Osmosis in potato

After checking out my plants in the balcony, the young girl in my class had a curious question "Aunty, how do the roots know when to absorb water?".
 "You mean, how does water get into roots ..right?" asked my daughter walking towards the girl.
 "Yes, thats what I don't understand " replied the girl. Listening to their conversation I said "water enters the root hair cells by 'osmosis' and we will do an experiment to understand osmosis today".  And that's how we decided to do an experiment on osmosis.
 

What is Osmosis?

Plants absorb water from the soil by 'osmosis'. Osmosis is the movement of water through a semi permeable membrane. Water moves into plant cells through osmosis by making them turgid or stiff to hold the plants upright. Osmosis is passive transport, meaning it does not require energy to be applied.  

We also discussed about how osmosis is helpful for photosynthesis as well. Otherwise the plants would wilt. After a long discussion it was time for experiment to explain the theory of osmosis.

Osmosis experiment with potato

Materials needed

1. Potato - 1

2. Knife

3. Bowls -2 

4. Salt -  2 tsp

5. Water - 1/2 cup

Procedure

1. Cut  2 equal slices of potato using the knife.

2. Take 1/4 cup Water in each bowl.

3. Add the salt to  one of the bowls  of water and mark it as ' salt water'.

4. Put one potato slice in each bowl.

5. Leave it for 30 mins.

Observation

After 30 mins when kids saw the potato slice, they could see the difference in the appearance of the potato slices. The potato slice that was inside the salt water had turned light brown color and had also become soft where as the potato in 'plain water' was same and felt crisp .  The potato slice soaking in salt water looked substantially different from the original and the other slice. It seems to have wilted, gotten very soft and flexible.

What really happened?

Potato slice has a semi permeable membrane.  Due to Osmosis , water from the higher concentration (potato slice) has moved to the lower concentration ( salt water). This is how water from the roots move up to the leaves which uses it in  photosynthesis and respiration.

The potato is made up of tiny, living units called cells. Each cell is surrounded by a cell membrane which acts like our skin does. It keeps the cells parts inside and keeps the other things outside protecting the cell.

While the membrane stops most things, water can pass through it. The water tends to move towards higher concentrations of dissolved chemicals. This means that if the water outside the cell is saltier than the water inside, water will  move from the inside of the cell to the outside. As more and more of the cells lost water, the slice of potato became soft.

The potato slice in normal water was more firmer and bright looking. This is because, there was more salt and other dissolved chemicals within the potato than the surrounding water . This means that the water has moved into the potato.

It was great to hear the little girl say " Now, I easily understand the process of osmosis in plants, Aunty" . With a smile on my face I simply nodded with satisfaction.