Cell Organization

Prokaryotic Structures

Prokaryotic cells lack membrane-bound organelles but include other structures such as the nucleoid, where the cell's genetic material is stored.
Prokaryotes lack membrane-bound organelles, including a nucleus. However, they still carry out all the functions of life. Prokaryotes generally have smaller DNA molecules than eukaryotes, and these are suspended within the cell in a region called the nucleoid. Prokaryotic cells are enclosed by a cell membrane that is surrounded by a strong, rigid cell wall. Some prokaryotic cells move via the action of a flagellum, which is a whiplike appendage. Some have short hairlike structures called pili that help them move or obtain food. Shorter pili called fimbriae can help attach a bacterium to a surface. Many prokaryotic cells obtain nutrients by engulfing organic molecules, forming vesicles similar to eukaryotic lysosomes. Some prokaryotes contain photosynthetic pigments and make their own food using light energy. Others build organic molecules using the energy from Earth's heat.
In prokaryotic cells, the nucleoid is a membrane-bound organelle that contains all of the genetic content for the cell.
Some prokaryotes orient themselves along Earth's magnetic fields. These bacteria, including archaea, contain structures that help them sense magnetic fields. The magnetosome is a membranous pocket within the bacterial cell membrane that contains iron-rich magnetic particles. These structures help the bacterium orient itself and move when environmental conditions become unfavorable. Most prokaryotes reproduce through the action of binary fission by doubling their DNA, growing, and then dividing. However, sometimes prokaryotes transfer DNA between cells. This process is known as conjugation and entails the formation of a "bridge" between the cells, in which their cell membranes connect via a pilus. Additionally, prokaryotic cells can take up DNA from their environment by engulfing it, similarly to how they obtain food. This process is called transformation. The environmental DNA is incorporated into the prokaryote's DNA. Finally, prokaryotes occasionally have DNA forced into them by viruses called bacteriophages. Once the bacteriophage DNA has entered the prokaryotic cell, it is incorporated into the cell's own DNA strand. This process is known as transduction.
Pili are short, spiky structures that help one prokaryotic cell stick to another. The sex pilus is a structure that transfers DNA between two prokaryotic cells during conjugation, which is the transfer of genetic material between cells.
Structures in Different Cell Types
Organelle Prokaryotes Eukaryotes
Cell wall Present Absent in animals but present in plants, fungi, and some protists
Nucleus Absent Present
Chloroplasts Absent Absent in animals, fungi, and very few protists, but present in plants
Cell membrane Present Present
Large vacuoles Present Present in plants
Centrioles Absent Present only in animal cells
Cilia or flagella Flagella only Sometimes present
Mitochondria Absent Present
Endoplasmic reticulum Absent Present
Ribosomes Present Present
Golgi apparatus Absent Present

Prokaryotes and eukaryotes differ in their types of organelles.