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We know, that cancer cell can travel across an organism.
Is this ABSOLUTELY impossible for NORMAL cells?
For example, is it EXACTLY ZERO probability to find some bone cells inside liver or some skin cells inside brain?
In non-living things and with molecules this is called "diffusion". Any two contacting things will penetrate inside each other after hundreds and thousands years.
Is something similar happening in living thing?
May be very slow?
Was this phenomenon searched for?
Are there some protection mechanisms against appearing wrong tissue cells inside another tissues?
I understand that cells appear in their places during grow, but in mature state I think it is not impossible for cells to travel.
I know that there are many cells that travel by nature, like blood cells.
The question is namely about "static" cells, those ones we normally regard as non-travelling.
Also, I understand that there are some mechanisms, making cells to prefer their normal places when they divide and grow.
But the question is what about all these mechanisms failed? And how often it happens?
For example, I am 40 y.o. So, how many bone cells from my legs are currently in my brains? Zero? One? Million?
In a healthy body, cells travel. But this is very dependent of the cell type. Some cell type will travel in all the body (lymphocytes, plasmocytes, etc) while other do not have any migration capability.
The concept of diffusion is wrong to apply here, indeed the scales at which you are looking are not molecular.
Moreover, there are very active mechanism to keep cell type in place, for example, a concept called preferential attachment will maintain the cells grouped by cell type. If you take two balls A and B and A attached more to A and B to B, and you shake, you will see appearance of clusters of A and clusters of B. But that is only one such mechanism and a lot of other take play here.
see this: http://www.ncbi.nlm.nih.gov/books/NBK26937/
Thus, the probability of finding a cell type NOT at its normal position (that is one that does not travel) is maybe not zero, but so close to it that you will not find a cell outside of their normal position.
I hope this answers your question ?
Cooper GM. The Cell: A Molecular Approach. 2nd ed. Sunderland (MA): Sinauer Associates; 2000
Structure of the Membrane
Properties of the Cell Membrane
- The cell membrane is semi permeable- The pores that occur on the cell membrane allows the passage of the small size molecules but does not allow the passage of the large sized molecules. Such a membrane is said to be selectively permeable or semi-permeable. In particular, when a cell is surrounded by a dilute sugar solution, the small sized water molecules will enter the cell but the larger sugar molecules will not pass through the cell membrane. In contrast, the cell wall is permeable as it allows both sugar and water molecules to pass through it it has larger pores. This property of selectively permeability enables the cell membrane to select what enters and leaves the cell.
- The cell membrane is sensitive to changes in temperature and pH- Cell membranes are made up of protein. Proteins are adversely affected by extreme changes in temperature and pH. Changes in temperature and pH will alter the structure of the cell membrane thereby hindering the normal functioning of the cell membrane. High temperature denatures (destroys) the proteins thereby impairing the functions of the cell membrane.
- The cell membrane possesses electric charges- The cell membrane has both positive and negative charges. These charges affect the manner in which substances move in and out of the ells. The charges also enable the cell to detect changes in the environment.
Facilitated Diffusion through Cell Membrane (With Diagram)
A variety of compounds including sugars and amino acids pass through the plasma membrane and into the cell at a much higher rate than would be expected on the basis of their size, charge, distribution coefficient, or magnitude of the concentration gradient.
The increased rate of transport through the membrane is believed to be facilitated by specific membrane carrier substances and is called facilitated (or mediated) diffusion.
During facilitated diffusion, the rate at which the solute permeates the membrane increases with increasing solute concentration up to a limit.
Above this limiting concentration, no increase in the rate of transport across the membrane is observed. In other words, facilitated diffusion exhibits saturation kinetics (Fig. 15-38) and is therefore similar to the relationship between reaction rate and substrate concentration in enzyme-catalyzed reactions.
Other characteristics of facilitated diffusion are also similar to enzyme catalysis. Transport is specific for example, in the erythrocyte, the inward diffusion of glucose, but not fructose or lactose, is facilitated. The rate of solute permeation can also be affected by the presence of structurally similar chemical compounds, much as in competitive enzyme inhibition. Facilitated diffusion exhibits pH dependency.
Although facilitated diffusion results in a more rapid attainment of concentration equilibrium across the membrane than passive diffusion, the normal equilibrium concentrations are not altered. Substances are not transported through the membrane against a concentration gradient. Although facilitated diffusion is not affected by chemicals that act as metabolic inhibitors, it is affected by enzyme inhibitors such as sulfhydryl blocking agents.
Facilitated diffusion is believed to result from the interaction of solute with specific membrane molecules, presumably proteins, thereby forming, a carrier-solute complex. The complex undergoes a positional change within the membrane in such a way that the solute now faces the other membrane surface and is released from the carrier (Fig. 15-39).
An alternative suggestion is that the carrier may be a small molecule with the solute-carrier complex formed by an enzyme-catalyzed reaction within the membrane. Once formed, the solute-carrier complex diffuses to the other side of the membrane where the solute is released in a second reaction.
A good and well-defined example of facilitated diffusion occurs in the bacterium Escherichia coli. The sugar lactose does not readily permeate E. coli cells and cannot be hydrolyzed by cytoplasmic extracts of cells grown in the absence of lactose. However, when E. coli is in a medium containing lactose, the enzyme P-galactosidase, which hydrolyzes lactose, soon appears in the cell cytoplasm, and a specific transport protein appears in the membrane.
The presence of the substrate in the growth medium is said to induce the formation of the enzymes by the cells. Such cells are able to transport and subsequently hydrolyze the lactose thereby forming galactose and glucose. Certain mutants of E. coli can be induced in this way to form β-galactosidase even though lactose remains impermeable and cannot be. incorporated by the cells. Finally, other E. coli mutants can be found that are able to remove galactose from the medium but cannot metabolize it once it is inside the cell.
It is now clear from these studies that in wild-type cells the presence of lactose in the growth medium induces both the formation of the hydrolytic enzyme and a carrier system—called a permease or translocase. The sets of genes controlling the formation of the permease and the hydrolytic enzyme are coordinately induced in the presence of lactose. The loss or alteration of either set of genes (as in the E. coli mutants) results in a corresponding inability to induce the formation of the enzyme or the permease.
"Bulk flow" is the movement/flow of an entire body due to a pressure gradient (for example, water coming out of a tap). "Diffusion" is the gradual movement/dispersion of concentration within a body, due to a concentration gradient, with no net movement of matter. An example of a process where both bulk motion and diffusion occur is human breathing. 
First, there is a "bulk flow" process. The lungs are located in the thoracic cavity, which expands as the first step in external respiration. This expansion leads to an increase in volume of the alveoli in the lungs, which causes a decrease in pressure in the alveoli. This creates a pressure gradient between the air outside the body at relatively high pressure and the alveoli at relatively low pressure. The air moves down the pressure gradient through the airways of the lungs and into the alveoli until the pressure of the air and that in the alveoli are equal, that is, the movement of air by bulk flow stops once there is no longer a pressure gradient.
Second, there is a "diffusion" process. The air arriving in the alveoli has a higher concentration of oxygen than the "stale" air in the alveoli. The increase in oxygen concentration creates a concentration gradient for oxygen between the air in the alveoli and the blood in the capillaries that surround the alveoli. Oxygen then moves by diffusion, down the concentration gradient, into the blood. The other consequence of the air arriving in alveoli is that the concentration of carbon dioxide in the alveoli decreases. This creates a concentration gradient for carbon dioxide to diffuse from the blood into the alveoli, as fresh air has a very low concentration of carbon dioxide compared to the blood in the body.
Third, there is another "bulk flow" process. The pumping action of the heart then transports the blood around the body. As the left ventricle of the heart contracts, the volume decreases, which increases the pressure in the ventricle. This creates a pressure gradient between the heart and the capillaries, and blood moves through blood vessels by bulk flow down the pressure gradient.
The concept of diffusion is widely used in: physics (particle diffusion), chemistry, biology, sociology, economics, and finance (diffusion of people, ideas and of price values). However, in each case the substance or collection undergoing diffusion is "spreading out" from a point or location at which there is a higher concentration of that substance or collection.
There are two ways to introduce the notion of diffusion: either a phenomenological approach starting with Fick's laws of diffusion and their mathematical consequences, or a physical and atomistic one, by considering the random walk of the diffusing particles. 
In the phenomenological approach, diffusion is the movement of a substance from a region of high concentration to a region of low concentration without bulk motion. According to Fick's laws, the diffusion flux is proportional to the negative gradient of concentrations. It goes from regions of higher concentration to regions of lower concentration. Sometime later, various generalizations of Fick's laws were developed in the frame of thermodynamics and non-equilibrium thermodynamics. 
From the atomistic point of view, diffusion is considered as a result of the random walk of the diffusing particles. In molecular diffusion, the moving molecules are self-propelled by thermal energy. Random walk of small particles in suspension in a fluid was discovered in 1827 by Robert Brown, who found that minute particle suspended in a liquid medium and just large enough to be visible under an optical microscope exhibit a rapid and continually irregular motion of particles known as Brownian movement. The theory of the Brownian motion and the atomistic backgrounds of diffusion were developed by Albert Einstein.  The concept of diffusion is typically applied to any subject matter involving random walks in ensembles of individuals.
In chemistry and materials science, diffusion refers to the movement of fluid molecules in porous solids.  Molecular diffusion occurs when the collision with another molecule is more likely than the collision with the pore walls. Under such conditions, the diffusivity is similar to that in a non-confined space and is proportional to the mean free path. Knudsen diffusion, which occurs when the pore diameter is comparable to or smaller than the mean free path of the molecule diffusing through the pore. Under this condition, the collision with the pore walls becomes gradually more likely and the diffusivity is lower. Finally there is configurational diffusion, which happens if the molecules have comparable size to that of the pore. Under this condition, the diffusivity is much lower compared to molecular diffusion and small differences in the kinetic diameter of the molecule cause large differences in diffusivity.
Biologists often use the terms "net movement" or "net diffusion" to describe the movement of ions or molecules by diffusion. For example, oxygen can diffuse through cell membranes so long as there is a higher concentration of oxygen outside the cell. However, because the movement of molecules is random, occasionally oxygen molecules move out of the cell (against the concentration gradient). Because there are more oxygen molecules outside the cell, the probability that oxygen molecules will enter the cell is higher than the probability that oxygen molecules will leave the cell. Therefore, the "net" movement of oxygen molecules (the difference between the number of molecules either entering or leaving the cell) is into the cell. In other words, there is a net movement of oxygen molecules down the concentration gradient.
In the scope of time, diffusion in solids was used long before the theory of diffusion was created. For example, Pliny the Elder had previously described the cementation process, which produces steel from the element iron (Fe) through carbon diffusion. Another example is well known for many centuries, the diffusion of colors of stained glass or earthenware and Chinese ceramics.
In modern science, the first systematic experimental study of diffusion was performed by Thomas Graham. He studied diffusion in gases, and the main phenomenon was described by him in 1831–1833: 
". gases of different nature, when brought into contact, do not arrange themselves according to their density, the heaviest undermost, and the lighter uppermost, but they spontaneously diffuse, mutually and equally, through each other, and so remain in the intimate state of mixture for any length of time."
The measurements of Graham contributed to James Clerk Maxwell deriving, in 1867, the coefficient of diffusion for CO2 in the air. The error rate is less than 5%.
In 1855, Adolf Fick, the 26-year-old anatomy demonstrator from Zürich, proposed his law of diffusion. He used Graham's research, stating his goal as "the development of a fundamental law, for the operation of diffusion in a single element of space". He asserted a deep analogy between diffusion and conduction of heat or electricity, creating a formalism similar to Fourier's law for heat conduction (1822) and Ohm's law for electric current (1827).
Robert Boyle demonstrated diffusion in solids in the 17th century  by penetration of zinc into a copper coin. Nevertheless, diffusion in solids was not systematically studied until the second part of the 19th century. William Chandler Roberts-Austen, the well-known British metallurgist and former assistant of Thomas Graham studied systematically solid state diffusion on the example of gold in lead in 1896. : 
". My long connection with Graham's researches made it almost a duty to attempt to extend his work on liquid diffusion to metals."
In 1858, Rudolf Clausius introduced the concept of the mean free path. In the same year, James Clerk Maxwell developed the first atomistic theory of transport processes in gases. The modern atomistic theory of diffusion and Brownian motion was developed by Albert Einstein, Marian Smoluchowski and Jean-Baptiste Perrin. Ludwig Boltzmann, in the development of the atomistic backgrounds of the macroscopic transport processes, introduced the Boltzmann equation, which has served mathematics and physics with a source of transport process ideas and concerns for more than 140 years. 
In 1920–1921, George de Hevesy measured self-diffusion using radioisotopes. He studied self-diffusion of radioactive isotopes of lead in the liquid and solid lead.
Yakov Frenkel (sometimes, Jakov/Jacob Frenkel) proposed, and elaborated in 1926, the idea of diffusion in crystals through local defects (vacancies and interstitial atoms). He concluded, the diffusion process in condensed matter is an ensemble of elementary jumps and quasichemical interactions of particles and defects. He introduced several mechanisms of diffusion and found rate constants from experimental data.
Sometime later, Carl Wagner and Walter H. Schottky developed Frenkel's ideas about mechanisms of diffusion further. Presently, it is universally recognized that atomic defects are necessary to mediate diffusion in crystals. 
Henry Eyring, with co-authors, applied his theory of absolute reaction rates to Frenkel's quasichemical model of diffusion.  The analogy between reaction kinetics and diffusion leads to various nonlinear versions of Fick's law. 
Osmosis and Diffusion
The cell membrane maintains the cell a separate entity it holds the cell contents within, and acts as a barrier to the external environment. It is selectively permeable and has various mechanisms to allow for the exchange of gases and nutrients. These mechanisms allow for the intake of anything that is required and allows for the expulsion of waste and toxins. This membrane does not resemble a sheet or bag rather, it is many molecules of Phospholipid Bilayers held together by the combined forces of attraction and repulsion. They are comprised of a Phosphate head which is hydrophilic (water-loving), and a Lipid (fatty acid) tail which is hydrophobic (repelled by water). As the internal and external environments of a cell are aqueous, these molecules arrange themselves into two layers one with the Phosphate heads oriented out into the external fluid, and the other with the heads oriented inwards into the internal fluid (the Cytoplasm). The Lipid tails are between the two layers of Phosphate heads thereby, protected from the water, and the strength of this attraction/repulsion mechanism keeps the molecules together as though the membrane were a single entity.
In this practical, dialysis tubing is used as a surrogate cell membrane for a visual demonstration of osmosis and diffusion. A solution containing large molecules (Starch) and small molecules (Glucose) is placed inside the tubing which is then placed in a solution containing iodine. Students are able to observe as the solution inside the tubing turns dark blue, while the surrounding solution it is submerged in does not. From this, students can use their prior knowledge of the Starch-Iodine complex to surmise that Iodine is able to pass through the membrane while starch is not. The Glucose-testing strips indicate that glucose has been able to pass out of the tubing and into the external fluid. Thus proving the tubing allows movement in both directions.
This inexpensive and simple experiment provides students with a clear visual result that effectively demonstrates how the size of a molecule can affect its ability to be transported into or out of a cell. It also illustrates the mechanics of diffusion and osmosis by which a cell will attempt to create homeostasis, or equilibrium between its inner and outer environments.
PREPARATION - BY LAB TECHNICIAN
- Cut the dialysis tubing into 15cm lengths and soak for 15 minutes in a beaker filled with room temperature distilled water. Prepare one length of tubing per student or group. However, it is best to prepare extra strips for students, as some strips may tear or leak through handling.
- To create the Starch solution, dissolve 2g of Starch in 100mL of boiling hot water (2% solution) on a hot plate until the Starch powder has been fully dissolved. Stir as required.
- To create the Glucose solution, dissolve 30g of Glucose in 100mL water (30% solution) and continue stirring until the glucose has been fully dissolved.
- Combine the Starch and Glucose solutions in a single beaker. Use a stirring rod to mix well.
METHOD - STUDENT ACTIVITY
Glucose/ Starch Solution
- Measure 5-10 mL of the Glucose/Starch mixture in a small beaker or test tube.
- To determine the initial concentration within the Starch/ Glucose solution, you will first need to dilute a sample of the mixture in water. To do this, collect 1mL of your mixture using a transfer pipette and add to a test tube filled with 9mL of water. Mix using a clean stirring rod.
- Measure the diluted Starch/Glucose by placing a Glucose-testing strip in the solution, immediately removing it and waiting 60 seconds to observe any colour change. Using the colour guide on the testing strip container, determine the approximate Glucose levels, and record the results.
- Fill a large beaker with 100mL water, and add 1mL of Iodine/KI solution. The solution should appear a yellowish colour.
- Measure the Glucose levels of the Iodine solution with another strip following the same procedure as before. Ensure you record the results.
Preparing the "cell" tubing
- Retrieve your soaked piece of dialysis tubing and tie a knot in one end as though you are tying a balloon.
- Using a transfer pipette, half-fill the tubing with your undiluted Starch/Glucose solution and tie the other end to create a &ldquocell&rdquo.
- Submerge the &ldquocell&rdquo tubing into the Iodine solution.
Observing changes in the &ldquocell&rdquo
- After 15 minutes, observe any colour changes in the tubing and in the beaker solution.
- Measure the Glucose levels in the Iodine solution.
- Carefully open the tubing and pour the contents into a clean beaker.
- To dilute the tubing contents for Glucose testing, collect 1mL of the contents using a pipette and deposit into a test tube filled with 9mL of water.
- Measure the Glucose levels in the diluted contents using a Glucose testing strip following the same procedure as before.
- Record the results of the Glucose testing.
- Compare the changes in Glucose levels before and after the 15 minute interval.
OBSERVATION AND RESULTS
- Provide students with the information that you prepared a 100mL solution of 2% starch and a 100mL solution of 30% Glucose. Based on this information, ask your students to calculate the concentration of each in the combined solution. Students should understand that double the volume without extra solute means half the concentration, so what was 2g of Starch in 100mL (2%) is now 2g of Starch in 200mL (1%) and what was 30g of Glucose in 100mL (30%) is now 30g of Starch in 200mL (15%).
- Ask students to identify what occurred the Starch, based on the fact that the blue colour is found inside the cell but not outside of it, students should be able to identify that the Starch has not been able to pass through the tubing, while the Iodine has. Students should understand that the Starch-Iodine complex has therefore been confined to the area where both Starch and Iodine are found, that is, the inside of the cell.
- Ask students to describe what is suggested by the Glucose results. The appearance of Glucose into the previously Glucose-free solution in the beaker should inform students that Glucose has been able to pass through the membrane.
- To provide students with a deeper understanding surrounding the molecular size of Glucose and Iodine, you may provide students with the information that our dialysis tubing typically allows passage to molecules of up to 12,000 to 14,000 daltons (g/mol). This should provide some guidance of the sizes that Starch molecules can reach. Remind students, however, that the shape of a molecule may affect the passage as a large linear molecule may be able to pass through more easily than a smaller but globular molecule.
The concentration of Glucose in this practical is quite high to enable shorter waiting times for students. This allows them to more readily measure the glucose which has diffused out of the &ldquocell&rdquo using their test strips. However, this also means that the initial concentration is too high to show that the concentration inside the cell has decreased in line with the increase outside the cell. To manage this, students are asked to take a sample of the original combined Glucose/ Starch solution prior to being placed in the &ldquocell&rdquo and also a sample of the now-blue solution inside the &ldquocell&rdquo at the end of the prac. Both solutions are diluted by a factor of ten to bring the Glucose concentration into the range of the Uriscan strips.
To observe the process of cell diffusion and osmosis over an extended period of time, make an extra &ldquocell&rdquo and keep it in solution until the next class. By the beginning of next class, the Glucose inside and outside the cell should have somewhat equalised. This could be conducted as a class demonstration, or each student may make an extra cell. Once again, dilute both solutions by a factor of ten prior to measuring.
Prepare extra dialysis strips for students, as some strips may tear or leak through handling as students attempt to tie them.
4.1 Studying Cells
By the end of this section, you will be able to do the following:
- Describe the role of cells in organisms
- Compare and contrast light microscopy and electron microscopy
- Summarize cell theory
A cell is the smallest unit of a living thing. Whether comprised of one cell (like bacteria) or many cells (like a human), we call it an organism. Thus, cells are the basic building blocks of all organisms.
Several cells of one kind that interconnect with each other and perform a shared function form tissues. These tissues combine to form an organ (your stomach, heart, or brain), and several organs comprise an organ system (such as the digestive system, circulatory system, or nervous system). Several systems that function together form an organism (like a human being). Here, we will examine the structure and function of cells.
There are many types of cells, which scientists group into one of two broad categories: prokaryotic and eukaryotic. For example, we classify both animal and plant cells as eukaryotic cells whereas, we classify bacterial cells as prokaryotic. Before discussing the criteria for determining whether a cell is prokaryotic or eukaryotic, we will first examine how biologists study cells.
Cells vary in size. With few exceptions, we cannot see individual cells with the naked eye, so scientists use microscopes (micro- = “small” -scope = “to look at”) to study them. A microscope is an instrument that magnifies an object. We photograph most cells with a microscope, so we can call these images micrographs.
The optics of a microscope’s lenses change the image orientation that the user sees. A specimen that is right-side up and facing right on the microscope slide will appear upside-down and facing left when one views through a microscope, and vice versa. Similarly, if one moves the slide left while looking through the microscope, it will appear to move right, and if one moves it down, it will seem to move up. This occurs because microscopes use two sets of lenses to magnify the image. Because of the manner by which light travels through the lenses, this two lens system produces an inverted image (binocular, or dissecting microscopes, work in a similar manner, but include an additional magnification system that makes the final image appear to be upright).
To give you a sense of cell size, a typical human red blood cell is about eight millionths of a meter or eight micrometers (abbreviated as eight μm) in diameter. A pin head is about two thousandths of a meter (two mm) in diameter. That means about 250 red blood cells could fit on a pinhead.
Most student microscopes are light microscopes (Figure 4.2a). Visible light passes and bends through the lens system to enable the user to see the specimen. Light microscopes are advantageous for viewing living organisms, but since individual cells are generally transparent, their components are not distinguishable unless they are colored with special stains. Staining, however, usually kills the cells.
Light microscopes that undergraduates commonly use in the laboratory magnify up to approximately 400 times. Two parameters that are important in microscopy are magnification and resolving power. Magnification is the process of enlarging an object in appearance. Resolving power is the microscope's ability to distinguish two adjacent structures as separate: the higher the resolution, the better the image's clarity and detail. When one uses oil immersion lenses to study small objects, magnification usually increases to 1,000 times. In order to gain a better understanding of cellular structure and function, scientists typically use electron microscopes.
In contrast to light microscopes, electron microscopes (Figure 4.2b) use a beam of electrons instead of a beam of light. Not only does this allow for higher magnification and, thus, more detail (Figure 4.3), it also provides higher resolving power. The method to prepare the specimen for viewing with an electron microscope kills the specimen. Electrons have short wavelengths (shorter than photons) that move best in a vacuum, so we cannot view living cells with an electron microscope.
In a scanning electron microscope, a beam of electrons moves back and forth across a cell’s surface, creating details of cell surface characteristics. In a transmission electron microscope, the electron beam penetrates the cell and provides details of a cell’s internal structures. As you might imagine, electron microscopes are significantly more bulky and expensive than light microscopes.
Link to Learning
For another perspective on cell size, try the HowBig interactive at this site.
The microscopes we use today are far more complex than those that Dutch shopkeeper Antony van Leeuwenhoek, used in the 1600s. Skilled in crafting lenses, van Leeuwenhoek observed the movements of single-celled organisms, which he collectively termed “animalcules.”
In the 1665 publication Micrographia, experimental scientist Robert Hooke coined the term “cell” for the box-like structures he observed when viewing cork tissue through a lens. In the 1670s, van Leeuwenhoek discovered bacteria and protozoa. Later advances in lenses, microscope construction, and staining techniques enabled other scientists to see some components inside cells.
By the late 1830s, botanist Matthias Schleiden and zoologist Theodor Schwann were studying tissues and proposed the unified cell theory , which states that one or more cells comprise all living things, the cell is the basic unit of life, and new cells arise from existing cells. Rudolf Virchow later made important contributions to this theory.
Have you ever heard of a medical test called a Pap smear (Figure 4.4)? In this test, a doctor takes a small sample of cells from the patient's uterine cervix and sends it to a medical lab where a cytotechnologist stains the cells and examines them for any changes that could indicate cervical cancer or a microbial infection.
Cytotechnologists (cyto- = “cell”) are professionals who study cells via microscopic examinations and other laboratory tests. They are trained to determine which cellular changes are within normal limits and which are abnormal. Their focus is not limited to cervical cells. They study cellular specimens that come from all organs. When they notice abnormalities, they consult a pathologist, a medical doctor who interprets and diagnoses changes that disease in body tissue and fluids cause.
Cytotechnologists play a vital role in saving people’s lives. When doctors discover abnormalities early, a patient’s treatment can begin sooner, which usually increases the chances of a successful outcome.
Organismic Biology: Notes on Organismic Biology
The Organismic view is rather popular among biologists of the twentieth century. Those who accept this general frame of reference tend to reject both vitalism and mechanism on the ground that neither provides an adequate explanation of the life process. Organ- ismic- biologists object particularly to mechanists’ tendency to explain the behaviour of living creatures in terms of a collection of reflex arcs.
They also oppose vitalism, but because of its postulation of an unproved life force. A central feature of the Organismic outlook is an emphasis on study of life forms by wholes, rather than in terms of a collection of response connections. Accordingly, Organismic biologists are interested in ho w a total creature, or organism, behaves as it interacts with its environment.
It is true that some who call themselves Organismic biologists regard life forms as machines. These biologists are likely to adopt a behaviouristic psychology. But others, and probably the great majority, avoid entirely the machine analogy. One of the originators and most prominent figures of Organismic biology was Ludwig von Bertalanffy. He describes three points on which the Organismic position diverges from traditional mechanism. These are as follows.
1. Life is a “system” rather than a collection of cooperating parts:
To state the idea differently, life is characterized by organization—an interdependence of parts and some kind of coordinating agency. Further, a living organism is a “system of systems.” That is, a single cell is a system in itself, although it is under die general control of the organ of which it is a part likewise, a single organ, such as the stomach, is a system but under the general control of the entire organism and a total organism is a system whose behaviour is influenced by a still larger system—the “organism-in-its-environment.” An analogy of systems within systems might be our solar system. The sun with all its planets behaves as a unit. Nevertheless, each planet has its own system of interacting forces, as does each moon or manmade satellite of each planet.
2. Life is dynamic rather than static:
That is, life is purposive in the sense that it tries to maintain and better itself in its environment. Its fundamental pose is “activity/exploration, movement.” Life does not wait for environmental stimuli to impinge upon it and thus excite it to action action is its normal state. Purposiveness of higher life forms is likely to involve conscious design, growing from unpredictable wants, and not at all times being related to physiological drives.
3. Life is interactive rather than reactive:
Since the fundamental orientation of an organism is toward exploration and manipulation of its environment, it is unlike a slot machine, which is passive until someone pulls the lever. On the other hand, life forms are modified by environment: living is a two-way process in which a life form and its environment exert simultaneous influence upon each other. This viewpoint contrasts with the traditional mechanists’ view that life is reactive, that, its fundamental pose is waiting, that it acts only after stimulation.
In addition to what Bertalanffy states, it should be noted that Organismic biologists have devoted much study to that side of life which we call “psychological”—in contrast to a mechanist’s almost exclusive interest in the physical side of life. Although Organismic biologists do not presuppose substantive minds, in one sense the Organismic and vitalistic views are alike.
They both have no objection to studying the physical aspects of life—in fact, they would insist that much which is valuable may be learned only in this way. However, they insist that we can achieve useful definitions of-the life process only if we include theories of “central control,” i.e., purposiveness, which we can do only through study of the mental as well as the physical characteristics of life forms.
An eminent American biologist, H. S. Jennings, asserts that, with reference to human beings, biologists have two kinds of data to work from: one kind results from observation of how living beings act (outer, or behavioural, data), the other from observation of mental lives (inner, or cognitive, data). It is proper, Jennings says, to label what we can discover through the use of outer data physical, and what we can discover through inner data mental. Jennings then points out that many biologists and some psychologists have largely ignored the mental aspect of life.
He obviously thinks those scientists who have dismissed mind as unimportant, or non-existent, are misguided. “The universe,” says Jennings, “is a system that brings forth life, sensation, and emotion, thought.” With the development of life, the universe”… begins to become conscious of itself, it begins to feel, to think, to have ideas and purposes and ideals.”
How do Organismic biologists conceive of mind? They regard it as a function, but in a way very different from that of mechanists. To an Organismic biologist, mind is a function that arises in interactive situations—that is, situations in which experience is incurred. The role of this function is to further an organism’s purposes which consist of the shaping and fulfilling of organic need and the creating and fulfilling of nonorganic need.
To be more explicit, mind is the capacity of an organism, in an interactive situation, to see and be guided by meanings. Meaning is the “sign-quality” or “pointing-quality” which objects come to have as a result of an organism’s having experienced them. For example, the “sign-quality”, of a hot stove soon becomes apparent to a small child a stove means. “Touch me and you will get burned.”
To extend the illustrations: growling dogs mean bite, thunderheads mean rain, paddles mean spank, the smell of cooking food means dinner. A human being probably begins acquiring his first meanings during his prenatal period, long before he can verbalize them. But mind becomes of major effectiveness in the life of a human person only after he is old enough to verbalize meanings and only after he has learned to socialize with others.
The foregoing reference to human beings should not be construed to mean that mind, as defined above, is confined to humans. It is evident that all the higher animals may acquire meanings as they interact with their environments. Furthermore, although the “mental” experiences of an amoeba must be very different from those of a Harvard professor, all forms of life may do likewise.
In a single paragraph C. W. Morris, drawing mainly from the thinking of John Dewey, sums up a definition of mind which probably would be satisfactory to most Organismic biologists:
When the ongoing activity of the organism is blocked there arises a situation with the character which Dewey calls “doubtful” or “tensional.” It is in such situations that mind and consciousness make their appearance, serving the purpose of resolving this ambiguity so that the situation can be controlled in the service of the frustrated organic demands or interests.
It should be noted that this view does not make thought instrumental to sheer activity but to specific interests. Nor does it specify the limits of such interests—they may range from the need of food to a solution of the problem of mind. The insistence is simply that thought is inseparably linked with the demands of interested behaviour, and is instrumental to the satisfaction of such demands.
Although the Organismic approach may present enormous complexities, it is also probably a fairly good safeguard against some of the oversimplification of the atomistic, piecemeal approach. If we understand the Organismic principle, and apply it, we never cease looking for relevant factors in a web which we recognize at the outset as being very complex.
Letting Concentration Do the Work
Sometimes cells are in an area where there is a large concentration difference. For example, oxygen molecule concentrations could be very high outside of the cell and very low inside. Those oxygen molecules are so small that they are able to cross the lipid bilayer and enter the cell. There is no energy needed for this process. In this case, it's good for the cell because cells need oxygen to survive. It can also happen with other molecules that can kill a cell.
Sponges lack complex digestive, respiratory, circulatory, reproductive, and nervous systems. Their food is trapped when water passes through the ostia and out through the osculum. Bacteria smaller than 0.5 microns in size are trapped by choanocytes, which are the principal cells engaged in nutrition, and are ingested by phagocytosis. Particles that are larger than the ostia may be phagocytized by pinacocytes. In some sponges, amoebocytes transport food from cells that have ingested food particles to those that do not. For this type of digestion, in which food particles are digested within individual cells, the sponge draws water through diffusion. The limit of this type of digestion is that food particles must be smaller than individual cells.
All other major body functions in the sponge (gas exchange, circulation, excretion) are performed by diffusion between the cells that line the openings within the sponge and the water that is passing through those openings. All cell types within the sponge obtain oxygen from water through diffusion. Likewise, carbon dioxide is released into seawater by diffusion. In addition, nitrogenous waste produced as a byproduct of protein metabolism is excreted via diffusion by individual cells into the water as it passes through the sponge.
Failure of proliferation control
Cancer can arise when the controlling factors over cell growth fail and allow a cell and its descendants to keep dividing at the expense of the organism. Studies of viruses that transform cultured cells and thus lead to the loss of control of cell growth have provided insight into the mechanisms that drive the formation of tumours. Transformed cells may differ from their normal progenitors by continuing to proliferate at very high densities, in the absence of growth factors, or in the absence of a solid substrate for support.
Major advances in the understanding of growth control have come from studies of the viral genes that cause transformation. These viral oncogenes have led to the identification of related cellular genes called protooncogenes. Protooncogenes can be altered by mutation or epigenetic modification, which converts them into oncogenes and leads to cell transformation. Specific oncogenes are activated in particular human cancers. For example, an oncogene called RAS is associated with many epithelial cancers, while another, called MYC, is associated with leukemias.
An interesting feature of oncogenes is that they may act at different levels corresponding to the multiple steps seen in the development of cancer. Some oncogenes immortalize cells so that they divide indefinitely, whereas normal cells die after a limited number of generations. Other oncogenes transform cells so that they grow in the absence of growth factors. A combination of these two functions leads to loss of proliferation control, whereas each of these functions on its own cannot. The mode of action of oncogenes also provides important clues to the nature of growth control and cancer. For example, some oncogenes are known to encode receptors for growth factors that may cause continuous proliferation in the absence of appropriate growth factors.
Loss of growth control has the added consequence that cells no longer repair their DNA effectively, and thus aberrant mitoses occur. As a result, additional mutations arise that subvert a cell’s normal constraints to remain in its tissue of origin. Epithelial tumour cells, for example, acquire the ability to cross the basal lamina and enter the bloodstream or lymphatic system, where they migrate to other parts of the body, a process called metastasis. When cells metastasize to distant tissues, the tumour is described as malignant, whereas prior to metastasis a tumour is described as benign.