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Organelles or Compartments in Bacteria and Eukaryotic Cells

Linda Crampton has an honors degree in biology and many years of teaching experience. She finds the study of microorganisms fascinating.

A bacterial cell (Some bacteria don't have a flagellum, capsule, or pilli. They may also have a different shape.)

A bacterial cell (Some bacteria don't have a flagellum, capsule, or pilli. They may also have a different shape.)

Bacterial Compartments

In animal and plant cells, organelles are compartments surrounded by membrane that have a particular function in the cell’s life. Until quite recently, it was thought that bacterial cells were much simpler and that they didn’t have any organelles or internal membranes. Recent research has shown that these ideas are wrong. At least some bacteria do have internal compartments surrounded by a boundary of some kind, including membrane. Some researchers are calling these compartments organelles.

Animal cells (including ours) and the cells of plants are said to be eukaryotic. Bacterial cells are prokaryotic. For a long time, bacteria were thought to have comparatively primitive cells. Researchers now know that the organisms are more complex than they realized. Studying the structure and behavior of bacteria is important for advancing scientific knowledge. It’s also important because it might indirectly benefit us.

A plant cell has a wall made of cellulose and chloroplasts that perform photosynthesis.(The true extent or number of some of the organelles isn't shown in the illustration.)

A plant cell has a wall made of cellulose and chloroplasts that perform photosynthesis.(The true extent or number of some of the organelles isn't shown in the illustration.)

Eukaryotic and Prokaryotic Cells

Facts About Eukaryotic Cells

In the five-kingdom system, living things (with the exception of monerans) have eukaryotic cells. Eukaryotic cells are covered by a cell membrane, which is also called a plasma or a cytoplasmic membrane. Plant cells have a cell wall outside the membrane.

Eukaryotic cells also contain a nucleus that is covered by two membranes and contains the genetic material. In addition, they have other organelles surrounded by membrane and specialized for various tasks. The organelles are embedded in a fluid called cytosol. The entire contents of the cell—organelles plus cytosol—is referred to as cytoplasm.

Facts About Prokaryotic Cells

Monerans include bacteria and cyanobacteria (once known as blue-green algae). This article specifically refers to the features of bacteria. Bacteria have a cell membrane and a cell wall. Though they have genetic material, it’s not enclosed in a nucleus. They also contain fluid and the chemicals (including enzymes) needed to maintain life. As in eukaryotic cells, the cytosol moves and circulates the chemicals.

Enzymes are vital substances in living things. They control reactions involving chemicals known as substrates. In the past, bacteria were sometimes referred to as a "bag of enzymes" and were thought to contain very few specialized structures. This model of bacterial structure is now inaccurate because compartments with specific functions have been discovered in some of the organisms. The number of known compartments is increasing as more research is performed.

Organelles in Eukaryotic Cells

A brief overview of some major organelles in eukaryotic cells and their functions is given in the three sections below. Bacteria can perform similar jobs, but they may perform them in different ways from eukaryotes and with different structures or materials. Although bacteria lack some of the eukaryotic cell's structures, they have some unique ones of their own. I mention related bacterial structures in my description of the eukaryotic cell's organelles.

Some people restrict the definition of "organelle' to internal structures that are surrounded by membrane. Bacteria do contain these structures, as I describe below. The microbes appear to make use of pockets that were formed from their cell membrane instead of creating new membranes, however.

An animal cell doesn't have a cell wall or chloroplasts. Many animal cells don't have a flagellum, either.

An animal cell doesn't have a cell wall or chloroplasts. Many animal cells don't have a flagellum, either.

Four Eukaryotic Organelles or Structures

Nucleus

The nucleus contains the chromosomes of the cell. Human chromosomes are made of DNA (deoxyribonucleic acid) and protein. The DNA contains the genetic code, which depends on the order of chemicals called nitrogenous bases in the molecule. Humans have twenty-three pairs of chromosomes. The nucleus is surrounded by a double membrane.

A bacterium has no nucleus, but it has DNA. Most bacteria have a long chromosome that forms a looped structure in the cytosol. Linear chromosomes have been found in some types of bacteria, however. A bacterium may have one or more small, circular pieces of DNA that are separate from the main chromosome. These are known as plasmids.

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Ribosomes

Ribosomes are the site of protein synthesis in a cell. They are made of protein and ribosomal RNA, or rRNA. RNA stands for ribonucleic acid. The DNA code in the nucleus is copied by messenger RNA, or mRNA. The mRNA then travels through the pores in the nuclear membrane to the ribosomes. The code contains instructions for making specific proteins.

The ribosomes aren't surrounded by a membrane. This means that some people call them an organelle and others don't. Bacteria have ribosomes as well, though they aren't completely identical to the ones in eukaryotic cells.

Endoplasmic Reticulum

The endoplasmic reticulum or ER is a collection of membranous tubes that extend through the cell. It's classified as rough or smooth. Rough ER has ribosomes on its surface. (Ribosomes are also found unattached to ER.) The endoplasmic reticulum is involved in the manufacture, modification, and transport of substances. Rough ER focuses on proteins and smooth ER on lipids.

Golgi Body, Apparatus, or Complex

The Golgi body can be thought of as a packaging and secretion plant. It's composed of membranous sacs. It accepts substances from the endoplasmic reticulum and changes them into their final form. It then secretes them for use within the cell or outside it. At the moment, highly membranous structures such as the ER and Golgi body haven't been found in bacteria.

Mitochondria Facts

The mitochondria produce most of the energy needed by a eukaryotic cell. A cell may contain hundreds or even thousands of these organelles. Each mitochondrion contains a double membrane. The inner one forms folds called cristae. The organelle contains enzymes that break down complex molecules and release energy. The ultimate source of the energy is glucose molecules.

Energy released by mitochondrial reactions is stored in chemical bonds in ATP (adenosine triphosphate) molecules. These molecules can be quickly broken down to release energy when the cell needs it.

Anammoxosomes have been found in some bacteria. They have a different structure from mitochondria and perform different chemical reactions, but as in mitochondria, energy is released from complex molecules inside them and stored in ATP. An anammoxosome is surrounded by membrane.

Chloroplasts, Vacuoles, and Vesicles

Chloroplasts

Chloroplasts carry out photosynthesis. In this process, plants turn light energy into chemical energy, which is stored in the chemical bonds in molecules. A chloroplast contains stacks of flattened sacs known as thylakoids, Each stack of thylakoids is called a granum. The fluid outside the grana is called the stroma.

Chlorophyll is located in the membrane of the thylakoids. The substance traps light energy. Other processes involved in photosynthesis occur in the stroma. Some bacteria contain chlorosomes that contain the bacterial version of chlorophyll and enable them to perform photosynthesis.

Vacuoles and Vesicles

Eukaryotic cells contain vacuoles and vesicles. Vacuoles are larger. These membranous sacs store substances and are the site of certain chemical reactions. Bacteria have gas vacuoles that have a wall made of protein molecules instead of membrane. They store air. They are found in aquatic bacteria and enable the microbes to adjust their buoyancy in the water.

Structures in Prokaryotic Cells

Bacteria are unicellular organisms and are generally smaller than animal and plant cells. Without the required equipment and techniques, it has been hard for biologists to explore their interior structure. The apparently unspecialized structure of bacteria meant they were regarded as lesser organisms in terms of evolution for a long time. Though bacteria could obviously perform the activities needed to keep themselves alive, it was thought that for the most part these activities happened in undifferentiated cytoplasm inside the cell instead of in specialized compartments.

The new equipment and techniques that are available today are showing that bacteria are different from eukaryotic cells, but they are not as different as we once thought. They have some interesting organelle-like structures that are reminiscent of eukaryotic organelles and other structures that seem to be unique. Some bacteria have structures that other ones lack.

A representation of the cell membrane of a eukaryotic cell

A representation of the cell membrane of a eukaryotic cell

Bacterial Cell Membrane and Wall

The Cell Membrane

Bacterial cells are covered by a cell (or plasma) membrane. The structure of the membrane is very similar but not identical in prokaryotes and eukaryotes. As in eukaryotic cells, the bacterial cell membrane is made of a double layer of phospholipids and contains scattered protein molecules.

The Cell Wall

Like plants, bacteria have a cell wall as well as a cell membrane. The wall is made of peptidoglycan instead of cellulose. In Gram-positive bacteria, the cell membrane is covered with a thick cell wall. In Gram-negative bacteria, the cell wall is thin and is covered by a second cell membrane.

The terms "Gram positive" and "Gram negative" refer to the different colors that appear after a special staining technique is used on the two types of cells. The technique was created by Hans Christian Gram, which is why the word "Gram" is often capitailzed.

Different outer layers in Gram positive and Gram negative bacteria

Different outer layers in Gram positive and Gram negative bacteria

Bacterial Microcompartments or BMCs

Structures involved in the metabolic processes that occur in bacteria are sometimes called bacterial microcompartments or BMCs. Microcompartments are useful because they concentrate the enzymes needed in a particular reaction or reactions. They also isolate any harmful chemicals made during a reaction so that they don't harm a cell.

The fate of any harmful chemicals made in microcompartments is still being investigated. Some appear to be transient—that is, they are made in one step of the overall reaction and then used up in another. The passage of materials into and out of the compartment is also being investigated. The protein shell or lipid envelope surrounding a bacterial microcompartment may not be a complete barrier. It often allows the passage of materials under specific conditions.

The names of the first four bacterial compartments described below end in "some," which is a suffix meaning body. The suffix rhymes with the word home. The similar names are related to the fact that the structures were once—and sometimes still are—known as inclusion bodies or inclusions.

By limiting diffusion to a confined space, concentrations of enzymes and substrates can be optimized to promote specific enzymatic reactions. In turn, sequestration of activities within compartments protects the cell from toxic byproducts of such reactions.

— Elias Cornejo, Nicole Abreu, and Arash Komeili, US National Library of Medicine

Carboxysomes in a bacterium named Halothiobacillus neopolitanus (A: within the cell and B: isolated from the cell)

Carboxysomes in a bacterium named Halothiobacillus neopolitanus (A: within the cell and B: isolated from the cell)

Carboxysomes and Anabolism

Carboxysomes were first discovered in cyanobacteria and then in bacteria. They are surrounded by a protein shell that has a polyhedral or roughly icosahedral shape, and they contain enzymes. The illustration on the right below is a model based on discoveries made so far and is not intended to be completely biologically accurate. Some researchers have pointed out that protein shell of a carboxysome looks similar to the outer covering of some viruses.

Carboxysomes are involved in anabolism, or the process of making complex substances from simpler ones. They make compounds from carbon in a process called carbon fixation. The bacterial cell absorbs carbon dioxide from the environment and converts it into a usable form. Each tile of the protein shell of a carboxysome appears to have an opening to allow for the selective passages of materials.

Carboxysomes (on the left) and a representation of their structure (on the right)

Carboxysomes (on the left) and a representation of their structure (on the right)

Anammoxosomes and Catabolism

Anammoxosomes are compartments in which catabolism occurs. Catabolism is the breakdown of complex molecules into simpler ones and the release of energy during the process. Though they have a different structure and different reactions, both anammoxosomes and mitochondria in eukaryotic cells produce energy for the cell.

Anammoxosomes break down ammonia to obtain energy. The term "anammox" stands for anaerobic ammonia oxidation. An anaerobic process occurs without the presence of oxygen. As in mitochondria, the energy produced in anammoxosomes is stored in ATP molecules. Unlike carboxysomes, anammoxosomes are surrounded by a lipid bilayer membrane.

Magnetite magnetosomes in a bacterium

Magnetite magnetosomes in a bacterium

The open access image above appears in Pósfai M, Lefèvre CT, Trubitsyn D, Bazylinski DA, Frankel RB - Frontiers in Microbiology (2013)

Magnetosomes in Bacteria

Some bacteria contain magnetosomes. A magnetosome contains a magnetite (iron oxide) or a greigite (iron sulfide) crystal. Magnetite and greigite are magnetic minerals. Each crystal is enclosed by a lipid membrane produced from an invagination of the bacterium's cell membrane. The enclosed crystals are arranged in a chain that acts as a magnet.

The magnetic crystals are produced inside the bacteria. Fe(lll) ions and other required substances move into a magnetosome and contribute to the growing particle. The process is intriguing for researchers not only because the bacteria can make magnetic particles but also because they are able to control the size and shape of the particles.

Bacteria that contain magnetosomes are said to be magnetotactic. They live in aquatic environments or in the sediments at the bottom of a body of water. Magnetosomes enable the bacteria to orient themselves in a magnetic field in their environment, which is believed to benefit them in some way. The benefit may be related to a suitable concentration of oxygen or the presence of suitable food.

A cartoon representation of a chlorosome

A cartoon representation of a chlorosome

Chlorosome illustration from "Chlorophylls, Symmetry, Chirality, and Photosynthesis" by Mathias O. Senge, Aoife A. Ryan, Kristie A. Letchford, Stuart A. MacGowan, and Tamara Mielke, via MDPI Open Access

Chlorosomes for Photosynthesis

Like plants, some bacteria perform photosynthesis. The process occurs in structures called chlorosomes and their attached reaction center. It involves the capture of light energy by a pigment and its conversion into chemical energy in the reaction center. Researchers who are exploring the chlorosome say that it's an impressive light-harvesting structure.

The pigment that absorbs the light energy is called bacteriochlorophyll. It exists in different varieties. The energy that it absorbs is passed to other substances. The specific reactions that occur during bacterial photosynthesis are still being studied.

The rod model and the lamellar model for the internal structure of the chlorosome are depicted in the illustration above. Some evidence suggests that the bacteriochlorophyll is arranged in a group of rod elements. Other evidence suggests that it's arranged in parallel sheets, or lamellae. It's possible that the arrangement is different in different groups of bacteria.

The chlorosome has a wall made of a single layer of lipid molecules. As the illustration shows, the cell membrane is made of a lipid bilayer. The chlorosome is attached to the reaction center in the cell membrane by a protein base plate and FMO protein. The FMO protein isn't present in all types of photosynthetic bacteria. In addition, the chlorosome isn't necessarily oblong in shape. It's often ellipsoidal, conical, or irregularly-shaped.

The PDU Microcompartment

Bacteria contain other interesting compartments/organelles. One of these can be found in some strains of Escherichia coli (or E. coli). The bacterium uses the compartment to break down a molecule called 1,2 propanediol in order to obtain carbon (a vital chemical) and perhaps energy.

The picture on the left above shows an E.coli cell expressing PDU (propanediol utilization) genes. "Expressing" means the genes are active and triggering protein production. The cell is making PDU microcompartments, which have walls of protein. They are visible as dark shapes in the bacterium and in a purified form in the right picture.

The microcompartment encapsulates the enzymes required for the breakdown of 1,2 propanediol. The compartment also isolates those chemicals made during the breakdown process that could be harmful for the cell.

Researchers have also found PDU microcompartments in a bacterium named Listeria monocytogenes. This microbe can cause foodborne illness. It sometimes causes serious symptoms and even death. Understanding its biology is therefore very important. The study of its microcompartments may lead to better ways to prevent or treat infections by the living bacterium or to prevent harm from the bacterium's chemicals.

Listeria monocytogenes has multiple flagella on its body..

Listeria monocytogenes has multiple flagella on its body..

Increasing Our Knowledge of Bacteria

Many questions surround the bacterial structures that have been discovered. For example, were some of them forerunners to eukaryotic organelles or did they evolve along their own line? The questions become more tantalizing as more organelle-like structures are found.

Another interesting point is the wide variety of organelles that are present in bacteria. Illustrators can create a picture that represents all animal cells or all plant cells because each group has organelles and structures in common. Though some animal and plant cells are specialized and have differences from other ones, their basic structure is the same. This doesn't seem to be true for bacteria because of the apparent variation in their structure.

Bacterial organelles are useful for them and could be useful for us if we make use of the microbes in some way. Understanding how certain organelles work may enable us to create antibiotics that attack harmful bacteria more effectively than current medications. That would be an excellent development because antibiotic resistance is increasing in bacteria. In a few cases, however, the presence of the bacterial organelles might be harmful for us. The quote below gives one example. Tuberculosis is caused by members of the genus Mycobacterium.

The deadly pathogen that causes tuberculosis, for example, scavenges fatty molecules from our own bodies and stores them as energy reserves in organelles, helping the pathogen to persist for years in our lungs, compromising treatment and making the emergence of drug resistance likely

— Chris Greening, Monash University

Organelles, Compartments, or Inclusions

At the moment, some researchers appear to have no problem referring to certain bacterial structures as organelles and do it frequently. Others use the word compartment or microcompartment instead of or sometimes alternating with the word organelle. The term "organelle analog" is also used. Some documents that are older but still available use the terms inclusion bodies or inclusions for the structures in bacteria.

The terminology can be confusing. In addition, it may suggest to casual readers that one structure is less important or less complex than another based on its name. Whatever terminology is used, the structures and their nature are fascinating and potentially important for us. I'm looking forward to seeing what else scientists discover about the organelle-like structures inside bacteria.

References

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2020 Linda Crampton

Comments

Linda Crampton (author) from British Columbia, Canada on August 18, 2020:

Thank you, Jeremiah. I appreciate your visit.

Jeremiah on August 18, 2020:

Good work! The article is detailed.

Linda Crampton (author) from British Columbia, Canada on August 18, 2020:

Thank you very much, Umesh.

Umesh Chandra Bhatt from Kharghar, Navi Mumbai, India on August 18, 2020:

It is a great resource to refer anytime. Well explained.

Linda Crampton (author) from British Columbia, Canada on August 12, 2020:

Hi, Denise. I think there's a high probability that the discoveries will help us fight harmful bacteria. I'm not so sure about viruses. It's possible that the studies will be indirectly useful in increasing our understanding of viruses, though.

Blessings to you as well, Denise.

Denise McGill from Fresno CA on August 12, 2020:

I feel like a biology student. These studies will help us better understand and combat bacterial infections and maybe even fight viruses, yes? I think we have come a long way since I was in school last. So many new things to study and new words to learn.

Blessings,

Denise

Linda Crampton (author) from British Columbia, Canada on August 12, 2020:

Thank you for the visit and the comment, Nell.

Nell Rose from England on August 12, 2020:

Really interesting. I could have done with studying your articles back in college. Really useful for schools etc.

Linda Crampton (author) from British Columbia, Canada on August 12, 2020:

Thank you very much, Moondot. I appreciate your kindness.