Living Cells and Life Processes

Osmosis

Now we have established the general features of both animal and plant cells we must also remember that many cells do look different and this is because they have their own special jobs to do. These cells display specialised features that make them suited to carry out their specific function.

As we know each cell is surrounded by a cell membrane. This membrane has tiny holes in it which allow small molecules to pass through but not large ones. Due to this nature the cell membrane is described as being partially (or selectively) permeable.

Osmosis is a special kind of diffusion involving water molecules. It occurs when two solutions are separated by a partially permeable membrane. The definition of osmosis is as follows:

Osmosis is the movement of water molecules from an area of high water concentration (weak/dilute solution) to an area of low water concentration (strong/concentrated solution) through a partially permeable membrane.

Osmosis is the movement of water molecules from an area of high water concentration (weak/dilute solution) to an area of low water concentration (strong/concentrated solution) through a partially permeable membrane.

Water is actually moving both ways to try to even up the concentration, however more water molecules are moving from the pure water towards the sugar solution because there is a higher concentration of water molecules in the pure water solution than the sugar solution. The result is a net transfer of molecules down a concentration gradient from the water to the sugar solution. Eventually the level of water on the lower water concentration side rises while that on the more concentrated side falls. When the concentration of water is the same on both sides the movement of water will be the same in both directions at this point the net exchange of water is zero and the system is in equilibrium.

Osmosis in animal cells

As you will remember animal cells have partially permeable cell membrane. This means that if they are placed in pure water because their cytoplasm is a stronger solution than the pure water, water will pass into the cells by osmosis. The cells will therefore swell up. However animal cells have no cell wall to stop them swelling too much so they keep swelling until they burst! We call this haemolysis.

Osmosis in plant cells

Osmosis is the way in which plants take up water. This is how. Root hairs of the plant take in water from the soil by osmosis. The cell membrane of the root hair cell acts a partially permeable membrane (the cell wall is fully permeable) and because the cell sap inside the vacuole is a strong solution (low water concentration) water passes from the soil (high water concentration) into the root hair cell by osmosis. The concentration of the sap in the vacuole is now weaker as there is a high concentration of water. Water will now pass from this area of high concentration to the next cell which has a low water concentration by osmosis. In this way water continues to move along the cells of the root up the xylem to the leaf. all the time water is moving to areas of lower water concentration

As water enters plant cells it makes the cell swell up. The water moves into the plant cell vacuole and pushes against the cell wall. Eventually the cell contains as much water as it can hold. The strong cell wall stops the cell bursting. We say that cell is turgid. Turgid cells are useful implants as the give the plant support as they keep the stems of plants upright.

When plants are placed into a strong sugar or salts solution water will pass out of the cells by osmosis. As water passes out, the sap vacuole starts to shrink. These cells are no longer firm they are limp. We say that they are flaccid and the plant will wilt.

If a lot of water leaves the cells then the cytoplasm starts to peel away from the cell wall. We say that the cell has undergone plasmolysis.