Phospholipid bilayers are selectively permeable membranes that control the movement of molecules into and out of cells through passive and active transport mechanisms. Passive transport includes diffusion, facilitated diffusion, and osmosis, which allow molecules to move down their concentration gradients without energy expenditure. Active transport requires energy in the form of ATP to move molecules against their concentration gradients, such as sodium-potassium pumps that repolarize neurons after firing. Membrane transport is essential for cellular functions like nutrient absorption, waste removal, and nerve signal transmission.

1. 1.4. Membrane transport
Miltiadis-Spyridon Kitsos
Platon IB Diploma
Edited from
http://education.uoit.ca/lordec/ID_LORDEC/diffusion_osmosis/garib_diffusion_osmosis.html

2. The official IB Diploma Biology guide
Essential idea: Membranes control the composition of cells by active and passive transport
https://ibpublishing.ibo.org/server2/rest/app/tsm.xql?doc=d_4_biolo_gui_1402_1_e&part=3
&chapter=1

3. Phospholipid bilayers and transport of molecules
Phospholipid bilayers are selectively permeable
Selectively permeable: Certain molecules may
pass through the membrane and others may
not
+ + + + + + + + + + + + + + + + + + + +
- - - - - - - - - - - - - - - - - - - -
Modes of transport
Polarity

4. Diffusion
Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active
transport.
Does not require
energy
Overall direction of movement (given that all molecules
are moving all the time in all directions)
Diffusion is the passive net movement of molecules from areas of high concentration to
areas of low concentration (that is down the concentration gradient).
Semi-permeable membrane
Red molecules will move towards the right, that is
down the concentration gradient, through the semi-
permeable membrane.
Purple molecules are bigger. Deduce their possible
diffusion pattern.
Slide edited from

5. Simple
Diffusion
http://commons.wikimedia.org/wiki/File:Scheme_simple_diffusion_in_cell_membrane-en.svg
Diffusion
Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active
transport.

6. Stephen Taylor http://sciencevideos.wordpress.com
Diffusion
Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active
transport.
Slide from

7. Stephen Taylor http://sciencevideos.wordpress.com
Diffusion
Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active
transport.
Slide from

8. Stephen Taylor http://sciencevideos.wordpress.com
Diffusion
Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active
transport.
Slide from

9. Slide from
Diffusion
Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active
transport.

10. Diffusion
Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active
transport.
Slide edited from

11. Facilitated Diffusion
Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active
transport.
Facilitated Diffusion is the passive net movement of molecules from areas of high
concentration to areas of low concentration (that is down the concentration gradient)
through carrier proteins of the membrane.
Semi-permeable membrane
Mode of transfer for polar molecules which may not
pass through the phospholipid bilayer.
Protein specificity, only a specific molecule may
pass through the channel.
Integral channel proteins.
Slide edited from

12. Facilitated Diffusion
Structure and function of sodium–potassium pumps for active transport and potassium channels
for facilitated diffusion in axons.
A potassium channel is an integral protein
consisting of 4 protein subunits having a
narrow pore (0.3 nm)
Potassium ions, when dissolved, become
attached within a cluster of water
molecules.
As soon the ion with the water molecules
enters the pore, bonds with the water
molecules are broken, and a new series
of bonds is formed between the ion and
the amino acids.
These bonds are again broken, and new
bonds are formed with water molecules.
Structure of a subunit.
http://www.mun.ca/biology/desmid/brian/BIOL2060/BIOL2060-
13/13_08.jpg

13. Facilitated Diffusion
Structure and function of sodium–potassium pumps for active transport and potassium channels
for facilitated diffusion in axons.
http://hyperphysics.phy-astr.gsu.edu/hbase/biology/imgbio/actpot4.gif
http://physiologyonline.physiology.org/content/nips/13/4/
177/F1.large.jpg
Potassium channels are voltage gated and triggered when the external side membrane becomes
more positive than the inside.

14. Facilitated Diffusion
Media
http://highered.mheducation.com/sites/0072495855/student_view0/chapter2
/animation__how_facilitated_diffusion_works.html http://highered.mheducation.com/sites/0072943696/student_view0/chapter8/ani
mation__voltage-gated_channels_and_the_action_potential__quiz_1_.html
Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active
transport.

15. Simple & facilitated diffusion are passive.
• No energy input is required
There is a net movement of molecules from one side of the membrane to the other.
• The motion of molecules is random (Brownian motion)
• But there is an overall general movement in one direction.
This net movement is down the concentration gradient.
• From areas of high concentration to low concentration.
Movement is across a selectively or partially permeable membrane
Dependent on size or properties, some molecules can cross and not others.
Simple Diffusion
Occurs when the molecule’s properties allow them
pass across the membrane.
Facilitated Diffusion
Some molecules cannot cross easily, for example if
they are polar the phospholipids of the bilayer will
repel them.
The rate of diffusion is affected by:
• magnitude of concentration gradient
• SA:Vol ratio (more membranes, more transport
per unit volume)
• Length of diffusion pathway (longer journey
gives slower diffusion).
Channel proteins are integral membrane proteins
that pass through the membrane.
Their properties allow molecules to pass through
(e.g. polar molecules or ions).
Activation of these channels might be voltage-
gated (e.g. in neurons) or binding-activated.
Compare and distinguish Simple and Facilitated Diffusion
Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active
transport.

16. Active Transport
Require energy in
the form of ATP
Active transport is the active movement of molecules from areas of low concentration to areas of
high concentration (that is against the concentration gradient).
Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active
transport.

17. Why do
this?
To move molecules against the concentration
gradient or to create a large concentration gradient
across a membrane.
Protein
pumps
These are integral, passing through the membrane.
They are specific – only working with the target
molecule.
What
happens?
1. Target molecules bind to the pump.
2. ATP also binds to the pump. ATP is broken,
releasing energy and causing a conformational
(shape) change in the protein pump.
3. This conformational change pushes the
molecules across the membrane.
4. The molecule unbinds, and the pump reverts
back to the original shape.
Examples • Sodium-potassium pump is used to re-polarise
neurons after an action potential, ready to fire
again.
• Proton pumps in mitochondria generate a high
concentration gradient of H+ ions, ready for
chemiosmosis through ATP synthase, used for
generating ATP.
Active Transport
Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active
transport.

18. Active Transport
Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active
transport.

19. Active Transport
Structure and function of sodium–potassium pumps for active transport and potassium channels
for facilitated diffusion in axons.
Sodium potassium pumps follow a repeating cycle of events. Each cycle requires a
molecule of Adenosine Triphosphate (ATP)
1. Pump opens to the interior and 3 Na+ bind to the three binding
sites
1. Binding of Na+ activates the phosphorylation of the pump
protein
http://study.com/cimages/multimages/16/nak_pump.jpg

20. Active Transport
Structure and function of sodium–potassium pumps for active transport and potassium channels
for facilitated diffusion in axons.
Sodium potassium pumps follow a repeating cycle of events. Each cycle requires a
molecule of Adenosine Triphosphate (ATP)
3. Phosphorylation induces a conformational change in the
structure of the protein. Protein closes in the interior and
opens to the exterior releasing the three Na+ ions
4. K+ from the cell’s exterior binds to the two specific sites and this
triggers the release of the phosphate group from the protein.
http://study.com/cimages/multimages/16/nak_pump.jpg

21. Active Transport
Structure and function of sodium–potassium pumps for active transport and potassium channels
for facilitated diffusion in axons.
Sodium potassium pumps follow a repeating cycle of events. Each cycle requires a
molecule of Adenosine Triphosphate (ATP)
5. Loss of the phosphate group restores the protein to initial
state.
6. K+ ion are released to the interior of the cell, the protein
pump is again susceptible to receive Na+ ions and the cycle
starts over again.
http://study.com/cimages/multimages/16/nak_pump.jpg

22. The Na+-K+ pump
Structure and function of sodium–potassium pumps for active transport and potassium channels
for facilitated diffusion in axons.
The sodium-potassium pump is responsible for building concentration gradients of Na+ and
K+ across membranes. These gradients are used for the transition of nerve impulses
across the nerve cells.
Na+-K+ is also essential for
 Absorption of minerals in the small intestine cells.
 Regulation of Na+ in the kidney.
 regulating cardiac contractions.
https://upload.wikimedia.org/wikipedia/commons/8/83/Blausen_0818_Sodium-
PotassiumPump.png

23. Osmosis
Osmosis is the passive net movement of WATER molecules from areas of high
concentration (low solute concentration) to areas of low concentration (high
solute concentration) through a partially permeable membrane
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
Slide edited from

24. Osmosis
Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active
transport.
Slide from

25. Osmosis in cells
Animal cells exposed to hypertonic or hypotonic solutions change
If a red blood cell is placed in distilled water, water
molecules enter the cell by osmosis. This is because the
cytoplasm is hypertonic (higher solute concentration) in
relation to environment which is hypotonic (low solute
concentration). This increases the osmotic pressure
within the cell.
The volume of the cell may grow up to a certain size and
then the cell bursts, since the plasma membrane is quite
weak.
However, when red blood cells are placed in a hypertonic
solution they shrink. This makes the cells appear wrinkled.
http://1.bp.blogspot.com/-
pveNOT3ckRg/UG8ySTId5fI/AAAAAAAAADw/gWCtYBem
smc/s1600/Tonicidad+de+disoluciones.png
http://1.bp.blogspot.com/-
pveNOT3ckRg/UG8ySTId5fI/AAAAAAAAADw/gWCtYBemsmc/s1600/Toni
cidad+de+disoluciones.png

26. 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
organs to be used in medical procedures must be bathed in a solution with the same osmolarity as
asm to prevent osmosis.
m

27. Osmosis in cells
Plant cells exposed to hypertonic or hypotonic solutions undergo plasmolysis
As you remember, the plant cell wall protects the cells
from changing shape and prevents bursting.
If a plant cell is placed in a hypotonic solution, water
enters the cell by osmosis. This increases the osmotic
pressure within the cell and thus its turgidity.
When a plant cell is placed in a hypertonic solution, then the
plasma membrane detaches from the cell wall and the cell
undergoes plasmolysis.
http://1.bp.blogspot.com/-
pveNOT3ckRg/UG8ySTId5fI/AAAAAAAAADw/gWCtYBemsmc/s1600/Toni
cidad+de+disoluciones.png
http://www.one-
school.net/Malaysia/UniversityandCollege/SPM/revisioncard/biology/movementacrossmemb
rane/images/hypertonicplantcell.png
https://youtu.be/4JyT__Dea8Q

28. Osmosis in cells
Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions.
(Practical 2)
Osmotic concentration, or osmolarity, is the measure of solute concentration,
defined as the number of osmoles (Osm) of solute per litre (L) of solution.
Slide edited from

29. Endocytosis
The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis.
Vesicles move materials within cells.
Endocytosis is the active transfer of large molecules within the cell, via an engulfment of the plasma
membrane.
When endocytosis?
• to transfer larger molecules needed by
the cell (fetus absorbs mother antibodies)
• Food digestion by certain protozoa
• White blood cells kill bacteria or viruses
https://upload.wikimedia.org/wikipedia/commons/thumb/1/1a/Endocytosis_types.svg/
672px-Endocytosis_types.svg.png
https://highered.mheducation.com/olcweb/cgi/plugin
op.cgi?it=swf::535::535::/sites/dl/free/0072437316/12
068/bio02.swf::Endocytosis%20and%20Exocytosis
https://youtu.be/JnlULOjUhSQ

30. Endocytosis
The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis.
Vesicles move materials within cells.
Describe endocytosis.
endocytosis occurs when a membrane encloses a target particle;
fluidity of membrane permits movement of membrane;
membrane sinks inwardly/forms pit/invaginates to enclose particle;
membrane seals back on itself / edges fuse;
one membrane layer / two phospholipid layers enclose particle
making
vesicle;
inner phospholipid layer of (original) membrane becomes outer
phospholipid layer of vesicle membrane;
outer phospholipid layer of (original) membrane becomes inner
phospholipid layer of vesicle membrane;
vesicle breaks away from membrane/moves into cytoplasm;
changes in membrane shape require energy;
specific example of endocytosis (e.g. pinocytosis, phagocytosis);
From the IB Question bank
http://www.mun.ca/biology/desmid/brian/BIOL2060/BIOL2060-12/12_13.jpg

31. Slide from
Endocytosis
The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis.
Vesicles move materials within cells.

32. Slide from
Endocytosis
The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis.
Vesicles move materials within cells.
http://www.sumanasinc.com/webcontent/animations
/content/vesiclebudding.html

33. Slide from

34. Exocytosis
The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis.
Vesicles move materials within cells.
Exocytosis is the active transfer of large molecules outside the cell, via vesicles fusing with the plasma
membrane.
When endocytosis?
• To release materials from the cell (e.g.,
release of neurotransmitters.
• Production of secretory enzymes
• Waster products removed (e.g., Para-
mecium’s contractile vacuole)
https://highered.mheducation.com/olcweb/cgi/pluginp
op.cgi?it=swf::535::535::/sites/dl/free/0072437316/120
068/bio02.swf::Endocytosis%20and%20Exocytosis
https://youtu.be/qXThS39gLso

35. Annotate this diagram to explain vesicle transport & exocytosis.
Animated tutorial: http://bcs.whfreeman.com/thelifewire8e/content/cat_040/0504003.html
Slide from

36. www.stepsinbiology.com