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


Click here for a much more elaborate picture of a cabinet membrane.

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

You are watching: Which term refers to the diffusion of water through a membrane

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


In simple diffusion, tiny noncharged molecules or lipid dissolve molecules pass between the phospholipids to go into or leaving the cell, moving from locations of high concentration to areas of low concentration (they movedown their concentration gradient). Oxygen and carbon dioxide and most lipids enter and leave cell by basic diffusion.

Illustrations of basic diffusion.

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


Osmosis is a type of simple diffusion in i m sorry water molecule diffuse v a selectively permeable membrane from areas of high water concentration to locations of reduced water concentration. (Note that the much more particles liquified in a solution, the less water there is in it, therefore osmosis is sometimes defined as the diffusion of water from areas of low solute concentration to locations of high solute concentration).

Illustration that Osmosis. Assume that the membrane is permeable to water, but not to sucrose (represented by the little black squares). The sucrose molecules will certainly not leave the cell due to the fact that they can not pass v the membrane. However, due to the fact that there is less water ~ above the side with the sucrose, water will enter the cell by osmosis.

Another method to describe the two solutions in the example of over is to usage the state hypertonic and hypotonic. A hypertonic systems has more solutes and also less water 보다 a hypotonic solution. So, in the instance above, the solution inside the cabinet is hypertonic come the solution outside the cell. Throughout osmosis, water move from the hypotonic systems (more water, much less solutes) come the hypertonic solution (less water, an ext solutes).

In every of the examples displayed below, i beg your pardon of the services is hypertonic?




In facilitated diffusion, building materials move into or the end of cells down their concentration gradient through protein networks in the cabinet membrane. Straightforward diffusion and facilitated diffusion are similar in the both involve movement down the concentration gradient. The distinction is just how the substance gets with the cabinet membrane. In basic diffusion, the substance passes between the phospholipids; in assisted in diffusion there are a committed membrane channels. Charged or polar molecules that cannot fit in between the phospholipids typically enter and also leave cells through promoted diffusion.

Illustrations of facilitated diffusion.

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



The species of membrane transport discussed so far always involve substances relocating down their concentration gradient. It is also feasible to move substances across membranes against their concentration gradient (from areas of short concentration to areas of high concentration). Since this is one energetically unfavorable reaction, energy is required for this movement. The resource of power is the malfunction of ATP. If the energy of ATP is directly used come pump molecules versus their concentration gradient, the carry is dubbed primary energetic transport.

Illustration of primary active transport.

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


In some cases, the use of ATP may be indirect. For example, if a cell supplies ATP come pump the end Na+ and also then provides the Na+ concentration gradient to carry in glucose, the move of glucose would be an example of secondary energetic transport.

On the left side of the snapshot below, a problem (represented by one X) is being transported native the within of the cell to the outside even though over there is more of that substance on the exterior (indicated by the letter X being bigger on the external of the cell. This is primary energetic transport.

In the snapshot on the right side, problem S, already at greater concentration in the cell, is carried into the cell with substance X. Due to the fact that S is gift transported without the direct use the ATP, the deliver of S is an example of second active transport. Because that substance X primary energetic transport that X is occurring. The high concentration of X outside the cabinet is being offered to carry in substance S versus its concentration gradient.



It is feasible for large molecules to get in a cabinet by a process called endocytosis, where a small piece the the cabinet membrane wraps about the particle and is brought into the cell. If the fragment is solid, endocytosis is likewise called phagocytosis. If liquid droplets room taken in, the processes is referred to as pinocytosis.

Illustration the endocytosis. Note that the particle gone into the cell surrounding by a piece of cabinet membrane.

The opposite of endocytosis is exocytosis. Cells use exocytosis to secrete molecule too big to pass v the cell membrane by any other mechanism.

Other Links and animations:


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


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




Click top top the button above to open up a problem solver to aid you exercise your understanding of membrane transport through the following examples:

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

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

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

The pH of the cytosol (fluid inside the cells) that stomach wall cells is approximately 7. (Recall that a low pH method high hydrogen ion concentrations).

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

5. Salivary gland cells produce the enzyme outstanding amylase and secrete it into the salivary ducts to be ceded 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 loses water through its cabinet membrane.

7. Some bacteria usage the energy of ATP to pump H+ the end of their cells. They usage the H+ concentration gradient to drive the transfer of sugars into the cabinet

against their concentration gradients. What system of transport ideal describes exactly how the sugars room entering bacterial cells?

8. Part cells engulf droplets the extracellular fluid. What system of deliver would this be?