Cell Membrane + Transport

- cell membrane controls what enters the cell and what leaves the cell
- cell membrane = 2 phospholipid + proteins
- cell membrane is semi-permeable (only allows small molecules through)
- cell membrane is impermeable to polar substances due to the fact that most of the bilayer is nonpolar and hydrophobic
- 4 components of the cell membrane are protein molecules, carbohydrates, cholesterol and phospholipid
- cholesterol is what holds the cell membrane together
- protein is polar on the two sides but nonpolar in the middle
- cytoskeleton holds the cell, move it, transfer material in and out, and help cell maintain its structure
- gatekeepers: open and close to let materials (ex.ions) in and out
- receptors (protein molecules) transport materials in and out

transport proteins: proteins that allow specific ions and polar molecules to pass through. It is specific to its type of substance.
There are 2 types of transport proteins:
1. channel proteins: certain molecules or atomic ions use it as a tunnel to get through to the membrane at a faster rate
2. carrier proteins: it change shape to allow their passengers to cross through.

There are 2 types of transports:
1. passive transport: movement of materials across cell membrane without spending energy

Diffusion is a type of passive transport where molecules go from an area of high concentration to an area of low concentration because of Brownian motion (random movement of molecules)

Osmosis is the movement of a solvent through semi-permeable barrier until an equilibrium is reached (diffusion). Water follows the concentration gradient (the difference in number of solute or ions in two adjoining regions).
There are 3 types of osmosis:
1. isotonic solution: solute and solvent inside the cell = solute and solvent outside the cell
2. hypertonic solution: solute inside < solute outside, solvent inside > solvent outside, solvent diffuses outward, shrinking the cell
3. hypertonic solution: solute inside > solute outside, solvent inside < solvent outside, solvent diffuses inward, expanding the cell (may burst)

- Tonicity, which is the ability of a surrounding solution to cause a cell to gain or lose water, it mainly depends on the concentration of solute from the two sides.
- Osmoregulation, the control of solute concentration and water balance.

Facilitated Diffusion is when molecules uses protein carrier molecules (for noncharged molcules) or channel proteins (for charged molecules) to diffuse in and out of a cell

2. active transport: movement of materials across cell membrane using energy (ATP)

active transport pump is a cell membrane protein that actively pumps ions across the membrane against their concentration gradient.

bulk transport is used when some materials are too large to pass in or out of the membrane on their own, so the membrane will actively engulf these materials, using energy in process.
An example would be endocytosis, where the membrane engulf the material found outside and traps it in a vesicle or a vacuole.
There are three types of endocytosis:
1. pinocytosis: cells take up solution with dissolved molecules
2. phagocytosis: solid particles are engulfed
3. Receptor-mediated endocytosis: engulfs receptors

Another example would be exocytosis, where the vesicles form inside the cell, fuse with the cell membrane and release its materials into the external environment

-membrane potential: voltage across a membrane
-There are 2 forces that drive diffusion of ions across a membrane
  1. chemical force (ion's concentration gradient)
  2. electrical force (effect of the membrane potential on the ion's movement)
-when these 2 forces combined together, it is called electrochemical gradient

- electrogenic pump: transport protein that generates voltage across a membrane
- proton pump: transports hydrogen ions (H+) out of cell
- cotransport: the simultaneous movement of two substances across a membrane