3.2 Osmosis
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Substances can move through membranes in 3 different ways; diffusion, osmosis and active transport. In this section, we are going to be looking at osmosis.
Osmosis is the net movement of water molecules through a partially permeable membrane from a dilute solution to a more concentrated solution. It happens when the total concentration of all dissolved substances inside and outside the cell are different. When this is the case, water will move across the membrane from a dilute solution to a more concentrated one. Osmosis still follows the concentration rule from diffusion. The water moves from a place where it is in high concentration (hence the dilute solution) to where there is a lower concentration of water (where the solution is concentrated). Osmosis can only occur if the cell membrane is partially permeable, meaning that it lets through certain substance (i.e. water) and does not let through other substances (i.e. other solutes).
Osmosis is important in moving water from cell to cell and from the blood to tissue. It is important that cells in the body are in a solution that has the correct concentration of solutes. If this is not the case, the cells can be damaged by osmosis and unable to perform their intended function. For example, if red blood cells are placed in water. Water will move through the blood cells membrane and into the blood cell, resulting in the cell swelling up and eventually bursting. However, if we placed red blood cells in a salty solution, the water will leave the red blood cell into the salty solution. This results in the cell shrinking.
Osmosis is the net movement of water molecules through a partially permeable membrane from a dilute solution to a more concentrated solution. It happens when the total concentration of all dissolved substances inside and outside the cell are different. When this is the case, water will move across the membrane from a dilute solution to a more concentrated one. Osmosis still follows the concentration rule from diffusion. The water moves from a place where it is in high concentration (hence the dilute solution) to where there is a lower concentration of water (where the solution is concentrated). Osmosis can only occur if the cell membrane is partially permeable, meaning that it lets through certain substance (i.e. water) and does not let through other substances (i.e. other solutes).
Osmosis is important in moving water from cell to cell and from the blood to tissue. It is important that cells in the body are in a solution that has the correct concentration of solutes. If this is not the case, the cells can be damaged by osmosis and unable to perform their intended function. For example, if red blood cells are placed in water. Water will move through the blood cells membrane and into the blood cell, resulting in the cell swelling up and eventually bursting. However, if we placed red blood cells in a salty solution, the water will leave the red blood cell into the salty solution. This results in the cell shrinking.
Example
Let’s have a look at an example of visking tubing. Visking tubing has holes in it that allows water to pass through it (permeable to water) but is not permeable to larger molecules. For example, visking tubing will not let sugar sucrose through. We can perform an experiment whereby we have a sucrose solution inside visking tubing. The tubing is then placed inside a beaker that contains water.
Let’s have a look at an example of visking tubing. Visking tubing has holes in it that allows water to pass through it (permeable to water) but is not permeable to larger molecules. For example, visking tubing will not let sugar sucrose through. We can perform an experiment whereby we have a sucrose solution inside visking tubing. The tubing is then placed inside a beaker that contains water.
The sucrose molecules are too large to pass through the partially permeable membrane, but the water particles are small enough and are able to pass through the membrane in either direction. The water molecules in the sucrose solution (which is inside the Visking tubing), are attracted to the sucrose molecules. As the water molecules are attracted to the sucrose molecules the kinetic energy of the water molecules reduces, which results in fewer water molecules moving from inside to outside the visking tubing (in the diagram, this is from right to left). Whilst this is happening, water molecules are moving from outside to inside the visking tubing (left to right). The overall effect is that more water molecules move inside the visking tubing, which is the same as saying the water molecules are moving from a more dilute solution to a less dilute solution (where the more dilute solution is pure water and the less dilute solution is sucrose solution).
Rate of Osmosis
Like with diffusion, the rate of osmosis is dependent on a variety of factors.
Like with diffusion, the rate of osmosis is dependent on a variety of factors.
- Temperature – the greater the temperature, the more kinetic energy the water particles have, which results in a greater rate of osmosis.
- Concentration gradient – the lower the concentration of the solute within a solvent, the faster osmosis will occur.
- Surface area to volume ratio – the greater this ratio is, the faster the rate of osmosis
- Water potential – water potential refers to how free the water molecules are to move. Pure water has the freest molecules and therefore has the highest water potential. A more concentrated solution has a lower water potential. This is because there are more solutes for water molecules to be attracted to. In the example that we had above, water is moving from a solution with a high water potential (which is pure water) to a solution with a lower water potential (which is the sucrose solution).