The Command Center
for Deep Work.

1. Cell Theory — Historical Foundations

• All living organisms are composed of one or more cells
• The cell is the basic unit of structure, function, and organization
• All cells arise from pre-existing cells through division (Virchow, 1855)
• Schleiden & Schwann (1838-39) established the first two tenets

2. Prokaryotic vs. Eukaryotic Cells

• Prokaryotes — No membrane-bound nucleus; circular DNA; ribosomes (70S); examples: bacteria, archaea
• Eukaryotes — Membrane-bound nucleus; linear chromosomes; ribosomes (80S); examples: plants, animals, fungi
• Both share: plasma membrane, cytoplasm, ribosomes, DNA as genetic material
• Endosymbiotic theory — Mitochondria and chloroplasts likely evolved from engulfed prokaryotes (Lynn Margulis, 1967)

3. Key Organelles & Their Functions

• Nucleus — Contains chromatin/chromosomes; site of DNA replication and transcription; bounded by double membrane with nuclear pores
• Mitochondria — Produces ATP via cellular respiration (glycolysis, Krebs cycle, oxidative phosphorylation); has own DNA
• Rough ER — Studded with ribosomes; synthesizes and folds secretory and membrane proteins
• Smooth ER — Lipid synthesis, detoxification, calcium storage; no ribosomes
• Golgi Apparatus — Modifies, sorts, and packages proteins into vesicles; cis (receiving) and trans (shipping) faces
• Lysosomes — Contain hydrolytic enzymes (pH ~5); digest macromolecules, old organelles, and pathogens
• Chloroplasts — (Plants only) Site of photosynthesis; contains thylakoids, grana, and stroma; has own DNA
• Central Vacuole — (Plants) Maintains turgor pressure; stores water, ions, and waste products

4. Cell Membrane — Fluid Mosaic Model

• Phospholipid bilayer with hydrophilic heads and hydrophobic tails
• Integral proteins — Span the membrane (channels, transporters, receptors)
• Peripheral proteins — Attached to membrane surface (enzymes, structural support)
• Cholesterol regulates membrane fluidity across temperatures
• Glycoproteins & glycolipids on extracellular surface enable cell recognition

5. Transport Across Membranes

• Passive transport — Diffusion, osmosis, facilitated diffusion; no ATP required; moves down concentration gradient
• Active transport — Requires ATP; moves against concentration gradient (e.g., Na⁺/K⁺ pump)
• Endocytosis — Phagocytosis (cell eating), pinocytosis (cell drinking), receptor-mediated
• Exocytosis — Vesicles fuse with membrane to release contents extracellularly

6. Key Vocabulary

• Cytoplasm — Gel-like substance inside the cell membrane
• Cytoskeleton — Network of microfilaments, intermediate filaments, and microtubules
• Tonicity — Hypertonic, hypotonic, and isotonic solutions
• Selectively permeable — Membrane allows only certain molecules to pass

7. Review Questions

1. Compare and contrast prokaryotic and eukaryotic cells. Include at least three structural differences.
2. Explain the role of the mitochondria in ATP production. Reference all three stages of cellular respiration.
3. Describe the fluid mosaic model. How do cholesterol and proteins contribute to membrane function?
4. A red blood cell is placed in a hypertonic solution. Predict what will happen and explain why.
5. Provide evidence supporting the endosymbiotic theory for the origin of mitochondria.