Introduction to Prokaryotes: Videos & Practice Problems

Prokaryotic cells, including bacteria and archaea, lack a nucleus and membrane-bound organelles, making them simpler and smaller, typically around 1 micrometer. In contrast, eukaryotic cells possess a nucleus and can be unicellular or multicellular, averaging 10 to 100 micrometers in size. Bacterial DNA is circular and found in the nucleoid, while eukaryotic DNA is linear. Prokaryotes reproduce via binary fission, whereas eukaryotes undergo mitosis. Both cell types share essential biomolecules and a cell membrane, highlighting their fundamental similarities despite significant structural differences.

Prokaryotic & Eukaryotic Cells

Prokaryotic and eukaryotic cells represent the two fundamental categories of life, distinguished primarily by their structural characteristics. Prokaryotic cells encompass two domains: bacteria and archaea. A defining feature of prokaryotic cells is the absence of a nucleus, which is a membrane-bound organelle that houses genetic material. In contrast, eukaryotic cells possess a nucleus and other membrane-bound organelles, allowing for more complex cellular functions.

The term "karyo" originates from the Greek word for "kernel," referring to the nucleus. The prefix "pro-" means "before," indicating that prokaryotic cells evolved before the development of a nucleus. Conversely, "eu-" means "good" or "true," signifying that eukaryotic cells have a well-defined nucleus.

To further differentiate these cell types, consider the following characteristics:

• Domains of Life: Prokaryotic cells belong to the domains Bacteria and Archaea, while eukaryotic cells are classified under the domain Eukarya.
• Cell Type: Prokaryotic cells are unicellular, meaning they consist of a single cell. Eukaryotic cells can be unicellular or multicellular, with multicellular organisms, such as humans, being composed of many eukaryotic cells.
• Nucleus: Prokaryotic cells lack a nucleus, whereas eukaryotic cells have a nucleus that is present.
• Organelles: Prokaryotic cells do not contain membrane-bound organelles, while eukaryotic cells have various organelles that perform specialized functions.
• Cell Size: Prokaryotic cells are generally much smaller, averaging around 1 micrometer in diameter, compared to eukaryotic cells, which can be up to 100 micrometers or more.

Understanding these distinctions is crucial for studying cellular biology, as they lay the foundation for more complex topics related to cell function and organization. As we progress, we will explore the various organelles found in eukaryotic cells and their specific roles in cellular processes.

Which domains of life are classified as prokaryotes?

• A. Bacteria & Eukarya.
• B. Archaea & Fungi.
• C. Bacteria & Archaea.
• D. Bacteria & Protista.

Answer: (C)

Features of Bacterial Cells

Bacterial cells are fascinating prokaryotic organisms that lack a nucleus and are recognized as the most abundant and diverse life forms on Earth. Their ancient lineage contributes to their prevalence. One of the defining characteristics of bacteria is their circular DNA, which is located in a specific region known as the nucleoid. This area is crucial as it houses the genetic material necessary for the cell's functions and reproduction.

Like all cells, bacteria contain ribosomes, which are essential for protein synthesis. However, bacterial ribosomes are notably smaller than those found in eukaryotic cells, specifically referred to as 70S ribosomes. The "S" denotes a unit of measurement related to the size of the ribosome, while the "70" indicates its specific size category. These ribosomes play a vital role in translating genetic information into proteins, which are fundamental for cellular processes.

When it comes to reproduction, bacteria utilize a straightforward method known as binary fission. This process allows a single bacterial cell to divide into two identical daughter cells, contrasting with the more complex division mechanisms seen in eukaryotic cells.

In summary, understanding the structure and function of bacterial cells, including their circular DNA, nucleoid region, and 70S ribosomes, provides a foundation for exploring the differences between prokaryotic and eukaryotic cells in future studies.

In bacteria, DNA will be found in _______________.

• A. A membrane-enclosed nucleus.
• B. Mitochondria.
• C. The nucleoid.
• D. Ribosomes.

Answer: (C)

Recap: Prokaryotic vs. Eukaryotic Cells

Understanding the differences between prokaryotic and eukaryotic cells is fundamental in biology. Prokaryotic cells are characterized by the absence of a nucleus, while eukaryotic cells possess a defined nucleus. This distinction is crucial as it highlights the complexity of eukaryotic cells, which are generally larger, ranging from 10 to 100 micrometers, compared to the much smaller prokaryotic cells, typically between 1 to 10 micrometers.

Prokaryotic cells are exclusively unicellular, meaning they consist of a single cell, whereas eukaryotic cells can be either unicellular or multicellular, with all multicellular organisms classified as eukaryotic. The structure of DNA also differs significantly; prokaryotic cells contain circular DNA, while eukaryotic cells have linear DNA. Additionally, prokaryotic cells lack membrane-bound organelles, which are present in eukaryotic cells, contributing to their greater complexity.

When it comes to cell division, prokaryotic cells undergo binary fission, a simpler process, while eukaryotic cells divide through mitosis and cytokinesis, which are more intricate processes. Furthermore, the ribosomes in prokaryotic cells are smaller, referred to as 70S ribosomes, in contrast to the larger 80S ribosomes found in eukaryotic cells.

Despite these differences, prokaryotic and eukaryotic cells share some similarities. Both types of cells have a cell membrane, which serves as a barrier and regulator of substances entering and exiting the cell. Additionally, they both contain essential biomolecules, including carbohydrates, proteins, nucleic acids, and lipids, which are vital for various cellular functions.

This overview of prokaryotic and eukaryotic cells lays the groundwork for further exploration of cellular biology, emphasizing the importance of understanding these fundamental concepts as we advance in our studies.

Which of the following is/are a primary difference(s) between all prokaryotes and eukaryotic cells?

• A. Prokaryotes have DNA that floats freely in the cytoplasm.
• B. Prokaryotes are significantly smaller than eukaryotic cells.
• C. Prokaryotes do not have ribosomes like eukaryotic cells.
• D. a and b.
• E. a, b, and c

Answer: (D)

Which of the following is not a common feature shared by all types of cells?

• A. Contains DNA as the genetic material.
• B. Contains a protective cell wall or cell membrane.
• C. Contains a fluid-like portion called the cytoplasm.
• D. Contains a nucleus that stores the genetic material.

Answer: (D)

You isolate a cell with the following characteristics:(1) no nucleus, (2) a cell wall, and the cell is (3) 2 μm in size. This cell could be a/an:

• A. Bacterium.
• B. Plant cell.
• C. Animal cell.
• D. Bacterial cell or a plant cell.
• E. Plant cell or an animal cell.

Answer: (A)

Introduction to Archaea

Archaea, a plural term with the singular form being archaeon, represent one of the three domains of life, alongside bacteria and eukarya. Both archaea and bacteria are classified as prokaryotes, meaning they are unicellular organisms that lack a nucleus. However, it is crucial to understand that archaea and bacteria are fundamentally different from each other, as well as from eukaryotes. One of the key distinctions lies in their ribosomal RNA (rRNA) sequences, which are unique to archaea and significantly different from those found in bacteria and eukarya.

Another major difference is in their cell wall composition. Archaea do not possess peptidoglycan, a molecule that is characteristic of bacterial cell walls. This absence of peptidoglycan is a defining feature that sets archaea apart from bacteria, despite both being categorized as prokaryotes.

Archaea are particularly notable for their ability to thrive in extreme environments, earning them the designation of extremophiles. These organisms can survive in conditions that are highly saline, extremely hot, or under high pressure. For instance, Haloquadratum walsbyi, known as the salt square, can flourish in very salty environments. Other examples include Pyrolobus fumarii, the fire lobe, and Pyrococcus furiosus, the furious fireball, both of which can endure extremely high temperatures.

Despite their reputation for thriving in extreme conditions, some archaea are capable of living in more moderate environments, including soils and even as part of the human microbiota. This adaptability highlights the diversity within the domain of archaea.

In summary, while archaea and bacteria share the prokaryotic classification, they exhibit significant differences in their genetic makeup, cell wall structure, and environmental adaptability. Understanding these distinctions is essential for appreciating the complexity of life forms within the microbial world.

Many species of Archaea are known to thrive in environments in which most living things would not survive. We call these Archaea… 

• A. Extremophiles.
• B. Thermophiles.
• C. Halophiles.
• D. Acidophiles.
• E. Alkaliphiles.

Answer: (A)

If you wanted to increase your chances of obtaining a member of Archaea (rather than a member of another domain), which would be the best site to obtain a sample?

• A. Inside a human intestine.
• B. On the surface of human skin.
• C. A 95°C (203°F) hot spring in Yellowstone.
• D. A 22°C (72°F) freshwater spring in Hawaii.
• E. On the surface of a raw hamburger patty.

Answer: (C)

Here’s what students ask on this topic:

What are the main differences between prokaryotic and eukaryotic cells?

Prokaryotic cells, such as bacteria and archaea, lack a nucleus and membrane-bound organelles, making them simpler and smaller, typically around 1 micrometer. Their DNA is circular and found in a region called the nucleoid. They reproduce via binary fission. In contrast, eukaryotic cells possess a nucleus and membrane-bound organelles, making them more complex and larger, averaging 10 to 100 micrometers. Eukaryotic DNA is linear and housed within the nucleus. Eukaryotic cells can be unicellular or multicellular and reproduce through mitosis. Despite these differences, both cell types share essential biomolecules like carbohydrates, proteins, nucleic acids, and lipids, and have a cell membrane.

What are the characteristics of bacterial cells?

Bacterial cells are prokaryotic, meaning they lack a nucleus and membrane-bound organelles. Their DNA is circular and located in a region called the nucleoid. Bacteria have small ribosomes, known as 70S ribosomes, which are essential for protein synthesis. They reproduce through a simple process called binary fission. Bacteria are the most abundant and diverse organisms on Earth, capable of thriving in various environments. They are also known for their ancient origins, being among the earliest forms of life on the planet.

How do archaea differ from bacteria?

Although both archaea and bacteria are prokaryotes, they have significant differences. Archaea have unique ribosomal RNA (rRNA) sequences that differ from those of bacteria and eukaryotes. Their cell walls lack peptidoglycan, a molecule found in bacterial cell walls. Archaea are known for thriving in extreme environments, such as high salinity, temperature, or pressure, earning them the name extremophiles. However, some archaea can also live in moderate environments. These differences highlight the distinct evolutionary paths and adaptations of archaea and bacteria.

What is binary fission and how does it differ from mitosis?

Binary fission is a simple form of cell division used by prokaryotic cells, such as bacteria. During binary fission, the cell's circular DNA replicates, and the cell splits into two identical daughter cells. This process is straightforward and efficient. In contrast, mitosis is a more complex form of cell division used by eukaryotic cells. Mitosis involves multiple stages, including prophase, metaphase, anaphase, and telophase, followed by cytokinesis. During mitosis, the cell's linear DNA is carefully divided into two daughter cells, ensuring each cell receives an identical set of chromosomes. This complexity allows for greater regulation and accuracy in eukaryotic cell division.

What are extremophiles and where can they be found?

Extremophiles are organisms, particularly archaea, that thrive in extreme environmental conditions. These conditions can include high salinity, extreme temperatures, high pressure, and acidic or alkaline environments. Examples of extremophiles include haloquadratum walsbyi, which grows in extremely salty environments, and pyrolobus fumarii, which thrives in extremely hot environments. While many extremophiles are found in harsh environments, some archaea can also live in more moderate conditions, such as soil or as part of the human microbiota. Extremophiles demonstrate the remarkable adaptability and resilience of life in diverse habitats.