Cell Classification and Types

Study Notes

Organisms with one cell in their body are called unicellular organisms (bacteria, blue-green algae, some algae, Protozoa, etc.).
Organisms with many cells in their body are called multicellular organisms (fungi, most plants and animals).

Prokaryote means "before nucleus" in Greek.
Prokaryotes include all cells that lack a nucleus and other membrane-bound organelles.
Examples of prokaryotes include Mycoplasma, viruses, bacteria, and cyanobacteria or blue-green algae.
Most prokaryotes range between 1 µm to 10 µm in size, but they can vary in size from 0.2 µm to 750 µm (Thiomargarita namibiensis).
Prokaryotes belong to two taxonomic domains: bacteria and archaea.
Most prokaryotes are unicellular, with exceptions being myxobacteria which have multicellular stages in their life cycles.

Prokaryotes lack cell organelles but have internal structures such as cytoskeletons and ribosomes.
Membranous organelles are known in some groups of prokaryotes, such as vacuoles or membrane systems devoted to special metabolic properties (e.g., photosynthesis or chemolithotrophy).
Some species also contain protein-enclosed microcompartments, which have distinct physiological roles (carboxysomes or gas vacuoles).

Gram-positive bacteria possess a thick cell wall containing many layers of peptidoglycan and teichoic acids.
Gram-negative bacteria have a relatively thin cell wall consisting of a few layers of peptidoglycan surrounded by a second lipid membrane containing lipopolysaccharides and lipoproteins.

Cell membrane: surrounds the cell's cytoplasm and regulates the flow of substances in and out of the cell.
Cytoplasm: a fluid in nature that fills the cell and is composed mainly of 80% water that also contains enzymes, salts, cell organelles, and various organic molecules.
Ribosomes: organelles responsible for protein synthesis.
Nucleoid Region: the area of the cytoplasm that contains the bacterial DNA molecule.

Plasmid: a DNA molecule (mostly in bacteria) that is separate from and can replicate independently of the chromosomal DNA.
Pili: hair-like structures on the surface of the cell that help attach to other bacterial cells.
Types of pili include conjugative pili (allow the transfer of DNA between bacteria) and type IV pili (generate motile forces).

Prokaryotic cells have various shapes, including cocci (spherical), bacilli (rod-shaped), spirochaete (spiral-shaped), and vibrio (comma-shaped).

Prokaryotes live in nearly all environments on Earth, including extreme conditions (thermophiles or halophiles).
Some archaea and bacteria thrive in extreme conditions.
Prokaryotes can form stable aggregated communities in a stabilizing polymer matrix called "biofilms".
Cells in biofilms often show distinct patterns of gene expression (phenotypic differentiation) in time and space.

A colony of bacteria is a group of individual organisms of the same species living closely together in mutualism.
The shape of the colony can be circular and irregular.
Bacterial colonies are frequently shiny and smooth in appearance.

Bacteria and archaea reproduce through asexual reproduction known as binary fission.
Binary fission is an asexual mode of reproduction in which the genomic DNA undergoes replication and the original cell is divided into two identical cells.