Intermediate 2 Biology
Unit 1: Living Cells
DIFFUSION AND OSMOSIS IN
PLANT AND ANIMAL CELLS
Diffusion
Learning Objectives
• Name the method by which substances move
in and out of cells.
• Define the term ‘diffusion’
• State when diffusion will stop.
• Identify concentration differences and
predict the direction of movement of
substances by diffusion in a given diagram.
• State which substances enter and leave cells
by diffusion.
• Explain the importance of diffusion to cells.
Diffusion
• Diffusion is the net movement of
molecules of a substance from a
region of high concentration to an
region of low concentration until the
concentrations become equal.
• The difference in concentration is
called the concentration gradient.
Diffusion in Organisms
Unicellular organisms
• Oxygen diffuses into
the cell
• Carbon dioxide diffuses
out of the cell
• Diffusion of food
substances into the
cytoplasm
• Examples
– Paramecium
– Ameoba
Multicellular organisms
• Diffusion in the alveoli
– O2 into the blood
– CO2 into the alveoli
• Diffusion between the
blood and body cells
– O2 from the blood into
the body cells
– CO2 from the body cells
into the blood
Role of the cell membrane
• The cell surface membrane is
selectively permeable
– Small molecules can pass freely e.g.
Oxygen, water, carbon dioxide
– Some molecules e.g. Glucose can pass
through more slowly
– Large insoluble molecules e.g. Starch can
not pass through
Osmosis
Learning Objectives
• Name the method by which water passes into and out
of cells.
• Explain what a selectively permeable membrane is.
• Explain what is meant by a concentration gradient.
• Define osmosis using the terms selectively permeable
membrane and concentration gradient.
• Identify water concentration gradients when given
percentage solute concentrations.
• Predict the direction of water movement between 2
solutions of known concentration of solute
• Describe a visking tubing experiment involving 2 solutions
of different concentration and predict its results
Osmosis
• Osmosis is the net movement of water
from a region of high water
concentration to a region of low water
concentration (down a water
concentration gradient) across a
selectively permeable membrane.
Relative water concentrations
• Hypotonic
– Solution with the higher water
concentration
• Isotonic
– Solutions of equal water concentrations
• Hypertonic
– Solution with the lower water
concentration
Osmosis in plant and animal cells
Learning Objectives
• Explain why osmotic effects are different on plant cells
compared with animal cells.
• State the meaning of the terms: hypertonic, hypotonic
and isotonic.
• Describe the effects on plant cells of immersion in
different solutions.
• State the meanings of the words: flaccid, turgid and
plasmolysed.
• Predict the effect of different solutions on plant cells.
• Recognise and label plant cells in various conditions.
• Describe the effects on animal cells of immersion in
different solutions.
• Predict the effect of different solutions on animal cells.
Osmosis in red blood cells
(pg 33 Int 2 Bio 1st Edition)
• If a red blood cell is placed in a
hypotonic solution it will burst
• If a red blood cell is placed in an
isotonic solution there is no net
movement of water and the cell
remains unchanged
• If a red blood cell is placed in a
hypertonic solution is will shrink
(crenate)
Plant cells
(pg 33 Int 2 Bio 1st Edition)
• If a plant cell is placed in a hypotonic
solution it becomes turgid
• If a plant cell is placed in an isotonic
solution there is no net movement of
water and the cell remains
unchanged
• If a plant cell is placed in a hypertonic
solution it becomes plasmolysed
Osmoregulation in Paramecium
• Paramecium lives in fresh water, as a
result the organism is continuously gaining
water by osmosis
• The cell is prevented from bursting by the
presence of contractile vacuoles
• The contractile vacuoles have canals
that collect excess water, when swollen
the vacuole contracts and discharges it
contents through a pore
• There are two contractile vacuoles and
they discharge their contents alternately