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Wize University Biology Textbook > The Plasma Membrane

Passive Transport

Thermodynamics of Membrane FormationPrevious Section
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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?
checklist
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  3. View the solution and report whether you got it right or wrong.

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.
Extra Practice
Facilitated diffusion
Define facilitated diffusion (do the best you can!)
Which of the following graphs represents the transport of lactose by cells that use both facilitated diffusion and active transport for lactose? The inhibitor added is an ATP inhibitor.
What factors allow a molecule to enter a cell by diffusion? Select all that apply
Practice: Transport
How would each of the following molecules be transported into the cell? (aimple diffusion, facilitated diffusion, primary active transport, or secondary active transport).
1.
2.
Practice: Membrane Transport 3
You are running an experiment in the lab to determine if your bacterial cells take up lactose using active transport, facilitated diffusion, or both. You add an ATP inhibitor to the culture and measure the ratio of lactose inside the cell vs. outside the cell over time. Based on the graph below, which type of transport is used by the cells?
Practice: Membrane Transport 1
By what method would you predict that these molecules would enter the cell?
a) Oxygen:
b) Potassium:
Which of the following is true regarding membrane transport?
Which of the following are accurate statements regarding membrane transport?
i. The entropy of the system decreases during active transport which is why it requires the use of ATP
ii. Passive diffusion functions because of an increase in entropy
How do hydrophobic molecules (such as O2, steroids etc.) pass through phospholipid bilayers?
How is the action of carrier proteins best described?
Voltage gated describes what type of transport?
What type of membrane transport is shown below?
Which of the following requires ATP?
How do inorganic ions pass through a cell membrane?