Prokaryotic and Eukaryotic Cells: Structure, Function, and Comparison

Prokaryotic vs Eukaryotic Cells

Overview of Cell Types

Cells are the fundamental units of life and can be classified into two major types: prokaryotic and eukaryotic cells. Understanding their differences is essential for studying cell biology.

• Prokaryotic cells lack a nucleus and membrane-bound organelles. Examples include Bacteria and Archaea.

• Eukaryotic cells possess a nucleus and various membrane-bound organelles. Examples include Animalia, Plantae, Fungi, and Protista.

Key Differences

• Nucleus: Eukaryotes have a true nucleus; prokaryotes have a nucleoid region.

• Organelles: Eukaryotes have membrane-bound organelles; prokaryotes do not.

• DNA Structure: Eukaryotic DNA is linear and associated with histone proteins; prokaryotic DNA is circular and not associated with histones.

• Cell Wall: Most prokaryotes have a cell wall made of peptidoglycan; plant eukaryotes have cell walls made of cellulose, fungi of chitin, and animal cells lack cell walls.

Cell Structure and Membranes

Plasma Membrane

The plasma membrane is a universal feature of all cells, providing a barrier and regulating the movement of substances.

• Composed of a phospholipid bilayer with embedded proteins.

• Functions include protection, communication, and transport.

Cell Wall

• Prokaryotic cell walls: Made of peptidoglycan (bacteria).

• Plant cell walls: Made of cellulose.

• Fungal cell walls: Made of chitin.

• Animal cells: No cell wall.

Genetic Material and Organization

Eukaryotic DNA

Eukaryotic DNA is organized into chromosomes within the nucleus.

• DNA is linear and wrapped around histone proteins.

• Paired chromosomes allow for sexual reproduction and genetic diversity.

Prokaryotic DNA

• DNA is found in a nucleoid region, typically as a single circular, double-stranded chromosome.

• May contain plasmids: small, circular DNA molecules that can be transferred between cells.

Protein Production

Ribosomes

Ribosomes are the sites of protein synthesis in all cells.

• Eukaryotic ribosomes: 80S (composed of 60S and 40S subunits).

• Prokaryotic ribosomes: 70S (composed of 50S and 30S subunits).

• Made of protein and ribosomal RNA (rRNA).

Endomembrane System

Components and Functions

The endomembrane system in eukaryotic cells is responsible for the synthesis, sorting, and transport of proteins and lipids.

• Nuclear envelope: Double membrane surrounding the nucleus; contains pores for mRNA transport.

• Endoplasmic reticulum (ER):

  • Rough ER: Studded with ribosomes; site of protein synthesis and folding.

  • Smooth ER: Lacks ribosomes; involved in lipid synthesis, detoxification, and calcium storage.

• Golgi apparatus: Modifies, sorts, and packages proteins and lipids for delivery.

• Vesicles: Transport materials between organelles and to the cell membrane.

Energy Production

Eukaryotic Organelles

• Mitochondria: Site of cellular respiration in animal, plant, and fungal cells.

• Chloroplasts: Site of photosynthesis in plant cells.

Prokaryotic Energy Production

• Occurs across the plasma membrane; no membrane-bound organelles.

Cellular Recycling and Storage

Lysosomes and Vacuoles

• Lysosomes: Vesicles containing digestive enzymes; break down food, waste, and damaged organelles (mainly in animal cells).

• Vacuoles: Storage organelles; maintain cell tonicity, store ions, and waste (large central vacuole in plant cells).

Peroxisomes

• Break down fatty acids and amino acids, producing hydrogen peroxide.

• Also involved in cholesterol synthesis for cell membranes.

Cell Motility

Cilia and Flagella

• Cilia: Short, hair-like structures for movement (mainly in eukaryotes).

• Flagella: Long, whip-like structures; present in both prokaryotes and eukaryotes (single or multiple).

• Prokaryotes do not have cilia.

Compartmentalization and Specialization

Advantages in Eukaryotes

Compartmentalization allows eukaryotic cells to maintain different environments for specialized functions, increasing efficiency and compatibility of cellular processes.

• Organelles such as the nucleus, mitochondria, and ER create distinct regions for specific tasks.

• Specialization of function is possible due to compartmentalization.

Endosymbiotic Theory

Origin of Mitochondria and Chloroplasts

The endosymbiotic theory explains the origin of mitochondria and chloroplasts as formerly free-living bacteria engulfed by ancestral eukaryotic cells.

• Both organelles contain circular DNA similar to prokaryotes.

• They reproduce independently within the cell.

• Have double membranes and their own ribosomes.

• Antibiotics that block bacterial protein synthesis also affect these organelles.

Surface Area to Volume Ratio

Importance for Cells

The surface area to volume ratio (SA:V) is crucial for cell function, affecting nutrient uptake and waste removal.

• As cell size increases, volume grows faster than surface area.

• High SA:V ratio allows efficient exchange of materials.

• Cells may adopt unique shapes (elongated, flattened, biconcave) to increase SA:V.

Formulas:

• Volume of a sphere: $V = \frac{4}{3}\pi r^3$

• Volume of a rectangular solid: $V = lwh$

• Volume of a cylinder: $V = \pi r^2 h$

• Volume of a cube: $V = a^3$

Cytoskeleton

Structure and Function

The cytoskeleton provides structural support, enables cell movement, and organizes organelles.

• Microtubules: Hollow rods made of tubulin; involved in cell shape, transport, and chromosome separation during division.

• Microfilaments: Thin fibers made of actin; involved in cell movement and muscle contraction.

• Intermediate filaments: Provide mechanical strength; made of keratin and other proteins.

Cell Junctions

Types and Functions

Cell junctions connect cells and facilitate communication and structural integrity in tissues.

Junction Type	Structure	Function	Location
Tight Junctions	Belt-like protein seals	Prevent leakage of extracellular fluid	Animal cells
Desmosomes	Rivet-like protein complexes	Provide mechanical strength	Animal cells
Gap Junctions	Protein channels	Allow ions and small molecules to pass	Animal cells
Plasmodesmata	Channels in cell walls	Allow cytosol and small molecules to pass	Plant cells

Summary Table: Prokaryotic vs Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	No	Yes
DNA Structure	Circular, single chromosome	Linear, multiple chromosomes
Organelles	Absent	Present
Cell Wall	Peptidoglycan (bacteria)	Cellulose (plants), chitin (fungi), none (animals)
Ribosomes	70S	80S
Reproduction	Asexual (binary fission)	Sexual and asexual

Example:

Escherichia coli is a prokaryotic bacterium, while Homo sapiens cells are eukaryotic.

Additional info: Some details, such as the specific protein names and drug interactions with microtubules, were inferred and expanded for academic completeness.