27.1 & p. 514-517

1. 27.1 & p. 514-517 Prokaryotes and Eukaryotes

2. Overview: Masters of Adaptation • Prokaryotes thrive almost everywhere, including places too acidic, salty, cold, or hot for most other organisms • Most prokaryotes are microscopic, but what they lack in size they make up for in numbers • There are more in a handful of fertile soil than the number of people who have ever lived

3. They have an astonishing genetic diversity • Prokaryotes are divided into two domains: bacteria and archaea

4. Concept 27.1: Structural and functional adaptations contribute to prokaryotic success • Most prokaryotes are unicellular, although some species form colonies • Most prokaryotic cells are 0.5–5 µm, much smaller than the 10–100 µm of many eukaryotic cells • Prokaryotic cells have a variety of shapes • The three most common shapes are spheres (cocci), rods (bacilli), and spirals

5. Fig. 27-2 2 µm 5 µm 1 µm (a) Spherical (cocci) (b) Rod-shaped (bacilli) (c) Spiral

6. Cell-Surface Structures • An important feature 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 • Eukaryote cell walls are made of cellulose or chitin • Bacterial cell walls contain peptidoglycan, a network of sugar polymers cross-linked by polypeptides

7. Archaea contain polysaccharides and proteins but lack peptidoglycan • Using the Gram stain, scientists classify many bacterial species into Gram-positive and Gram-negative groups based on cell wall composition • Gram-negative bacteria have less peptidoglycan and an outer membrane that can be toxic, and they are more likely to be antibiotic resistant • Many antibiotics target peptidoglycan and damage bacterial cell walls

8. Fig. 27-3 Carbohydrate portion of lipopolysaccharide Outer membrane Peptidoglycan layer Cell wall Cell wall Peptidoglycan layer Plasma membrane Plasma membrane Protein Protein Gram- positive bacteria Gram- negative bacteria 20 µm (b) Gram-negative: crystal violet is easily rinsed away, revealing red dye. (a) Gram-positive: peptidoglycan traps crystal violet.

9. A polysaccharide or protein layer called a capsule covers many prokaryotes Capsule

10. Some prokaryotes have fimbriae (also called attachment pili), which allow them to stick to their substrate or other individuals in a colony • Sex piliare longer than fimbriae and allow prokaryotes to exchange DNA Fimbriae

11. Motility • Most motile bacteria propel themselves by flagella that are structurally and functionally different from eukaryotic flagella • In a heterogeneous environment, many bacteria exhibit taxis, the ability to move toward or away from certain stimuli

12. Fig. 27-6 Flagellum Filament 50 nm Cell wall Hook Basal apparatus Plasma membrane

13. Internal and Genomic Organization • Prokaryotic cells usually lack complex compartmentalization • Some prokaryotes do have specialized membranes that perform metabolic functions

14. Fig. 27-7 1 µm 0.2 µm Respiratory membrane Thylakoid membranes (a) Aerobic prokaryote (b) Photosynthetic prokaryote

15. The prokaryotic genome has less DNA than the eukaryotic genome • Most of the genome consists of a circular chromosome • Some species of bacteria also have smaller rings of DNA called plasmids

16. Fig. 27-8 Chromosome Plasmids 1 µm

17. Reproduction and Adaptation • Prokaryotes reproduce quickly by binary fission and can divide every 1–3 hours • Many prokaryotes form metabolically inactive endospores, which can remain viable in harsh conditions for centuries Endospore

19. Fig. 27-10 EXPERIMENT • Prokaryotes can evolve rapidly because of their short generation times Daily serial transfer 0.1 mL (population sample) New tube (9.9 mL growth medium) Old tube (discarded after transfer) RESULTS 1.8 1.6 Fitness relative to ancestor 1.4 1.2 1.0 0 Generation

20. The First Single-Celled Organisms • The oldest known fossils are stromatolites, rock-like structures composed of many layers of bacteria and sediment • Stromatolites date back 3.5 billion years ago • Prokaryotes were Earth’s sole inhabitants from 3.5 to about 2.1 billion years ago

21. Photosynthesis and the Oxygen Revolution • O2 produced by oxygenic photosynthesis reacted with dissolved iron and precipitated out to form banded iron formations • The source of O2 was likely bacteria similar to modern cyanobacteria

22. By about 2.7 billion years ago, O2 began accumulating in the atmosphere and rusting iron-rich terrestrial rocks • This “oxygen revolution” from 2.7 to 2.2 billion years ago • Posed a challenge for life • Provided opportunity to gain energy from light • Allowed organisms to exploit new ecosystems

23. The First Eukaryotes • The oldest fossils of eukaryotic cells date back 2.1 billion years • The hypothesis of endosymbiosis proposes that mitochondria and plastids (chloroplasts and related organelles) were formerly small prokaryotes living within larger host cells • An endosymbiont is a cell that lives within a host cell

24. The prokaryotic ancestors of mitochondria and plastids probably gained entry to the host cell as undigested prey or internal parasites • In the process of becoming more interdependent, the host and endosymbionts would have become a single organism • Serial endosymbiosis supposes that mitochondria evolved before plastids through a sequence of endosymbiotic events

25. Fig. 25-9-4 Cytoplasm Plasma membrane DNA Ancestral prokaryote Endoplasmic reticulum Nucleus Nuclear envelope Aerobic heterotrophic prokaryote Photosynthetic prokaryote Mitochondrion Mitochondrion Ancestral heterotrophic eukaryote Plastid Ancestral photosynthetic eukaryote

26. Key evidence supporting an endosymbiotic origin of mitochondria and plastids: • Similarities in inner membrane structures and functions • Division is similar in these organelles and some prokaryotes • These organelles transcribe and translate their own DNA • Their ribosomes are more similar to prokaryotic than eukaryotic ribosomes