Membranes control the composition of cells through active and passive transport. Materials move across membranes via simple diffusion, facilitated diffusion, osmosis, and active transport. The sodium-potassium pump uses active transport to move ions against their gradients in axons. Vesicles transport materials within cells from the ER to the Golgi and plasma membrane by budding off and fusing. Endocytosis transports materials into cells while exocytosis releases them out of cells.

1. By Chris Paine
https://bioknowledgy.weebly.com/
1.4 Membrane transport – Part 2
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. Primary active transport requires ATP.
Integral protein pumps use the energy from the hydrolysis of
ATP to move ions or large molecules across the cell
membrane.
Molecules are moved against their concentration gradient
http://commons.wikimedia.org/wiki/File:Scheme_sodium-potassium_pump-en.svg
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/animation__how_the_sodium_potassium_pump_works.html

5. Link to Other Topics – Topic 2.
All Macromolecules are broken down to Monomers to be absorbed and
used by the body
Many biological molecules are polymers
Monomer Polymer
Sugars carbohydrates
Amino acids Proteins
Nucelotides DNA RNA ATP

6. Link to Other Topics – Topic 2.

7. Link to Other Topics – Topic 2.

8. 1.4.U1 Particles move across membranes by simple diffusion, facilitated diffusion,
osmosis and active transport.
Active transport uses energy to
move molecules against a
concentration gradient. This
energy may either be generated
by:
• The direct hydrolysis of ATP
(primary active transport)
• Indirectly coupling transport
with another molecule that is
moving along its gradient
(secondary active transport)

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

10. 1.4.A1 Structure and function of sodium–potassium pumps for active transport and
potassium channels for facilitated diffusion in axons.
The axons of nerve cells transmit electrical impulses by translocating ions to create
a voltage difference across the membrane
At rest, the sodium-potassium pump expels sodium ions from the nerve cell, while
potassium ions are accumulated within
When the neuron fires, these ions swap locations via facilitated diffusion via
sodium and potassium channels

11. 1.4.A1 Structure and function of sodium–potassium pumps for active transport and
potassium channels for facilitated diffusion in axons.
1. Three sodium ions bind to intracellular sites on the sodium-potassium pump
2. A phosphate group is transferred to the pump via the hydrolysis of ATP
3. The pump undergoes a conformational change, translocating sodium across the
membrane
4. The conformational change exposes two potassium binding sites on the extracellular
surface of the pump
5. The phosphate group is released which causes the pump to return to its original
conformation
6. This translocates the potassium across the membrane, completing the ion exchange

12. 1.4.A1 Structure and function of sodium–potassium pumps for active transport and
potassium channels for facilitated diffusion in axons.
http://bioserv.fiu.edu/~walterm/human_online/nervous/nervous_system_files/image012.gif
1. At one stage during a nerve
impulse there are relatively more
positive charges inside.
2. This voltage change causes
potassium channels to open,
allowing potassium ions to diffuse
out of the axon.
3. Once the voltage conditions
change the channel rapidly closes
again.
Potassium channels in axons are voltage gated. They enable the facilitated
diffusion of potassium out of the axon
n.b. other positively charged ions that we might expect to pass
through the pore are either too large to fit through or are too small to
form bonds with the amino acids in the narrowest part of the pore -
this explains the specificity of the channel.

13. Vesicles are small spheroidal
packages that bud off of the
RER and the Golgi apparatus
They carry proteins produced
by ribosomes on the RER to the
Golgi apparatus, where they are
prepared for export from the
cell via another vesicle
http://www.sumanasinc.com/webcontent/animation
s/content/vesiclebudding.html
1.4.U3 Vesicles move materials within cells.
Use the animated tutorial
to learn more about the
formation and use of
vesicles in cells

14. Materials destined for secretion are transported around the cell in
membranous containers called vesicles
Endoplasmic Reticulum
• The endoplasmic reticulum is a membranous network that is
responsible for synthesizing secretory materials
• Rough ER is embedded with ribosomes and synthesizes
proteins destined for extracellular use
• Smooth ER is involved in lipid synthesis and also plays a role
in carbohydrate metabolism
Materials are transported from the ER
when the membrane bulges and then
buds to create a vesicle surrounding
the material

15. Golgi Apparatus
• Materials move via vesicles
from the internal cis face of the
Golgi to the externally
oriented trans face
• While within the Golgi
apparatus, materials may be
structurally modified (e.g.
truncated, glycosylated, etc.)
• Material sorted within the Golgi
apparatus will either be
secreted externally or may be
transported to the lysosome

16. Plasma Membrane
• Vesicles containing materials destined for extracellular use will
be transported to the plasma membrane
• The vesicle will fuse with the cell membrane and its materials
will be expelled into the extracellular fluid
Materials sorted by the Golgi apparatus may be either:
• Released immediately into the extracellular fluid (constitutive
secretion)
• Stored within an intracellular vesicle for a delayed release in
response to a cellular signal (regulatory secretion)

17. Endocytosis: The taking in of
external substances by an
inward pouching of the plasma
membrane, forming a vesicle
Exocytosis: The release of
substances from a cell
(secretion) when a vesicle joins
with the cell plasma membrane.
http://highered.mcgraw-hill.com/olc/dl/120068/bio02.swf
1.4.U2 The fluidity of membranes allows materials to be taken into cells by endocytosis
or released by exocytosis.

18. 1.4.U2 The fluidity of membranes allows materials to be taken into cells by endocytosis
or released by exocytosis.
There are two main types of endocytosis:
• Phagocytosis – The process by which solid substances
are ingested (usually to be transported to the
lysosome)
• Pinocytosis – The process by which liquids / dissolved
substances are ingested (allows faster entry than via
protein channels)

19. http://commons.wikimedia.org/wiki/File:Endocytosis_types.sv
g
“Cell eating” “Cell drinking”
1.4.U2 The fluidity of membranes allows materials to be taken into cells by endocytosis
or released by exocytosis.

20. 1.4.U2 The fluidity of membranes allows materials to be taken into cells by endocytosis
or released by exocytosis.

21. Bibliography /
Acknowledgments
Jason de Nys