Recall that membranes have two major components: phospholipids arranged in a bilayer, and also membrane proteins.


Click right here for a more elaborate snapshot of a cabinet membrane.

One of the features of membrane is to regulate what passes into and also out that the cell. In this module friend will testimonial mechanisms that membrane transport.

You are watching: Similarities between diffusion and facilitated diffusion

There are several different types of membrane transport, relying on the attributes of the substance gift transported and the direction the transport.


In an easy diffusion, small noncharged molecule or lipid dissolve molecules pass between the phospholipids to enter or leave the cell, relocating from areas of high concentration to locations of short concentration (they movedown their concentration gradient). Oxygen and carbon dioxide and also most lipids enter and also leave cell by basic diffusion.

Illustrations of straightforward diffusion.

Note the the arrows show that the problem is relocating from wherein there is more of the substance to wherein there is less of it, and also that the substances are passing in between the phospholipids of the membrane.


Osmosis is a form of straightforward diffusion in which water molecule diffuse through a selectively permeable membrane from areas of high water concentration to areas of reduced water concentration. (Note that the more particles liquified in a solution, the less water over there is in it, so osmosis is sometimes described as the diffusion that water from locations of low solute concentration to locations of high solute concentration).

Illustration the Osmosis. Assume that the membrane is permeable come water, yet not to sucrose (represented by the small black squares). The sucrose molecules will certainly not leaving the cell due to the fact that they cannot pass v the membrane. However, due to the fact that there is much less water on the side v the sucrose, water will go into the cabinet by osmosis.

Another way to define the two options in the example of above is to use the terms hypertonic and hypotonic. A hypertonic systems has an ext solutes and also less water 보다 a hypotonic solution. So, in the instance above, the solution inside the cabinet is hypertonic to the solution external the cell. During osmosis, water moves from the hypotonic systems (more water, less solutes) come the hypertonic systems (less water, more solutes).

In every of the examples displayed below, which of the options is hypertonic?




In facilitated diffusion, building material move right into or the end of cells under their concentration gradient through protein channels in the cell membrane. An easy diffusion and also facilitated diffusion are similar in that both involve motion down the concentration gradient. The difference is how the problem gets through the cabinet membrane. In basic diffusion, the substance passes between the phospholipids; in facilitated diffusion there are a specialized membrane channels. Charged or polar molecules the cannot fit between the phospholipids usually enter and leave cells through assisted in diffusion.

Illustrations of assisted in diffusion.

Note the the problem is moving down the concentration gradient with a membrane protein (not in between the phospholipids)



The types of membrane transport discussed so far always involve substances relocating down their concentration gradient. That is also feasible to move substances throughout membranes versus their concentration gradient (from areas of low concentration to locations of high concentration). Due to the fact that this is an energetically unfavorable reaction, power is necessary for this movement. The source of energy is the failure of ATP. If the energy of ATP is directly used come pump molecules versus their concentration gradient, the transfer is referred to as primary energetic transport.

Illustration that primary active transport.

Note that the problem (indicated by the triangles) is gift transported native the next of the membrane with little of the problem to the side of the membrane through a many the substance with a membrane protein, and also that ATP is being broken down to ADP.


In some cases, the usage of ATP may be indirect. Because that example, if a cell provides ATP come pump out Na+ and also then supplies the Na+ concentration gradient to carry in glucose, the carry of glucose would certainly be an example of secondary active transport.

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

In the picture on the right side, problem S, currently at greater concentration in the cell, is carried into the cell v substance X. Because S is being transported without the direct use the ATP, the move of S is an example of an additional active transport. For substance X primary energetic transport the X is occurring. The high concentration the X exterior the cabinet is being supplied to lug in problem S versus its concentration gradient.



It is possible for big molecules to get in a cabinet by a procedure called endocytosis, where a little piece that the cabinet membrane wraps roughly the particle and also is lugged into the cell. If the bit is solid, endocytosis is also called phagocytosis. If fluid droplets are taken in, the procedures is called pinocytosis.

Illustration the endocytosis. Note the the particle gotten in the cell surrounded by a item of cell membrane.

The the opposite of endocytosis is exocytosis. Cells usage exocytosis come secrete molecules too big to pass v the cabinet membrane by any kind of other mechanism.

Other Links and animations:


For an computer animation of active transport, endocytosis, exocytosis, see:


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




Click on the button above to open up a problem solver to aid you exercise your knowledge of membrane transport with the following examples:

1. A white blood cell engulfs a bacterium as you fight turn off an infection.

2. Carbon dioxide (a little uncharged gas molecule) beginning the lungs (where it is much less concentrated) from the blood (where it is more concentrated).

3. Cells of the stomach wall transport hydrogen ions v a ATP-dependent membrane protein come the inside of the stomach, producing a pH that 1.5.

The pH of the cytosol (fluid within the cells) that stomach wall surface cells is roughly 7. (Recall the a short pH way high hydrogen ion concentrations).

4. The lung cells of a victim that drowned in fresh water room swollen due to water start the cells.

5. Outstanding gland cells produce the enzyme outstanding amylase and also secrete it into the outstanding ducts to be delivered to the mouth.

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

7. Some bacteria use the energy of ATP come pump H+ out of your cells. They usage the H+ concentration gradient to drive the transport of sugars right into the cell

against their concentration gradients. What mechanism of transport finest describes exactly how the sugars room entering bacter cells?

8. Part cells engulf droplets of extracellular fluid. What mechanism of carry would this be?