1. Osmosis
Noadswood Science, 2016

2. Osmosis
Wednesday, January 02, 2019
To identify the effect of osmosis due to concentration gradients

3. Precise Learning
Water may move across cell membranes via osmosis. Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane (recognise, draw and interpret diagrams that model osmosis)
Students should be able to:
use simple compound measures of rate of water uptake
use percentiles
calculate percentage gain and loss of mass of plant tissue.
Students should be able to plot, draw and interpret appropriate graphs.

4. Osmosis Eggs
Look at the different ‘osmosis eggs’ – why do they appear different and what caused their appearance to be this way?

5. Osmosis Eggs
The eggs have been placed in different conditions – they were all soaked in vinegar to remove the shell…
One has been placed in sugar syrup – this has decreased in size
One has been placed in water – this has increased in size
This is all due to osmosis…

6. Osmosis
Water can move across cell membranes because of osmosis – osmosis is the net movement of water molecules across a partially permeable membrane from a high concentration to a low concentration
For osmosis to happen you need: -
Two solutions with different concentrations
A partially permeable membrane to separate them (a partially permeable membrane lets some substances pass through them, but not others

7. Osmosis
Eventually the level on the more concentrated side of the membrane rises, while the one on the less concentrated side falls
When the concentration is the same on both sides of the membrane, the movement of water molecules will be the same in both directions – at this point, the net exchange of water is zero and there is no further change in the liquid levels

8. Importance Of Osmosis
Osmosis is important to plants – they gain water by osmosis through their roots as well as controlling the water content of some specialised cells
Stomata are tiny holes which allow CO2 into the leaf (they also allow water to escape) – they are pores which open and close automatically controlled by guard cells
When there is lots of water they become turgid so the pore opens
When there is not much water they become flacid which closes the pore preventing water loss but it also stops CO2 getting in so photosynthesis stops too!

9. Rich sugar / salt solution
Osmosis Experiment
Set up the potato tubes experiment – after the experiment is set up predict what will happen to the potatoes
Experiment 1 – place some potato tubes in pure water
Experiment 2 – place some potato tubes in rich sugar / salt solution
Potato tube
Potato tube
Rich sugar / salt solution
Pure water
Experiment 1
Experiment 2

10. Rich sugar / salt solution
Explanation
Explain your results for the potato tube experiment – why did the potato tubes swell / shrink? How could you have been more precise in this experiment?
The mass before / after the experiment could have been recorded, and doing more than one experiment would increase the reliability and validity of the experiment
Potato tube
Potato tube
Rich sugar / salt solution
Pure water
Experiment 1
Experiment 2

11. Rich sugar / salt solution
Osmosis Experiment
In pure water the potato tubes swell because water enters the cells via osmosis (water moves from the [high] in the beaker to the [low] in the potato)
In rich sugar / salt solution the potato tubes shrink because water leaves the cells via osmosis (water moves from the relative [high] in potato to the [low] in the beaker solution)
Potato tubes swell
Potato tubes shrink
Rich sugar / salt solution
Pure water
Experiment 1
Experiment 2

12. Osmosis
Osmosis is the net movement of water molecules across a partially permeable membrane from a region of high water concentration to a region of low water concentration
Water moves into and out of cells by osmosis

13. Sports Drinks
Sports drinks contain sugars to replace the sugar used during exercise, as well as water and ions to replace those lost during sweating
If the water and ions are not replaced the ion/water balance of the body is disturbed which means the cells do not work as efficiently (and in the worst case can lead to serious health issues)…

14. It can lead to confusion, headaches and a fatal swelling of the brain.
A 22-year-old man died after completing his first London Marathon because he drank too much water. David Rogers collapsed at the end of the race and died yesterday in Charing Cross Hospital.
Today it emerged the fitness instructor from Milton Keynes died from hyponatraemia, or water intoxication.
This is when there is so much water in the body that it dilutes vital minerals such as sodium down to dangerous levels.
It can lead to confusion, headaches and a fatal swelling of the brain.

15. Practice Questions What is osmosis
Explain how a cell that is short of water gains water from the surrounding solution

16. Answers
What is osmosis – the movement of water molecules across a partially permeable membrane from a region of higher water concentration to a region of lower water concentration.
Explain how a cell that is short of water gains water from the surrounding solution – if a cell is short of water the surrounding solution will usually have a higher concentration of water molecules / be more dilute that the solution inside the cell meaning water molecules will move by osmosis from the surrounding solution into the cell

17. Practice Questions – Application
An experiment is carried out to investigate osmosis – a potato is cut into cylinders of the same known length and width. Some are placed into pure water and others into differing concentrations of sugar. They are left for 24 hours and their length and mean change calculated as below
Describe what happened to the potato cylinders in the 0.00M and 0.25M solutions (explain in terms of osmosis)
Predict the mean change in length for the 1.00M solution
What the dependent and independent variable in the experiment and give two variables that had to be controlled
Concentration of solution (M)
Mean change in length (mm)
0.00
+3.1
0.25
+2.1
0.50
-1.2
0.75
-2.6
1.00 ?

18. Concentration of solution (M) Mean change in length (mm)
Answers
Describe what happened to the potato cylinders in the 0.00M and 0.25M solutions (explain in terms of osmosis) – they increased in length (3.1mm in the 0.00M solution and 2.1mm in the 0.25M solution) as the solutions had higher water concentrations so water molecules moved into the potato
Predict the mean change in length for the 1.00M solution – 3-4mm
What the dependent and independent variable in the experiment and give two variables that had to be controlled – DV: length; IV: concertation sugar; control: volume, temperature, type of sugar, time left etc…
Concentration of solution (M)
Mean change in length (mm)
0.00
+3.1
0.25
+2.1
0.50
-1.2
0.75
-2.6
1.00 ?