Osmosis is the net movement of water molecules from an area of high water concentration to an area of lower water concentration through a selectively permeable membrane. It's 'cause the sugar is too fat to fit through the membrane.
Variables include concentration of sucrose, volume of the solution, mass of potato, size of potato pieces (surface area), time given for osmosis to occur, temperature...
When the amount of water leaving the potato cells is the same as the amount of water entering the cells, there is said to be a dynamic equilibrium.
All food webs start with an autotroph or producer, which is either a plant or kind of bacteria. Above the autotrophs are the heterotrophs, which have to eat something else to survive.
Food webs show how all the food chains in a habitat are linked together, eg. a pond habitat. The arrows go from what is being eaten to what is eating it. For example, if I wanted to say lions eat zebras, I'd do:
Zebra --------------------> Lion
Food webs can be changed by environmental changes, not necessarily caused by humans. Less rain means less lettuces means less snails. Simple.