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Starter
Explain why active transport allows
substances to be accumulated in an
area where as facilitated diffusion
doesn`t.
Answer
• Active transport uses energy to ensure
that materials can pass only one way
through the membrane- against a
concentration gradient. Facilitated
diffusion allows the movement of
molecules both ways and so the
concentration will reach an equilibrium
over time.
Active transport
• Make a flow diagram to show the steps
involved in active transport
• e.g. Start with
The cell needs to contain a higher level of
potassium ions in the cell than the
concentration outside the cell
Objectives
• Recall a definition for osmosis
• Explain osmosis in terms of water potential
• Recognise and evaluate the effects of
solutions of different water potentials on
plant and animal cells
Osmosis
• Osmosis is the
diffusion of water from
a region of high water
concentration to a
region of low water
concentration through
a partially permeable
membrane
Osmosis
• Osmosis is the
diffusion of water from
a region of high water
potential to a region
of low water potential
through a partially
permeable membrane
Osmosis
• Water molecules can
diffuse easily across
membranes
• Many solute molecules,
e.g. Na+, cannot diffuse
across membranes
(without help)
• Osmosis is the diffusion
of water, down water
potential (Ψ) gradient,
across a selectively
permeable membrane
Water potential (Ψ)
• Ψ is the tendency of a
solution to lose water, it is
decreased by the addition
of a solute, it is increased
by external pressure
• Ψ is measured in kPa
• Ψ is always negative, i.e.
Ψ of pure water is 0, Ψ of
sugar solution may be 2000 kPa
• Water always moves to the
lowest (most negative) Ψ
• Hypotonic = higher Ψ
than cell
• Hypertonic = lower Ψ
than cell
• Isotonic = same Ψ as cell
Task 1
• Complete the spaces on the osmosis
sheet
Task 2
• In pairs Summarise what happens to plant an
animal cells in the following solutions,
1. Isotonic
2. Hypotonic
3. Hypertonic
Use pages 31and 32. 10 minutes time limit.
Then teach each other the other ½ (10 minutes)
Plenary
• Fresh water amoebae must continually
move large volumes of water out of the
well. Explain why this is the case and
suggest how they might carry this out.
Answer
• The water potential of the cell is lower than that
of the surrounding water- so water moves into
the cell.
• Amoebae move excess water into a membrane
bound vacuole called a `contractile vacuole`.
This vacuole empties its contents out of the cell
via process called exocytosis. His is an active
process.