I could not figure out what the sentence shown in below means. What are the meanings of "as" and "they swim up a gradi­ent of nutrients"?

Also what is the meaning of "gradient" in here?

Even humble motile bacteria, as they swim up a gradi­ent of nutrients toward the point of maximum concentration, can be said to have an internal model of their environment.

(from Philosophy and Simulation: The Emergence of Synthetic Reason by Manuel De Landa (2011))

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    No no no, "as" in this sentence doesn't mean "when". It means "since" or "because". It could be rephrased, "Because of the fact that they swim blah blah, we can say that even humble bacteria have an internal model". The swimming is being presented as a logical justification for a general statement. "When" would be more like a condition of time or circumstances: when they swim, we say it of that bacterium, and when they stop swimming we stop saying it. – Steve Jessop Dec 26 '15 at 22:05
  • @SteveJessop - I agree that at the highest level it means "because", but I did not want to confuse the asker. It seems that the immediate meaning is closer to "while", a kind of illustrative "while" -- "look at these humble bacteria while they swim; doesn't it occur to you that they are acting consciously?" – CowperKettle Dec 27 '15 at 5:44
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    I think it's worth pointing out that this is a challenging passage even for native speakers. "Motile" is a technical term that I suspect most English speakers don't know. "Gradient" is a technical term. The concept of "moving up a gradient" is familiar and even intuitive to people with mathematical or scientific training, but not to everyone. So you shouldn't feel bad that this was hard to interpret on a first reading. – Mark Foskey Dec 27 '15 at 16:49

"As" could be roughly glossed as "when" or "while".

When we look upon a bacterium that is swimming in an environment where nutrients are distributed in a non-uniform manner, and we observe how this bacterium is moving in a purposeful way to the area where the concentration of nutrients is higher, we can deduce that the bacterium has an internal model of its enviroment.

That is, that the bacterium is able to "understand" what goes on around it, and to act based on that understanding.

"gradient of nutrients": at one side, you have the maximum concentration of nutrients. An another side, you have the minimum concentration of nutrients. And in the space between these two sides, the concentration changes gradually. This change is gradient. The bacterium swims towards the place where the concentration is maximal. It "feels" the gradient.

Here's a black-white gradient -- Imagine that there are more nutrients in the black part. The bacterium swims towards the black part to get more nutrients:

enter image description here

A definition of "gradient" from Merriam-Webster:

gradient (2): change in the value of a quantity (as temperature, pressure, or concentration) with change in a given variable and especially per unit distance in a specified direction

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    "Gradient" is a mathematical term which is often used in technical writing. Its dictionary definition above is imprecise, but should be sufficient for rough understanding of the quoted text. @verdery, if you want a more detailed definition, please indicate. – laugh salutes Monica C Dec 26 '15 at 20:02
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    It is better to think of "as" like "while" rather than "when". Both 'as' and 'while' are happening right now. 'When' can be some point in the future. – Shane Dec 26 '15 at 21:31

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