Cells fall into one of two broad categories: prokaryotic and eukaryotic. Bacteria are classified as prokaryotes (pro– = before; –karyon– = nucleus). Animal cells, plant cells, fungi, and protists are eukaryotes (eu– = true).
Components of Prokaryotic Cells
All cells share four common components: 1) a plasma membrane, an outer covering that separates the cell’s interior from its surrounding environment; 2) cytoplasm, consisting of a gel-like region within the cell in which other cellular components are found; 3) DNA, the genetic material of the cell; and 4) ribosomes, the part of the cell that creates proteins.
Prokaryotes differ from eukaryotic cells in several important ways. A prokaryotic cell is a simple, single-celled (unicellular) organism that lacks a nucleus, or any other membrane-bound organelle. We will shortly come to see that this is significantly different in eukaryotes. Prokaryotic DNA is found in the central part of the cell: a darkened region called the nucleoid (Figure 1).
Unlike eukaryotes, bacteria have a cell wall made of peptidoglycan, comprised of sugars and amino acids, and many have a polysaccharide capsule (Figure 1). The cell wall acts as an extra layer of protection, helps the cell maintain its shape, and prevents dehydration. The capsule enables the cell to attach to surfaces in its environment. 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.
Components of Eukaryotic Cells
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. The principle “form follows function” is found in many contexts. 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 (Figure 2 and 3). The word eukaryotic means “true kernel” or “true nucleus,” pointing to the presence of the membrane-bound nucleus in these cells. The word “organelle” means “little organ,” and, as already mentioned, organelles have specialized cellular functions, just as the organs of your body have specialized functions.
At 0.1–5.0 μm in diameter, prokaryotic cells are significantly smaller than eukaryotic cells, which have diameters ranging from 10–100 μm (Figure 4). 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.
Across the bottom are different size labels from 0.1nm to 1m on the right. Above this is a label “relative sizes on a logarithmic scale. Farthest left over 0.1nm is an illustration of an atom. Above 1nm are phospholipids shown as red circles with two yellow lines extending down. Proteins are above 10nm. A purple blob labeled flu virus is located above 100nm. An oval-shaped mitochondrion and a green oval-shaped bacteria are above 1 micrometer. Diagrams of animal and plant cells are between 10 and 100 micrometers. A purple circle inside a peach circle is labeled frog egg and is above 1mm. A white egg shape labeled chicken egg is between 10mm and 100mm. A diagram of an adult human is above 1m.
Text adapted from: OpenStax, Concepts of Biology. OpenStax CNX. May 18, 2016 http://email@example.com