[[Cell Structure and Function]]

Membrane transport is how cells move materials across the nuclear membrane. 

Passive transport includes simple diffusion, facilitated diffusion, and osmosis.

Active transport and bulk transport, including endocytosis and exocytosis use ATP to move substances against gradients or move large amounts of material.

# Summary
* Selective permeability comes from hydrophobic membrane interior. 
* Passive transport moves molecules down a concentration with no metabolic energy. Active transport uses ATP.
* Concentration gradients store potential energy and let cells keep internal conditions different from outside.
* Endocytosis and exocytosis move a lot of material and require energy.
* Direction of movement (high to low/low to high) is the fastest clue to if it's using active or passive.
* Cells use multiple transport types to maintain levels of solute and water.

Larger movements of water require proteins called aquaporins.

| Molecule Type         | Examples             | Can Cross Freely?   | Needs Transport Protein? |
| --------------------- | -------------------- | ------------------- | ------------------------ |
| Small nonpolar        | O₂, CO₂, N₂          | Yes                 | No                       |
| Small polar uncharged | H₂O, NH₃             | Yes (small amounts) | Yes (large amounts)      |
| Large polar           | Glucose, amino acids | No                  | Yes                      |
| Ions                  | Na⁺, K⁺, Cl⁻, Ca²⁺   | No                  | Yes                      |
# Concentration Gradients
Selective permeability allows cells to build concentration gradients, which exist when a substance has different concentrations of a substance on either side. 

Allows for storing potential energy, drive passive transport, and maintain internal conditions separate from the outside.

# Passive Transport
1. Simple Diffusion
	* Molecules move straight through the membrane
	* Works for small non-polars
2. Facilitated Diffusion
	* Still moves down gradient (high to low)
	* Requires transport or channel proteins
	* Used for polar molecules or ions
3. Osmosis
	* Diffusion of water across the selectively permeable membrane
	* Moves from low to high (refers to solute concentrations)
	* Aquaporins speed up water movement

# Active Transport
Requires direct input of energy (usually ATP).

* Uses membrane proteins, often called pumps
* Important for processes like nerve function and nutrient absorption
* Uses ATP
* Can create concentration gradients

|Transport Type|Energy Required?|Direction|Examples|
|---|---|---|---|
|Simple Diffusion|No|High to Low|O₂, CO₂|
|Facilitated Diffusion|No|High to Low|Glucose, amino acids|
|Active Transport|Yes (ATP)|Low to High|Na⁺/K⁺ pump, calcium pumps|
1. Selective permability creates control
2. Active transport allows for creating concentration gradients
3. Passive transport allows for equilibrium
4. Maintaining water and solute levels is vital to an organism

# Transport of Large Molecules

![[Pasted image 20260817123835.png]]

Exocytosis (moving stuff out):
* Internal vesicles fuse to plasma membrane
* Requires energy
* Is then secreted outside
* Used for secreting things like hormones

Endocytosis (bringing stuff in):
* Plasma membrane folds inwards and create vesicle
* Requires energy
* Vesicle pinches off and enters
Three main types of endocytosis:
1. Phagocytosis
	* Cell engulfs large molecules or microorganisms
	* Creates food vacuoles
	* Common for white blood cells that take in bacteria
2. Pinocytosis
	* Cell takes in liquid with dissolved substances
	* Non specific: brings in whatever is liquid
3. Receptor-Mediated Endocytosis
	* Specific: only brings in molecules that bind to receptors
	* Receptors clustered in coated pits that form vesicles
	* Examples: cholesterol uptake, insulin uptake