Prokaryotes vs Eukaryotes: Cell Structure and Solutions to Cellular Size

Prokaryotes vs Eukaryotes

Overview of Cell Types

Cells are classified into two major types: prokaryotes and eukaryotes. Understanding their structural differences is fundamental in biology, as these differences underpin many cellular functions and evolutionary processes.

• Prokaryotes are simpler cells, lacking a nucleus and most organelles. Examples include Bacteria and Archaea.

• Eukaryotes possess a nucleus and various membrane-bound organelles. Examples include plants, animals, fungi, and protists.

Structural Features of Prokaryotic and Eukaryotic Cells

Both cell types share some basic features, but eukaryotes have additional complexity due to their organelles and larger size.

Feature	Prokaryotes	Eukaryotes
Nucleus	Absent (nucleoid region)	Present
Organelles	Few (e.g., ribosomes)	Many (e.g., mitochondria, ER, Golgi)
Cell Size	Typically 0.1–5 μm	Typically 10–100 μm
Cell Wall	Usually present (peptidoglycan)	Present in plants/fungi (cellulose/chitin), absent in animals
Examples	Escherichia coli, Corynebacterium diphtheriae	Human cells, plant cells

Key Structures in Prokaryotes

• Nucleoid: Region containing circular DNA.

• Ribosomes: Sites of protein synthesis.

• Plasma Membrane: Controls entry/exit of substances.

• Cell Wall: Provides structural support.

• Fimbriae: Hair-like structures for attachment.

• Flagella: Used for movement.

• Glycocalyx: Protective outer layer.

Key Structures in Eukaryotes

• Nucleus: Contains genetic material (DNA).

• Endoplasmic Reticulum (ER): Rough ER (protein synthesis), Smooth ER (lipid synthesis).

• Golgi Apparatus: Modifies, sorts, and packages proteins.

• Mitochondria: Site of cellular respiration and energy production.

• Cytoskeleton: Microfilaments, intermediate filaments, microtubules for structure and transport.

• Other Organelles: Peroxisomes, lysosomes, centrosomes, microvilli.

Cell Size and the Problem of Being Big

Relative Sizes of Biological Structures

Eukaryotic cells are generally much larger than prokaryotic cells, often by several orders of magnitude. This size difference presents unique challenges for cellular function, especially regarding transport and communication within the cell.

Structure	Approximate Size
Human height	~1.7 m
Human egg	~100 μm
Most plant/animal cells	~10–100 μm
Most bacteria	~1 μm
Viruses	~100 nm
Ribosomes	~20 nm
Small molecules	~1 nm
Atoms	~0.1 nm

Diffusion and Brownian Motion

One of the main challenges for large cells is the efficient movement of molecules. Diffusion is the process by which molecules move from areas of high concentration to low concentration due to random motion, also known as Brownian motion.

• Diffusion: Random movement of molecules from point A to point B.

• Brownian Motion: The random movement of particles suspended in a fluid, resulting from collisions with fast-moving molecules in the fluid.

• As cell size increases, diffusion becomes less efficient for moving substances across the cell.

Equation for Diffusion Time:

$t = \frac{R^2}{2D}$

Where t is the time required for diffusion, R is the distance, and D is the diffusion coefficient.

• For small cells (e.g., E. coli, R = 1 μm), diffusion is fast.

• For large cells (e.g., human cells, R = 20 μm), diffusion is slower.

• For very large cells (e.g., neuronal axons, R = 1 cm), diffusion is extremely slow.

Solutions to the Problem of Large Cell Size

Compartmentalization and Organelles

Eukaryotic cells have evolved specialized structures to overcome the limitations of diffusion and maintain efficient cellular function.

• Endomembrane System: Includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and vesicles. These compartments localize and specialize cellular processes.

• Cytoskeleton: Provides structural support and facilitates transport within the cell.

• Compartmentalization: Allows for distinct microenvironments, optimizing conditions for specific biochemical reactions.

Endoplasmic Reticulum (ER)

The ER is a network of membranes involved in biosynthesis and transport.

• Rough ER: Studded with ribosomes; site of protein synthesis for export or membrane insertion.

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

• Transport Vesicles: Move proteins and other molecules between compartments.

Ribosomes and Protein Synthesis

Ribosomes are complexes of ribosomal RNA and proteins that synthesize polypeptides. In eukaryotes, ribosomes can be free in the cytoplasm or bound to the ER.

• Free Ribosomes: Synthesize proteins for use within the cytoplasm.

• Bound Ribosomes: Synthesize proteins for export or for use in membranes and organelles.

• Compartmentalization of translation ensures proper folding and chemical environment for proteins.

The Nucleus: Information Central

Structure and Function

The nucleus is the control center of eukaryotic cells, containing most of the cell's genetic material organized as chromatin.

• Nuclear Envelope: Double membrane with nuclear pores for transport of molecules.

• Chromatin: DNA and associated proteins, organized into chromosomes.

• Nucleolus: Site of ribosome assembly.

Gene expression involves transcription of DNA to mRNA in the nucleus, followed by translation of mRNA to protein in the cytoplasm.

Summary Table: Prokaryotes vs Eukaryotes

Characteristic	Prokaryotes	Eukaryotes
DNA Location	Nucleoid	Nucleus
Cell Division	Binary fission	Mitosis/meiosis
Complexity	Simple	Complex

Example

Example: The bacterium Corynebacterium diphtheriae is a prokaryote, while a human liver cell is a eukaryote. The human cell contains a nucleus, mitochondria, ER, and other organelles, whereas the bacterium does not.

Additional info: Some context and explanations have been expanded for clarity and completeness, including the diffusion equation and the functional significance of compartmentalization.