1. 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 and BioNinja

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

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. The direct hydrolysis of ATP (primary active transport)
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.
Beyond the Scope

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.
Three sodium ions bind to intracellular sites on the sodium-potassium pump
A phosphate group is transferred to the pump via the hydrolysis of ATP
The pump undergoes a conformational change, translocating sodium across the membrane
The conformational change exposes two potassium binding sites on the extracellular surface of the pump
The phosphate group is released which causes the pump to return to its original conformation
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.
Potassium channels in axons are voltage gated. They enable the facilitated diffusion of potassium 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.
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. 1.4.U3 Vesicles move materials within cells.
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
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. 1.4.U2 The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis.
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.

18. There are two main types of endocytosis:
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. 1.4.U2 The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis.
“Cell eating”
“Cell drinking”

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