Chapter 8: Movement
through cell membranes
Leaving Certificate Biology
Higher Level
Selective Permeability
• Cell membranes are selectively permeable
meaning they can control what enters and
leaves the cell
– Selective permeability is controlled by proteins
embedded in the cell membrane
Glucose transporter
Glucose (in blood)
Cell membrane
(made of
phospholipids)
Glucose (in cell)
Diffusion
• Diffusion is the movement of particles
from a region of high concentration to a
region of low concentration
(i.e. down a concentration gradient)
– E.g. oxygen and carbon dioxide diffuse across
the alveolus membranes from regions of
relative high concentration to regions of
relative low concentration
Osmosis
• Osmosis is the movement of water
molecules from a region of high water
concentration to a region of low water
concentration across a semi-permeable
membrane
– E.g. water is continuously moving across the
membrane of an Amoeba by osmosis from
the freshwater (high water concentration) to
the interior of the cell (lower water
concentration)
Application of Osmosis
• Osmosis is used by food industry for
preservation of food
– E.g. tinned fruit/vegetables are placed in a
concentrated sugar (syrup) or salt solution –
this causes any bacteria/fungi present to die
due to water leaving the cells (Bacterial and
fungal cells have a high water concentration
relative to the syrup/salt solution)
Turgor and Plasmolysis
• Turgor is the pressure exerted by the cells
contents on the cell wall of a plant cell
H2O
Roots
H2O
Leaf
H2O
Wilted leaf
Turgor and Plasmolysis
Experiment to demonstrate
osmosis
• Apparatus:
Visking tubing, beakers, water,
sucrose solution, balance
• Method: set up as follows:
Control
Water Water
Control
Test
60%
60%
sucrose sucrose
60%
Water sucrose
Experiment to demonstrate
osmosis
• Result:
Visking tubes
remained same weight
Control
Water Water
Visking tube
swelled
Control
Test
80%
80%
sucrose sucrose
80%
Water sucrose
Experiment to demonstrate
osmosis
• Conclusion:
– The two control Visking tubes did not change
weight due to the fact that equal diffusion of
water particles was occurring – i.e. osmosis
was occurring in equal measure in both
directions
– The test Visking tube gained water by
osmosis due to a water concentration
gradient, i.e. water moved by osmosis from
the beaker (high water concentration) into the
Visking tube (low water concentration)