2026-08-17 12:43:46: Membrane Transport

biology/Cell Structure and Function/Membrane Transport.md

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+[[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