The Membrane

Cystic Fibrosis is a respiratory disease which results in a build-up of mucus in the lungs. An inherited disease, the defective gene which leads to symptoms encodes an ion channel. Ion channels are membrane proteins that allow ions to flow from high to low concentration. Defects in this particular ion channel lead to decreases in chloride ion transport which results in excessively thick mucus produced from epithelial cells. Due to research into the function of this protein mean survival rate for people with CF has gone from 6 months in 1959 to 37.4 years in 20081. The life of a CF sufferer is still quite challenging. Because CF is relatively common, research on this topic is extremely active which may result in more effective treatment of symptoms.In this lesson you will learn about ion channels as well as other cell membrane components. How they function will determine how the cells of the nervous system function. In order to fully understand how drugs influence brain function, thinking about the cellular and even sub cellular effects drugs is necessary.

ObjectivesAfter successfully completing this lesson you will be able to:

to understand biological membranes – composition and function

to understand how pumps and channels allow regulated and specific transport

to learn the cell types of the nervous system

to learn how neurons are structured and how they function – at rest and in action

1 Wikipedia.com1

The Membrane

Before you begin! PretestAt rest, a neuron is negative on the inside and has more positively charged sodium on the outside. What are TWO reasons why sodium will move inside a neuron when allowed?

To move from positive to negativeTo move down its concentration gradient

What are the properties of a phospholipid?Has a hydrophobic tail region and a hydrophilic head region.

Which is a correct statement about channels and pumps?Pumps require energy to work and push material from high concentration to

lowChannels require energy to work and push material from high concentration

to lowPumps require energy to work and push material from low concentration to

highChannels require energy to work and push material from low concentration to

lowNeither requires energy, but pumps go from low to high while channels go

from high to lowLabel the parts of a neuron

Label axon, cell body, dendrite, and synaptic bulbs

Lesson 3: The MembraneGuiding Questions

1. How are cells organized in a way that allows them to regulate their contents?2. What molecules help a plasma membrane be selectively permeable?3. When materials move into or out of a cell, what forces direct that movement?4. How do protein transporters work, depending upon concentration gradient?5. How selective are protein transporters?6. Why is a neuron “electrically active”?7. What is resting potential and how is it established?8. What is an action potential and what forces move it down an axon?

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The Membrane Key Terms

Ion Diffusion Concentration gradient Lipid bilayer; phospholipid Hydrophobic Membrane protein Ion channel; chloride-, sodium-, potassium channels Membrane pump: sodium/ potassium pump Receptor Neuron Neuroglial cell Blood brain barrier Axon Dendrite Cell body Synaptic bulb Resting potential Action potential Depolarization Repolarization Threshold

Activity One: Membrane Composition

Start

We live in a watery world, and our cells manage to ensure that they can keep inside separate from outside thanks to the plasma membrane. All cells, for every living cell on earth, are surrounded by a plasma membrane. One of the primary concerns for a cell membrane is distinguishing inside and outside, but the membrane also needs to transport materials from in to out and from out to in. This becomes clear when you think about cells

producing waste, just like any living organism. That waste must be removed. Similarly, nutrients must be taken in. Two quite different membrane molecule types enable these very different functions.To distinguish inside vs. outside, membrane lipids known as phospholipids, are key. Like other lipid molecules, phospholipids are water-fearing, or hydrophobic. In a watery world, these hydrophobic phospholipids ensure that water soluble molecules outside the cell cannot pass into the cell and vice versa. The phospholipids in the membrane are arranged in a bilayer

(see diagram). An individual phospholipid has a head region and a tail region. The head (drawn as an oval) is water-loving, or hydrophilic but the tail region is hydrophobic. The

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tail

head

The Membranebilayer is held together because the hydrophobic tails assemble toward each other to avoid the watery spaces inside and outside the cell.

Now that the barrier is established, the cell must use another means to transport materials in and out. For this, membrane proteins are key. Membrane proteins reside in the phospholipid venue and enable materials to be transported through the protein into or out of the cell.

Activity Two: Concentration Gradients

How materials move through the membrane proteins depends upon where those materials are concentrated. Consider glucose, the fuel for most cells. The concentration of glucose is generally higher outside the cell compared to inside where every glucose molecule gets put into use right away. We might diagram that by looking at the cell (left) or the concentration of glucose (right).

From the perspective of the membrane, there is a concentration gradient for glucose. We speak of gradients as having high and low sides (here high concentration side is outside) and use that to talk about molecules moving DOWN their concentration gradient (with) or UP their concentration gradient (against).

In our glucose scenario, to transport glucose into the cell would require little energy. Flowing down the concentration gradient is easy because molecules tend to diffuse (move from high concentration to low concentration). In contrast, moving against the concentration gradient takes effort. Cellular energy in the form of ATP supplies that energy.

So depending upon the direction that materials are moving, with or against their concentration gradient, the type of protein needed is different.

Test of content

Imagine the following situation where the concentration of some molecule inside and outside the cell is represented by the density of the color. Which way would the pink color molecule move by diffusion (without energy)?

From inside to outside4

Gluc

ose

Dens

ity

The MembraneFrom outside to insideFrom left to rightFrom right to leftFrom front to backFrom back to front

Activity Three: Protein TransportersThe protein transporter that moves a molecule from high concentration side to low concentration side of a membrane is called a channel. Channels that are open allow molecules to flow by diffusion (which means from high to low concentration).

In contrast, in order to move material against its concentration gradient, energy is needed and a protein pump uses that energy to shove the material against its concentration gradient.

So while pumps and channels are quite different from the perspective of concentration gradients, pumps and channels have a lot in common. Can you think of two things you already know about that they have in common (both are proteins, both are located in the membrane)? Start by thinking about some specific pumps and channels, the ones we will need to talk about repeatedly. The only pump we’ll work with will be the sodium-potassium pump. The channels we’ll need to know about are the potassium channel, the sodium channel, and the chloride channel. For future reference, the atomic symbol for each is sodium = Na, potassium = K, and chloride = Cl. Also, for future reference, both sodium and potassium are positively charged (Na+ and K+, chloride is negatively charged (Cl-).

As their names imply, pumps and channels are specialists – they will not transport just any material. Chloride channels will ONLY transport chloride ions.

One might wonder how there can even BE concentration gradients if there are channels which allow materials to diffuse down their concentration gradients. The explanation is that channels are frequently gated. The “gate” on the channel could be open or closed. Many cell processes are directed by the regulated opening or closing of gates on channels.

Test of content

When the gate on a Chloride channel opens, chloride flows into a neuron (a cell of the nervous system). What does this mean about the concentration gradient of chloride?

Chloride concentration is equal inside and outside the neuron.Chloride concentration is higher on the inside of the neuron.Chloride concentration is higher on the outside of the neuron.We have insufficient information to determine anything about the chloride

concentration gradient.

Activity Four: Neurons and Neuroglial Cells5

The MembraneIn the nervous system (including the brain) there are two cell types. We will focus on the exciting (and excitatory) cell called the neuron in this class. But of equal importance is the neuroglial cell. Neurons are electrically active cells, and they have specialized (and LONG) shapes that allows their electrical messages to be transported for long distances. We’ll talk more about the neuron later.

Neuroglial cells outnumber neurons approximately 3:1. Among their jobs are the nurturing or supporting of neurons. They also insulate neurons (which, like an electrical cable can result in speedier electrical signal transmission). Neuroglial cells also respond to conditions, including neuron damage, in an attempt to protect neurons. Neuroglial cells also protect the brain as a whole. Consider the blood brain barrier. To protect our brain against invasion by poisons, neuroglial cells help to create a barrier prohibiting many molecules from passing through.

The blood brain barrier is a functional term that has as structural explanation. Two differences in the circulatory system that nourishes the brain exist to promote this protection. One is that capillary vessels, which typically are fairly porous to allow materials to enter and leave the circulatory system are much more restrictive in the brain. By analogy, consider the bouncer of an all-ages club. You might not be allowed to enter if you are carrying a weapon, for instance, but entry is not especially restrictive. IN capillaries that enter the brain, the “bouncer” is more like a 21-and-over club. Many fewer would-be clubbers can enter because the bouncer is more selective about who gets in. The porous-ness of the capillaries is determined in part by having closely connected cells. But glial cells are part of the story as well. Capillaries that enter the brain are covered with glial cells, whose fatty phospholipid membranes wrap around the capillary vessels and insulate them, restricting the flow of materials into and out of these capillaries into brain fluid.

Better reference for a high school audience?

TEST OF CONTENT

Drug delivery, for therapeutic purposes, can be challenging because of the blood-brain barrier. Many therapeutic molecules cannot cross (such as dopamine which might be used to treat Parkinson’s Disease otherwise). Recently, the use of microbubbles and ultrasound waves have been used to disrupt the blood brain barrier and enable therapeutics to enter2. What might be a down-side of this technique?

Toxins can enter as wellOthers?

Activity Five: The NeuronTurn your attention now to the star of the brain – the neuron.

2 Many sources, first of which is Hynynen 2001. 6

The MembraneView the linked presentation to investigate neuron anatomy and function.

TEST OF CONTENT

Where would you find dendrites in of a neuron connecting the thalamus to the visual cortex (hint – this is the WHOLE length of that neuron). In the thalamus – explanation – the visual cortex is where sight information ends up so we know the neuron goes FROM thalamus TO visual cortex. A neuronal signal travels down the axon from dendrites to synaptic bulbs.

Activity Six: Neurons at Rest – Resting PotentialIn order to understand how a neuron can be electrically active, we have to understand its potential to act – its resting potential. Consider a battery first. You know if you put a battery into your electronic devise backward, the device doesn’t work. So you pay attention to the little – and + symbols when inserting the battery. The battery is “polarized”: it has a plus end and a minus end. A neuron is also polarized. But unlike the battery, the minus “end” is actually the inside of the neuron. A neuron at rest is negatively charged on the inside compared to the outside. We know this in part because recording the inside charge of a neuron is possible in the axons of the giant squid. An electrode placed inside these axons reads a charge, relative to the outside of approximately -65 millivolts.

In addition to having a charge gradient (we can have a gradient to molecules (= chemical gradient) or charge (=electrical gradient) a neuron has a chemical gradient. The inside of a neuron has relatively more potassium than the outside and relatively little sodium. A clever student suggested that a way to remember this relationship was to write the word “NAKED”. 3 NA (Na+) is found on the outside of the word and the outside of the cell. K (K+) is more concentrated on the inside.

Since both sodium and potassium are positively charged, it might seem counterintuitive that there is a negative charge inside the neuron. But there are two reasons why this is true. One is that there are other ions we will not ask you to learn about. The other reason is that the imbalance of sodium is not exactly equal to the imbalance of potassium.

TEST OF CONTENTBased on what you know about the chemical gradient of a neuron, which

membrane component will sodium travel through to go from outside to inside?3 Natalie Crawford

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The MembraneA phospholipidThe sodium – potassium pumpA sodium channelA potassium channelWe don’t have enough information to know

Activity Seven: Channels and a Pump Help Make the Resting Potential

The neuron at rest:Is more negative on the inside (compared to the outside)Has more sodium (Na+) on the outside (compared to the inside)Has more potassium (K+) on the inside (compared to the outside)

How does this come to be?First, consider the sodium-potassium pump. Visit http://highered.mcgraw- hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html and note the RELATIVE distribution of positive charges every time the sodium-potassium pump works. Imagine that we start out with an even distribution of positive charges on inside as outside (no relative charge difference) and the pump goes through FIVE cycles. Compute how many additional positive charges are on the outside now compared to the inside. (Five) Consider now that there are so many of these pumps in a cell, that 1/3 of the cell’s energy (more if the cell is a neuron) is consumed by operating the pump. Each pump pushes more positive charge out than in every time it works.

Next consider the sodium and potassium channels. Both channel types are specific for their ion and both channels are gated (for the most part). But one class of potassium channels has very leaky gates. This means that although potassium is more concentrated on the inside of neurons, through these leaky potassium channels, a little positive charge escapes. The sodium channels are not leaky. Over time, there is considerable accumulation of positive charge on the outside due to one-directional leakiness.

TEST OF CONTENTWhat are TWO reasons a neuron is negative inside relative to its outside.

More positive charge pumped out than in, positive charge leaks out, but not in.

Activity Eight: Action Potential

Because the neuron at rest has both charge and electrical gradients, it has the potential to respond to this polarized state – like a loaded spring. When a neuron is stimulated (by an environmental stimulus or by another neuron) it responds in a way that allows these polarized ions to diffuse down their concentration gradients. This means that a neuron in action works by opening channels. For some, it is counterintuitive to think about a neuron at work and using channels (which do not require energy). But the work of a neuron is in establishing its potential, much like the work of acquiring potential energy by climbing to

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The Membranethe top of a slide. Going down the slide takes no effort, you just have to let go of the handles (or in the case of the neuron – open the gates on your channels).

The first thing that happens to a stimulated neuron is that the gates on the sodium channels open. Predict how a neuron will respond to that event. Remember, sodium is at higher concentration outside the neuron than inside and the neuron is more negative inside than outside. Which way will sodium move and why? Sodium moves from outside to in to go down the concentration gradient and to move toward negative because it is positive.

The movement of sodium toward the inside of the neuron is called “depolarization” because the polarized neuron is inverted. Depolarization results in the inside of the neuron becoming more positive. Once it is positive enough, threshold is reached and an action potential begins. At the peak of its positive charge, potassium gates respond and all come open, not just the leaky ones. Which way will potassium move when this happens? Come up with TWO reasons why. Potassium will move from inside to outside to move down its concentration gradient and to move away from the positive charge inside. This half of the action potential is called “repolarization” because the neuron goes back to its negative charge inside. The resetablishment of sodium and potassium gradients happens more slowly and requires more effort from the pumps.

TEST OF CONTENTThe chart to the right shows the events

of an action potential. Label the axes and the upward and downward slope marked with “A” and “B”. X axis is time, Y axis is charge with zero being the blue horizontal line. A = depolarization, B = repolarization

Activity Nine: ReadingREQUIRED READING

From any Introductory textbook Biological membranes Passive and active transport (pumps and channels) Neuron structure and function Resting and action potentials

From Internet http://en.wikipedia.org/wiki/Cell_membrane

o functiono structureo composition

http://highered.mcgraw- hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html (if we are allowed to use it, it’s not password protected…)

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A B

The Membrane http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/

animation__receptors_linked_to_a_channel_protein.html (same) http://faculty.washington.edu/chudler/bbb.html

SUPPLEMENTAL READING

From Internet http://www.ionchannels.org/ http://www.sciencemag.org/cgi/content/summary/286/5439/388 https://health.google.com/health/ref/Cystic+fibrosis

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