1. 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.

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. Transport across the membrane
Passive Transport moves molecules across membrane without the use of energy by cell.
simple diffusion
facilitated diffusion
osmosis
Active Transport uses energy (ATP) to move molecules across membrane.
protein pumps
exocytosis
endocytosis (phagocytosis & pinocytosis)

4. Selectively Permeable Membranes
Selectively permeable membranes = semipermeable membranes
Only some molecules can pass through the membrane via diffusion.
Often based on size or charge
Cell membranes are semipermeable

5. Passive Transport

6. 2.4.4 Diffusion
The net movement of particles from an area of high concentration to low concentration (down their concentration gradient) due to continuous random movement.

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

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

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

10. Factors Affecting Diffusion
1.4.U1 Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.

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

12. Simple Diffusion across a membrane
1.4.U1 Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.

13. 2.4.5 Facilitated Diffusion
Protein channels (Carrier Proteins) allow certain substances to cross different membranes
often the large or polar/charged molecules who couldn’t enter by simple diffusion
The substances still moving down their concentration gradient.
Does NOT require use of the cell’s energy
Outside the Cell
Protein
Solute
Cytoplasm

14. Animations

15. Potassium Channels (We’ll cover this again next year!)
Potassium channels in axons of nerve cells are voltage gated. They enable the facilitated diffusion of potassium ions out of the axon
At one stage during a nerve impulse there are relatively more positive charges inside.
This voltage change causes potassium channels to open, allowing potassium ions to diffuse out of the axon.
Once the voltage conditions change the channel rapidly closes again.
1.4.A1 Structure and function of sodium–potassium pumps for active transport and potassium channels for facilitated diffusion in axons.
(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.)

17. Osmosis
The diffusion of water through a selectively permeable membrane.
Water flows from a region of low solute concentration to high solute concentration (high water concentration to low water concentration).

18. OSMOSIS

19. Animation

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

21. Osmolarity
When solutes bond with water, the “free water molecules” on one side of a membrane are reduced, thus causing osmosis.
We call these solutes “osmotically active.”
Glucose
Sodium ions
Chloride ions
Potassium ions

22. Osmolarity
Osmolarity is the total concentration of osmotically active solutes. [the solute concentration]
Units: osmoles or miliosmoles (mOsm)
Human cells and tissues are normally around 300 mOsm.

23. The Importance of Osmotic Control
Hypertonic – higher solute concentration (osmolarity) outside than inside the cell
Isotonic – equal solute concentration (osmolarity) inside and outside the cell
Hypotonic – lower solute concentration (osmolarity) outside than inside the cell
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.

24. The importance of osmotic control
preventing damage to cells and tissues
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
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.

25. Kidney Dialysis
Kidney Dialysis artificially mimics the function of the human kidney by using appropriate membranes and diffusion gradients.
What do our kidneys do?
Filter our blood of waste products. The waste is then excreted in urine.

26. Normal Kidney
Water and various solutes are forced to diffuse out of the blood by concentration gradients.
Some are reabsorbed into the blood, others stay in kidneys and get released to the bladder as urine.

27. Dialysis

28. Dialysis
Membrane must be appropriately semi-permeable so waste molecules (urea, creatine, etc.) can exit, but other molecules like glucose stay in the blood.
Dialysis fluid must have an appropriate osmolarity (total concentration of osmotically active solutes) so not too much water is lost or added to the blood.

29. Animation Passive Transport vs. Active Transport

30. 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

31. Adenosine Triphosphate (ATP)
1.4.U1 Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.

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

33. Sodium-potassium pump
The sodium–potassium pump follows a repeating cycle of steps that result in three sodium ions being pumped out of the axon (of neurons) and two potassium ions being pumped in. Each time the pump goes round this cycle it uses one ATP. The cycle consists of these steps:
1.4.A1 Structure and function of sodium–potassium pumps for active transport and potassium channels for facilitated diffusion in axons. 1
The interior of the pump is open to the inside of the axon; three sodium ions enter the pump and attach to their binding sites.

34. Sodium–potassium pump3
The interior of the pump opens to the outside of the axon and the three sodium ions are released.
1.4.A1 Structure and function of sodium–potassium pumps for active transport and potassium channels for facilitated diffusion in axons. 2
ATP transfers a phosphate group from itself to the pump; this causes the pump to change shape and the interior is then closed.

35. Sodium–potassium pump4
Two potassium ions from outside can then enter and attach to their binding sites.
1.4.A1 Structure and function of sodium–potassium pumps for active transport and potassium channels for facilitated diffusion in axons. 5
Binding of potassium causes release of the phosphate group; this causes the pump to change shape again so that it is again only open to the inside of the axon. 6
The interior of the pump opens to the inside of the axon and the two potassium ions are released. Sodium ions can now enter and bind to the pump again (#1).

36. Vesicles
Vesicles are small spherical packages that bud off of the membranes of 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.
1.4.U3 Vesicles move materials within cells
Use the animated tutorial to learn more about the formation and use of vesicles in cells

38. Budding Vesicles

39. How do membranes fuse?
1.4.U2 The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis.

41. Endocytosis & Exocytosis
Endocytosis: The taking in of external substances by an infolding of the plasma membrane, forming a vesicle
Exocytosis: The release of substances from a cell (secretion) when a vesicle fuses with the cell plasma membrane.
1.4.U2 The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis.

42. Endocytosis

43. Endocytosis: Phagocytosis & Pinocytosis
1.4.U2 The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis.
“Cell eating”
“Cell drinking”

44. PINOCYTOSIS PHAGOCYTOSIS
EXTRACELLULAR
FLUID
Cytoplasm extension
CYTOPLASM
“Food” or
other particle
Food
vacuole
1 µm
Amoeba’s extension of cytoplasm
Bacterium
An amoeba engulfing a bacterium via
phagocytosis (TEM).
PINOCYTOSIS
Pinocytosis vesicles
forming (arrows) in
a cell lining a small
blood vessel (TEM).
0.5 µm
Plasma
membrane
Vesicle
PHAGOCYTOSIS

45. Exocytosis

47. Homework Answer the TOK question on pg. 36.
Answer the data-based questions in the boxes on pg. 36 – 37
Read osmosis in potatoes lab from your Topic 1 booklet.

48. Bibliography / Acknowledgments
Jason de Nys