Movement of Molecules Across a Membrane: Diffusion & Osmosis

Diffusion

Movement of molecules from an area of higher concentration to an area of lower concentration.

Equilibrium

molecules are evenly dispersed

Osmosis

Diffusion of water through a selectively permeable membrane. Driven by imbalance of dissolved molecules (solutes) on either side of the membrane

Simple Diffusion

molecules that can cross phospholipid bilayer will simply move through membrane

What molecules can cross Phospholipid Bilayer

small and nonpolar molecules

Facilitated diffusion

molecules that cannot cross phospholipid bilayer can move through protein channel (aquaporins)

What molecules cannot get through Phospholipid Bilayer

large, polar or charged molecules (ex. ions, sugars, amino acids)

aquaporins

water channel proteins that facilitate the diffusion of water across the membrane

What direction will water move?

in an out of cell in a balanced way (along its concentration gradient)

higher amount of salt draws the water out

more salt = less water (salt & sugar suck)

Why doesn't salt just diffuse?

salt breaks into sodium & chloride ions (charged) that must go through specific protein channels. Channel not available = water compensates by moving in and out to create equilibrium

Tonicity

the dissolved solutes in a solution

high tonicity

high concentration of (dissolved) solutes

low tonicity

low concentration of (dissolved) solutes

Isotonic Solutions

2 solutions have equal solute concentrations. (

Hypertonic solution

higher solute concentration than solution on other side of membrane; cell loses water (shrinks)

Hypotonic solution

Solute concentration is less than that inside the cell; cell gains water

Which way does water go (hypo/hyper)

water moves from hypotonic to hypertonic; direction of higher solute con. (bc there is less water. osmosis)

salt & water + other solutes

HYPERTONIC

endocytosis

process in which large molecules/substances enter a cell

exocytosis

process which large molecules/substances leave cell