Cell Structure and Function: Prokaryotic and Eukaryotic Cells

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Cell Structure and Function

Overview of Prokaryotic and Eukaryotic Cells

Cells are the fundamental units of life and are classified into two main types: prokaryotic and eukaryotic cells. These classifications are based on structural and functional differences, particularly the presence or absence of a nucleus and membrane-bound organelles.

Prokaryotic cells lack a true nucleus and membrane-bound organelles. They include members of the domains Bacteria and Archaea.
Eukaryotic cells possess a nucleus and various membrane-bound organelles. They include organisms in the domain Eukarya (e.g., plants, animals, fungi, and protists).

Table comparing domains of life: Bacteria, Archaea, Eukarya

Key Differences Between Prokaryotic and Eukaryotic Cells

Nucleus: Prokaryotes lack a nucleus; eukaryotes have a nucleus.
Organelles: Prokaryotes do not have membrane-bound organelles; eukaryotes do.
Cell Size: Prokaryotic cells are generally smaller (1–10 μm) than eukaryotic cells (10–100 μm).
Cellularity: Prokaryotes are almost always unicellular; eukaryotes can be unicellular or multicellular.
DNA Structure: Prokaryotic DNA is typically circular and found in a region called the nucleoid; eukaryotic DNA is linear and housed within the nucleus.
Ribosomes: Prokaryotes have 70S ribosomes; eukaryotes have 80S ribosomes.
Cell Division: Prokaryotes divide by binary fission; eukaryotes divide by mitosis and cytokinesis.

Comparison table of prokaryotic and eukaryotic cells

Structure of Bacterial and Eukaryotic Cells

Bacterial cells are prokaryotic and have a simple structure, while eukaryotic cells are more complex.

Bacterial Cell: Contains circular DNA in the nucleoid, small 70S ribosomes, and lacks membrane-bound organelles.
Eukaryotic Cell: Contains linear DNA within a nucleus, large 80S ribosomes, and various organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus.

Diagram of a bacterial cell Diagram of a eukaryotic cell

Shared Features of All Cells

All cells have a cell membrane composed of a phospholipid bilayer.
All cells contain genetic material (DNA), ribosomes for protein synthesis, and cytoplasm.

Prokaryotic Cell Structures

Structures Outside and Inside the Cell

Prokaryotic cells possess various structures that contribute to their survival and function.

Location	Structure	Description
Outside the Cell	Cell Wall	Rigid layer that protects the cell from damage.
Outside the Cell	Glycocalyx	Sticky layer secreted by the cell to form capsules or slime layers.
Outside the Cell	Flagella	Long protein filaments used for cell motility.
Outside the Cell	Pili/Fimbriae	Protein filaments that extend from the cell surface for attachment or DNA transfer.
Inside the Cell	Ribosomes	Site of protein synthesis in the cytoplasm.
Inside the Cell	Inclusions	Molecular aggregates for storage in the cytoplasm.
Inside the Cell	Endospores	Dormant cell type resistant to damaging conditions.

Table of prokaryotic cell structures and their functions

The Glycocalyx: Capsules and Slime Layers

The glycocalyx is a gelatinous, sticky substance that surrounds the outside of some bacterial cells. It is composed mainly of polysaccharides and sometimes polypeptides.

Capsule: Highly organized and firmly attached to the cell wall; provides protection against desiccation and immune defenses.
Slime Layer: Loosely organized and easily removed; aids in attachment to surfaces and formation of biofilms.

Glycocalyx in Gram-positive and Gram-negative cells Capsule vs. slime layer structure

Biofilms

Biofilms are structured communities of microorganisms encased in a self-produced matrix of extracellular polymeric substances (EPS) and attached to a surface. Biofilms provide protection from environmental threats such as toxins, UV light, and antibiotics.

Biofilms are important in medical and environmental contexts due to their resistance to antimicrobial agents.
Development occurs in stages: attachment, colonization, maturation, and dispersal.

Diagram of a biofilm and its protective matrix Stages of biofilm development Microscopic image of biofilm structure Microscopic image of Staphylococcus aureus biofilm

Quorum Sensing

Quorum sensing is a cell-to-cell communication process that enables bacteria within a biofilm to detect and respond to cell population density by producing and sensing signaling molecules called autoinducers. This process regulates gene expression and coordinates group behaviors such as biofilm formation and virulence.

Diagram of quorum sensing and signal molecule concentration

Prokaryotic Motility: Flagella

Flagellar Structure and Function

Flagella are long, whip-like appendages that provide motility to many prokaryotic cells. The arrangement and number of flagella can be used to classify bacteria.

Monotrichous: Single flagellum at one pole.
Lophotrichous: Tuft of flagella at one or both poles.
Amphitrichous: One flagellum at each pole.
Peritrichous: Flagella distributed over the entire cell surface.
Atrichous: No flagella.

Bacterial flagella and motility Types of flagellar arrangements Monotrichous flagella Lophotrichous flagella Amphitrichous flagella Peritrichous flagella Atrichous cell

Flagellar Structure

A typical prokaryotic flagellum consists of three main parts:

Filament: Composed of the protein flagellin; forms the long, helical structure.
Hook: Flexible segment that connects the filament to the basal body.
Basal Body: Anchors the flagellum to the cell wall and plasma membrane; acts as a rotary motor.

Structure of a prokaryotic flagellum Gram-positive flagellum basal body structure

Summary Table: Prokaryotic Cell Structures

Structure	Function
Cell Wall	Provides shape and protection
Glycocalyx	Attachment, protection, and biofilm formation
Flagella	Motility
Pili/Fimbriae	Attachment, DNA transfer (conjugation)
Ribosomes	Protein synthesis
Inclusions	Storage of nutrients and other molecules
Endospores	Dormant, resistant cell type