Biological Organization and Scientific Inquiry

Levels of Biological Organization

Biological systems are organized hierarchically, from the largest ecological scale to the smallest chemical scale. Each level exhibits emergent properties—new characteristics that arise from the arrangement and interaction of components.

• Ecosystem: Includes a biological community and its nonliving environment.

• Community: Populations of different species living together.

• Population: Individuals of one species in a specific area.

• Organism: A single living individual.

• Organ System: Group of organs working together.

• Organ: Structure composed of tissues.

• Tissue: Group of organized cells.

• Cell: Fundamental unit of life.

• Organelle: Specialized structure within a cell.

• Molecule: Group of bonded atoms.

• Atom: Smallest unit of an element.

Energy flows through ecosystems and is eventually lost as heat, requiring continuous input. Matter cycles between organisms and the environment.

Cell Types and Classification

Cells are classified as prokaryotic or eukaryotic, distinguished by structural features and domain classification.

The four traditional eukaryotic kingdoms are Protista (historical grouping), Fungi, Plantae, and Animalia. Modern classification divides protists among multiple lineages.

Evolution by Natural Selection

Charles Darwin proposed that evolution occurs by natural selection. Individuals vary in heritable traits, and those with advantageous traits tend to leave more offspring, causing populations to evolve over generations.

• Descent with modification: Species change over time, giving rise to new species.

• Natural selection: Individuals with beneficial traits are more likely to survive and reproduce.

• Individuals are selected; populations evolve.

Scientific Inquiry

Science uses observation and experimentation to understand natural phenomena. It distinguishes between discovery science (describing patterns) and hypothesis-driven science (testing explanations).

• Hypothesis: Testable explanation.

• Prediction: Expected result.

• Theory: Broad, well-supported explanatory framework.

• Variables: Independent (manipulated), dependent (measured), controlled (held constant).

• Control group: Used for comparison.

• Replication: Measures variability.

• Random assignment: Reduces bias.

• Correlation does not imply causation.

Atoms, Bonds, and Water

Atomic Structure and Energy

Atoms consist of protons, neutrons, and electrons. Elements are defined by their proton number. Compounds are formed from two or more elements in fixed ratios. Living matter is mostly carbon, hydrogen, oxygen, and nitrogen.

• Proton: Positive charge; in nucleus.

• Neutron: Neutral; in nucleus.

• Electron: Negative; in orbitals.

• Atomic number: Number of protons.

• Mass number: Protons + neutrons.

• Isotopes: Same element, different neutron number.

• Ions: Atoms that gain or lose electrons.

• Energy: Capacity to do work; potential energy depends on position or structure.

• Valence electrons: Outermost shell; influence bonding.

Chemical Bonds and Molecular Interactions

Chemical bonds and interactions determine molecular structure and function.

Electronegativity is the attraction for shared electrons. Covalent bonds provide stability; weaker interactions allow reversible binding and flexibility. Molecular shape and chemical groups determine function.

At chemical equilibrium, forward and reverse reaction rates are equal; concentrations remain constant but not necessarily equal.

Water and Hydrogen Bonding

Water is a polar molecule due to unequal electron sharing and its bent shape. Each water molecule can form up to four hydrogen bonds.

• Cohesion: Attraction among water molecules; supports surface tension.

• Adhesion: Attraction to other surfaces.

• Transpiration: Pulls water through plant xylem; cohesion and adhesion maintain the column.

• High specific heat: Moderates temperature change.

• High heat of vaporization: Enables evaporative cooling.

• Ice is less dense than liquid water: Hydrogen bonding causes expansion.

• Versatile solvent: Dissolves ions and polar substances.

Temperature reflects average molecular kinetic energy. Heat is energy transferred due to temperature difference. Specific heat is energy required to raise a given mass by one degree.

Solutions, Acids, Bases, and Buffers

Solutions and Concentration

A solution is a homogeneous mixture. The solvent is the dissolving medium; the solute is the dissolved substance. Hydrophilic substances interact with water; hydrophobic substances do not.

• Mole: entities.

• Molarity (M):

• Moles:

• Grams:

To prepare a solution: calculate solute, dissolve in less than final volume, then adjust to required volume.

Water Dissociation and pH

Water dissociates into hydrogen and hydroxide ions. pH measures hydrogen ion concentration.

• Water dissociation: or

• In pure water at 25°C:

• pH formula:

• at 25°C

• Acidic: pH < 7; Neutral: pH = 7; Basic: pH > 7

• Each pH unit = tenfold difference in [H+]

• Acid: donates H+; Base: accepts H+ or supplies OH-

Buffer Action

A buffer resists changes in pH by accepting or donating hydrogen ions. Buffers have limited capacity and do not necessarily make a solution neutral.

• Bicarbonate system:

• Acid addition: bicarbonate accepts H+, forming carbonic acid.

• H+ removal: carbonic acid dissociates, replacing H+.

• Excess acid/base can exceed buffer capacity.

Acid Precipitation

Sulfur dioxide and nitrogen oxides form acids in the atmosphere, causing acid precipitation. This lowers soil and water pH, leaches nutrients, mobilizes toxic metals, and damages organisms. Ordinary rain is slightly acidic due to dissolved carbon dioxide.

Essential Distinctions to Review

• Atomic number vs. mass number

• Isotope vs. ion

• Polar covalent bond vs. hydrogen bond

• Cohesion vs. adhesion

• Heat vs. temperature

• Solute vs. solvent

• Hydrophilic vs. hydrophobic

• Acid vs. base

• Constant concentrations vs. equal concentrations at equilibrium

Carbon, Carbohydrates, Nucleic Acids, Proteins, and Lipids

Carbon Functional Groups and Isomers

Carbon forms four covalent bonds, enabling chains, branches, and rings. Functional groups determine chemical behavior. Hydrocarbon regions are nonpolar.

Isomers have the same molecular formula but different structures:

• Structural isomers: Different covalent arrangement.

• Cis-trans isomers: Different arrangement around double bonds.

• Enantiomers: Mirror images, not superimposable.

Macromolecule Assembly

Four major classes: carbohydrates, proteins, nucleic acids, lipids. Monomers form polymers via covalent links. Dehydration forms links by removing water; hydrolysis breaks links by adding water. Lipids are not true polymers.

Carbohydrates

Carbohydrates include monosaccharides, disaccharides, and polysaccharides. Monosaccharides are single sugars; disaccharides are two sugars joined by glycosidic linkage; polysaccharides are many sugar units.

Linkage geometry controls shape, function, and enzyme recognition. Human enzymes hydrolyze starch but not cellulose.

Nucleic Acids and ATP

A nucleotide contains a five-carbon sugar, nitrogenous base, and phosphate group(s). Phosphodiester bonds form the sugar–phosphate backbone. DNA contains deoxyribose and A, T, G, C; RNA contains ribose and A, U, G, C.

• DNA: Antiparallel double helix; A–T and G–C base pairing.

• RNA: Usually single-stranded; A–U and G–C pairing.

• DNA stores genetic information; RNA participates in expression, regulation, and catalysis.

• Transcription: DNA → RNA; Translation: mRNA → polypeptide.

• ATP: Adenine, ribose, three phosphates; hydrolysis releases energy.

Amino Acids and Protein Structure

Each amino acid has a central carbon, amino group, carboxyl group, hydrogen, and R group. Peptide bonds link amino acids into polypeptides.

Sequence influences folding and function. Denaturation disrupts shape but usually leaves primary sequence intact.

Proteins function in catalysis, transport, structure, signaling, movement, and defense. Enzymes lower activation energy but do not change overall free-energy difference or equilibrium.

Lipids

Lipids include triglycerides, phospholipids, and steroids. Triglycerides are glycerol plus three fatty acids joined by ester linkages. They are energy-rich and hydrophobic.

• Saturated fatty acids: No double bonds; pack tightly.

• Cis unsaturated: Kinks reduce packing; increase fluidity.

• Trans unsaturated: Straighter tails; remain unsaturated.

• Phospholipids: Glycerol, two fatty acids, phosphate head; amphipathic; form bilayers.

• Cholesterol: Four fused rings; membrane fluidity buffer; steroid precursor.

Comparison of Building Components and Bonds

Review and Study Strategies

Use assigned eText, lecture slides, and MasteringBiology resources to review weak topics. Rework assignments and use study modules and animations. Be able to explain terms, identify structures, compare concepts, and perform calculations independently.

Additional info: This guide is based on Chapters 1–3 of Campbell Biology in Focus and BI-183 course context. For full exam scope, consult course resources.