BIOLOGY 2e Chapter 2: The Chemical Foundation of Life

Celebrating Biologists

• Adriana Chader, MD/PhD

• Lydia Villa-Komaroff, PhD

• Bernardo Alberto Houssay, MD

Atoms: The Building Blocks of Molecules

Matter and Elements

• Matter: Anything that occupies space and has mass.

  • Living organisms are entirely composed of matter.

• Elements: Unique forms of matter that cannot be broken down into smaller substances by ordinary chemical means.

  • Possess specific chemical properties.

  • Possess specific physical properties.

  • Each element is designated by a unique chemical symbol (one or two letters), e.g., Sulfur = S, Calcium = Ca.

Essential Elements for Life

• The four most common elements in living organisms are:

  • Carbon (C)

  • Hydrogen (H)

  • Oxygen (O)

  • Nitrogen (N)

• Comparison of Elements in Living Organisms (Humans) vs. Atmosphere and Earth's Crust:

  • Oxygen (O): $65\%$ in life, $21\%$ in the atmosphere, $46\%$ in Earth's crust.

  • Carbon (C): $18\%$ in life, trace in atmosphere, trace in Earth's crust.

  • Hydrogen (H): $10\%$ in life, trace in atmosphere, $0.1\%$ in Earth's crust.

  • Nitrogen (N): $3\%$ in life, $78\%$ in atmosphere, trace in Earth's crust.

Atomic Structure

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

• Two primary regions within an atom:

  1. Nucleus: The central part of the atom, containing protons and neutrons.

  2. Outermost Region (Orbitals): Holds electrons in orbit around the nucleus.

• Sub-atomic particles: Protons, neutrons, and electrons are the fundamental particles that make up atoms.

Key Properties of Sub-atomic Particles

Atomic Number vs. Atomic Mass

• Atomic Number:

  • The number of protons in an atom's nucleus.

  • Each element has a distinct and unique atomic number.

  • It defines the element.

• Atomic Mass:

  • The total mass of an atom, roughly determined by the sum of its protons and neutrons.

  • Electrons have negligible mass and are generally not included in atomic mass calculations.

  • Expressed in atomic mass units (amu) or grams per mole ($g/mol$).

• Calculating Neutrons: The number of neutrons in an atom can be calculated by subtracting the atomic number (number of protons) from the atomic mass.

  • $\text{Number of Neutrons} = \text{Atomic Mass} - \text{Atomic Number}$

• Example (Carbon):

  • Carbon has an atomic number of $6$ (meaning $6$ protons).

  • Carbon has stable isotopes with mass numbers of $12$ and $13$.

  • Carbon-12: Atomic mass is approximately $12.011$ amu.

    • Number of neutrons: $12 - 6 = 6$

  • Carbon-13: Has $7$ neutrons.

Isotopes

• Definition: Atoms of the same element that have the same number of protons (same atomic number) but different numbers of neutrons, resulting in different mass numbers (and thus different atomic masses).

• Examples of Hydrogen Isotopes:

  • $^{1}H$ (Protium): Has $0$ neutrons.

  • $^{2}H$ (Deuterium): Has $1$ neutron.

  • $^{3}H$ (Tritium): Has $2$ neutrons.

• Some isotopes are stable, while others (radioisotopes) are unstable.

  • For example, Hydrogen-2 is stable, whereas Hydrogen-4 is unstable.

Radioisotopes as Research Tools

• Radioisotopes: Isotopes that are unstable and emit energy (in the form of subatomic particles) as they decay over time to a more stable form.

• Half-life: The time it takes for half of the original concentration of a radioisotope to decay back to its more stable form.

  • Example: The half-life of Carbon-14 ($^{14}C$) is $5,730$ years.

• Radiometric Dating (Carbon Dating): A technique that utilizes the known decay rates (half-lives) of radioisotopes to estimate the age of objects.

  • Mechanism: Living organisms continuously exchange carbon with the atmosphere, maintaining a ratio of $^{14}C$ to $^{12}C$ similar to the atmosphere.

  • Upon death, this exchange stops, and the $^{14}C$ within the organism begins to decay to Nitrogen-14 ($^{14}N$).

  • Researchers can compare the $^{14}C$ remaining in a fossil or artifact to the atmospheric $^{14}C$ levels to estimate its age.

    • If $^{14}C$ in a dead organism A $= 0.5$ x (Atmospheric $^{14}C$), it's one half-life old ($5,730$ years).

    • If $^{14}C$ in a dead organism B $= 0.25$ x (Atmospheric $^{14}C$), it's two half-lives old ($11,460$ years).

    • If $^{14}C$ in a dead organism C $= 0.125$ x (Atmospheric $^{14}C$), it's three half-lives old ($17,190$ years).

  • Application: Carbon dating is effective for carbon-containing remains (like the pygmy mammoth) that are less than approximately $50,000$ years old.

  • For older objects: Other isotopes with longer half-lives are used, such as uranium (which decays to lead), potassium, or rubidium.

The Periodic Table

• The periodic table is an organized display of all known elements.

• It provides key information for each element:

  • The atomic number (number of protons) typically appears above the chemical symbol.

  • The approximate atomic mass typically appears below the chemical symbol.

Molecules

• Molecules: Formed when two or more atoms are chemically bonded together.