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The Eukaryotic Cell - Lecture Notes Flashcards

The Eukaryotic Cell: Overview

  • Includes algae, protozoa, fungi, higher plants, and animals.
  • Are a lot larger and much more complex than bacteria – Average size: 10-100 \,\mu\mathrm{m} in diameter.
  • Come in many shapes and arrangements – Unlike bacteria, we do not have standard names for the morphology of eukaryotic cells.

Size and Diversity

  • Eukaryotic cells are typically 10–100 μm in diameter (as stated in the source).
  • Prokaryotes are much smaller, typically 0.2–2.0 μm in diameter.
  • Eukaryotic cells contain membrane-enclosed organelles; prokaryotes do not.

Eukaryotic Cell Features (Composite Cell Diagram)

  • Plant cell features include: Peroxisome, Mitochondrion, Golgi complex, Microfilament, Vacuole, Microtubule, Chloroplast, Cytoplasm, Ribosome, Smooth ER, Rough ER, Plasma membrane, Cell wall, Nucleolus, Nucleus (as shown in a half-plant/half-animal composite).
  • Animal cell features include: Flagellum, Nucleus, Nucleolus, Golgi complex, Cytoplasm, Basal body, Microfilament, Lysosome, Centrosome (Centriole, Pericentriolar material), Ribosome, Microtubule, Peroxisome, Rough and Smooth ER, Plasma membrane.
  • Both cell types share basic organelles and themes, but differ in the presence/absence of cell wall, chloroplasts, large central vacuoles, etc.

Comparative Look: Prokaryotes vs. Eukaryotes (Chemistry and Organization)

  • Chemically similar in many ways: nucleic acids, protein, lipid, and carbohydrate synthesis; major metabolic pathways (catabolism and anabolism) are shared.
  • Major differences lie in structural organization and membrane-bound compartments.
  • Prokaryotes: simpler organization; Eukaryotes: compartmentalized with organelles.

The Major Differences (Table 4.2: Principal Differences)

  • Characteristic: Size of Cell
    • Prokaryotic: Typically 0.2-2.0 \,\mu\mathrm{m} in diameter.
    • Eukaryotic: Typically 10-100 \,\mu\mathrm{m} in diameter.
  • Nucleus
    • Prokaryotic: Typically no nuclear membrane or nucleoli (except Gemmata).
    • Eukaryotic: True nucleus with nuclear membrane and nucleoli.
  • Membrane-Enclosed Organelles
    • Prokaryotic: Relatively few; no true membrane-bound organelles (except simple inclusions).
    • Eukaryotic: Present; include nuclei, lysosomes, Golgi, ER, mitochondria, chloroplasts, etc.
  • Flagella
    • Prokaryotic: Present; simple structure with different mechanism (not described here as 9+2 structure).
    • Eukaryotic: Present; complex 9+2 microtubule arrangement.
  • Glycocalyx
    • Prokaryotic: Usually present as a capsule or slime layer.
    • Eukaryotic: Also present in some cells; may include sterols and carbohydrates that serve as receptors.
  • Cell Wall
    • Prokaryotic: Usually present; bacteria walls are often peptidoglycan.
    • Eukaryotic: When present, chemically simple (cellulose or chitin in plants/fungi); some cells lack a wall.
  • Plasma Membrane
    • Prokaryotic: Sterols generally absent; membranes with different protein/lipid composition.
    • Eukaryotic: Contains sterols and carbohydrates; receptors for signaling and adhesion.
  • Cytoplasm
    • Prokaryotic: Cytoskeleton limited; no cytoplasmic streaming.
    • Eukaryotic: Cytoskeleton (microfilaments, intermediate filaments, microtubules); cytoplasmic streaming present in some cells.
  • Ribosomes
    • Prokaryotic: Smaller size (70S) overall.
    • Eukaryotic: Larger cytoplasmic ribosomes (80S); organelles such as mitochondria/chloroplasts have 70S ribosomes.
  • Chromosome (DNA)
    • Prokaryotic: Typically a single circular chromosome; lacks histones.
    • Eukaryotic: Multiple linear chromosomes with histones.
  • Cell Division
    • Prokaryotic: Binary fission.
    • Eukaryotic: Sexual recombination via meiosis; cell division via mitosis.

Locomotion in Eukaryotes

  • Not all eukaryotic cells are motile; motility can be developmentally regulated.
  • Motility structures:
    • Flagella: Long, fiber-like appendages; usually few in number.
    • Cilia: Short, hair-like; usually high in number.

Locomotion Cont.: Structure and Mechanics

  • Basal body anchors both cilia and flagella to the plasma membrane.
  • Both flagella and cilia are composed of microtubules in a 9+2 arrangement.
  • Microtubules are long hollow tubes made of tubulin; they function as a flexible skeleton for the fibers.
  • In many species, the movement produces wave-like motion (especially in eukaryotic flagella).

Flagella vs. Prokaryotic Flagella

  • Prokaryotic flagella rotate like a propeller.
  • Eukaryotic flagella beat in a wave pattern.
  • Eukaryotic flagella are large, around 200 \,\mathrm{nm} in diameter.

Cell Walls in Eukaryotes

  • Many eukaryotic cells have cell walls to maintain shape; algae and plant walls are mainly cellulose.
  • Fungal walls are largely chitin (polymer of N-acetylglucosamine, NAG).
  • Yeast walls primarily contain glucan and mannan.
  • Protozoa lack a conventional cell wall, having a flexible pellicle instead.
  • Some eukaryotic cells are covered with a glycocalyx, a layer of sticky carbohydrates with proteins/lipids anchored to the membrane, aiding in attachment and cell-cell recognition.

Endomembrane System and the Plasma Membrane

  • Plasma membrane: Similar in function to bacteria; protein content differs; contains carbohydrates used for signaling, binding, and as receptors; also sites of adhesion.
  • Eukaryotic membranes contain sterols, strengthening against osmotic lysis.
  • Substances enter by diffusion, facilitated diffusion, active or passive transport, and endocytosis.

Endocytosis

  • Endocytosis enables engulfing large particles; three types:
    • Phagocytosis: Pseudopods extend to surround and internalize target.
    • Pinocytosis (cellular drinking): Membrane folds inward to form vesicles.
    • Receptor-mediated endocytosis: Ligand binds to receptor, enabling vesicle formation; a common route for viral entry into eukaryotic cells.

Cytoplasm and Cytoskeleton

  • Cytoplasm: Everything inside the plasma membrane and outside the nucleus.
  • Major difference from prokaryotes: presence of a complex cytoskeleton (microfilaments, intermediate filaments, microtubules).
  • Cytoskeleton maintains shape, organization, and movement of cellular products.

Organelles: General Concept

  • Organelles are membrane-bound structures with specific shapes and functions.
  • Not every cell has every organelle; diversity exists across cell types.

The Nucleus

  • Found only in eukaryotic cells.
  • Spherical or oval; usually the largest organelle.
  • Contains most genetic material; surrounded by a double membrane (nuclear envelope).
  • DNA associated with histones to condense DNA and regulate transcription; when not replicating, DNA forms chromatin.
  • Histones help regulate transcription by keeping DNA accessible or inaccessible.
  • Nucleolus(s): Sites of rRNA synthesis; rRNA is essential for ribosome formation.
  • Nuclear pores: Allow access to DNA-synthesizing machinery; transcription and translation are physically separated in eukaryotes (unlike prokaryotes).

Nucleus: Visual Components

  • Nucleolus, Nuclear envelope, Nuclear pore, Chromatin, Ribosomes can be associated with the outer surface of the nucleus during synthesis.

Ribosomes

  • Function: Synthesize proteins.
  • Structure: Larger in eukaryotes; composed of subunits: 40S + 60S = 80S ribosome.
  • Location: Free ribosomes synthesize proteins for the cell; rough ER ribosomes synthesize proteins to be exported or integrated into membranes.
  • Mitochondria contain their own ribosomes: 70S, hinting at prokaryotic origin.

Endoplasmic Reticulum (ER)

  • ER is a network of membranous cisternae attached to the nucleus.
  • Two sections:
    • Rough ER: Studded with ribosomes; synthesizes proteins into the ER lumen, where they are modified (folded, attached to lipids or carbohydrates).
    • Smooth ER: Lacks ribosomes; synthesizes phospholipids, fats, and sterols.

Golgi Complex

  • Structure: Stacks of cisterns (cisternae) resembling pita bread; acts as the cell’s post office.
  • Transport: Proteins move from cis to trans via transport vesicles; vesicles bud off from one cisterna and fuse with the next.
  • Function: Further modify proteins within cisterns; secretory transport vesicles carry finished products to their destinations.

Mitochondria

  • Ubiquitous in cytoplasm; number varies by cell type.
  • Structure: Double membrane with highly folded inner membrane (cristae) and a matrix in the inner compartment.
  • Function: Generate ATP energy via cellular respiration; large surface area supports energy-producing reactions.
  • Contain their own DNA and ribosomes, supporting endosymbiotic origins.

Lysosomes

  • Formed by budding from the Golgi complex.
  • Single membrane and no internal structures.
  • Contain over 40 digestive enzymes capable of breaking down organic molecules.
  • Important components of immune defenses (to be discussed later).

Peroxisomes

  • Similar to lysosomes but smaller and arise from division of other peroxisomes.
  • Detoxify cells by oxidizing toxic compounds; convert toxins to less-toxic or non-toxic forms.
  • Example: Alcohol metabolism pathway represented as: \mathrm{Alcohol} \rightarrow[\text{oxidation}] H2O2 \rightarrow[\text{conversion}] 2H2O + O2

Vacuoles

  • Membrane-bound cavities in the cytoplasm; size varies from 5% to 90% of total cell size depending on cell type.
  • Derived from the Golgi complex.
  • Functions vary: storage of sugars, organic acids, proteins, energy sources, waste products, toxins, and water.

Chloroplasts

  • Found in algae and green plants; contain chlorophyll and the enzymes for photosynthesis.
  • Structure: Flattened membranous sacs called thylakoids; stacks of thylakoids are grana.
  • Photosynthesis occurs in thylakoid membranes; chlorophyll stores energy.
  • Like mitochondria, chloroplasts have 70S ribosomes, their own DNA, and can replicate independently.

Endosymbiotic Theory: Overview

  • Proposes that eukaryotic cells evolved from prokaryotic cells through endosymbiosis.
  • Early prokaryotic ancestors developed internal membranes and became hosts for engulfed bacteria.
  • This process could explain the origin of organelles such as mitochondria, chloroplasts, and possibly flagella.
  • The theory helps explain the complexity of eukaryotic cells and the presence of organelles with prokaryotic-like features.

Endosymbiotic Theory: Timeline and Process

  • Start: Prokaryotic cells existed first (~3.5–4.5 billion years ago).
  • A predatory prokaryotic cell engulfed smaller bacteria but did not digest them.
  • Engulfed bacteria replicated at similar rates and were maintained within the host; over time, many lost DNA and became dependent on the host.
  • Through co-evolution, these engulfed bacteria evolved into organelles such as mitochondria and chloroplasts.
  • Multiple engulfment events could explain the origin of other organelle-like structures (e.g., flagella).

Endosymbiotic Theory: Evidence

  • Mitochondria and chloroplasts resemble bacteria in size and shape.
  • They contain circular DNA, typical of prokaryotes.
  • They can replicate independently from the host cell.
  • Mitochondrial and chloroplast ribosomes resemble prokaryotic ribosomes and are inhibited by antibiotics that target prokaryotic ribosomes.

Practical and Conceptual Implications

  • Antibiotics targeting peptidoglycan (absent in eukaryotes) selectively affect bacteria; eukaryotes generally tolerate these antibiotics due to absence of peptidoglycan in their cell walls (penicillins and cephalosporins).
  • The endomembrane system and organelle evolution illustrate how compartmentalization supports complex cellular functions and specialization.
  • The separation of transcription and translation in the nucleus has implications for gene regulation and RNA processing in eukaryotes.

Key Takeaways for Exam Preparation

  • Eukaryotes are larger and structurally compartmentalized, with true nuclei and membrane-bound organelles.
  • Major structural differences from prokaryotes include the presence of a nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, and (in plants/algae) chloroplasts.
  • The cytoskeleton enables shape control, intracellular transport, and cell movement; 9+2 microtubule arrangement underlies the beating of cilia and flagella.
  • Endocytosis types (phagocytosis, pinocytosis, receptor-mediated endocytosis) explain how cells internalize materials and pathogens.
  • Endosymbiotic theory provides a coherent explanation for the origin of mitochondria and chloroplasts, supported by evidence such as circular DNA, 70S ribosomes, and independent replication.