The tenets of cell theory indicate that cells represent the basic building units of all life, and all life consists of one or more cells. At this point in the study of all life forms, there are only two types of cells: eukaryotes and prokaryotes. Prokaryotic cells differ from eukaryotes in that they do not have a separated nucleus or organelles bound inside a membrane within the cell, as DNA and other genetic material exists in the central part of the cell called the nucleoid.
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Prokaryotic cells, for the most part, are too small to see with the naked eye (there are a few exceptions) and exist in the Bacteria and Archaea domains of the Linnaean system of taxonomy classification that biologists, microbiologists and other scientists use to categorize and rank all life on the planet.
Oldest Form of Life on Earth
On an Earth that is at least 4 billion or more years old, researchers discovered evidence of prokaryotic bacterial cells from about 3.5 million years ago in micro-fossils and in large fossilized structures. They also discovered that these bacterial prokaryotic cells look like the prokaryotic bacterial communities of today.
Archaea, a special type of prokaryotic bacteria cell living on the edge of volcanic vents deep in the ocean and other places in the world, also dates to this time. Eukaryotic cells only showed up about 1.2 billion years ago. Even though evidence points to different evolutionary pathways for cellular life, scholars posit that all life arose on the planet from a single and universal common ancestor. Humans, animals, plants, most fungi and protists catalogued under the Eukarya kingdom are typically multicellular, though some single-celled eukaryotes do exist.
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Where Prokaryotic Cells Set up House
Prokaryotic cells live everywhere on the planet; in the coldest regions of the planet to some of the hottest areas, like hot springs found near calderas or volcanoes. They can even survive deep in the ocean where the extreme pressure might kill other lifeforms. Scientists even discovered single-celled archaea – related to both bacteria and eukaryotic cells – living near volcanic vents deep under the ocean.
The human body serves as a home to multiple single-celled prokaryotes in the form of bacteria, which, according to the National Institutes of Health, outnumber human cells 10 to one. But recent studies suggest the ratio could be closer to one-to-one. With 37.2 trillion cells in the human body alone, that means prokaryotic bacteria cells that make their homes on or within the human body also number at least 37.2 trillion – or possibly ten times that.
Common Prokaryotic Cell Characteristics
Prokaryotic and eukaryotic cells share four common characteristics:
- All cells have a plasma-membrane outer covering the separates what's inside the cell from the environment outside the cell.
- Material within the cell, called cytoplasm, in which the cell's other components reside.
- Genetic material – deoxyribonucleic acid, abbreviated as DNA.
- Ribosomes – minute particles consisting of ribonucleic acid, abbreviated as RNA, and its related proteins.
Typical prokaryotic bacteria cells have:
- An internal, cytoplasmic membrane contained by a cell wall and possibly an outer membrane.
- A liquid-like interior (about 80 percent water) with a region that contains nuclear material and multiple ribosomes called the nucleoid.
- A single, circular piece of DNA called a plasmid, attached to the cell membrane (in some cases) and directly contacting the cytoplasm contains the genetic material for reproduction.
- The plasmids in some prokaryotic bacteria can transfer between cells, which allows them to share antibiotic-resistant characteristics with other cells.
- Multiple external structures like flagella, glycocalyx and pili.
Large Surface-to-Volume Ratio
Most prokaryotic cells require a microscope to view them. Because of their small size, prokaryotic cells have a larger surface-to-volume ratio – the surface area of the prokaryotic cell compared to its volume – which allows nutrients to easily and quickly reach all parts of the cell's interior. Prokaryotic cells are also simpler in their makeup when compared with the more complex eukaryotic cells.
Prokaryotic Cell Wall Composition
The material that makes up the prokaryotic cell's exterior walls are different when compared with eukaryotic cells. Surrounded by a capsule, a loose slime layer or both, the cell's outer wall and layer helps it attach to surfaces in the environment using short hair-like filaments called fimbriae. Prokaryotic cells in the Bacteria domain consist of peptidoglycan, a tight mesh-like wall consisting of amino sugar chains linked by peptides (two or more amino acids linked in a chain). Prokaryotic cells in the Archaea domain consist of proteins, complex carbohydrates or distinctive molecules that resemble, but are not the same as peptidoglycan.
Cytoplasmic Membranes of Prokaryotic Cells
Inside some prokaryotic cell walls, there exists a skin-like cytoplasmic layer, consisting of a two-layer organic compound – lipids – typically insoluble in water and lacking steroid alcohols. Some bacteria do have cell compartmentalization, where these membranes do enclose parts of the interior of the cell like groups of DNA or ribosomes, akin to the characteristics found in eukaryotic cells.
Because this cytoplasmic membrane is semi-absorbent, it governs which molecules can enter or leave the cell. All cells require the ability to pull in and retain multiple chemicals to aid metabolic processes – chemical procedures that occur in all cells to maintain life. Ingredients move across this membrane by one of three ways: active transport, facilitated diffusion and passive diffusion.
How Prokaryotic Cells Make Food
Prokaryotic cells, like all living entities, require organic compounds for energy like molecules containing carbon or hydrogen. Organic nutrients include carbohydrates – starches and sugars – lipids and proteins.
Prokaryotic cellular organisms are either autotrophs, cells that make their own food, or heterotrophs, cells that consume food present in their environments.
Prokaryotic autotrophs fall into one of two categories: those that make food using the sun (like plant photosynthesis), called photosynthetic autotrophs, and chemosynthetic autotrophs, cells that make food using energy from inorganic chemicals.
Biologists classify heterotrophic prokaryotic cells by their feeding modes: saprotrophic, parasitic or mutualisitic. Saprotrophic prokaryotic cells operate as decomposers, playing an important role in releasing or recycling nutrients bound up in the bodies of dead organisms upon which they feed.
Parasitic prokaryotic cells operate in a symbiotic relationship and feed off a host organism, generally without killing the host. Mutualistic prokaryotes cells operate in a beneficial-to-both species relationship, like the nitrogen-fixing bacteria cells that live in nodules attached to plant roots. The prokaryotic bacteria convert atmospheric nitrogen in the atmosphere to a structure usable by plants, while plants provide these single-celled organisms with carbohydrates. ****
A Nucleoid Instead of a Membrane-Bound Nucleus
Prokaryotic cells do not have a separate area inside them enclosed in a casing to hold genetic material. Instead, the nuclear body within a prokaryotic cell, called a nucleoid, usually contains one circular chromosome consisting of DNA. Prokaryotic cells do not have a dense spherical structure called a nucleolus that contains the nucleus. The DNA inside the prokaryotic cell becomes the blueprint for the daughter cells when the prokaryotic cell reproduces.
Prokaryotic Cells Reproduce by Binary Fission
The DNA in prokaryotic cells exist in a single circular DNA structure called a plasmid inside the cytoplasm. Reproduction starts with chromosome replication, where it makes a copy of itself, forming new DNA, that attaches to the plasma membrane. At this point, each chromosome moves to opposite ends of the cell while a membrane in the middle grows between the two chromosomes until it separates them into different sections. Each section contains genetic material for a separate cell. Once the membrane grows to separate each part of the cell with its individual genetic material, it then divides in the center to form two new daughter cells. Being more complex, eukaryotic cells reproduce by means of mitosis.
Types and Shapes of Prokaryotic Cells
As varied and incredibly abundant minute lifeforms, microbiologists typically catalogue bacteria by three basic, but distinct shapes: coccus, rod or spiral.
- Coccus: Appear as oval- or spherical-shaped cells.
- Rod: Also called bacillus, these are as they sound, shaped like a rod.
- Spiral: These bacteria cells look one of three ways beneath the lens of a microscope: vibrios or comma-shaped; spirillum, a thick corkscrew-like cell; or a spirochete with a thin, more flexible corkscrew shape.
But these are not the only shapes that single-celled bacteria have. Other shapes include lobed, filamentous, multiple shapes of various types, sheathed, spindle-shaped, stalked, star-shaped and trichome-forming bacteria.
Prokaryotic Bacteria Cell Sensitivity to Antibiotics
The gram-staining process, originally developed by Danish physician Hans Christian Gram, is another method a microbiologist uses to identify unknown bacteria. This process has two results: gram-negative or gram-positive. The test involves using differently colored stains that signal the cell's ability to absorb the stain or not. The chemical makeup of the cellular walls of the prokaryotic bacteria cell determines what color the bacteria cells become.
After inserting a colony of cells onto a slide, the microbiologist adds multiple chemicals to the group of cells at various stages of the process, starting with adding a purple-colored chemical to the slide and iodine to set the stain. Ethanol washes the purple dye away to allow the addition of a red-colored dye. Gram-positive cells turn purple, while gram-negative cells turn pink on the slides examined under a microscope. Gram-positive bacteria have very permeable walls, which allow specific antibiotics in, whereas gram-negative bacteria are not as susceptible. An example of a gram-positive prokaryotic cell is a spirochete responsible for syphilis.
Cyanobacteria Prokaryotic Cells
Researchers used to think that a prokaryotic cell now called cyanobacteria belonged in the Eukarya domain in the plant kingdom. On closer examination they discovered that the prokaryotic cell did not have a distinct nucleus and that it also lacked chloroplasts, small parts of plants that contain units called plastids in which photosynthesis occurs.
Prokaryotic Cells in the Archaea Domain and Kingdom
Before researchers discovered that archaea were distinctively different from bacteria, they called them archaebacteria, as opposed to bacteria. These singled-celled organisms exist between the Bacteria and Eukarya kingdoms, sharing some cell characteristics of both while having unique characteristics of their own.
The membranes in archaea prokaryotic cells consist of branched hydrocarbon chains and their cellular walls do not contain any peptidoglycans. Some prokaryotic archaea cells respond to antibiotics that affect cells in the Eukarya domain, but don't respond to some of the antibiotics some bacteria are sensitive to. The rRNA material in archaea is totally different from the genetic material found in prokaryotic bacteria cells. Most of the other characteristics of archaea cells liken unto the traits of bacteria prokaryotic cells.
In-between Prokaryotic Cells
A special group of prokaryotic bacteria cells – a superphylum – includes three members in its classification: planctomycetes, verrucomicrobia and chlamydiae, shortened to PVC. These cells, categorized in the Bacteria domain, exhibit traits found in both Archaea and Eukarya cells. Some of them have peptidoglycan-lacking cell wells, akin to eukaryotes and archaea and some have membranes that surround parts of the cell's genetic material, features typical in eukaryotic cells. Some PVC prokaryotic cells divide through a budding process or contain sterols in their membranes, unlike most prokaryotic bacteria.