Symbol	Percentage of Body Mass
Oxygen	O	65%
Carbon	C	18.5%
Hydrogen	H	9.5%
Nitrogen	N	3.3%
Other elements	-	3.7%

Distribution in Earth's Crust vs. Human Body

• Elements abundant in the Earth's crust (e.g., silicon, aluminum) are not the same as those in living organisms.

• Biological importance is determined by the ability of elements to form stable, diverse bonds (especially carbon).

Chemical Bonds in Biology

Types of Chemical Bonds

• Covalent Bonds: Atoms share pairs of electrons. Strong and stable; form the backbone of organic molecules.

• Ionic Bonds: Atoms transfer electrons, resulting in charged ions that attract each other. Example: NaCl.

• Hydrogen Bonds: Weak attractions between a hydrogen atom and an electronegative atom (e.g., oxygen or nitrogen). Important in water and DNA structure.

• Van der Waals Interactions: Weak, transient attractions due to temporary charge differences.

Covalent Bonds: Polar vs. Nonpolar

• Nonpolar Covalent Bonds: Electrons are shared equally between atoms. Example: H2.

• Polar Covalent Bonds: Electrons are shared unequally, creating partial charges. Example: H2O.

Electronegativity

• Electronegativity: The tendency of an atom to attract electrons in a covalent bond.

• Differences in electronegativity lead to polar covalent bonds.

Chemical Reactivity and Stability

Valence Electrons and Bond Formation

• Atoms with unpaired valence electrons are reactive.

• Atoms form bonds to fill their valence shells and achieve stability (octet rule).

$\text{Octet rule: Atoms tend to form bonds until they have 8 electrons in their valence shell}$

Functional Groups in Organic Molecules

Role and Examples

• Functional groups: Specific groups of atoms within molecules that confer particular chemical properties.

• Examples include hydroxyl (-OH), carboxyl (-COOH), amino (-NH2), and phosphate (-PO4).

• Functional groups determine the reactivity and interactions of organic molecules.

Summary Table: Types of Chemical Bonds

Bond Type	Description	Strength	Example
Covalent	Electron sharing between atoms	Strong	H2O, CH4
Ionic	Electron transfer, attraction between ions	Moderate	NaCl
Hydrogen	Weak attraction involving hydrogen	Weak	Between water molecules, DNA base pairs
Van der Waals	Transient, weak attractions	Very weak	Between nonpolar molecules

Applications and Examples

• Water: Polar covalent bonds and hydrogen bonding give water its unique properties (cohesion, adhesion, high specific heat).

• DNA: Hydrogen bonds hold complementary base pairs together.

• Proteins: Structure depends on covalent, ionic, and hydrogen bonds.

Conclusion

Understanding the atomic structure, chemical bonds, and the major elements in biological molecules is foundational for further study in biology. These principles explain the stability, diversity, and reactivity of molecules essential for life.