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April 2014 Teacher's Guide for

Skin Color: A Question of Chemistry


Table of Contents


About the Guide 2

Student Questions 3

Answers to Student Questions 4

Anticipation Guide 5

Reading Strategies 6

Background Information 8

Connections to Chemistry Concepts 15

Possible Student Misconceptions 16

Anticipating Student Questions 17

In-class Activities 18

Out-of-class Activities and Projects 18

References 19

Web Sites for Additional Information 19

About the Guide

Teacher’s Guide editors William Bleam, Donald McKinney, Ronald Tempest, and Erica K. Jacobsen created the Teacher’s Guide article material. E-mail: bbleam@verizon.net


Susan Cooper prepared the anticipation and reading guides.
Patrice Pages, ChemMatters editor, coordinated production and prepared the Microsoft Word and PDF versions of the Teacher’s Guide. E-mail: chemmatters@acs.org
Articles from past issues of ChemMatters can be accessed from a DVD that is available from the American Chemical Society for $42. The DVD contains 30 years of ChemMatters—all ChemMatters issues from February 1983 to April 2013.
The ChemMatters DVD also includes an Index—by titles, authors and keywords—that covers all issues from February 1983 to April 2013, and all Teacher’s Guides from their inception in 1990 to April 2013.
The ChemMatters DVD can be purchased by calling 1-800-227-5558.
Purchase information can be found online at www.acs.org/chemmatters.

Student Questions


(for “Skin Color: A Question of Chemistry”)


    1. What is the function of melanin in our skin?

    2. Why do we want protection from the sun’s ultraviolet (UV) radiation?

    3. Why is it a problem to have unpaired electrons in our DNA?

    4. What is one mechanism by which it is thought melanin molecules protect our cells, particularly the DNA inside the cells?

    5. What are the differences in melanin content between dark skin, light skin, and the skin of albinos?

    6. If all people (except albinos) have the same number of melanocytes (that produce melanin), why do people with different skin color have different amounts of melanin in their skin?

    7. How are the amounts of melanin in the skin and the production of vitamin D related?

    8. Why do we need vitamin D?


Answers to Student Questions


(for “Skin Color: A Question of Chemistry”)


      1. What is the function of melanin in our skin?

Melanin protects our skin from ultraviolet (UV) radiation.

      1. Why do we want protection from the sun’s ultraviolet (UV) radiation?

When UV photons strike our cells, they eject electrons from the DNA inside our cells.

      1. Why is it a problem to have unpaired electrons in our DNA?

Having an unpaired electron in the DNA molecule makes the molecule unstable and the instructions in the DNA cannot be read correctly for execution, possibly creating cellular havoc leading to such things as skin cancer.

      1. What is one mechanism by which it is thought melanin molecules protect our cells, particularly the DNA in the cells?

When UV light is absorbed by melanin, it is transformed into heat. This way, UV light is not absorbed by DNA. (When UV light is absorbed by DNA, it causes the loss of electrons, and it makes parts of the DNA molecule unstable.)

      1. What are the differences in melanin content between dark skin, light skin, and the skin of albinos?

Dark skin possesses the most melanin, light skin lesser amounts, and albino skin is devoid of melanin.

      1. If all people (except albinos) have the same number of melanocytes (that produce melanin), why do people with different skin color have different amounts of melanin in their skin?

The amount of melanin produced in the melanocytes is determined genetically. Genetic instructions differ from person to person so skin color, under genetic control, also varies.

      1. How are the amounts of melanin in the skin and the production of vitamin D related?

Vitamin D is produced through the absorption of UV light by a special molecule called 7-dehydrocholesterol which initiates a series of steps needed to synthesize the vitamin D molecule. The more melanin in the skin, the more UV light is blocked, not reaching the 7-dehydrocholesterol to start the vitamin D synthesis. Thus, the more melanin, the less vitamin D is produced.

      1. Why do we need vitamin D?

We need vitamin D for producing healthy bones and a strong immune system. (The article does not explain the fact that vitamin D promotes the uptake of calcium and phosphorus which are essential in the growth and maintenance of bone tissue.)

Anticipation Guide


Anticipation guides help engage students by activating prior knowledge and stimulating student interest before reading. If class time permits, discuss students’ responses to each statement before reading each article. As they read, students should look for evidence supporting or refuting their initial responses.
Directions: Before reading, in the first column, write “A” or “D,” indicating your agreement or disagreement with each statement. As you read, compare your opinions with information from the article. In the space under each statement, cite information from the article that supports or refutes your original ideas.


Me

Text

Statement







  1. Twins always have the same amount of melanin in their skin.







  1. Exposure to UV radiation can change the DNA in our skin cells, which can lead to skin cancer.







  1. Melanin protects us from UV radiation by blocking the sun’s rays.







  1. The complete chemical structure of melanin has been determined.







  1. The more melanin you have in your skin, the better the UV protection.







  1. Melanin does a better job than sunscreens at converting UV radiation to heat.







  1. Melanin and most typical sunscreens have aromatic rings in their chemical structures.







  1. Everyone, regardless of skin color, has about the same number of cells that make melanin.







  1. People with a high amount of melanin produce a lot of Vitamin D in their skin.







  1. People with darker skin have a higher risk of bone fractures than people with light-colored skin.

Reading Strategies


These graphic organizers are provided to help students locate and analyze information from the articles. Student understanding will be enhanced when they explore and evaluate the information themselves, with input from the teacher if students are struggling. Encourage students to use their own words and avoid copying entire sentences from the articles. The use of bullets helps them do this. If you use these reading strategies to evaluate student performance, you may want to develop a grading rubric such as the one below.

Score

Description

Evidence

4

Excellent

Complete; details provided; demonstrates deep understanding.

3

Good

Complete; few details provided; demonstrates some understanding.

2

Fair

Incomplete; few details provided; some misconceptions evident.

1

Poor

Very incomplete; no details provided; many misconceptions evident.

0

Not acceptable

So incomplete that no judgment can be made about student understanding



Teaching Strategies:


  1. Links to Common Core Standards for writing: Ask students to revise one of the articles in this issue to explain the information to a person who has not taken chemistry. Students should provide evidence from the article or other references to support their position.




  1. Vocabulary that is reinforced in this issue:




  • Solvent

  • Amphoteric compounds

  • Semiconductor

  • Structural formulas

  • Polymerization




  1. To help students engage with the text, ask students which article engaged them most and why, or what questions they still have about the articles.

Directions: As you read, complete the graphic organizer below describing what you learned about melanin when reading the article.


3

Write three new things you learned about melanin from reading this article that you would like to share with your friends.
1.

2.

3.



2

Share two things you learned about chemistry from the reading the article.
1.

2.


1

Did this article change your views about the importance of melanin? Explain in one sentence.

Contact!

Describe a personal experience about melanin that connects to something you read in the article—something that your personal experience validates.



Background Information


(teacher information)
More on the evolution of skin color
The question arises as to why or how skin color of very different shades has come to be. The first point is that the question of “why” is actually related to the “how”, which in turn is related to the basic mechanism of evolution called “natural selection”. For skin color, what force has been acting to select humans with different skin color? To begin with, there is much evidence to suggest that the human stock originated on the African continent. More than likely, this population was very dark skinned which was suitable to the intense sunlight of this environment, particularly near the equator. One of the benefits of having darker skin is to prevent the absorption of too much UV light which in turn can destroy an important biochemical, folic acid, which is an essential nutrient for the development of healthy fetuses. While UV rays can cause skin cancer, it probably had little effect on the evolution of skin color because evolution favors those changes that improve reproductive success. Preventing the destruction of folic acid in darker skinned people means survivability of the next generation of that line of humans. As people migrated out of Africa to both the Asian and European continents above the equator, they located in areas with lower light intensity (including seasonal variation which is not a part of the equator region).
Having darker skin in those areas north of the equator was now a disadvantage, particularly in relation to producing vitamin D since they would have to have longer periods of daily exposure to sunlight in order to produce enough vitamin D (which is not readily available in most food sources). Any individuals with lighter skin in these lower light environments would be favored over darker skinned people because of their ability to produce more vitamin D. But, if these people have a diet rich in seafood (fatty fish such as salmon and mackerel), they have a good source of vitamin D. For some Arctic peoples (natives of Alaska and Canada for instance), their dark skin is not a disadvantage in a region considerably north of the equator because of their vitamin D food source (i.e., fatty fish—salmon and mackerel). And in the summer months, they are protected from excess UV exposure that comes from UV rays reflected from the snow and ice. That is biological success! (See TED lecture by Nina Jablonski, Penn State University professor, at: http://www.ted.com/talks/nina_jablonski_breaks_the_illusion_of_skin_color.html, and her written article on the same subject at: http://scienceline.org/2012/02/the-skin-were-in/)
More on melanin and skin color
Scientists have figured out that several genes are involved in skin color. One of these genes is the melanocortin 1 receptor (MC1R). When MC1R is working well, it has melanocytes convert pheomelanin into eumelanin. If it's not working well, then pheomelanin builds up.

Most people with red hair and/or very fair skin have versions of the MC1R gene that don't work well. This means they end up with lots of pheomelanin, which leads to lighter skin.” (For more information on MC1R and red hair, see http://genetics.thetech.org/ask/ask288.)


Two other skin color genes are SLC24A5 and Kit Ligand gene (kitlg). East Asians get their skin color mostly from a non-working version of kitlg. Northern European people with lighter skin often have a poorly working version of SLC24A5. A small number of pale northern Europeans get their skin color from a non-working MC1R gene.
There are three types of UV light with different functions: UV-A (315 to 400 nm), UV-B (280 to 315 nm), and UV-C (100 to 280 nm). Of the solar UV energy reaching the equator, 95% is UV-A and 5% is UV-B. UV-A activates melanin pigment already in the upper skin (dermis) cells, creating that quick tan. Because UV-A penetrates into the deeper skin layers, it can cause loss of skin elasticity and eventually, wrinkles! Thus, large doses of UV-A cause premature aging of the skin and probably enhance the development of skin cancers.
UV-B stimulates the production of new melanin as well as new skin cells that develop a thicker epidermis. But UV-B rays are also the ones that usually burn the superficial layers of the skin.
UV-C has the shortest wave lengths of all the UV rays, hence is the most energetic. But it is not damaging to our skin because UV-C is absorbed by the ozone layer in the upper atmosphere.
More on importance of vitamin D on various aspects of health
There is evidence that vitamin D plays an important role in a variety of biological functions in humans. Vitamin D is needed to facilitate the absorption of calcium and phosphorus from the gut and into the blood stream. There is statistical data that indicates a significant portion of the population does not have enough vitamin D in their bodies on a daily basis. It is estimated that a billion people worldwide have inadequate levels of vitamin D in their blood—a situation that cuts across all ethnicities and ages. In the U.S. there are a variety of reasons for this situation. One is that people simply do not get outside long enough to generate some vitamin D. Additionally, African-Americans and others with dark skin have much lower levels of vitamin D, as well as the elderly and the obese.
There is medical research that supports the belief that this deficiency or low levels of vitamin D may impact on the health of individuals, including the increased risk of contracting a number of chronic diseases such as osteoporosis, heart disease, some cancers, as well as infectious diseases such as tuberculosis and possibly seasonal flu. What people are debating is how much vitamin D is really needed daily. A report in 2010 recommended tripling the daily vitamin D intake for children and adults to 600 IU per day and changing the upper limit from 2,000 to 4,000 IU per day. Some experts feel that even this increase in recommended minimums is still not enough for bone health and chronic disease prevention.
Sources of vitamin D besides vitamin supplements include dairy products and breakfast cereals fortified with vitamin D, along with fatty fish such as salmon and tuna. Ten non-dairy calcium-rich foods include bok choy, kale, sea vegetables, broccoli, almonds, Brazil nuts, tofu, figs and sesame seeds.
Vitamin D, regardless of origin, is an inactive pro-hormone and must first be metabolized to its hormonal form before it can function. Once vitamin D enters the circulation from the skin or from the lymph, it is cleared by the liver or storage tissues within a few hours.
The chemical steps in synthesizing vitamin D from 7-dehydrocholesterol follows, below.


(http://www.vivo.colostate.edu/hbooks/pathphys/endocrine/otherendo/vitamind.html)
More on the multiple functions of the skin
Skin is the largest organ in our body, weighing twice as much as our brain.

Functions of the skin:




  • Provides a protective barrier against mechanical, thermal and physical injury and
    hazardous substances

  • Prevents loss of moisture

  • Reduces harmful effects of UV radiation

  • Acts as a sensory organ (touch, detects temperature)

  • Helps regulate temperature

  • As an immune organ, detects infections, etc

  • Produces vitamin D

Regulating body temperature is an important function of the skin. For an interactive diagram showing physical changes in skin to regulate body temperature when the environmental temperature changes, go to: http://www.abpischools.org.uk/page/modules/skin/skin3.cfm?coSiteNavigation_allTopic=1.


The skin’s structure includes special glands called sweat glands. Normally, the body cools itself by dilating blood vessels close to the skin’s surface to allow for heat transfer into the atmosphere. At the same time, sweat glands secrete moisture onto the surface of the skin (through pores), from which evaporative cooling provides heat transfer into the atmosphere. On humid days, evaporative cooling is not as efficient and less cooling takes place. It is the loss of these sweat glands in burn victims that contributes to the victim’s body overheating, which is a dangerous situation. And, with the loss of the epidermis and dermis in burn victims, it means that the tissue is subject to drying, loss of evaporative cooling and, of greatest concern, risk of developing infections that cannot be easily treated.
More on artificial skin
In the 1970s, artificial skin was developed to provide a cover to protect badly damaged skin (severe burns) as it regenerates itself. This product was developed by Dr. John Burke, a Harvard surgeon, and Ioannas Yannis, a materials engineer at MIT. Their collaboration produced a skin cover for burn victims that would hydrate the burned area (actually the burned skin is removed, an important step), protect it from drying, and reduce the threat of infection. The material that Burke and Yannis developed is called Integra Dermal Regeneration Template (Integra DRT). Using an artificial product rather than skin, from whatever source, has advantage—including the fact that the product will not be rejected by the recipient (if the grafted skin is not from the patient), and that is free of viruses and bacteria. Again, the main function of the DRT is to induce dermal regeneration, providing a scaffold onto which the patient’s own skin cells can regenerate the dermal layer. The DRT consists of two layers. The bottom layer consists of a matrix of interwoven collagen (from cow protein) and sticky carbohydrate molecules—glycosaminoglycan. This matrix is attached to a flexible silicon sheet. The resulting product looks like a translucent plastic wrap. After the material is placed on a wound, the patient’s own cells infiltrate the sheet, (over a two to four week period), and the top layer of the DRT is removed, to be replaced by a very thin sheet of the patient’s own epithelial cells. Normal epidermis (without hair follicles) develops and the matrix disintegrates over time.
The majority of biomaterials in use today are based on natural or extracted collagen. The basic point of artificial skin is to induce dermal regeneration and supply a protective covering and a pliable scaffold onto which the patient's own skin cells can "regenerate" the lower, dermal layer of skin that was damaged or destroyed.
A current procedure relates to “a method of skin regeneration of a wound or burn in an animal or human. This method comprises the steps of initially covering the wound with a collagen glycosaminoglycan matrix, allowing infiltration of the grafted GC matrix by mesenchymal cells and blood vessels from healthy underlying tissue and applying a cultured epithelial autograft sheet grown from epidermal cells taken from the animal or human at a wound free site on the animal's or human's body surface. The resulting graft has excellent take rates and has the appearance, growth, maturation and differentiation of normal skin.”

(http://www.google.com/patents/US5489304)


One of the more well-developed skin covers (epidermal covers) has the commercial name of “Myskin”. This is a synthetic polymer of acrylic acid that is coated with medical grade silicon. Cultivated keratinocytes (from the patient) readily attach themselves to this polymer. (Keratinocytes make up 95 % of the epidermis, basically acting as a barrier to the environment, preventing excess drying of the skin and blocking penetration of toxins and pathogens into the interior of the skin. They also perform a structural function by keeping the nerves of the skin in place.) When the sheet is placed on a wound bed, the keratinocytes leave the sheet and become incorporated into the wound bed, eventually forming new epidermis. This sheet also provides moisture retention for the damaged skin. (See a video that shows actual skin cultivation [from keratinocytes] onto a protective sheet at http://www.youtube.com/watch?v=WERRXcBRSs4.)
Currently there continues to be research into the techniques for quickly cultivating new skin that can be applied to a burn victim and is not rejected by the recipient, depending on the source (embryonic stem cells, the patient’s non-embryonic stem cells, or layers of skin from the patient).
What are the goals for cultivated or lab-grown skin? “Tissue-engineered skin needs to:

(a) provide a barrier layer of renewable keratinocytes (the cells that form the upper barrier layer of our skin), which is (b) securely attached to the underlying dermis, (c) well vascularized, and (d) provides an elastic structural support for skin.” (http://www.sciencedirect.com/science/article/pii/S1369702108700877)


While lab grown skin is often discussed in the context of burn victims, it has multiple applications. People with unhealed wounds or ulcers could benefit from the product, as could animals used in laboratory testing. L'Oreal holds a patent for lab-grown skin derived from cells discarded during plastic surgery that can be formed into a skin substance. The substance can then be used instead of animals to test reactions to cosmetics.” (See article and video at: http://science.howstuffworks.com/innovation/everyday-innovations/lab-grown-skin.htm.)
A newer technique for growing skin is spraying skin cells directly onto the wound area rather than first growing them in the lab on a foundation material. This treatment is meant only for second degree burns (third degree burns still require skin grafts) in which the top two layers of skin (epidermis and dermis) are damaged, but the subcutaneous layer is still okay. There is actually a “kit” that has been developed for use by surgeons that essentially makes instant spray-on skin cells. The source of the cells would be from what are known as skin progenitor cells found between a person’s epidermal and dermal layers in an area close to the wound. Cells from this region are harvested in the operating room, placed in an incubator the size of a large sunglasses case that contains an enzyme solution. This solution “loosens” the cells between the epidermal and dermal layers of the skin swatch, after which the surgeon can scrape off the cells into another solution where they are suspended. The cell mixture contains the three important types of cells for growth—keratinocytes for healing, fibroblasts for skin structure, and melanocytes for skin color. The mixture can then be sprayed onto a wound for growth and repopulation of the burn site. (See a video about this process at: http://www.youtube.com/watch?v=em_I33KSEKQ.)
Being able to grow new skin outside the body in order to make use of it on various body locations to replace skin that has been damaged by burns remains a very important goal. Burn victims with areas of skin that no longer function are in a state that can prove fatal primarily due to the onset of bacterial infection.
One approach is to use embryonic stem cells. Previously, skin stem cells were collected from a patient’s body and cultivated in a biologically supportive growth medium to produce enough to cover the burn area. But this takes at least three weeks. To cut the growing time, a different approach utilizes embryonic stem cells which are grown on a scaffold at least 40 days in advance of any utilization. Growing the cells on a scaffold, in the right pharmacological mixture of chemicals and proteins, produces multiple-layered tissue. The interesting thing is that by having the bottom layer of cells on the scaffold exposed to the growth medium and the top layer exposed to air, the stem cells form a multiple-layered epithelium that resembles skin and has the same biological properties. Depending on the source of the stem cells, newly reproduced epithelial cells would normally be rejected by the recipient if the proteins on the surface of the new cells are not identical to the recipient’s cell surface proteins. Stem cells from embryos lack the protein that is involved in tissue rejection. Clinical studies are in progress to evaluate this approach to growing new skin.
Finally, a surprisingly simple approach to growing new skin in third-degree burn situations involves nothing more than applying a hydrogel that contains only water and dextran, a polymer made from glucose. The investigators, Goming Sun and Sharon Gerecht of Johns Hopkins University, do not know why this hydrogel, when applied to burn areas, grows new skin in 21 days, complete with hair follicles, blood vessels and skin oil glands. Some ideas as to what might be happening include that fact that the hydrogel may be attracting bone marrow stem cells that are circulating in the blood stream. These stem cells are then “signaled” (chemical stimulus) by the hydrogel to form into skin cells and blood vessels. The investigators have found that inflammatory cells first penetrate and degrade the gel, allowing the infiltration of special cells such as endothelial progenitor cells (derivatives of stem cells) that form blood vessels. The presence of blood vessels supports new tissue growth.
One of the remaining major problems with artificial skin is its vulnerability to infection. It can take a week or two for blood vessels, which carry the immune system’s infection-fighting machinery, to connect to the newly growing dermis. Without blood vessels, bacteria can grow and cause infection, and may destroy the graft and open the wound once more.
More on animal camouflage, a different skin situation
Although skin color is more a static situation in humans, some animals can actively manipulate their skin color for camouflage purposes. The skin color can change according to the environment, as is the case for certain cephalopods such as cuttlefish and squid. There is interest in understanding the mechanisms for skin color in these animals in order to make use of the technique for military camouflage. There are both mechanical and chemical components to some of these camouflage mechanisms.
In the case of the squid, the strategy is to be completely reflective. Like a piece of metal foil …
… the skin of a squid is mirrored to reflect back as much of its surrounding environment as possible. Reflective light includes both visible and infrared. But the way in which the squid’s skin is reflective is not simple metallic reflectivity.
Rather there is a combination of reflective skin layers and pigment sacs for creating color. And in the featureless environment of the ocean, those mirrors become almost invisible.
But what is it that enables the squid to do this? Its secret lies in soft optical materials and, more specifically, in the layer of cells called iridophores that lurk below the colored pigment sacs (acting as filters) in the squid's skin. These contain proteins with a very particular structure responsible for producing an iridescent sheen much like the structures in some birds' feathers and butterfly wings. [Iridescent comes from the Greek word, “iris” which means rainbow.] …
So does structural color or iridescence work for camouflage as it does for the brightly colored displays of butterflies? Sönke Johnsen, a biologist at Duke University, Durham, NC, who last year received a US navy grant of $7.5 million to study cephalopod camouflage, explains what's special about the squid – they can do it dynamically; they realign the protein structures responsible for their iridescence in order to match their surroundings. In the rapidly fluctuating light fields near the sea surface, this can mean constant readjustment. “They can change it on a dime.” says Johnsen. “They switch from one optical characteristic to another, so they could be reflecting blue light and then they can tell their cells to change and all of a sudden they're reflecting green light.” This ability to adapt instantaneously to environmental changes requires softer, more flexible materials than the hard chitin found in butterfly scales and is clearly one that would be of interest to military funders.
(http://www.rsc.org/chemistryworld/Issues/2010/June/HowToDisappearCompletely.asp )
Alison Sweeney, a collaborator with Johnsen, further explains the squid’s ability to adapt,
'There are iridescent cells and then darkly pigmented cells on top of those, and the two of those working in concert are responsible for these dynamic camouflage changes,' explains Alison Sweeney, currently at the University of California at Santa Barbara. In the skin of the squid, she explains, the arrangement of the proteins in the iridescent cells is completely disordered. (This is fairly unusual since cell proteins tend to have definite architectures such as helices or sheets). But chemical stimulation by a neurotransmitter causes the polymers, which are otherwise repelled from each other by their positive charge, to gain negative phosphates that allow them to agglomerate. In more neutral conditions, aromatic interactions begin to dominate and the proteins organize themselves into stacked, plate-like structures - essentially, they 'switch on' iridescence.”
(http://www.rsc.org/chemistryworld/Issues/2010/June/HowToDisappearCompletely.asp )
Iridescence, also known as goniochromism) is the property of certain surfaces that appear to change color as the angle of view or illumination changes. This is essentially refraction or thin film interference as seen in soap bubbles, the surface of a CD or DVD, crumpled cellophane as examples. When light that passes onto or through a medium is reflected back, the different wavelengths in the light come off the reflective surface at different angles producing a separation of these different wave-lengths, thus the “rainbow” effect.”
“More recently, [investigators have] proved that varying the thickness of the platelets could produce color shifts right across the visible spectrum. They also went on to suggest that soft protein materials such as these could have biomedical applications, for instance in smart artificial lenses with self-correcting focal lengths. But this is not the first time the potential of so-called 'reflectin' proteins has been recognized. In a 2007 Nature paper, scientists at the Air Force Research Laboratory in Dayton, US, cast reflectin proteins - engineered to be manufactured in bacteria - in films of varying thickness, resulting in a range of different structural colors. They also showed it was possible to induce dynamic iridescence by exposing reflectins to water vapor, which makes them swell and changes their reflectance - shifting from one wavelength to another.”

(http://www.rsc.org/chemistryworld/Issues/2010/June/HowToDisappearCompletely.asp )


Then there is the color change in the skin of a chameleon. These animals change skin color but not in response to environmental colors (as with squid). Rather, color change is a reflection of the animal’s “mood”. Changing color is a signal, a visual signal of mood and aggression, territory and mating behavior.
A chameleon’s colorful beauty is truly skin deep. Under the transparent outer skin are two cell layers that contain red and yellow pigments, or chromatophores. Below the chromatophores are cell layers that reflect blue and white light. Even deeper down is a layer of brown melanin (which gives human skin its various shades). Levels of external light and heat, and internal chemical reactions cause these cells to expand or contract. A calm chameleon, for example, may exhibit green, because the somewhat contracted yellow cells allow blue-reflected light to pass through. An angry chameleon may exhibit yellow, because the yellow cells have fully expanded, thus blocking off all blue-reflected light from below.
(http://www.pbs.org/edens/madagascar/creature3.htm)

Connections to Chemistry Concepts


(for correlation to course curriculum)


  1. Electromagnetic Radiation (EMR)—Biological material interacts with various parts of the spectrum of electromagnetic radiation (EMR) including both the visible and the invisible, such as IR and UV portions of the spectrum. Several vitamins are involved with light interaction. The article mentions vitamin D formation but vitamin A (retinol, an unsaturated primary alcohol) is part of a visual pigment molecule found in the light sensitive cells of our eyes that transforms EMR energy into nerve impulses, destined for our brain so that we see.

  2. Exothermic—For various energy transformations, there is always some energy that becomes heat energy, an exothermic reaction. In the case of the skin absorbing EMR of different wavelengths, in particular infrared (IR) and ultraviolet (UV), that which does not become chemical potential energy from bond formation will become kinetic energy or heat.

  3. Aromatic Compounds—Organic compounds such as a portion of the melanin molecule and 7-dehydrocholesterol (the molecule that is converted into a vitamin D compound) contain aromatic rings which interact with light, with different outcomes. In the case of the melanin molecule, the molecule is stable when interacting with UV radiation, possibly because of π-electron delocalization that is associated with a resonant structure which seems to confer some kind of stability. Although 7-dehydrocholesterol does contain some aromatic rings, it is not as stable as the melanin molecule and does change its molecular structure when interacting with UV radiation. (See “More on importance of vitamin D …” for a diagram of the steps involved in the chemical changes to 7-dehydrocholesterol initiated by exposure to light.)

  4. Photoionization—This effect is responsible for the change in biological molecules such as DNA when enough energy in the form of photons (minimum amount necessary) is absorbed by a molecule, causing the loss of an electron, creating an unstable molecule (ion, really) due to an unpaired electron. Such a molecule is known as a free radical. The absorption by chlorophyll molecules of specific wavelengths of light in the blue and red regions provides enough energy to raise the energy levels of some electrons in the hydrogen of water molecules, producing hydrogen ions (and an oxygen molecule). The electrons of higher energy become part of a reduction process as the excited electrons join several different molecules including ATP, an energy transfer molecule.

  5. Vitamins—These are organic substances which are absolutely necessary for an animal’s growth and health. They are also substances that cannot be synthesized (vitamin D the exception) and must be supplied through an animal’s diet. Vitamins are of two categories: fat-soluble and water-soluble. An overdose of vitamins (vitaminosis) occurs with the fat-soluble types because they accumulate in the fat tissue (particularly the liver) unlike water-soluble vitamins that are regularly excreted if not used.



Possible Student Misconceptions


(to aid teacher in addressing misconceptions)


  1. Dark skinned people do not have to worry about either sunburn or developing skin cancer—lucky them!” Dark skinned people can develop sunburn but not at the rate of light-skinned people. And contrary to what people might believe, dark skinned people can develop various types of skin cancer because of overexposure to sunlight. This is particularly true if a dark-skinned person overexposes the lighter pigmented areas of the hands and feet to the sun.



Anticipating Student Questions


(answers to questions students might ask in class)


  1. Why can’t you get a suntan (or burn) when sitting behind a glass window?” Glass normally blocks the transmission of UV light which is important for tanning.

  2. Why is it better or safer to wear sunglasses with glass lenses rather than plastic lenses?” Sunglasses with plastic lenses allow UV light to pass through to the eye. And more light, which includes UV, gets into the eye in the first place with plastic lenses, because the pupil dilates more with the darkening effect of the sun glasses than without the sunglasses. (The same is true with glass lenses but the UV would still be blocked.)

  3. What is the basis for naming vitamins by letters?” The vitamins have been assigned letters based on the order in which they were discovered. The exception to this rule is that vitamin K was assigned its “K” from the word “Koagulation” (a Danish word) by the Danish researcher, Henrik Dam. Vitamin K is known as the coagulation or anti-hemorrhagic vitamin because it is essential for the production of pro-thrombin, the precursor of the blood-clotting enzyme, thrombin.

  4. Why can some vitamins be taken in large, even excessive amounts and others are dangerous when taken beyond recommended doses?” It all comes down to what type of vitamin we are talking about. Water-soluble vitamins can be taken in larger doses than is recommended, while fat-soluble vitamins should not be taken in amounts beyond what is recommended. The reason for this is that water-soluble vitamins (the B-complex vitamins and C) are excreted when they build up in the blood system beyond what is taken into the cells. Fat-soluble vitamins (A, D, E, and K) in excess become deposited in the fat tissue, particularly liver fat, where they can become toxic beyond a certain amount. The most recent research suggests that taking vitamin supplements does not provide any benefit and is a waste, that a balanced diet provides adequate amounts of needed vitamins.

  5. Do people with dark skin suffer from sunburn because of an overextended exposure after not being in the sun for a period of time, like being indoors most of the winter months?” Yes, people with dark skin can get sunburn but not as easily (in terms of how long they are exposed) as lighter-skinned persons. An extended period of time away from direct sunlight reduces their protection because they lose their “tan”. Further, darker-skinned people can also develop a variety of skin cancers but in particular, melanomas. Darker-skinned persons have lightly pigmented areas such as the palms and soles of their extremities which can develop melanomas when exposed for a longer time than is “healthy” and/or without adequate protection such as sunblock or clothing.

  6. Specifically, how does vitamin D relate to healthy bones and teeth? Where does calcium fit into the picture? ” Vitamin D is known as a hormone that facilitates the uptake of calcium from the gut. Vitamin D stimulates the expression of a number of proteins involved in transporting calcium from the lumen of the intestine, across the epithelial cells and into blood. It is the calcium that is needed both for strong bones and teeth, as well as a properly functioning immune system.

  7. As a light-skinned person who develops a good, deep tan, am I protected from developing skin cancer?” Depending on how often and how long you are exposed to UV rays, you may still develop skin damage was well as skin cancer in spite of a deep tan. The reason these changes take place is because the effects of UV light on the skin are dependent on the total time of continuous exposure, similar to the way that the danger of exposure to radioactivity is, in part, related to the total time of exposure.

  8. Does exposure to UV light have any effect on the eye?” UV light transmission through the pupil of the eye can affect the lens of the eye, producing cataracts in people with extended exposure to intense sunlight containing UV-B rays. According to the World Health Organization (WHO), “Every year some 16 million people in the world suffer from blindness due to a loss of transparency in the lens. WHO estimates suggest that up to 20 per cent of cataracts may be caused by overexposure to UV radiation and are therefore avoidable. … Cataracts are the leading cause of blindness in the world.” (http://www.who.int/uv/faq/uvhealtfac/en/index3.html)

  9. If the cells of our skin are regularly replaced, why do scars and tattoos persist indefinitely?” The cells in the superficial or upper layers of skin (the epidermis) are constantly replacing themselves. But the deeper layers of skin (dermis) do not go through this cellular turnover and do not replace themselves. Foreign bodies such as tattoo dyes, implanted in the dermis, will remain.


In-class Activities


(lesson ideas, including labs & demonstrations)


  1. There are a number of well-documented lab exercises that test the effectiveness of various sunblock products with different SP ratings. See

  1. http://www.sciencebuddies.org/science-fair-projects/project_ideas/MatlSci_p015.shtml and http://www.sciencebuddies.org/science-fair-projects/project_ideas/MatlSci_p015.shtml#procedure.

  2. A source for an inexpensive UV meter to determine the degree of UV transmission in the sunblock experiments can be found at http://www.amazon.com/gp/product/B00481APE8/ref=as_li_ss_tl?ie=UTF8&tag=sciencebuddie-20&linkCode=as2&camp=1789&creative=390957&creativeASIN=B00481APE8

  3. A third approach to this type of experiment, but using so-called UV beads, which are not as quantitative, is found at http://www.stevespanglerscience.com/lab/experiments/uv-reactive-beads.

  1. Students can design their own experiment to measure the difference in the transmission of UV light through glass vs. plastic. The UV meter mentioned in #1 would be a useful tool for making quantitative measurements through each medium.

  2. If students want to make their own light-sensitive (UV) paper, using Prussian Blue and test their product, consult this reference: http://www.cwu.edu/~petersj/chem101labf08/cyanotype.pdf. For a commercial source of the light-sensitive paper, and for another explanation of how the water-soluble chemical, Berlin Green [iron(III) hexacyanoferrate(III)] is converted to the insoluble Iron(III) hexacyanoferrate(II), better known as Prussian Blue, visit a page from Steve Spangler’s Web site: http://www.stevespanglerscience.com/lab/experiments/sun-sensitive-paper-experiment#sthash.ViCjmMDt.dpuf. He also provides a series of suggested exercises to test sunscreen SPF ratings using the light-sensitive paper at http://www.stevespanglerscience.com/lab/experiments/sun-screen-spf-test.



Out-of-class Activities and Projects


(student research, class projects)


  1. A student project to create a sunblock lotion is found at http://www.virtualsciencefair.org/2012/dosunt. Note that this Web site (Introduction) contains other sections of the project, including procedure, which you can click on to activate.

  2. Students could research the techniques for lightening skin—a practice in many societies such as Egypt where women with darker tan skins desire lighter or even white skin.

  3. Students could research the causes and outcomes for two types of skin cancer—basal and squamous cell skin cancers versus melanoma skin cancer. What are the treatments for each type of skin cancer? Can they be avoided or are they genetically determined? A starting Web reference from the American Cancer Society is found at http://www.cancer.org/cancer/cancercauses/sunanduvexposure/skin-cancer-facts.

References


(non-Web-based information sources)
In the April 1998 issue of ChemMatters, the article “Sun Alert” delves into all aspects of sun (and tanning salon) exposure, including how an SPF (Sun Protection Factor) is calculated for a particular sunblock lotion. Students might find this of interest. In addition, the structural formulas of the more common chemicals in sunblock are illustrated. Note all those aromatic rings! (Baxter, R. Sun Alert. ChemMatters 1998, 16 (2), pp 4–6)

Web Sites for Additional Information


(Web-based information sources)
Web sites about the structure and function of skin
A complete PowerPoint presentation that can be used in class to illustrate various aspects about skin includes its microstructure and function, various skin conditions that can develop. The slides include the several categories of skin cancer (basal and squamous cell skin cancer and melanoma skin cancer). There is also information about some of the more popular beauty treatments for skin, including collagen and Botox injections. Refer to the following Web site: http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCUQFjAA&url=http%3A%2F%2Fdrmagrann.com%2FAnatomy%2F4%2520skin.ppt&ei=_PfvUvSzCdSssAT4u4HYCQ&usg=AFQjCNHCKPXrUx0GdykN8Bc7CnvdGWuavg&sig2=1QNzdckrNxfvp_ZWaCT7ng&bvm=bv.60444564,d.cWc.
A second reference discusses extensively how the evolution of skin color may have come about. Included is a discussion of some experimental data related to the role of folate, its interaction with strong light, and survivability of humans with different degrees of pigmentation. It also provides very graphic illustrations of the interaction of UV light (A, B, and C) with the different structural components of skin. Refer to the following: http://physics.scsu.edu/~dscott/gen/ScientificAmerican/Skin%20Deep.pdf.
Web sites on evolution of skin color and inheritance patterns
A useful Web site for students, if they are interested in the genetics of skin color and inheritance patterns, is found at http://www.saasta.ac.za/Media-Portal/download/bio_fs14.pdf. (Interestingly enough, the Web site comes from South Africa.) There is also reinforcement of the basic ideas behind the right amount of exposure to sunlight for production of vitamin D but avoiding overexposure to sunlight by a pregnant woman thereby destroying the important biochemical folate, needed for a developing fetus.
Web sites on culturing new skin
All the details of how artificial skin is made (with illustrations) can be found at http://www.burnresearchcenter.org/brcpublicwebsite/artificialskin.htm. This reference includes the work of Dr. John Burke, a surgeon (Harvard), and Ioannas Yannis, a materials engineer (MIT), who collaboratively developed the first effective artificial skin called Integra DRT in the 1970s.
A text and video from the Discovery Channel answers the question as to how new skin is grown in the lab. Included in the video is a discussion about the evolution of skin color. Refer to http://curiosity.discovery.com/question/scientists-grow-new-existing-skin.
A detailed article about the biomaterials used in tissue-engineered skin is found at http://www.sciencedirect.com/science/article/pii/S1369702108700877. An important part of this article is the details of the various components of skin and their functions.
Web sites on how radiation affects cells
An article that clearly explains how ionizing radiation affects cells (and the damage that might occur) is found at http://www.rerf.jp/radefx/basickno_e/radcell.htm.
A complementary article with more detailed explanations, particularly with respect to DNA damage (with some diagrams), is found at http://teachnuclear.ca/contents/cna_bio_effects_rad/direct_indirect/.
Web sites on all aspects of skin function
A very comprehensive PowerPoint reference on the structure and function of skin is found at http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCUQFjAA&url=http%3A%2F%2Fdrmagrann.com%2FAnatomy%2F4%2520skin.ppt&ei=_PfvUvSzCdSssAT4u4HYCQ&usg=AFQjCNHCKPXrUx0GdykN8Bc7CnvdGWuavg&sig2=1QNzdckrNxfvp_ZWaCT7ng&bvm=bv.60444564,d.cWc. This reference is well illustrated—good for class use.
Web sites on the details of vitamin D
All you wanted to know about vitamin D is found in this extensive technical discussion about the vitamin. It includes sources, metabolism, functions and physiological actions, and vitamin D action over a human’s life cycle. Refer to

http://www.ncbi.nlm.nih.gov/books/NBK56061/.
Another site that provides extensive information about the relationship between vitamin D and various health issues is found at http://www.hsph.harvard.edu/nutritionsource/vitamin-d/.
The worldwide status of vitamin D nutrition in different populations is found at http://www.ncbi.nlm.nih.gov/pubmed/20197091. There are also nutritional guidelines and food sources of various vitamins, particularly vitamin D, for people without readily available dairy products—as is the case on the African continent.
Non-dairy sources for calcium (for those who are lactose intolerant) are listed in the following short article: http://www.thenational.ae/lifestyle/food/food-for-thought-10-best-non-dairy-sources-of-calcium#ixzz2sTkuPYTI.
An interesting alternative to a vitamin D source uses specially treated mushrooms which generate vitamin D in much the same way as in our skin when it is exposed to sunlight. Refer to http://www.thenational.ae/business/industry-insights/retail/suntanned-mushroom-answer-to-uae-vitamin-d-deficiency.
The problem of vitamin overload is put into context by the medical establishment in this reference: http://www.thenational.ae/lifestyle/well-being/vitamin-overload. Recent medical research news (2014) has concluded that any vitamin supplements are a waste and ineffective, except in those cases where specific vitamin deficiencies are known to exist.


www.acs.org/chemmatters



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