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The Selective Permeability of The Protein Channels in The Cell Membrane

Proteins do not float freely in the cell; their movements within the cell are tightly controlled. Inside the cell, there are compartments just like rooms in a house. The walls of the compartments within the cell are equipped with a gate and chemical receptors. If a protein with the correct identification tag approaches, the receptor gate opens and lets the protein pass through. If a protein with the wrong tag approaches, the gate remains closed. In order for this passage to take place, the gate, receptor and tag all need to be present at once. The organ where these processes can be observed most clearly is the liver, the largest organ in the body and which controls the levels of essential nutrients, such as proteins, in the blood. If the gate, receptor and tag were not present at the same time in the liver's cell membranes, then the liver itself and consequently, the body would be unable to survive. Moreover, this is only one of the preconditions for life.

hayvanlar

A living cell is a marvel of detailed and complex architecture. Seen through a microscope there is an appearance of almost frantic activity. On a deeper level it is known that molecules are being synthesized at an enormous rate. — Carl Sagan

In the previous chapters, you read how some of the proteins in the cell membrane serve as channels. The passage of substances through them varies according to the diameter of the channel, its shape and internal electrical charge. As the result of experiments he carried out with the support of the US National Academy of Sciences, the biochemist Phillip Klebba discovered that the cell membrane proteins that regulate entry to the cell behave like gateways and that they recognize the substances necessary for the cell to grow. Furthermore, it has been established that these gateways—after permitting substances to enter the cell and preventing unnecessary, harmful materials from entering—absorb the molecules they need. The results, published in the May 23 1997 issue of Science, reveal that these entrance portals recognize substances that the cell needs for growth. They actively open to allow their entrance, and then close. In this way, cells obtain the molecules they need. 42

hücre zarı kanalları

1- Channel in neutral position.
2- Channel in closed position.
3- Channel in open position.

The cell membrane channel in a neutral position

(1) is polarized because of a change in the electric current around the membrane

(2). This later opens in such a way as to permit ions to pass through the channel

(3). Movement of ions along the channel is of great importance for the survival of the cell and for it to perform its functions.

In short, the gates control what proteins will pass along the channels. Scientists' accounts of this employ verbs such as choosing, feeling, perceiving, permitting, recognizing, as if they were referring to a conscious system. Certainly the components comprising the cell—atoms, amino acids, proteins—are all unconscious, whatever their size and function. Yet the emerging mechanism evidences conscious activity. This superior Intellect we encounter belongs to our Almighty Lord, the Creator of all things, Who pervades all places.

Sensitive Selection by Ion Channels

The cell membrane is selectively permeable for ions, and for many other substances. (Ions are atoms or molecules that carry an electrical charge, having lost or gained an electron.) In addition to the cell membrane's phospholipid structure, it also repels ions in the extracellular fluid. Therefore, ions can enter or leave the cell only by way of the special proteins in the cell membrane. However, ions do not pass at random along these protein channels: The channels behave most selectively on which ions will pass.

  deniz ev
 

We created man from a mingled drop to test him, and We made him hearing and seeing.
(Surat al-Insan, 2)

Ions are generally mobile, in order to balance their electrical charges. In any solution under typical conditions, there are as many negatively charged ions as positively charged ones. So long as this equilibrium is not disrupted, no potential difference in charge arises. (Potential difference is the difference in electrical tension between two points.) If that equilibrium is disrupted, the + and – charged ions in the solution will move in order to keep the solution neutral.

The passage of ions through the cell membrane takes place via this mechanism. Since the content of the intracellular fluid is different from that outside the cell, ions cross over in order to restore equilibrium between these fluids. The channels by which the ions pass assume the form of liquid pores in the cell membrane. In this way, some ions—particularly sodium, potassium, calcium and chloride—are permitted to enter the cell.

 
nöronlar

1. Nerve cell
2. Message-transmitting molecules
3. Membrane channels

Despite such processes as recognition and selection, the passage of ions through the channels takes place very quickly. Indeed, ions are transported so rapidly that the nerve cells carry messages to any point in the body in as little as 1/1000th of a second.

One of the ion channels' most important features is that they are capable of selecting different ions. It is of course extraordinary for one atom to recognize another and permit it to pass. It's impossible to imagine that any atoms assumed such a duty of their own accord, functioning like conscious doormen without ever making a mistake. It is also irrational to maintain that atoms assembled flawlessly by coincidence and gave rise to such a vital function. Anyone with reason and good conscience will appreciate that the obvious order here is the work of Allah, and that He is the sole Ruler of all. As Allah reveals in the Qur'an, "He knows everything in the land and sea. No leaf falls without His knowing it. There is no seed in the darkness of the Earth, and nothing moist or dry which is not in a Clear Book" (Surat al-An'am, 59). Allah possesses knowledge of everything.

Research has revealed that the ion channels are not always open, but work like gates or circuit breakers, allowing passage to just one variety of ion. Eric Young, a professor of biomedical engineering at Johns Hopkins University, refers to the selectivity of ion channels in these terms:

The most striking property of ion channels is their selectivity for different ions. Channels are classed as potassium, sodium, calcium or chloride channels, based on the ions that can permeate. Often channels are able to select between chemically almost identical ions (e.g., sodium and potassium) ... At present, the portions of the protein molecule responsible for the selectivity of several types of channels are known, but detailed theories to explain the selectivity have not appeared. Some aspects of ion selectivity of sodium can be accounted for by charge and size. But neither of these effects can count for the relative selectivity of sodium, potassium and calcium channels. For example, the sodium ion Na+ is smaller than the potassium ion K+ and has the same charge, but potassium channels discriminate against sodium by a factor of between 10 and 100. 43

As the above quotation emphasizes, the selection mechanism in ion channels possesses a very complex system. The unconscious molecules making up the channel recognize atoms' chemical structures and can distinguish a sodium ion (Na+) from a potassium ion (K+), leaving scientists with questions in their minds. These channels have an effective control mechanism that lets them open and close under particular conditions. For instance, some channels open as the result of changes in the electrical charge around the cell membrane, while others open in response to chemical transmitters and hormones.

These messages are transmitted at great speed. Despite the recognition and selection processes, passage through the ion channels takes place very rapidly. There is no delay or deceleration during the course of selection. Indeed, ions are carried so rapidly that messages are transmitted to any point in the body in just a few thousandths of a second. For example, movement is very high in a nerve cell, and millions of ion flows take place in a millisecond (1 thousandth of a second).44 Bearing in mind that ions enter and leave the ion channels at every point in your body 24 hours a day, the magnitude of their movement in your body can be better comprehended.

Unconcious Atoms Cannot Establish The Complex Order In Our Bodies Themselves: This Flawless Creation Belongs To Allah …

hücre

1. Extracellular Fluid
2. Closed Potassium Gate
3. Cell Membrane
4. Closed Sodium Gate
5. Sodium Gates
6. Potassium Gates
7. The Sodium-Potassium Pump

a. Resting Potential
b. The Sodium Gate Is Opened.
c. The Potassium Gate Is Opened.

A. When a nerve cell is not receiving a message, its sodium (Na+) and potassium (K+) channels are closed.

B. When movement potential is established, the sodium (Na+) gates are opened and sodium ions enter.

C. When sodium (Na+) gates are closed and potassium (K+) gates are opened, potassium (K+) ions begin to exit.

D. The sodium (Na+) and potassium (K+) gates are closed, but still ready to open with a new signal.

When sodium ions enter the cell through the membrane, the interior of the cell becomes positively charged. The channels in the cell membrane, now also positively charged, start working. Potassium ions start flowing out of the cell, and the positive charge inside it declines. Due to this order inside your cells, you are able to perceive our surroundings and act accordingly. Whether you know it or not, this system in the cell membrane constantly functions in order for you to survive. This system never forgets, becomes tired or is caught unawares. That is because Allah, the Compassionate and Merciful, creates that order and causes it to continue in all of your cells.

There are countless preconditions for our survival. We have touched on only a few of them here, but they are all ready and constantly fulfilled in our bodies on our behalf. Indeed, before we are even born; even as a single cell, the data for these systems were encoded in our genes.

Human beings make no contributions to the Creation of this order, to its construction, nor to its functioning. Allah reveals His compassion for human beings as follows in the Qur'an:

". . . He has given you everything you have asked Him for. If you tried to number Allah's blessings, you could never count them ... " (Surah Ibrahim, 34)

Electricity Production in Ion Channels

iyon kanalları

1. Ion
2. Cell membrane
3. Ion channel

The diagram at the side shows the passage of ions through channels in the cell membrane. This passage is most important in terms of the body's electrical activities. This special structure in the membrane shows evidence of the most superior Intelligence that belongs to our Lord, Who manifests it in every cell in your body, reflecting His infinite knowledge.

The passage of ions through the channels is vitally important for the functions and maintenance of the life of the cells, and thus to life itself. As ions enter and depart from the cell via these channels, they set up small electrical currents that enable your nerve cells to function and permit communication between cells. All the vital functions in the body are regulated by the information carried by these electrical signals. Without proteins, the cell membranes would be in a state of electrical slumber, which would end any signaling within the body. From that point of view, the proteins constituting the ion channels in the cell membrane are vital to the body's electrical activities.

When an ion channel is opened, positively-charged sodium ions immediately enter the cell. This movement starts the events that form the power in nerves and muscles. In that regard, sodium channels also possess a vital importance. The entry of calcium into a cell via special channels enables nerve transmissions between cells and the secretion of hormones. 45

The movement of ions in these channels takes place very quickly and selectively. For example, when a cell membrane opens a sodium-selecting channel, it permits sodium to enter the cell and gives the internal cell voltage (or electrical tension) a positive charge. When a potassium-selecting channel opens, it permits potassium to leave the cell and reduces voltage to a negative value. In this way, voltage changes constantly and very rapidly.

Cellular electricity is a very important topic in biology. As phosphate compounds, amino acids or ions are carried through the cell membrane, their movement sets up an electrical current, which creates a voltage differential along the cell membrane, known as cell membrane potential. This electrical potential formed in the cell membrane balances electrical build-up by storing energy inside the cell.

When the flow of ions in the cell membrane changes, the membrane disrupts this potential, allowing the sodium channels to open. The dimensions of the sodium channels is between 0.3 and 0.5 nanometers (A nanometer is 1 millionth of a millimeter). As the opened channel attracts sodium ions, there is a large change in the cell membrane's potential, and the cell becomes electrically active. When in a state of rest, the sodium channels in nerve and muscle cells remain tightly closed. If there is a fall in cell membrane potential—if the charge within the cell becomes more negative compared to the outside—the sodium channels are opened. Channels of this kind are known as voltage-gated ion channels.

hayvanlr

He created the heavens and the Earth with truth and formed you, giving you the best of forms. And He is your final destination.
(Surat at-Taghabun, 3)

Voltage-Gated Ion Channels

hücre sıvıları

1. Extracellular fluid
2. Intracellular fluid

The above diagram shows the closed voltage ion channels in the cell membrane.

(A) Normally the closed voltage sodium channel is closed.

(B) When the electrical current reaches a specific level, the voltage gate is opened and sodium is permitted to enter the cell. After a certain time, the gates are again neutralized and the channels are closed.

The movement of ion channel in the form of gates depends upon the electrical charge of the cell membrane. For example, when a strong negative charge exists on the inner side of the cell membrane, the sodium channels' outer sides are tightly closed. When the inner side of the membrane loses its negative charge, these gates suddenly open and large amounts of sodium enter the cell. Potassium gates, on the other hand, open when the inner side of the cell membrane becomes positively charged.

You might compare the gates opening and closing to doors that are opened and closed under the control of a security officer. In the same way that the officer opens the door only when he recognizes people or approves their identity cards, so the ion channels open only when they recognize the appropriate ions. However, every opening and closing in the cell membrane takes place in a few millionths of a second—an exceptionally brief space of time. Were it any longer, then all your bodily functions would slow down, and you would be late in perceiving our surroundings and in reacting to those perceptions. With that decelerated mode of life, it would be impossible for the cells—and therefore, for you—to survive. In that regard, the speed at which your systems function is just as important as those systems themselves. If entry and departure through just one cell membrane was slower than it should be, then the order in your body would be disrupted. Therefore, every detail in your body represents a refutation of the theory of evolution's claims of gradual development.

Scientists who first measured the voltage changes that occur in the ion channels arrived at astonishing results. The 16 December, 2000 edition of Nature magazine announced that the amino acids in the voltage receptor did not make simple two-way movements, as had previously been thought. On the contrary, behave like keys turning in a lock.

Professor of Physics Paul Selvin of Illinois University refers to the results of their research:

The nerve cell membrane contains special cavities or channels that regulate the flow of sodium and potassium ions. The channels open and close like gates, depending on the voltage in the membrane, for which reason they control the manufacture and dissemination of nerve impulses. Our aim in this study was to discover how the ion channels detect voltage change and how the amino acids in the voltage detectors in the channels behave as these open and close ... In our view, the amino acids form cleft-like folds on the cell membrane. The rotation amends the chemical entry of the charge inside the cell to the charges outside it. Thus a small action can have very considerable consequences. 46

Francisco Bezanilla of California University refers to the complex structure of the voltage-gates in ion channels:

We marked specific amino acids in the ion channel and measured the change in distance according to the function of the voltage on the membrane... Astonishingly, some of these amino acids moved away while other drew closer; and some did not move at all. These movements cannot be explained as simple actions with the up-and-down motion of the pump in the cell membrane. This movement is like the turning of a lock and is totally compatible with the data. 47

As the above quotations express, the events taking place in the cell membrane's ion channels are not simple individual mechanisms. The entry and exit from the cell, which we have examined only in fairly general terms, show that the entire system has been created as a whole. It benefits the body only when all its components are operating together flawlessly. Otherwise, life would not be possible at all.

The voltage-gate potassium channels are a part of the signaling in the cell membrane. Signaling proteins have gaps that allow millions of ions every second through the cell membrane. These gaps allow ions to pass with an extraordinary selectivity and speed. Also, a perception mechanism identifies voltage changes in the gateways and whenever it detects any change in voltage, the gates open and close in as little as a millisecond.

According to Gary Yellen, of the Harvard Medical School Neurology Department: "The architectural modules and functional components of these sophisticated signaling molecules are becoming clear, but some important links remain to be elucidated." 48

The Amazing Order Invisible to the Naked Eye

Voltaj kapılı sodyum kanalı

1. Channel Is Closed
2. The Channel Opens And Sodium Enters Inside The Cell
3. The Channel Closes Again

4. Sodium Channel With Voltage-Gate
5. The Exterior of The Cell
6. The Interior of The Cell

7. Region Sensitive To The Voltage
8. Voltage Gate
9. Particle Neutralizing The Channel

The structure and functions of the closed voltage sodium channel:

Scientists recently discovered that the amino acids in the voltage preceptors do not make simple back-and-forth motions, as they previously imagined, but act like keys turning in a lock. The complex structure of the cell membrane, so difficult for scientists to unravel, clearly reveals that there is no room for coincidence at the molecular level.

The cell membrane's complex structure, whose other processes scientists are having great difficulty unraveling, clearly reveals that coincidence has no place at the molecular level. Incredible speed, perfect order and flawlessness are all operating at dimensions too small to be seen with the naked eye. The components of this order are unconscious atoms, and such an astonishing system could never come about by these same atoms combining together at random. According to those blindly devoted to Darwinism, however, this sublime Creation is the product of coincidence. Seeing this obvious Creation but calling it purposeless and describing it as coincidence, is nothing more than denying the facts. Even a few superficial pieces of information about the cell membrane are sufficient to demonstrate that claims of coincidence are illogical, irrational, and impossible.

  kelebek
 

He makes night merge into day and day merge into night, and He has made the Sun and Moon subservient, each one running until a specified time. That is Allah, your Lord. The Kingdom is His. Those you call on besides Him have no power over even the smallest speck.
(Surah Fatir, 13)

The Sodium-Potassium Pump

iyonlar

(A) Channels' Ion Selectivity
(B) Gate Formation in The Channel
(C) Neutralization of The Channel

1. A Potassium Ion Combined with Water Molecules
2. Closed
3. Open
4. Ion Binding Region
5. Neutralizing Particle
6. Cell Membrane

These diagrams show how the closed voltage ion channels select the potassium ion.

(A) 1- Negative charges at the entrance to the channel attract positive charges. 2- The diameter of the channel restricts the amount of ions that will pass through. 3- After the ions have been selected they separate from the water molecules.
(B) When the electrical state of the membrane changes, the structure of the channel changes, and the channel gate opens.
(C) After ion passage has taken place, the channel is again closed by a neutralizing particle.

In addition, protein pumps requiring energy are also used to transport ions. One of the best known pumping systems is the sodium-potassium type. The protein constituting the channel in the cell membrane uses up a third of the cell's total energy production. This protein works non-stop, day and night, pumping sodium ions to the outside, while attracting potassium ions to replace them. During each pumping process, three sodium ions (Na+) are sent outside the cell, and two potassium ions (K+) brought in.49 Thus, different concentrations linked to the sodium (Na+) and potassium (K+) ions arise in the cell. These pumps present in all the body's cells, are used to ensure concentration of ions inside the cell and to control cell volume.

One side of the transport protein forms a protrusion towards the inside of the cell. On it are three receptors for the binding of sodium ions. When three sodium ions are attached to the inner side of the transport protein, the protein's ATP-az (an enzyme inside ATP—adenosine triphosphate, the cellular energy used directly by living things) is activated. This enzyme breaks down the energy-bearing ATP and turns it into ADP (adenosine diphosphate, a compound emerging when the phosphate group separates from ATP). As the energy is freed, a change occurs in the transport protein molecule that causes sodium ions to head for the outside and potassium ions to enter.

Human Beings Need The Order Created in The Cell by Allah, Cystic Fibrosis is One Example To Be Borne in Mind

A. Normal cells and the departure of chloride ions
B. Defect in the cells and chloride ion departure of someone with cystic fibrosis.

1. Cells extending along an air cavity
2. Air cavity

hücre zarı

The fatal disease of cystic fibrosis results in closure of airways in the lungs due to an increase in fibrous connective tissue between cells. At the same time, this disease may also lead to blocked channels in the liver and pancreas. There is currently no cure. The researcher Paul Quinton determined that the cause of this disease is the faulty working of a protein in the cell membrane.

In healthy individuals, sodium ions enter the channel in the cell membrane by being transported by the sodium-potassium pump. Chloride ions follow through a passive channel. In cystic fibrosis sufferers, the potassium pump carries sodium ions into the channel, but chloride ions cannot enter through the cell membrane. In other words, the passive chloride channels do not function properly. (Sandra S. Gottfried, Biology Today, Mosby-Year Book Inc., ABD, 1993, p. 70.)

As you have seen, a defect in just one passage through the cell membrane can have fatal consequences. Countless conditions need to be met at the same time for you to survive.

The ion-pumping system we have described in general terms is only one of the complex processes in the cell membrane that scientists have spent years studying. There is great wisdom in all the details that emerge under an electron microscope. Allah has created humans to be in need of every one of these systems. The information discovered in recent years offers us an important opportunity to appreciate the omniscience of Allah, Who pervades all places. In the Qur'an it is revealed that:

"... My Lord encompasses all things in His knowledge, so will you not pay heed?" (Surat al-An'am, 80)

 

Footnotes

42. http://www.nsf.gov/od/lpa/news/press/pr9740.htm; National Science Foundation Press Release.

43. http://www.bme.jhu.edu/courses/580.439/notes/Notes_channels.pdf

44. http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v419/

45. Wray, D., "Ion Channels: Molecular Machines par Excellence," Science Spectra, 2000, No. 23, pp. 64-71.

46. A. Cha, G.E. Snyder, P. R. Selvin, F. Bezanilla, "Atomic scale movement of the voltage-sensing region in a potassium channel measured via spectroscopy," Nature, No. 402, 16 December, 1999, pp. 809-813; http://www.hhmi.org/news/mackinnon4.html

47. Ibid.

48. Gary Yellen, "The voltage-gated potassium channels and their relatives," Nature, No. 419, 5 September 2002, pp. 35-42.

 

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