CRYOGENICS HAZARD, RISKS AND DANGER BASIC INFORMATION
Cryogenics, or low-temperature technology, is the science of producing and maintaining very low temperatures usually below 120 K, as distinct from traditional refrigeration which covers the temperature range 120 to 273.1 K. At or below 120 K, the permanent gases including argon, helium, hydrogen, methane, oxygen and nitrogen can be liquefied at ambient pressure.
Any object may be cooled to low temperatures by placing it in thermal contact with a suitable liquefied gas held at constant pressure. Applications can be found in food processing, rocket propulsion, microbiology, electronics, medicine, metal working and general laboratory operations.
Cryogenic technology has also been used to produce low-cost, high-purity gases through fractional condensation and distillation. Cryogens are used to enhance the speed of computers and in magnetic resonance imaging to cool high conductivity magnets for non-intrusive body diagnostics. Low-temperature infrared detectors are used in astronomical telescopes.
Typical insulating materials include purged rockwool or perlite, rigid foam such as foam-glass or urethane, or vacuum. However, because perfect insulation is not possible heat leakage occurs and the liquefied gas eventually boils away. Uncontrolled release of a cryogen from storage or during handling must be carefully considered at the design stage. The main hazards with cryogens stem from:
• The low temperature which, if the materials come into contact with the body, can cause severe tissue burns. Flesh may stick fast to cold uninsulated pipes or vessels and tear on attempting to withdraw it. The low temperatures may also cause failure of service materials due to embrittlement; metals can become sensitive to fracture by shock.
• Asphyxiation (except with oxygen) if the cryogen evaporates in a confined space.
• The very large vapour-to-liquid ratios so that a large cloud, with fog, results from loss of liquid.
• Catastrophic failure of containers as cryogen evaporates to cause pressure build-up within the vessel beyond its safe working pressure (e.g. pressures ≤280 000 kPa or 40 600 psi can develop when liquid nitrogen is heated to ambient temperature in a confined space).
• Flammability (e.g. hydrogen, acetylene, methane), toxicity (e.g. carbon dioxide, fluorine), or chemical reactivity (fluorine, oxygen).
• Trace impurities in the feed streams can lead to combination of an oxidant with a flammable material (e.g. acetylene in liquid oxygen, solid oxygen in liquid hydrogen) and precautions must be taken to eliminate them.
• Several materials react with pure oxygen so care in selection of materials in contact with oxygen including cleaning agents is crucial.
CHEMICAL STORAGE GENERAL PRINCIPLES BASIC INFORMATION
• Store minimum quantities
• Control stock, i.e. first-in/first-out, move redundant stock
• Segregate chemicals, e.g. from water, air, incompatible chemicals, sources of heat, ignition sources
• Segregate ‘empties’, e.g. cylinders, sacks, drums, bottles
• Monitor stock, e.g. temperature, pressure, reaction, inhibitor content, degradation of substance, deterioration of packaging or containers/corrosion, leakages, condition of label, expiry date, undesirable by products (e.g. peroxides in ethers)
• Spillage control; bund, spray, blanket, containment. Drain to collection pit
• Decontamination and first-aid provisions, e.g. neutralize/destroy, fire-fighting
• Contain/vent pressure generated to a safe area
• Store in ‘safest’ form, e.g. as pre-polymers, as chemical for generation of requirements (e.g. hypochlorites for chlorine) in dilute form
• Handle solids as prills or pellets rather than powders to minimize the possibility of dust formation
• Split-up stocks into manageable lots, e.g. with reference to fire loading/spillage control. Limit stack heights; generally chemicals should be stored off the ground (e.g. to facilitate cleaning, to keep above any ingress of water in the event of flooding)
• Select correct materials of construction; allow for reduction in resistance due to dilution/concentration, presence of impurities, catalytic effects
• Transport infrequently to minimize stocks for both safety and to reduce costs and environmental hazards arising from the need to dispose of surplus or expired material
• Ensure appropriate levels of security, hazard warning notices, fences, patrols. Control access including vehicles
• Segregate/seal drains
• Appropriate gas/vapour/fume/pressure venting, e.g. flame arrestors, scrubbers, absorbers, stacks
• Ensure adequate natural or forced general ventilation of the storage area
• Provide adequate, safe lighting
• Label (name and number); identify loading/unloading/transfer couplings
• Facilitate sampling (for quality assurance and stock monitoring)
• Provide appropriate fire protection (sprinkler, dry powder, gas)
• Consider spacings from buildings, road, fence
• Ensure adequate access for both normal and emergency purposes with alternative routes
• Protect from vehicle impact, e.g. by bollards
• Assign responsibility for administration, maintenance, cleaning and general housekeeping
WHY IS THE SKY BLUE?
The sky is actually a little bit on the violet side. It only looks blue because your eyes are much more sensitive to blue light than to violet light.
When white light from the sun travels through clear air, it hits the molecules of nitrogen and oxygen and gets scattered a little bit, so it travels in a slightly different direction. Since there are miles of air between you and the sun, the light will scatter many times.
But how much the light is scattered depends on the color. Blue light is scattered about 10 times more than red light. This is called Rayleigh scattering, after the man who worked out the math.
But your eyes are not very sensitive to violet light. They are very sensitive to blue light. They are also sensitive to green light and red light. A little bit of the violet light excites the red light sensing cones in your eyes, which is why violet looks like blue with a little red in it. It is also why the sky looks light blue instead of deep blue.
When the sun is near the horizon, there is more air between it and your eyes. The light near the sun has more red and yellow because light that is scattered only one or two times does not change direction much. There is also more dust and smog, which scatter more red and yellow light. So sunsets are red, yellow, orange, and pink.
WHAT IS PLASMA? - PLASMA BASIC INFORMATION
Plasma is a gas that is so hot that the molecules or atoms lose some of their electrons. On Earth, the most common forms of matter are solid, liquid, and gas. But in the universe as a whole, the most common form of matter is plasma.
Plasmas consist of electrons and the positively charged atoms the electrons were stripped from. These atoms that are missing electrons are called ions, and we say that the gas that has become a plasma has been ionized.
Gases that have only a small percentage of their atoms stripped of electrons are said to be weakly ionized. When more of the atoms are affected, the plasma is said to be highly ionized.
Most flames are weakly ionized plasmas. Sparks and lightning can be highly ionized.
Plasmas can emit colors just like flames do. The color of a neon light is due to excited electrons falling back into their normal energy levels, emitting light to lose the extra energy.
Other plasmas emit other colors, since their atoms or molecules have different energy levels. Helium plasmas are pink. Plasmas made from sodium vapor are the characteristic yellow of sodium.
STEM CELLS IN ALZHEIMER'S RESEARCH BASIC INFORMATION
Because the mouse models of Alzheimer’s don’t truly duplicate the disease, researchers have tried making the genetic changes that cause early-onset Alzheimer’s in a variety of cells grown in the lab. This technique has been helpful in letting scientists understand how these genes work normally and what happens when they carry the mutations that cause Alzheimer’s.
However, until recently, scientists weren’t able to make and study these genetic changes in human neurons because, before the advent of human embryonic stem cells and technologies for reprogramming other cells, making human neurons in the lab was, for all practical purposes, impossible.
Scientists can use human stem cells — either embryonic stem cells or induced pluripotent stem cells from tissues — or neuronal stem cells from fetal tissue to grow human neurons that have the genetic mutations that lead to Alzheimer’s. Then they can study what’s different about those neurons and perhaps come up with drugs that repair the damage or keep the damage from spreading.
Similarly, scientists can take other types of cells — say, from the skin — from people with sporadic Alzheimer’s, reprogram them to become pluripotent stem cells, and grow neurons from them. (Larry’s lab at the University of California–San Diego is using this technique.) Then they can compare the behavior of human neurons with the genetic architecture of hereditary Alzheimer’s, sporadic Alzheimer’s, and normal neurons to see where the similarities and differences lie.
This comparison may help researchers determine why some people are less susceptible to Alzheimer’s than others, as well as identify the triggers and mechanisms of the disease — which may, in turn, lead to new therapies for treating Alzheimer’s. (Scientists are using similar approaches to study other diseases, too, such as Lou Gehrig’s and Parkinson’s.)
Scientists also are trying to find ways to use the brain’s own stem cells to replace damaged cells in Alzheimer’s and other diseases. Of course, growing neurons isn’t the only thing you can do with stem cells.
You can also
✓ Use them in cell transplant experiments. Several labs are experimenting with transplanting healthy cells into animals to see whether they replace or rescue damaged or defective cells in mouse models of Alzheimer’s.
✓ Use them to deliver material to specific regions of the brain. In Alzheimer’s patients, their brains may have enough of certain material, such as growth factors, but the material doesn’t get to the regions of the brain that need it. Scientists are exploring ways of using stem cells and other methods to deliver these potentially important materials to the appropriate parts of the brain.
✓ Use them to develop potential drug therapies. Scientists can test drug therapies on cells with hereditary or sporadic Alzheimer’s to see whether the therapies make the cells behave more normally.
Unfortunately, the idea of manipulating the brain’s own stem cells to solve problems outside their normal purview is easy to draw on a blackboard, but not so easy to put into practice. But it’s an exciting possibility with implications for all kinds of neurodegenerative diseases, so the scientific community is eagerly pursuing it.
For decades, researchers thought brains in humans and other mammals were devoid of stem cells. But the human brain (and animal brains, for that matter) does contain two small populations of stem cells. One cache supports the olfactory system (the tissues and organs involved in sensing smell), and the other is in a
region of the brain that’s involved in processing information and forming new memories.
Many researchers are trying to figure out whether these indigenous brain stem cells can be induced to provide rescue activity to regions that are damaged in Alzheimer’s and other neurodegenerative diseases. For example, perhaps these stem cells could be programmed to spawn new neurons to replace damaged or dead ones.
HEATS OF REACTION AND CALORIMETRY BASIC INFORMATION
Another type of energy that contributes to the internal energy of a system is chemical energy. This is energy associated with chemical bonds and intermolecular attractions.
If we think of a chemical reaction as a process in which some chemical bonds are broken and others are formed, then, in general, we expect the chemical energy of a system to change as a result of a reaction.
Furthermore, we might expect some of this energy change to appear as heat. A heat of reaction, is the quantity of heat exchanged between a system and its surroundings when a chemical reaction occurs within the system at constant temperature.
One of the most common reactions studied is the combustion reaction. This is such a common reaction that we often refer to the heat of combustion when describing the heat released by a combustion reaction.
If a reaction occurs in an isolated system, that is, one that exchanges no matter or energy with its surroundings, the reaction produces a change in the thermal energy of the system the temperature either increases or decreases.
Imagine that the previously isolated system is allowed to interact with its surroundings. The heat of reaction is the quantity of heat exchanged between the system and its surroundings as the system is restored to its initial temperature
In actual practice, we do not physically restore the system to its initial temperature. Instead, we calculate the quantity of heat that would be exchanged in this restoration. To do this, a probe (thermometer) is placed within the system to record the temperature change produced by the reaction.
Then, we use the temperature change and other system data to calculate the heat of reaction that would have occurred at constant temperature.
Two widely used terms related to heats of reaction are exothermic and endothermic reactions. An exothermic reaction is one that produces a temperature increase in an isolated system or, in a nonisolated system, gives off heat to the surroundings.
For an exothermic reaction, the heat of reaction is a negative quantity In an endothermic reaction, the corresponding situation is a temperature decrease in an isolated system or a gain of heat from the surroundings by a nonisolated system.
In this case, the heat of reaction is a positive quantity Heats of reaction are experimentally determined in a calorimeter, a device for measuring quantities of heat.
PITOT TUBE BASIC INFORMATION
WHAT IS A PITOT TUBE AND WHAT ARE THE
USES OF PITOT TUBE?
The pitot tube is used to measure the
difference between the impact and static pressures in a fluid. It
normally consists of two concentric tubes arranged parallel to the
direction of flow; the impact pressure is measured on the open end of
the inner tube.
The end of the outer concentric tube is
sealed and a series of orifices on the curved surface give an
accurate indication of the static pressure. The position of these
orifices must be carefully chosen because there are two disturbances
which may cause an incorrect reading of the static pressure.
These are due to:
(1) the head of the instrument;
(2) the portion of the stem which is at
right angles to the direction of flow of the fluid.
These two disturbances cause errors in
opposite directions, and the static pressure should therefore be
measured at the point where the effects are equal and opposite. If
the head and stem are situated at a distance of 14 diameters from
each other as on the standard instrument/41 the two disturbances are
equal and opposite at a section 6 diameters from the head and 8 from
the stem.
This is, therefore, the position at
which the static pressure orifices should be located. If the distance
between the head and the stern is too great, the instrument will be
unwieldy; if it is too short, the magnitude of each of the
disturbances will be relatively great, and a small error in the
location of the static pressure orifices will appreciably affect the
reading.
The two standard instruments are shown
in Figure 6.13; the one with the rounded nose is preferred, since
this is less subject to damage. For Reynolds numbers of 500-300,000,
based on the external diameter of the pitot tube, an error of not
more than 1 per cent is obtained with this instrument.
A Reynolds number of 500 with the
standard 7.94 mm pitot tube corresponds to a water velocity of 0.070
m/s or an air velocity of 0.91 m/s. Sinusoidal fluctuations in the
flowrate up to 20 per cent do not affect the accuracy by more than 1
per cent, and calibration of the instrument is not necessary.
A very small pressure difference is
obtained for low rates of flow of gases, and the lower limit of
velocity that can be measured is usually set by the minimum
difference in pressure that can be measured. This limitation is
serious, and various methods have been adopted for increasing the
reading of the instrument although they involve the need for
calibration.
Correct alignment of the instrument
with respect to the direction of flow is important; this is attained
when the differential reading is a maximum.
For the flow not to be appreciably
disturbed, the diameter of the instrument must not exceed about
one-fiftieth of the diameter of the pipe; the standard instrument
(diameter 7.94 mm) should therefore not be used in pipes of less than
0.4 m diameter.
An accurate measurement of the impact
pressure can be obtained using a tube of very small diameter with its
open end at right angles to the direction of flow; hypodermic tubing
is convenient for this purpose. The static pressure is measured using
a single piezometer tube or a piezometer ring upstream at a distance
equal approximately to the diameter of the pipe: measurement should
be made at least 50 diameters from any bend or obstruction.
The pilot tube measures the velocity of
only a filament of fluid, and hence it can be used for exploring the
velocity distribution across the pipe section. If, however, it is
desired to measure the total flow of fluid through the pipe, the
velocity must be measured at various distances from the walls and the
results integrated.
The total flowrate can be calculated
from a single reading only if the velocity distribution across the
section is already known. Although a single pitot tube measures the
velocity at only one point in a pipe or duct, instruments such as the
averaging pitot tube or Annubar, which employ multiple sampling
points over the cross-section, provide information on the complete
velocity profile which may then be integrated to give the volumetric
flowrate. An instrument of this type has the advantage that it gives
rise to a lower pressure drop than most other flow measuring devices,
such as the orifice meter.
ENERGY REQUIREMENTS FOR DILUTE PHASE CONVEYING
The energy required for conveying can
conveniently be considered in two parts: that required for the flow
of the air alone, and the additional energy necessitated by the
presence of the particles. It should be noted, however, that the
fluid friction will itself be somewhat modified for the following
reasons: the total cross-sectional area will not be available for the
flow of fluid; the pattern of turbulence will be affected by the
solids; and the pressure distribution through the pipeline will be
different, and hence the gas density at a given point will be
affected by the solids.
The presence of the solids is
responsible for an increased pressure gradient for a number of
reasons. If the particles are introduced from a hopper, they will
have a lower forward velocity than the fluid and therefore have to be
accelerated.
Because the relative velocity is
greatest near the feed point and progressively falls as the particles
are accelerated, their velocity will initially increase rapidly and,
as the particles approach their limiting velocities, the acceleration
will become very small.
The pressure drop due to acceleration
is therefore greatest near the feed point. Similarly, when solids are
transported round a bend, they are retarded and the pressure gradient
in the line following the bend is increased as a result of the need
to accelerate the particles again.
In pneumatic conveying, the air is
expanding continuously along the line and therefore the solid
velocity is also increasing. Secondly, work must be done against the
action of the earth's gravitational field because the particles must
be lifted from the bottom of the pipe each time they drop.
Finally, particles will collide with
one another and with the walls of the pipe, and therefore their
velocities will fall and they will need to be accelerated again.
Collisions between particles will be less frequent and result in less
energy loss than impacts with the wall, because the relative velocity
is much lower in the former case.
The transference of energy from the gas
to the particles arises from the existence of a relative velocity.
The particles will always be travelling at a lower velocity than the
gas. The loss of energy by a particle will generally occur on
collision and. thus be a. discontinuous process.
The acceleration of the particle will
be a gradual process occurring after each collision, the rate of
transfer of energy falling off as the particle approaches the gas
velocity.
The accelerating force exerted by the
fluid on the particle will be a function of the properties of the
gas, the shape and size of the particle, and the relative velocity,
it will also depend on the dispersion of the particles over the
cross-section and the shielding of individual particles.
The process is complex and therefore it
is not possible to develop a precise analytical treatment, but it is
obviously important: to know the velocity of the particles.
GASOLINE AND OTHER COMMON FUELS CHEMICAL PROPERTIES BASIC INFORMATION
Gasoline is a mixture of the lighter liquid hydrocarbons that distills within the range of 38 to 204 OC (100 to 400 OF). Commercial gasolines are a mixture of straight -run, cracked, reformed, and natural gasolines.
It is produced by the fractional distillation of petroleum; by condensation or adsorption from natural gas; by thermal or catalytic decomposition of petroleum or its fractions; by the hydrogenation of producer gas or by the polymerization of hydrocarbons of lower molecular weight Gasoline produced by the direct distillation of crude petroleum is known as straight-run gasoline.
It is usually distilled continuously in a bubble tower, which separates the gasoline from the other fractions of the oil having higher boiling points, such as kerosene, &el oil, lubricating oil, and grease. The range of temperatures in which gasoline boils and is distilled off is roughly between 38 and 205 OC (100 and 400 OF).
The yield of gasoline from this process varies from about 1 percent to about 50 percent, depending on the petroleum. Straight-run gasoline now makes up only a small part of gasoline production because of the superior merits of the various cracking processes. The flash point of gasoline is well below -17.8 OC (0 OF) at atmospheric pressure. In atmospheric burning smoke production normally occurs.
In some instances natural gas contains a percentage of natural gasoline that may be recovered by condensation or adsorption. The most common process for the extraction of natural gasoline includes passing the gas as it comes from the well through a series of towers containing a light oil called straw oil.
The oil absorbs the gasoline, which is then distilled off. Other processes involve adsorption of the gasoline on activated alumina, activated carbon, or silica gel. High-grade gasoline can be produced by a process known as hydrofining, that is, the hydrogenation of refined petroleum oils under high pressure in the presence of a catalyst such as molybdenum oxide.
Hydrofining not only converts oils of low value into gasoline of higher value but also at the same time purifies the gasoline chemically by removing undesirable elements such as sulfur. Producer gas, coal, and coal-tar distillates can also be hydrogenated to form gasoline.
Kerosene or sometimes referred to as Fuel Oil # 1 is a refined petroleum distillate. Kerosenes usually have flash points within the range of 37.8 OC to 54.4 OC (100 OF to 130 OF).
Therefore unless heated, kerosene will usually not produce ignitable mixtures over its surface. In atmospheric burning smoke production normally occurs. In some applications it is treated with sulfuric acid to reduce the content of aromatics, which burn with a smoky flame. It is commonly used as a fire and a solvent.
Diesel or sometimes referred to Fuel Oil #2 is the fraction of petroleum that distills after kerosene; which is in the family of gas oils. In atmospheric burning smoke production normally occurs. Several grades of diesel are produced depending on the intended service.
The combustion characteristics of diesel fbels are expressed in terms of a centane number, which is a measure of ignition delay. A short ignition delay, i.e., the time between injection and ignition is desirable for a smooth running engine.
GLYCEROL CHEMICAL PROPERTIES BASIC INFORMATION
WHAT ARE GLYCEROL?
Glycerol (glycerin, melting point: 18oC, boiling point: 290oC, density: 1.2620, flash point: 177oC) is a clear, nearly colorless liquid having a sweet taste but no odor.
Glycerol may be produced by a number of different methods, such as:
1. The saponification of glycerides (oils and fats) to produce soap.
2. The recovery of glycerin from the hydrolysis, or splitting, of fats and oils to produce fatty acids.
3. The chlorination and hydrolysis of propylene and other reactions from petrochemical hydrocarbons.
Natural glycerol is produced as a coproduct of the direct hydrolysis of triglycerides from natural fats and oils in large continuous reactors at elevated temperatures and pressures with a catalyst. Water flows countercurrent to the fatty acid and extracts glycerol from the fatty phase.
The sweet water from the hydrolyzer column contains about 12% glycerol. Evaporation of the sweet water from the hydrolyzer is a much easier operation than with evaporation of spent soap lye glycerin in the kettle process.
The high salt content of soap lye glycerin requires frequent soap removal from the evaporators. Hydrolyzer glycerin contains practically no salt and is readily concentrated.
The sweet water is fed to a triple-effect evaporator where the concentration is increased from 12% to 75 to 80% glycerol. After concentration of the sweet water to hydrolyzer crude, the crude is settled for 48 hours at elevated temperatures to reduce fatty impurities that could interfere with subsequent processing. Settled hydrolyzer crude contains approximately 78% glycerol and 22% water.
The settled crude is distilled under vacuum at approximately 200oC. A small amount of caustic is usually added to the still feed to saponify fatty impurities and reduce the possibility of codistillation with the glycerol.
The distilled glycerin is condensed in three stages at decreasing temperatures. The first stage yields the purest glycerin, usually 99% glycerol and lower-quality grades of glycerin are collected in the second and third condensers. Final purification of glycerin is accomplished by carbon bleaching, followed by filtration or ion exchange.
There are several synthetic methods for the manufacture of glycerol. One process involves chlorination of propylene at 510oC (950oF) to produce allyl chloride in seconds in amounts greater than 85 percent of theory (based on the propylene). Vinyl chloride, some disubstituted olefins, and some 1,2 and 1,3-dichloropropanes are also formed.
Treatment of the allyl chloride with hypochlorous acid at 38oC (100oF) produces glycerin dichlorohydrin (CH2ClCHClCH2OH), which can be hydrolyzed by caustic soda in a 6% Na2CO3 solution at 96oC. The glycerin dichlorohydrin can be hydrolyzed directly to glycerin, but this takes two molecules of caustic soda; hence a more economical procedure is to react with the cheaper calcium hydroxide, taking off the epichlorohydrin as an overhead in a stripping column. The epichlorohydrin is easily hydrated to monochlorohydrin and then hydrated to glycerin with caustic soda.
CH3CH=CH2 + C12 → CH2ClCH=CH2 + HCl
CH2ClCH=CH2 + HOCl → CH2ClCHClCH2OH
CH2ClCHClCH2OH + 2NaOH → CH2OHCHOHCH2OH + 2NaCl
POSSIBILITIES AND LIMITATIONS OF EMBRYONIC STEM CELLS
Embryonic stem cells hold a great deal of promise in treating or even curing a range of devastating diseases. But potential isn’t reality, and, even with all their promise, embryonic stem cells can’t do some things — at least, we don’t think they can.
Unfortunately, these nuances are often missed or blurred when a promising idea or test captures headlines. So here’s a summary of what we really know about embryonic cells: what they can do, what they can’t do, and what we think they may be able to do — not today, but in the relatively near future.
To figure out how cells develop normally and what goes wrong when they don’t, you need a lot of cells to observe and test. Perhaps the most useful property of embryonic stem cells, at least for today’s researchers is that when you grow them properly, you can make lots and lots and lots of embryonic stem cells.
And you can, in turn, use those embryonic stem cells to make lots of specific types of cells. If you want to figure out why pancreatic beta cells misbehave in some types of diabetes, for example, and then find ways to repair or replace them, you need a lot of pancreatic beta cells.
Several research groups have already started testing drugs using cells derived from embryonic stem cells, and a number of researchers are well on their way to transplanting such derivative cells into animals to test the cells’ capacity to change a disease — reduce symptoms or reverse damage. (Scientists need to do these types of tests on animals, and the results have to meet certain benchmarks before they can conduct similar tests on humans.)
Embryonic stem cells seem to be able to make all types of cells in the adult body, which makes them particularly useful in investigating the causes of and possible treatments for a wide range of diseases, from central nervous system disorders like ALS to chronic conditions like diabetes and heart disease.
What embryonic stem cells can’t do?
Even with all their wondrous abilities, embryonic stem cells have their limitations.
For example, they can’t make a baby. That’s because embryonic stem cells are derived from the inner cell mass of a blastocyst. Those inner cells can’t make the placenta that provides nourishment for a developing baby, or the umbilical cord that delivers nutrients to the fetus. Embryonic stem cells can’t be used to clone an adult (at least, as far as we know today).
And they can’t cure disease in and of themselves. You can’t just inject a syringe-full of embryonic stem cells into a mouse or a human and expect them to identify and correct a problem. You have to know what kind of cells to make from embryonic stem cells, how to purify them, and where to put them in the body.
Plus, in order to make testing any potential treatments safe (and to give those potential treatments a strong likelihood of being effective), you need a lot of reliable information about both the cells and the potential
treatment.
Perhaps most important, embryonic stem cells can’t solve all our medical problems and issues overnight. Even with all the exciting things we can do with them now, it’s going to take time — quite a lot of it — to fulfill many of the promises these fascinating cells seem to hold.
What embryonic stem cells may be able to do?
With good ideas and rigorous research, scientists see virtually no limit to what we may be able to do with embryonic stem cells a generation or two from now. The ideas are the key: Creative ideas lead to realities that nobody ever imagined. (After all, do you think the guys who invented the first computer foresaw the Internet?)
Some of the things scientists are daring to imagine now:
✓ Creating cells and tissues that can be transplanted into humans
✓ Developing drugs that are cheaper to make and more effective in treating disease
✓ Testing toxins and environmental factors to see how they affect human development and health
✓ Growing “replacement parts” — new limbs, organs, and tissues that the body accepts as its own, without suppressing the immune system or running the risk of infection
We’re years away from realizing many of these dreams, of course. Human embryonic stem cell research is only about 10 years old; work with adult stem cells, on the other hand, is more than 40 years old, which is why we know so much about some kinds of adult stem cells and have been able to develop some practical applications for them.
BROMINE CHEMICAL PROPERTIES BASIC INFORMATION
Bromine (freezing point: –7.3oC, boiling point: 58.8oC, density: 3.1226) is a member of the halogen family and is a heavy, dark-red liquid.
Bromine is produced from seawater, in which bromine occurs in concentrations of 60 to 70 ppm, and from natural brine, where the concentration of bromine may be as high as 1300 ppm. It can also be produced from waste liquors resulting from the extraction of potash salts from carnallite deposits.
Bromine is isolated from sea water by air-blowing it out of chlorinated seawater.
2NaBr + Cl2 → 2NaCl + Br2
In ocean water, where the concentration of bromine is relatively dilute, air has proved to be the most economical blowing-out agent. However, in the treatment of relatively rich bromine sources such as brines, steaming out the bromine vapor is more satisfactory.
The steaming-out process (Fig. 1) process involves preheating the brine to 90oC in a heat exchanger and passing it down a chlorinator tower. After partial chlorination, the brine flows into a steaming-out tower, where steam is injected at the bottom and the remaining chlorine is introduced.
The halogen-containing vapor is condensed and gravity separated. The top water-halogen layer is returned to the steaming-out tower, and the crude halogen (predominantly bromine) bottom layer is separated and purified.
Crude bromine is purified by redistillation or by passing the vapors over iron filings that remove any chlorine impurity.
Bromine is used for the production of alkali bromides that cannot be manufactured by the action of caustic soda on bromine because hypobromites and bromates are also produced. Thus, the van der Meulen process from the production of potassium bromide involves treating bromine with potassium carbonate in the presence of ammonia.
K2CO3 + 3Br2 + 2NH3 → 6KBr + N2 + 3CO2 + 3H2O
Manufacture of bromine from brine.
EMBRYONIC STEM CELLS PROPERTIES BASIC INFORMATION
In the movie “Star Trek IV: The Voyage Home,” the crew of the Enterprise travels back in time to 20th-century Earth, and Dr. McCoy helps a woman undergoing dialysis. As she’s being wheeled down a hospital corridor, she joyfully tells everyone, “Doctor gave me a pill, and I grew a new kidney!”
Okay, that kind of biological miracle is still the stuff of science fiction. But embryonic stem cells hold enormous potential for giant leaps in medical treatments because of their unique properties:
✓ They can divide and grow more or less indefinitely.
✓ They can develop into any cell type found in the adult body.
✓ They have an incredibly lengthy shelf life, so they can be stored for very long periods without losing their potency.
Growing and growing and growing . . .
Left to their own devices, cells in the blastocyst eventually differentiate, or begin developing special characteristics of job-specific cells. Some become neurons, for example — the brain cells that control thought and movement.
Others spin off into red or white blood cells, or heart muscle cells, or liver cells, and so on. DNA, which provides the blueprints for cell development, and RNA, the user’s manual stating which part of the blueprint to follow, determine which job each cell takes on.
In the lab, though, scientists can delay the reading and implementation of the genetic instructions that tell cells to specialize by isolating the inner cells from blastocysts, transferring them to Petri dishes, and feeding them a mixture of appropriate nutrients (called a growth medium or culture medium) so they can
continue to grow.
Under the correct conditions, embryonic stem cells read the genetic instructions for self-renewal and continue to grow until they’re exposed to signals that tell them to read the genetic instructions for specialization.
Cells like to touch other cells, so scientists usually line the Petri dish with alayer of other cells — most commonly embryonic skin cells from mice that have been treated so they won’t grow. The cells in this layer are called feeder cells.
The inner cells from a blastocyst are placed on top of the feeder cells, and a growth medium (a broth rich in the nutrients the cells need to thrive) is added. Depending on what’s in the growth medium, these inner cells can continue dividing into more embryonic stem cells, or they can begin differentiating into specific categories or types of cells.
One of the risks associated with using mouse feeder cells to grow human embryonic stem cells is that the human cells may be infected with viruses or contaminated with other unwanted material from the mouse cells. Researchers have come up with other ways to grow embryonic stem cells, but only time will tell us whether these alternative methods are as useful and reliable as using mouse cells.
Assuming the relocation of the blastocyst’s inner cell mass into a Petri dish is successful — and that’s not always the case — the embryonic stem cells grow until they crowd the dish. Then they’re removed from the original dish, divided up, and placed into several new dishes with fresh growth media.
This process is called subculturing, and each round of subculturing is called a passage. Scientists can separate, freeze, and store batches of cells at any stage of the subculturing process. They also can ship them to other researchers after they’ve verified that the cells are stable and usable.
It takes at least six months and several passages to create an embryonic stem cell line — that is, millions of cells derived from the original inner cell mass of the blastocyst that meet two critical conditions:
✓ They retain their ability to grow into any kind of cell in the adult body.
✓ They appear to have no genetic defects.
Scientists use a method called karyotyping to make sure that stem cells have the correct number of chromosomes. Normal human cells have a total of 46 chromosomes, paired in sets of two; you inherit one of each set, or 23 chromosomes, from each parent.
Embryonic stem cells’ ability to grow and divide practically indefinitely under the right conditions is an important property for medical research and developing useful treatments. It takes millions of cells to conduct reliable experiments, and one of the biggest eventual challenges in in using stem cells to treat illness is creating enough of them to do the job.
FEATURES OF PERIODIC TABLE OF ELEMENTS
Guide In Using The Periodic Table of Elements.
In the periodic table, elements are listed according to increasing atomic number starting at the upper left and arranged in a series of horizontal rows. This arrangement places similar elements in vertical groups, or families.
For example, sodium and potassium are found together in a group labeled 1 (called the alkali metals). We should expect other members of the group, such as cesium and rubidium, to have properties similar to sodium and potassium. Chlorine is found at the other end of the table in a group labeled 17.
Some of the groups are given distinctive names, mostly related to an important property of the elements in the group. For example, the group 17 elements are called the halogens, a term derived from Greek, meaning salt former.
Each element is listed in the periodic table by placing its symbol in the middle of a box in the table. The atomic number (Z) of the element is shown above the symbol, and the weighted-average atomic mass of the element is shown below its symbol. Some periodic tables provide other information, such as density and melting point, but the atomic number and atomic mass are generally sufficient for our needs.
Elements with atomic masses in parentheses, such as plutonium, Pu (244), are produced synthetically, and the number shown is the mass number of the most stable isotope. It is customary also to divide the elements into two broad categories metals and nonmetals. In figure below, colored backgrounds are used to distinguish the metals (tan) from the nonmetals (blue and pink).
Except for mercury, a liquid, metals are solids at room temperature. They are generally malleable (capable of being flattened into thin sheets), ductile (capable of being drawn into fine wires), and good conductors of heat and electricity, and have a lustrous or shiny appearance.
The properties of nonmetals are generally opposite those of metals; for example, nonmetals are poor conductors of heat and electricity. Several of the nonmetals, such as nitrogen, oxygen, and chlorine, are gases at room temperature.
Some, such as silicon and sulfur, are brittle solids. One bromine is a liquid. Two other highlighted categories in the figure are a special group of nonmetals known as the noble gases (pink), and a small group of elements, often called metalloids (green), that have some metallic and some nonmetallic properties.
The horizontal rows of the table are called periods. (The periods are numbered at the extreme left in the periodic table inside the front cover.) The first period of the table consists of just two elements, hydrogen and helium.
This is followed by two periods of eight elements each, lithium through neon and sodium through argon. The fourth and fifth periods contain 18 elements each, ranging from potassium through krypton and from rubidium through xenon.
The sixth period is a long one of 32 members. To fit this period in a table that is held to a maximum width of 18 members, 15 members of the period are placed at the bottom of the periodic table. This series of 15 elements start with lanthanum and these elements are called the lanthanides.
The seventh and final period is incomplete (some members are yet to be discovered), but it is known to be a long one. A15-member series is also extracted from the seventh period and placed at the bottom of the table. Because the elements in this series start with actinium they are called the actinides.
The labeling of the groups of the periodic table has been a matter of some debate among chemists. The 1-18 numbering system used is the one most recently adopted. Group labels previously used in the United States
consisted of a letter and a number, closely following the method adopted by Mendeleev, the developer of the periodic table.
As seen in the figure, the A groups 1 and 2 are separated from the remaining Agroups (3 to 8) by B groups 1 through 8. The International Union of Pure and Applied Chemistry (IUPAC) recommended the simple 1 to 18 numbering scheme in order to avoid confusion between the American number and letter system and that used in Europe, where some of the Aand B designations were switched! Currently, the IUPAC system is officially recommended by the American Chemical Society (ACS) and chemical societies in other nations. Because both numbering systems are in use, we show both in the figure and in the periodic table inside the front cover.
TOP UNANSWERED QUESTIONS TO EXPLORE ABOUT STEM CELLS
Scientists have been studying adult stem cells for more than 40 years and embryonic stem cells for more than 20 years. They’ve uncovered a lot about both kinds of stem cells, but there’s a lot they still don’t know.
Questions researchers are still seeking answers to include the following:
✓ How many kinds of adult stem cells are there?
✓ Where do adult stem cells live in specific tissues?
✓ What control mechanisms do stem cells use to maintain their selfrenewal capabilities?
✓ What genetic mechanisms control stem cells’ ability to make one or more kinds of differentiated cells?
✓ Why don’t adult stem cells differentiate automatically when they’re surrounded by differentiated cells?
✓ Why can embryonic stem cells grow and make more of themselves in the lab for a year or more, while most adult stem cells have far more limited self-renewing capabilities in a Petri dish?
✓ How do stem cells know when to make more of themselves and when to make cells for specific tissues?
✓ Why don’t all stem cells “home in” to their proper location the way blood-forming stem cells do when they’re transplanted into a living body
✓ If you introduce stem cells into specific tissues in a living body, do they stay where you put them, or do they wander aimlessly around the body’s tissues?
✓ How long do transplanted stem cells stay in the body?
✓ If you reprogram adult cells to behave like embryonic stem cells, are the reprogrammed cells completely normal, or does the reprogramming process mess with the genetic instructions?
✓ In their normal environments (known as niches), can adult stem cells really make differentiated cells for tissues other than their tissue of origin?
✓ Is there a master adult stem cell — one that, like embryonic stem cells, can make any type of cell in the body?
Modern stem cell science is pretty young, so it’s not surprising that researchers still don’t know the answers to some relatively basic questions. As Lao Tzu, the father of Taoism, is credited with saying, “The wise man knows he doesn’t know.”
DNA STRUCTURE AND REPLICATION BASIC INFORMATION
DNA molecules are large, with RMMs up to one trillion (1012). Experimental work by Chargaff and other workers led Crick and Watson to propose that the three dimensional structure of DNA consisted of two single molecule polymer chains held together in the form of a double helix by hydrogen bonding between the same pairs of bases, namely the adenine–thymine and cytosine–guanine base pairs.
In other words, at the ends of the structure one chain has a free 3’-OH group whilst the other chain has a free 5’-OH group. X-Ray diffraction studies have since confirmed this as the basic three dimensional shape of the polymer chains of the B-DNA, the natural form of DNA.
This form of DNA has about 10 bases per turn of the helix. Its outer surface has two grooves, known as the minor and major grooves respectively, which act as the binding sites for many ligands. Electron microscopy has shown that the double helical chain of DNA is folded, twisted and coiled into quite compact shapes.
A number of DNA structures are cyclic, and these compounds are also coiled and twisted into specific shapes. These shapes are referred to as supercoils, supertwists and superhelices as appropriate.
DNA molecules are able to reproduce an exact replica of themselves. The process is known as replication and occurs when cell division is imminent. It is believed to start with the unwinding of the double helix starting at either the end or more usually in a central section, the separated strands acting as templates for the formation of a new daughter strand.
New individual nucleotides bind to these separated strands by hydrogen bonding to the complementary parent nucleotides. As the nucleotides hydrogen bond to the parent strand they are linked to the adjacent nucleotide, which is already hydrogen bonded to the parent strand, by the action of enzymes known as DNA polymerases.
As the daughter strands grow the DNA helix continues to unwind. However, both daughter strands are formed at the same time in the 5’ to 3’ direction. This means that the growth of the daughter strand that starts at the 3’ end of the parent strand can continue smoothly as the DNA helix continues to unwind. This strand is known as the leading strand.
These sections, which are known as Okazaki fragments after their discoverer, are joined together by the enzyme DNA ligase to form the second daughter strand. Replication, which starts at the end of a DNA helix,continues until the entire structure has been duplicated.
The same result is obtained when replication starts at the centre of a DNA helix. In this case unwinding continues in both directions until the complete molecule is duplicated. This latter situation is\more common.
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