1. Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings The earliest cells??? Membranes may have separated various aggregates of self-replicating molecules –Which could be acted on by natural selection LM 650  Membrane Polypeptide RNA Figure 16.6B, C

2. Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings PROKARYOTES 16.7 Prokaryotes have inhabited Earth for billions of years Prokaryotes are the oldest life-forms –And remain the most numerous and widespread organisms Colorized SEM 650  Figure 16.7

3. Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings 16.8 Bacteria and archaea are the two main branches of prokaryotic evolution Domains Bacteria and Archaea –Are distinguished on the basis of nucleotide sequences and other molecular and cellular features

4. Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Domain Archaea 16.12 Archaea thrive in extreme environments— and in other habitats Archaea are common in –Salt lakes, acidic hot springs, deep-sea hydrothermal vents Figure 16.12A, B

5. Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Differences between Bacteria and Archaea Table 16.8

6. Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings 16.9 Prokaryotes come in a variety of shapes Prokaryotes may be shaped as –Spheres (cocci) –Rods (bacilli) –Curves or spirals Colorized SEM 12,000  Colorized SEM 9,000  Colorized SEM 3,000  Figure 16.9A–C

7. Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings External Structures The cell wall –Is one of the most important features of nearly all prokaryotes –Is covered by a sticky capsule Colorized TEM 70,000  Capsule Figure 16.10A

8. Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings The Cell Wall Made of peptidoglycan- a carbohydrate matrix (polymers of sugars) cross-linked by short polypeptide subunits Depending on the thickness of the cell wall, bacteria are classified as gram positive or gram negative Danish microbiologist Hans Christian Gram developed the staining procedure

9. Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Gram positive bacteria Have a thick peptidoglycan layer which accepts the purple gram stain

10. Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Gram negative bacteria Have a multilayered and complex cell wall The outside layer is a membrane made of lipopolysaccharide (sugars and lipids) and a thin peptidoglycan layer inside which does not accept the purple gram stain These layers often make them resistant to antibiotics

11. Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Some prokaryotes –Stick to their substrate with pili –It is also used to transfer DNA from one bacterium to another during conjugation (sex) Colorized TEM 16,000  Pili Figure 16.10B

12. Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Capsule Jelly like coating surrounding the cell wall Prevents the cell from drying out Helps the cells stick together or on host organisms Helps them slide on surfaces Prevents the host from killing them

13. Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Motility Many bacteria and archaea –Are equipped with flagella, which enable them to move –They swim by rotating their flagella Flagellum Plasma membrane Cell wall Rotary movement of each flagellum Colorized TEM 14,000  Figure 16.10C

14. Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Some prokaryotes can withstand harsh conditions –By forming endospores TEM 34,000  Endospore Figure 16.10D

15. Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Internal Organization Some prokaryotic cells –Have specialized membranes that perform metabolic functions Figure 16.10E Respiratory membrane Thylakoid membrane TEM 45,000  TEM 6,000 