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Chapter 2: The Chemical Context of Life – Study Notes

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Chapter 2: The Chemical Context of Life

Introduction to Chemistry in Biology

Chemistry forms the foundation for understanding biological processes. All living organisms rely on chemical principles to survive, grow, and interact with their environment. For example, Formica rufa (wood ants) use formic acid, a chemical compound, for defense.

  • Key Point: Biological phenomena are governed by chemical interactions and properties.

  • Example: Wood ants shoot formic acid to ward off enemies.

The Elements of Life

Life depends on a select group of elements out of the 92 naturally occurring ones. These elements are classified as essential, with a few making up the majority of living matter.

  • Essential Elements: About 20–25% of elements are required for life.

  • Major Elements: Carbon (C), Hydrogen (H), Oxygen (O), and Nitrogen (N) constitute 96% of living matter.

  • Minor Elements: Calcium (Ca), Phosphorus (P), Potassium (K), and Sulfur (S) make up most of the remaining 4%.

  • Trace Elements: Required in minute quantities (e.g., iron, iodine).

Element

Symbol

Percentage of Body Mass

Oxygen

O

65.0%

Carbon

C

18.5%

Hydrogen

H

9.5%

Nitrogen

N

3.3%

Calcium

Ca

1.5%

Phosphorus

P

1.0%

Potassium

K

0.4%

Sulfur

S

0.3%

Sodium

Na

0.2%

Chlorine

Cl

0.2%

Magnesium

Mg

0.1%

Atoms and Atomic Structure

Atoms are the basic units of matter, composed of protons, neutrons, and electrons. The atomic number is the number of protons, while atomic mass is the sum of protons and neutrons.

  • Electron Shells: Electrons occupy shells with distinct energy levels and distances from the nucleus.

  • Energy Absorption and Loss: Electrons can move to higher shells by absorbing energy and fall to lower shells by releasing energy.

  • CHNOPS: The outer electron shells of Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, and Sulfur are crucial for biological molecules.

Electron Distribution and Chemical Properties

The arrangement of electrons in shells determines an atom's chemical behavior. The periodic table reflects this distribution, with each row corresponding to the sequential addition of electrons and protons.

  • Valence Electrons: Electrons in the outermost shell (valence shell) are key to chemical reactivity.

  • Chemically Inert Elements: Atoms with full valence shells do not readily react.

  • Atoms Interact: Atoms tend to interact to complete their valence shells.

Chemical Bonds

Atoms form chemical bonds to achieve stable electron configurations. The main types of bonds in biology are covalent, ionic, hydrogen, and van der Waals interactions.

Covalent Bonds

  • Covalent Bond: Sharing of a pair of valence electrons between two atoms.

  • Single Bond: Sharing one pair of electrons.

  • Double Bond: Sharing two pairs of electrons.

  • Example: , , ,

Molecule

Electron Distribution

Lewis Structure

Space-Filling Model

Hydrogen ()

H-H

H:H

Two spheres

Oxygen ()

O=O

O::O

Two red spheres

Water ()

H-O-H

H:O:H

One red, two white spheres

Methane ()

H-C-H

H:C:H

One black, four white spheres

Electronegativity and Bond Polarity

  • Electronegativity: An atom's ability to attract electrons in a covalent bond.

  • Nonpolar Covalent Bond: Electrons are shared equally.

  • Polar Covalent Bond: Electrons are shared unequally, resulting in partial charges ( and ).

  • Example: Water () is a polar molecule.

Ionic Bonds

  • Ionic Bond: Attraction between oppositely charged ions formed when electrons are transferred.

  • Cation: Positively charged ion.

  • Anion: Negatively charged ion.

  • Example: Sodium chloride () forms from and ions.

  • Bond Strength: Ionic bonds are stronger in air (chemists' view), but covalent bonds are stronger in aqueous solutions (biologists' view).

Weak Chemical Interactions

  • Weak Bonds: Many biological molecules are stabilized by weak bonds, which allow flexibility and reversibility.

  • Hydrogen Bonds: Form when a hydrogen atom covalently bonded to one electronegative atom is attracted to another electronegative atom (usually O or N).

  • Van der Waals Interactions: Weak attractions due to transient local partial charges; collectively can be strong (e.g., gecko's toe hairs).

Chemical Reactions

Chemical reactions involve the making and breaking of chemical bonds, transforming reactants into products.

  • Reactants: Starting molecules in a chemical reaction.

  • Products: Resulting molecules after the reaction.

  • Example Equation:

  • Photosynthesis: An essential biological reaction:

    • Reactants:

    • Products:

    • Equation:

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