Task Answers
Topic 1, Key Area 3
Task 42
Before we begin, reflect on your own understanding of the phrase "fluid mosaic model" with reference to the cell membrane. Perhaps you have learned about this before and can remember some key facts. Consider what the terms "hydrophilic" and "hydrophobic" mean.
Task 43
Lucas was reminding Dr McRobbie again of his need to leave early for football training. "Right, okaydokey, no problem - but first, now we have remembered that membranes are comprised of a bilayer of phospholipid molecules and a patchwork of protein molecules, I would like you to sketch a labelled phospholipid in your notes - then you can go".
Task 44
The new information was building links in their understanding about membranes. David had his new set of stationary lined up ready to go so Dr McRobbie decided to take advantage of it. "Can you all now draw a diagram of a cell membrane with a fully integral membrane protein, a transmembrane protein and a peripheral protein. Colour these to show the distribution of hydrophobic (blue) and hydrophilic (green) R groups".
David was happy to use his new stationary but didn't really know where to begin with this. He sneaked a glance over Olivia's shoulder. Sketch in your own response to this question.
Task 45
"Right everyone, it is time to think about transport across membranes. We have stumbled across this before and hopefully a few things are coming to mind. Have a look at these questions and write a brief response to each of them". Big David whispered in Andra's ear, "I'm glad she said "brief"!" Some suggested responses are shown here.
Diffusion: The passive movement of molecules, e.g. glucose, from an area of higher to lower concentration across a selectively permeable membrane.
Based on the model of the phospholipid bilayer, polar molecules and charged ions are unlikely to pass freely across due to the presence of the hydrophobic tails of phospholipids. This includes molecules like water and charged ions.
Transport of these molecules will probably rely on membrane proteins.
Task 46
"Right team, let's think about Aquaporin in a bit more detail now. This is an example of a channel protein found in the cell membrane. Can you make your own notes about what this protein does, it's structure, tissues it is found in and the impact it has on a cell. You may wish to use the bioinformatic tools highlighted in the video above".
Suggested response:
By using the Protein Data Bank and Uniprot, we can find out many of these answers. "Aquaporin water channels are found in almost every organism from humans to bacteria. In humans, 13 classes of AQPs control water and glycerol homeostasis" - PDB. The structure is composed of an alpha helical barrel structure that extends across the membrane. Uniprot reveals that "AQP1 forms a water-specific channel that provides the plasma membrane of red blood cells and kidney proximal tubules with high permeability to water, thereby permitting water to move in the direction of an osmotic gradient". It has an impact on the water concentration of the blood by influencing the rate of water reabsorption from kidney tubules.
Task 47
"Time for some deep thinking here to wake these guys up", thought Dr McRobbie. It alright learning loads of stuff but now it is time to apply it. She asked the class to apply their new understanding of aquaporin recruitment to the membranes of the kidney collecting ducts to predict the cellular and physiological consequences of dehydration. Produce a flowchart to outline the step-by-step events that would take place.
Your answer should make reference to:
ADH levels
AQP2 recruitment
Water reabsorption rates from the collecting duct
Blood water concentrations
Urine production - volume and concentration
Suggested response:
Task 48
Lucas and Steve were chatting away, plotting how they could get Dr McRobbie to bring more cake into class. Always a soft touch she said, "Right boys, we can have cake next double period - IF you can both sketch a diagram of a ligand-gated and voltage-gated channel protein in your own notes and explain how they work". The stakes had never been higher. Have you help them out to ensure they get their cake reward.
Suggested diagram: The diagram shows a phospholipid bilayer with a ligand-gated (left) and voltage-gated (right) transmembrane channel. The left channel is gated by a neurotransmitter - when this binds to the channel, a conformational change would take place and permit the passage of a specific molecule/ion, e.g. calcium ions. The right channel is gated by a change in membrane potential - when the membrane potential reaches a certain threshold, the channel undergoes a conformational change, permitting the passage of particular ions/molecules.
Ligand-gated channel
Voltage-gated channel
Task 49
The Glucose Transporter protein is an important transport protein in mammalian cells. Research the GLUT4 transporter and include the following:
A diagram, video or animation showing how the protein works
Where the protein is located
The physiological role of the protein
Suggested response:
The Human GLUT4 protein is an insulin-regulated glucose transporter, which plays a key role in removal of glucose from the bloodstream. In response to insulin, its intracelluar location changes.
As shown in the diagram opposite, GLUT4 is stored within intracellular vesicles. However, when insulin binds to its cell surface receptor, GLUT4 is recruited to the cell surface where it can transports glucose from the excellular environment into the cells.
The GLUT4 transporter is found in skeletal and cardiac muscle cells and also fat (adipose) cells. It has a transmembrane helical structure.`
Task 50
"One final short piece of research for you all now", announced Dr McRobbie. "This is quite an important one - it will be something we revisit in the next section so it is worth taking the time to understand it. The Glucose Symport Channel is involved in movement of glucose. Research this channel and include the following information in your notes:
A diagram, video or animation showing how the protein works
Where the protein is located
The physiological role of the protein
A detailed description on why it is known as a "symport" channel".
Provide an outline of how the class might have responded to this task.
The glucose symporter is located in the small intestine. It is involved in the co-transport of sodium (passive transport) and glucose (active transport) from the lumen of the small intestine into the epithelial cells lining the small intestine. The concentration of sodium is kept low in the epithelial cells (to facilitate passive transport) by the Na/K pump.
The Glucose that arrives in the epithelial cells is then removed from this space and taken into the bloodstream via the GLUT transporter, as shown in the diagram (referred to as glucose permease in the animation available via the pink button below).
It is known as symport as glucose and sodium ions are both moved at the same time and in the same direction.
Physiological role: This transport protein is the primary route for the transport of dietary glucose from the intestine into the bloodstream.
Starter
The answer to the question below is B.
Task 51
Sketch the axon of a neuron, showing the resting membrane potential recorded using an electrode.
Task 52
Reflect back on your own notes about glucose symport from task 50 in Key Area 3a. Can you add anything else in now to support your understanding of this membrane transport protein?
Suggested response - Look back to the answer for Task 50 above and make sure you are clear on what you have written and understand by this transport protein,
Task 53
"The Na/K pump plays a critical role in the absorption of glucose from the small intestine". Do you agree or disagree with this statement? Explain your answer.
Suggested response: You should be able to agree with this statement. The absorption of glucose from the small intestine is coupled to the passive transport of sodium from the intestinal lumen into the epithelial cells. The only way for this to happen is if the concentration of sodium is kept low within the epithelial cells - this is achieved by the Na/K pump.
Task 54
In your notes, include a step-by-step flowchart to illustrate the mechanism of the Na/K pump.
Memory Platform
Answer the following questions to try and link your knowledge from the past few areas:
How would you define the Na/K-pump in terms of it being a "membrane protein"? (peripheral/integral/transmembrane/channel/transporter?)
Considering your response to question 1, can you predict the types of amino acids you might expect to find in different regions of the protein? Explain your prediction.
Reflect on the role of the Na/K-pump in maintaining the resting membrane potential of a neuron. During an action potential, this membrane potential must change. Can you make a prediction about how this might happen by reflecting on what you have learned during Key Area 3a.
Suggested answers:
The Na/K pump is a transmembrane transporter and ATPase that hydrolyses ATP, becomes phosphorylated and alternates between 2 conformations.
The Na/K pump is likely to have a cluster of polar R groups at either end of the pump in contact with the intra- and extracellular environment. The pump is also likely to have clusters of hydrophobic R groups around the main "alpha helical barrel" in contact with the phospholipid tails.
For the membrane potential to change, there must be movement of ions across the channel. During transmission of an impulse, an influence of positive ions occurs through, firstly, ligand-gated ion channels and then, secondly, voltage-gated ion channels. This changes the voltage across the membrane such that the intracellular environment is positive relative to the outside.