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.

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

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

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

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

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

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

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

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

Viruses

Viruses Viruses are not considered living things.

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

Viruses They have no cell membrane They have no cytoplasm

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

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

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