Prokaryotic Cells

Prokaryotic Cells

Prokaryotic Cells

INTRODUCTION

  • A prokaryotic cell is a cell that does not have a true nucleus.
  • The nuclear structure is called a nucleoid.
  • The nucleoid contains most of the cell’s genetic material and is usually a single circular molecule of DNA.
  • A prokaryotic organism, such as a bacterium, is a cell that lacks a membrane- bound nucleus or membrane-bound organelles.
  • The exterior of the cell usually has glycocalyx, flagellum, fimbriae, and pili.

PARTS OF PROKARYOTIC CELLS

Glycocalyx

  • Glycocalyx is a sticky, sugary envelope composed of polysaccharides and/or polypeptides that surround the cell.
  • Glycocalyx is used by a prokaryotic cell to protect it against attack from the body’s immune system.
  • This is the case with Streptococcus mutans, which is a bacterium that colonizes teeth and excretes acid that causes tooth decay.
  • Normally the body’s immune system would surround the bacterium and eventually kill it, but that doesn’t happen with Streptococcus mutans.
  • It has a glycocalyx capsule state, which prevents the Streptococcus mutans from being recognized as a foreign microorganism by the body’s immune system.

Flagella

Flagella are made of protein and appear “whip-like”.

They are used by the prokaryotic cell for mobility.

Flagella can exist in the following forms:

  • Monotrichous: One flagellum.
  • Lophotrichus: A clump of flagella, called a tuft, at one end of the cell.
  • Amphitrichous: Flagella at two ends of the cell.
  • Peritrichous: Flagella covering the entire cell.
  • Endoflagellum: A type of amphitrichous flagellum that is tightly wrapped around spirochetes. A spirochete is a spiral-shaped bacterium that moves in a corkscrew motion.

Fimbriae

  • Fimbriae are proteinaceous, sticky, bristle-like projections used by cells to attach to each other and to objects around them.
  • Neisseria gonorrhoeae, the bacterium that causes gonorrhea, uses fimbriae to adhere to the body and to cluster cells of the bacteria.

Pili

  • Pili are tubules that are used to transfer DNA from one cell to another cell similar to tubes used to fuel aircraft in flight.
  • Some are also used to attach one cell to another cell.
  • The tubules are made of protein and are shorter in length than flagella and longer than fimbriae.

CELL WALL

  • The prokaryotic cell’s cell wall is located outside the plasma membrane and gives the cell its shape and provides rigid structural support for the cell.
  • The cell wall also protects the cell from its environment.
  • The cell wall of many bacteria is composed of peptidoglycan, which covers the entire surface of the cell.
  • Peptidoglycan is made up of a combination of peptide bonds and carbohydrates, either N-acetylmuramic acid, commonly referred to as NAM, or N-acetylglucosamine, which is known as NAG.

The wall of a bacterium is classified in two ways:

difference between gram positive and gram negative bacterial cell

Gram-positive: 

  • A gram-positive cell wall has many layers of peptidoglygan that retain the crystal of violet dye when the cell is stained. 
  • This gives the cell a purple color when seen under a microscope.

Gram-negative: 

  • A gram-negative cell wall is thin.
  • The inside is made of peptidoglycan.
  • The outer membrane is composed of phospholipids and lipopolysaccharides.
  • The cell wall does not retain the crystal of violet dye when the cell is stained.
  • The cell appears pink when viewed with a microscope.

CYTOPLASMIC MEMBRANE

  • The prokaryotic cell has a cell membrane called the cytoplasmic membrane that forms the outer structure of the cell and separates the cell’s internal structure from the environment.
  • The cytoplasmic membrane is a membrane that provides a selective barrier between the environment and the cell’s internal structures.
  • The cytoplasmic membrane provides a selective barrier, allowing certain substances and chemicals to move into and out of the cell.
  • The cytoplasmic membrane is a bilayer of phospholipids that has polar and nonpolar parts, which is referred to as being amphipathic.
  • The nonpolar parts share electrons of atoms equally. The polar parts share electrons unequally.
  • Each polar part has a head that contains phosphate and is hydrophilic (“water-loving”).
  • Each nonpolar part has two tails composed of long fatty acids that are hydrophobic (“water-fearing”).

Cytosol and Cytoplasm

  • The cytosol is the intracellular fluid of a prokaryotic cell that contains proteins, lipids, enzymes, ions, waste, and small molecules dissolved in water, commonly referred to as semifluid.
  • The cytosol also contains a region called the nucleoid, which is where the DNAof the cell is located.
  • A prokaryotic microorganism has a single chromosome that isn’t contained within a nuclear membrane or envelope.
  • Cytosol is located in the cytoplasm of the cell.
  • Cytoplasm also contains the cytoskeleton, ribosomes, and inclusions.

Ribosomes

  • A ribosome is an organelle within the cell that synthesizes polypeptide. There are thousands of ribosomes in the cell.
  • A ribosome is comprised of subunits consisting of protein and ribosomal RNA, which is referred to as rRNA.
  • Ribosomes in prokaryotic cells are uniquely identified by the number of proteins and rRNA molecules contained in the ribosome and by sedimentation rate.
  • Sedimentation rate is the rate at which ribosomes are drawn to the bottom of a test tube when spun in a centrifuge.
  • Sedimentation rate is expressed in Svedberg (S) units.
  • A sedimentation rate reflects the mass, size, and shape of a ribosome and its subunits.
  • Prokaryotic ribosomes are relatively small and less dense than ribosomes of other microorganisms.
  • For example, bacterial ribosomes have a sedimentation rate of 70S compared to the 80S sedimentation rate of a eukaryotic ribosome.

Inclusions

An inclusion is a storage area that serves as a reserve for lipids, nitrogen, phosphate, starch, and sulfur within the cytoplasm.

Scientists use inclusions to identify types of bacteria.

Inclusions are usually classified as granules.

Granule inclusion: 

Membrane-free and densely packed, this type of inclusion has many granules each containing specific substances.

Vesicle inclusion: 

This is a protein membrane inclusion commonly found in aquatic photosynthetic bacteria and cyanobacteria such as phytoplankton, which suspends freely in water.

These bacteria use vesicle inclusions to store gas that give the cell buoyancy to float at a depth where light, carbon dioxide (CO2), and nutrients all required for photosynthesis are available.

Prokaryotic Cells

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