Brief Notes: Cell Theory & Cell Biology

Cell theory: unifying concept in biology.
Founders: Schleiden (1838) and Schwann (1839) concluded that all plants and animals consist of cells.
Virchow (1855): new cells form only by division of previously existing cells.
Weismann (1880): all living cells share a common origin.
Classical cell theory (3 points):
1. Cells are the basic living units of organization and function in all organisms.
2. All organisms are composed of one or more cells.
3. All cells came from other cells.
Modern cell theory (3 additions):
- DNA is passed between cells during cell division.
- Cells of all organisms within a similar species are mostly the same structurally and chemically.
- Energy flow occurs within cells.
Organization and size: small cell size helps maintain homeostasis; basic organization is similar across cells.

Plasma membrane: selective barrier; separates cell contents from the external environment; maintains internal milieu; exchanges materials; allows energy stores.
Cells have internal structures (organelle) for metabolic activities: energy conversion, synthesis, and manufacturing.
DNA: genetic instructions; concentrated in a limited region in cells; nucleus (in eukaryotes) vs nucleoid (prokaryotes).
Central concept: all cells share a basic plan but differ in membrane-bound compartments (in eukaryotes).
Central dogma (brief): DNA → RNA → Protein; replication copies DNA; transcription and translation build proteins.

Prokaryotic cells:
- No membrane-bound organelles; DNA in the nucleoid (not a nucleus).
- Plasma membrane; most have cell walls; some have flagella for locomotion; fimbriae for adhesion.
- Typically smaller: $1-10\,\mu\text{m}$ .
Eukaryotic cells:
- Membrane-bound organelles; true nucleus.
- Cytoplasm (cytosol) and cytoskeleton for shape/transport.
- Typically larger: $10-30\,\mu\text{m}$ ; mitochondria ~size of small bacteria; chloroplasts ~ $5\,\mu\text{m}$ .
Similarities: plasma membrane; DNA; ribosomes; basic metabolism.

Plant cells:
- Chloroplasts for photosynthesis; cell wall external to plasma membrane; large central vacuole.
Animal cells:
- Lack cell wall; various organelles (e.g., lysosomes); centrosomes with centrioles in many animals.
General note: both contain nucleus, cytoplasm, mitochondria, ER, Golgi, ribosomes, etc.

Endomembrane system performs synthesis, modification, and trafficking of proteins:
- Rough ER: ribosomes synthesize proteins destined for secretion or organelles.
- Golgi apparatus: processes, sorts, and packages proteins into vesicles.
- Smooth ER: lipid synthesis; detoxification (e.g., in liver); carbohydrate metabolism.
Quality control: molecular chaperones assist folding; misfolded proteins targeted to proteasomes for degradation.

Microscopy scales:
- Light microscopy vs electron microscopy.
- Typical component sizes: most cell components are in the range of $1-100\,\mu\text{m}$ , while molecular components are in the nm range.
Scale relationships:
- $1\ \mu\text{m} = 1000\ \text{nm}$
- $1\ \text{nm} = 10^{-3}\ \mu\text{m}$
Common organelle sizes:
- Nucleus ~ $5\ \mu\text{m}$ in diameter.
- Mitochondria ~ $1\ \mu\text{m}$ .
- Chloroplasts up to several micrometers.
Cell fractionation: separates cellular components for study to determine function.
Key structure/function recap:
- Nucleus stores DNA; nuclear envelope with pores.
- Ribosomes synthesize proteins; rough ER bears ribosomes.
- Golgi modifies/sorts proteins; lysosomes digest macromolecules.
- Lysosomes: digest unwanted material; Tay-Sachs disease illustrates Hex-A deficiency leading to GM2 lipid accumulation.
- Peroxisomes: detoxification and lipid metabolism (briefly referenced in context).