Prokaryotes By Reiss Amoah

What are they?  Prokaryotes are single-celled organisms which lack many of the membrane bound organelles found in Eukaryotic cells such as a Nucleus.  They were the first living organisms to inhabit the Earth.  Although they are much smaller than Eukaryotic cells, they are able to live in some very extreme conditions such as 1. In very hot environments such as volcanic springs (Thermophiles) 2. In water with a very high Salinity (Halophiles) 3. In very low acidity for example in the human stomach (Acidophiles) 4. In places with anaerobic conditions such as the intestines of animals (Methanogens)  Their versatility is what has made them thrive in almost all environments on Earth for the last 3.8 billion years.

General Structure

But what do all of these mean?  Let’s take a closer look at the structure and function of all of these parts.

Capsule  Thick mucus layer consisting of starch, proteins and Glycolipids which protects cell from Phagocytosis from White blood cells.  It also prevents the cell from becoming dehydrated by preventing water leakage in dry conditions.

Cell Wall  Cell Wall: Similar to Eukaryotic plant cells. It is made up of Chitin molecules woven together which fortifies the cell, keeping it Turgid.  Gram Negative bacteria have thinner Single-layered cell walls whereas Gram Positive bacteria are Multi-layered cell walls.  It consists of Two layers in Gram Negative bacteria: 1. Periplasm (Gel-like substance between layers) 2. Peptidoglycan  It consists of Four layers in Gram Positive bacteria: 1. Two layers of Periplasm 2. Peptidoglycan 3. Outer Membrane (Lipopolysaccharide)

Cell Membrane  Similar to Eukaryotes, consists of a Phospholipid Bilayer with a multitude of membrane bound proteins, carbohydrates and glycoproteins.  This structure allows the diffusion of small chemicals into and out of the cell.  There are also Channel Proteins which allow the movement of large or charged molecules through the membrane.

Fimbriae  Small hair-like structures which protrude from the surface of prokaryotes (most commonly Gram Negative bacteria) and are required for the attachment of bacteria to organic tissue in order to Colonise.  A very specialised sex pilus is also used by E. Coli to stabilise mating bacteria during Conjugation. This is used to share DNA between Bacteria, in order to spread mutations which make the bacteria resistant to antibiotics.

Flagella  A long, thin, whip-like structure which propels the cell through solution.  They are long filaments consisting of many subunits of Flagellin, a structural protein, arranged in a ring of microtubules.  The Flagellin is surrounded by plasma membrane to form the structure known as the Axoneme (Flagellum).  The movement is given energy through the Hydrolysis of ATP.

Cytoplasm  Consists mainly of water, enzymes, proteins and various other cell components such as Food Stores, Ribosomes and Organelles.  Where the majority of chemical reactions occur within a cell.

Nucleoid  Similar to a Eukaryotic cell’s Nucleus, the Nucleoid is where the majority of a Prokaryotes genetic material may be found.  Unlike a Nucleus, the Nucleotide is not membrane bound. This makes it easier for Ribosomes and Enzymes to reach.  The genetic material is organised into a Circle of Super coiled Double- Stranded DNA.

Plasmids  Small circular DNA molecules within the cell which are separate from the chromosome in the Nucleoid, as a result they can replicate independently.  Most commonly found in bacteria, although they can also occur in some Eukaryotes such as Yeast.  Bacteria can exchange genetic material in the form of plasmids through the process of conjugation in order to increase genetic diversity within a species as well as passing on a mutation which makes them resistant to antibiotics.  Scientists use plasmids as Vectors as a means to clone, transfer or manipulate genes as they are able to enter a cell easily.

Ribosomes  A globular protein consisting of two subunits, one larger than the other.  The larger subunit (50S) is the bonding site for Amino acid-carrying tRNA molecules onto the mRNA strand as well as Ribozymes which aid in the process of Protein Synthesis.  whereas the smaller subunit (30S) is the one that initially locates and binds onto an mRNA strand before fixing onto a larger ribosome subunit.  Ribosomes translate mRNA from the nucleoid to form long sequences of Amino acids which correspond to triplet codons on mRNA strand. These amino acids bond through the formation of Peptide bonds to form a protein.  The Ribosomes in prokaryotes are considerably smaller than those of Eukaryotes. They are also all free-floating in the cytoplasm.

The Two Domains of Prokaryotes  Prokaryota can be split into Two Domains, Bacteria and Archaea.  Although the two may appear very similar in terms of general structure, there are some rather important chemical differences which change some of the processes within the cell.

Similarities of Archaea and Bacteria  Both single-celled organisms.  Generally have very similar shape, size and appearance.  Both can reproduce Asexually through Binary Fission, Budding and Fragmentation.  Both contain small Ribosomes (70S).  Both groups contain Cell Walls.  Neither contain organelles.

Differences between Archaea and Bacteria  Archaea are Extremophiles (Thrive in extreme conditions) whereas Bacteria are Ubiquitous (inhabit all parts of the Earth).  The Cell Wall of Archaea consists of Pseudopeptidoglycan whereas the cell wall of Bacteria consists of Peptidoglycan and Lipopolysaccharide.  Bacteria can produce Spores to remain dormant for years whereas Archaea cannot.  Generally, Archaea do not contain Plasmids. Bacteria on the other hand almost always contain them.  Some Archaea (especially Extremophiles) have a Lipid Monolayer instead of a Lipid Bilayer as found in all Bacteria and Eukaryotic cells.  The RNA polymerase found in Archaea are far more complex than those found in Bacteria.  Archaea have a different Metabolic Pathway compared to Bacteria. (No Kreb’s Cycle).  Generally, Archaea are Non-Pathogenic whereas most Bacteria are.  In Archaea cell membrane, carbon bonds to lipids through Ether bonds whereas in Bacteria they bond through Ester bonds.