Wize AP Biology Textbook > The Plasma Membrane

Passive Transport

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Membrane Permeability
Cell membranes are said to have selective permeability. This is the idea that it will allow only certain molecules through, while others need assistance to get into the cell. Which molecules manage to squeeze through the lipid bilayer depends on the molecule's properties:
Size and polarity affect the permeability of molecules through the phospholipid bilayer.
The lipid bilayer has a largely non-polar interior, therefore, non-polar molecules are more permeable than polar molecules.
The smaller the molecule, the easier it can cross the membrane.
Exception: polar water can cross the membrane very quickly due to numerous aquaporins (water channels) in the membrane.

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Introduction and Passive Transport
In order for molecules or ions to get into a cell they must face the cell membrane. There are a few mechanisms by which they can get through this barrier:

(Simple) Diffusion
Simple diffusion occurs due to random thermal motion of molecules. When you add a drop of red dye to a glass of water, some time later the drop will have spread and the entire water will be pink. This is diffusion! 
Diffusion always occurs from high to low concentration regions (driving force), also known as the concentration gradient.
When the concentrations are equal throughout, this system is said to be at equilibrium. 
In order for diffusion to occur across a membrane, the membrane must be permeable to the molecule. 

Diffusion of Non-Electrolytes 
Small, uncharged and lipophilic molecules can cross right through the lipid bilayer.


Photo by Rice University / CC BY


Facilitated Diffusion
Molecules that cannot diffuse through the membrane require additional help of transport proteins to get into a cell. 
No energy required just like simple diffusion but instead uses the help of a membrane protein.
Down the concentration gradient (i.e. molecules go from areas of high to low concentration).
Two types of membrane proteins that help in facilitated diffusion: Channel Proteins and Carrier Proteins
Channel proteins
Specific for a certain molecule;
Can be open all the time or need a trigger ("gated").
Example: channels for Na+ or Cl- ions.
Carrier proteins
Not just a hollow channel: when its specific molecule binds, it changes shape (conformation) and enables the passage of the molecule inside.
Example: glucose transporters.

Practice: Types of Passive Transport
Which of the following statement(s) are FALSE?

A. Ions like Na+ are small enough to cross the cell membrane through the lipid bilayer

B. Facilitated diffusion requires the input of energy

C. Small, uncharged and lipophilic molecules can enter the cell by diffusing through the lipid bilayer

D. Ion channels are selective and can be gated

E. Availability of channels is not important in the ability of an ion to diffuse into a cell

I don't know

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Practice: Molecules Crossing the Lipid Bilayer
Place the following compounds under the appropriate category on their ability to cross a phospholipid bilayer: Fast, Slow, Can't Cross. 

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