BIOL-102 6/26 (Unit 3)

Prokaryotic vs. Eukaryotic Fundamentals

• Shared feature: All cells possess a plasma membrane, cytoplasm, ribosomes, and genetic material.
• Key difference: Presence of membrane-bound organelles (especially a nucleus) in eukaryotes.
• Nucleoid (prokaryotes)
  • DNA concentrated in a single, circular region.
  • No surrounding membrane – therefore not isolated from cytoplasm.
• Ribosomes are the great exception
  • Present in both cell types, highlighting their universal role in protein synthesis.

Relative Size & Visualization Cues

• Instructor’s “tiered brain reference” for size hierarchy (helps visual learners):
  • \text{Atom} \ll \text{Cell Membrane Thickness} \ll \text{Whole Cell}
• Memorization tip: Pair similar-sounding items (e.g., “1,500 vs. 1,700 nm”) together on flash cards so you don’t forget one of them.

Plant vs. Animal Cells — Clearing Up Misconceptions

• Both possess mitochondria.
• Only plants (and some protists) have chloroplasts.
• Plants perform \text{photosynthesis} and \text{cellular respiration}; animals only respire.
• Structural add-ons in plants: cell wall (varied composition), central vacuole, and chloroplasts.

Plasma (Cell) Membrane

• Basic structure: Phospholipid bilayer ➔ amphipathic barrier that is flexible yet selectively permeable.
• Embedded components:
  • Integral & peripheral proteins (channels, pumps, receptors).
  • Glycolipids, lipoproteins, cholesterol (modulate fluidity & cell identity).
• Functions preview (full detail postponed until “transport” unit):
  • Regulates H_2O flow, ion gradients, nutrient uptake, waste export.
  • Resists minor mechanical stress but can rupture under strong force.

Cytoplasm & Cytoskeleton

• Cytoplasm ≈ cytosol (gel) + suspended organelles.
  • Mostly water with a colloidal, “Jell-O-like” semi-solid consistency.
• Cytoskeletal trinity
  1. Microfilaments (actin)
  • Thin, flexible; provide scaffolding just under the membrane.
  1. Intermediate filaments
  • Rope-like; anchor organelles (e.g., keep the nucleus centered).
  1. Microtubules (briefly mentioned indirectly)
  • Hollow rods; tracks for vesicle movement, form spindle fibers.
• Lipid bubble + gel needs internal bracing; cytoskeleton supplies rigidity, positioning, and motility.

The Nucleus – Cellular HQ

• Metaphor: “Corporate headquarters / mission-control.”
• DNA stored as chromatin (unwound; accessible) rather than visible chromosomes (condensed).
• Nuclear envelope
  • Double phospholipid bilayer studded with nuclear pores.
  • Pores connect directly to Endoplasmic Reticulum (ER) lumen.
• Governs the cell by regulating gene expression (i.e., which proteins are produced, when, and in what quantity).
• Exception fascination: Mature red blood cells (RBCs) eject their nucleus ➔ biconcave, more hemoglobin space, cannot synthesize new proteins.

Endoplasmic Reticulum (ER)

• Extension of the nuclear envelope forming a labyrinth of tubules & sacs.
• Lumen = cisternal space ("empty pipe interior").
• Double membrane ➔ organelle qualifies as “membrane-bound.”

Rough ER (RER)

• Studded with ribosomes; first stop for most protein synthesis.
• Major jobs
  • Translate mRNA into polypeptide chains.
  • Begin protein folding & modification (glycosylation, disulfide bonds) inside the lumen.
  • Manufacture phospholipids for all cellular membranes.
• Vesicle budding
  • RER pinches off small membrane bubbles (vesicles) loaded with newly made proteins/lipids.
  • Necessitates continuous phospholipid production to “replace” pinched-off segments.

Smooth ER (SER)

• Lacks ribosomes; visually “smooth.”
• Functions are cell-type specific
  • Lipid & carbohydrate synthesis (e.g., steroid hormones, phospholipids, glycogen).
  • Detoxification (especially abundant in liver cells).
  • Storage & release of Ca^{2+} in muscle cells.
• Cells that detoxify or distribute many molecules up-regulate SER & Golgi numbers.

Ribosomes – The Molecular Machines

• Composition: \text{rRNA} + \text{protein}; large + small subunit.
• Two populations
  1. Free in cytosol ➔ proteins that stay in cytoplasm.
  2. Bound to RER ➔ proteins destined for membranes, secretion, or specific organelles.
• Primary protein structure = linear chain of amino acids held via peptide bonds \left(\text{C–N (amide) linkage}\right).

Golgi Apparatus – Cellular UPS/FedEx

• Series of flattened membrane sacs (cis ➔ medial ➔ trans faces).
• Core workflow (analogy: package delivery)
  1. Receive vesicles from ER ("factory outputs").
  2. Sort & tag cargo with small molecular “labels” (lipids or short proteins ≈ shipping labels).
  3. Modify (e.g., add complex polysaccharides, finalize glycoproteins).
  4. Package into new vesicles and dispatch to correct destination (plasma membrane, lysosome, secretion, etc.).
• Also synthesizes certain polysaccharides (e.g., plant cell wall components).

Lysosomes – Acidic Recycling Centers

• Small vesicles containing hydrolytic enzymes + \text{pH} \approx 5 acid.
• Functions
  • Digest worn-out organelles, macromolecules, or pathogens.
  • Fuse with endocytic vesicles in phagocytic cells (e.g., macrophages) to destroy invaders “alive.”
• Efficiency mantra: “Waste not, want not” – salvage monomers for reuse.

Vesicles & Vacuoles

• Vesicle
  • Generic term for small, membrane-bound transport bubbles.
  • Types: transport, secretory, recycling, peroxisomes, etc.
• Vacuole (plant focus)
  • Larger, longer-lived, multifunctional (storage, turgor pressure maintenance, pigment, toxin sequestration).

Common Exceptions & Instructor’s Exam Hints

• Professor loves anomalies; expect test questions on them.
  1. Anucleate RBCs (animal cell without nucleus).
  2. Ribosome presence in prokaryotes (organelle shared despite other differences).
  3. Misconception traps (e.g., “plants lack mitochondria” – false).
• “Helium story” cue: if you don’t recall the helium anecdote, study further — it illustrates exception-based questioning.

Big-Picture Takeaways & Study Strategies

• Pair similar terms ("1500 nm vs 1700 nm") in flash cards to prevent confusion.
• Visual metaphors (factory, UPS, recipe book, Jell-O cytoplasm) anchor abstract ideas.
• Understand process flow (DNA → mRNA → ribosome (protein) → RER → Golgi → destination).
• Always connect structure with function (e.g., why SER abundance matches detox roles).
• Remember ethical/practical angle: cellular efficiency/recycling mirrors sustainable engineering.