Glycogen and starch inclusions are cytoplasmic structures found in some prokaryotic cells that store excess carbon in a polymerized form. Cells can access these stored polymers for energy when needed. Storing these nutrients in a polymerized state is advantageous because it prevents the buildup of osmotic pressure that would otherwise occur if the cell accumulated individual solute molecules.

Glycogen and Starch Inclusions

Volutin granules, also known as metachromatic granules due to their staining characteristics, are a type of cytoplasmic inclusion that stores polymerized inorganic phosphate. This stored phosphate can be utilized in cellular metabolism and assists in the formation of biofilms. These granules are found in various microbes, including the archaea Methanosarcina, the bacterium Corynebacterium diphtheriae, and the unicellular eukaryotic alga Chlamydomonas.

Volutin Granules

Sulfur granules are a type of cytoplasmic inclusion found in certain sulfur bacteria, such as those in the genus Thiobacillus. These granules store elemental sulfur, which the bacteria can subsequently utilize for their metabolism.

Sulfur Granules

Polyhydroxybutyrate (PHB) inclusions are specialized cytoplasmic storage structures that are surrounded by a phospholipid monolayer embedded with proteins. These inclusions are produced by certain bacterial species, such as Bacillus and Pseudomonas. In industrial applications, the PHB harvested from these inclusions is utilized as a source of biodegradable polymers to manufacture bioplastics.

Polyhydroxybutyrate (PHB) Inclusions

Gas vacuoles are accumulations of small, protein-lined vesicles of gas found in some prokaryotic cells. These inclusions allow the cells to alter their buoyancy, enabling them to adjust their vertical location within a water column.

Gas Vacuoles

Magnetosomes are cytoplasmic inclusions found in magnetotactic bacteria, such as Magnetospirillum magnetotacticum. They consist of magnetic iron oxide or iron sulfide surrounded by a lipid layer, which allows the cells to align along a magnetic field to aid their movement.

Magnetosomes

Carboxysomes are cytoplasmic inclusions composed of an outer shell made of thousands of protein subunits. Produced by cyanobacteria like Anabaena cylindrica and other bacteria such as Halothiobacillus neapolitanus, their interior is filled with carbonic anhydrase and ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) for use in carbon metabolism.

Carboxysomes

Cytoplasmic inclusions are structures often found in prokaryotic cells that serve to compartmentalize their cytoplasm, despite their general lack of complex membrane-bound organelles. They can store nutrients or gas needed for the cell's metabolism or buoyancy, and they play an important role in reducing the buildup of osmotic pressure as the cell accumulates compounds.

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Prokaryotic cells possess a variety of specialized internal and external structures. A typical rod-shaped bacterium features a thick outer capsule, a thinner cell wall, and an innermost plasma membrane. Externally, it may have long projections called flagella, a shorter projection called a pilus, and numerous very short projections known as fimbriae. Internally, the plasma membrane encloses a fluid called cytoplasm, which contains ribosomes, small spheres called inclusions, a large folded loop of DNA known as the nucleoid, and a small loop of DNA called a plasmid. Other structures sometimes present include mesosomes and endospores.

Bacterial Structures

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The cell wall is a structure found in most prokaryotes and some eukaryotes that envelopes the cell membrane. Working in combination with cytoskeletal elements, the cell wall maintains the cell's morphology and protects it from changes in osmotic pressure.

Cell Wall

The cytoplasm consists of organic and inorganic solutes and free floating ribosomes. Chromosomal DNA and plasmids are located in this region.

Cytoplasm

Mesosomes store respiratory enzymes and aid in cellular reproduction. These vesicles are formed by the invagination of the plasma membrane into the cytoplasm. 

Mesosomes

Cytoplasmic Inclusions

Flagella (singular: flagellum) are tail-like cellular structures used for locomotion by some bacteria, archaea, and eukaryotes. Because they are extremely thin, they typically cannot be observed under a standard light microscope without specialized staining techniques.

Flagella

The plasma membrane, also referred to as the cell membrane or cytoplasmic membrane, is a fundamental structure found in all prokaryotic and eukaryotic cells. It encloses the cytoplasm and acts as a semipermeable barrier, exhibiting selective permeability to control the flow of metabolites by allowing specific molecules to enter or leave the cell while restricting the passage of others.

Plasma Membrane

Fimbriae are short, bristle-like protein appendages that project from the bacterial cell surface by the hundreds. Because they are structurally similar to pili, the terms are often used interchangeably, but fimbriae are primarily responsible for enabling a cell to adhere to surfaces and other cells. For pathogenic bacteria, this adherence is critical for colonization, infectivity, and virulence, and it also plays an essential role in biofilm formation.

Fimbriae

The nucleoid is a region within a prokaryotic cell where its DNA and DNA-associated proteins are concentrated. Because prokaryotic cells lack a complex nuclear membrane, their genetic material—which is typically a single, circular, and haploid (unpaired) chromosome—is housed in this specific nucleoid area rather than a true nucleus.

Nucleoid

Ribosomes are made up of RNA and proteins. They are responsible for translating mRNA into proteins. Ribosomes are present in the cytoplasm.

Ribosomes

Endospores are highly resilient, dormant structures formed by certain vegetative bacterial cells when nutrients are depleted or environmental conditions become unfavorable. They protect the bacterial genome and allow the cell to survive for extended periods—up to thousands of years—enduring harsh conditions such as extreme temperatures, radiation, chemicals, and lack of food or water.

Endospores

Some bacteria are capable of forming inclusion bodies which are used for holding excess materials like nutrients or proteins. These cell structures decrease osmotic pressure in the cell by containing these compounds in inclusion bodies. 

Inclusion Bodies

Plasmids are small, circular, double-stranded molecules of extrachromosomal DNA, meaning they are not part of an organism's main chromosome. While most commonly found in bacteria, they can also be present in archaea and eukaryotic organisms. A single cell can contain hundreds of plasmids, which often carry genes that confer advantageous traits, such as antibiotic resistance, that are critical for the organism's survival.

Plasmids

A capsule is a specific type of glycocalyx characterized as an organized layer located immediately outside of the bacterial cell wall. It is typically composed of polysaccharides or proteins. By forming a distinct, clear boundary around the cell, capsules help protect the microbe and facilitate its adherence to surfaces.

Capsule

Many bacterial cells feature protein appendages that are embedded within their cell envelopes and extend outward, enabling them to interact with their external environment. These filamentous appendages, which include fimbriae, pili, and flagella, facilitate crucial functions such as attaching to surfaces, transferring DNA between cells, and providing cellular movement.

Filamentous Protein Appendages

Pili (singular: pilus) are long, relatively sparse protein appendages on the surface of some bacterial cells that primarily facilitate attachment to external surfaces.

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