The sucrose molecules will not leave the cell because they cannot pass through the membrane. However, since there is less water on the side with the sucrose, water will enter the cell by osmosis. Another way to describe the two solutions in the example of above is to use the terms hypertonic and hypotonic.
A hypertonic solution has more solutes and less water than a hypotonic solution. So, in the example above, the solution inside the cell is hypertonic to the solution outside the cell. During osmosis, water moves from the hypotonic solution more water, less solutes to the hypertonic solution less water, more solutes. In each of the examples shown below, which of the solutions is hypertonic?
In facilitated diffusion , substances move into or out of cells down their concentration gradient through protein channels in the cell membrane. Simple diffusion and facilitated diffusion are similar in that both involve movement down the concentration gradient.
The difference is how the substance gets through the cell membrane. In simple diffusion, the substance passes between the phospholipids; in facilitated diffusion there are a specialized membrane channels.
Charged or polar molecules that cannot fit between the phospholipids generally enter and leave cells through facilitated diffusion. Note that the substance is moving down its concentration gradient through a membrane protein not between the phospholipids. The types of membrane transport discussed so far always involve substances moving down their concentration gradient.
It is also possible to move substances across membranes against their concentration gradient from areas of low concentration to areas of high concentration. Since this is an energetically unfavorable reaction, energy is needed for this movement.
The source of energy is the breakdown of ATP. If the energy of ATP is directly used to pump molecules against their concentration gradient, the transport is called primary active transport. Note that the substance indicated by the triangles is being transported from the side of the membrane with little of the substance to the side of the membrane with a lot of the substance through a membrane protein, and that ATP is being broken down to ADP.
The main difference is that diffusion does not involve particular structures while facilitated diffusion does. Diffusion refers to the movement of molecules along concentration gradient i. However, unlike water, larger molecules cannot simply diffuse in and out of cell through plasma membrane and require specific structure to help them move. These are transport proteins also called carrier proteins.
How would you explain the difference between diffusion and facilitated diffusion? Shahzada B. Jan 30, The rate of diffusion increases linearly as we add more particles to one side of the membrane. If the particles can only pass through protein channels, then the rate of diffusion is determined by the number of channels as well as the number of particles.
Once the channels operate at their maximal rate, a further increase in particle numbers no longer increases the apparent rate of diffusion. At this limited rate we describe the protein channel as being saturated. The cartoon illustrates several points about facilitated diffusion.
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