General Biology Study Guide: The Chemical Context and Molecular Diversity of Life

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Chapter 2: The Chemical Context of Life

Chemical Elements and Compounds

Understanding the basic chemical principles is essential for studying biological systems. Living organisms are composed of elements and compounds, each with unique properties.

Element: A substance that cannot be broken down into other substances by chemical means.
Compound: A substance consisting of two or more different elements combined in a fixed ratio. Compounds can be broken down into their constituent elements.

Exploring Life on Its Many Levels

Six elements make up about 96% of living matter. These are essential for the structure and function of biological molecules.

Carbon (C)
Nitrogen (N)
Oxygen (O)
Phosphorus (P)
Sulfur (S)
Hydrogen (H)

Atoms and Molecules

Atoms are the basic units of matter, and their structure determines the properties of elements and compounds.

Atomic number: Number of protons in an atom.
Mass number: Sum of protons and neutrons in an atom.
Isotopes: Atoms of the same element with different numbers of neutrons.
Valence electrons: Electrons in the outermost shell, important for chemical bonding.
Radioactive isotopes: Unstable isotopes that decay, emitting radiation; used in biological research and medicine.
Covalent bond: A chemical bond formed when two atoms share one or more pairs of electrons.
Ionic bond: A bond formed by the transfer of electrons from one atom to another, resulting in oppositely charged ions.
Hydrogen bond: A weak bond between a hydrogen atom covalently bonded to an electronegative atom and another electronegative atom.
Van der Waals interactions: Weak attractions between molecules or parts of molecules that result from transient local partial charges.
Electronegativity: The tendency of an atom to attract electrons in a covalent bond.

Example: Water molecules are held together by polar covalent bonds and form hydrogen bonds with each other, giving water its unique properties.

Effects of Water's Polarity

Water's structure and polarity give rise to its unique properties, which are essential for life.

Polarity: Water is a polar molecule with a partial negative charge near the oxygen atom and a partial positive charge near the hydrogen atoms.
Hydrogen bonding: The polarity of water molecules allows them to form hydrogen bonds with each other.
Cohesion: Water molecules stick together due to hydrogen bonding.
Adhesion: Water molecules can also stick to other substances.
Surface tension: The cohesive forces at the surface of water create a "skin" that resists external force.

Properties of Water

Ability to moderate temperature: Water can absorb or release large amounts of heat with only a slight change in its own temperature due to its high specific heat.
Expansion upon freezing: Water expands as it freezes, making ice less dense than liquid water.
Versatility as a solvent: Water can dissolve a wide variety of substances due to its polarity.
Specific heat: The amount of heat required to raise the temperature of 1 gram of water by 1°C is high, allowing water to buffer temperature changes.
High heat of vaporization: Water requires a large amount of energy to change from liquid to gas, which helps organisms cool off through evaporation.

Table: Properties of Water and Their Biological Importance

Property	Biological Importance
Cohesion	Helps transport water against gravity in plants
Adhesion	Helps water stick to cell walls, aiding transport
High specific heat	Stabilizes temperature in organisms and environments
Expansion upon freezing	Prevents bodies of water from freezing solid, protecting aquatic life
Versatile solvent	Facilitates chemical reactions in cells

Solutions, Solvents, and Solutes

Water is known as the "universal solvent" because it can dissolve many substances, facilitating chemical reactions in living organisms.

Solution: A homogeneous mixture of two or more substances.
Solvent: The dissolving agent (e.g., water).
Solute: The substance that is dissolved.
Aqueous solution: A solution in which water is the solvent.
Hydrophilic: Substances that have an affinity for water and dissolve easily.
Hydrophobic: Substances that do not have an affinity for water and do not dissolve easily.

Dissociation of Water Molecules

Water can dissociate into ions, which is important for many biological processes.

Dissociation of water: Water molecules can break apart into hydrogen ions (H+) and hydroxide ions (OH-).

Equation:

Acids: Substances that increase the hydrogen ion concentration of a solution (pH < 7).
Bases: Substances that reduce the hydrogen ion concentration of a solution (pH > 7).
pH scale: Measures the concentration of H+ ions in a solution; lower pH is more acidic, higher pH is more basic.

Chapter 3: Carbon and the Molecular Diversity of Life

The Importance of Carbon

Carbon is the backbone of biological molecules, allowing for a diversity of structures and functions.

Carbon skeletons can vary in length, branching, and double bond position, contributing to molecular diversity.

Functional Groups

Functional groups are specific groups of atoms within molecules that have characteristic properties and reactivity.

Phosphate group
Carboxyl group
Hydroxyl group
Sulfhydryl group
Methyl group

Polymer Principles

Many biological molecules are polymers, made by joining smaller units called monomers.

Polymer: A long molecule consisting of many similar or identical building blocks linked by covalent bonds.
Monomer: The repeating unit that serves as the building block of a polymer.
Condensation (dehydration) synthesis: Monomers are joined by removing a water molecule.
Hydrolysis: Polymers are broken down by adding a water molecule.

Major classes of macromolecules:

Carbohydrates
Lipids
Proteins
Nucleic acids

Carbohydrates: Fuel and Building Material

Carbohydrates include sugars and polymers of sugars, serving as energy sources and structural materials.

Monosaccharide: Simple sugar (e.g., glucose).
Disaccharide: Two monosaccharides joined by a glycosidic bond.
Polysaccharide: Polymers of many monosaccharides (e.g., starch, cellulose).
When monosaccharides join, they release H2O in a process called dehydration synthesis.
Polysaccharides can be broken apart by hydrolysis, which adds water to break the bonds.

Lipids: Diverse Hydrophobic Molecules

Lipids are a diverse group of hydrophobic molecules that do not form true polymers.

Fats: Constructed from glycerol and fatty acids; used for energy storage.
Phospholipids: Major component of cell membranes, with hydrophilic heads and hydrophobic tails.
Steroids: Lipids with a carbon skeleton consisting of four fused rings (e.g., cholesterol).
Saturated fatty acids: No double bonds between carbon atoms; solid at room temperature.
Unsaturated fatty acids: One or more double bonds; liquid at room temperature.

Proteins: Many Structures, Many Functions

Proteins are polymers of amino acids and perform a wide variety of functions in cells.

Amino acid: Building block of proteins, consisting of an amino group, carboxyl group, hydrogen atom, and R group (side chain).
Polypeptide: A polymer of amino acids linked by peptide bonds.
Primary structure: The unique sequence of amino acids in a protein.
Secondary structure: Coiling or folding of the polypeptide chain (e.g., alpha helix, beta sheet) stabilized by hydrogen bonds.
Tertiary structure: The overall 3D shape of a polypeptide, determined by interactions among side chains.
Quaternary structure: Association of multiple polypeptide chains.

Example: Hemoglobin is a protein with quaternary structure, consisting of four polypeptide subunits.

Table: Levels of Protein Structure

Level	Description
Primary	Sequence of amino acids
Secondary	Alpha helices and beta sheets formed by hydrogen bonding
Tertiary	3D folding due to side chain interactions
Quaternary	Association of multiple polypeptide chains

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