Tour of the Eukaryotic Cell – Vocabulary Flashcards

Components of the Eukaryotic Cell

• External & Internal Architecture
  • Cell (plasma) membrane – comparable to prokaryotes but embedded in a complex intracellular system.
  • Possible cell wall – composition and presence vary by kingdom.
  • Appendages – flagella or cilia anchored to cytoskeleton.
  • Internal structures – nucleus, membrane‐bound organelles, elaborate cytoskeleton.

Cell Wall

• Function
  • Provides mechanical strength & rigidity.
  • Counters \Delta \Psi_{osmotic} (osmotic pressure changes).
• Distribution
  • Present in plants, fungi, some protists.
  • Absent in animal cells.
• Composition by lineage
  • Plantae: cellulose microfibrils with large openings ⇒ wall is porous and non-selective.
  • Fungi: chitin + additional polysaccharides.

Plasma (Cell) Membrane

• Chemical make-up: phospholipid bilayer + diverse integral/peripheral proteins.
• Primary role: selectively permeable barrier.
  • Passive diffusion of small neutrals \text{H}2\text{O},\;\text{CO}2,\;\text{O}_2.
  • Larger / polar solutes require protein transporters or bulk transport.
• Localization
  • Sole outer boundary in animals & many protists.
  • Lies just internal to wall in plants/fungi.

Bulk Membrane Traffic

• Exocytosis – vesicle fusion expels secretory proteins or wastes into extracellular milieu.
• Endocytosis – plasma membrane invaginates, pinches into a vacuole. Types:
  • Phagocytosis – ‘cell eating’ of particles/cells.
  • Pinocytosis – ‘cell drinking’ of extracellular fluid.
• Central challenge: import/export large items without forming catastrophic holes.

Vacuoles

• Fluid‐filled membrane sacs; function diverges by kingdom.
  • Animal cells – numerous, small; shuttle material internally or from plasma membrane.
  • Plant tonoplast – single, dominant vacuole:
  • Maintains turgor & osmotic balance.
  • Recycles macromolecules.
  • Sequesters toxic ions/molecules.

Lysosomes

• Specialized vacuoles packed with hydrolytic enzymes (optimal at acidic pH).
• Execute intracellular digestion & recycling via hydrolysis reactions.
• Present in both animal and plant cells.

Microbodies (Peroxisomes)

• Single-membrane vesicles containing oxidative enzymes.
• Abundant in hepatocytes (liver).
• Oxidize amino acids, fatty acids, alcohol → by-product \text{H}2\text{O}2.
• Catalase converts \text{H}2\text{O}2 \rightarrow \text{H}2\text{O} + \tfrac12\text{O}2.

Cytosol vs. Cytoplasm

• Cytosol – aqueous matrix of nutrients, ions, soluble proteins surrounding organelles.
• Cytoplasm – cytosol + every component between nuclear envelope & plasma membrane.
• In casual usage the terms often interchange.

Nucleus

• Central command center; stores genetic blueprint (chromatin = DNA + histones).
• Double membrane nuclear envelope with nuclear pores ⇒ regulated trafficking (e.g., mRNA export, protein import).
• Nucleolus – rRNA synthesis & ribosome subunit assembly.

Ribosomes

• Non-membranous ribonucleoprotein complexes translating mRNA → polypeptide.
  • Free in cytosol – synthesize cytosolic proteins.
  • Bound to rough ER – synthesize secretory / membrane proteins.
• Composition: rRNA + proteins; two subunits.
  • Prokaryotic: 70\,\text{S} = 50\,\text{S} + 30\,\text{S}.
  • Eukaryotic: 80\,\text{S} = 60\,\text{S} + 40\,\text{S}.
  • rRNA lengths (bases) – Prokaryote 5\text{S}(120),\;16\text{S}(1540),\;23\text{S}(2900); Eukaryote 5\text{S}(120),\;5.8\text{S}(160),\;18\text{S}(1900),\;28\text{S}(4800).

Endomembrane System

• Continuous or vesicle-linked membranes coordinating protein & lipid traffic.
  • Nuclear envelope
  • ER (Endoplasmic Reticulum)
  • Rough ER – studded with ribosomes; co-translational import, folding, glycosylation, dispatch of secretory proteins.
  • Smooth ER – no ribosomes; lipid/steroid synthesis, ion (Ca^{2+}) storage, detox, vesicle budding.
  • Golgi apparatus – stacked cisternae (‘pancakes’):
  • Cis face (receiving) accepts ER vesicles.
  • Trans face (shipping) dispatches processed proteins/lipids to lysosome, membrane, or exocytosis routes.
  • Lysosomes – degradative endpoint.
  • Transport vesicles – shuttle cargo; mediate protein trafficking percentage referenced: 15\% of some cellular budget (slide context).

Energy-Converting Organelles

• Mitochondria – ‘powerhouse’; hosts Krebs cycle & electron transport chain, generating \text{ATP} from organic substrates.
• Chloroplasts (plants) – photosynthetic energy capture; thylakoid stacks with chlorophyll convert photons → glucose → ATP.
• Leucoplasts – non-pigmented plastids specialized for starch storage.

Endosymbiotic Theory

• Step 1: Ancestor eukaryote engulfed aerobic bacterium ⇒ proto-mitochondrion (ancestral heterotroph).
• Step 2: Cell with mitochondrion engulfed photosynthetic bacterium ⇒ proto-chloroplast (ancestral photosynthesizer).
• Supporting Evidence
  • Own DNA (often circular).
  • Independent binary-fission style replication.
  • 70S ribosomes, bacterial size.
  • Double membranes.
  • Organelle size ≈ modern bacteria.
• Criticisms / Open Questions
  • Some mitochondria possess linear DNA with telomeres (eukaryote-like).
  • Many mitochondrial proteins encoded in nucleus ⇒ mechanism of gene transfer not fully resolved.
  • 3-D ultrastructure of mitochondria ≠ simple spheres; networked tubules challenge ‘bacterial-like’ view.

Cytoskeleton & Motility

• Protein fiber triad
  • Microtubules – \alpha/\beta-tubulin cylinders; organelle tracks; mitotic spindle.
  • Actin (microfilaments) – cell cortex, motility, cytokinesis.
  • Intermediate filaments – tensile stability.
• Centrioles – microtubule triplet barrels; organize spindle in animals.
• Cilia & Flagella
  • 9+2 microtubule axoneme; dynein-powered bending.
  • Flagella: long, undulatory; found chiefly in sperm.
  • Cilia: short, numerous; sweep mucus in respiratory epithelium.
• Comparison with Bacterial Flagellum
  • Bacterial: helical filament rotated 360^{\circ} by basal motor.
  • Eukaryotic: bending wave/whip motion.

Comparative Overview

• Plants vs. Animals
  • Cell wall (cellulose) & large central vacuole, chloroplasts, plasmodesmata – plants only.
  • Centrioles, lysosomes prominent in animals.
• Eukaryotes vs. Prokaryotes
  • DNA packaging: linear chromosomes in membrane-bound nucleus vs. circular nucleoid.
  • Ribosome size (80S vs. 70S).
  • Compartmentalization via endomembranes absent in bacteria.

Analogy Exercise (Teaching Activity)

• Students asked to design a creative analogy for the eukaryotic cell (e.g., ‘city’ model where ribosomes = factories, Golgi = post office, mitochondria = power plants).
• Purpose: reinforce functional relationships by mapping biology onto familiar systems.