A cell is about ten times larger than an atom
Keith S. Taber
Bert was a participant in the Understanding Science Project. Bert was interviewed in Y10 and asked about the topics he had been studying, which included circulation in biology, static electricity in physics, and oxidation in chemistry.
He talked about protons, electrons and atoms in both chemistry and physics, and was asked if this could also link with biology. Bert suggested that the nucleus comes up in chemistry (in the context of atomic structure) and physics (in the context of static electricity), and in biology in the context of cells (see 'The nucleus is the brain of the cell'). From the perspective of the science teacher, there is little basis for confusing the nucleus of an atom with that of a cell: obviously a cell is a complex entity with many components, each of which has itself a complex supra-molecular structure – so clearly the atomic nucleus is on a scale many orders of magnitude smaller than a cell nucleus.
However, the expert perspective is based on relating a lot of knowledge that the novice may not yet have, or at least, may not yet be coordinating. In Bert's case, he was only just starting to coordinate these ideas, as became clear when Bert was asked about the relative sizes of atomic and cell nuclei:
Which do you think is bigger, an atom or a cell, or are they both about the same size?
I'd say a cell.
A correct, but hardly confident and definitive, response. I followed up:
Which do you think is bigger, an atom or a cell, or are they both about the same size?
I'd say a cell.
So which do you think is bigger, the nucleus of a cell or the nucleus of an atom, or do you think they're both about the same size?
I think they're both about, well I should, oh. (Laughs) I'd say the one in the cell is probably bigger.
Why do you think that?
Well it's a cell, I'd have thought it was bigger than the atom. And you know, if the nucleus is kind of the main part of it, then it would probably be about, it would be the • same sort of – If the atom was brought to the size of the cell then the nucleus would be the same size I would have thought. So if the atom is smaller then the nucleus is a lot smaller.
I see, so you are sort of like scaling it, accordingly?
Yeah.
I see. So any idea roughly, just very roughly, how much bigger a cell is than an atom?
Erm oh, it's, they're both really microscopic so, I couldn't really say how much bigger they are than each other.
So it seems that Bert would "have thought [the cell] was bigger than the atom", but he did not seem entirely certain of this, whereas from the scientific perspective the difference in scale is considered vast and highly significant. Although cells are generally microscopic entries, they are more like familiar macroscopic objects that we can handle in everyday life than quanticles such as atoms which do not behave like familiar objects. (So, there is sense in which it is meaningless to talk about the size of atoms as they have no edges or surfaces but rather fade away to infinity.)
Erm oh, it's, they're both really microscopic so, I couldn't really say how much bigger they are than each other.
Mm. No, okay. So if I said a cell was ten times bigger than an atom, a hundred times bigger than an atom, a thousand times bigger than an atom?
I wouldn't say that, I'd say, I'd probably go with the first one you said, ten times bigger.
So roughly ten times bigger than an atom. So a nucleus of a cell you'd expect to be roughly ten times bigger than the nucleus of an atom?
Yeah.
But you're not really sure?
Well no, there are a lot more parts in a cell than there is in an atom. So I'd say the nucleus is… if they're both brought to the same size again, I'd say the nucleus of the atom would be bigger than the cell. But I could be totally wrong.
Oh I see, so you've got two arguments there. That because they, because they both have a nucleus in the middle, that in terms of scale, if the cell is quite a bit bigger than the atom, you'd expect the nucleus of the cell would be quite a bit bigger than the atom. But an atom is quite a simple structure, whereas a cell has a lot more things in it, it's a lot more complex.
Yeah.
So maybe there's not so much room for the nucleus of the cell as there is for an atom because you've got to fit so much more in.
Yeah.
Is that what you're thinking?
Yeah.
Bert's thinking here is quite reasonable, within the limits of his knowledge. He suggests that a cell nucleus will be larger than an atomic nucleus, because a cell is larger than an atom. However, he only think the cell nucleus will be about ten times the size of the atomic nucleus as he suspects the cell is only about ten times the size of an atom – after all they are both "really microscopic".
However, he also points out that a cell seems to have a more a lot more components to be fitted in, which would suggest that perhaps there is less space to fit the nucleus, so perhaps it would not be as much as ten times bigger than the atomic nucleus.
So Bert is able to consider a situation where there may be several factors at work (the size of the cell versus the size of the atom; the multitude of cellular components versus the sparsity of atoms) and appreciate how they would operate in an opposite sense within his argument so one could compensate for the other. (This type of thinking is needed a lot in studying science. One example is comparisons of ionisation enthalpies between different atoms and ions. I also recall physics objective examination questions that asked students to compare, say, the conductance of two wires with different resistivity, length and area.)
It is not reasonable to expect Bert to know just how much larger a typical cell nucleus is to an atomic nucleus, however, it is likely the science teacher would expect Bert to be aware that the nucleus is one small part of the atom, which is a constituent of the molecules and ions that are the chemical basis for the organelles such as nuclei found in cells. Bert had told me "there are lots of atoms in you", but he did not seem to have understood the role those atoms played in the structure of all tissues. This would seem to be an example of a fragmentation learning impediment, where a learner has not made the connections between topics and ideas that a science teacher would have intended and expected.
