The Chemical Context of Life: Foundations for General Biology

The Chemical Context of Life

Introduction to Chemistry in Biology

Understanding biology requires a foundation in chemistry, as all living organisms are composed of matter and undergo chemical processes. The study of atoms, elements, and molecules is essential for grasping how cells are built and how life functions at the molecular level.

• Matter: Anything that occupies space and has mass.

• Element: A substance that cannot be broken down into other substances by chemical reactions. There are 92 natural elements.

• Atom: The smallest unit of matter that retains the properties of an element.

• Subatomic particles: Atoms are composed of protons, neutrons, and electrons.

• Essential elements: About 25 elements are essential for life, with a few (such as carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur) making up the majority of living matter.

Elements of Life

Living organisms are primarily composed of a small subset of elements. The most abundant elements in the human body are:

• Oxygen (O)

• Carbon (C)

• Hydrogen (H)

• Nitrogen (N)

• Phosphorus (P)

• Sulfur (S)

Trace elements such as copper (Cu) and iodine (I) are required in minute amounts but are vital for proper biological function.

Atomic Structure

Atoms consist of a nucleus containing protons and neutrons, surrounded by electrons in electron shells.

• Proton: Positively charged particle found in the nucleus.

• Neutron: Neutral particle found in the nucleus.

• Electron: Negatively charged particle found in electron shells around the nucleus.

• Atomic number: Number of protons in an atom; defines the element.

• Mass number: Total number of protons and neutrons in an atom.

Example: Carbon has 6 protons, 6 neutrons, and 6 electrons.

Isotopes and Radioactivity

Isotopes are different forms of an element with the same number of protons but different numbers of neutrons.

• Stable isotopes: Do not change over time.

• Radioisotopes: Unstable isotopes that decay, emitting radiation.

Applications of radioactive isotopes include dating fossils, tracing atoms through metabolic processes, and medical diagnostics.

Electron Shells and Energy Levels

Electrons are arranged in shells around the nucleus. The chemical behavior of an atom is determined by the distribution of electrons in these shells.

• The first shell can hold up to 2 electrons.

• Subsequent shells can hold up to 8 electrons each.

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

Atoms are most stable when their outermost shell is full.

Chemical Bonds

Atoms bond to achieve full valence shells, resulting in the formation of molecules. The main types of chemical bonds are:

• Covalent bonds: Electrons are shared between atoms.

• Polar covalent bonds: Electrons are shared unequally, leading to partial charges.

• Ionic bonds: Electrons are transferred from one atom to another, creating charged ions (cations and anions) that attract each other.

• Hydrogen bonds: Weak attractions between a hydrogen atom in one molecule and an electronegative atom in another.

• Van der Waals interactions: Weak attractions due to transient local charges.

Electronegativity

Electronegativity is the tendency of an atom to attract electrons in a covalent bond. Oxygen is highly electronegative, making water a polar molecule.

Ionic Bonds Example

Formation of sodium chloride (NaCl):

• Sodium (Na) loses an electron to become Na+ (cation).

• Chlorine (Cl) gains an electron to become Cl- (anion).

• Na+ and Cl- attract to form NaCl.

Hydrogen Bonds Example

Hydrogen bonds are crucial in water and biological molecules like DNA. In water, the partial positive charge of hydrogen is attracted to the partial negative charge of oxygen in another molecule.

Molecular Shape and Function

The shape of a molecule is determined by the arrangement of its atoms and the types of bonds present. Molecular shape is critical for biological function, such as enzyme-substrate interactions and receptor binding.

• Example: The double helix structure of DNA allows for the storage and transmission of genetic information.

Practice Questions

• What is lithium’s atomic mass?

• How many protons does lithium have?

• How many electrons?

• How many neutrons?

• What is its atomic number?

• How many electrons are in its valence shell?

• What are these different forms of lithium called?

• Which is the anion, Mg or Cl?

Summary Table: Types of Chemical Bonds

Bond Type	Description	Example
Covalent	Electrons are shared between atoms	H2O (water)
Polar Covalent	Electrons are shared unequally	H2O (water)
Ionic	Electrons are transferred; ions attract	NaCl (sodium chloride)
Hydrogen	Weak attraction between H and electronegative atom	Between water molecules
Van der Waals	Weak, transient attractions	Between nonpolar molecules

Key Equations

• Atomic number: $\text{Atomic number} = \text{Number of protons}$

• Mass number: $\text{Mass number} = \text{Number of protons} + \text{Number of neutrons}$

Additional info: The images provided (DNA double helix, ants) illustrate the molecular basis of life and the importance of chemical interactions in biological systems.