Prokaryote Structure

expressed in groups rather than in separate genes. The groups are called “operons”. They are later divided into separate proteins.

Prokaryotes also have a larger surface area to volume ratio. This means that they are more highly metabolically active when compared to eukaryotes. They divide faster and have a shorter generation time (which is the time from cell division to another cell division).

Prokaryotes are “haploid”, meaning they have just one copy of the genes. On the contrary, eukaryotes are diploid, having two copies of a particular gene. They do not have histones, which are the proteins that condense the genetic material. They have their own condensing proteins and supercoil the circular piece of DNA in order to condense it. Transcription and translation into proteins happen at the same time in prokaryotes but not in eukaryotes.

PROKARYOTE STRUCTURE

There are many different structures of bacteria. These are small cells—only about a tenth of the size of most eukaryotic cells, being less than 5 micrometers in length. The main shapes of bacteria include spherical, called “cocci” bacteria, rod-shaped, called bacilli, comma-shaped, called vibrio, and spiral-shaped, called spirochetes. There are very rarely other shapes, including star-shaped bacteria. The cell wall and the intracellular cytoskeleton determine what the shape of bacterial species is. The shape of the bacterial organism determines many things, including the mobility of the organism.

While bacteria are essentially single-celled organisms, they often form multicellular shapes. Streptococcus bacteria form chains of varying lengths, while Neisseria species form pairs or diploid configurations. Staphylococcus species are rarely singular and come in bunches, looking like spherical bunches of grapes. Myxobacteria, Actinobacteria, and Streptomyces form aggregates, filaments, and hyphae, respectively. Figure 12 shows what different bacterial shapes look like:

Bacterial species can also form complex states, depending on the conditions they find themselves in. They communicate with each other through the act of what’s called “quorum sensing”, in which they migrate together in unfavorable circumstances. This happens with Myxobacteria, forming aggregates that have large fruiting bodies that have certain cells going into dormancy states, known as “myxospores”. These myxospores are resistant to adverse environmental conditions, such as situations of dryness, which normally doesn’t support bacterial growth. Another common bacterial phenomenon is the biofilm formation. This is the attachment of bacteria to surfaces, such as the lining of the bladder or GI tract in dense aggregations. There are larger aggregates that will form a microbial mat. These mats can be really thick, containing other types of microorganisms in the form of an ecosystem. There can be microcolonies, that have networks of channels that enable the enhanced diffusion of cellular nutrients. Bacteria in these biofilms and mats will be more difficult to kill when compared to individual bacterial organisms.

Bacteria have a cell membrane that is made mainly of phospholipids. This membrane completely encloses the cell and defines the intracellular space. Inside this space is cytoplasm and a notable absence of membrane-bound structures seen in eukaryotic cells. There are, however, some protein-bound organelles that define different areas of

the bacterial metabolism. The carboxysomes, for example, are protein-bound structures that concentrate carbon dioxide inside the shell by using localized areas of carbonic anhydrase activity, which makes carbon dioxide from the bicarbonate that diffuses into the carboxysome.

Bacteria also have a cytoskeleton that creates their structure. It also localizes the different proteins and nucleic acids within the cell. The cytoskeleton also manages the process of cellular division. It is not unlike the cytoskeleton of eukaryotes, that maintains certain compartments within the cell. Because there are no membranous structures inside the bacterial cell, those reactions that need a membrane must be done across the cell membrane. Electron transport, an important way of gaining cellular energy, happens between the cytoplasm inside the cell and the periplasm outside the cell. Figure 13 shows the basic structure of a bacterium:

A few bacteria engage in photosynthesis and will have highly folded plasma membrane structures, filling the cell with multiple layers of light-gathering membrane complexes. These will sometimes form lipid-enclosed structures known as chlorosomes. These are particularly seen in green sulfur bacterial species.

The DNA or genetic material inside a bacterium is called a nucleoid rather than being membrane bound in a nucleus. The nucleoid generally consists of a circular piece of

DNA along with RNA and proteins necessary for protein synthesis. They do contain ribosomes, which are protein-creating structures; however, the structure of these ribosomes is somewhat different than is seen in eukaryotic cells.

There are also nutrient storage granules within the bacterial cell. These may contain polyhydroxyalkanoates, sulfur, polyphosphate, and glycogen granules. There are also gas vacuoles inside Cyanobacteria, which help regulate the ability of these bacterial organisms to be buoyant in an aqueous environment. This allows them to move into layers of a body of water in order to regulate light exposure and the nutrient environment.

Bacteria are surrounded by cell walls consisting of peptidoglycan. These are sugar chains (polysaccharide chains) that are crosslinked by small proteins or “peptides”. These are greatly different from the cell walls of plants (which are made from cellulose) and fungi (which are made from chitin). Archaea, smaller organisms we will discuss later, also have cell walls but these don’t consist of peptidoglycan. Cell walls are of two major categories, depending on whether they stain positively or negatively with the Gram staining technique, which is a popular way to see bacteria under a microscope. There are Gram-positive species and Gram-negative species. The Gram-positive bacteria have very thick cell walls containing both peptidoglycan and teichoic acids. They take up the Gram-stain and appear red under the microscope. The Gram-negative bacteria consist of thin cell walls that do not take up the stain and that are surrounded by a second lipid membrane consisting of lipoproteins and lipopolysaccharides. There are some bacterial species that aren’t of either classification, such as Mycobacteria species, which have a thick cell wall but also have a second layer of lipids.

Many species of bacteria also have what’s called an S-layer, which is an array of protein molecules on the outside of the cell wall. These will both chemically and physically protect the cell surface, preventing the passage of macromolecules into the cell. There is no full understanding of the nature of this layer except that, in some cases, they confer virulence in certain bacterial species.