IB Topic 1.4

1. By Chris Paine
https://bioknowledgy.weebly.com/
1.4 Membrane transport
Essential idea: Membranes control the composition
of cells by active and passive transport.
The background image is a piece of artwork inspired by the complexity of an E. Coli.
Complexity in cell structure is much greater still in Eukaryotes and this only possible
through the compartmentalisation and the selective transport membranes allow.
Edited and Revised by Eran Earland
Acknowledgements to Chris Paine https://bioknowledgy.weebly.com/ and BioNinja
http://onlinelibrary.wiley.com/doi/10.1002/bmb.20345/full#fig2

2. Understandings, Applications and Skills
Statement Guidance
1.4.U1 Particles move across membranes by simple
diffusion, facilitated diffusion, osmosis and active
transport.
1.4.U2 The fluidity of membranes allows materials to be
taken into cells by endocytosis or released by
exocytosis.
1.4.U3 Vesicles move materials within cells.
1.4.A1 Structure and function of sodium–potassium pumps
for active transport and potassium channels for
facilitated diffusion in axons.
1.4.A2 Tissues or organs to be used in medical
procedures must be bathed in a solution with the
same osmolarity as the cytoplasm to prevent
osmosis.
1.4.S1 Estimation of osmolarity in tissues by bathing
samples in hypotonic and hypertonic solutions.
(Practical 2)
Osmosis experiments are a useful opportunity
to stress the need for accurate mass and
volume measurements in scientific
experiments.

3. 1.4.U1 Particles move across membranes by simple diffusion, facilitated diffusion,
osmosis and active transport.

4. 1.4.U1 Particles move across membranes by simple diffusion, facilitated diffusion,
osmosis and active transport.
Cellular membranes possess two key qualities:
• They are semi-permeable (only certain materials may freely cross –
large and charged substances are typically blocked)
• They are selective (membrane proteins may regulate the passage of material that cannot
freely cross)
Movement of materials across a biological membrane may occur either actively or passively

5. 1.4.U1 Particles move across membranes by simple diffusion,
facilitated diffusion, osmosis and active transport.
Passive Transport
• Passive transport involves the movement of
material along a concentration gradient
(high concentration ⇒ low concentration)
• Because materials are moving down a
concentration gradient, it does not require
the expenditure of energy (ATP hydrolysis)
• There are three main types of passive
transport:
– Simple diffusion – movement of small or lipophilic
molecules (e.g. O2, CO2, etc.)
– Osmosis – movement of water molecules
(dependent on solute concentrations)
– Facilitated diffusion – movement of large or
charged molecules via membrane proteins (e.g.
ions, sucrose, etc.)

6. 1.4.U1 Particles move across membranes by simple diffusion, facilitated diffusion,
osmosis and active transport.
http://highered.mheducation.com/sites/0072495855/student_view0/chapter2/anima
tion__how_diffusion_works.html
http://www.phschool.com/science/biology_place/labb
ench/lab1/concepts.html

7. 1.4.U1 Particles move across membranes by simple diffusion, facilitated diffusion,
osmosis and active transport.

8. 1.4.U1 Particles move across membranes by simple diffusion, facilitated diffusion,
osmosis and active transport.
http://www.northland.cc.mn.us/biology/Biology1111/animations/transpor
t1.html

9. 1.4.U1 Particles move across membranes by simple diffusion, facilitated diffusion,
osmosis and active transport.

10. What is
osmosis?
http://www.flickr.com/photos/luchilu/399970490/
1.4.U1 Particles move across membranes by simple diffusion, facilitated diffusion,
osmosis and active transport.

11. http://www.flickr.com/photos/luchilu/399970490/
1.4.U1 Particles move across membranes by simple diffusion, facilitated diffusion,
osmosis and active transport.
Osmosis is the net movement of water molecules across a semi-permeable membrane
from a region of low solute concentration to a region of high solute concentration
(until equilibrium is reached)
Water is considered the universal solvent – it will associate with, and dissolve, polar or
charged molecules (solutes)
• Because solutes cannot cross a cell membrane unaided, water will move to equalize
the two solutions
• At a higher solute concentration there are less free water molecules in solution as
water is associated with the solute
Osmosis is essentially the diffusion of free water molecules and hence occurs from
regions of low solute concentration
*** low solute
concentration = higher
water concentration
so water is going
DOWN its gradient
and thus Osmosis is
Passive

12. Osmosis may occur when there is a partially
permeable membrane, such as a cell
membrane.
When a cell is submerged in water, the water
molecules pass through the cell membrane
from an area of low solute concentration
(outside the cell) to one of high solute
concentration (inside the cell) (Wikipedia)
Aquaporin is an integral protein that, as
it’s name suggests, acts as a pore in the
membrane that speeds the movement
of water molecules
http://opm.phar.umich.edu/protein.php?pdbid=1sor
1.4.U1 Particles move across membranes by simple diffusion, facilitated diffusion,
osmosis and active transport.
http://highered.mcgraw-
hill.com/sites/0072495855/student_vie
w0/chapter2/animation__how_osmosis
_works.html

13. 1.4.S1 Estimation of osmolarity in tissues by bathing samples in hypotonic and
hypertonic solutions. (Practical 2)
Osmolarity is a measure of solute concentration, as defined by the number of osmoles of
a solute per litre of solution (osmol/L)
Solutions may be loosely categorised as hypertonic, hypotonic or isotonic according to
their relative osmolarity
• Solutions with a relatively higher osmolarity are categorised as hypertonic
• (high solute concentration ⇒ gains water)
• Solutions with a relatively lower osmolarity are categorised as hypotonic
• (low solute concentration ⇒ loses water)
• Solutions that have the same osmolarity are categorised as isotonic
• (same solute concentration ⇒ no net water flow)

14. 1.4.S1 Estimation of osmolarity in tissues by bathing samples in hypotonic and
hypertonic solutions. (Practical 2)
Estimating Osmolarity - The osmolarity of a tissue may be interpolated by bathing the
sample in solutions with known osmolarities
The tissue will lose water when placed in hypertonic solutions and gain water when
placed in hypotonic solutions
Water loss or gain may be determined by weighing the sample before and after bathing
in solution
Tissue osmolarity may be inferred by identifying the concentration of solution at which
there is no weight change (i.e. isotonic)

15. 1.4.S1 Estimation of osmolarity in tissues by bathing samples in hypotonic and
hypertonic solutions. (Practical 2)
http://www.saps.org.uk/secondary/teaching-resources/286-measuring-the-
water-potential-of-a-potato-cell
http://www.nuffieldfoundation.org/practical-biology/investigating-effect-
concentration-blackcurrant-squash-osmosis-chipped-potatoes
The two lab protocols shown suggest
different ways of measuring the dependent
variable.
This is an ideal opportunity to practise and
improve your understanding of the
following IA criteria:
• Analysis
• Evaluation
• Communication
n.b. estimation of osmolarity should be limited to statements such as “this tissue has an osmolarity
equivalent to a 2% sucrose solution”. Molar concentrations maybe used in place of % and other solution
such as sodium chloride maybe used in place of glucose.
Estimation of osmolarity is
a simple lab with many
possible variations.

16. http://commons.wikimedia.org/wiki/File:Osmotic_pressure_on_blood_cells_diagram.svg
http://commons.wikimedia.org/wiki/File:Turgor_pressure_on_plant_cells_diagram.svg
Tissues or organs to be used in medical procedures must be kept in solution to
prevent cellular desiccation
This solution must share the same osmolality as the tissue / organ (i.e. isotonic) in
order to prevent osmosis from occurring
Uncontrolled osmosis will have negative effects with regards to cell viability:
• In hypertonic solutions, water will leave the cell causing it to shrivel (crenation)
• In hypotonic solutions, water will enter the cell causing it to swell and potentially
burst (lysis)
In plant tissues, the effects of uncontrolled osmosis are moderated by the
presence of an inflexible cell wall
• In hypertonic solutions, the cytoplasm will shrink (plasmolysis) but the cell wall
will maintain a structured shape
• In hypotonic solutions, the cytoplasm will expand but be unable to rupture
within the constraints of the cell wall (turgor)
1.4.A2 Tissues or organs to be used in medical procedures must be bathed in a solution
with the same osmolarity as the cytoplasm to prevent osmosis.

17. http://commons.wikimedia.org/wiki/File:Osmotic_pressure_on_blood_cells_diagram.svg
http://commons.wikimedia.org/wiki/File:Turgor_pressure_on_plant_cells_diagram.svg
The
importance
of osmotic
control
1.4.A2 Tissues or organs to be used in medical procedures must be bathed in a solution
with the same osmolarity as the cytoplasm to prevent osmosis.
http://highered.mcgraw-
hill.com/sites/0072495855/student_vie
w0/chapter2/animation__how_osmosis
_works.html

18. The importance of osmotic control
preventing damage to cells and tissues
1.4.A2 Tissues or organs to be used in medical procedures must be bathed in a solution
with the same osmolarity as the cytoplasm to prevent osmosis.
Common medical procedures in
which an isotonic saline
solution is useful:
• fluids introduction to a
patient’s blood system via an
intravenous drip, e.g for
rehydration
• used to rinse wounds, skin
abrasions etc.
• keep areas of damaged skin
moist before applying skin
grafts
• eye drops/wash
• frozen and used pack donor
organs for transportation
http://www.defenseimagery.mil/imageRetrieve.action?guid=8c9d5fade029a4f5a68fe667d1ae802ba9f30dd5&t=2

19. http://commons.wikimedia.org/wiki/File:Scheme_facilitated_diffusion_in_cell_membrane-en.svg
Facilitated Diffusion:
Large and polar molecules can’t get across the membrane via
simple diffusion
Transmembrane (polytopic) proteins recognise a particular
molecule and help it to move across the membrane. The
direction it moves is dependent on the concentration gradient.
Watch the animation
1.4.U1 Particles move across membranes by simple diffusion, facilitated diffusion,
osmosis and active transport.
http://highered.mcgraw-
hill.com/sites/0072495855/student_view0/chapter2/ani
mation__how_facilitated_diffusion_works.html

20. Facilitated diffusion is the passive movement of molecules across the cell membrane via the aid of a membrane
protein
• It is utilized by molecules that are unable to freely cross the phospholipid bilayer (e.g. large, polar molecules
and ions)
• This process is mediated by two distinct types of transport proteins – channel proteins and carrier proteins
Carrier Proteins
• Integral glycoproteins which bind a solute and undergo a conformational change to translocate the solute
across the membrane
• Carrier proteins will only bind a specific molecule via an attachment similar to an enzyme-substrate interaction
• Carrier proteins may move molecules against concentration gradients in the presence of ATP (i.e. are used in
active transport)
• Carrier proteins have a much slower rate of transport than channel proteins (by an order of ~1,000 molecules
per second)
Channel Proteins
• Integral lipoproteins which contain a pore via which ions may cross from one side of the membrane to the
other
• Channel proteins are ion-selective and may be gated to regulate the passage of ions in response to certain
stimuli
• Channel proteins only move molecules along a concentration gradient (i.e. are not used in active transport)
• Channel proteins have a much faster rate of transport than carrier proteins

22. 1.4.U1 Particles move across membranes by simple diffusion,
facilitated diffusion, osmosis and active transport.
Active Transport
• Active transport involves the
movement of materials against a
concentration gradient (low
concentration ⇒ high concentration)
• Because materials are moving against
the gradient, it requires the
expenditure of energy (e.g. ATP
hydrolysis)
• There are two main types of active
transport:
• Primary (direct) active transport –
Involves the direct use of metabolic
energy (e.g. ATP hydrolysis) to
mediate transport
• Secondary (indirect) active transport –
Involves coupling the molecule with
another moving along an
electrochemical gradient

23. 1.4.U1 Particles move across membranes by simple diffusion, facilitated diffusion,
osmosis and active transport.

24. 1.4.U1 Particles move across membranes by simple diffusion, facilitated diffusion,
osmosis and active transport.