Key Area 3
(a) Movement of molecules across membranes
In this section, we will learn about the structure of the cell membrane and the role of proteins in the movement of molecules across the membrane.
Why should you be interested in membranes?
Understanding more about membranes is of fundamental importance in the fight against antibiotic resistance. Watch the video from the University of Birmingham on understanding Membrane Permeability by clicking the button below.
Before we get stuck in, let's start with a quick 6-mark quiz. Click on the Google form below by clicking the small icon in the top corner to open in a new tab.
Task 42
Steve arrived in class on Thursday. "Hi Miss, guess what! I won my badminton tournament!".
"Amazing Steve, we all knew you would!". Steve was still high on adrenaline and asking loads of questions about the new topic. "Miss, I think I remember some stuff about membranes from before. Is that all the stuff about proteins and phospholipids from National 5? I remember you talking about that a few years ago".
Dr McRobbie reflected on the years with the class before and smiled. "Yes, Steve, well remembered. Before we begin today, let's reflect on your own understanding of the phrase "fluid mosaic model", thinking carefully about what this means with respect to the cell membrane. Perhaps you have learned about this before and can remember some key facts. Also, can you remember what the terms "hydrophilic" and "hydrophobic" mean?"
Steve's hand shot up. "Steve, let's write it down first".
You should do the same and then check suggested answers here.
The Fluid Mosaic Model
Cell membranes are composed of a bilayer of phospholipid molecules and a patchwork of protein molecules.
The phospholipid (shown left) is an essential part of the cell membrane. It is a hybrid molecule:
The head region of a phospholipid molecule is charged and, therefore, hydrophilic (attracted to water).
The tail region is uncharged and non-polar. Therefore, it is hydrophobic (repelled by water).
This hybrid nature dictates the "bilayer" arrangement with hydrophobic tails shielded to the interior of the membrane structure and hydrophilic heads exposed to either the intra- or extracellular environment.
The phospholipids are constantly changing position, giving the membrane its fluid and dynamic quality.
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".
Can you replicate what Lucas might have drawn and then check your answers here.
Cell Membrane Structure
Types of membrane protein
Membranes are composed of a phospholipid bilayer and a patchwork arrangement of proteins. There are two main types of protein molecules found within the membrane:
Integral
Peripheral
Integral proteins
Regions of hydrophobic R groups allow strong hydrophobic interactions that hold integral membrane proteins within the phospholipid bilayer. These integral membrane proteins interact extensively with the hydrophobic region of the phospholipids.
Peripheral proteins
Peripheral membrane proteins have hydrophilic R groups on their surface and are bound to the surface of membranes, mainly by ionic and hydrogen bond interactions. Many peripheral membrane proteins interact with the surfaces of integral membrane proteins.
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 and check your answer here.
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.
Transport of molecules across the membrane
Simple Diffusion
The phospholipid bilayer is a barrier to ions and most uncharged polar molecules. Some small molecules, e.g. oxygen and carbon dioxide, pass through the bilayer by simple diffusion, as shown in the image below.
Facilitated Diffusion
Facilitated diffusion
Facilitated diffusion is the passive transport of substances across the membrane through specific transmembrane proteins. The majority of these proteins in animals and plant cells are highly selective. To perform specialised functions, different cell types have different channel and transporter proteins.
Facilitated diffusion through channel proteins
Channels are multi-subunit proteins with the subunits arrange to form water-filled pores that extend across the membrane as shown in the diagram.
Aquaporin - a channel protein
An example of a channel protein that facilitates diffusion is aquaporin. This channel protein was shown to be so vital to life that, in 2003, the Nobel Prize in Chemistry was awarded to Professor Agre for his discovery of the channels.
Click on the video below for a quick tour around some basic bioinformatics tools to learn a bit more about proteins like Aquaporin.
Task 46
"Right team, let 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 answers to this are shown here.
A moment to reflect on the Kidneys
In Scotland, there is often very little focus in BGE and Senior CfE courses on the Kidneys. However, some basic knowledge at this point will prove helpful at key points in Topic 1. The kidney contains 1000s of filtering units called nephrons. In the nephron, water, salts, urea and glucose are filtered out of the blood into the glomerulus (see diagram opposite of a nephron).
The substances filtered out of the blood pass along the kidney tubule where all glucose, some water and some salts are reabsorbed.
Remaining urea, water and salts form urine, which passes down the collecting duct and eventually reaches the bladder.
Binding of a hormone called ADH (which we will revisit later), to its receptor in collecting ducts of the kidney triggers the recruitment of the channel protein Aquaporin 2 (AQP2). AQP provides a highly efficient route for water to across the membrane. Recruitment of AQP2 allows control of water balance in terrestrial vertebrates.
The diagram here shows ADH binding to its receptor. This triggers are signal transduction cascade within the collecting duct cells. This means that various reactions proceed, with one reaction triggering the next.
The ultimate consequence of this cascade of events is that AQP2 is recruited to the cell membrane of the collecting duct, enabling the facilitated transport of water across the membrane and back into the bloodstream.
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
Answers are found here.
Facilitated diffusion via ligand-gated and voltage-gated channel proteins
Some channel proteins are gated and change conformation to allow or prevent diffusion. Gated channels respond to a stimulus which causes them to open or close. The stimulus may be chemical (ligand-gated) or electrical (voltage-gated).
Ligand-gated channel protein
Ligand-gated channels are controlled by the binding of signal molecules. In the example shown above, the transmembrane channel protein facilitates diffusion of various ions (which is quite unusual as most are highly selective): sodium, potassium and calcium. But the channel is gated - it only opens in response to binding of a particular ligand. In this example, binding of the neurotransmitter Acetylcholine triggers a conformational change in the channel protein and the channel opens. This enables the diffusion of ions from a high to lower concentration.
Voltage-gated channel protein
Voltage-gated channels are controlled by changes in ion concentration. Often they will work in a manner depicted in the image on the left.
Imagine an ion, such as potassium (carrying a positive charge) enters the cytosol via a ligand-gated ion channel. As it enters the cytosol, it brings more positive charge into the cytosol. This changes the membrane potential and as the intracellular environment reaches a threshold voltage, the voltage-gated channel then opens.
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.
Check your answers here.
Transporter Proteins
Transporter proteins bind to the specific substance to be transported and undergo a conformational change to transfer the solute across the membrane. Transporters alternate between 2 conformations so that the binding site for a solute is sequentially exposed on one side of the bilayer, then the other.
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 answers to this are shown here.
Active Transport
Active transport uses pump proteins that transfer substances across the membrane against their concentration gradient. Pumps that mediate active transport are transporter proteins coupled to an energy source. A source of metabolic energy is required for active transport. Some active transport proteins hydrolyse ATP directly to provide the energy for the conformational change required to move substances across the membrane.
The Sodium-potassium pump is a protein we will return to later in much greater detail.
But, briefly, you can see the pump protein is an ATPase that hydrolyses ATP, becomes phosphorylated, which triggers a conformational change. This ultimately facilitates the movement of a solute (sodium or potassium) across the membrane.
Co-transport of multiple solutes
Proteins can sometimes transfer one solute (uniport) or they might couple the transport of one solute to another - this second solute might travel in the same direction as the first (symport) or might travel in the opposite direction (antiport). This following diagram illustrates this.
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.
Suggested answers to this are shown here.
Watch this very useful video to summarise and review the Topic 1, Key Area 3A content.
Now go to SCHOLAR "3.1 Movement of molecules across membranes" for consolidation.
Your teacher might now issue you with Learner Check 9 to check your learning of the Topic 1 (Key Area 3a) content so far.
