Learning Objectives
By the end of this section, you will be able to:
- Compare and contrast prokaryotic cells and eukaryotic cells
- Describe the relative sizes of different kinds of cells
Cells fall into one of two broad categories: prokaryotic and eukaryotic. The predominantly single-celled organisms of the Domains Bacteria and Archaea are classified as prokaryotes (pro– = before; –karyon– = nucleus). Animal cells, plant cells, fungi, and protists are eukaryotes (eu– = true; –karyon- = nucleus).
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, a jelly-like region within the cell in which other cellular components are found
3) DNA, the genetic material of the cell
4) ribosomes, particles that synthesize proteins, but with some differences
![In this illustration, the prokaryotic cell has an oval shape. The circular chromosome is concentrated in a region called the nucleoid. The fluid inside the cell is called the cytoplasm. Ribosomes, depicted as small circles, float in the cytoplasm. The cytoplasm is encased in a plasma membrane, which in turn is encased by a cell wall. A capsule surrounds the cell wall. The bacterium depicted has a flagellum protruding from one narrow end. Pili are small protrusions that extend from the capsule in all directions.](https://s3-us-west-2.amazonaws.com/courses-images/wp-content/uploads/sites/198/2016/11/23213516/4-1-1.jpeg)
Figure 1. This figure shows the generalized structure of a prokaryotic cell.
A prokaryotic cell is a simple, single-celled (unicellular) organism that lacks a membrane-bound nucleus, or any other membrane-bound organelle. Prokaryotic DNA is found in the central part of the cell in a darkened region called the nucleoid (Figure 1).
Unlike Domain Archaea and eukaryotes, bacteria have a cell wall comprised of sugars and amino acids and many also have a polysaccharide capsule (Figure 1). The cell wall acts as an extra layer of protection. It helps the cell maintain shape and prevents dehydration. The capsule enables the cell to attach to environmental surfaces. Other structures found in bacterial cells include flagella, pili, or fimbriae. Flagella are appendages for locomotion. Pili are used to exchange genetic material during conjugation, a type of reproduction. Fimbriae are protein appendages used by bacteria to attach to other cells.
Eukaryotic Cells
In nature, the relationship between form and function is apparent at all levels, including the cellular level. The principle “form follows function” is found throughout all life forms. Birds and fish have streamlined bodies that allow them to move quickly through their environment. Much of life is based on how something is made in order to achieve what it is made for.
A eukaryotic cell is a cell that has a membrane-bound nucleus and other structures. Organelles, “little organs”, are membrane-bound compartments which have specialized functions within the cell.
Cell Size
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 2). 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 through which materials must be transported declines.
![Relative sizes on a logarithmic scale, from 0.1 nm to 1 m, are shown. Objects are shown from smallest to largest. The smallest object shown, an atom, is about 1 nm in size. The next largest objects shown are lipids and proteins; these molecules are between 1 and 10 nm. Bacteria are about 100 nm, and mitochondria are about 1 µm. Plant and animal cells are both between 10 and 100 µm. A human egg is between 100 µm and 1 mm. A frog egg is about 1 mm, a chicken egg and an ostrich egg are both between 10 and 100 mm, but a chicken egg is larger. For comparison, a human is approximately 1 m tall.](https://s3-us-west-2.amazonaws.com/courses-images/wp-content/uploads/sites/198/2016/11/23213600/4-1-2-1024x639.jpeg)
Figure 2. This figure shows the relative sizes of different kinds of cells and cellular components. An adult human is shown for comparison.
Section Summary
Prokaryotes are predominantly single-celled organisms of the Domains Archaea and Bacteria. All prokaryotes have plasma membranes, cytoplasm, DNA, and ribosomes. Prokaryotic cells may have other outer layers but lack membrane-bound organelles. Prokaryotic cells range in diameter from 0.1–5.0 µm.
Like a prokaryotic cell, a eukaryotic cell has a plasma membrane, cytoplasm, DNA and ribosomes. However, eukaryotic cells have a membrane-bound nucleus and other membrane-bound organelles allowing for compartmentalization of functions. Eukaryotic cells tend to be 10 to 100 times the size of prokaryotic cells.
Additional Self Check Question
1. Describe the structures that are characteristic of a prokaryote cell.
Answer