Cell Structure & Function – Lecture Vocabulary

Plasma Membrane & Molecular Composition

Plasma membrane = phospholipid bilayer → semi-permeable boundary that encloses cytosol (“cell soup”).
Relative abundance of biomolecules inside cells is unequal:
- Proteins = bulk of cellular mass.
- Followed (in descending order) by water, nucleic acids, carbohydrates, etc.
Phospholipid tails differ across domains:
- Bacteria: fatty-acid tails attached to glycerol ("standard" structure).
- Archaea: isoprenoid chains → increased stability in extreme temperatures (supports extremophiles).

Three domains: Bacteria, Archaea, Eukarya.
Distinctive feature that originally separated groups = presence/absence of a membrane-bound nucleus:
- Prokarya (Bacteria & Archaea): no nucleus ⇒ DNA located in nucleoid.
- Eukarya: possess membrane-bound nucleus.
Course focus = finer details of eukaryotic cell biology, but prokaryotes used as baseline for comparison.

Electron microscopy: allows visualization down to individual organelles (e.g., chloroplast surface) rather than just whole cells.
Advances in imaging → deeper understanding of sub-cellular architecture & function.

Usually one circular chromosome (largest visible structure → “spaghetti”).
DNA highly super-coiled to fit inside small volume.
- Demonstration: lysed E.\ coli cell spreads DNA over area far larger than intact cell.
Additional DNA = plasmids (small circular molecules):
- Carry accessory genes (e.g., antibiotic resistance, GFP for “glow-in-the-dark” traits).
- Widely used in genetic engineering & CRISPR workflows.

Cytosol = aqueous interior packed with ribosomes.
Ribosomes (protein + rRNA) have large & small subunits → assemble only during translation.
Responsible for synthesizing “primary structure” (beads-on-a-string polypeptide).

Outside plasma membrane; composed of peptidoglycan (NOT cellulose).
- Peptide bonds (not hydrogen bonds) ⇒ greater mechanical strength.
“Test question”: “What molecule makes up bacterial cell walls?” → Peptidoglycan.

Many prokaryotes form internal photosynthetic membranes (infolded to increase surface area):
- Hold pigments (chlorophyll) & enzymes.
- Example: green algae mats on water surfaces.
Biological trend: increase internal surface area rather than becoming one huge cell.
- Counter-example = slime mold (single giant cell).
- Larger single cells suffer diffusion limits; infolding mitigates low surface-to-volume ratio.

Flagella: long whip-like appendages; rotate as propellers for swimming.
Fimbriae: short needle/Velcro-like projections enabling adhesion to host tissues.
- Analogy: person in Velcro suit sticking to Velcro wall.
- Essential for pathogenic colonization of throat, gut, etc.

Smaller cytosolic volume despite overall larger cell size due to organelles filling space.
Separate incompatible reactions, store ions (Ca^{2+}), toxins, or magnetite crystals.
Increases metabolic efficiency & complexity.

Nucleus: dominant oval structure; genomic “library.”
- Surrounded by nuclear envelope (double membrane).
Rough Endoplasmic Reticulum (RER):
- Studded with ribosomes → protein synthesis.
- Translocates nascent polypeptides into ER lumen.
Smooth Endoplasmic Reticulum (SER):
- Lacks ribosomes; lipid & membrane phospholipid synthesis, detoxification.
Golgi Apparatus:
- “Packing & shipping” center.
- Modifies, sorts, directs proteins/lipids to destinations (secretion vs. retention).
Peroxisomes / Lysosomes:
- Contain enzymes for redox reactions & degradation of macromolecules/toxins.
Cytoskeleton:
- Microfilaments, intermediate filaments, microtubules provide shape & motility.
Mitochondria:
- ATP production via oxidative phosphorylation; inner-membrane cristae increase surface area (parallels bacterial infolding).

Cell Wall (cellulose) gives rigid rectangular shape.
Central Vacuole (large, pushes nucleus to periphery):
- Stores water, nutrients, soluble sugars produced during daytime photosynthesis.
Chloroplasts: photosynthetic organelles (contain chlorophyll → green pigment).
Key reminder: Plants possess both chloroplasts AND mitochondria (common misconception to exclude mitochondria).

Genetic engineering: plasmid vectors enable insertion of foreign genes (e.g., jellyfish GFP into mice, fish, bacteria).
Spread of antibiotic resistance: plasmid transfer of ampicillin-resistance genes among bacteria.
CRISPR: modern tool to transplant genes across species (e.g., resurrection of thylacine traits).
Importance for medicine: understanding fimbrial adhesion informs therapies against infections; membrane differences exploited by antibiotics.

Identify composition of bacterial cell wall (peptidoglycan).
Distinguish RER (protein synthesis) vs. SER (lipid synthesis).
Explain why membrane infolding enhances metabolic capacity & overcomes diffusion limits.
Compare chromosome numbers in prokaryotes vs. humans.
Describe roles of flagella vs. fimbriae in bacterial pathogenesis.
Recognize that chloroplast-containing cells still require mitochondria for ATP during dark periods.

Cellular architecture is driven by form–function relationships: structure (membranes, infoldings, cytoskeleton) dictates biochemical capability.
Evolution trends toward maximizing surface area while minimizing diffusion distances.
Compartmentalization underpins eukaryotic complexity, enabling multicellularity and specialized tissues.
Advances in microscopy & molecular tools continuously refine our understanding of intracellular life.