Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: pirunner on 01/12/2007 22:00:23
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So...everything in nature has color. This color comes from the way light interacts with the molecules in a object. My question is: Are there specific molecules in natural things that really don't serve any purpose other than creating that color....or do the functional molecules of things just happen to produce the colors that we eventually see as distinctive? (i.e. the yellow of a banana)
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well i suppose you could say pigmints, or light itself, it creates color, but in other words:i dont believe anything is ment to serve a purpose, we use it as something that we found out works for it. i.e. chicken wasn't meant to eat, some caveman decided to eat it, and now we eat it. or lets say dogs, they could have been meant to eat, but now we use them as pets.. understand what i mean?
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well i suppose you could say pigmints, or light itself, it creates color, but in other words:i dont believe anything is ment to serve a purpose, we use it as something that we found out works for it. i.e. chicken wasn't meant to eat, some caveman decided to eat it, and now we eat it. or lets say dogs, they could have been meant to eat, but now we use them as pets.. understand what i mean?
I would disagree on the matter of chickens.
The ancestors of chickens, jungle fowl, were not for eating; but then early man (much later than what we would regard as cave man) captured, and selectively bred the jungle fowl, and what we have now is a species that was strongly modified for humans to farm, and the only reason humans farm chickens is to eat eggs or meat.
The same is probably true of many pigments. It may be that the original pigments (for instance, the colour of a flower or fruit) was accidental. From then, there would have been some co-development, where a particular species of bird or insect might have been able to see the plant better with this colour, and was useful to the plant in order to pollinate or to disperse its seeds. So the plant would selectively breed to be particularly visible for those animals that it might find useful to it, while it would change colour to reduce its visibility to those animals that it felt was harmful to it. At the same time, animals that found it useful to see the plant would also change their vision to better see those plant it found useful (some plants and animals could find it mutually beneficial for the plant to be seen by the animal, while in other cases, it is in the animals interest to see the plant, but not in the plants interest to be seen by the animal). In these cases, what might have started as a slight accidental colour, can then progress to be a modification purely for the purpose on enhancing colour.
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So...everything in nature has color.
Everything has a colour, depending on your eyesight. Most humans would be hard put to find much colour in transparent glass. The same is true of air (but ofcourse, the reason why we say air does not have colour is because our eyes have developed only to see that spectrum of light to which air is transparent).
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Two thirds or so of the earth is covered by water which is, in principle colourless. The whole of the earth's surface is covered by air which, again is colourless. In order to work the eye has to have a lens, cornea and such which are colourless. Most proteins are pretty much colourless; so are DNA and RNA. The carbohydrates are colourless too.
There seems to be something of a contradiction between those observations and the unevinced assertion that "So...everything in nature has color."
A lot of things that are coloured just happen to be so- like the red colour of blood (in many animals). For a very few things, most commonly the green chlorophyl in plants, the colour is an intrinsic part of the way they work. The other common reasons for things being coloured are uv screening (like skin pigmentaion in humans and the colours of many fungi) or the colours of fruits and flowers that act as signals.
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Two thirds or so of the earth is covered by water which is, in principle colourless. The whole of the earth's surface is covered by air which, again is colourless. In order to work the eye has to have a lens, cornea and such which are colourless. Most proteins are pretty much colourless; so are DNA and RNA. The carbohydrates are colourless too.
There seems to be something of a contradiction between those observations and the unevinced assertion that "So...everything in nature has color."
I think it depends on what you mean by colour (which is why I qualified my statement by saying it was colourless to the human eye). Air and water do react to electromagnetic radiation, and so sensors that can detect other regions of the electromagnetic spectrum could detect the 'colour' of air, but that 'colour' is not detectable by the more limited parameters of the human eye.
In the case of air, there are probably no natural eyes that will detect the colour of air; but there are many other colours which can be sensed by the eyes of other species that cannot be sensed by the human eye, thus one has to ask whether you are restricting your definition of colour to that which is detectable by the human eye, or that detectable by any natural eye, or that which might be detectable by any eye, natural or otherwise?
A lot of things that are coloured just happen to be so- like the red colour of blood (in many animals).
Although the actual colour of blood (as defined by the range of electromagnetic wavelengths it reflects) may be an incidental consequence of the iron in haemoglobin; but the fact that our eyes can detect the colour of blood (more specifically, that our eyes can easily detect dilated capillaries in the skin, such as in blushing) may not be such a coincidence. Colour only has meaning in the context of the particular sensors which is seeing it (and the brain used to process signals from those sensors). We see blood as a colour we call red, and that we distinguish from other colours that we use for other purposes; but had there not been a benefit for us to see the colour as red, we could just as easily have evolved eyes that did not see blood as having any colour at all (it does not mean we would have developed such eyes, but that we could have - some evolutionary biologists have speculated that the only reason primates developed sight in the red region of the spectrum was to detect dilated capillaries).
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If you take the view that everything has a colour then it becomes a bit meaningless. Everything apart from a vacuum will interact with em radiation of some sort.
The original question was "Are there specific molecules in natural things that really don't serve any purpose other than creating that color".
The red colour of blood is essentially accidental. The red colour of holly berries is so they atract other animals. To that end they have to be coloured and I don't think the red pigment serves any other purpose.
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If you take the view that everything has a colour then it becomes a bit meaningless.
Agreed, but that was an extreme position to take.
But the point is that if one steps back from saying that everything has colour, then one has to recognise that colour is not a manifestation of the object, but a perception of that object, and one cannot treat colour independently of the means of perceiving it (the eye and the brain).
The red colour of blood is essentially accidental. The red colour of holly berries is so they atract other animals. To that end they have to be coloured and I don't think the red pigment serves any other purpose.
I don't agree on either count.
I have already said that there is speculation that the red colour of blood (most mammals have no receptors for red, and would probably just see blood as being dark) developed in primates specifically to identify blood - or to be more precise, to identify expanded virginal swellings that many female primates have when they are on heat, and this is caused by blood below the skin - in humans, a comparable colour is the reddening of the lips, and blushing in the cheeks.
In terms of hollies - right idea, but wrong signal. Holly berries are poisonous to humans, and many other animals, and the red is an indicator of that poison. Many birds have good vision in the ultra violet, and I would expect that they can see the holly berry in ultra violet, and will know it as good food from that image they have of the berry.
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http://www.bio.bris.ac.uk/research/vision/4d.htm
Exploring the Fourth Dimension
The vivid colours of many animals and plants are not only a source of inspiration to artists and poets, but are of great interest to evolutionary biologists. Ever since Darwin, biologists have used colour variation to test the theory of natural selection itself, and more specific evolutionary theories of signalling, crypsis, mimicry and warning coloration, to name but a few. But in the last decade, the hottest area of research has been sexual selection, Darwin's theory of how female mating preferences can lead to the elaboration of colourful ornaments such as the peacock's tail. Indeed, birds have been the most popular group for such research, but scientists in the Ecology of Vision group at the School of Biological Sciences (University of Bristol) have argued that much of this work is fundamentally flawed.
Introduction
If you watched a wildlife series with, say, the red light source of your television removed (or if you were red-green "colour-blind") and you then came up with conclusions about colour variation in the natural world, would anyone believe you? Probably not, but then that is what we humans are doing every time we think we are seeing the colour world of non-human animals. Unlike other variables such as length, width, mass, or time of day, colour is not an inherent property of the object; it is a property of the nervous system of the animal perceiving the light. In an interdisciplinary collaboration Andrew Bennett, Innes Cuthill and Julian Partridge have been combining techniques from visual physiology and behavioural ecology to investigate the colour world of birds.
Colour Vision in Humans
The sensation of colour stems from the differential stimulation of the different types of photoreceptors in the retina. Each cone type produces an output, and it is their differences in output at a particular point on the retina which underlies the sensation of colour. In humans there are only three types of cones, absorbing maximally in different regions of the spectrum. Due to the appearance (to humans) of monochromatic light at these wavelengths, these three cone types are called "red", "green" and "blue" respectively. Consequently, for humans, all hues can be produced by mixing red, green and blue light. This is how a colour television set works; a mixture of three wavelengths produces several million apparent "colours". This mechanism has a number of consequences. (1) Different wavelength spectra can produce the same hue; as long as the output from the three types of cone remains the same, the hue is the same. (2) The same wavelength spectra will produce different hues to animals that differ in the absorption spectra of their cone types. (3) Humans have trichromatic, or three-dimensional, colour vision because we have three interacting cone types. Animals with two interacting cone types, such as most mammals other than old-world primates, have two-dimensional colour vision (similar perhaps to the faulty colour TV set mentioned earlier). It is harder to imagine what colour vision with more dimensions than three might be like, but animals with 4 and 5-dimensional colour vision exist.
human-visible peacock feather UV peacock feather
Colour Vision in Birds
Bird colour vision differs from that of humans in two main ways. First, birds can see ultraviolet light. It appears that UV vision is a general property of diurnal birds, having been found in over 35 species using a combination of microspectrophotometry, electrophysiology, and behavioural methods. So, are birds like bees? Bees, like humans, have three receptor types, although unlike humans they are sensitive to ultraviolet light, with loss of sensitivity at the red end of the spectrum. This spectral range is achieved by having a cone type that is sensitive to UV wavelengths, and two that are sensitive to "human visible" wavelengths. Remember, because 'colour' is the result of differences in output of receptor types, this means that bees do not simply see additional 'UV colours', they will perceive even human-visible spectra in different hues to those which humans experience. Fortunately, as any nature film crew knows, we can gain an insight to the bee colour world by converting the blue, red and green channels of a video camera into UV, blue and green channels. Bees are trichromatic, like humans, so the three dimensions of bee colour can be mapped onto the three dimensions of human colour. With birds, and indeed many other non-mammalian vertebrates, life is not so simple. As well as seeing very well in the ultraviolet, all bird species that have been studied have at least four types of cone. They have four, not three, dimensional colour vision. Recent studies have confirmed tetra-chromacy in some fish and turtles, so perhaps we should not be surprised about this. It is mammals, including humans, that have poor colour vision! Whilst UV reception increases the range of wavelengths over which birds can see, increased dimensionality produces a qualitative change in the nature of colour perception that probably cannot be translated into human experience. Bird colours are not simply refinements of the hues that humans, or bees, see, these are hues unknown to any trichromat.
Investigating animal colours
Why have so many behavioural and evolutionary ecologists failed to recognise that human colour perceptions are irrelevant for their studies? One reason is the feeling, long dismissed by philosophers, that perception mirrors an objective reality. Yet whilst reflectance spectra can be quantified independent of the observer, colours cannot. Second, until relatively recently it was thought that humans had amongst the best colour vision of any animal, and that most animals' spectral sensitivities lay within the human-visible spectrum. This misapprehension still persists outside the visual sciences. A corollary is that we have long accepted that birds have (human-like) colour vision because many birds are colourful to us. The notion that plumages may be even more colourful to birds, or simply colourful in different ways, has not been widely considered, even though we readily accept that a dog's sense of smell is far richer than our own. The fact that human colour experience can only really be applied to other old world primates has important consequences for testing evolutionary hypotheses. Many of the objects to which evolutionary hypotheses apply, reflect in the UV. Many fruit, flowers, and seeds contrast with their background much more strongly in UV than human-visible wavelengths. Furthermore, and of particular interest to research on sexual selection and mate choice, so do many species of birds' plumage. For example, male and female blue tits look similar to us, but there is a significant sex difference in the UV reflection of several plumage regions (e.g. the 'blue' crest). Some 'human-white' feathers are UV-reflecting, some are not. Reflectance spectroradiometry and multi-spectral cameras that extend into the ultraviolet allow the Bristol team to quantify plumage patterns objectively. By using selective filters to cut out particular wavelengths of light, behavioural experiments have already shown that the ultraviolet component of plumage colours is important in mate choice decisions for species such as starlings and zebra finches. The Bristol approach is to link behavioural ecology to a research program in colour cognition, for if we wish to understand evolutionary hypotheses involving colour we need to understand how animals perceive colour. Ultimately this will not only improve our understanding of animal signals, but might give us an insight to the function and evolution of the different colour vision systems that exist across the animal kingdom.
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Sorry for the lack of clarity; the other I meant in "other animals" that holly beries attract is "other than human"; particularly birds that do eat them. The red spot on seagulls faces and the red plumage of many birds (I guess the robin is the best known) sugests to me that birds can see red and use it for signaling.
The red colour of blood is not required for its function; there are artificial blood substitutes that are colourless.
How humans (and other animals) react to the colour red is another matter entirely. There may be evidence that we picked up the ability to see red so we could tell is out kin were blushing. It's quite likely that we wanted to see if apples were ripe and/ or to avoid holly beries.
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The red colour of blood is not required for its function; there are artificial blood substitutes that are colourless.
No problem with that - and other animals that do not use haemoglobin can also have blood that looks a different colour to human eyes.
How humans (and other animals) react to the colour red is another matter entirely. There may be evidence that we picked up the ability to see red so we could tell is out kin were blushing. It's quite likely that we wanted to see if apples were ripe and/ or to avoid holly beries.
But the point I am making is that if we cannot see red, then red does not exist - it is only red because it looks like red, and for no other reason.
I accept that the question as to whether red was first used for selecting fruits and berries, or first used to see blood is very much still debated; but the point is that if we had not modified our sight to see red, then red would not exist in human vision (and we would not be able to ask other animals whether they see red or not - or even to understand what they would mean by seeing a colour red).
When one looks at most of the electromagnetic spectrum, where we do not have our own subjective senses to give an idea of colour. In those cases, we may objectively describe a 'colour' - but only insofar as we may talk about the frequency/wavelength/energy of the radiation reflected. What is different about subjective colour is that we don't actually have a unique frequency associated with a given colour (yes, we can see the colour yellow - but is yellow a single frequency, or a compound of many frequencies of light - this is how subjective colour is different from simply saying that an object reflects a specific frequency of radiation).
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Red is only called red because we can call it red. True but it generalises to such an extent as to be ridiculous. Soup is only called soup because we call it soup. We only call it red because we can see it as a distinct colour and, if we lacked the apropriate receptor, we wouldn't be able to. On the other hand I'm quite happy to talk of bees seeing UV. (With apologies for the anthropomorphism) from a bee's point of view UV is a colour. Yellow is a perception, and it can indeed be produced by yellow light or a combination of reg and green. I don't see how this matters to the original question (and, for what it's worth I'm not sure if a banana reflects just yellow light or a mixture of red and green; my best bet would be that it reflects all 3).
However the question seemed to be about molecules that exist solely as dyes (like the red berries)rather than, for example, carotene which are used as something else (a free radical trap among other things in this case) and which are coloured my coincidence.
A holly tree needs to produce berries that birds eat- it almost certainly did that before there were any humans to call colours by names. If you look at polination of coloured floweres by insects with colour vision then I think the problem was sorted out by them a long time before there were mammals, never mind primates.
The fact is that the eye is made of water, proteins, carbohydrates and such. The visible region of the spectrun isn't a totally free choice fron the em spectrum. You can only use radiation that you can get through tissue (water absorbs if you go into the IR; water and even more so proteins absorb if you go into the UV. The bees can see UV mainly because their eyes are small so there's not much of the proteins and such in the way to absorb it. Some people can see UV too- those whose lenses and coreneas have been replaced by artificial ones. No animal sees xrays or radio; there's no true sight with IR.
The choice of what we can see is largely determined by what we (and all the other life forms)are made from. To me that seems to be an important aspect of the original question otherwise the simpe answer is that most rocks are coloured but it doesn't serve a purpose.
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Red is only called red because we can call it red. True but it generalises to such an extent as to be ridiculous. Soup is only called soup because we call it soup.
Not quite comparable. Soup is still soup, even to a blind man; but red has no meaning to a blind man, and may have something of an ambiguuous meaning to someone who is red/green colour-blind.
I don't see how this matters to the original question (and, for what it's worth I'm not sure if a banana reflects just yellow light or a mixture of red and green; my best bet would be that it reflects all 3).
However the question seemed to be about molecules that exist solely as dyes (like the red berries)rather than, for example, carotene which are used as something else (a free radical trap among other things in this case) and which are coloured my coincidence.
A holly tree needs to produce berries that birds eat- it almost certainly did that before there were any humans to call colours by names. If you look at polination of coloured floweres by insects with colour vision then I think the problem was sorted out by them a long time before there were mammals, never mind primates.
In terms of the question about the 'purpose' of colour, if any, I agree it is not directly pertinent; but it is relevant to asking what the colour is, or even whether a thing has colour at all.
The point I was trying to make was to say that something that may appear to have one colour to us (or no colour at all), may well have a different colour to a different species, so we cannot talk about a fixed position of saying this thing has this colour.
Most humans may agree that the colour of a banana is yellow, but that is not an innate attribute of the banana, but an interaction that is specific between bananas and humans, and that interaction may be very different for other species of animal.
This line of debate really came out of the question as to whether the premise "everything in nature has colour" has any meaning - and to answer that, one needs to start by asking what is the colour a thing has. Ofcourse, this is further complicated by whether you even regard black or white as colours (I seem to remember that chemists regard white as a non-colour).
The fact is that the eye is made of water, proteins, carbohydrates and such. The visible region of the spectrun isn't a totally free choice fron the em spectrum. You can only use radiation that you can get through tissue (water absorbs if you go into the IR; water and even more so proteins absorb if you go into the UV. The bees can see UV mainly because their eyes are small so there's not much of the proteins and such in the way to absorb it. Some people can see UV too- those whose lenses and coreneas have been replaced by artificial ones. No animal sees xrays or radio; there's no true sight with IR.
The problem with X-rays is that they are difficult to focus, but the eye does respond to X-rays (I was reading recently that the early experimenters did see a glow from an intense x-ray transmitter, but that the experiment has not been reproduced in recent times because of safety concerns).
UV is visible not only to insects, but to birds (with larger eyes).
The choice of what we can see is largely determined by what we (and all the other life forms)are made from. To me that seems to be an important aspect of the original question otherwise the simpe answer is that most rocks are coloured but it doesn't serve a purpose.
Although I would agree with you conclusion, in most cases, but not necessarily with the logic.
The fact that rocks do not evolve, and so from the rocks perspective, it reflects what light it reflects, without any adaptation. On the other hand, what colour we see a rock is peculiar to us, and while the identification of rocks have generally not been important to human evolution, it is not inconceivable that to some animal, such identification might be more important, and so for that animal, the colours attributable to the rocks may not be so accidental.