Scientific Method

Overview

The scientific method is a systematic approach used to justify and compare results through experimentation, with the goal of proving or disproving a hypothesis.

• Steps:

  1. Ask question about an observation

  2. Background research

  3. Construct hypothesis (must be falsifiable)

  4. Set predictions

  5. Construct experiment to test

  6. Collect data

  7. Analyze data

  8. Support or reject hypothesis (refine hypothesis if needed)

Key Concepts

• Observation: Gathering information using senses or tools; can use human knowledge or data from surroundings.

• Background research: Involves scientific methods, community knowledge, and/or indigenous knowledge.

• Hypothesis: A testable explanation based on observations; leads to predictions that can be tested. Can be wrong; does not need to be unique.

• Prediction: Statement about what will happen under specific conditions, derived from a hypothesis.

• Hypothesis vs. Theory: A theory is broader, supported by a large body of evidence, and generates new hypotheses.

• Experiment: Testing hypotheses under controlled conditions, with control and experimental groups.

• Variables:

  • Independent variable (IV): Changed or controlled by the experimenter.

  • Dependent variable (DV): Measured outcome, affected by IV.

  Data can be qualitative or quantitative.

• Analyzing Data: Use statistical tests; calculate mean ().

• Standard Deviation: Measures spread of data from the mean.

• Standard Error of the Mean: Indicates accuracy of mean compared to population; decreases as sample size increases.

• Inductive Reasoning: Drawing general conclusions from specific observations.

Levels of Biological Organization

Hierarchy from Broad to Specific

• Biosphere: All life on Earth, including lithosphere, hydrosphere, atmosphere.

• Ecosystem: All living things in a particular area plus abiotic components.

• Communities: All living things in a particular area; mixture of life.

• Populations: Conspecifics (same species) interacting and producing offspring.

• Organisms: Individual living things.

• Organs: Collection of tissues with function.

• Tissues: Integrated group of cells with common structure/function.

• Cells: Fundamental unit of structure and function.

• Organelles: Membrane-enclosed structures with specialized functions.

• Molecules: Two or more atoms held together by covalent bonds.

DNA and Genetic Information

Structure and Function

• DNA: Macromolecule carrying genetic instructions for growth, development, and reproduction.

• Double helix structure; composed of four types of chemical building blocks:

  • Adenine (A)

  • Thymine (T)

  • Cytosine (C)

  • Guanine (G)

  Sugar + phosphate backbone.

• Genes: Stretches of DNA that provide blueprints for proteins.

• Gene expression: Process by which DNA directs synthesis of proteins or RNAs.

• Genome: Complete genetic material of an organism or virus.

Cell Types and Classification

Prokaryotic vs. Eukaryotic Cells

• Prokaryotic cells: Small, no nucleus, most have cell wall (e.g., Bacteria, Archaea).

• Eukaryotic cells: Larger, double membrane-enclosed nucleus, DNA organized into chromosomes (e.g., Plant and Animal cells).

Classification of Life

• Approx. 2 million species identified; thousands more each year.

• Bacteria: Most recently recognized, ~80% of life on Earth.

Taxonomy and Nomenclature

• Taxonomy: Rules and structure for naming/classifying organisms.

• Binomial nomenclature: Two-part scientific naming system (Genus species), e.g., Homo sapiens, Canis familiaris.

Hierarchical Classification

• Domain

• Kingdom

• Phylum

• Class

• Order

• Family

• Genus

• Species

Domains and Kingdoms of Life

Three Domains

• Domain Bacteria: Most diverse and widespread prokaryotes.

• Domain Archaea: Prokaryotes living in extreme environments.

• Domain Eukarya: Includes 3 kingdoms distinguished by nutrition:

  • Plantae: Multicellular, photosynthetic, autotrophic

  • Fungi: Multicellular/unicellular, absorb nutrients, heterotrophic

  • Animalia: Multicellular, ingest other organisms, heterotrophic

  • Protista: Mostly unicellular

All Cells Share

• Cytoplasm

• Enclosed by a membrane

• Use DNA as genetic material

Cell Structure and Organelles

Major Organelles and Functions

• Nucleus: Contains DNA, controls cell activities.

• Mitochondria: Site of aerobic respiration.

• Chloroplasts: Site of photosynthesis (plants).

• Endoplasmic Reticulum (ER): Protein and lipid synthesis; rough ER has ribosomes, smooth ER does not.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

• Ribosomes: Protein synthesis; can be free or attached to ER.

• Cytoskeleton: Network of fibers for cell shape and movement; microtubules important for chromosome separation.

Comparison of Plant and Animal Cells

Cell Division

Overview

Cell division is essential for growth, repair, and reproduction. It includes mitosis (somatic cells) and meiosis (gametes).

• Mitosis: Division of nucleus resulting in two identical daughter cells.

• Meiosis: Division resulting in gametes with half the genetic material.

• Chromatin: DNA plus proteins (histones).

• Chromosome: Condensed chromatin, higher order of DNA organization.

• Somatic cells: Non-reproductive, diploid ().

• Gametes: Reproductive cells, haploid ().

• Homologous chromosomes: Similar but not identical; same genes in same order, alleles may differ.

The Cell Cycle

• Interphase: 90% of cell cycle; includes G1 (growth), S (DNA synthesis), G2 (preparation for division).

• Mitosis Phases (PPMAT):

  1. Prophase: Chromatin condenses, nucleoli disappear, spindle forms.

  2. Prometaphase: Nuclear envelope fragments, chromosomes condense further.

  3. Metaphase: Chromosomes align at metaphase plate.

  4. Anaphase: Sister chromatids separate and move to opposite poles.

  5. Telophase: Nuclear envelopes reform, chromosomes decondense.

• Cytokinesis: Division of cytoplasm; cleavage in animal cells, cell plate in plant cells.

Regulation of the Cell Cycle

• Timing and rate vary with cell type.

• Checkpoints for cell size, nutrients, energy, DNA damage, growth factors.

Evolutionary History and Multicellularity

Key Events

• Stromatolites: Earliest form of life (~3.7 bya), single-celled organisms.

• Niche: Specific environmental/ecological conditions available for life.

• First eukaryotes: ~1.8 bya.

• Endosymbiotic theory: Mitochondria and chloroplasts originated from engulfed prokaryotic cells.

• Multicellular organisms: ~1.2 bya.

• Cambrian explosion: 510–530 mya; rapid diversification of animal phyla.

Additional info:

• Standard deviation formula:

• Standard error of the mean:

• DNA base pairing: pairs with , pairs with

• Cell cycle checkpoints ensure proper division and prevent errors.