Module 3 Study Guide - Lecture notes 3 PDF

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Module 3 Study Guide Overview / Introduction This module will cover membrane transport, carrier proteins and channel proteins. Use this study guide as a starting point for learning this material. You can think of the study guide as a written equivalent to a lecture in a typical face to face course. Once you have read through the study guide, read through the chapter from the textbook to fill out your understanding of the material. You are responsible for information presented in the study guides, the chapter readings, and the lab exercises you perform in this course. Required Reading: Chapter 12 Learning Objectives • • • • • • •

Understand the semi-permeable nature of cell membranes and how ions would move across this membrane unaided Understand the importance of membrane transport Be able to describe the function of membrane carriers, channel proteins and gated channels Be able to describe the difference between passive transport, facilitated transport, and active transport Understand the importance of maintaining unequal concentrations of various materials across the cell membrane Be able to describe the function and mechanism of a ligand gated ion channels Be able to give examples and explain the mechanisms of all three types of ion channels

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Study Guide The phospholipid bilayer forms a semi-permeable barrier to the passage of many types of molecules including just about anything that is water soluble or carries a charge. However, many of these materials do need to be able to pass through the cell membrane for the cell to get the materials it needs to survive and to get rid of material that are potentially toxic. Movement of these important particles is achieved using channel proteins and transport proteins. Through the regulation of movement through these passageways the concentrations of material inside the cell may maintain at a different concentration than outside the cell. The table below shows these concentration differences for some important ions.

Not only do these differences in concentration exist between the inside and outside of the cell, they are critically important for the cell’s ability to operate enzymes, and to generate and use energy. Permeability of the lipid bilayer is determined by molecule size, polarity, and charge of the molecule. Large particles are simply too big to squeeze between the phospholipids in a phospholipid bilayer. However, some molecules that are small, uncharged and hydrophobic can move through the lipid bilayer with ease. Examples would be O2 and CO2. Even some larger hydrophobic molecules can do this. For example, steroid hormones, those that are based on cholesterol, can diffuse right through the plasma membrane. Some small uncharged polar molecules can also move easily through the plasma membrane. Examples would be water (H2O) and ethanol.

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The movement of ions across the membrane drives many processes within the cell, so maintaining the concentration gradient of these ions is an important priority for the cell. Since ions and polar molecules cannot diffuse freely, they must be assisted by proteins that help them across the plasma membrane. Transport proteins can be classified as transporters or channel proteins. Transport proteins require a molecule to bind in a binding site on the transporter. A conformational change then moves the molecule from one side of the lipid bilayer to the other where it is then released. Channels on the other hand will let the molecule move through a small space from one side of the membrane to the other. For a molecule to use a channel it must be the right size and charge to pass through. Both transporters and channels can be tightly regulated by the cell. Below is a figure that shows the basic difference between a transporter and a channel.

Transporter proteins can be passive or active depending on whether they use energy to accomplish the job. Passive transport occurs when molecules move from an area of high concentration to low concentration. What drives the movement is the concentration gradient and no energy is required on the part of the cell to operate the transporter. Conversely, active transport requires the input of energy, and can move molecules against their concentration gradient. These transporters maintain the concentration gradient between the inside and outside of the cell.

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The sodium potassium pump is an example of a carrier protein. Its job is to maintain the sodium gradient between the inside and outside of the cell. The sodium gradient is used to regulate cell volume and drive sugar and amino acid transport. The sodium potassium pump is more properly called a sodium/potassium ATPase because the energy used to drive the pump is derived from ATP-transfer of the terminal phosphate group from ATP to the transport protein. This changes the conformation of the carrier protein and allows the ions to be moved across the plasma membrane. The actions of the sodium potassium ATPase are critically important for the survival and function of the cell and, as it turns out, is very expensive. Much of the energy in our bodies goes toward the operation of this ATPase. The following figures show the basic principles behind the sodium potassium pump. We know from the table provided earlier that sodium is in high concentration outside the cell and at low concentration inside the cell. Potassium on the other hand has a reversed situation with a high concentration inside the cell and a low concentration outside the cell. From the standpoint of electrical charge, the inside of the cell is more negative than the outside. This is largely due to large negatively charged proteins that reside inside of cells. The overall negative charge inside the cell helps pull positive ions into the cell but represents a resistance to moving positive ions out of the cell. Overall, how easily the ion moves from one side of the membrane to the other is a combination of the concentration force and the electrical force.

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In the sodium potassium pump, sodium and potassium are being moved against their electrochemical gradients. However, the movement of sodium out of the cell is against a much stronger electrochemical gradient than the movement of potassium into the cell. Not surprisingly it is the transport of sodium out of the cell that requires phosphorylation by ATP. The operation of the pump is as follows. Sodium ions bind to cytoplasmic side of carrier protein. Binding of sodium along with phosphorylation of carrier causes a change in conformation which results in the carrier protein opening to the extracellular side. Sodium ions are released from the carrier which allows potassium ions to bind to the carrier. Binding of potassium causes the carrier to de-phosphorylate, which changes the carrier proteins conformation back to its original conformation and the potassium ions are released to the cytoplasmic side.

The sodium potassium pump maintains a low concentration of sodium inside cells. This concentration gradient can then be used to drive the movement of other materials into the cell. For example, the sodium/glucose co-transporter (a symport) uses the concentration gradient of Na+ to bring glucose into the cell. The sodium potassium pump is an example of an antiport. In this kind of transporter one ion or molecule is traded for another.

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Sodium/glucose co-transporter is an example of a symport. It is located on the apical surface of gut epithelial cells. In this area the glucose concentration is higher inside intestinal epithelial cells than in the intestinal lumen, or in the blood. Therefore, glucose cannot diffuse freely across the membrane. However, the flow of sodium down its concentration gradient into the cell through the sodium/glucose co-transporter is so electrochemically favoured, it brings glucose along with it. Sodium is removed from the basal surface using the sodium potassium pump which maintains the low sodium concentration inside the cell, and glucose diffuses out of the intestinal cell on the basal side through facilitated transport mediated by GLUT4. See below.

Channel proteins are different from transporters in that they do not bind individual molecules, but rather form aqueous pores that allow the controlled movement of materials down their electrochemical gradients. They are passive in that the movement of individual molecules does not require the expenditure of ATP. Also, the molecules can move very quickly as compared to transporters. Most channel proteins are selective as to which molecules can pass through them and most are gated. For the most part channels are only used for small ions as opposed to large molecules. They can be opened or closed depending on the needs of the cell. Additionally, most but not all channels allow the movement of ions in both directions. Below is an example of a 6

channel protein for potassium ions. Note that the ions essentially squeeze though the channel. This allows for tighter regulation of the channel.

Ion channels change conformation between open and closed. What triggers the conformational change depends on the transporter itself. Some respond to changes in membrane electrical potential (difference between charge inside the cell and outside the cell), some respond to binding chemical mediators, like those that change due to binding hormones. Some respond to physical pressure or movement.

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Cystic fibrosis (CF) is the result of a faulty chloride ion transporter. The transporter is ligand gated with ATP binding as a requirement for conformational change. Normal goblet cells, which are mucous producing cells, regulate the movement of water out of the cell through the movement of chloride ions. Movement of more chloride ions causes more water to move as well. This causes the mucous to be thinner.

In CF, chloride ion channels do not work, and chloride ions are not moved out of the cell and into the mucous. Therefore, water is also not added to the mucous and the mucous becomes thick and sticky.

The defect is in a protein called CFTR which is part of the chloride channel protein that binds ATP. In normal proteins, the chloride channel is blocked by a regulatory domain, until ATP binds the proteins and the regulatory domain becomes phosphorylated. This causes the channel to open and chloride to flow out. Because the faulty chloride channel of CF cannot bind ATP, the channel does not open, and chloride is not released. Normally, water would diffuse freely out of the cell following the release of chloride. With the faulty chloride channel, water does not leave the cell either, and the mucous secretions are thick and sticky. People with cystic fibrosis experience severe respiratory problems as well as other problems throughout their bodies as a result of the accumulation of thick mucous secretions. 8

Hearing: the ion channels in the ear are operated mechanically. The ear contains “hair cells” which are the sensory cells responsible for responding to sound vibration. Sound causes the underlying basal membrane of the cochlea in the ear to vibrate. The cilia on the apical surface of the hair cell moves relative to each other and this mechanical change in position causes the opening and closing of ion channels.

Neural transmission of an action potential down the length of a neuron is due to the opening and closing of voltage gated channels. Much of the work on neurons has been done using the neurons that run the length of the mantel of squids.

These animals need very fast neural transmission from their brains to the far end of the mantel when they need to escape. The way they achieve this speed of transmission is using enormous neurons with axons up to 1 mm thick.

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These large axons allowed for the study of the details surrounding the transmission of nerve signals by allowing scientists to measure the electrical potential inside and outside the cell membrane within the axon. The following figure depicts the changes of membrane potential across a membrane. Most cells have an unequal charge distribution along the plasma membrane with more negative ions inside and more positive outside. This situation is accentuated in excitable cells like neurons and muscle cells. The ion channels in these membranes usually respond to changes in this electrochemical gradient.

Neurons are especially dependent on the movement of potassium ions. Remember, potassium is at a higher concentration inside the cell than outside. Potassium moves across the plasma membrane, both ways, constantly through potassium leak channels. Potassium is drawn into the cell to balance the negatively charged macromolecules, but also leaks out down its concentration gradient until and equilibrium is reached.

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The movement of potassium is a large contributor to the potential difference across animal plasma membranes of -20 mV to -200 mV (usually around -65mV in humans), but other ions contribute as well. The membrane is said to be polarized because of this potential difference. The resting membrane will be more negative on the cytosolic side and more positive on the extracellular side of the plasma membrane.

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Excitable cells like to conduct impulses by rapidly changing the membrane permeability to different ions. As the ions move across the plasma membrane the difference between the charge inside the cell and outside the cell changes. As the membrane potential becomes more positive inside the cell (compared to its resting state), it is said to be depolarized. If the membrane becomes more negative than the resting membrane potential it is said to be hyperpolarized and when the membrane potential returns to resting membrane potential it is said to be repolarised. The membrane potential changes occur by the flow of ions through membrane channels. Some channels are normally open, and others closed. For example, the potassium leakage channels discussed earlier are always open. Closed channels have molecular gates that can be opened. Voltage-gated channels are opened by changes in membrane potential.

The action potential is a wave of membrane potential change along the axon. The wave is formed by the rapid depolarization of the membrane by the sodium influx, followed by the rapid repolarization by potassium efflux. Both the sodium channels and potassium channels involved are triggered by changes in membrane potential.

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The figure below shows the activity of the voltage gated sodium channels during an action potential. A small depolarizing change in membrane potential is needed to initiate the action potential. This causes the opening of voltage gated sodium channels. As the positive sodium ions rush into the cell, the cell becomes more positive inside and is depolarized. When the membrane potential moves closer to zero, the voltage gated sodium channels close. What is not shown here is that potassium voltage channels open about the same time the sodium channels close. The outflux of potassium causes the inside of the cell to become more negative and the membrane potential repolarizes. It takes a short period of time after the action potential for potassium leakage channels and the sodium potassium pump to move all the ions back to starting positions. This time is called the refractory period because no action potential can occur then. Even the sodium voltage gated channels enter an inactivated period where changes in membrane potential do not result in opening or closing of the channels.

As each section of axon becomes slightly depolarized by the action potential in the preceding segment, it too will experience an action potential. This is how the action potential spreads in one direction along the axon.

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When the action potential reaches the end of an axon, the signal is generally transmitted to another cell via a synapse. A typical chemical synapse uses calcium influx into the terminal end of the axon as a result of the change in membrane potential. Calcium then binds with vesicles located nearby which then fuse with the plasma membrane at the end of the axon. When the vesicles fuse, they release chemicals called neurotransmitters which then bind to channel proteins on the adjacent cell. These chemicals then bind to channels of their own and cause changes in the cell.

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One example of a neurotransmitter receptor is the acetylcholine receptor on muscle cells. This receptor is usually in a closed conformation unless it binds acetylcholine. Once acetylcholine has bound the receptor opens and sodium rushes into the cell. This makes the inside of the cell more positive which then causes the muscle cell to contract.

When you are finished reading through this module you should proceed to Chapter 12 in the textbook.

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