What is osmosis? (GCSE question)

Osmosis is the movement of water from an area of high concentration to an area of low concentration across a semi-permeable membrane. 

A semi-permeable membrane acts like a barrier that allows only small molecules through, such as water, and not larger molecules like sugar molecules.  The water will move across this membrane until equilibrium is reached.  For example, osmosis occurs in the kidneys of animals and allows water to move into the roots of plants. For a clearer explanation imagine a container of water with a membrane down the middle.  On one side there is 80% water and 20% sugar (therefore it has a high concentration of water molecules) and on the other side there is 40% water and 60% sugar (it has a low concentration of water molecules).  Osmosis will occur and the water molecules will move from the high concentration side (90% water) to the low concentration side (40% water) until it becomes even and reaches equilibrium.  This equilibrium is reached because neither side of the container is high or low in water concentration - it is now equal.