# Understand Osmosis

In this worksheet, students will define osmosis.

Key stage:  KS 4

GCSE Subjects:   Biology: Single Subject, Biology: Combined Science

GCSE Boards:   OCR 21st Century

Curriculum topic:   Living Together: Food and the Ecosystem

Difficulty level:

### QUESTION 1 of 10

Osmosis is the diffusion of water molecules from a low concentration solution to a high concentration solution, across a partially permeable membrane.

A partially permeable membrane has holes or pores in it that allow water molecules through, but are too small to allow larger molecules through.

During osmosis, water molecules diffuse from pure water or dilute solution to more concentrated solutions.

Dilute solutions have a high concentration of water molecules.

Concentrated solutions have a low concentration of water molecules.

Osmosis in plants

Plant cells are surrounded by a partially permeable cell membrane. This allows water and other small molecules to diffuse across. Plant cells have a strong cell wall surrounding the membrane, which offers support and protection.

Plants require water in order to photosynthesise. The roots of a plant contain root hair cells, which are specialised cells that increase the surface area for maximum absorption of water by osmosis. In pure water, plant cells will take in water via osmosis and become firm or turgid. In a concentrated solution (not much water present), the cell loses water and starts to shrink and becomes flaccid.

Calculations Involving Osmosis

Osmosis can be demonstrated using cubes of potatoes of roughly the same mass. By placing the cubes in different concentrations of sugar solutions, the cubes might gain or lose mass, or might even stay the same mass.

Scientists will be able to calculate the change in mass to see how much mass was gained or lost by using the following equation:

Final mass (g) – initial mass (g) = change in mass (g)

In the following activity, you will define and describe osmosis.

Plant and animal cells use the process of osmosis to transport water into or out of their cells.

Describe osmosis.

Cell membranes are partially permeable.

What is a partially permeable membrane?

A partially permeable membrane does not have holes in it

A partially permeable membrane has holes in it

It only allows waste products through

It allows water molecules through but the holes are too small to allow larger molecules through

Lisa is drinking orange squash. She finds it too sweet so adds more water to her glass of squash.

What has she done to her orange squash?

She has diluted it

She has made it less concentrated

She has made it more concentrated

Water is absorbed by root hair cells in plants. The water will move from a high concentration to a low concentration.

Why do plants need water?

For absorbing water

For photosynthesis

For metabolism

What is the name of the structure of the plant where water is absorbed by osmosis?

For absorbing water

For photosynthesis

For metabolism

Plants take in water through their root hair cells. If too much water is taken into the cells, the cells swell up.

If there is too little water, what might happen?

Cell gains water

Cell loses water

Cell becomes flaccid

Cell stays the same

 Cell A < Permeable membrane   Cell B

The diagram above shows two cells separated by a permeable membrane. Cell A has four molecules of water and Cell B has eight molecules of water.

In which direction will the water move?

From Cell A to Cell B

From Cell B to Cell A

No movement

Sally was preparing potatoes to make some chips. She wanted to wash her raw potato chips, so she placed them in a bowl of pure water.

What is likely to happen to the mass of the potato chips?

The mass stays the same

The mass increases

The mass decreases

What do the following terms mean?

## Column B

Turgid
The cell shrinks as water is moving out of it
Flaccid
Allows certain substances to pass through and not ...
Partially permeable
The cell swells as more water is moving into it

Osmosis can be demonstrated using cubes of potatoes of roughly the same mass. By placing the cubes in different concentrations of sugar solutions, the cubes might gain or lose mass, or might even stay the same mass.

Lara did an experiment at school on the effect of sugar solutions on potato pieces.  The potato pieces were of equal size. She measured the mass of each potato cube and placed each one in a beaker containing different concentrations of sugar solution. After two hours, each potato cube's mass was measured.

 Beaker 1 2 3 4 5 Sugar concentration (M) 0.2 0.4 0.6 0.8 1 Initial mass (g) 2 2 2 2 2 Final mass (g) 2.2 2.15 1.96 1.8 1.73

Calculate the change in mass in beaker 2 and beaker 3.

+0.16 g

+0.15 g

+0.04 g

-0.04 g

• Question 1

Plant and animal cells use the process of osmosis to transport water into or out of their cells.

Describe osmosis.

EDDIE SAYS
How did you find this first question? Did you remember that all-important definition from the Introduction? Osmosis is really important for cells - they're able to get the water they need for different processes like photosynthesis. Like gases in diffusion, water molecules move from a less concentrated (more water) solution to a more concentrated (less water) solution.
• Question 2

Cell membranes are partially permeable.

What is a partially permeable membrane?

A partially permeable membrane has holes in it
It allows water molecules through but the holes are too small to allow larger molecules through
EDDIE SAYS
Did you find this one a bit easier than the first one because there were no words to write in yourself?! Even if you weren't sure about the correct choices, you might have been able to have a guess that there must be some holes in it so that substances could pass through, and that it had something to do with water! A partially permeable membrane is selective and only allows water and small particles to pass through it.
• Question 3

Lisa is drinking orange squash. She finds it too sweet so adds more water to her glass of squash.

What has she done to her orange squash?

She has diluted it
She has made it less concentrated
EDDIE SAYS
There were three correct options this time - did you get them all? Adding water to orange squash means you are diluting your drink and making it less concentrated. This means that the orange squash has more water to mix with, which will make it taste less sweet.
• Question 4

Water is absorbed by root hair cells in plants. The water will move from a high concentration to a low concentration.

Why do plants need water?

For photosynthesis
EDDIE SAYS
It might seem obvious that plants need water but have you ever stopped to ask yourself why? Water, as well as carbon dioxide and oxygen, is needed for photosynthesis to occur. Without these three things, plants won't be able to respire and grow.
• Question 5

What is the name of the structure of the plant where water is absorbed by osmosis?

EDDIE SAYS
Did you get this one right? There were several options that were permitted here, but it was necessary to include the word roots in some form to gain the mark. The perfect answer would have named the root hair cells. The roots and root hair cells are really important - they are the structure where water enters for photosynthesis. Water passes into the root hair cells using osmosis.
• Question 6

Plants take in water through their root hair cells. If too much water is taken into the cells, the cells swell up.

If there is too little water, what might happen?

Cell loses water
Cell becomes flaccid
EDDIE SAYS
Did you remember the key fact about osmosis? Water moves from a higher concentration to a lower concentration. If the plant's surroundings are dry, then there is a higher concentration of water in the cells than in the surroundings. This will mean that water will leave the plant cells making the cells flaccid. This is like when you forget to water your plants, they start to wilt!
• Question 7
 Cell A < Permeable membrane   Cell B

The diagram above shows two cells separated by a permeable membrane. Cell A has four molecules of water and Cell B has eight molecules of water.

In which direction will the water move?

From Cell B to Cell A
EDDIE SAYS
Did you remember it?!! Water will move from a higher concentration to a lower concentration. There are more water molecules in Cell B than Cell A, so water will move from Cell B, through the partially permeable membrane and into Cell A until there are an equal number of molecules in each cell.
• Question 8

Sally was preparing potatoes to make some chips. She wanted to wash her raw potato chips, so she placed them in a bowl of pure water.

What is likely to happen to the mass of the potato chips?

The mass increases
EDDIE SAYS
You need to apply what you know about osmosis to this problem. What can you remember? You know that water molecules move from a high concentration to a lower concentration. So where is there a higher concentration of water - in the potato or in the bowl? The answer is in the bowl. The water in the bowl is pure, so the cells of the potato will contain less water than their surroundings. This means that water will move into the potato chips, increasing their mass.
• Question 9

What do the following terms mean?

## Column B

Turgid
The cell swells as more water is ...
Flaccid
The cell shrinks as water is movi...
Partially permeable
Allows certain substances to pass...
EDDIE SAYS
How did you do with this one? Hopefully, you had no problem choosing the right definition for the partially permeable option, but did you get the other two round the right way? One way to help you to get them sorted is to think of flaccid as meaning floppy (both start with f), and turgid is a bit like the word rigid. Take the time to learn these key terms as you are often asked about them in exams.
• Question 10

Osmosis can be demonstrated using cubes of potatoes of roughly the same mass. By placing the cubes in different concentrations of sugar solutions, the cubes might gain or lose mass, or might even stay the same mass.

Lara did an experiment at school on the effect of sugar solutions on potato pieces.  The potato pieces were of equal size. She measured the mass of each potato cube and placed each one in a beaker containing different concentrations of sugar solution. After two hours, each potato cube's mass was measured.

 Beaker 1 2 3 4 5 Sugar concentration (M) 0.2 0.4 0.6 0.8 1 Initial mass (g) 2 2 2 2 2 Final mass (g) 2.2 2.15 1.96 1.8 1.73

Calculate the change in mass in beaker 2 and beaker 3.

+0.15 g
-0.04 g
EDDIE SAYS
A maths question to finish off with - don't panic though, just use the formula given to you in the Introduction. To calculate the change in mass: (Final mass – initial mass). For Beaker 2: 2.15 - 2 = 0.15 g or +0.15 g For Beaker 3: 1.96 - 2 = -0.04 g Well done for completing this activity. Hopefully, you're feeling more confident with this topic now.
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