1. AH Biology: Unit 1 Proteins Topic 3
Membrane Proteins

2. Membrane Proteins LOs
Specific transmembrane proteins, which act as channels or transporters, control ion concentrations and concentration gradients.
To perform specialised functions, different cell types and different cell compartments have different channel and transporter proteins.
Passage of molecules through channel proteins is passive (e.g. aquaporin).
Some channel proteins are gated and change conformation to allow or prevent diffusion (e.g. sodium channels, potassium channels).
‘Gated’ channels can be controlled by signal molecules (ligand-gated channels) or changes in ion concentrations (voltage-gated channels).
Transporter proteins change conformation to transport molecules across a membrane.

3. Plasma membrane
The plasma membrane consists mainly of a fluid mosaic bilayer of phospholipids and protein.
Hydrophilic head groups align to the extracellular space or cytoplasm, and the hydrophobic tails point inwards.

4. Membrane structure
The hydrophobic area of the plasma membrane prevents ions and most uncharged polar molecules from passing through it.
Only hydrophobic molecules and some small molecules (e.g. oxygen and carbon dioxide) are able to pass through unaided.
For all other molecules to pass through the membrane, protein molecules are required.

5. Phospholipid bilayer
The phospholipid bilayer is a barrier to charged or polar ions and molecules as they will not cross into the hydrophobic core of the bilayer. The phospholipids themselves have lateral movement within their layer, but do not ‘flip-flop’ to the other side. Fluid yet stable.

6. Movement of molecules across membranes
Hydrophobic molecules such as O2, CO2, N2 and steroid hormones pass readily across such bilayers
Small uncharged polar molecules (eg H2O, urea and glycerol) pass through readily but at a lower rate as they are capable of dissolving in the bilayer.
Large uncharged polar molecules (eg glucose and sucrose) cannot pass through.
Protein-free bilayers are impervious to all ions, eg H+, Mg2+, Ca2+ and HCO3–.

7. Membrane proteins
Many transmembrane proteins act as channels or transporters to aid and control the movement of substances.
They help control ion concentrations and concentration gradients.
There are a huge variety of different types of these proteins, each to perform a specialised function.
To allow different cell types (and different cell compartments) carry out different functions, they have different channel and transporter proteins in their membranes.

8. Uses of transmembrane proteins
Movement across the membrane can be controlled in many ways.
Small molecules that could move only slowly by passive diffusion can be speeded through the membrane using transmembrane channel proteins that are specifically shaped to that molecule.
This is called facilitated diffusion, and is a passive process.
It is important to remember that inside a eukaryotic cell there are many layers of membrane, enclosing specific organelles that need different concentration of substances.
E.g. of a channel protein- aquaporin

9. Uses of transmembrane proteins
Some channel proteins are always held open, but an additional control is often used, using change in the conformation of the protein to open the channel.
These are called gated channels.
e.g. sodium channels, potassium channels

10. Ligand gated channel
A ligand gated channel use the binding of a signal molecule to change the conformation of the channel and so open it e.g. sodium or potassium channels.

11. Voltage gated channel
A voltage gated channel uses changes in ion concentration to determine the opening and closing of the channel, often found in nerve cells.

12. Use your notes to answer the questions….
What is the phospholipid bilayer a barrier to and why?
What is facilitated diffusion?
What is a gated channel?
What is a ligand gated channel?
What is a voltage gated channel?

13. Signal transduction

15. Membrane Proteins LOs
Specific transmembrane proteins, which act as ________ or ____________, control ion concentrations and concentration gradients.
To perform specialised functions, different cell types and different cell compartments have different channel and transporter proteins.
Passage of molecules through channel proteins is passive (e.g. _____________).
Some channel proteins are __________ and change conformation to allow or prevent diffusion (e.g. ______ ________, __________ ________).
‘Gated’ channels can be controlled by signal molecules (_______-______ _______) or changes in ion concentrations (________-_______ __________).
____________ proteins change conformation to transport molecules across a membrane.

16. Membrane Proteins Key Concepts
Specific transmembrane proteins, which act as channels or transporters, control ion concentrations and concentration gradients.
To perform specialised functions, different cell types and different cell compartments have different channel and transporter proteins.
Passage of molecules through channel proteins is passive (e.g. aquaporin).
Some channel proteins are gated and change conformation to allow or prevent diffusion (e.g. sodium channels, potassium channels).
‘Gated’ channels can be controlled by signal molecules (ligand-gated channels) or changes in ion concentrations (voltage-gated channels).
Transporter proteins change conformation to transport molecules across a membrane.

17. Membrane Proteins Key Concepts
Transport can be facilitated-
Glucose examples:
Maintaining the osmotic balance in animal cells.
Generation of the ion gradient for glucose symport in the small intestine.
Generation and long-term maintenance of ion gradient for resting potential in neurons.
Generation of ion gradient in kidney tubules.
The maintenance of ion gradients by Na/KATPase accounts for a significant part of the basal metabolic rate (up to 25% in humans).

18. Transporter proteins
Transporter proteins change shape (conformation) in order to transport molecules across membranes.
This may be passive (simply facilitate diffusion down a gradient) or active (transport against the gradient).
This energy can come from:
ATP, eg Na+/K+ ATPase
electrochemical gradients, eg glucose/Na+ symport
light-driven pumps, eg bacterial rhodopsin.

19. Transport- Glucose Symport
Transporter proteins change conformation to transport molecules across a membrane.
Facilitated transport (e.g. glucose symport - both in same direction across membrane) is found where a transmembrane protein provides a channel for transport, but does not require energy to move the substance.

20. Glucose Symport

21. Glucose/Na+ symport
There are two classes of binding sites, one for Na+ and the other for glucose. Binding of either molecule enhances the binding of the other. As this system is driven by the Na+ gradient generated by the Na+/K+ ATPase it is described as secondary active transport.
When all binding sites are filled a conformational change in the protein delivers both molecules across the membrane. Later the sodium is pumped back out of the cell by the Na+/K+ ATPase. Because the conformational change relies on both sets of sites being filled or not, the switch between states only happens if all sites are full or empty. This transport protein exists in two states A and B.

22. Glucose/Na+ symport
Because of the much higher extracellular than intracellular Na+ levels, glucose is more likely to bind to the molecule in the A state than the B state. More glucose and Na+ enter the cell by A–B transitions than are lost by the reverse. This is an example of cooperative co-transport.
This net flow results in an accumulation of glucose against its concentration gradient. The sodium ions flow down their electrochemical gradient while the glucose molecules are pumped up their concentration gradient.
The Na+/glucose symport is used to actively transport glucose out of the intestine and also out of the kidney tubules and back into the blood.

23. Sodium potassium pump Sodium Potassium Pump animation Task…
Use your resources to review the sodium potassium pump.
 CfE Higher Text Book page 119 , Advanced Higher Bright Red Book, own notes or printed notes booklet.
2. In small groups draw and describe the sodium potassium pump
on a big show me board

24. Na+/K+ ATPase
This transporter is responsible for maintaining an Na+/K+ gradient across the plasma membrane.

25. Na+/K+ ATPase
The transporter has binding sites with high affinity for three Na+ ions.
When Na+ binds, ATP is reduced, transferring a phosphate to the transporter. The resulting conformational change opens the transporter to the extracellular side, pumping the Na+ ions across the membrane.
The conformational change means two binding sites with affinity for K+ are then exposed to the extracellular side.

26. Na+/K+ ATPase Two K+ ions bind from the extracellular side.
This results in the transporter being dephosphorylated and the phosphate being released.
The transporter returns to its initial state.
The K+ is pumped into the cytoplasm.
The affinity for Na+ on the inside of the membrane is restored

27. Sodium potassium pump animation

28. Sodium Potassium Pump Key Concepts
The sodium potassium pump transports ions against a steep concentration gradient using energy directly from ATP.
Sodium potassium pumps functional steps include;
The transporter protein has high affinity for sodium ions inside the cell
binding occurs;
Phosphorylation by ATP;
conformation changes;
affinity for ions changes;
sodium ions released outside of the cell, potassium ions bind outside the cell;
dephosphorylation;
potassium ions taken into cell;
affinity returns to start.

29. Membrane Proteins LOs 2 Transport can be facilitated-
Glucose examples:
Maintaining the osmotic balance in animal cells.
Generation of the ion gradient for glucose symport in the small intestine.
Generation and long-term maintenance of ion gradient for resting potential in neurons.
Generation of ion gradient in kidney tubules.
The maintenance of ion gradients by Na/KATPase accounts for a significant part of the basal metabolic rate (up to 25% in humans).

30. Membrane Proteins LOs 3 Ion channels and nerve transmission
Nerve transmission is a wave of depolarisation of the resting potential of a neuron. This can be stimulated when an appropriate signal molecule, such as a neurotransmitter, triggers the opening of ligand-gated ion channels.
If sufficient ion movement occurs, then voltage-gated ion channels will open and the effect travels along the length of the nerve. Once the wave of depolarisation has passed, these channel proteins close and others open to allow the movement of ions in the opposite direction to restore the resting potential.

31. Nerve Impulses
The electrical impulse travels along a nerve cell as channel proteins open to allow sodium ions to rush in down a concentration gradient.
A tiny gap (synapse) prevents the nerve impulse jumping directly to the next nerve cell.
synaptic transmission

32. Neurotransmitters, such as acetylcholine and noradrenaline, are responsible for carrying the signal across the gap.
On the other side of the gap, protein receptors are linked to closed ion channel proteins.
Binding of the neurotransmitter, opens the ion channels to allow sodium ions to rush in, and the signal travels down the next neuron.
action of acetylcholine

33. When a neurotransmitter binds to a protein receptor a conformation change causes the ion channel to open and allow sodium ions to rush in.
The neurotransmitter binding to its receptor is an example of a ligand gated channel.

34. Voltage-gated ion channels
The flow of ions into the post synaptic membrane as a result of neurotransmitter binding leads to a change in charge across the membrane.
This change in charge is known as depolarisation.
This change in charge triggers the opening of further ion channels (Na+/K+ ATPase) along the axon of the nerve(voltage gated ion channels).
This results in the charge moving (propagating) along the axon towards the next nerve.

35. After depolarisation
The wave of depolarisation eventually reaches the end of the nerve and triggers the release of a neurotransmitter.
Once the wave of depolarisation has passed, the sodium channels close and others open to move ions in the opposite direction and reset the neuron to its resting potential (charge before depolarisation).

36. Propagation of a nerve impulse
Na+ channels open and Na+ flows into the axon, causing depolarisation.
Na+ channels close when the action potential is reached. K+ channels open, allowing efflux of K+ to repolarise membrane.
Axon becomes hyperpolarised. K+ channels close and the Na+/K+ ATPase pumps potassium back into the axon and sodium out during the refractory period. Sodium and potassium also leak down electrochemical gradients, aiding a return to the resting state. 1 2 3
Graph by en:User:Chris 73, updated by en:User:Diberri, [GFDL ( via Wikimedia Commons

38. Membrane Proteins Key Concepts
Ion channels and nerve transmission
Nerve transmission is a wave of depolarisation of the resting potential of a neuron. This can be stimulated when an appropriate signal molecule, such as a neurotransmitter, triggers the opening of ligand-gated ion channels.
If sufficient ion movement occurs, then voltage-gated ion channels will open and the effect travels along the length of the nerve. Once the wave of depolarisation has passed, these channel proteins close and others open to allow the movement of ions in the opposite direction to restore the resting potential.

39. Membrane Proteins Summary
You must complete…
Check all of your notes are up to date.
Ensure you have all new key terms added to your unit 1 glossary.
You should complete…
Create 5 key word quiz cards: word on one side meaning on the another.
Review your learning outcomes for key area 3 membrane proteins.
Complete the extended response questions from page 94 of the SCHOLAR study guide.
You could complete…
Topic 3 end of topic test page 95 of the SCHOLAR study guide.
Exercise 3 – Membrane Proteins from Unit 1 Questions Booklet
Homework: Due Friday 13 Jan – Exercise 4 from Unit 1 Homework Booklet

40. Key Area 1.3 Membrane Proteins
Past Paper Practice
Advanced Higher
Key Area 1.3
Membrane Proteins

41. Describe the transport of sodium and potassium ions across the plasma membrane.
(5)

42. 1. Protein that spans the membrane
2. Works against concentration gradient/by active transport
3 ATP provides phosphate
4. Phosphate attaches to pump/protein/protein phosphorylated
5. Phosphorylation/ dephosphorylation alters conformation/shape of protein OR description
6. Different conformations have different affinity for sodium/potassium
7. (3) Sodium ions (pumped) out of cell and (2) potassium in
Any five
Not channel or carrier
Diagrams might be useful here but must be annotated correctly

51. Discuss the movement of ions across membranes under the following headings:
i) mechanism and functions of Na/K ATPase