BMS 1021 Lecture 4 Notes

Introduction to Eukaryotic Cells

• Overview of cell components; focus on eukaryotic cells and their organelles.
• Discussion on endosymbiosis - the evolutionary theory that explains the origin of organelles like mitochondria and chloroplasts.

Importance of the Lectures

• Assessments will only cover lecture content; readings supplement understanding.

Historical Overview of Cell Observation

• Early microscopy revealed plant cells with organelles (e.g., chloroplasts).
• Initial confusion regarding organelles as independent organisms led to the endosymbiosis theory.

Endosymbiosis Theory

• Eukaryotic cells arise from the symbiosis of at least two cell types, specifically between ancestral archaea and independent bacteria.
• Ancestral archaea features and their similarities to eukaryotes discussed.
• Key examples include:
  • Mitochondria
  • Chloroplasts
• Lynn Margulis proposed this theory in the 1960s and faced skepticism.

Evidence Supporting Endosymbiosis

• Two prominent testing avenues:
  1. DNA Structure:
  • Prokaryotes: Circular genomes vs. Eukaryotes: Linear genomes.
  • Mitochondria and chloroplasts contain circular DNA, supporting ancestral prokaryotic origin.
  1. Membrane Structure:
  • Double membranes observed in chloroplasts and mitochondria:
    • Inner membrane: Prokaryotic-like
    • Outer membrane: Eukaryotic-like

Mitochondria

• Functions as the powerhouse of the cell:
  • Varying numbers across cell types based on energy needs.
• Features of mitochondria:
  • Outer membrane resembling eukaryotic structure.
  • Inner membrane features:
    • Convoluted, increasing surface area for ATP production.
    • Houses enzyme complexes; ATP synthase plays a key role in ATP synthesis.
  • Contains circular DNA coding for energy production enzymes, with many genes being transferred to the nucleus over evolutionary time.

Chloroplasts

• Main function: Photosynthesis
• Key characteristics:
  • Green color due to chlorophyll.
  • Contains enzymes essential for sugar production.
  • Features double membranes and a third set of membranes called thylakoids for photosynthesis.

Eukaryotic Cell Components

Nucleus

• Contains genetic material (DNA) and is the cell's command center.
• Nucleolus: Region for ribosomal RNA (rRNA) synthesis, essential for ribosome assembly.
• Surrounded by the nuclear pore complex to control substance transport.

Ribosomes

• Site of protein synthesis, translating mRNA to proteins.
• Associated with the endoplasmic reticulum (ER) for protein processing.

Endoplasmic Reticulum (ER)

• Types:
  • Rough ER: Studded with ribosomes; protein synthesis and initial modifications occur here.
  • Smooth ER: Functions in lipid synthesis and carbohydrate metabolism.
• Provides a compartmentalized environment for complex molecule production, unlike prokaryotes.

Golgi Apparatus

• Structured into cis (entry) and trans (exit) sides.
• Modifies, sorts, and packages proteins received from the ER into vesicles for transport or secretion.

Endomembrane System Summary

• A cohesive system where:
  1. DNA is transcribed to mRNA in the nucleus.
  2. mRNA traverses nuclear pores to reach ribosomes for protein synthesis.
  3. Proteins enter the ER for initial processing then migrate to the Golgi for further modifications.
  4. Golgi vesicles transport proteins to various destinations.

Lysosomes and Vacuoles

Lysosomes

• Function as recycling centers, digesting unwanted materials through:
  1. Phagocytosis: Engulfing food
  2. Autophagy: Breaking down damaged organelles.
• Contain enzymes (acid hydrolases) that digest cellular components.

Vacuoles

• Storage compartments for food, water, or waste.
• Important for compartmentalization and safeguarding the cell’s integrity.

Cytoskeleton

• Unique to eukaryotes, provides structural support and mechanisms for motility.
• Three main components:
  • Microtubules: Thickest, composed of tubulin dimers; vital for shape, organization, and motility.
  • Intermediate Filaments: Provide resilience and tensile strength.
  • Microfilaments: Actin filaments support cellular movement.

Microtubule Dynamics

• Composed of tubulin dimers with distinct growth rates on the plus (growth) and minus (shrinkage) ends.
• Microtubules originate from microtubule organizing centers (e.g., centrosomes).

Motility Structures

• Flagella: Fewer and longer, provide propulsion with undulating motion.
• Cilia: Shorter and more numerous, beat rhythmically to move substances across cell surfaces.
• Flagella exhibit a classic "9 + 2" microtubule arrangement, facilitating movement through dynein activity.