1. Early studies led to the development of the cell theory.
The Cell theory has three principles.
All organisms are made of cells.
All existing cells are produced by other living cells.
The cell is the most basic unit of life.

2. All cells share certain characteristics.
Cells tend to be microscopic.
Bacterium
(colored SEM; magnification 8800x)

3. All cells share certain characteristics.
Cells tend to be microscopic.
All cells are enclosed by a membrane.
cell membrane
Bacterium
(colored SEM; magnification 8800x)

4. All cells share certain characteristics.
Cells tend to be microscopic.
All cells are enclosed by a membrane.
All cells are filled with cytoplasm.
cell membrane
Bacterium
(colored SEM; magnification 8800x)
cytoplasm

5. There are two cell types: eukaryotic cells and prokaryotic cells.
Eukaryotic cells have a nucleus.
nucleus
cell membrane

6. There are two cell types: eukaryotic cells and prokaryotic cells.
Eukaryotic cells have a nucleus.
Eukaryotic cells have
membrane bound
organelles
nucleus
organelles
cell membrane

7. There are two cell types: eukaryotic cells and prokaryotic cells.
Prokaryotic cells do not have a nucleus.
nucleus
organelles
cell membrane

8. There are two cell types: eukaryotic cells and prokaryotic cells.
Prokaryotic cells do not have a nucleus.
Prokaryotic cells do not have membrane-bound organelles.
nucleus
organelles
cell membrane
cytoplasm

9. Evolution
Most biologists believe prokaryotes evolved before eukaryotes for two reasons:
Prokaryotes tend to be much smaller
Prokaryotes are much less complex

10. Viruses

11. Viruses
Viruses are not considered living things.

12. Viruses They have no cell membrane Viruses are wrapped in a
protein coat

13. Viruses
They have no cell
membrane
They have no
cytoplasm

14. Viruses They have no cell membrane cytoplasm They depend on
their host for
energy

15. Viruses reproduction They have no cell membrane They have no cytoplasm
They depend on
their host for
energy
reproduction

16. Viruses Characteristics of life Characteristics of Viruses
Need energy for life processes.
2. The ability to reproduce
3. Responds to the environment.
4. Passes heritable traits through DNA.
Must depend on host.
2. Must depend on host.
3. Does not respond to the environment.
4. Passes heritable traits through DNA or RNA