Recall the membranes have two major components: phospholipids i ordered it in a bilayer, and also membrane proteins.


Click here for a an ext elaborate snapshot of a cabinet membrane.

One the the attributes of membranes is to manage what overcome into and out that the cell. In this module girlfriend will testimonial mechanisms of membrane transport.

You are watching: Compare and contrast diffusion and facilitated diffusion

There are several different varieties of membrane transport, depending upon the attributes of the substance being transported and the direction of transport.


In simple diffusion, small noncharged molecules or lipid dissolve molecules pass between the phospholipids to get in or leave the cell, relocating from areas of high concentration to areas of low concentration (they movedown their concentration gradient). Oxygen and carbon dioxide and most lipids enter and also leave cells by straightforward diffusion.

Illustrations of straightforward diffusion.

Note the the arrows indicate that the substance is relocating from whereby there is much more of that substance to where there is much less of it, and also that the substances space passing between the phospholipids of the membrane.


Osmosis is a kind of basic diffusion in i beg your pardon water molecule diffuse v a selectively permeable membrane from locations of high water concentration to areas of lower water concentration. (Note that the more particles liquified in a solution, the much less water there is in it, so osmosis is sometimes described as the diffusion of water from areas of low solute concentration to locations of high solute concentration).

Illustration the Osmosis. Assume the the membrane is permeable to water, yet not come sucrose (represented by the tiny black squares). The sucrose molecules will not leaving the cell because they can not pass with the membrane. However, since there is much less water ~ above the side v the sucrose, water will go into the cabinet by osmosis.

Another way to explain the two services in the example of over is to use the state hypertonic and hypotonic. A hypertonic systems has an ext solutes and less water 보다 a hypotonic solution. So, in the instance above, the systems inside the cabinet is hypertonic come the solution exterior the cell. Throughout osmosis, water move from the hypotonic solution (more water, much less solutes) to the hypertonic systems (less water, much more solutes).

In every of the examples shown below, which of the services is hypertonic?




In facilitated diffusion, substances move into or the end of cells down their concentration gradient through protein networks in the cabinet membrane. Basic diffusion and also facilitated diffusion are similar in the both involve activity down the concentration gradient. The difference is exactly how the substance gets v the cell membrane. In simple diffusion, the substance passes in between the phospholipids; in helped with diffusion there space a committed membrane channels. Charged or polar molecules that cannot fit between the phospholipids generally enter and leave cells through promoted diffusion.

Illustrations of facilitated diffusion.

Note that the substance is moving down that is concentration gradient through a membrane protein (not in between the phospholipids)



The types of membrane transport discussed so far constantly involve substances moving down their concentration gradient. The is also possible to move substances throughout membranes versus their concentration gradient (from locations of low concentration to locations of high concentration). Because this is one energetically unfavorable reaction, energy is needed for this movement. The resource of power is the malfunction of ATP. If the energy of ATP is straight used to pump molecules against their concentration gradient, the move is called primary energetic transport.

Illustration of primary active transport.

Note that the substance (indicated by the triangles) is being transported indigenous the side of the membrane with small of the problem to the side of the membrane v a most the substance v a membrane protein, and also that ATP is being broken down come ADP.


In some cases, the use of ATP might be indirect. For example, if a cell offers ATP come pump out Na+ and then supplies the Na+ concentration gradient to carry in glucose, the move of glucose would be an instance of secondary active transport.

On the left side of the snapshot below, a problem (represented by an X) is gift transported indigenous the inside of the cell to the outside even though over there is an ext of the substance ~ above the outside (indicated by the letter X being larger on the outside of the cell. This is primary energetic transport.

In the picture on the best side, substance S, currently at higher concentration in the cell, is carried into the cell through substance X. Due to the fact that S is gift transported there is no the direct use that ATP, the transfer of S is an example of an additional active transport. For substance X primary active transport of X is occurring. The high concentration that X outside the cell is being used to bring in substance S against its concentration gradient.



It is possible for large molecules to get in a cell by a process called endocytosis, where a little piece the the cabinet membrane wraps about the particle and is carried into the cell. If the particle is solid, endocytosis is likewise called phagocytosis. If fluid droplets are taken in, the processes is called pinocytosis.

Illustration the endocytosis. Note that the particle entered the cell surrounded by a piece of cell membrane.

The the contrary of endocytosis is exocytosis. Cells use exocytosis to secrete molecules too big to pass through the cell membrane by any kind of other mechanism.

Other Links and animations:


For an animation of energetic transport, endocytosis, exocytosis, see:


For more information on second active transport, inspect out: http://www.lib.mcg.edu/edu/eshuphysio/program/section1/1ch2/s1ch2_36.htm




Click ~ above the button over to open up a difficulty solver to aid you practice your expertise of membrane transport through the adhering to examples:

1. A white blood cabinet engulfs a bacterium together you fight off an infection.

2. Carbon dioxide (a small uncharged gas molecule) enters the lungs (where the is much less concentrated) native the blood (where it is an ext concentrated).

3. Cells of the stomach wall surface transport hydrogen ions with a ATP-dependent membrane protein to the inside of the stomach, creating a pH that 1.5.

The pH the the cytosol (fluid within the cells) the stomach wall surface cells is approximately 7. (Recall the a low pH way high hydrogen ion concentrations).

4. The lung cells of a victim who drowned in fresh water are swollen because of water entering the cells.

5. Salient gland cells develop the enzyme outstanding amylase and secrete it into the salivary ducts come be delivered to the mouth.

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6. A Paramecium (a solitary celled organism) swims into an area of braided water.. The Paramecium shrivels up together it loser water with its cell membrane.

7. Some bacteria use the energy of ATP to pump H+ the end of your cells. They use the H+ concentration gradient to journey the deliver of sugars into the cell

versus their concentration gradients. What mechanism of transport best describes how the sugars room entering bacterial cells?

8. Some cells engulf droplets that extracellular fluid. What device of carry would this be?