Eukaryotic Cells: Structure, Function, and Comparison with Prokaryotes

Eukaryotic Cells

Introduction to Eukaryotic Cells

Eukaryotic cells are a fundamental topic in microbiology, distinguished by their complex structure and compartmentalization. Unlike prokaryotic cells, eukaryotes possess membrane-bound organelles and a defined nucleus, allowing for specialized cellular functions and greater structural complexity.

• Eukaryotic cells are typically larger and more structurally complex than prokaryotic cells.

• They contain a double membrane-enclosed nucleus that houses genetic material.

• Key organelles include: mitochondria, Golgi complex, endoplasmic reticulum, lysosomes, microtubules, microfilaments, and chloroplasts (in phototrophs).

• Some eukaryotes have motility structures such as flagella or cilia.

• Cell membranes contain sterols for structural integrity.

Why Study Eukaryotic Cells?

Evolutionary and Systematic Relationships

Eukaryotic cells are essential for understanding the diversity of life and the evolutionary relationships among organisms. They are a mosaic of features derived from various prokaryotic ancestors, as illustrated by endosymbiotic theory and the origin of organelles.

• Endosymbiotic theory explains the origin of mitochondria and chloroplasts as descendants of ancient prokaryotes engulfed by ancestral eukaryotic cells.

• Eukaryotes display a collection of features from different prokaryotic lineages.

• Systematic relationships among eukaryotes are often traced through organelle structure and genetic evidence.

• Example: Plasmodium (malaria-causing organism) and its mosquito vector are both eukaryotes, but with distinct evolutionary origins.

Eukaryotic Cell Structure and Function

General Features

Eukaryotic cells are characterized by extensive internal compartmentalization, allowing for specialized metabolic processes and efficient regulation of cellular activities.

• Membrane-bound nucleus separates genetic material from the cytoplasm.

• Intracellular organelles perform specific functions (energy production, synthesis, transport, degradation).

• Cytoplasm is a complex matrix, not homogenous, supporting various cellular activities.

Cytoplasmic Matrix

The cytoplasmic matrix is the site of many metabolic reactions and provides a medium for organelle suspension and movement.

• Composed of 70-85% water (free and bound forms).

• Contains proteins, ions, and cytoskeletal elements.

• pH typically ranges from 6.8-7.1.

The Cytoskeleton

The cytoskeleton is a dynamic network of protein filaments that maintains cell shape, enables movement, and organizes intracellular transport.

• Microfilaments (7 nm): Polymers of actin, involved in cell shape and movement.

• Intermediate filaments (8-12 nm): Fibrous proteins (e.g., keratin), provide structural support.

• Microtubules (25 nm): Tubulin polymers, maintain cell shape, facilitate movement, and transport organelles.

• Prokaryotes lack a true, organized cytoskeleton.

Membranous Organelles

Endoplasmic Reticulum (ER)

The ER is a network of membranous tubules and sacs (cisternae) involved in synthesis and transport of biomolecules.

• Rough ER (RER): Studded with ribosomes; synthesizes secreted and membrane proteins.

• Smooth ER (SER): Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

Golgi Apparatus

The Golgi apparatus modifies, packages, and secretes proteins and lipids received from the ER.

• Composed of stacks of cisternae.

• Involved in the formation of lysosomes and other vesicles.

• Modifies ER products by adding specific groups (e.g., carbohydrates).

Lysosomes, Vacuoles, and Vesicles

Lysosomes are single-membrane organelles containing hydrolytic enzymes for intracellular digestion. Vacuoles and vesicles are membrane-bound compartments for storage and transport.

• Lysosomes: Digest macromolecules, formed in RER and Golgi.

• Vacuoles: Larger compartments for storage and digestion.

• Vesicles: Smaller transport compartments.

• Major forms of endocytosis:

  • Phagocytosis: Uptake of large particles into phagosomes.

  • Pinocytosis: Uptake of fluids and small molecules into pinosomes.

  • Receptor-mediated endocytosis: Selective uptake via coated pits (clathrin).

  • Caveolae-mediated endocytosis: Nonselective uptake, enriched in cholesterol and caveolin.

Proteasome

The proteasome is a non-lysosomal protein degradation system present in eukaryotes, bacteria, and archaea.

• Degrades ubiquitin-tagged proteins into small peptides.

• Structure: Cylindrical 26S proteasome complex.

• Used for recycling proteins and antigen processing.

Ribosomes

Eukaryotic Ribosomes

Ribosomes are the site of protein synthesis. Eukaryotic ribosomes are larger (80S) than prokaryotic ribosomes (70S) and consist of two subunits (60S and 40S).

• RER-associated ribosomes: Synthesize secretory and membrane proteins.

• Free ribosomes: Synthesize cytoplasmic proteins.

• Polysomes: Complexes of mRNA with multiple ribosomes.

Mitochondria

Structure and Function

Mitochondria are the site of ATP production via electron transport and oxidative phosphorylation. They are bounded by two membranes and contain their own DNA and ribosomes.

• Inner membrane: Highly folded into cristae, contains electron transport chain enzymes.

• Matrix: Contains mitochondrial DNA, ribosomes, and enzymes for the tricarboxylic acid (TCA) cycle and fatty acid oxidation.

• F1 particles: Site of ATP synthase.

• Mitochondria can replicate independently of the cell.

Chloroplasts

Structure and Function

Chloroplasts are the site of photosynthesis in phototrophic eukaryotes. They are a type of plastid and contain their own DNA and ribosomes.

• Enclosed by two membranes.

• Stroma: Gelatinous matrix containing DNA, ribosomes, lipid droplets, starch granules, and thylakoids.

• Thylakoids: Flattened sacs where light reactions occur (ATP, NADPH, and oxygen generation).

• Grana: Stacks of thylakoids.

• Site of dark reactions: Formation of carbohydrates from water and carbon dioxide.

The Nucleus

Structure and Function

The nucleus is the most visible organelle in eukaryotic cells, housing genetic material and directing cellular activities.

• Nuclear envelope: Double membrane with nuclear pores for transport.

• Chromatin: DNA-protein complex; condenses to form chromosomes during cell division.

• Euchromatin: Loosely packed, actively expressed DNA.

• Heterochromatin: Densely packed, less active DNA.

• Nucleolus: Site of ribosomal RNA (rRNA) synthesis and ribosome assembly; not membrane-bound.

Cell Division

Mitosis and Meiosis

Eukaryotic cells divide by mitosis (producing genetically identical cells) or meiosis (producing gametes with half the chromosome number).

• Mitosis: Maintains chromosome number (2N to 2N).

• Meiosis: Reduces chromosome number by half (diploid to haploid).

External Cell Coverings

Cell Wall and Pellicle

Eukaryotic cells may have external coverings for protection and structural support.

• Cell wall: Rigid layer outside the plasma membrane; composition varies (cellulose in algae, chitin in fungi, silica in diatoms).

• Pellicle: Flexible layer beneath the plasma membrane, common in protozoa.

• Cell membranes contain sterols for added strength.

Cilia and Flagella

Structure and Function

Cilia and flagella are motility organelles with similar ultrastructure, enabling movement and environmental interaction.

• Cilia: Short, numerous, move in coordinated waves.

• Flagella: Longer, fewer, move in undulating fashion.

• Both have a membrane-bound cylinder with an axoneme (9+2 arrangement of microtubules).

• Basal body: At the base, directs synthesis and growth of axoneme.

• Dynein: Motor protein driving motility.

• Structurally and functionally distinct from prokaryotic flagella.

Comparison of Prokaryotic and Eukaryotic Cells

Key Differences

The following table summarizes the main differences between prokaryotic and eukaryotic cells:

Feature	Prokaryotic Cells	Eukaryotic Cells
Genetic Material	No nucleus; DNA in nucleoid region	Membrane-bound nucleus
Organelles	Absent	Present (mitochondria, ER, Golgi, etc.)
Cytoskeleton	Absent	Present (microfilaments, microtubules, intermediate filaments)
Cell Wall Composition	Peptidoglycan (bacteria)	Cellulose (algae), chitin (fungi), silica (diatoms), or absent
Ribosomes	70S	80S (cytoplasm), 70S (organelles)
Cell Division	Binary fission	Mitosis and meiosis
Size	Typically 0.5-5 μm	Typically 10-100 μm

Basic Chemical and Genetic Features

• Both cell types share basic chemical composition and genetic code.

• Both perform similar metabolic processes, but with different organizational complexity.

Additional info: Some details on endocytosis, proteasome function, and organelle replication were expanded for academic completeness.