Download Osmosis practical - Nuffield Foundation

Practical Work for Learning: Model-based inquiry
Using a ‘pot model’ to represent osmosis
Teacher guidance
This lesson is designed to exemplify a model-based inquiry approach to practical work, in which
students use and evaluate a model for osmosis.
Component resources:
 Teacher guidance, including lesson plan
 Student sheet and assessment item
 PowerPoint
Overview:
Osmosis is an important process which plants use to take up water. A model can be used to help
explain how this process works.
Summary of lesson:
During the lesson students will
 be introduced to the model of osmosis
 make a ‘pot model’ using beans of different sizes to represent solvent and solute molecules,
and a net to represent the cell membrane
 use the model to predict and explain what happens when they place potato pieces into a
concentrated sucrose solution and distilled water, measuring the change in mass
The lesson follows this basic structure:
 Share the learning outcomes with students
 Set the context
 Revise relevant prior learning
 Define the key scientific terms
 Construct a ‘pot model’
 Make a prediction and explain reasoning
 Test the prediction by manipulating variables and collecting data
 Analyse data
 Explain the data using the model
 Evaluate the fit of the model with the data
 Assess and evaluate learning
Age range: 14-16
Timing: 50 mins
Curriculum links: Osmosis is a key concept in GCSE Additional Science.
Prior knowledge:
Students will already know:
 all matter consists of molecules or atoms which are constantly moving
 the random motion of atoms or molecules in liquids means that they spread from areas of
high concentration to areas of low concentration
 plants need water to grow, and they get this water from the soil
Practical Work for Learning: Model-based inquiry

cells are surrounded by a partially-permeable cell membrane. This allows some chemicals
through, but is a barrier to other chemicals. The plant cell wall is freely permeable.
Learning outcomes:
Students will be able to:
 explain the overall movement of water into and out of plant cells
 construct and apply a model of osmosis
Link to practical on Practical Biology / Chemistry / Physics:
http://www.nuffieldfoundation.org/practical-biology/investigating-effect-concentration-blackcurrantsquash-osmosis-chipped-potatoes
Background information:
The model of osmosis can be defined as follows:
When solutes dissolve in water, weak bonds form between the water and solute. For this reason,
water molecules in a solution are less free to move across a partially permeable compared with
water molecules in pure water.
Osmosis is the result of molecules colliding with pores in the membrane, water molecules going
through, some solute molecules not.
Osmosis is the overall movement of water by diffusion through a partially-permeable membrane,
from a solution of lower concentration to a solution of higher concentration of dissolved solutes.
Scientific terms:
The scientific terms which are needed to understand and use this model are:
 atom – the smallest particle of a defined element
 molecule– a group of two or more atoms held together by covalent bonds
 partially-permeable membrane – a membrane which will only allow certain molecules (or
ions) to pass through it by diffusion
 diffusion – the spread of particles through random motion from regions of higher
concentration to regions of lower concentration
 solution – a mixture in which a solute (e.g. sucrose) is dissolved (forms weak bonds with) a
solvent (e.g. water)
 concentration – the amount of solute (e.g. sucrose) in a certain volume of solvent (e.g.
water)
 model - a simplified version of a theory which allows the theory to be discussed and used to
solve problems. A model allows predictions to be made and tested through scientific inquiry.
Lesson outline:
Step
Timing
Share learning
outcomes with
students
2 min
Details
Students will be able to:
 explain the overall movement of water into
and out of plant cells
 construct and apply a model of osmosis
Resources
PowerPoint
presentation
Practical Work for Learning: Model-based inquiry
Set the context 3 min
Ask the question - when you water plants, the
water goes in. How?
PowerPoint
presentation
This is the phenomenon being studied.
Review
relevant prior
learning
5 min
Review plant cell structure so students understand
that this lesson concerns the passage of water
and solutes in and out of plant cells.
PowerPoint
presentation
Review a simple model of osmosis. This does not
involve consideration of the pressure from plant
cell walls or the reason why water passes from
dilute to more concentrated solutions. Define the
key scientific terms as part of this review.
Emphasise how water passes in both directions
across a membrane, so osmosis relates to the
overall (net) flow over time.
Students
10 min
construct and
test a simple
model for
osmosis
(alternatively
this could be a
demonstration)
Show students the presentation describing how
the model is made.
PowerPoint
presentation
Students make their models.
Equipment for
making pot
models
Refine the
model of
osmosis
Introduce the refined model for osmosis – then
introduce the potato practical which can sit in
solutions while students make the revised pot
model.
5 mins
The student sheet asks them to sketch their
findings and explain how the pot model represents
osmosis.
The slide showing the simple model of osmosis
can stimulate discussion about how well the pot
model represents osmosis.
The next two slides in the presentation introduce
the idea that this pot model can’t account for the
fact that water from a dilute solution is more likely
to pass through a membrane than water from a
concentrated solution. The more sophisticated
diagram explains why this is.
Ask for suggestions on how the pot model can be
revised. Provide hints e.g. why did you need to
turn the pots so they were equally on top and on
the bottom while you shook?
Lead to the idea that gravity can be used to
represent the pressure (diffusion gradient)
produced when solutions of different
concentrations are separated by a partially
permeable membrane. This pressure drives the
overall flow of water in osmosis.
PowerPoint
presentation
Practical Work for Learning: Model-based inquiry
Set up the
potato
practical and
make the
revised pot
model
Make
prediction and
explain
reasoning
20 min
The potatoes can soak while students discuss and
make their revised pot models. In this case the
practical is set up before the prediction is made as
students can be thinking while the potato pieces
soak.
You could allocate different pairs of students to
carry out the potato practical with one solution only
– and make the related pot model. The pairs could
then join up with others who have carried out the
practical with the different solution.
Groups of four then discuss their results and pot
models.
Weigh potato samples (two per working group/
pair) first – all approximately the same mass,
around 15 g or 20 g. <teacher/ technician>
Put samples of potato (one per working group/
pair) into two concentrations of solution – one a
very high concentration of sugar (= low
concentration of water (<concentration to be
inserted>) and the other a very low concentration
of sugar: (distilled water = 100% water).
While the potato is soaking, students make the
prediction and its justification – with reference to
the revised model of osmosis. Structure questions
to scaffold the prediction. Encourage discussion in
pairs or small groups. Keep ‘minds on’ while the
potato is soaking.
Questions for prompting group discussion:
What do the different parts of the model
represent?
How do water molecules move?
Will the sugar molecules be able to cross the
membrane?
(Note to teachers: would glucose and salt work?)
PowerPoint
presentation
<it might be
necessary to
start potatoes
soaking ahead
of the lesson,
and have them
pre-soaked. To
be tested during
trialling phase:
how quickly do
potato pieces
react?>
Practical Work for Learning: Model-based inquiry
Analyse data
and explain
using the
model
10 min
If students only carried out the experiment with
one concentration, get pairs together into groups
of four so they can discuss both sets of data.
PowerPoint
presentation
Discuss which potato sample has lost mass and
which has gained mass. Link change in mass to
net movement of water.
Make sure students have engaged with the model
to explain their results.
e.g. The potato piece has shrivelled up/ reduced in
mass. This is because water molecules moved
from the more dilute solution in the potato cells
into the more concentrated sucrose solution. The
solute molecules and water molecules form weak
bonds in a solution. This means there are less
water molecules to move in and out of a cell when
solute molecules are present.
Assess and
evaluate
learning
For
homework/
next
lesson
The questions ask students to transfer their
understanding to a 2D diagram of osmosis. They
are asked about how the model can be used to
predict outcomes for the potato experiment, and
how it can be applied to plants taking up water
from the soil.
Differentiation / optional extra activities:
 Be the molecules – students can be ‘water’ or ‘sucrose’ – differentiated by coloured bands or
hats. When a sucrose molecule becomes surrounded by three or four water molecules
forming a ring around them, they can’t get through the gaps in your semi-permeable
membrane (made with chairs or other barriers). Unattached water molecules can get through
the pores. Difficult to mimic random movement. Simple message: Sugar can’t cross the
membrane, water can.
Taking it further:
 A full inquiry - Find the concentration equal to the concentration of the potato cells you have
(or other plant tissue or red blood cells?)
Links to related practical activities on Practical biology / Chemistry / Physics:
Investigating osmosis in chickens’ eggs
Observing osmosis, plasmolysis and turgor in plant cells.
A closer look at blood
Practical Work for Learning: Model-based inquiry
Marking criteria:
Answers
1)
sucrose solution
inside potato cell
free water molecule
sucrose molecule
surrounded by water
molecules
cell membrane
overall movement of water
by osmosis
Water moves in both directions across the partially permeable
membranes of the potato cells.
The sucrose molecules can’t move across the membrane.
The overall movement of water is from the cells into the more
concentrated solution.
2) The model shows that the potato will lose water, so the prediction is that it will lose mass.
3) Water in the soil is less concentrated than the solution inside plant cells. The overall
movement of water by osmosis is into the plant from the soil water.
4) Encourage students to think about how the revised model allowed it to simulate solutions
with different concentrations either side of the membrane.