

Cell size and scale - araneae
http://learn.genetics.utah.edu/content/begin/cells/scale/

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mrshoe
That zoom effect is great. It reminds me of the ZUI (zooming user interface)
described in Jef Raskin's book, The Humane Interface. Oh how I wish someone
would build a desktop OS with a ZUI.

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cloudkj
Also reminds me of Seadragon from Microsoft Research.

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camccann
A carbon atom is bigger than a water molecule? Interesting.

Speaking of the carbon atom, there is no way that what seems to be its nucleus
is even _remotely_ to scale.

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amoeba
Best I can tell, they have it wrong. They might be interpreting size
differently than I expect but Carbon should be around 220pm and Water around
310pm.

Rough estimates for diameter are based on molecular mass and density; density
being variable. I'm curious how they did their calculations.

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araneae
I'm not sure if it's actually wrong, but rather a function of how they draw
atoms versus molecules.

Diameter is a funny concept for atoms and molecules because electrons orbit at
different distances around the nucleus probabilistically. The depiction of
this carbon atom I think represents this, with the lighter color areas
denoting areas of lower probability.

However, drawing such a diagram for a molecule isn't very pretty, because the
orbitals for each atom in the molecule overlap. They probably drew the water
molecule using intermolecular distances, and then filled in the distances so
that each atom was a sphere. In actuality the electrons are found with some
probability outside this area.

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amoeba
If they're drawing Carbon with respect to its orbitals then they are
completely wrong.

Carbon has both S and P orbitals whereas water has hybridized orbitals. This
would mean both their carbon and water are drawn incorrectly.

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araneae
Actually, as I look at carbon more closely, that's exactly what they're
drawing. The inner circle (s orbital) has two yellow dots (electrons) on it,
and the outer circle (p orbital) has 4 yellow dots. It does look like they
totally ripped off this lady's drawing though: [http://www.dorlingkindersley-
uk.co.uk/nf/ClipArt/Image/0,,_1...](http://www.dorlingkindersley-
uk.co.uk/nf/ClipArt/Image/0,,_1584216,00.html)

Clearly they aren't drawing the orbitals for water, which I already stated.

But you're right, carbon is probably too big. Wolfram alpha says the diameter
of carbon is 134pm, and we all know WA is right about everything.

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araneae
So, I actually submitted feedback about this on the site. Their response:

"Thanks for your response to our website.

Depicting the size of atoms definitely has its challenges. We've been getting
a lot of comments and feedback about this.

The radius of a carbon atom is not a measure of the size of its electron
cloud, but rather the distance between the nuclei of two adjacent atoms. See
<http://www.chemguide.co.uk/atoms/properties/atradius.html> The size of the
carbon atom in our activity is based on its van der Waals radius (170 pm), not
the covalent or ionic radius (around 77 pm). See
<http://www.webelements.com/carbon/atom_sizes.html>

The sizes of the atoms in the water molecule have been adjusted to make them
consistent with other atoms depicted in the activity, although the overall
diameter of the molecule is the same.

Sincerely,

Sheila Avery Genetic Science Learning Center"

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ugh
I didn’t know that chromosomes are so big. The photoreceptor (or sperm, for
that matter) seems way too small to fit 46 (23). What am I missing?

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araneae
DNA only exists in the form of chromosomes at particular parts of a cell's
life (namely, cell division) and is a highly organized structure consisting of
DNA and of proteins.

Neither sperm nor photoreceptors undergo cell division, so for all its
existence its DNA exists in an undifferentiated mass inside the cell. DNA is
long, but skinny, so it compacts quite nicely.

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ugh
Cool. But that also means that photoreceptor cells and sperm divide from cells
quite unlike themselves, right? Or does that work differently?

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amoeba
Stem cells are totipotent and can give rise to any type of specialized cell in
your body. Cell signaling induces horribly complex signaling pathways that
lead to profound changes in transcription of that cell's DNA.

As an embryo, your gonads start in your chest and, through signals from your
brain, end up in your pelvic region.

Erythrocytes (Red blood cells) are a great example of specialization. Before
they become erythrocytes they have a nucleus. Their nucleus is ejected, their
shape completely changes from spherical to flat-with-a-dimple and they are not
a red blood cell. Obviously, erythrocytes cannot divide.

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ugh
Thanks for the explanation!

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amoeba
Cells are great. You're welcome.

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kirubakaran
No wonder you, amoeba, think so.

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lsb
It's like a choose-your-own-adventure version of the Eames Brothers' "Powers
of Ten" movie!

