Prokaryotic and Eukaryotic Cells | Botany Optional for UPSC PDF Download

Prokaryote Definition

Prokaryotes are single-celled organisms characterized by the absence of membrane-bound structures, with the nucleus being the most prominent one missing. Prokaryotic cells are typically small and uncomplicated, with a diameter ranging from about 0.1 to 5 μm.
The key structures present in a prokaryote cellDespite the lack of membrane-bound structures, prokaryotic cells have specific regions within them. In these cells, genetic material is clustered together in a region known as the nucleoid. Prokaryotes can be categorized into two major domains: bacteria and archaea. Within prokaryotic cells, various molecules like proteins, DNA, and metabolites coexist in the cytoplasm without any membrane separation. In certain primitive organelles found in bacteria, there is some degree of compartmentalization, contributing to a semblance of organization in the cell.

Prokaryotic Cell Features

Here is a breakdown of the components typically found in a prokaryotic bacterial cell:

• Nucleoid: The central region of the cell housing its DNA.
• Ribosome: Responsible for protein synthesis.
• Cell Wall: Provides structural support and protection; commonly composed of peptidoglycans, which are carbohydrates and proteins.
• Cell Membrane: Also known as the plasma membrane, it separates the cell from its external environment.
• Capsule: Some bacteria possess a carbohydrate layer surrounding the cell wall known as the capsule, aiding in attachment to surfaces.
• Fimbriae: Thin, hair-like structures that assist in cellular attachment.
• Pili: Rod-shaped structures with various functions, including attachment and DNA transfer.
• Flagella: Thin, tail-like appendages that enable movement.

Examples of Prokaryotes

Prokaryotes are primarily divided into two groups: bacteria and archaea. Notably, they lack a true nucleus and other membrane-bound organelles like the endoplasmic reticulum.

Do Prokaryotes Have a Nucleus?

No, prokaryotic cells do not possess a nucleus. Instead, their DNA is bundled but free-floating within the nucleoid region. This DNA is typically found as a single circular chromosome. Additionally, prokaryotes lack other membrane-bound structures such as the endoplasmic reticulum.

Do Prokaryotes Have Mitochondria?

No, mitochondria are exclusive to eukaryotic cells and are not present in prokaryotes. This holds true for other membrane-bound structures like the nucleus and the Golgi apparatus.
One hypothesis regarding eukaryotic evolution suggests that mitochondria were originally prokaryotic cells that existed inside other cells. Over time, evolution led to these separate entities functioning together as eukaryotes.

Eukaryote Definition

Eukaryotes are organisms whose cells possess a nucleus and other organelles enclosed by a plasma membrane. These cells are notably larger (typically 10 to 100 μm) and more complex than prokaryotic cells. While many eukaryotes are multicellular, some exist as single-celled organisms.
The key structures present in a eukaryote cell

Eukaryotic Cell Features

In a eukaryotic cell, various membrane-bound structures perform specific cellular functions.
Here is an overview of the primary components commonly found in eukaryotic cells:

• Nucleus: Stores genetic information in the form of chromatin.
• Nucleolus: Located within the nucleus, it is responsible for ribosomal RNA production.
• Plasma Membrane: A phospholipid bilayer surrounding the cell and its organelles.
• Cytoskeleton or Cell Wall: Provides structural support, facilitates cell movement, and aids in cell division.
• Ribosomes: Involved in protein synthesis.
• Mitochondria: Known as the cell's "powerhouses," they produce energy.
• Cytoplasm: The region between the nuclear envelope and plasma membrane.
• Cytosol: A gel-like substance within the cell containing organelles.
• Endoplasmic Reticulum: An organelle responsible for protein maturation and transport.
• Vesicles and Vacuoles: Membrane-bound sacs involved in transport and storage.

Other organelles may include the Golgi apparatus, chloroplasts (in plant cells), and lysosomes, although these are not universally present in all eukaryotic cells.

Examples of Eukaryotes

Eukaryotes encompass a wide range of organisms, including animals, plants, fungi, algae, and protozoans. These organisms exhibit a nucleus and membrane-bound organelles, distinguishing them from prokaryotes.

Comparing Prokaryotes and Eukaryotes

• All life on Earth can be categorized as either eukaryotic or prokaryotic. Scientists believe that eukaryotes evolved from prokaryotes approximately 2.7 billion years ago. The primary distinction between these two groups lies in the presence of a membrane-bound nucleus. 
• Eukaryotes store their genetic information within the nucleus, while prokaryotes have DNA bundled within the nucleoid but not enclosed by a membrane.
• Furthermore, eukaryotic cells possess numerous membrane-bound organelles, while prokaryotes lack these structures. Additionally, the DNA in eukaryotes is in the form of multiple molecules of double-stranded linear DNA, whereas prokaryotes have double-stranded circular DNA.

Key similarities between prokaryotes and eukaryotes

A comparison showing the shared and unique features of prokaryotes and eukaryotes

Despite their differences, all cells, whether prokaryotic or eukaryotic, share four fundamental features:

• DNA
• Plasma membrane
• Cytoplasm
• Ribosomes

Transcription and Translation in Prokaryotes vs. Eukaryotes

• In prokaryotic cells, transcription and translation are coupled, meaning translation starts during mRNA synthesis. 
• Conversely, in eukaryotic cells, transcription occurs in the nucleus, resulting in the production of mRNA. The mRNA then exits the nucleus, and translation takes place in the cytoplasm.

Key Differences Between Prokaryotes and Eukaryotes

Prokaryotes and eukaryotes vary in several important ways - these differences include structural variation - whether a nucleus is present or absent, and whether the cell has membrane-bound organelles, and molecular variation, including whether the DNA is in a circular or linear form.
The differences are summarized in the table below: