Cellular Biology

Prokaryotic Cell Strructure



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Prokaryotes are evolutionarily ancient, for billions of years the only form of life. Here's a summary of the basic "parts & pieces" of these primitive cells.

Prokaryotes

Pro = "before", karyon = "nucleus"

Prokaryotes were the first type of cell to evolve, and unlike eukaryotic cells, still lack a membrane bound nucleus. Their genetic material is naked within the cytoplasm, ribosomes their only type of organelle.

Prokaryotes are most always single-celled, except when they exist in colonies. These ancestral cells, now represented by members of the domains Archaea and Eubacteria, reproduce by means of binary fission, duplicating their genetic material and then essentially splitting to form two daughter cells identical to the parent.

Structure of Prokaryotic Cells

Plasmids

This is a small extra piece of genetic material, typically 5 - 100 genes that are not critical to everyday functions. Plasmids can provide "bonus" genetic information, such as antibiotic resistance, virulence factors (molecules produced by pathogen that helps the invader thrive) or can promote conjugation (transfer of genetic material between bacteria).

Cytoplasm

This gel matrix is also known as protoplasm, and consists of water, enzymes, nutrients, wastes, gases and cell structures. It is the location of growth, metabolism, and replication.

Granules

These masses of substance are how prokaryotes store nutrients such as glycogen, polyphosphate, sulfur or the lipid poly-B-hydoxybuteric (PBH). PBH is one of most common energy reserve products stored in the granules of prokaryotes.

Ribosomes

These ribose sugar bodies are made of rRNA and proteins and exist either free within cytoplasm or attached to the plasma membrane. Ribosomes translate genetic code into proteins. They are composed of a small (30S) subunit and a large (50S) subunit and each cell may contain thousands.

Cytoskeleton

A major advance in prokaryotic cell biology in the last decade has been the discovery of a prokaryotic cytoskeleton. This is the cellular "scaffolding" or "skeleton" within the cytoplasm previously thought to be a feature only of eukaryotic cells.

Plasma Membrane

A phospholipid bilayer separates the cell from its environment. Phospholipid molecules are oriented so that hydrophilic (water-loving) heads are directed outward and hydrophobic (water-hating) tails are directed inward. Proteins and other molecules make up the fluid mosaic moving among the two layers of lipids. The plasma membrane is selectively permeable to allow substances to pass into and out of the cell.

Cell Wall

This rigid structure of prokaryotes contains peptidoglycan giving the cell shape and surrounding the cytoplasmic membrane. The cell wall provides the cell with protection from environment.

LPS Membrane

Gram-negative bacteria have this outer membrane, similar to the plasma membrane, but less permeable and composed of lipopolysaccharides (LPS). LPS is a harmful substance classified as an endotoxin.

Glycocalyx

Some bacteria have an additional layer called the glycocalyx. This additional layer can come in one of two forms.

A slime layer consists of glycoproteins loosely associated with the cell wall. Slime layers cause bacteria to adhere to solid surfaces.

Capsules are formed from polysaccharides firmly attached to the cell wall.They adhere to solid surfaces and to nutrients in the environment.The adhesive power of capsules is a major factor in the initiation of some bacterial diseases.

Endospores

These dormant, tough, non-reproductive structures are produced by small number of bacteria from Firmicute family. The primary function of most endospores is to ensure survival of bacteria through periods of environmental stress, such as radiation, desiccation, lysozymes, temperature, starvation, and chemical disinfectants.

Surface Appendages

Some prokaryotes have distinct appendages that allow them to move about or adhere to solid surfaces. These appendages consist of delicate stands of proteins and come in three main forms.

Flagella: long, thin extensions that allow bacteria to move about freely in aqueous environments Axial filaments or endoflagella: wind around bacteria causing movement in waves Pili or fimbriae: shorter, finer appendages that surround the cells of some gram-negative bacteria. They have no role in motility, but permit microbes to adhere to solid surfaces.

Sources

Bauman, R. (2005) Microbiology.

Park Talaro, K. (2008) Foundations in Microbiology.

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