Prokaryotic Cell Structure (SL IB Biology)

• The cell structure of organisms determines whether they are prokaryotic or eukaryotic
• Prokaryotes have the simplest cell structure, being the first organisms to evolve on Earth and have been classified into two domains:
  • Bacteria or Eubacteria - 'true' bacteria, includes commonly known bacteria such as E.coli and Helicobacter
  • Archaebacteria or Archaea - typically found in extreme environments such as high temperatures and salt concentrations and include methanogens (organisms that exist in anaerobic conditions and produce methane gas)

• Prokaryotic cells are small, ranging from 0.1µm to 5.0µm
• Prokaryotes have cells that lack a nucleus (the Greek roots of prokaryote are 'pro' = before and 'karuon' = nut or kernel, relating to 'before the nucleus')

Structure of prokaryotic cells

• The cytoplasm of prokaryotic cells is not divided into compartments, it lacks membrane-bound organelles
• Structures that are common to most prokaryotes include:
  • 70S ribosomes
  • DNA in a loop
  • Cytoplasm
  • Plasma membrane
  • Cell wall

Ribosomes

• Prokaryotic ribosomes are structurally smaller (70S) in comparison to those found in eukaryotic cells (80S)
• The function of these ribosomes is the binding and reading of mRNA during translation to produce proteins

DNA

• Prokaryotes do not have a nucleus, but they do have genetic material. This is generally in the form of a "naked" single circular DNA molecule (not associated with proteins) located in the nucleoid and in smaller loops called plasmids
• Plasmids are small loops of DNA that are separate from the main circular DNA molecule
  • Plasmids contain genes that can be passed between prokaryotes (e.g. genes for antibiotic resistance)

Cytoplasm

• Prokaryotic cytoplasm is very similar to the cytoplasm of any other cell
• It is the site of many cellular reactions
• This is where the 70S ribosomes are found
• A major component of the cytoplasm is a gel-like cytosol, a water-based solution that contains ions, small molecules, and macromolecules

Cell membrane

• The cell membrane of prokaryotes is composed of a lipid bilayer
• A group of prokaryotes, known as archaea, have their plasma membrane formed as a monolayer as opposed to a bilayer
• The role of the plasma membrane is to control substances entering and exiting the cell

Cell wall

• Most prokaryotes have a cell wall containing murein/peptidoglycan (a glycoprotein)
• The cell wall acts as protection, maintains the shape of the cell and prevents the cell from bursting
• Some bacteria are able to be classified because of their cell wall structure
  • Their ability to retain a dye called crystal violet classifies a group of bacteria as Gram positive, they appear blue/violet after exposure to the dye
  • Examples of gram positive bacteria are Bacillus and Staphylococcus
  • Bacteria that do not react with the dye are referred to as Gram negative bacteria

Additional structures

• In addition, many prokaryotic cells have a few other structures that differentiate the species from others and act as a selective advantage, examples of these are:
  • Plasmids
  • Capsules
  • Flagellum
  • Pili
• Some prokaryotes (e.g. bacteria) are surrounded by a final outer layer known as a capsule. This is sometimes called the slime capsule
  • It helps to protect bacteria from drying out and from attack by cells of the immune system of the host organism
• Flagellum (plural = flagella) are long, tail-like structures that rotate, enabling the prokaryote to move (a bit like a propeller)
  • Some prokaryotes have more than one
• Pili are shorter and thinner structures than flagella
  • They assist with movement, avoidance of attack by white blood cells, conjugation (the sexual mode for bacteria) and are commonly used to allow bacteria to adhere to cell surfaces

A diagram of the structure of prokaryotic cells

Prokaryotic cells are often described as being ‘simpler’ than eukaryotic cells, and they are believed to have emerged as the first living organisms on Earth