Prokaryotic Cells

Cells fall into one of two broad categories: prokaryotic and eukaryotic. We classify only the predominantly single-celled organisms Bacteria and Archaea as prokaryotes (pro- = “before”; -kary- = “nucleus”). Animal cells, plants, fungi, and protists are all 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 jelly-like cytosol within the cell in which there are other cellular components; 3) DNA, the cell’s genetic material; and 4) ribosomes, which synthesize proteins. However, prokaryotes differ from eukaryotic cells in several ways.

A prokaryote is a simple, mostly 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 in the cell’s central part: the nucleoid.

Extended description

A cutaway oval bacterium is drawn with its interior visible, and labels point to eight parts. Capsule: the outermost layer surrounding the cell. Cell wall: the layer just inside the capsule. Cell membrane: the innermost boundary layer, just inside the cell wall. Ribosomes: small dots scattered through the cytoplasm. Chromosome (DNA): a tangled coil filling the cell’s center. Nucleoid region: the area of cytoplasm where the chromosome is concentrated. Pili: short hair-like projections covering the cell’s surface. Flagellum: a single long, whip-like filament extending from one narrow end of the cell.

Most bacteria have a peptidoglycan cell wall and many have a polysaccharide capsule. 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. Pili exchange genetic material during conjugation, the process by which one bacterium transfers genetic material to another through direct contact. Bacteria use fimbriae to attach to a host cell.

Cell Size

At 0.1 to 5.0 µm in diameter, prokaryotic cells are significantly smaller than eukaryotic cells, which have diameters ranging from 10 to 100 µm. The prokaryotes’ small size allows ions and organic molecules that enter them to quickly diffuse to other parts of the cell. Similarly, any wastes produced within a prokaryotic cell can quickly diffuse. This is not the case in eukaryotic cells, which have developed different structural adaptations to enhance intracellular transport.

Extended description

A horizontal logarithmic scale runs from 0.1 nm to 1 m, marked at each power-of-ten tick: 0.1 nm, 1 nm, 10 nm, 100 nm, 1 µm, 10 µm, 100 µm, 1 mm, 10 mm, 100 mm, and 1 m. Above the scale, objects are placed at their approximate size, smallest to largest: an atom, about 0.1 nm; lipids and proteins, 1–10 nm; a flu virus, just under 100 nm; bacteria and a mitochondrion, about 1 µm; a plant cell and an animal cell, 10–100 µm; a human egg, 100 µm–1 mm; a frog egg, about 1 mm; a chicken egg and a larger ostrich egg, 10–100 mm; and an adult human, about 1 m. Below the scale, three overlapping brackets mark which instrument can resolve each size range: an electron microscope, from below 0.1 nm to almost 100 µm; a light microscope, from 100 nm to just over 1 mm; and the naked eye, from 100 µm upward.

Small size, in general, is necessary for all cells, whether prokaryotic or eukaryotic. Let’s examine why that is so. First, we’ll consider the area and volume of a typical cell. Not all cells are spherical in shape, but most tend to approximate a sphere. You may remember from your high school geometry course that the formula for the surface area of a sphere is $4\pi r^2$, while the formula for its volume is $4\pi r^3/3$. Thus, as the radius of a cell increases, its surface area increases as the square of its radius, but its volume increases as the cube of its radius (much more rapidly). Therefore, as a cell increases in size, its surface area-to-volume ratio decreases. This same principle would apply if the cell had a cube shape. If the cell grows too large, the plasma membrane will not have sufficient surface area to support the rate of diffusion required for the increased volume. In other words, as a cell grows, it becomes less efficient. One way to become more efficient is to divide. Other ways are to increase surface area by foldings of the cell membrane, become flat or thin and elongated, or develop organelles that perform specific tasks. These adaptations lead to developing more sophisticated cells, which we call eukaryotic cells.

Summary

Prokaryotes are single-celled organisms of the domains Bacteria and Archaea. All prokaryotes have plasma membranes, cytoplasm, ribosomes, and DNA that is not membrane-bound. Most have peptidoglycan cell walls and many have polysaccharide capsules. Prokaryotic cells range in diameter from 0.1 to 5.0 µm.

As a cell increases in size, its surface area-to-volume ratio decreases. If the cell grows too large, the plasma membrane will not have sufficient surface area to support the rate of diffusion required for the increased volume.

Key terms

• nucleoid — the central region of a prokaryotic cell where the chromosome is located
• prokaryote — unicellular organism that lacks a nucleus or any other membrane-bound organelle

Practice

Name examples of prokaryotic and eukaryotic organisms

Compare and contrast prokaryotic and eukaryotic cells

Describe the relative sizes of different cells

Explain why cells must be small

This section is adapted from Biology 2e, Section 4.2: Prokaryotic Cells by Mary Ann Clark, Jung Choi, Matthew Douglas, and OpenStax, © OpenStax, licensed under CC BY-NC-SA 4.0. Access the original for free at openstax.org. Changes: figures re-encoded as WebP, with the generalized prokaryotic-cell diagram re-kinded from the manifest’s file-extension guess of “photo” to “diagram” (it is a labeled line drawing, not a photograph), and given a longer extended description walking its eight labels, since a labeled diagram’s full reading is not carried by its caption; the size-comparison scale figure’s over-long source alt was shortened to what the image shows, with its full object-by-object and instrument-bracket walk-through moved into an extended description; the cube-and-sphere comparison figure’s alt corrected — the source says the same sphere is shown in both boxes, but the image draws a visibly larger sphere in the larger box, matching the cell-growth point the caption and prose make — and reported as a source defect; the surface-area and volume formulas set in KaTeX rather than as printed running text; the Visual Connection question and its two-part answer split across its two questions — “what advantages might small cell size confer” kept in the body immediately after its figure as a self-check, and “what advantages might large cell size have” placed in the closing Practice block under the objective it argues, since the source pairs one compound solution with two distinct questions; feature boxes (one Career Connection) rendered as a callout with its bold name; the end-of-section Review Questions and Critical Thinking Questions adapted into the closing interactive Practice block (multiple choice and self-check respectively); the “nucleoid” glossary definition’s duplicated “central part … central part” phrasing cleaned to “the central region … where the chromosome is located,” reported as a source defect; and two key-term recall items (prokaryote, nucleoid) added from the glossary, since the module’s own exercise sets do not otherwise cover the relative-sizes objective; rubric checkpoints added to each self-check (the body self-check on small cell size included), decomposing its model answer (the source solution) into check-off clauses with no new claims; and four summary-derived recall items added, one per objective, to raise every group to the raised floor (“Bacteria and Archaea,” “not membrane-bound,” “5.0 µm,” and “decreases,” each a cloze or multiple choice on a summary sentence the module’s own exercises did not otherwise cover).