General Biology: Properties of Life, Chemical Bonds, and Biological Macromolecules

Properties of Life and Experimental Design

Characteristics of Life

Living organisms share several fundamental properties that distinguish them from non-living matter.

• Adaptation to environment: The ability to change over time in response to environmental changes.

• Response to stimuli: Reacting to environmental signals (e.g., light, temperature, chemicals).

• Homeostasis: Maintaining a relatively constant internal environment, such as temperature or pH.

• Heredity: Passing genetic information to the next generation.

Example: Humans sweat to cool down when hot, demonstrating homeostasis.

Experimental Design in Biology

Scientific experiments require careful control and identification of variables.

• Control group: A group not exposed to the experimental variable, used for comparison.

• Independent variable: The factor that is changed or manipulated by the experimenter.

• Dependent variable: The factor that is measured or observed in response to changes in the independent variable.

Example: Testing if a sweetener is toxic to mice: the independent variable is the type of water (with or without sweetener), and the dependent variable is the level of toxicity observed.

Atoms, Elements, and the Periodic Table

Atomic Structure and Valence Electrons

Atoms consist of protons, neutrons, and electrons. The arrangement of electrons, especially in the outermost shell (valence shell), determines chemical properties.

• Valence electrons: Electrons in the outermost shell, important for chemical bonding.

• Cations and anions: Atoms that lose electrons become positively charged (cations); those that gain electrons become negatively charged (anions).

Example: Oxygen has 6 valence electrons and tends to gain 2 electrons to fill its shell, forming an anion.

Periodic Table Organization

The periodic table arranges elements by increasing atomic number and groups elements with similar properties together.

• Groups: Vertical columns; elements in the same group have similar valence electron configurations.

• Periods: Horizontal rows; elements in the same period have the same number of electron shells.

Chemical Bonds and Molecules

Types of Chemical Bonds

Atoms combine to form molecules through different types of chemical bonds.

• Ionic bonds: Formed when electrons are transferred from one atom to another, resulting in oppositely charged ions (e.g., NaCl).

• Covalent bonds: Formed when atoms share electrons. If the sharing is equal, the bond is non-polar; if unequal, the bond is polar.

• Hydrogen bonds: Weak attractions between a hydrogen atom covalently bonded to an electronegative atom (like O or N) and another electronegative atom.

Example: Water molecules are held together by hydrogen bonds, which are weaker than covalent bonds but important for biological structure.

Electronegativity and Bond Polarity

Electronegativity is an atom's ability to attract electrons in a bond.

• If two atoms have the same electronegativity, they form a non-polar covalent bond.

• If there is a difference in electronegativity, a polar covalent bond forms.

Properties of Water and Hydrogen Bonds

• Hydrogen bonds are responsible for water's high boiling point, surface tension, and its role as a universal solvent.

• Hydrogen bonds are weaker than covalent bonds but stronger than van der Waals interactions.

Macromolecules: Structure and Function

Proteins

Proteins are polymers of amino acids linked by peptide bonds.

• Primary structure: Sequence of amino acids.

• Secondary structure: Local folding into alpha helices and beta sheets, stabilized by hydrogen bonds.

• Functional groups: Amino group (-NH2), carboxyl group (-COOH), and side chains (R groups).

Example: The peptide bond is a covalent bond between the carboxyl group of one amino acid and the amino group of another.

Nucleic Acids

Nucleic acids (DNA and RNA) are polymers of nucleotides, which consist of a sugar, a phosphate group, and a nitrogenous base.

• DNA: Double-stranded, contains deoxyribose sugar, bases A, T, C, G.

• RNA: Single-stranded, contains ribose sugar, bases A, U, C, G.

• Phosphodiester bonds: Covalent bonds linking nucleotides in a strand.

• Base pairing: A pairs with T (or U in RNA), C pairs with G via hydrogen bonds.

Example: The complementary sequence to 5'-ATTGG-3' is 3'-TAACC-5'.

Carbohydrates

Carbohydrates are composed of monosaccharides (simple sugars) and serve as energy sources and structural components.

• Monosaccharides: Glucose, fructose, galactose.

• Polysaccharides: Starch, glycogen, cellulose.

Tables and Comparisons

Comparison of Bond Types

Bond Type	Strength	Example	Formation
Ionic	Strong (in dry conditions)	NaCl	Transfer of electrons
Covalent	Very strong	H2O, CO2	Sharing of electrons
Hydrogen	Weak	Between water molecules	Attraction between H and electronegative atom

DNA vs. RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Strands	Double	Single
Bases	A, T, C, G	A, U, C, G
Location	Nucleus	Cytoplasm, nucleus

Key Equations and Concepts

• Number of covalent bonds: Determined by the number of electrons needed to fill the valence shell.

• Hydrogen bonds in DNA: A-T pairs have 2 hydrogen bonds; C-G pairs have 3 hydrogen bonds.

Example calculation:

• For the sequence 5'-ATTGG-3' and its complement 3'-TAACC-5':

• A-T pairs: 2 hydrogen bonds each

• C-G pairs: 3 hydrogen bonds each

Summary

• Life is characterized by properties such as adaptation, response to stimuli, homeostasis, and heredity.

• Atoms form molecules via ionic, covalent, and hydrogen bonds, with properties determined by electron configuration and electronegativity.

• Macromolecules—proteins, nucleic acids, and carbohydrates—are essential for structure and function in living organisms.

• Understanding experimental design, chemical bonding, and macromolecular structure is foundational for biology.