1. Chapter 27
Prokaryotes

2. Overview: They’re (Almost) Everywhere!
Most prokaryotes are microscopic
But what they lack in size they more than make up for in numbers
The number of prokaryotes in a single handful of fertile soil is greater than the number of people who have ever lived!

3. Prokaryotes thrive almost everywhere
Including places too acidic, too salty, too cold, or too hot for most other organisms
Figure 27.1

4. Biologists are discovering
That these organisms have an astonishing genetic diversity

5. Most prokaryotes are unicellular
Concept 27.1: Structural, functional, and genetic adaptations contribute to prokaryotic success
Most prokaryotes are unicellular
Although some species form colonies

6. Prokaryotic cells have a variety of shapes
The three most common of which are spheres (cocci), rods (bacilli), and spirals
1 m
2 m
5 m
(a) Spherical (cocci)
(b) Rod-shaped (bacilli)
(c) Spiral
Figure 27.2a–c

7. Cell-Surface Structures
One of the most important features of nearly all prokaryotic cells
Is their cell wall, which maintains cell shape, provides physical protection, and prevents the cell from bursting in a hypotonic environment

8. Using a technique called the Gram stain
Scientists can classify many bacterial species into two groups based on cell wall composition, Gram-positive and Gram-negative
(a)
Gram-positive. Gram-positive bacteria have
a cell wall with a large amount of peptidoglycan
that traps the violet dye in the cytoplasm. The
alcohol rinse does not remove the violet dye,
which masks the added red dye.
(b)
Gram-negative. Gram-negative bacteria have less
peptidoglycan, and it is located in a layer between the
plasma membrane and an outer membrane. The
violet dye is easily rinsed from the cytoplasm, and the
cell appears pink or red after the red dye is added.
Figure 27.3a, b
Peptidoglycan
layer
Cell wall
Plasma membrane
Protein
Gram-
positive
bacteria
20 m
Outer
membrane
Lipopolysaccharide
negative

9. The cell wall of many prokaryotes
Is covered by a capsule, a sticky layer of polysaccharide or protein
200 nm
Capsule
Figure 27.4

10. Some prokaryotes have fimbriae and pili
Which allow them to stick to their substrate or other individuals in a colony
200 nm
Fimbriae
Figure 27.5

11. Motility Most motile bacteria propel themselves by flagella
Which are structurally and functionally different from eukaryotic flagella
Flagellum
Filament
Hook
Cell wall
Plasma
membrane
Basal apparatus
50 nm
Figure 27.6

12. In a heterogeneous environment, many bacteria exhibit taxis
The ability to move toward or away from certain stimuli

13. Internal and Genomic Organization
Prokaryotic cells usually lack complex compartmentalization.
Some prokaryotes do have specialized membranes that perform metabolic functions.
(a) Aerobic prokaryote
(b) Photosynthetic prokaryote
0.2 m
1 m
Respiratory
membrane
Thylakoid
membranes
Figure 27.7a, b

14. The typical prokaryotic genome
Is a ring of DNA that is not surrounded by a membrane and that is located in a nucleoid region
Figure 27.8
1 m
Chromosome

15. Some species of bacteria
Also have smaller rings of DNA called plasmids

16. Reproduction and Adaptation
Prokaryotes reproduce quickly by binary fission
And can divide every 1–3 hours

17. Many prokaryotes form endospores
Which can remain viable in harsh conditions for centuries
Endospore
0.3 m
Figure 27.9

18. What creates variation in prokaryotes?
Mutations are the main source of variation in prokaryotic populations
Rapid reproduction and horizontal gene transfer (ex. transduction, conjugation) facilitate the evolution of prokaryotes to changing environments as well.
Transduction – when a bacteriophage carries bacterial genes from one host cell to another
Conjugation – genetic material is transferred between two bacterial cells that are temporarily joined