Passive transport is the movement of molecules and ions (or solutes and water) through a cell membrane without the input of free energy.
It occurs where the concentration of a substance is greater on one side of the membrane than the other which is known as "moving with the concentration gradient."
There are three types of passive transport:
Diffusion
Osmosis
Facilitated diffusion
Diffusion
Diffusion is the random movement of particles in gas or liquid from an area of high concentration to low concentration.
Particles move randomly in all directions, but if there is an uneven distribution of particles in solution, then over time, the particles naturally spread out and become evenly distributed.
Example: A large drop of blue dye dropped into a container of water spreads out over time.
Example: If you spray a strong perfume in one corner of the classroom, it will have a strong smell in that corner, but students on the other side of the classroom won't be able to smell it, over time, the perfume molecules will diffuse to the rest of the classroom.
There is now more room for the same number of particles, so each particle has more places it can be which means the entropy of the perfume room system has increased.
No energy is required for this to occur.
Membranes often act as the barrier between areas of high and low concentrations of a solute.
Solute molecules diffuse in all directions, but the net movement is from areas of high concentration to areas of low concentration.
Molecules that are allowed to cross a membrane will diffuse until their concentration is at equilibrium, equal on both sides of the membrane.
O<em>2 and CO</em>2 molecules are small enough to cross the lipid bilayer membrane by diffusion.
The molecules follow the concentration gradient (from high to low).
Concentration gradients of different solutes are independent of one another.
Each type of molecule moves along its own concentration gradient.
Some may diffuse into the cell, others may diffuse out.
Diffusion is a spontaneous process and each type of molecule diffuses at its own rate.
Small molecules like O<em>2 and CO</em>2 readily diffuse through all cell membranes because they are small and nonpolar; they can squeeze between the phospholipids.
Larger molecules, polar molecules, and ions cannot squeeze between the phospholipids or they are repelled by the hydrophobic membrane interior, so they need the help of a transport protein.
Facilitated Diffusion
As in passive diffusion, particles undergoing facilitated diffusion move from an area of high to low concentration.
Unlike passive diffusion, particles cross the membrane with the help of an integral protein.
Since the substances are going with their concentration gradient, this is a type of Passive Transport and no energy is needed.
Water crosses membranes through aquaporins.
Protein channels make it possible for polar water molecules to diffuse across the membrane.
Types of Passive Transport Proteins
Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane (aquaporins are an example).
Carrier proteins bind to a specific solute and change shape, helping it move across the membrane.