# Unit 2: Biology

## Cellular Biology
A person contains roughly 100 trillion cells
- Cells are roughly 20 `μm` (micrometre, 10<sup>-6</sup> 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

## Eukaryotic versus Prokaryotic Cells
`Prokaryotic cell`: Meaning before/lacking nucleus

`Eukaryotic cell`: Means complete nucleus

|Factors|Prokaryotic|Eukaryotic|
|:-----------|:----------|:---------|
|DNA|In nucleoid region|Usually in membrane-bound nucleus|
|Size|Usually smaller|Usually larger|
|Organelles|Not membrane-bound, smaller|Membrane-bound, more complex|
|Organization|Usually singlecelled|Often form multicellular organisms|
|Metabolism|May not need oxygen|Usually need oxygen|

## Cellular Organelles

### Business Analogy

| In a business | In a cell | 
| ------------- | --------- |
| Building      | Cytoplasm & Cytoskeleton |
| Department Head | Organelles |
| Boss          | DNA |
| Workers       | Ribosomes & Enzymes |
| Waste management | Lysosomes |
| Storage       | Vacuoles | 
| Powerhouse    | Chloroplasts & Mitochondria |
| Security gate | Cell membrane controls entrance and exit from the cell |
| Transportation Department | IMPORT: Cell membranes and vesicles, INTERNAL: Endoplasmic reticulum & vesicles, EXPORT: Golgi apparatus, vesicles, and cell membrane |


### Cell Membrane
 - Controls what substances enter/leave the cell selectively via various receptors/osmosis
    - Allows nutrients to enter
    - Allows waste products to leave
 - Surrounds and holds other organelles in cell
 - Interact with outside chemicals (e.g., hormones) 

### Nucleus
 - Holds deoxyribonucleic acid (DNA) in form of chromatin
    - DNA is a double helix that make genes
        - Genes are a complete set of instructions to make a complete product (typically proteins)
        - `Genes`: one of the many sets of instructions, in your DNA, found on a **chromosome**, for making a functional product (mostly proteins)
    - Chromatin is DNA wrapped tightly in protein
    - Chromosomes are even more tightly wrapped chromatin used in cell division only
 - Surrounded by a double membrane
 - Substances enter and exit the nucleus via nuclear pores
 - Messenger RNA (mRNA) is encoded from DNA and sent to ribosomes to produce proteins


### Nucleolus
 - Dense region of DNA located in the nucleus
 - This area of DNA is specially for ribosomal DNA (rDNA), or DNA used to make ribosomes
 - Produce "large" and "small" subunits of ribosomes, which either form complete ribosomes in cytosol or mix with endoplasmic reticulum, forming rough endoplasmic reticulum (RER)
 - Is *not* surrounded by a membrane, literally is just same as rest of nucleus but denser

### Cytoplasm & Cytosol
 - Cytosol is the fluid cells contain
 - All organelles are suspended in cytosol
 - Cytoplasm is the cytosol along with everything in a cell, excluding the nucleus

## Rough Endoplasmic reticulum
The endoplasmic reticulum is a network of tubules and flattened sacs with a "rough" appearance because of the presence of ribosomes on the surface

### Function
- Protein synthesis
    - About half of the cell's proteins are made here, by the ribosomes
- Protein movement
    - Proteins are transported by vesicles throught the cell to other organelles, like the golgi apparatus
- Protein "Proof-Reading"
    - pre-existing proteins can enter the rough-ER for modification

## Golgi Apparatus (Golgi Complex)
- Receives, modifies and transports proteins that were produced by the rough-E.R
- Packages proteins into vesicles and sends them cell membrane for export