Lec 2: Evolution and the Cellular and Chemical Basis of Life

Evolution and the Cellular and Chemical Basis of Life

Fundamental Definitions and Concepts

  • Definition of Life

    • Life is defined by the laws of physics and chemistry.

    • All biological processes are dictated by the atomic and molecular constituents of cells and organisms.

  • Dependence on Water

    • Life depends on water, which is essential for all known living organisms.

    • The search for extraterrestrial life may hinge on discovering water on exoplanets.

  • Key Elements in Life

    • Life on Earth relies on a subset of elements, principally:

    • Hydrogen (H)

    • Oxygen (O)

    • Carbon (C)

    • Nitrogen (N)

    • Phosphorus (P)

Evolutionary Conservation

  • Conservation Across Species

    • There is evolutionary conservation from yeast to human.

    • This indicates that certain genetic mechanisms are preserved through evolution, suggesting a common ancestry.

Molecular Biology and Genetics

  • Cell Cycle Control Gene cdc2

    • Research on fission yeast led to the cloning of a human homologue of the cdc2 gene.

    • This was done by expressing a human cDNA library in fission yeast and selecting clones that could complement a mutant of cdc2.

    • The predicted protein sequence of the human homologue exhibits high similarity to that of the yeast cdc2 gene.

    • Findings suggest that elements controlling the cell cycle are conserved between yeast and humans.

Elements in the Human Body

  • Table of Elements A summary of elements and their percentages in body mass (including water):

    • 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%

  • Trace Elements

    • Elements that contribute less than 0.01% of mass include:

    • Boron (B), Chromium (Cr), Cobalt (Co), Copper (Cu), Fluorine (F), Iodine (I), Iron (Fe), Manganese (Mn), Molybdenum (Mo), Selenium (Se), Silicon (Si), Tin (Sn), Vanadium (V), Zinc (Zn).

Atomic Structure and Matter

  • Atomic Elements

    • Atomic elements serve as the building blocks of matter and cannot be chemically broken down.

    • Compounds

    • Compounds are formed by the combination of atoms in a fixed ratio; for example, NaCl (sodium chloride) consists of one sodium atom and one chlorine atom.

  • Molecules

    • Molecules consist of atoms held together by chemical bonds:

    • Covalent Bonds: Strong bonds resulting from shared electrons.

    • Hydrogen Bonds: Weaker bonds resulting from attraction between polar molecules.

  • Valence Shells of Important Elements

    • Valence electrons in the outer shell dictate chemical bonding behavior:

    • Hydrogen (H): valence = 1

    • Oxygen (O): valence = 2

    • Nitrogen (N): valence = 3

    • Carbon (C): valence = 4

Chemical Bonding

  • Role of Unpaired Electrons

    • Unpaired electrons in the valence shell determine potential chemical bonds.

    • Types of Bonds:

    • Covalent Bonds: Involve sharing of electrons.

      • An example is the formation of a covalent bond between H and O in water (H₂O).

    • Ionic Bonds: Involve the transfer of electrons from one atom to another.

      • Example: Sodium (Na) transfers an electron to Chlorine (Cl) to form Na⁺ and Cl⁻ ions.

  • Ionic Bond Example:

    • Sodium ion (Na⁺) and Chloride ion (Cl⁻) combine to form sodium chloride (NaCl).

Water and its Importance

  • Water as the Essential Element of Life

    • Water is characterized by its polar covalent bond:

    • Oxygen (due to high electronegativity) pulls electrons closer, creating a slight negative charge on O and slightly positive charges on H.

    • Cells are composed of 70-95% water and surrounded by water.

  • Molecular Structure of Water

    • The molecular formula for water is H₂O, characterized by:

    • Space-filling model, electron distribution diagrams, and Lewis dot structure.

    • Each water molecule's polar covalent bonds create a dipole moment with separated charges.

  • Properties of Water

    • Cohesion and Adhesion:

    • Hydrogen bonds provide cohesion, helping plants transport water against gravity.

    • Adhesion allows attraction to other polar molecules.

    • Temperature Moderation:

    • Water has high specific heat and high heat of vaporization which helps stabilize temperatures in organisms and oceans.

    • Solvent Properties:

    • Water is a versatile solvent and can dissolve many substances, exhibiting acid/base properties.

States of Water and Bonding

  • Hydrogen Bonding in Liquid vs. Frozen States

    • In liquid water:

    • Hydrogen bonds continually break and reform.

    • In solid state (ice):

    • Hydrogen bonds are more stable, maintaining a structured lattice.

Ionization and pH Scale

  • Ionization Equilibrium of Water

    • Represents the dissociation of water molecules into hydronium (H₃O⁺) and hydroxide (OH⁻) ions:

    • 2 H₂O ⇌ H₃O⁺ + OH⁻

  • pH Values

    • The pH scale represents the concentration of hydrogen ions

    • Acidic solutions: [H⁺] > [OH⁻] (Acids donate H⁺)

    • Neutral solutions: [H⁺] = [OH⁻] (pH = 7)

    • Basic solutions: [H⁺] < [OH⁻] (Bases donate OH⁻ or accept H⁺)

  • pH Scale Range

    • The scale ranges from 0 (most acidic) to 14 (most basic).

Properties of Water and Temperature Control

  • High Specific Heat

    • Water's specific heat is 1 cal/g/°C, contributing to:

    • High temperature retention, stabilizing ocean temperatures and organismal temperatures, crucial for maintaining homeostasis.