The structure of the plasma membrane is frequently described using the fluid mosaic model. This model illustrates the membrane as having a mosaic-like composition made up of a diverse array of lipid and protein components. It also emphasizes the fluid nature of the membrane, as these components possess the ability to move dynamically within the plane of the membrane.

Fluid Mosaic Model

A primary function of the plasma membrane is controlling the transport of molecules into and out of the cell. This continuous regulation allows the cell to maintain internal conditions within a specific range, regardless of changes in the external environment. Cells utilize various modes of transport, broadly categorized into passive and active mechanisms, to move substances across the plasma membrane.

Membrane Transport

Certain prokaryotic cells, specifically cyanobacteria and photosynthetic bacteria, possess specialized membrane structures that allow them to perform photosynthesis. These structures are formed by infoldings of the plasma membrane that enclose photosynthetic pigments like green chlorophylls and bacteriochlorophylls. The specific names for these structures vary by organism: they are called thylakoids in cyanobacteria, and chromatophores, lamellae, or chlorosomes in photosynthetic bacteria.

Prokaryotic Photosynthetic Membranes

The plasma membrane of eukaryotic cells is a phospholipid bilayer with embedded peripheral and integral proteins. Similar to prokaryotic plasma membranes, its components move within the plane of the membrane according to the fluid mosaic model, but unlike prokaryotic membranes, it contains sterols and specialized lipids.

Eukaryotic Plasma Membrane

The plasma membrane is composed primarily of a phospholipid bilayer, various embedded or associated proteins, and carbohydrate chains attached to lipids or proteins, forming glycolipids and glycoproteins, respectively.

Components of the Plasma Membrane

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.

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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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Microbiology covers the scope and sequence requirements for a single-semester microbiology course for non-majors. The book presents the core concepts of microbiology with a focus on applications for careers in allied health. The pedagogical features of the text make the material interesting and accessible while maintaining the career-application focus and scientific rigor inherent in the subject matter. Microbiology’s art program enhances students’ understanding of concepts through clear and effective illustrations, diagrams, and photographs.

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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 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.

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

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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