General Biology Study Guide: Foundations, Chemistry of Life, Cells, and Enzymes

The Scientific Method

Overview

The scientific method is a systematic approach used by scientists to investigate natural phenomena, develop hypotheses, and test predictions through experimentation and observation.

• Steps:

  1. Observation

  2. Hypothesis

  3. Prediction

  4. Experiment (test prediction)

  5. Conclusion (true/false prediction?)

  6. Revise or reject hypothesis

  7. Additional tests or alternative hypotheses

• Hypothesis: A proposed explanation or possible solution to a problem (educated guess). Must be testable and falsifiable.

  • Testable: "Colds are caused by viruses." / "Echinacea reduces severity of colds."

  • Not testable: "Spirits are watching you."

• Predictions: Use deductive reasoning (if/then). Example: If vitamin C decreases colds, then people taking supplements will get fewer colds than those who do not.

• Experiments: Controlled tests that change only one variable at a time.

  • Control group: Placebo, no treatment.

  • Experimental group: Receives treatment.

  • Random assignment: Reduces bias.

  • Double blind: Both researchers and participants unaware of group assignments.

• Correlation vs. Causation:

  • Correlation: Relationship between variables (e.g., ice cream sales ↑ with drowning deaths).

  • Causation: One variable directly affects another.

  • Correlation does not imply causation; may be coincidental or due to other factors.

• Statistics:

  • Statistical significance: Determines if results are due to chance.

  • Null hypothesis: Assumes no difference exists.

  • Statistical validity increases with large sample size, careful design, double blind, repetition, peer review.

• Sources:

  • Primary: Peer-reviewed journals (e.g., Science, Nature).

  • Secondary: Books, news, ads.

  • Anecdotal evidence: Personal stories ("worked for me").

  • Avoid anecdotal infomercials; use reputable sites (NIH, Mayo Clinic); check for bias; prefer peer-reviewed sources.

What Defines Life?

Properties of Living Things

Living organisms share several key characteristics that distinguish them from non-living matter.

• Organized (composed of cells)

• Metabolism (energy for growth, reproduction, response to stimuli, homeostasis)

• Reproduction

• Evolution/adaptation

Levels of Organization

• Molecule → Cell → Tissue → Organ → Organ System → Organism → Population → Community → Ecosystem → Biome → Biosphere

Chemistry of Life

Atoms

Atoms are the smallest units of elements, composed of subatomic particles.

• Protons (+): In nucleus

• Neutrons (0): In nucleus

• Electrons (–): Orbit nucleus in shells

• Atoms are reactive if outer shell is not full

Electrons

• Transfer energy in cells

• Shells: 1st holds 2, 2nd/3rd hold 8

• Ions: different numbers of protons and electrons

Free Radicals & Antioxidants

• Free radicals: Unstable, steal electrons, cause damage

• Antioxidants (vitamins C, E): Donate electrons, neutralize

Molecules & Bonds

• Ionic bonds: Electron transfer (NaCl), weak, break in water

• Covalent bonds: Electrons shared, strong, store energy (C6H12O6)

• Hydrogen bonds: Weak attractions, important in water and DNA

Properties of Water

• Universal solvent

• Cohesion/surface tension

• Resists temperature changes

• pH scale: acids (↑ H+), bases (↓ H+)

• Nonpolar molecules (oil) = hydrophobic

Organic vs. Inorganic

• Inorganic: No C–C bonds (H2O, O2, NaCl)

• Organic: Carbon-based (C–C bonds)

Macromolecules

Overview

Macromolecules are large, complex molecules essential for life, including carbohydrates, proteins, lipids, and nucleic acids.

• Carbohydrates: Energy & structure

  • Monosaccharides: Simple sugars

  • Disaccharides: 2 sugars

  • Polysaccharides: Long chains (starch, cellulose, glycogen)

• Proteins: Built from amino acids

  • Structural: Hair, muscle

  • Enzymes: Speed reactions (end in -ase)

  • Transport: Hemoglobin

  • Shape = function; change shape = different function

• Lipids: Hydrophobic molecules

  • Fats: Store energy, insulate, energy storage

  • Phospholipids: Make up cell membranes (hydrophilic head, hydrophobic tails)

• Nucleic Acids:

  • DNA: Double helix of nucleotides (sugar-phosphate backbone, base pairs by H-bonds)

  • Stores genetic info

• ATP: High-energy compound, immediate energy for cells

Cells

Prokaryotic vs. Eukaryotic Cells

• Prokaryotic Cells:

  • Bacteria

  • No nucleus or organelles

  • Have DNA/RNA, ribosomes, cytoplasm, plasma membrane, cell wall

  • Smaller than eukaryotes

• Eukaryotic Cells:

  • Plants, animals, fungi, protists

  • Have nucleus and organelles

  • Some have cell walls (plants, fungi)

Organelles

• Nucleus: Stores DNA

• Cytoplasm: Cytosol + organelles

• Mitochondria: Aerobic respiration → ATP

• Chloroplasts: Photosynthesis (plants/algae)

• Lysosomes: Digestion

• Ribosomes: Protein assembly (free or on ER)

• ER (Endoplasmic Reticulum): Protein/lipid production

• Golgi apparatus: Modify, store, package proteins

• Centrioles: Cell division

• Central vacuole (plants): Storage, pressure, rigidity

Membranes

• Fluid mosaic of lipids & proteins

• Regulate water:

  • Too much → swelling, burst

  • Too little → shrink

  • Plant/fungal cells use cell walls for protection

Diffusion & Enzymes

Diffusion

Diffusion is the movement of molecules from areas of high concentration to low concentration (down a concentration gradient).

• Passive transport: Does not require energy

• Osmosis: Diffusion of water across a selectively permeable membrane

• Facilitated diffusion: Transport proteins help large or charged molecules move across membrane (still passive)

Active Transport

• Moves molecules against concentration gradient (low → high)

• Requires energy (usually ATP)

Enzymes

• Definition: Proteins that speed up chemical reactions (biological catalysts)

• How they work:

  • Substrate binds to enzyme's active site (specific shape – lock-and-key or induced fit)

  • Reaction occurs → products released

  • Reusable: Enzymes are not consumed in the reaction

Factors Affecting Enzyme Activity

• Temperature:

  • Too low → reaction slows

  • Too high → enzyme denatures (loses shape)

• pH: Each enzyme works best at an optimal pH; too high/low → denaturation

• Substrate concentration: Higher substrate concentration → faster reaction (until saturation)

• Inhibitors: Block or reduce enzyme activity

  • Competitive inhibitors: Bind active site

  • Noncompetitive inhibitors: Bind elsewhere, change enzyme shape

Quick Review Questions

• What makes a hypothesis scientific?

• How do experiments avoid bias?

• Why doesn't correlation mean causation?

• What are signs of credible vs. non-credible sources?

• What properties define life?

• Difference between prokaryotic and eukaryotic cells?