# Unit 2 Biology
- Tissues
- Organs,
- Systems of living things

## Celular Biology
A person contains roughly 100 billion cells
- there are roughly to different types of cell s found in mammals
- cells are roughly 20 $`\um`$ (micrometer, $`10^{-6}`$ m), around 250 cells / cm

## Cell Theory
1. All living things are composed of cells 
2. cells are the basic units of living organisms
3. all cells came from pre-existing cells

## Types Of Cells
`Prokaryotic cell`: Meaning before/lacking

`Eukaryotic cell`: Has organelles, meaning true, complete.

|Part Of Cell|Prokaryotic|Eukaryotic|
|:-----------|:----------|:---------|
|DNA|In nucleoid region|Usually in membrane, bound nucleus|
|Size|Usually smaller|Usually larger|
|Organelles|No membrane, bound organelles|Membrane, bound organelle, the bigger, the more complex|
|Organization|Usually singler celled|Often multicellular|
|Metabolism|May not need oxygen|Usually needs oxygen to exist|