Cells

Prokaryotic Cells: Definition, Structure, Function (with Examples)

Scientists believe that prokaryotic cells were some of the first life forms on Earth. These cells are still abundant today and can be divided into bacteria and archaea.

A classic example of a prokaryotic cell is Escherichia coli (E. coli).

Prokaryotic cells are fundamental to mastering high school cell biology. Read on to learn about about the various cellular components of prokaryotes.

What Are Prokaryotes?

Prokaryotes tend to be simple, single-celled organisms without membrane-bound organelles or a nucleus. Eukaryotes have these structures.

Billions of years ago, prokaryotes may have evolved from membrane-bound organic molecules called protobionts. They may have been the first life forms on the planet.

You can divide prokaryotes into two domains: Bacteria and Archaea.

(Note that when you write about the domains, the names should be capitalized. However, you can leave them in lowercase when writing about the two groups in general.)

Both groups consist of small, single-celled organisms, but there are differences between them. Bacteria have peptidoglycans in their cell walls and archaea do not. In addition, bacteria have fatty acids in their plasma membrane lipids while archaea have phytanyl groups.

Some examples of common bacteria include E. coli and Staphylococcus aureus (better known as staph). Salt-dwelling halophiles are an example of archaea.

Bacteria: The Basics

Bacteria are one of the two domains that make up prokaryotic cells. They are diverse life forms and reproduce by binary fission.

There are three basic bacterial cell shapes: cocci, bacilli and spirilla. The cocci are oval or spherical bacteria, the bacilli are rod-shaped and the spirilla are spirals.

Bacteria play an important role in human disease and health. Some of these microbes, like Staphylococcus aureus, can cause infections in people. However, other bacteria are beneficial, such as Lactobacillus acidophilus, which helps your body break down lactose found in dairy products.

Archaea: The Basics

Initially classified as ancient bacteria and called "archaeobacteria," archaea now have their own domain. Many species of archaea are extremophiles and live in extreme conditions, such as boiling hot springs or acidic water, which bacteria cannot tolerate.

Some examples include hyperthermophiles that exist in temperatures above 176 degrees Fahrenheit (80 degrees Celsius) and halophiles that can live in salt solutions that range from 10 to 30 percent. The cell walls in archaea offer protection and allow them to live in extreme environments.

Archaea have many different shapes and sizes that range from rods to spirals. Some aspects of archaea's behavior, like reproduction, are similar to bacteria. However, other behaviors, such as gene expression, resemble the eukaryotes.

How Do Prokaryotes Reproduce?

Prokaryotes can reproduce in several ways. The basic types of reproduction include budding, binary fission and fragmentation. Although some bacteria have spore formation, it is not considered reproduction because there are no offspring formed through this process.

Budding happens when a cell makes a bud that looks like a bubble. The bud continues to grow while it is attached to the parent cell. Eventually, the bud breaks off from the parent cell.

Binary fission happens when a cell splits into two identical daughter cells. Fragmentation happens when a cell breaks into small pieces or fragments, and each piece becomes a new cell.

What Is Binary Fission?

Binary fission is a common type of reproduction in prokaryotic cells. The process involves the parent cell splitting into two cells that are identical. The first step in binary fission is to copy the DNA. Then, the new DNA moves to the opposite end of the cell.

Next, the cell begins to grow and expand. Eventually, a septal ring forms in the middle and pinches the cell into two pieces. The result is two identical cells.

When you compare binary fission to cell division in eukaryotic cells, you may notice some small similarities. For instance, both mitosis and binary fission create identical daughter cells. Both processes also involve the duplication of DNA.

Prokaryotic Cell Structure

The cell structure of prokaryotes can vary, but most organisms have several basic components. Prokaryotes have a cell membrane or plasma membrane that acts like a protective cover. They also have a rigid cell wall for added support and protection.

Prokaryotic cells have ribosomes, which are molecules that make proteins. Their genetic material is in the nucleoid, which is the region where DNA lives. Additional rings of DNA called plasmids float around the cytoplasm. It is important to note that prokaryotes do not have a nuclear membrane.

In addition to these internal structures, some prokaryotic cells have a pilus or flagellum to help them move. A pilus is a hairlike external feature, while a flagellum is a whiplike external feature. Some prokaryotes like bacteria have a capsule outside their cell walls. Nutrient storage can also vary, but many prokaryotes use storage granules in their cytoplasm.

Genetic Information in Prokaryotes

Genetic information in prokaryotes exists inside the nucleoid. Unlike eukaryotes, prokaryotes do not have a membrane-bound nucleus. Instead, the circular DNA molecules live in a region of the cytoplasm. For instance, the circular bacterial chromosome is one large loop instead of individual chromosomes.

DNA synthesis in bacteria starts with the initiation of replication at a specific nucleotide sequence. Then, elongation occurs to add new nucleotides. Next, termination happens after the new chromosome forms.

Gene Expression in Prokaryotes

In prokaryotes, gene expression happens in a different way. Both bacteria and archaea can have transcription and translation happen at the same time.

This means that cells can make amino acids, which are the building blocks of proteins, at any time.

The Prokaryotic Cell Wall

The cell wall in prokaryotes has several purposes. It protects the cell and offers support. In addition, it helps the cell maintain its shape and stops it from bursting. Located outside the plasma membrane, the overall structure of the cell wall is more complicated than the one found in plants.

In bacteria, the cell wall consists of peptidoglycan or murein, which is made up of polysaccharide chains. However, the cell walls differ among gram-positive and gram-negative bacteria.

Gram-positive bacteria have a thick cell wall, while gram-negative bacteria have a thin one. Since their walls are thin, gram-negative bacteria have an extra layer of lipopolysaccharides.

Antibiotics and other drugs can target the cell walls in bacteria without harming humans because people do not have these types of walls in their cells. However, some bacteria develop antibiotic resistance, and the drugs stop being effective.

Antibiotic resistance happens when bacteria evolve, and the ones with mutations that allow them to survive the medications are able to multiply.

Nutrient Storage in Prokaryotes

Nutrient storage is important for prokaryotes because some of them exist in environments that make it difficult to have consistent food supplies. Prokaryotes have developed specific structures for nutrient storage.

Vacuoles act as storage bubbles for food or nutrients. Bacteria can also have inclusions, which are structures for keeping reserves of glycogen or starches. Microcompartments in prokaryotes have protein shells and can hold enzymes or proteins. There are specialized types of microcompartments such as magnetosomes and carboxysomes.

What Is Antibiotic Resistance?

There is increasing concern about antibiotic resistance around the world. Antibiotic resistance happens when bacteria are able to evolve and no longer respond to drugs that previously destroyed them. This means that people taking an antibiotic will not be able to kill the bacteria inside their body.

Natural selection promotes resistance in bacteria. For example, some bacteria have random mutations that allow them to resist antibiotics. When you take a drug, it will not work on these resistant bacteria. Next, these bacteria can grow and multiply.

They can also give their resistance to other bacteria by sharing genes, creating superbugs that are difficult to treat. Methicillin-resistant Staphylococcus aureus (MRSA) is an example of a superbug that is resistant to antibiotics.

DNA replication occurs more quickly in prokaryotes than eukaryotes, so bacteria can reproduce at a much faster rate than humans can. The lack of checkpoints during replication in bacteria compared to eukaryotes also allows for more random mutations. All of these factors contribute to antibiotic resistance.

Probiotics and Friendly Bacteria

Although bacteria often cause human diseases, people also have symbiotic relationships with some microbes. Beneficial bacteria are important for skin, oral and digestive health.

For example, Bifidobacteria live in your intestines and help you break down food. They are crucial parts of a healthy gut system.

Prebiotics are foods that help the microflora in your gut. Some common examples include garlic, onion, leeks, bananas, dandelion greens and asparagus. Prebiotics provide the fiber and nutrients that beneficial gut bacteria need to grow.

On the other hand, probiotics are live bacteria that can help your digestion. You can also find probiotic organisms in foods such as yogurt or kimchi.

Gene Transfer in Prokaryotes

There are three main types of gene transfer in prokaryotes: transduction, conjugation and transformation. Transduction is horizontal gene transfer that happens when a virus helps move DNA from one bacterium to another.

Conjugation involves the temporary fusion of microbes to transfer DNA. This process usually involves a pilus. Transformation occurs when a prokaryote takes up pieces of DNA from its environment.

Gene transfer is important for disease because it allows microbes to share DNA and become resistant to drugs. For instance, bacteria that are resistant to an antibiotic can share genes with other bacteria. You may encounter gene transfer among microbes in your science classes, especially college laboratories, because it is significant for scientific research.

Prokaryote Metabolism

Metabolism in prokaryotes varies more than what you will find in eukaryotes. It allows prokaryotes like extremophiles to live in extreme environments. Some organisms use photosynthesis, but others can derive energy from inorganic fuel.

You can classify prokaryotes into autotrophs and heterotrophs. Autotrophs obtain carbon from carbon dioxide and make their own organic food from inorganic materials, but heterotrophs get carbon from other living things and cannot make their own organic food.

The main types of autotrophs are phototrophs, lithotrophs and organotrophs. Phototrophs use photosynthesis to get energy and make fuel. However, not all of them make oxygen like plant cells do during the process.

Cyanobacteria are an example of phototrophs. Lithotrophs use inorganic molecules as food, and they usually rely on rocks as the source. However, lithotrophs cannot get carbon from rocks, so they need air or other matter that has this element. Organotrophs use organic compounds to get nutrients.

Prokaryotes vs. Eukaryotes

Prokaryotes and eukaryotes are not the same because the types of cells they have differ greatly. Prokaryotes do not have the membrane-bound organelles and nucleus you find in eukaryotes; their DNA floats inside the cytoplasm.

In addition, prokaryotes have a smaller surface area compared to eukaryotes. Moreover, prokaryotes are single-celled despite some organisms being able to aggregate to form colonies.

Prokaryotic cells are less organized than eukaryotic cells. There are also differences in the levels of regulation, such as cell growth, in prokaryotes. You can see this in the mutation rates of bacteria because fewer regulations allow for rapid mutations and multiplication.

Since prokaryotes do not have organelles, their metabolism is different and less efficient. This prevents them from growing to a large size and sometimes limits their ability to reproduce. Nevertheless, prokaryotes are an important part of all ecosystems. From human health to scientific research, these small organisms matter and can affect you greatly.

References

About the Author

Lana Bandoim is a freelance writer and editor. She has a Bachelor of Science degree in biology and chemistry from Butler University. Her work has appeared on Forbes, Yahoo! News, Business Insider, Lifescript, Healthline and many other publications. She has been a judge for the Scholastic Writing Awards from the Alliance for Young Artists & Writers. She has also been nominated for a Best Shortform Science Writing award by the Best Shortform Science Writing Project.

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