Title: Introduction to AS biology
June 23, 2015
Learning objective:
• To understand the way water can cross a cell membrane
You should be able to…
• Define osmosis
in terms of water
potential
• Explain the different
behaviour of plant and
animal cells in
solutions of various
water potentials.
LANGUAGE FOR LEARNING
Osmosis, Water potential, Solute potential
• Calculate the water
potential of a plant
cell using
experimental
techniques
Osmosis
•  At GCSE you might have defined osmosis as the movement
of water from a weak solution to a strong solution through a
semi-permeable (partially permeable) membrane.
•  At advanced level you should explain osmosis as the
movement of water down its water potential gradient.
Osmosis
Weak solution separated from a strong solution
by a partially membrane.
Weak solution
(high water
potential)
Partially-permeable
membrane
Strong solution
(low water
potential)
Concentration gradient
for water molecules
Solute cannot get through the membrane so only the water moves
– it moves down its water potential gradient (from high to low).
Water potential (ψ)
•  Water potential is a measure of the tendency for water to move
from one place to another.
•  Water always moves from a region of higher water potential to
a region of lower potential.
•  By definition pure water has a water potential of 0.
•  Addition of solute lowers the water potential – it therefore
becomes negative. The more solute, the more negative water
potential is.
•  The amount by which solute molecules lower the water
potential is known as the solute potential (ΨS).
Solute potential of potato cells
•  Using the worksheet provided, carry out
an experiment to work out the solute
potential of the potato you have been
provided with.
•  Your work should be carried out with due
regard to health and safety, think about
the variables involved and produce a piece
of work that is of A LEVEL standard.
•  You will be assessed on this work in
September
Osmosis and Blood Cells …
If blood cells are placed in solutions of
different strength, they absorb or lose water
depending upon the water potential of the
solutions
1. Hypotonic solution
2.Isotonic solution
3.Hypertonic solution
What do each of these terms mean?
Would the response of the cell be different if it
was a plant cell rather than an animal one?
Why?