Biology Chapter: The Chemical Basis of Life

The Chemical Basis of Life

• Approximately 25 chemical elements are essential for life.
• Four elements make up about 96% of living matter: carbon (C), hydrogen (H), oxygen (O), and nitrogen (N).
• Four additional elements make up most of the remaining 4%: phosphorus (P), calcium (Ca), sulfur (S), and potassium (K).
• Everything is composed of matter, and matter is made up of atoms.

Bonding Properties

• Atoms will fill their outermost shell (valence shell) with electrons to become stable.
• The chemical behavior of an atom is determined by the number of electrons in its valence shell.
• Electrons determine the chemical behavior of atoms.
• Each electron shell fills from the inside out:
  • 1st shell: 2 electrons
  • 2nd shell: 8 electrons
  • 3rd shell: 8 electrons

Elements and Their Valence Shells

• Elements in the same row (period) of the periodic table have the same number of electron shells.
• Elements in the same column (group) of the periodic table have the same number of valence electrons and similar chemical properties.

Chemical Reactivity

• Atoms tend to:
  • Complete a partially filled valence shell.
  • Empty a partially filled valence shell.

Bonds in Biology

• Strong bonds:

  • Covalent bonds: Two atoms share a pair of electrons.

• Weak bonds:

  • Hydrogen bonds: Occur between a hydrogen atom covalently linked to a highly electronegative atom (like oxygen) and an electronegative atom on a different molecule.
  • Hydrophobic and hydrophilic interactions: These interactions involve the attraction or repulsion of water molecules. Hydrophilic molecules are attracted to water, while hydrophobic molecules are repelled by water.
  • Ionic bonds: Occur when one atom donates an electron to another atom, resulting in the formation of ions (charged atoms).

Covalent Bonds

• Two atoms share a pair of electrons.
• Both atoms hold onto the electrons.
• Forms molecules.

Multiple Covalent Bonds

• Pairs of electrons shared unequally by two atoms.
• Example: Water (H₂O): Oxygen has a stronger attraction for electrons than hydrogen, resulting in a polar molecule with positive and negative poles.

Hydrogen Bonding

• Polar water molecules form attractions between positive hydrogen atoms and negative oxygen atoms.
• Can occur wherever an -OH group exists in a larger molecule.

Life Needs Water

• Water is essential for all life, both inside and outside cells.

Special Properties of Water

• Cohesion and adhesion:

  • Cohesion: H-bonding between water molecules.
  • Adhesion: H-bonding between water molecules and other substances.

• Surface tension: Cohesion creates a strong surface tension.

• Capillary action: Adhesion allows water to climb up narrow spaces.

• Good solvent: Many polar molecules dissolve in water.

• Hydrophilic vs. hydrophobic: Hydrophilic molecules are attracted to water, while hydrophobic molecules are repelled by water.

• Lower density as a solid: Ice floats because water molecules form a crystalline structure when frozen, making it less dense than liquid water.

• High specific heat: Water can absorb a large amount of heat energy before its temperature changes significantly.

• High heat of vaporization: Water requires a significant amount of energy to vaporize because of the strong hydrogen bonds.

Cohesion & Adhesion

• Cohesion: H-bonding between H₂O molecules.
• Adhesion: H-bonding between H₂O and other substances.

Hydrophilic

• Substances attracted to water.
• Polar.

Hydrophobic

• Substances not attracted to water.
• Non-polar.

Ionization of Water & pH

• Water ionizes, meaning it can split into a hydrogen ion (H+) and a hydroxide ion (OH-).
• Neutral water has an equal concentration of H+ and OH-.
• Acidic water has a higher concentration of H+.
• Basic water has a higher concentration of OH-.

pH Scale

• Measures the acidity or basicity of a solution.
• pH 1-7: acidic
• pH 7: neutral
• pH 7-14: basic

Buffers & Cellular Regulation

• Cells need a stable pH of 7.
• pH affects the shape of molecules, which affects their function.
• pH affects cellular function.

Acids, Bases, and Buffers

• Acid: Donates a hydrogen ion (H+).
• Base: Accepts a hydrogen ion (H+).
• Buffer: A solution that resists changes in pH.

The Nature of Biological Molecules

• Biological molecules (biomolecules): Essential for life.
• Four main types:
  • Carbohydrates: Provide energy and structural support.
  • Proteins: Perform functions like enzyme activity, structure, and immune responses.
  • Lipids: Store energy, form cell membranes, and act as signaling molecules.
  • Nucleic Acids: Store and transmit genetic information (DNA and RNA).
• Biomolecules are often polymers (large molecules built from smaller units called monomers) and interact with water.
• Many reactions are catalyzed by enzymes.

Functional Groups

• Hydrocarbons: Organic molecules primarily made up of carbon and hydrogen. Relatively inactive chemically.
• Functional groups: Small groups of atoms within molecules that provide specific chemical properties, making hydrocarbons more soluble and reactive.

Common Types of Functional Groups

• Often contain electronegative atoms (N, P, O, S), which make organic molecules more polar, water-soluble, and reactive.

Classification of Biological Molecules by Function

• Macromolecules: Large, complex molecules essential for cell structure and function.
  • Made up of dozens to millions of carbon atoms.
  • Polymers are large molecules built from smaller units (monomers) through polymerization.
• Building blocks of macromolecules: Include the precursors needed to build new macromolecules.
• Metabolic intermediates (metabolites): Cellular molecules synthesized through metabolic pathways.
• Molecules of miscellaneous function: Molecules with diverse, but less abundant functions, compared to macromolecules and their precursors.

The Four Types of Biological Molecules

Carbohydrates

• Organic molecules composed of carbon, hydrogen, and oxygen.
• Crucial energy sources and structural components.
• Classified based on the number of sugar units:
  • Monosaccharides: Single sugar unit (e.g., glucose, fructose, galactose)
  • Disaccharides: Two monosaccharide units linked (e.g., sucrose, lactose, maltose)
  • Oligosaccharides: 3-10 monosaccharide units.
  • Polysaccharides: Many monosaccharide units (e.g., starch, glycogen, cellulose)

Lipids

• Hydrophobic molecules with diverse functions, including energy storage, cell membrane formation, and signaling.
• Classified into four main types:
  • Fatty acids: Long hydrocarbon chain with a carboxyl group (e.g., palmitic acid, oleic acid)
  • Triglycerides: Three fatty acids attached to a glycerol molecule (e.g., fats, oils)
  • Phospholipids: Two fatty acids and a phosphate group attached to a glycerol backbone (e.g., phosphatidylcholine), key component of cell membranes.
  • Steroids: Four fused carbon rings with various functions (e.g., cholesterol, testosterone, estrogen)

Nucleic Acids

• Primary information carriers in cells, directing protein synthesis.
• Long chains of nucleotides, each comprised of a sugar, phosphate group, and an organic base.
• Two main types:
  • DNA: The genetic blueprint, containing the bases adenine (A), cytosine (C), guanine (G), and thymine (T).
  • RNA: Essential for protein synthesis, containing the bases adenine (A), cytosine (C), guanine (G), and uracil (U).

Proteins

• Complex molecules essential for life, involved in a vast array of functions.
• Hold immense nutritional value.
• Essential for the chemical processes that sustain life.

This quiz covers the essential chemical elements necessary for life and their bonding properties. You will learn about atoms, electron configurations, and the periodic table's organization. Test your knowledge of how these elements contribute to life's chemical foundation.