There are two general categories of cells: prokaryotic and eukaryotic. Sometimes, organisms are referred to as prokaryotes or eukaryotes, based on the type of cell s that compose them.
Prokaryotic cells comprise bacteria and archaea. Prokaryotic cells are normally smaller than eukaryotic cells, with a typical size range of 0.
Prokaryotes are made up of a single cell, though they can pair up or cluster together to form mats. Like all other cells, bacteria have a cell membrane and a cytoplasm , and they contain genetic material. In addition to the plasma membrane, bacterial cells have a rigid cell wall that provides additional protection and helps keep them from becoming dehydrated. Some bacteria have another layer outside the cell wall, called the capsule.
It is a sticky coating that helps the bacteria attach to surfaces or other cells. Fimbriae and pili can often be found on the surface of bacterial cells. Fimbriae are short, hair-like projections that attach a bacterium to a substrate or other cells. Pili are similar structures that can serve many functions, including helping the bacterium move or helping it transfer DNA to another bacterium.
As a cell becomes larger, it becomes more and more difficult for the cell to acquire sufficient materials to support the processes inside the cell, because the relative size of the surface area across which materials must be transported declines.
Prokaryotes are predominantly single-celled organisms of the domains Bacteria and Archaea. All prokaryotes have plasma membranes, cytoplasm, ribosomes, a cell wall, DNA, and lack membrane-bound organelles. Many also have polysaccharide capsules. Prokaryotic cells range in diameter from 0. Like a prokaryotic cell, a eukaryotic cell has a plasma membrane, cytoplasm, and ribosomes, but a eukaryotic cell is typically larger than a prokaryotic cell, has a true nucleus meaning its DNA is surrounded by a membrane , and has other membrane-bound organelles that allow for compartmentalization of functions.
Eukaryotic cells tend to be 10 to times the size of prokaryotic cells. By the end of this section, you will be able to: Name examples of prokaryotic and eukaryotic organisms Compare and contrast prokaryotic cells and eukaryotic cells Describe the relative sizes of different kinds of cells. Previous: 3. Next: 3. Some prokaryotes have flagella, pili, or fimbriae.
Flagella are used for locomotion, while most pili are used to exchange genetic material during a type of reproduction called conjugation. In nature, the relationship between form and function is apparent at all levels, including the level of the cell, and this will become clear as we explore eukaryotic cells. For example, birds and fish have streamlined bodies that allow them to move quickly through the medium in which they live, be it air or water.
It means that, in general, one can deduce the function of a structure by looking at its form, because the two are matched.
A eukaryotic cell is a cell that has a membrane-bound nucleus and other membrane-bound compartments or sacs, called organelles , which have specialized functions. The small size of prokaryotes allows ions and organic molecules that enter them to quickly spread to other parts of the cell. Similarly, any wastes produced within a prokaryotic cell can quickly move out.
However, larger eukaryotic cells have evolved different structural adaptations to enhance cellular transport. Indeed, the large size of these cells would not be possible without these adaptations. In general, cell size is limited because volume increases much more quickly than does cell surface area. As a cell becomes larger, it becomes more and more difficult for the cell to acquire sufficient materials to support the processes inside the cell, because the relative size of the surface area across which materials must be transported declines.
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