Everyday uses of membrane transport
Understanding diffusion, osmosis and active transport explains many things we do at home, in cooking and in medicine. Most food-preservation methods work by using osmosis to remove water from microbes so that they cannot grow and spoil the food. The same ideas explain why plant material becomes crisp or limp and how doctors treat kidney failure and dehydration.
Preserving food by osmosis
- Salting fish and sugaring fruit surround the food with a very concentrated solution. Water leaves any bacterial cells by osmosis, so the microbes shrink and die and the food keeps longer.
- Pickling and making jam work in the same way.
Key idea
A concentrated (hypertonic) solution draws water OUT of cells by osmosis; a dilute (hypotonic) solution lets water IN.
Osmosis in the kitchen and body
- Limp vegetables become crisp again when soaked in fresh water, because water enters the cells by osmosis and makes them turgid.
- Sprinkling salt on cut vegetables draws water out, so they become soft and watery.
- Kidney dialysis uses diffusion across a partially permeable membrane to remove urea and excess salts from a patient's blood.
- An oral rehydration solution replaces water and ions lost in diarrhoea; the balanced salts and glucose are absorbed and water follows by osmosis.
- Freshwater fish keepers must not use very salty water, because water would leave the fish's cells by osmosis and harm them.
In every case the direction of water movement is decided by the concentration difference across the membrane, so the same rule of osmosis explains all of these everyday situations.
Example
A raisin (a dried grape) placed in water swells up as water enters by osmosis, while a fresh grape left in strong syrup shrinks as water leaves by osmosis.
Remember
Salt and sugar preserve food by pulling water out of microbes through osmosis.