BIO 141 Chapter 2: The Molecules of Cells – Study Notes

The Molecules of Cells

Matter and Energy

Understanding the molecular basis of cells begins with the concepts of matter and energy, which are foundational to all biological processes.

• Matter: Anything that occupies space and has mass (weight).

• Energy: The ability to do work. In biological systems, energy exists in several forms:

  • Chemical Energy: Stored in molecules such as ATP ($C_6H_{12}O_6$), used for cellular processes.

  • Electrical Energy: Involved in impulses and action potentials within nerve and muscle cells.

  • Mechanical Energy: Actual movement, such as muscle contraction.

  • Radiant Energy: Heat energy, important for maintaining body temperature.

Composition of Matter

Elements

Elements are the fundamental units of matter, and only a few are essential for life.

• Elements: Substances that compose all matter. There are 92 naturally occurring elements, with additional synthetic elements.

• Major Elements in the Human Body: 96% of the body is made from four elements:

  • Carbon (C)

  • Oxygen (O)

  • Hydrogen (H)

  • Nitrogen (N)

• Other important elements include phosphorus and sulfur.

Atoms

Atoms are the building blocks of elements and consist of subatomic particles.

• Nucleus: Contains protons ($p^+$) and neutrons ($n^0$).

• Electrons ($e^-$): Located outside the nucleus in electron shells.

Atomic Structure

Subatomic Particles

Atoms are composed of three main subatomic particles:

• Protons: Positively charged particles in the nucleus.

• Neutrons: Neutral particles in the nucleus.

• Electrons: Negatively charged particles orbiting the nucleus.

Example: A carbon atom has 6 protons, 6 neutrons, and 6 electrons.

Atomic Number and Atomic Mass

• Atomic Number: Number of protons in the nucleus; unique for each element.

• Atomic Mass: Sum of protons and neutrons in the nucleus.

For carbon: $\text{Atomic number} = 6$, $\text{Atomic mass} = 12$

Isotopes and Atomic Weight

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons.

• All isotopes have the same number of protons.

• Isotopes vary in the number of neutrons.

Examples:

• Hydrogen-1: 1 proton, 0 neutrons

• Deuterium (Hydrogen-2): 1 proton, 1 neutron

• Tritium (Hydrogen-3): 1 proton, 2 neutrons

• Carbon-12: 6 protons, 6 neutrons

• Carbon-14: 6 protons, 8 neutrons (used in radioactive dating)

Molecules and Compounds

Definitions

• Molecule: Two or more like or different atoms combined chemically.

• Compound: Two or more different atoms combined chemically.

Example: Water ($H_2O$) is a compound made of two hydrogen atoms and one oxygen atom.

Chemical Reactions and Chemical Bonds

Chemical Bonds

Atoms are united by chemical bonds, which are formed by interactions between electrons in the outer shell (valence shell).

• Ionic Bonds: Formed when electrons are completely transferred from one atom to another, resulting in charged particles called ions.

• Covalent Bonds: Formed when atoms share electrons to achieve stability.

• Hydrogen Bonds: Weak attractions between a hydrogen atom and a negative portion of another molecule.

Atoms dissociate from other atoms when chemical bonds are broken.

Valence Electrons and Bonding

• Bonding involves interactions between electrons in the outermost shell (valence shell).

• Atoms with full valence shells are stable and do not form bonds (inert elements).

• Atoms with incomplete valence shells are reactive and tend to gain, lose, or share electrons to achieve stability.

Inert vs. Reactive Elements

• Inert Elements: Have complete valence shells and are chemically stable (e.g., noble gases).

• Reactive Elements: Have incomplete valence shells and are chemically active, forming bonds to achieve stability.

Summary Table: Types of Chemical Bonds

Bond Type	Description	Example
Ionic	Electrons are transferred from one atom to another, forming ions	NaCl (Sodium chloride)
Covalent	Atoms share electrons to achieve stability	$H_2O$ (Water), $O_2$ (Oxygen gas)
Hydrogen	Weak attraction between hydrogen and a negative portion of another molecule	Between water molecules

Key Equations

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

Additional info:

• These foundational concepts are essential for understanding the molecular structure and function of cells in anatomy and physiology.

• Further topics in this chapter likely include the properties of water, acids and bases, organic molecules (carbohydrates, lipids, proteins, nucleic acids), and their roles in cellular function.