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

## Celular Biology
A person contains roughly 100 tillion cells
- 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 nucleus

`Eukaryotic cell`: Means complete nucleus

|Part Of Cell|Prokaryotic|Eukaryotic|
|:-----------|:----------|:---------|
|DNA|In nuclear region|Usually in membrane, bound nucleus|
|Size|Usually smaller|Usually larger|
|Organelles|Not membrane-bound, smaller|Membrane-bound, more complex|
|Organization|Usually single-celled|Often multicelled|
|Metabolism|May not need oxygen|Usually needs oxygen to exist|