05.06 Cell Membrane Permeability

Join NURSING.com to watch the full lesson now.

Included In This Lesson



  1. Cell Membrane Selectively Permeability- Controls what goes in and out of the cell
  1. Passive Transport- matter moves across the cell membrane from high concentration to low without the use of cell NRG (ATP)
    1. Simple Diffusion- matter moves down it’s concentation gradient by passing between phospholipids. Does not require NRG
      1. matter must be small, without a charge/non-polar
        1. ex. oxygen, carbon dioxide
    2. Facilitated Diffusion-matter moves down it’s concentation gradient by passing through a protein channel. Does not require NRG
      1. Polar/Large matter can diffuse with the help of protein
        1. ex. glucose, H+, Na+, K+
  2. Active Transport-matter moves across the cell membrane from low concentration to high concentration. Requires the use of cell NRG (ATP) ex. Na+/K+ Pump
    1. Endocytosis- Bulk movement of matter taken in the cell. Requires NRG.
      1. Pinocytosis- cell drinking
      2. Phagocytosis- cell eating
    2. Exocytosis- Bulk movement of matter taken out the cell. Requires NRG.
      1. ex. Cell secreting proteins


Today we are going to briefly discuss how the cell membrane functions.

The cell membrane is often thought of as this solid, protective border surrounding the cell, but really it is designed to be less like a wall and more like a gatekeeper. It is best described as being selectively permeable. So this means it is “picky” or selective about what leaves and what enters the cell. Much like this bouncer in this photo that is checking the woman’s ID to verify whether or not she should be permitted to enter the club. So too do membranes have methods for checking and controlling what and how things can enter or leave a cell.

So there are two main ways that things can move across a cell membrane in the first place. Passive transport is the movement of materials without utilizing any of the cell’s energy (like ATP). So as you may know, ALL matter is in motion. In fact that’s what helps differentiate between a solid, liquid and a gas is how fast the particles are moving and how close they are to one another. So since matter is already in motion….This is said to be  its kinetic energy, we can then understand that matter has a tendency to move to areas where it is not…. this whole 2nd law of thermodynamics that says things are becoming more unruly, or in disorder is being kept her what matter spreads out. So if there is a lot if “stuff’ on one side a membrane than on the other side of the membrane and said stuff can pass through somehow… it indeed will. That is what we call passive transport. Matter moving DOWN ITS CONCENTRATION GRADIENT. Much like a kid sliding down a playground slide doesn’t take energy on the kids part to do down, neither does it take NRG to move matter down or across its gradient. So a nice way to recall this is to say “matter moves from where there’s a lot to where it is not”……aka as “Down its concentration gradient”.  

Secondly matter can move from low to high (small amounts to where it already had more) or can even be moved in mass quantities/bulk, this is going to require cellular energy (aka ATP) and this process is known as Active transport. Moving up its concentration gradient (or the kid climbing up the slide) is going to require NRG!!!!!

So to delve into passive transport a bit more… you’ll need to understand that diffusion is matter spreading from areas of high to low concentration. The purple dots moving in this image are showing matter in motion and the gradient here is showing that matter spreading from where there was a lot if purple molecules to where there  weren’t any. This is diffusion. Usually its lipid-based or non-polar, small molecules that diffuse well across the phospholipid bilayer…. like oxygen and carbon dioxide. 

Diffusion can take place of larger or polar molecules but they need a little help or ‘facilitation’ to getting across. This is called Facilitated diffusion. Matter moves across the membrane from areas of high concentration to areas of lower concentration but they need a protein channel to get across the membrane. In this way a protein is facilitating the diffusion of the molecule.  So a perfect example of this is osmosis, which is the diffusion if water. Water can move from high concentrations to low through protein channels called aquaporins. And just a reminder this is still WITHOUT the use of the cell’s ENERGY, it’s just using a special channel to get across the membrane. 

And finally some matter just needs NRG to get where it needs to go. Sometimes matter needs to build up in concentration to  drive cellular processes….. so energy is going to need to be used. A cell will use ATP to pump certain molecules from low to high concentrations…. much like the sodium potassium pump for setting up an action potential in a neuron. 


Additionally cells may need to take in or release material in bulk amounts. Much like shopping at Sam’s, Costco or BJ’s… you’re buying in large amounts…. cells too may need to take in or secrete large quantities and this requires ATP energy.

When a cell takes in large amounts of liquid/solutions that is known as ‘cell drinking’ or pinocytosis. (I think of a drink of a popular wine known as pinot noir or pinot grigio). Cells taking in bulk quantities of solids is ‘cell eating’ also known as phagocytosis. A great example of this is the macrophages….white blood cells that engulf bacteria. 

And finally cells often times are making proteins that need to be shipped elsewhere… this is exocytosis, A nice example depicted here is a neuron secreting a bulk amount of neurotransmitters to communicate to a nearby neuron across the synapse.

So in summary today, know that cell membranes are picky or selective about what permeates to and from. And when items move in and out of cells it may or may not require cellular energy (aka ATP)  Passive does not use the cell’s energy but allows matter to move down its concentration gradient. Sometimes that matter may have a helper to diffuse and this is known as facilitated diffusion. And of course active transport, just like it sounds requires energy and in this case items are either being moved uphill or against the flow or in bulk…. exo bulk exit and endo bulk receiving in the cell. 

Thank you so much for joining us in this lesson to review cell membrane permeability. Now go out and be your best self today an as always happy nursing.