Prokaryotic and Eukaryotic Cells: A Comparison
Eukaryotes and Prokaryotes (EC / PC)
Plasma Membrane
The plasma membrane of prokaryotic and eukaryotic cells exhibits significant similarities in both basic function and structure. It acts as a permeability barrier, separating the cell’s interior from its external environment. This layer is composed of phospholipids and proteins, which can be organized in various ways.
Ribosomes
In prokaryotes, ribosomes are small structures composed of protein and RNA, serving as the site of protein synthesis. In eukaryotes, ribosomes are larger and denser. They are found attached to the surface of the rough endoplasmic reticulum and free in the cytoplasm. As in prokaryotes, they are the site of protein synthesis.
Nuclear Region
The nuclear region differs significantly between eukaryotic and prokaryotic cells. In prokaryotic cells, the nuclear area, called the nucleoid, contains a single, large, circular, double-stranded DNA molecule: the bacterial chromosome. This chromosome contains all the information necessary for cellular function and structure. The prokaryotic chromosome is linked to the plasma membrane, does not contain histones, and is not surrounded by a nuclear membrane.
In eukaryotic cells, there is a true nucleus, a region delimited by a nuclear membrane (the nuclear envelope) that separates the nucleus from the cytoplasm. The nucleus is generally the largest cellular structure, spherical or oval-shaped, and contains DNA molecules with all the genetic information organized into chromosomes. The nuclear membrane is structurally similar to the plasma membrane and is connected to the endoplasmic reticulum. It has pores that allow the entry and exit of substances. DNA replication and RNA synthesis occur within the nucleus.
Capsule
The capsule is a mucilaginous structure composed of mucopolysaccharides. Not all bacteria possess a capsule. It forms a loose, undefined network and functions to protect the cell and prevent desiccation.
Cell Wall
In prokaryotes, the cell wall is a rigid structure that surrounds the cytoplasmic membrane. It is responsible for the cell’s shape and provides protection against osmotic lysis. Many eukaryotic cells, such as algae and plants, also have cell walls. In algae and plants, the cell wall consists mainly of cellulose. In fungi, the cell wall is composed of cellulose and chitin. The cell wall is classified as Gram-positive (+) or Gram-negative (-).
- Gram (+): Composed of murein, teichoic acids, and peptidoglycans. They stain with crystal violet dye and form a single structure.
- Gram (-): Composed of murein, peptidoglycan, lipoprotein, lipopolysaccharide, and phospholipids. It has a double wall (internal and external). The internal wall contains murein and peptidoglycan; the outer wall contains lipoprotein and phospholipids. They do not stain with crystal violet.
Cytoplasm
The cytoplasm is comprised of approximately 90% water and 10% mineral salts, enzymes, proteins, nutrients, and internal cell structures. It is the site where chemical reactions occur.
Genetic Material
The area where DNA is located is called the nucleoid region. In prokaryotic cells, the DNA is in a naked, double-ring form. This single chromosome contains all the information needed for the structure and function of the bacterium.
Plasmid
Plasmids are extrachromosomal DNA, generally containing genes associated with resistance.
Flagella
Flagella are formed by microtubules. These long strands allow for the propulsion of the bacterium.
Cilia
Cilia are short filaments surrounding the bacteria that allow it to move.
Pili
Pili are tubular structures that serve to transfer bacterial DNA or plasmids to other bacteria in a process called conjugation, a form of sexual reproduction.
Mesosome
Mesosomes are invaginations or infoldings of the membrane involved in the process of reproduction.