Recall that membranes have two significant components: phospholipids i ordered it in a bilayer, and membrane proteins.


Click below for a more elaborate photo of a cell membrane.

One the the features of membrane is to control what passes into and out the the cell. In this module you will testimonial mechanisms of membrane transport.

You are watching: Which best describes how water moves during osmosis

There are numerous different varieties of membrane transport, depending on the qualities of the substance gift transported and the direction of transport.


In simple diffusion, tiny noncharged molecules or lipid soluble molecules pass between the phospholipids to enter or leaving the cell, moving from areas of high concentration to areas of short concentration (they movedown your concentration gradient). Oxygen and carbon dioxide and also most lipids enter and also leave cell by simple diffusion.

Illustrations of an easy diffusion.

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


Osmosis is a form of basic diffusion in i m sorry water molecule diffuse v a selectively permeable membrane from locations of high water concentration to areas of lower water concentration. (Note that the an ext particles liquified in a solution, the much less water over there is in it, therefore osmosis is sometimes explained as the diffusion that water from locations of short solute concentration to locations of high solute concentration).

Illustration the Osmosis. Assume the the membrane is permeable to water, yet not come sucrose (represented through the little black squares). The sucrose molecules will not leave the cell since they can not pass v the membrane. However, due to the fact that there is much less water ~ above the side through the sucrose, water will get in the cell by osmosis.

Another means to define the two remedies in the instance of above is to usage the terms hypertonic and hypotonic. A hypertonic solution has more solutes and also less water than a hypotonic solution. So, in the example above, the solution inside the cabinet is hypertonic to the solution exterior the cell. Throughout osmosis, water move from the hypotonic solution (more water, less solutes) to the hypertonic solution (less water, more solutes).

In each of the examples displayed below, i m sorry of the options is hypertonic?




In facilitated diffusion, substances move into or out of cells down their concentration gradient through protein networks in the cell membrane. An easy diffusion and also facilitated diffusion are comparable in that both involve motion down the concentration gradient. The distinction is just how the problem gets through the cell membrane. In basic diffusion, the problem passes between the phospholipids; in promoted diffusion there are a devoted membrane channels. Charged or polar molecules that cannot fit in between the phospholipids normally enter and leave cell through promoted diffusion.

Illustrations of facilitated diffusion.

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



The varieties of membrane transport disputed so far always involve substances relocating down your concentration gradient. It is also feasible to move substances across membranes versus their concentration gradient (from locations of short concentration to areas of high concentration). Because this is an energetically unfavorable reaction, energy is required for this movement. The source of power is the malfunction of ATP. If the power of ATP is straight used come pump molecules versus their concentration gradient, the transport is called primary energetic transport.

Illustration the primary energetic transport.

Note the the substance (indicated through the triangles) is being transported from the side of the membrane with tiny of the substance to the side of the membrane with a many the substance v a membrane protein, and also that ATP is being damaged down to ADP.


In part cases, the use of ATP might be indirect. Because that example, if a cell supplies ATP come pump out Na+ and also then offers the Na+ concentration gradient to bring in glucose, the move of glucose would be an example of secondary energetic transport.

On the left next of the photo below, a problem (represented by an X) is gift transported from the within of the cell to the outside also though there is more of that substance on the outside (indicated through the letter X being bigger on the outside of the cell. This is primary energetic transport.

In the snapshot on the appropriate side, substance S, currently at greater concentration in the cell, is brought into the cell v substance X. Due to the fact that S is being transported without the straight use the ATP, the carry of S is an instance of second active transport. Because that substance X primary active transport the X is occurring. The high concentration of X external the cabinet is being used to carry in problem S versus its concentration gradient.



It is feasible for big molecules to go into a cabinet by a procedure called endocytosis, wherein a little piece that the cabinet membrane wraps around the particle and also is brought into the cell. If the fragment is solid, endocytosis is additionally called phagocytosis. If fluid droplets space taken in, the procedures is called pinocytosis.

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

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

Other Links and animations:


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


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




Click on the button over to open a trouble solver to aid you exercise your expertise of membrane transport through the adhering to examples:

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

2. Carbon dioxide (a little uncharged gas molecule) start the lung (where that is less concentrated) from the blood (where that is more concentrated).

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

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

4. The lung cells of a victim who drowned in new water room swollen as result of water entering the cells.

5. Salient gland cells create the enzyme salivary amylase and secrete it into the salient ducts come be yielded to the mouth.

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

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

versus their concentration gradients. What device of transport best describes exactly how the sugars room entering bacter cells?

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