The Chemical Context of Life: Study Guide for General Biology

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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. Elements and compounds form the foundation of all matter, including living organisms.

Element: A pure substance consisting of only one type of atom, which cannot be broken down by chemical reactions. Example: Oxygen (O), Carbon (C).
Compound: A substance made up of two or more different elements combined in a fixed ratio. Compounds can be broken down into their constituent elements by chemical means. Example: Water (H2O).

Exploring Life on Its Many Levels

Top Six Elements in Living Matter: Sulfur, phosphorus, oxygen, nitrogen, carbon, and hydrogen make up about 96% of living matter.

Atoms and Molecules

Atoms are the fundamental units of matter. Their structure and interactions determine the properties of molecules and compounds.

Atomic Number: The number of protons in an atom's nucleus. Determines the element's identity.
Mass Number: The sum of protons and neutrons in the nucleus.
Atomic Weight (Atomic Mass): The average mass of all the isotopes of an element, weighted by their abundance.
Valence: The number of electrons an atom needs to gain, lose, or share to fill its outer shell.

Isotopes and Their Importance to Biologists

Isotopes: Atoms of the same element with different numbers of neutrons. Some isotopes are radioactive and decay over time, releasing energy as radiation.
Applications: Radioactive isotopes are used in biological research and medicine, such as in radiometric dating and medical imaging.

Chemical Bonds

Covalent Bonds: Atoms share pairs of electrons. Can be nonpolar (equal sharing) or polar (unequal sharing).
Ionic Bonds: Electrons are transferred from one atom to another, resulting in oppositely charged ions that attract each other.
Hydrogen Bonds: Weak attractions between a hydrogen atom covalently bonded to an electronegative atom (like oxygen or nitrogen) and another electronegative atom.
Van der Waals Interactions: Weak, transient interactions due to temporary shifts in electron density.

Importance of Weak Bonds in Living Organisms

Weak bonds, such as hydrogen bonds and van der Waals interactions, are crucial for the structure and function of biological molecules, including DNA and proteins.

Molecular Shape and Biological Function

The three-dimensional shape of a molecule determines its function in biological systems.

Example: The shape of an enzyme's active site allows it to bind specifically to its substrate, enabling catalysis.
Example: Hemoglobin's quaternary structure enables it to carry oxygen efficiently.

Effects of Water's Polarity

Structure and Geometry of a Water Molecule

A water molecule consists of two hydrogen atoms and one oxygen atom, with a bent shape due to the lone pairs on oxygen.

Polarity and Hydrogen Bonding

Water is a polar molecule: oxygen is more electronegative, so the shared electrons are pulled closer to oxygen, giving it a partial negative charge and hydrogen a partial positive charge.
This polarity allows water molecules to form hydrogen bonds with each other and with other polar molecules.

Emergent Properties of Water

Water's unique properties arise from its ability to form hydrogen bonds.

Cohesion: Water molecules stick together, aiding the transport of water in plants.
Adhesion: Water molecules stick to other substances, helping water move against gravity in plant vessels.
Surface Tension: Water has a high surface tension, allowing small insects to walk on its surface.
Moderation of Temperature: Water absorbs and releases heat slowly, stabilizing temperatures in organisms and environments.
Floating of Ice: Ice is less dense than liquid water, so it floats, insulating aquatic life in winter.
Water as a Solvent: Water dissolves many substances, making it the "universal solvent" for biological reactions.

Specific Heat, Heat of Vaporization, and Expansion Upon Freezing

High Specific Heat: Water can absorb or release large amounts of heat with little temperature change, helping organisms maintain stable internal temperatures.
High Heat of Vaporization: It takes a lot of energy to convert water from liquid to gas, which helps cool organisms through evaporation (e.g., sweating).
Expansion Upon Freezing: Water expands as it freezes, making ice less dense than liquid water.

Acids, Bases, and pH

Dissociation of Water: Water can dissociate into hydronium () and hydroxide () ions.
Equation:

Acids: Substances that increase the hydrogen ion concentration in a solution.
Bases: Substances that reduce the hydrogen ion concentration, often by accepting or donating .

Hydrophilic vs. Hydrophobic Substances

Hydrophilic: Substances that have an affinity for water (e.g., salts, sugars).
Hydrophobic: Substances that repel water (e.g., oils, fats).

Chapter 3: Carbon and the Molecular Diversity of Life

The Importance of Carbon

Carbon can form four covalent bonds, allowing for a diversity of stable organic molecules with various shapes and functions.
Variation in carbon skeletons (length, branching, double bonds, rings) contributes to the diversity of organic molecules.

Functional Groups

Functional Groups: Specific groups of atoms attached to carbon skeletons that confer particular properties.

Functional Group	Structure	Properties
Phosphate	-PO4	Energy transfer (e.g., ATP)
Carboxyl	-COOH	Acidic, found in amino acids
Amino	-NH2	Basic, found in amino acids
Hydroxyl	-OH	Polar, forms hydrogen bonds
Sulfhydryl	-SH	Forms disulfide bonds in proteins

Polymer Principles

Monomers: Small molecules that serve as the building blocks of polymers.
Polymers: Large molecules made by joining monomers through condensation (dehydration synthesis) reactions.
Hydrolysis: The process of breaking polymers into monomers by adding water.

Major Classes of Macromolecules

Carbohydrates
Lipids
Proteins
Nucleic acids

Carbohydrates: Fuel and Building Material

Monosaccharides: Simple sugars (e.g., glucose).
Disaccharides: Two monosaccharides joined together (e.g., sucrose).
Polysaccharides: Long chains of monosaccharides (e.g., starch, cellulose).

Lipids: Diverse Hydrophobic Molecules

Lipids: Not true polymers; include fats, phospholipids, and steroids.
Fats: Composed of glycerol and fatty acids; used for energy storage.
Phospholipids: Major component of cell membranes; have hydrophilic heads and hydrophobic tails.
Steroids: Lipids with a carbon skeleton of four fused rings (e.g., cholesterol).
Saturated Fats: No double bonds; solid at room temperature.
Unsaturated Fats: One or more double bonds; liquid at room temperature.

Proteins: Many Structures, Many Functions

Amino Acids: Building blocks of proteins; each has an amino group, carboxyl group, hydrogen atom, and R group (side chain).
Peptide Bond: Covalent bond formed between amino acids during protein synthesis.
Protein Structure: Four levels—primary (sequence), secondary (alpha helix, beta sheet), tertiary (3D folding), quaternary (multiple polypeptides).
Denaturation: Loss of protein structure due to changes in pH, temperature, or salt concentration.

Nucleic Acids: Information Molecules

DNA and RNA: Polymers of nucleotides; store and transmit genetic information.
Nucleotide: Consists of a sugar, phosphate group, and nitrogenous base.

Chapter 4: A Tour of the Cell

Limits to Cell Size

Cells must be large enough to contain all necessary components, but small enough for efficient transport of materials across the membrane.
Surface area-to-volume ratio limits cell size; as a cell grows, its volume increases faster than its surface area.

Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	No	Yes
Membrane-bound organelles	No	Yes
DNA location	Nucleoid region	Nucleus
Size	Generally smaller	Generally larger

The Nucleus and Ribosomes

Nucleus: Contains most of the cell's DNA; surrounded by a double membrane (nuclear envelope).
Ribosomes: Sites of protein synthesis; can be free in the cytoplasm or bound to the endoplasmic reticulum.

Protein Synthesis

Transcription: DNA is transcribed into messenger RNA (mRNA) in the nucleus.
Translation: mRNA is translated into protein at ribosomes in the cytoplasm.

Additional info: Some explanations and examples were expanded for clarity and completeness, based on standard General Biology curriculum.