user3340: The issue of unanswered question fraction is always in discussion (and it being a factor for site graduation).
In my opinion a lot of mechanisms are unknown in biology (even for some commonly observed processes). This is unlike the problems addressed in stackoverflow or any other discipline for which there will always exist a precise answer (or perhaps less number of unanswerable questions). 
Have a look at these two questions:

What are the side-effects of long-term liquid breathing?
How does Sodium Valproate cause neural plasticity

There may be more like these. They are good questions, logically posed and clear. The issue is that we don't know the answers for these; the research community doesn't know the answers (I assume that because for these two questions I could not find any research papers that answer them). 
What is/should be our policy on such currently unanswerable questions? They do increase our unanswered question count. It is justifiable to keep those questions until the answers are found out and someone posts them but I guess the purpose is to also give answers that are useful at present. One can always add new answers later. 

user338: 2 questions? No big deal - leave them open. I'd say if a large percentage of questions asked are unanswerable atm because of limited research, that would be a case for closing these questions as unclear or whatever, but as there are at most a handful of these right now, a better way to use your 'unanswered' time would be to leave those and answer some of the old forgotten questions no one paid attention to earlier. At least, if you think you can answer well.

user3340: I have an idea about this.
Some of these questions have informative comments. It would be appropriate to compile those comments (retaining the names of the people who posted them) and post them as as community-wiki answers.

user12909: I can tell you math has a lot of unanswered questions. As of yesterday at 18:06, by SE time, our bot told us we have 81963 unanswered questions. In fact, we have so many unanswered that we have a room dedicated to solving them, meet The Crusade of Answers. Additionally, we use a sql query on SEDE called crossbow which finds questions by user over 180days (I think) that are considered unanswered; that is, the question has an answer with no upvote. We can then read the answer and determine if it should be voted on, meet Crossbow. 
If you really feel the unanswered questions are getting outline, you can try what we do and start a room dedicated to finding, answering, and voting on worthy answers of unanswered questions. Presently, I don't think your unanswered is too crazy. I am not sure what SO's number of unanswered questions are but I would think higher than 100k.
The user currently known as Woodface noted that we can use the search feature to find questions with negative and zero score multiple answers with
answers:1 isanswered:no hasaccepted:no closed:no

You can add [your favorite tag] if you want to search in tags to the above query.

user3624: I recently looked at old unanswered questions, and many of these should be closed in my mind (and I voted to close on a number of them). There are certainly many good unanswered Qs (which should remain open), but I think we could in part adress the fraction of answered Qs by going through and closing Qs that are unclear/off-topic etc. This is an easy way to increase the answered%, but will also increase the quality of the site by either removing bad content or pushing askers to improve on their questions.

user1590: I can see two problems with closing questions which are good but have what we deem to be unanswerable due to a lack of suitable research or scientific material:
1) It is based on an assumption that we have enough breadth of expertise among the very regular visitors, some questions may be answerable just not by people that are visiting the site (at all or often enough). I simply don't believe that we have enough highly expert prolific contributors to cover every known facet of biology. I think we would have to increase the number of close votes required because in many cases (where research activity is low) I don't think 5 users could have the joint breadth and depth of knowledge to say that an answer doesn't exist.
2) There may well be an answer soon enough, I think it is perfectly valid to answer one of these questions by saying it is currently unknown while discussing related things are known and trying to infer some kind of expectation - it could even prompt someone in to doing the research!

user948: I have answered a number of questions on Stack Overflow by essentially saying "This is not possible" or "The software you're using doesn't have that feature" or something similar, and more often than not these get accepted as the correct answer. They aren't one-line answers, as I try to back up my assertion with proof, of course, citing any references I can. 
Here, I would think it's perfectly acceptable to write up an answer saying that no research has been done on this subject, or the pathway has not yet been elucidated, or whatever is appropriate for the question. Back this up by explaining what you've done to determine this, and perhaps link to groups that are working on this field, if you can find any. 
We've discussed that we tend to answer questions in the comments a lot, so even if there's a comment saying "This hasn't been studied" or whatever, these questions, as long as they're on-topic, deserve an answer, even if it's just expanding on the comment.

3340: <p>The issue of unanswered question fraction is always in discussion (and it being a factor for site graduation).</p>

<p>In my opinion a lot of mechanisms are unknown in biology (even for some commonly observed processes). This is unlike the problems addressed in stackoverflow or any other discipline for which there will always exist a precise answer (or perhaps less number of unanswerable questions). </p>

<p>Have a look at these two questions:</p>

<ol>
<li><a href="https://biology.stackexchange.com/questions/23074/what-are-the-effects-of-long-term-liquid-breathing">What are the side-effects of long-term liquid breathing?</a></li>
<li><a href="https://biology.stackexchange.com/questions/19856/how-does-sodium-valproate-cause-neural-plasticity">How does Sodium Valproate cause neural plasticity</a></li>
</ol>

<p>There may be more like these. They are good questions, logically posed and clear. The issue is that we don't know the answers for these; the research community doesn't know the answers (I assume that because for these two questions I could not find any research papers that answer them). </p>

<p><strong>What is/should be our policy on such currently unanswerable questions?</strong> They do increase our unanswered question count. It is justifiable to keep those questions until the answers are found out and someone posts them but I guess the purpose is to also give answers that are useful at present. One can always add new answers later. </p>
 338: <p>2 questions? No big deal - leave them open. I'd say if a large percentage of questions asked are unanswerable atm because of limited research, that would be a case for closing these questions as unclear or whatever, but as there are at most a handful of these right now, a better way to use your 'unanswered' time would be to leave those and answer some of the old forgotten questions no one paid attention to earlier. At least, if you think you can answer well.</p>
 3340: <p>I have an idea about this.</p>

<p>Some of these questions have informative comments. It would be appropriate to compile those comments (retaining the names of the people who posted them) and post them as as community-wiki answers.</p>
 12909: <p>I can tell you math has a lot of unanswered questions. As of yesterday at 18:06, by SE time, our bot told us we have 81963 unanswered questions. In fact, we have so many unanswered that we have a room dedicated to solving them, meet <a href="http://chat.stackexchange.com/rooms/9141/the-crusade-of-answers">The Crusade of Answers</a>. Additionally, we use a sql query on SEDE called crossbow which finds questions by user over 180days (I think) that are considered unanswered; that is, the question has an answer with no upvote. We can then read the answer and determine if it should be voted on, meet <a href="http://data.stackexchange.com/meta.math/query/265676/crusaders-crossbow-smite-unanswered-questions-by-finding-worthy-answers-by-use" rel="nofollow noreferrer">Crossbow</a>. </p>

<p>If you really feel the unanswered questions are getting outline, you can try what we do and start a room dedicated to finding, answering, and voting on worthy answers of unanswered questions. Presently, I don't think your unanswered is too crazy. I am not sure what SO's number of unanswered questions are but I would think higher than 100k.</p>

<p>The user currently known as <a href="https://math.stackexchange.com/users/147263/woodface">Woodface</a> noted that we can use the search feature to find questions with negative and zero score multiple answers with</p>

<pre><code>answers:1 isanswered:no hasaccepted:no closed:no
</code></pre>

<p>You can add [your favorite tag] if you want to search in tags to the above query.</p>
 3624: <p>I recently looked at old unanswered questions, and many of these should be closed in my mind (and I voted to close on a number of them). There are certainly many good unanswered Qs (which should remain open), but I think we could in part adress the fraction of answered Qs by going through and closing Qs that are unclear/off-topic etc. This is an easy way to increase the answered%, but will also increase the quality of the site by either removing bad content or pushing askers to improve on their questions.</p>
 1590: <p>I can see two problems with closing questions which are good but have what we deem to be unanswerable due to a lack of suitable research or scientific material:</p>

<p>1) It is based on an assumption that we have enough breadth of expertise among the very regular visitors, some questions may be answerable just not by people that are visiting the site (at all or often enough). I simply don't believe that we have enough highly expert prolific contributors to cover every known facet of biology. I think we would have to increase the number of close votes required because in many cases (where research activity is low) I don't think 5 users could have the joint breadth and depth of knowledge to say that an answer doesn't exist.</p>

<p>2) There may well be an answer soon enough, I think it is perfectly valid to answer one of these questions by saying it is currently unknown while discussing related things are known and trying to infer some kind of expectation - it could even prompt someone in to doing the research!</p>
 948: <p>I have answered a number of questions on Stack Overflow by essentially saying <em>"This is not possible"</em> or <em>"The software you're using doesn't have that feature"</em> or something similar, and more often than not these get accepted as the correct answer. They aren't one-line answers, as I try to back up my assertion with proof, of course, citing any references I can. </p>

<p>Here, I would think it's perfectly acceptable to write up an answer saying that no research has been done on this subject, or the pathway has not yet been elucidated, or whatever is appropriate for the question. Back this up by explaining what you've done to determine this, and perhaps link to groups that are working on this field, if you can find any. </p>

<p>We've <a href="https://biology.meta.stackexchange.com/questions/552/comments-with-answers">discussed</a> that we tend to answer questions in the comments a lot, so even if there's a comment saying <em>"This hasn't been studied"</em> or whatever, these questions, as long as they're on-topic, deserve an answer, even if it's just expanding on the comment.</p>
