Types of Cells

TYPES OF CELLS

  • Cells, as the fundamental units of life, display significant diversity in their structure and functions.
  •  Cells can be broadly categorized into two main types: prokaryotic and eukaryotic cells.
  •  Understanding these cell types is essential for comprehending the complexity of life forms on Earth.

Prokaryotic Cells:

- These cells lack a membrane-bound nucleus and are typically simpler in structure.

- They are commonly found in bacteria and archaea.

Eukaryotic Cells: 

- Eukaryotic cells have a defined nucleus enclosed by a membrane, and they are found in various multicellular organisms, including plants, animals, and fungi. 

  • In the following discussion, we will delve deeper into the characteristics and distinctions between prokaryotic and eukaryotic cells, highlighting their vital roles in the living world. 

Prokaryotic Cells

  • Prokaryotic cells are primarily found in organisms such as bacteria, blue-green algae, mycoplasma, and PPLO (Pleuro Pneumonia Like Organisms).

 

 

Eukaryotes and Prokaryotes  

  • Size and Reproduction:

- Prokaryotic cells are generally smaller than eukaryotic cells and reproduce at a faster rate. 

  • Diversity in Shape:

- Prokaryotic cells exhibit a wide range of shapes and sizes, with common shapes being bacillus (rod-like), coccus (spherical), vibrio (comma-shaped), and spirillum (spiral).

 

 

  • Basic Organization:

- Prokaryotic cells share fundamental structural features despite their varied shapes and functions. 

  • Cell Wall:

- All prokaryotes have a cell wall surrounding the cell membrane, except for mycoplasma. 

  • Cytoplasm:

- The interior of the cell is filled with a fluid matrix called cytoplasm.

  • Nucleus:

- Prokaryotic cells lack a well-defined nucleus.

- Their genetic material is essentially naked and not enclosed by a nuclear membrane. 

  • Plasmids:

- Many bacteria have small circular DNA structures called plasmids in addition to the genomic DNA.

- These plasmids can confer unique characteristics, including antibiotic resistance.

  • Organelles:

- Prokaryotic cells do not contain membrane-bound organelles like eukaryotic cells, except for ribosomes.

  • Inclusions:

- Prokaryotes have unique structures known as inclusions.

  • Mesosomes:

- Mesosomes are specialized, differentiated forms of the cell membrane found in prokaryotic cells, and they are essentially infoldings of the cell membrane. 

Cell Envelope and its Modifications

  • Complex Cell Envelope:

- Most prokaryotic cells, especially bacterial cells, possess a chemically complex cell envelope, which is a three-layered structure consisting of the outermost glycocalyx, followed by the cell wall, and then the plasma membrane.

- These layers function together as a protective unit.

  • Gram Classification:

- Bacteria can be classified into two groups based on differences in their cell envelopes and how they respond to the Gram staining procedure.

- Gram-positive bacteria take up the stain, while Gram-negative bacteria do not.

 

 

  • Glycocalyx:

- The composition and thickness of the glycocalyx can vary among different bacteria.

- It can be a loose sheath known as the slime layer in some, while in others, it may be thick and tough, forming a capsule. 

  • Cell Wall:

- The cell wall determines the shape of the bacterial cell and provides structural support, preventing bursting or collapsing.

  • Plasma Membrane: 

- The plasma membrane is selectively permeable and interacts with the external environment, sharing structural similarities with eukaryotic membranes.

  • Mesosome:

- A unique membranous structure formed by extensions of the plasma membrane into the cell.

 

 

 

- It can be in the form of vesicles, tubules, and lamellae.

- Mesosomes aid in various functions, including cell wall formation, DNA replication and distribution, respiration, secretion, and enzymatic processes. 

  • Chromatophores:

- In certain prokaryotes like cyanobacteria, membranous extensions into the cytoplasm known as chromatophores contain pigments. 

  • Motility:

- Bacterial cells may be motile or non-motile. Motile bacteria have thin filamentous extensions from their cell wall called flagella.

- The arrangement and number of flagella can vary.

A bacterial flagellum consists of three parts: filament, hook, and basal body.

  • Pili and Fimbriae:

- Pili and fimbriae are surface structures in bacteria but do not contribute to motility.

- Pili are elongated tubular structures made of a specific protein, while fimbriae are small bristle-like fibers that protrude from the cell.

- They are known to help attach bacteria to surfaces, including host tissues in some cases.

 

 

These modifications of the cell envelope and surface structures play crucial roles in bacterial functions, including motility, adhesion, and protection.

 Inclusion Bodies

  • Prokaryotic cells store reserve materials in the cytoplasm in the form of inclusion bodies.  
  • These structures are not enclosed by a membrane system and are free within the cytoplasm.
  • Examples of Inclusion Bodies: 

- Inclusion bodies can vary in composition and function. Examples include phosphate granules, cyanophycean granules, and glycogen granules. These structures serve as reservoirs of essential substances. 

  • Gas Vacuoles:

- Some prokaryotic cells, such as blue-green and purple and green photosynthetic bacteria, contain gas vacuoles.

- These gas-filled vacuoles help regulate buoyancy, allowing the cells to move within aquatic environments efficiently.

 Eukaryotic Cells

  • Eukaryotes encompass a wide range of organisms, including protists, plants, animals, and fungi. 
  • Compartmentalization:

- Eukaryotic cells exhibit extensive compartmentalization of their cytoplasm through the presence of membrane-bound organelles. These structures allow for specialized functions within the cell. 

  • Organized Nucleus:

- Eukaryotic cells have an organized nucleus with a nuclear envelope that separates the genetic material from the rest of the cell. 

  • Complex Structures:

- Eukaryotic cells possess a variety of complex locomotory and cytoskeletal structures, enabling them to perform various functions. 

  • Chromosomes:

- The genetic material in eukaryotic cells is organized into chromosomes, which carry the hereditary information of the cell. 

  • Cell Diversity:

- Eukaryotic cells are not uniform. Plant and animal cells, for example, have distinct differences. Plant cells are characterized by the presence of cell walls, plastids, and a large central vacuole, which are absent in animal cells. In contrast, animal cells contain centrioles, which are generally absent in plant cells.